<%BANNER%>

Design and Fabrication of Organic Semiconductor Photodiodes

Permanent Link: http://ufdc.ufl.edu/UFE0042771/00001

Material Information

Title: Design and Fabrication of Organic Semiconductor Photodiodes
Physical Description: 1 online resource (237 p.)
Language: english
Creator: HAMMOND,WILLIAM T
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: CELL -- DEPOSITION -- FABRICATION -- GAIN -- LUMINESCENCE -- ORGANIC -- PHOTODIODE -- PHOTOTRANSISTOR -- PHOTOVOLTAIC -- PROCESSING -- SEMICONDUCTOR -- SOLAR -- SPRAY
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic materials are synthetically numerous, mechanically flexible, light weight, processable at low temperature, and chromatically diverse. For those organic materials with favorable electronic structure and classified as semiconductors, these unique characteristics can be readily leveraged for functional electronic and optoelectronic applications. This thesis examines the wide scope of realizable device structures that are enabled by the processability and adaptability of these materials, focusing on photodiode devices designed for both the detection and the harvesting of optical energy. In the first part, two device structures are explored for their ability to output a photocurrent gain. Photocurrent gain, in which one incident photon gives rise to a multiple number of secondary photo-electrons cycled through an external circuit, is an important device property for applications that require the detection of low light signals and also those that require a more simplified electronic circuit. The field-effect transistor device structure is first studied, and it is found that although this structure can give rise to a small photocurrent gain up to G = 2, there are a number of trade-offs that may limit functionality. Next, a new multilayer organic photodiode structure is designed and studied that uses a carrier-selective confinement material to produce an extraordinarily efficient photoconductive gain mechanism, achieving G = 100-200 across the entire visible spectrum under a low applied bias (V = -3 V). The nature of the gain mechanism leads to relatively high bandwidth (f = 1 kHz), and optimized devices are found to produce record gain-bandwidth product, up to GBP = 10^5, among organic photodetectors operating above imaging-compatible bandwidth (>60 Hz). In the second part, the focus is shifted to organic photovoltaic devices. First, a new device structure is proposed that utilizes a photoluminescent external absorption antenna in order to down-convert part of the incident solar spectrum into an emission spectrum that is more efficiently absorbed by the solar cell. The application of a down-conversion structure to a few typical organic solar cells is computationally studied and it is found that the concept can enhance organic solar cell performance. Depending on the material and device structure, the enhancement to total solar cell absorption efficiency can reach 27%. Finally, as the ultimate advantage of organic solar cells may lie in their processability and commercial scalability, the final topic studied focuses on the processing of these cells by spray deposition. Spray deposition may enable low cost manufacture, however it inherently produces rough and non-ideal film morphology. Here it is proposed and confirmed that the addition of alkane diluents to a solution of polymer semiconductor and solvent can improve the uniformity of spray droplet deposition patterns. Furthermore, it is shown that the improvement in uniformity can significantly enhance solar cell power conversion efficiency (PCE) by up to 40%, reaching performance similar to that achieved by the highly uniform spin coating method.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by WILLIAM T HAMMOND.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0042771:00001

Permanent Link: http://ufdc.ufl.edu/UFE0042771/00001

Material Information

Title: Design and Fabrication of Organic Semiconductor Photodiodes
Physical Description: 1 online resource (237 p.)
Language: english
Creator: HAMMOND,WILLIAM T
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: CELL -- DEPOSITION -- FABRICATION -- GAIN -- LUMINESCENCE -- ORGANIC -- PHOTODIODE -- PHOTOTRANSISTOR -- PHOTOVOLTAIC -- PROCESSING -- SEMICONDUCTOR -- SOLAR -- SPRAY
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Organic materials are synthetically numerous, mechanically flexible, light weight, processable at low temperature, and chromatically diverse. For those organic materials with favorable electronic structure and classified as semiconductors, these unique characteristics can be readily leveraged for functional electronic and optoelectronic applications. This thesis examines the wide scope of realizable device structures that are enabled by the processability and adaptability of these materials, focusing on photodiode devices designed for both the detection and the harvesting of optical energy. In the first part, two device structures are explored for their ability to output a photocurrent gain. Photocurrent gain, in which one incident photon gives rise to a multiple number of secondary photo-electrons cycled through an external circuit, is an important device property for applications that require the detection of low light signals and also those that require a more simplified electronic circuit. The field-effect transistor device structure is first studied, and it is found that although this structure can give rise to a small photocurrent gain up to G = 2, there are a number of trade-offs that may limit functionality. Next, a new multilayer organic photodiode structure is designed and studied that uses a carrier-selective confinement material to produce an extraordinarily efficient photoconductive gain mechanism, achieving G = 100-200 across the entire visible spectrum under a low applied bias (V = -3 V). The nature of the gain mechanism leads to relatively high bandwidth (f = 1 kHz), and optimized devices are found to produce record gain-bandwidth product, up to GBP = 10^5, among organic photodetectors operating above imaging-compatible bandwidth (>60 Hz). In the second part, the focus is shifted to organic photovoltaic devices. First, a new device structure is proposed that utilizes a photoluminescent external absorption antenna in order to down-convert part of the incident solar spectrum into an emission spectrum that is more efficiently absorbed by the solar cell. The application of a down-conversion structure to a few typical organic solar cells is computationally studied and it is found that the concept can enhance organic solar cell performance. Depending on the material and device structure, the enhancement to total solar cell absorption efficiency can reach 27%. Finally, as the ultimate advantage of organic solar cells may lie in their processability and commercial scalability, the final topic studied focuses on the processing of these cells by spray deposition. Spray deposition may enable low cost manufacture, however it inherently produces rough and non-ideal film morphology. Here it is proposed and confirmed that the addition of alkane diluents to a solution of polymer semiconductor and solvent can improve the uniformity of spray droplet deposition patterns. Furthermore, it is shown that the improvement in uniformity can significantly enhance solar cell power conversion efficiency (PCE) by up to 40%, reaching performance similar to that achieved by the highly uniform spin coating method.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by WILLIAM T HAMMOND.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Xue, Jiangeng.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2012-04-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0042771:00001


This item has the following downloads:


Full Text

PAGE 1

DESIGNANDFABRICATIONOFORGANICSEMICONDUCTORPHOTODIODES By WILLIAMTHOMASHAMMOND ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 2011

PAGE 2

c 2011WilliamThomasHammond 2

PAGE 3

Tomyparentsandmyteachers 3

PAGE 4

ACKNOWLEDGMENTS ThisworkwouldnothavebeenpossiblewithoutthesupportandguidanceI receivedfromagreatnumberoffamily,friendsandcolleagues.First,Iwouldliketo thankmyMomandmyDadfortheirperpetualloveandsupportthroughoutmyextended academicjourney.IalsothankmybrotherKevinandallofmyfriendsfortheirsupport andencouragementthroughoutthesechallengingyears.Andaspecialthankyougoes toNoelleIdenitelycouldnothavedonethiswithoutyou. NextIwouldliketothankmyadvisor,Prof.JiangengXue,forhismentorship throughoutmytimeattheUniversityofFlorida.Histhoroughknowledgeandacademic rigorprovidedmewithaclearexampletofollow.IalsothankProf.StevePearton,Prof. FanRen,Prof.WolfgangSigmund,andProf.FrankySoforservingonmysupervisory committeeandfortheirguidancethroughmydoctoralwork. IalsowanttoexpressmygratitudetocolleaguesoftheXueResearchGroup, withwhomIhavebeenfortunatetosharesomuchtimeovertheyears.IthankTengKuanTsengandYingZhengforpioneeringourlab;Sang-HyunEomforsettinga greatexample;JasonMyersforsharingthejoysoflabengineering;RobelBekele, EdWrzesniewski,YixingYang,WeiranCao,andRenjiaZhoufortheirfriendship andcamaraderieovertheyears;JohnMudrick,NateShewmon,andMattRippefor embodyingthefutureexcellenceofourgroup;andVinnyCassidy,CaitlinDennis,and JessicaLedermanforacceptingmyimperfectresearchadvice.Finally,Ithankthe NationalScienceFoundationandFloridaEnergySystemsConsortiumforfundingthat enabledthiswork. 4

PAGE 5

TABLEOFCONTENTS page ACKNOWLEDGMENTS..................................4 LISTOFTABLES......................................9 LISTOFFIGURES.....................................10 ABSTRACT.........................................14 CHAPTER 1INTRODUCTIONTOORGANICSEMICONDUCTORS..............16 1.1AShortHistoricalPerspective.........................16 1.2OverviewofOrganicSolids..........................20 1.3MolecularBasisforChargeTransportinOrganicSolids..........22 1.3.1AtomicandMolecularOrbitals.....................22 1.3.2HybridizationandConjugationofCarbon...............24 1.4OptoelectronicProperties...........................25 1.4.1ExcitedStatesofOrganicSolids...................25 1.4.2ExcitonPhotophysics..........................28 1.4.3ExcitonEnergyTransferandMigration................33 1.4.4ExcitonDissociationandDonor-AcceptorHeterojunctions.....35 1.5ElectronicTransportProperties........................36 1.5.1ChargeInjectionatMetal-OrganicandOrganic-OrganicInterfaces37 1.5.2HoppingTransportandChargeMobility...............38 1.5.3EffectofMolecularMorphologyonChargeTransport........40 1.5.4BulkElectronicTransport.......................41 1.5.5TransportEnergyLevels........................43 2ORGANICPHOTODIODESASPHOTODETECTORS..............47 2.1OrganicPhotodetectorApplicationsandAdvantages............47 2.2OperationandCharacterizationofOrganicPhotodiodesforSignalDetection......................................49 2.2.1DarkDiodeCurrent...........................51 2.2.2SpectralResponsivity.........................51 2.2.3TemporalResponse..........................53 2.2.4Noise..................................54 2.2.5Sensitivity................................56 2.3PhotocurrentGenerationinOrganicSemiconductorDevices.......57 2.4DeviceStructureinOrganicPhotodetectors.................58 2.5PhotomultiplicationinOrganicSemiconductors...............60 5

PAGE 6

3ORGANICPHOTODIODESASSOLARCELLS..................63 3.1SolarEnergy..................................63 3.1.1SolarSpectrum.............................63 3.1.2SolarCells................................65 3.2OrganicPhotovoltaicCells...........................70 3.2.1SmallMolecules,Polymers,andProcessing.............71 3.2.2DeviceStructureinOrganicSolarCells...............73 3.3OperationandCharacterizationofSolarCells................77 3.3.1SolarSimulation............................78 3.3.2Current-VoltageCharacterization...................81 3.3.3EffectofParasiticResistances....................83 3.3.4SpectralResponse...........................85 3.3.5DegradationofOrganicSolarCells..................86 4ORGANICFIELDEFFECTPHOTOTRANSISTORS...............88 4.1Introduction...................................88 4.1.1Motivation................................88 4.1.2OrganicFieldEffectTransistors....................89 4.1.3OrganicPhototransistorGain.....................91 4.1.4OrganicPhototransistorLiteratureReview..............91 4.2Experiment...................................92 4.2.1DeviceFabrication...........................92 4.2.2TransistorCharacterization......................95 4.2.3PhototransistorPhotoresponseCharacterization..........96 4.3ResultsandDiscussion............................97 4.3.1TransistorOperationinDark......................97 4.3.2StaticPhotocurrentCharacterization.................98 4.3.3DynamicPhotocurrentCharacterization...............99 4.3.4PhototransistorPhotoresponseandInstability............102 4.4ConclusionsandFutureWork.........................104 5PHOTOMULTIPLICATIONBYCARRIERCONFINEMENTINORGANICPHOTODIODES......................................106 5.1OrganicPhotodiodeswithPhotocurrentGain................106 5.2CarrierConnement..............................107 5.3DeviceDesignandGainMechanism.....................110 5.4Experiment...................................113 5.4.1DeviceFabrication...........................113 5.4.2PhotocurrentandAbsorptionMeasurement.............114 5.4.3TransientPhotocurrentMeasurement.................115 5.4.4DarkCurrentandNoiseCurrentMeasurements...........115 5.5ResultsandDiscussion............................116 5.5.1EffectoftheBlockingLayerStructure.................116 6

PAGE 7

5.5.2EffectofBulkHeterojunctionMixingRatio..............120 5.5.3TemporalResponse..........................122 5.5.4DarkCurrentandOperationalDegradation..............125 5.6Conclusions...................................129 6DOWN-CONVERSIONFORORGANICSOLARCELLABSORPTIONEFFICIENCYENHANCEMENT.............................130 6.1Introduction...................................130 6.1.1Motivation................................130 6.1.2Concept:Down-ConversioninAdvancedDeviceArchitecture...131 6.2OpticalSimulation...............................134 6.2.1ConceptualOverview..........................134 6.2.2ModelDesign..............................135 6.3LuminescentEnhancementLayer.......................138 6.3.1DesignandMaterialSelection.....................138 6.3.2Solution-DepositedLuminescentEnhancementLayers.......140 6.4Down-ConversioninOrganicSolarCells...................142 6.4.1ModeledAbsorptioninaStandardC 60 :PbPcSolarCell......144 6.4.2Down-ConversionDeviceStructures.................146 6.4.3SimulatedDependenceonDeviceStructure.............148 6.4.4SimulatedDependenceonDonorMaterialAbsorption.......151 6.5ConclusionsandFutureWork.........................152 7SPRAYDEPOSITEDPOLYMERSOLARCELLS:EFFECTOFSOLUTION PROPERTIES....................................158 7.1Introduction...................................158 7.1.1Motivation................................158 7.1.2DenitionofProblem..........................159 7.2TheoryandHypothesis............................160 7.2.1FloodSprayandMulti-PassSpray..................160 7.2.2Multi-PassSprayandtheCoffeeRingEffect.............162 7.2.3MarangoniFluidFlowinDroplets...................164 7.2.4AlkaneAdditivesforSprayDeposition................166 7.3Experiment...................................170 7.3.1CustomSpraySystem:ConstructionandOptimization.......170 7.3.2FilmandDeviceFabrication......................175 7.3.3FilmAbsorptionandDeviceQuantumEfciencyCharacterization.177 7.3.4ThicknessMappingUsingTransmissionOpticalMicroscopy....179 7.3.5DeviceCurrent-VoltageCharacterization...............182 7.4ResultsandDiscussion............................182 7.4.1FilmsSprayedfromPureSolvent...................182 7.4.2Solvent-AlkaneSolutionsandDropletBehavior...........185 7.4.3FilmMorphologyStudies........................187 7.4.4EffectofAlkaneAdditivesonDevicePerformance..........192 7

PAGE 8

7.4.5ActiveLayerThicknessinSpunandSprayedDevices.......198 7.4.6EffectofAnnealingConditions.....................201 7.5Conclusions...................................204 8CONCLUSIONSANDFUTUREWORK......................206 8.1OrganicFieldEffectPhototransistors.....................206 8.2CarrierConnementinOrganicPhotodiodesforPhotocurrentGain....207 8.3Down-ConversionforEnhancedOrganicSolarCellAbsorptionEfciency208 8.4AlkaneDiluentsforImprovedPerformanceofSprayedPolymerSolarCells209 APPENDIX AMODELINGOLEDOUTCOUPLINGEFFICIENCY................212 A.1Introduction...................................212 A.2OLEDOutcouplingRayTracing........................213 A.3VericationofModel..............................215 A.4ResultsandConclusions...........................215 BLISTOFPUBLICATIONSANDPRESENTATIONS................220 REFERENCES.......................................222 BIOGRAPHICALSKETCH................................237 8

PAGE 9

LISTOFTABLES Table page 1-1Theatomicorbitalcongurationofelectronsforthersttworowsoftheperiodictable........................................23 4-1Literaturesummaryofreportedorganicphototransistorperformance......93 6-1ModeledabsorptiondataforC 60 :PbPcsolarcellsofvaryingLELstructure...146 6-2ModeledabsorptiondataforsolarcellsusingCuPcandsquarainedonors...155 7-1Chemicalpropertiesofspeciesusedinsprayedpolymersolarcells.......168 7-2Spraysystemcomponents..............................171 7-3Spraysystemadjustableparameters........................175 7-4Performancecharacteristicsofsprayedpolymersolarcells............195 7-5Deviceperformancecharacteristicsfordeviceswithdifferentspuncoatactive layerthickness....................................200 9

PAGE 10

LISTOFFIGURES Figure page 1-1Chemicalstructureoforganicsemiconductorswithvaryingcomplexity.....18 1-2Hybridizationofcarbonatomicorbitals.......................25 1-3Diagramofradiativeandnon-radiativeenergytransitionsinanorganicsolid..29 1-4Absorptionspectraforafeworganicsemiconductorscomparedtosilicon...30 1-5AbsorptionandFluorescenceFranck-Condontransition.............32 1-6Excitondissociationbydonor-acceptorheterojunction..............36 1-7Donororganicsemiconductormaterialelectronicstructure............44 1-8Acceptororganicsemiconductormaterialelectronicstructure..........46 2-1Conceptualdrawingoforganicphotodetectoradvantages............50 2-2Equivalentcircuitforanidealphotodiode.....................50 2-3Multi-stepphotocurrentgenerationprocessinorganicsemiconductordevices.58 2-4Conceptualdrawingshowingsimplicationofdetectordesignusinginternal gain..........................................61 3-1Solarspectrumincidentonearth..........................64 3-2AM1.5irradiance,solarux,andultimateefciencyofsinglejunctionsolarcells66 3-3Historicdevelopmentofsolarcellefciency....................68 3-4Recentdeclineintheinstalledcostforsolarpower................69 3-5Worldwideinstalledphotovoltaiccellenergycapacitybyyear,1990-2010...70 3-6Cross-sectionschematicsforthedonor-acceptorheterojunctiontypesused fororganicphotovoltaics...............................74 3-7Equivalentcircuitforsolarcell............................78 3-8Uniformityofsolarsimulatorattestcellposition..................79 3-9Variationoftheoutputofasolarsimulator.....................80 3-10Current-voltagecharacteristicsofasolarcellinthedarkandunderillumination82 3-11Effectofparasiticresistanceonsolarcelloperation................84 3-12OrganicsolarcellsC 60 :CuPcencapsulatedbyCytopuoropolymer......86 10

PAGE 11

4-1Devicestructureusedtoproducepentaceneeldeffecttransistors.......94 4-2Typicalelectronicoutputandtransfercharacteristicsforpentacenethinlm transistors.......................................98 4-3TransfercharacteristicsofapentaceneOFETinthedarkandunderillumination99 4-4PhototransistorEQEasdeterminedbystaticmethodology............100 4-5Transientphototransistoroutputundermodulatedillumination..........101 4-6Pentacenephototransistorquantumefciencyasmeasuredbythedynamic lock-intechnique...................................101 4-7Correlationofbiasstressinstabilitywithphototransistorphotoresponse.....104 5-1Holeconnementphotodiodedevicestructure..................108 5-2Proposedmechanismresponsibleforphotocurrentgainintheconnement photodiode......................................111 5-3Externalquantumefciencyincontrolphotodiodeswithnoblockinglayers...116 5-4Effectoftheblockinglayerstructureondevicespectralquantumefciency...117 5-5Voltageandspectraldependenciesofthequantumefciencyforoptimalconnementphotodiodestructure............................119 5-6Effectofthebulkheterojunctionmixingratioonthegainandbandwidth....121 5-7Optimalconnementphotodiodestructureresponseasfunctionofbandwidth.124 5-8Temporalresponseoftheoptimalconnementphotodiodestructure......126 5-9Darkcurrent-voltagecharacteristicsforphotodiodewithandwithoutblocking layers.........................................127 5-10Darkcurrent-voltagecharacteristicsfordevicesbeforeandafterdegradation.128 6-1Schematicdiagramshowingthedown-conversionabsorptionantennaconcept132 6-2AbsorptionandphotoluminescenceofDCJTB:Alq3inPMMA..........142 6-3ExternalQuantumEfciencyinPhthalocyanine:FullereneSolarCells......143 6-4ModeledabsorptionefciencyforPbPc:C 60 solarcellswithnoLEL.......145 6-5Substrateandsuperstratedown-conversiondevicegeometries.........147 6-6RaytracingmodelresultsforC 60 :PbPcsolarcellswithsubstrateLELstructure149 11

PAGE 12

6-7Effectofdevicesizeonlateralwaveguidedlossandactivelayerabsorptionin theITOonglassLELstructure...........................150 6-8RaytracingmodelresultsforCuPc:C 60 solarcells................153 6-9RaytracingmodelresultsforCuPc:C 60 solarcells................154 7-1Schematicshowingthedropletdryingdynamicsthatproducethecoffeering effect.........................................163 7-2Schematicshowingmulti-componentMarangonidropletuidowtocounteractthecoffeeringeffect...............................165 7-3Chemicalstructureofspeciesusedinsprayedpolymersolarcells.......169 7-4Spraysystemschematic...............................172 7-5Theeffectofcarriergasonlmuniformity.....................174 7-6TheeffectofsolventchoiceonsprayedP3HT:PCBMlmquality.........183 7-7Severesurfaceroughnessofsprayedlms.....................184 7-8Opticalmicrographsoflmssprayedfromtoluenewithandwithoutalkane diluent.........................................188 7-9Opticalmicrographsoflmssprayedfromchlorobenzenewithandwithout alkanediluent.....................................189 7-10Effectofiso-octaneonmacroscopiclmmorphology:lmthicknessmaps...191 7-11Sprayeddeviceperformance:toluenevs.toluene:iso-octane...........194 7-12Sprayeddeviceperformance:chlorobenzenevs.chlorobenzene:iso-octane...196 7-13Thicknessdependenceofspuncoatsolarcellperformance............201 7-14EffectofannealingondeviceswithspuncoatactivelayersandLiF/Alcathodes.203 7-15Effectofannealingondeviceswithsprayedactivelayers.............204 A-1Extractionefciencyasfunctionofmicrolensindexofrefraction.........216 A-2Extractionefciencyasfunctionofemissionwavelength.............216 A-3Extractionefciencyasafunctionoflenscontactangle.............217 A-4Microlensspacingarrangementsusedinopticalsimulations...........218 A-5Extractionefciencyasafunctionofmicrolenspackingarrangementandencapsulationthickness................................218 12

PAGE 13

A-6Extractionefciencyasafunctionofmicrolensspacingforclose-packedarrangement......................................219 13

PAGE 14

AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulllmentofthe RequirementsfortheDegreeofDoctorofPhilosophy DESIGNANDFABRICATIONOFORGANICSEMICONDUCTORPHOTODIODES By WilliamThomasHammond May2011 Chair:JiangengXue Major:MaterialsScienceandEngineering Organicmaterialsaresyntheticallynumerous,mechanicallyexible,lightweight, processableatlowtemperature,andchromaticallydiverse.Forthoseorganicmaterials withfavorableelectronicstructureandclassiedassemiconductors,theseunique characteristicscanbereadilyleveragedforfunctionalelectronicandoptoelectronic applications.Thisthesisexaminesthewidescopeofrealizabledevicestructuresthat areenabledbytheprocessabilityandadaptabilityofthesematerials,focusingon photodiodedevicesdesignedforboththedetectionandtheharvestingofopticalenergy. Intherstpart,twodevicestructuresareexploredfortheirabilitytooutputa photocurrentgain.Photocurrentgain,inwhichoneincidentphotongivesrisetoa multiplenumberofsecondaryphoto-electronscycledthroughanexternalcircuit,is animportantdevicepropertyforapplicationsthatrequirethedetectionoflowlight signalsandalsothosethatrequireamoresimpliedelectroniccircuit.Theeld-effect transistordevicestructureisrststudied,anditisfoundthatalthoughthisstructure cangiverisetoasmallphotocurrentgainuptoG=2,thereareanumberoftradeoffsthatmaylimitfunctionality.Next,anewmultilayerorganicphotodiodestructureis designedandstudiedthatusesacarrier-selectiveconnementmaterialtoproducean 14

PAGE 15

extraordinarilyefcientphotoconductivegainmechanism,achievingG=100across theentirevisiblespectrumunderalowappliedbias V =-3V.Thenatureofthegain mechanismleadstorelativelyhighbandwidth f 3 dB =1kHz,andoptimizeddevices arefoundtoproducerecordgain-bandwidthproduct,uptoGBP=10 5 ,amongorganic photodetectorsoperatingaboveimaging-compatiblebandwidth > 60Hz. Inthesecondpart,thefocusisshiftedtoorganicphotovoltaicdevices.First,a newdevicestructureisproposedthatutilizesaphotoluminescentexternalabsorption antennainordertodown-convertpartoftheincidentsolarspectrumintoanemission spectrumthatismoreefcientlyabsorbedbythesolarcell.Theapplicationofadownconversionstructuretoafewtypicalorganicsolarcellsiscomputationallystudiedandit isfoundthattheconceptcanenhanceorganicsolarcellperformance.Dependingonthe materialanddevicestructure,theenhancementtototalsolarcellabsorptionefciency canreach27%. Finally,astheultimateadvantageoforganicsolarcellsmaylieintheirprocessabilityandcommercialscalability,thenaltopicstudiedfocusesontheprocessingofthese cellsbyspraydeposition.Spraydepositionmayenablelowcostmanufacture,howeverit inherentlyproducesroughandnon-ideallmmorphology.Hereitisproposedandconrmedthattheadditionofalkanediluentstoasolutionofpolymersemiconductorand solventcanimprovetheuniformityofspraydropletdepositionpatterns.Furthermore,it isshownthattheimprovementinuniformitycansignicantlyenhancesolarcellpower conversionefciencyPCEbyupto40%,reachingperformancesimilartothatachieved bythehighlyuniformspincoatingmethod. 15

PAGE 16

CHAPTER1 INTRODUCTIONTOORGANICSEMICONDUCTORS 1.1AShortHistoricalPerspective Recentyearshaveshownaremarkablegrowinginterestinorganicsemiconductor research.Whiletheeldonlycametofruitionasamainstreamresearchtopicatlarge researchuniversitiesinthepasttwodecades,ithasarichandlonghistorythattraces backtothe19thcentury. Asubstantialamountofthepresentinterestinorganicsemiconductorsisfocused ontheiruniqueopticalproperties,asusedinoptoelectronicdevicestoeitherconvert lighttoelectricityor viceversa .Itisttingthenthattheoriginsofthisacademicdiscipline lieintheiropticalproperties.Forcenturiesbeforeanyrigorousscientictreatment, peoplefoundexamplesoforganicmaterialsthatuoresce,suchasthekidneywood usedbytheAztecsthatwhenusedasavesselgavewaterabluishcolor 1 .George Stokesrstidentiedthephysicalmechanismresponsibleforthisphenomenonin1852 astheabsorptionofshortwavelengthlightandre-emissionoflongerwavelengthlight, coiningthetermuorescenceafterthemineraluor-sparfromwhichheobserved theeffect 2 .Meanwhile,organicchemistryhadexperiencedarevolutionsinceFriedrich Wohlerrstsynthesizedureain1828,demonstratingthatorganicmaterials,previously denedasthosematerialsthatwereexclusivelyofbiologicalorigin,caninfactbe articiallysynthesizedfrominorganicprecursors 3 .NotlongafterStokes'discovery, therstsyntheticallyderivedorganicdyewassynthesizedbyWilliamHenryPerkina compoundcalledmauveine,in1856.Syntheticorganicdyes,desiredfortheirvaried 16

PAGE 17

andbrilliantcolorsaswellastheiruorescence,foundmanyindustrialapplicationsin thelate19thcenturymanyofwhichremainwithusinthepresentday. Althoughtheopticalpropertiesoforganicmaterialswerewidelystudiedand practicallyemployedduringthistime,theunderstandingoftheirelectricalproperties wouldprovemoreelusiveforthebetterpartofthe20thcentury.Nevertheless,the rstnotablesemiconductingeffectinorganicmaterialswasdiscoveredin1906,when Pocchetinomeasuredaphotoconductiveeffectinorganiccrystalsofanthracene C 14 H 10 ,seeFigure1-1 4 .FamedinventorChesterCharlsonwouldlaterleverage organicphotoconductorswithhisinventionoftheelectrophotographiccopyprocessthe Xeroxin1938,althoughitwouldtakeanadditionaltwentyyearsforhisinventionto becometherstwidelysuccessfulcommercialapplicationofanorganicsemiconducting effect 5 .In1958,aweakphotovoltaiceffectwasrstobservedintheorganicdye magnesiumphthalocyanine 6 .Untilthistime,organiccrystalswerestillthoughttobe mostlyinsulatingbynature,butin1960KallmannandPopediscoveredohmiccontacts thatenabledinjectionof10 )]TJ/F20 8.9664 Tf 6.967 0 Td [(5 Ampsofcurrentintoanthracenecrystals 7 .Thisledthe waytothediscoveryoforganicelectroluminescentdevicesusingbipolarinjectionand radiativerecombination 8 .Atthispointintime,thefundamentaldiscoverieshadbeen madethatwouldleadtotheorganiclight-emittingdevicesOLEDsandphotovoltaics OPVsthatarepresentlycommercializedandstillimproving.However,itwouldtake decadesbeforethesetechnologiescouldproducepracticalfunctionality. Thisbegantochangein1986whenTangpublishedhisrstpaperdescribing asimplebilayerheterojunctionorganicsolarcellthatexhibited1%solarpower conversionefciencyPCE 9 ,andthenwentontopublishwithVanSlykein1987 17

PAGE 18

AnthracenePentacenePhthalocyanine Small molecules PPVP3HTPEDOT:PSS Polymers Adenine-ThymineChlorophyllc1DNAportion Biologicalmolecules Figure1-1.Chemicalstructureofrepresentativeorganicsemiconductingsmall moleculesandpolymermonomerunits,including poly,4-phenylenevinylenePPVandpoly-hexylthiophene P3HTandtheconductivemixture poly,4-ethylenedioxythiophene:polystyrenesulfonate PEDOT:PSS.Afewbiologicalmaterialsarealsoshownfor comparisonsuchasdeoxyribonucleicacidDNAthatformhighly complexstructurescontainingsequencesofmoleculessuchas adenineandthymine. 18

PAGE 19

therstdemonstrationofabilayerheterojunctionOLEDthatexhibited1%quantum efciency 10 .ManyadvanceshavebeenmadeinOPVsandOLEDssincethistime, withcurrentstate-of-the-artOPVdevicesnowreaching8.3%PCE 11 andOLEDdevices reachingnearly100%internalquantumefciency 12 ;however,thisbasicheterojunction structureremainsthebasisonwhichimprovementshavebeenmadeusingorganic smallmoleculesthatcanbereadilydepositedusingvacuumthermalevaporation. Inadditiontothesmallmoleculesthatwereusedintheseearlymaterialanddevice studies,polymersalsohaveplayedaprominentroleintheeldsincethediscovery in1977ofmetallicconductionindopedpolyacetylene 13 .Theextendedchainlength ofpolymersisadvantageoustoprocessingasitenablesdissolution,butisalso disadvantageoustodevicestructureengineeringasitpreventsthermalevaporation. However,spontaneousphasesegregationofpolymerdonorsandfullerene-based PCBMacceptorsasdepositedfrommixedsolution,rstdiscoveredintheMDMOPPV:PCBMsystem1.3%PCE 14 ,notonlyleveragessolutionprocessingadvantagesbut alsocanproducenearlyidealsemiconductormorphology. InadditiontoOPVsandOLEDs,manyadditionaldeviceshavebeendeveloped usingbothsmallmoleculesandpolymers,includingtransistors,photodetectors,and manyothers.Organictransistorscannowbefabricatedthatmatchandevenexceed thetransistorperformanceofamorphoussilicon.Furthermore,organicphotodetectors usingmultipleorganicmaterialsystemscanbemadewithsensitivitysimilartodevices employingcrystallinesilicon 15,16 Whileresearchcontinuesatuniversitiesworldwide,examiningeverythingfrom materialphysicsandchemistrytodeviceoptimization,therearealsomanyapplications 19

PAGE 20

ofthesetechnologiespresentlybeingcommercialized,inbothnewandmaturestages, fordevicesrangingfromOLEDsformobiledisplaystopolymersolarcellsforindoorand outdoorenergyharvesting,andlow-costprintedelectronicsusedinforinstanceradio frequencyidenticationdevices. Insummary,organicsemiconductormaterialsexhibitveryinterestingphysical propertiesthathaveenticedalonghistoryoffundamentalresearchwork.Through discoveryandinnovation,researchershaveproducedtechnologiessuchasorganic solarcellsandlightemittingdevicesthatpromisetoreducethecostofenergy generationandincreasetheefciencywithwhichenergyisused.Motivatedby technologicalapplicationofscience,thisdissertationexaminesnewdevicestructures thatimproveapplicationoforganicsemiconductortechnologiestophotodetectionand solarenergycollection. 1.2OverviewofOrganicSolids Organicmaterialsareconstructedofprimarilycarbon-basedmolecules.The uniquebondingpropertiesofcarbon,includingtheabilitytoformextendedpolymeric chains,leadstothevastarrayofbiologicalmaterialsinnatureandalsotothemillions ofmoleculesandpolymerssyntheticallycreatedbymodernchemistry.Thecomplexity ofthemolecularunits,bothnaturallyoccurringandsynthetic,variesfromsimplesmall moleculeswithahandfulofcarbonatomstopolymericchainsofsimplemonomer units,andtomorecomplexstructuressuchasoligomersandbiologicalmolecules. Organicsemiconductorsaremostlyorthesmallmoleculeandpolymertypes,however representativemoleculesfromnearlyallclassesoforganicsolidshavebeenused 20

PAGE 21

successfullyinsemiconductordevices.Figure1-1displaysthemolecularstructurefor afewtypicalorganicsmallmoleculeandpolymersemiconductors,aswellassome commonbiologicalmoleculesforcomparison. Inorganicsolids,thesemolecularunitsareboundtogetherbyweakVanderWaals forces.Thepresenceofdiscrete,non-bondedmolecularunitscarriesfarreaching consequencesforthepropertiesofsuchmaterials.First,theweakintermolecular forceallowsmoleculestomoverelativelyeasilywithrespecttoeachother,enabling mechanicalductility.Thesameweakintermolecularbindingforceallowsmolecules tomoreeasilyseparateunderthermalenergy,causingthesematerialstohavelow meltingpoints.Infact,manyofthesmallersizedmoleculesaregasesandliquidsat roomtemperature,suchasthecomponentsofnaturalgasandorganicliquidsolvents. Bothoftheseproperties,ductilityandlowmeltingpoint,makeorganicmaterialsless robustagainstenvironmentalforces,yetatthesametimeallowthemtobeprocessed witheaseandwithlessthermalenergyinput.Theoverarchingtechnologicaladvantage, wellleveragedintheplasticsindustry,isthatthesematerialsarehighlymanufacturable. Theweakintermolecularforcealsoinuencestheelectronicpropertiesoforganic solids.Ingeneral,thelocalizedbondingorbitalswithinmolecularunitscombinedwith thephysicalseparationbetweenunitslimitselectronmobilityandgenerallymakes thesematerialsinsulators.Nevertheless,thesubclassoforganicmaterialsthatare semiconductorsaredenedbya p -bondedconjugatedstructurethatcanovercomethe moretypicalelectronlocalizationoforganicmaterials.Theremainderofthischapter exploresfourmaintopicsdescribingthepropertiesoforganicsemiconductors:athe 21

PAGE 22

originsofmolecularchargetransport,bphotophysicalandexcitonicproperties,andc electronicandtransportproperties. 1.3MolecularBasisforChargeTransportinOrganicSolids Nearlyallorganicmaterialshaveaband-gapthatseparatesoccupiedfrom unoccupiedelectronenergylevels.Theoreticallyitispossibletoinjectfreecarriers intoanysuchmaterial,regardlessofthesizeofthebandgap,ifacontactwithsuitable Fermienergyisavailable 17 .However,thefundamentaldifferencethatseparates organicmaterialsthatexhibitsemiconductivityfromthosethatdonotisthemobilitywith whichfreeelectronsorholescanbetransportedacrossthebulkmaterial.Inorderto understandwhysomeorganicmaterialshavethisabilitywhileothersareinsulating,we examineinthissectionthemolecularbondingorbitalsofcarbonthatleadtoconjugation andhowconjugationleadstoelectrondelocalizationandnon-negligablecarriermobility. 1.3.1AtomicandMolecularOrbitals Accordingtoquantummechanics,electronsboundtoanatomlieindistinct atomicorbitals.Theelectronswhichexhibitwave-particledualitycanbedescribed bycontinuouswavefunctions, ,whichdenethespatialandtemporaldistributionof electronswithinanorbital.Eachelectronoccupiesone-particlehydrogenicorbitalstates underapotentialdeterminedbythechargeofthenucleus qZ andisdescribedbya uniquesetofquantumnumbers n l m .Theprinciplequantumnumber` n 'denotesthe shellofelectronsandcorrespondstotheseriesnumberoftheelementintheperiodic table n 1;theazimuthalquantumnumber` l 'denotesthesubshell l =0... n )]TJ/F20 11.9552 Tf 11.075 0 Td [(1;and themagneticquantumnumber` m 'qualiestheorbitalwithinthesubshell )]TJ/F58 11.9552 Tf 9.289 0 Td [(l m l 22

PAGE 23

Asfermions,electronsfollowthePauliexclusionprincipleandthereforeoneorbitalmay bedoublyoccupiedbytwoelectronsofoppositespins s = )]TJ/F20 8.9664 Tf 10.484 4.71 Td [(1 2 ,+ 1 2 .Orbitalsarelled inthemannerthatminimizesfreeenergy,andorbitalsofequalenergyaresinglylled followingHund'srulebeforeelectronsofoppositespinpairinthesameorbital.These rulesareillustratedwiththeprogressionoforbitalelectronspinoccupancyfortherst tworowsoftheperiodictableinTable1-1,wherethethecommonatomicconguration nomenclatureisalsoshownascribingtheletterss,p,dtothe l =1,2,3subshells, respectively. Table1-1.Theatomicorbitalcongurationofelectronsforthersttworowsofthe periodictable. ZElementCongurationOrbitalelectronspin 1Hs 1 s 2Hes 2 "# 1 s 3LiHes "# 1 s 2 s 4BeHes 2 "# 1 s "# 2 s 5BHes 2 p "# 1 s "# 2 s 2 p 6CHes 2 p 2 "# 1 s "# 2 s 2 p 7NHes 2 p 3 "# 1 s "# 2 s 2 p 8OHes 2 p 4 "# 1 s "# 2 s "# 2 p 9FHes 2 p 5 "# 1 s "# 2 s "# 2 p "# 10NeHes 2 p 6 "# 1 s "# 2 s "# 2 p "# "# Molecularorbitaltheorystatesthattheorbitalstateswithinmoleculesfollowalinear combinationofthecontainedatomicorbitals.However,theatomicorbitalsthemselves arechangedunderbondingwithneighboringatoms.Incarbon-basedmolecules, bondingisgenerallyoftwotypes: s bondsthatshareelectronsin s orbitalsand p bonds 23

PAGE 24

thatshareelectronsin p orbitals.Justasforatoms,thesemolecularorbitalsarelledin themannerinwhichthetotalfreeenergyofthesystemisminimized. 1.3.2HybridizationandConjugationofCarbon Bondingincarbonprovidesauniquestudyinmolecularorbitaltheoryonaccount ofitshalf-lledvalenceshellcombinedwithalackof l > 2orbitals.Theseproperties enablethehybridizationofits2sorbitalswith1,2,or3ofits2porbitalstoformsp,sp 2 andsp 3 hybridizedorbitals,respectively.Figure1.3.2comparestherelativeatomic orbitalenergiesofcarbonasthesandporbitalsareincreasinglyhybridized. Theelectronwavefunctionsofthe3p-orbitalsinisolatedcarbonoccupyorthogonal spaceinthex,y,andzdirectionsaboutthenucleus.Inthespecialcaseofsp 2 hybridizationindouble-bondedcarbon,asinglep z orbitalremainsineachcarbon atomandthesharingofelectronsbetweentheseorbitalsconstitutesthe p bond. Molecularchainsthatalternate,orconjugate,thisC-Cdoublebondexhibitanextended delocalizationofthe p electronsandcanresultinalongerrangeelectronpolarizability. Conjugation,inwhichcarbonatomsarejoinedbyalternatingsingleanddouble bonds,isanimportantpropertyoforganicmoleculesforavarietyofpurposes. ConjugationreducesthefreeenergyofC-Cbondsandthereforeformsmorestable structures.Forinstance,alkenesthatcontainasingle p bondreadilyundergo bromination,whileconjugatedstructuresresistreactionwithbromine.Conjugated structurefurthermoreleadstoaweakdelocalizationofelectronicstructurealongthe molecularchain.Thisistheessentialelectronicpropertythatenableslong-range electronmobilityinorganicsemiconductors.Organicsemiconductorsgenerallycontain 24

PAGE 25

Figure1-2.Hybridizationofcarbonatomicorbitals,includingnon-hybridizedasfoundin non-bondedcarbon,sp-hybridizedasfoundintriple-bondedcarbon, sp 2 -hybridizedasfoundindouble-bondedcarbon,andsp 3 hybridizedas foundinsingle-bondedcarbon. well-conjugatedstructuressuchasbenzeneringsandthiophenerings,asevidentinthe moleculesshowninFigure1-1. 1.4OptoelectronicProperties Theinteractionoforganicsemiconductorswithlightisgovernedbythenature oftheexcitedstateswithinorganicsolids.Inthissection,weintroducethesestates andexplorethewaysinwhichexcitonicstatesarepopulatedandtransported.We thendescribetheenergetictransitionssuchstatesundergoandthewaysinwhichthis inuencestheabsorptionandemissionoforganicsemiconductors. 1.4.1ExcitedStatesofOrganicSolids Section1.3discussedthebasicmolecularorbitalenergystatesofcarbon-based materials.ThehighestoccupiedmolecularorbitalHOMOinsuchamoleculeisthe p bondedorbitalandisthegroundstate.Uponinteractionwithopticalenergy,underan appliedbias,orsimplyduetosteady-statethermalgeneration-recombinationprocesses, 25

PAGE 26

thesemolecularenergygroundstatescanbeexcitedintohigherenergystates;the lowestaccessibleofsuchstatesisthe p anti-bondedorbitalandisreferredtoasthe lowestunoccupiedmolecularorbitalLUMO. Inisolatedmolecules,i.e.inthegaseousstate,the p statesarewell-dened, discreteenergiesasdescribedbyquantummechanics.Inadisorderedsolutionof moleculesdissolvedinanorganicsolvent,thesestatesbecomebroadenedbythe disorderofthesystemintoGaussian-typebands,andtheenergyoftheexcitedstates areloweredbymolecularinteraction.Fordefect-freeorganicsinglecrystals,these discreteexcitedenergystatesareloweredinenergytoalargermagnitudeduetosolidstatesolvation,buttheorderofthesystemretainsdiscreteenergylinespectra.The molecularinteractionandmoleculardisorderoforganicamorphousandpolycrystalline solids,typicaloforganicsemiconductors,liebetweenthesetwoextremesofhighly orderedsinglecrystallinestatesandthoseofdisorderedsolutionstates. Regardlessofthelevelofdisorderinthesesystems,however,theweaklevel ofelectronenergydelocalizationretainswell-denedspinstateswithinwell-dened orbitals.FollowingthePauliexclusionprinciple,anexcitedstatewithtwoelectronsof totalcompositespin s =1canoccurinoneoffourpermutations,threeofwhichare tripletsandoneofwhichisasingletinthenotation j sm i : 8 > > > > > > > < > > > > > > > : j 11 i = "" j 10 i = 1 p 2 "# + #" j 1-1 i = ## 9 > > > > > > > = > > > > > > > ; s =1triplet 26

PAGE 27

j 00 i = 1 p 2 "#)-222(#" s =0singlet. ThisfundamentalratioarisesintheappliedtechnologyofOLEDs,whereasimilarratio hasbeenshownrelatinguorescentemissionfromsingletstatestophosphorescent emissionfromtripletstates 18 Shiftingtothelanguageofsemiconductormaterials,theseorganicsolidexcited statesconsistofaholeintheHOMOandanelectronintheLUMO,similartothecase ofinorganicsemiconductorswhereintheexcitationofanelectronfromthevalence bandtotheconductionbandleavesafreeholeinthevalenceband.Inbothorganic andinorganicmaterials,theinitialexcitationofanelectroncreatesanexcitonconsisting ofaneutralelectron-holepair.Theenergythatbindsthisexcitontotheoriginating atomormoleculedependsonthematerialproperties,andingeneralwerefertothree typesofexcitons:Wannier-Mott,Frenkel,andcharge-transfer.Formaterialswithstrong atomicinteractionsandhighdielectricconstant,i.e.inorganicsemiconductors,thefreed chargeisscreenedanddelocalized,resultinginabindingenergysimilartothethermal voltagemeVandaBohrradiusontheorderof10nm.Formaterialswithweak intermolecularinteractionsandsmalldielectricconstant,i.e.organicsemiconductors, thefreedchargeistightlyboundtotheoriginatingmoleculeornextneighbor,resulting inabindingenergythatisinsurmountablebyambientthermalenergy.1.0eV andaBohrradiusontheorderof1nm.Thecharge-transferexcitonisessentiallya specialcase,Frenkel-like,excitonthatissharedbetweendonorandacceptormolecules atamolecularinterface,andislikewisestronglyboundtothedonor-acceptorpair. Note,however,thatasisthecaseformanysuchclassications,theFrenkeland 27

PAGE 28

WannierdenitionsofexcitonbindingenergyandBohrradiusaregeneralizedcases andmanyrealsystemsexhibitintermediateproperties.Forinstance,excitonsin polydiacethylenepolymerchainsarestronglypolarizedanddelocalized,resultingina Wannierclassication 19 1.4.2ExcitonPhotophysics Excitonsareformedinorganicsolidsbyeitherdirectopticalabsorptionorbythe recombinationofinjectedfreeelectronsandholes.Thebehaviorofexcitonsgoverns theabsorptionandemissionpropertiesoforganicsemiconductors;therefore,thestudy ofthegenerationandrecombinationofexcitonsiscrucialtounderstandtheoptical transitionsinorganicsolidsthatenableoptoelectronicdevices. Thetransitionsbetweenenergystatesthatexcitonsundergoinanorganicsolid areafunctionofthetotalelectronenergyinanygivenstate,whichisacombination ofelectrical,vibrational,androtationalenergies.Theelectronictransitionenergies aretypicallyontheorderoftheenergyofvisibleandultra-violetphotons,whilethe vibrationalandrotationaltransitionenergiesareontheorderofnearinfraredandfar infraredphotonenergies,respectively.Thus,theopticaltransitionsinorganicsolidsare dominatedbyelectronictransitions,howeverthevibrationalandrotationaltransitions openupadditionalenergystatesandacttobroadentheabsorptionandemission spectra. Opticalabsorptionusuallyinvolvestheinteractionofaphotonwithadoubly occupiedsingletgroundstateHOMO, S 0 .Thisinteractionpromotesanelectron fromtheHOMOintotheLUMOwithconservedspin,andthereforetheresultingexciton 28

PAGE 29

Figure1-3.Diagramofradiativeandnon-radiativeenergytransitionsinanorganicsolid. Theopticaltransitionsofabsorption,uorescence,andphosphorescence areshowntogetherwiththenon-raditivetransitionsofinternalconversion, inter-systemcrossing,andthermalvibrationalrelaxation,withrespecttothe singletgroundstate, S 0 ,theexcitedsingletstates S 1 and S 2 ,andtheexcited tripletstates T 1 T 2 ,and T 3 isalwaysofthesinglettype S 1 S 2 ,etc..However,anexcitedstateitselfcanabsorb photonicenergy,andthisinteractioncaninvolvebothsingletsandtriplets,forinstance ofthetype S 1 S 2 or T 1 T 2 .Theseenergytransitionsareschematicallydrawn amongotherpossibleenergetictransitionsinFigure1-3.Figure1-4plotstheabsorption coefcient, a a =ln T = d ,where T isthetransmittancethroughasampleofthickness d ,forseveralrepresentativeorganicsemiconductorsincomparisontosilicon.The absorptionintotheGaussian-likevibronicbroadeningofthedensityofLUMOstates inorganicsemiconductorssharplycontrastswiththeabsorptionintotheexponentially risingdensityofstatesabovethebandgapininorganicsemiconductors,asseenfor instanceinsilicon. 29

PAGE 30

Figure1-4.Absorptioncoefcientspectraforafewtypicalorganicsemiconductors, includingpentacene,copperphthalocyanineCuPc,and perylene-3,4,9,10-tetracarboxylic-dianhydridePTCDA,comparedtothatof theinorganicsemiconductorsilicon. Excitonsmayalsobeformedbytherecombinationofafreeelectronwithafree hole, h + + e )]TJ/F20 11.9552 Tf 7.465 -4.34 Td [(.Duetothebindingenergyoftheexciton,theexcitonstateisata lowerenergythanthefreeelectronstateandsotheinitialrecombinationresultsin highlyexcitedstatewhichthenthermallyrelaxestothegroundstateoftheexciton. Furthermore,thespinoftheelectronisnotconstrainedbythatoftheholeandsothe generatedexcitonmaybeeitherofthesingletortriplettype.Theoretically,theratioof singlettotripletsgeneratedis1:3followingthesinglet:tripletpermutationratio. Oncegenerated,excitonscanundergoradiativetransitions.Singletexcitonscan relaxtothesingletgroundstatewiththeemissionofaphoton,resultinginuorescence. Tripletexcitonscanrelaxtothesingletgroundstateinmoleculesthatallowintersystem crossing,resultinginphosphorescence.Thetimescalesofthesetwoprocessesare 30

PAGE 31

verydifferent,however,owingtothefactthatuorescenceisasingleprocesswhile phosphorescencerequiresspin-orbitcouplingbeforeaphotoncanbeemitted.The radiativelifetimeofuorescenceandphosphorescenceistypically1psand0.1 ms,respectively.Nevertheless,bothoftheseprocessesoccuronalongertimescale thanvibrationalrelaxation 0.1ps,andthereforetheemissionofaphotonistypically producedbyatransitionfromthelowestvibrationalmodeoftheexcitonvibrational manifoldfollowingthermalrelaxation 20 .Mostorganicmoleculesexhibitamarkedredshiftofemissioncomparedtoabsorption,wheretheshiftinthepeakofabsorptionto emissionspectraiscalledtheStokesShift.Thisoffsetinenergycanbeexplainedby theFranck-Condoneffect,showninFigure1-5,whereinthenuclearcoordinationofthe excitedmoleculeincomparisontothelowerenergystateisshifted.Therefore,photon emissionrequirestheadditionalemissionofaphononandtheenergyoftheemitted photonisreducedincomparisontothatabsorbed. Thereareanumberofnon-radiativeprocessesthatanexcitoncanundergoafter generation.Ifanexcitonoccupiesavibrationalnode n > 0thisenergycanthermally relaxtothevibrationalgroundstate.Excitonscanalsoundergointersystemcrossing fromthesinglettothetripletmanifoldand viceversa .Thesetransitionsaregenerally forbiddenduetospinconservation,however,theycanbeinducedbyanincreasein spin-orbitcouplingforcertainorganometalliccompoundsthatcontainheavymetal atoms 21 .Insomesolidssuchaspentacene 22 ,thelowesttripletstateislessthanhalf theenergythanthelowestsingletstate,andconsequentlysingletssioncanoccur resultingintwotripletexcitons.Finally,excitonsmaysimplyrecombinenon-radiatively duetoforinstanceitsinteractionwithdefectstatesoradonor-acceptorinterface. 31

PAGE 32

Figure1-5.AbsorptionandFluorescenceFranck-Condontransition.Inboththelower andexcitedenergymanifolds,multiplevibronicmodesareshown,andared shiftinemissionisdepictedasarisingduetotheoffsetbetweenthe coordinationcoordinateofthemoleculeintheexcitedstatecomparedtothe groundstate. Insummary,thereareavarietyoftransitionsthatanexcitoncanundergowithinan organicsolid.ThesearesummarizedintheJablonskidiagramshowninFigure1-3.For technologicalapplicationitiscriticaltodiscouragethosetransitionsthatreducedevice efciency;forinstanceinOLEDsnon-radiativerecombinationcompeteswithemission, whileinsolarcellsradiativeandnon-radiativerecombinationofphoto-generated excitonsreducesthenumberofelectronsandholescollectedasphotocurrent. Therefore,inOLEDsitisimportanttoensurethatthedevicematerialsdonotcontain ahighdensityofquenchingsites,whileinsolarcellsitisimportanttoensurerapid quenchingbythesplittingofexcitonsintofreeelectronsandholes. 32

PAGE 33

1.4.3ExcitonEnergyTransferandMigration Excitonsinorganicsolidsarerelativelystronglyboundtotheirhostmoleculeand furthermorearechargelessparticlesmostlyunaffectedbyanappliedelectriceld. Nevertheless,thesestatescanmoveaboutamaterialusingoneofthreeenergytransfer mechanisms:radiativetransfer,F orstertransfer,andDextertransfer.Radiativetransfer involvestheuorescenceofaphotonthatisthenabsorbedatanotherlocationwithinthe materialgeneratinganewexciton.Inorderforradiativetransfertooccur,theemission spectrumofthedonormoleculemustoverlaptheabsorptionspectrumoftheacceptor molecule.Thelengthscaleoverwhichradiativetransferoccursvariesaccordingtothe absorptionefciencyofthematerial,butgenerallybecomesanimportantconsideration fordistancesgreaterthan10nm,beyondwhichothertransfermechanismsarenot active. F orstertransfer,orresonanttransfer,alsorequiresanoverlapoftheemission spectrumofthedonormoleculewiththeabsorptionspectrumoftheacceptor.However, thisenergytransferdoesnotinvolvearealphoton,butratherwhatisreferredtoas a`virtual'photon.Thevirtualphotonisarepresentationofaresonantdipole-dipole interactionbetweenthemolecules,whichmediatesthetransferoftheexcitationenergy fromthedonortotheacceptormoleculewithouttheemissionofaphotonandwithout directelectrontransfer.Whiletheinteractionstrengthofdipole-dipoleinteractionsis proportionalto R 3 ,where R istheseparationdistancebetweenmolecules,F orsterrst showedthatthedipoleinteractionandrateofenergytransferfalloffwiththesixthpower 33

PAGE 34

oftheseparationdistance: K F orster D A 1 t D 1 R 6 Z c 4 w 4 n 4 0 F D w s w d w where t D isthedonorlifetime, n 0 thesolventindexofrefraction, F D theuorescence spectrumofthedonor,and s A theabsorptioncross-sectionoftheacceptor 20 DexterenergytransferinvolvesthedirecttransferofanelectronfromtheLUMOof thedonormoleculetotheLUMOoftheacceptormoleculeinconcertwiththetransfer ofaholefromtheHOMOoftheacceptortothatofthedonor.Thismechanismisactive oververyshortdistances l < 1 )]TJ/F20 11.9552 Tf 10.943 0 Td [(2nm,andisallowedforbothsinglet-singlettransitions aswellastriplet-triplettransitions.Thisinteractionreliesontheoverlapofdonorand acceptorelectronwavefunctionsaswellastheoverlapofemissionandabsorption spectra,andtherateoftransferfallsoffexponentiallyas: K Dexter D A exp )]TJ/F20 11.9552 Tf 9.289 0 Td [(2 R L Z F D w s w d w where L istheeffectiveBohrradius 23 Excitonsmigratethroughanorganicsolidbytheadditivecombinationofmany oftheseindividualenergytransferprocesses.Becausetheseindividualprocesses arenotdirectionspecici.e.,thepresenceofanelectricormagneticelddoesnot inuencedirectionalitythismigrationcanbedescribedbyrandomhoppingdiffusive motiontheory.Phenomenolgically,then,themigrationofexcitonswithtimefollowsFick's SecondLawofDiffusion, d r dt r = )]TJ/F58 11.9552 Tf 9.289 0 Td [(D d r dr 34

PAGE 35

where r istheconcentrationproleofexcitonsand D isthediffusioncoefcient. Thediffusionlength, l ,isrelatedto D throughtheexcitonlifetime, t ,bytherelationship l = p ZD t ,where Z =6forthreedimensions.Theexcitonlifetimeanddiffusionlength areimportantquantitiesfororganicsemiconductordevices,aswillbecoveredinthe followingchapters.Thesequantitiescanbeinferredexperimentallybymethodssuch asluminescencequenchingstudiesasafunctionoftheconcentrationofaquenching moleculeorbymonitoringtheevolutionofluminescenceintensitywiththethicknessofa materialinabilayerheterojunction 24 1.4.4ExcitonDissociationandDonor-AcceptorHeterojunctions Duetothestrongbindingforceofexcitonsinorganicsolids,theyarenotreadily split,ordissociated,intofreeelectronsandholes.Forinstance,aFrenkelexcitonwith aBohrradiusof1nmandbindingenergyof0.5eVgeneratedwithinthebulkofa100 nmthicksemiconductorlayerwouldrequireapproximately50Vappliedacrossthe deviceinordertoeld-dissociatetheexcitonassumingauniformelectriceldwithin thesemiconductor.Indevicessuchasorganicphotodetectorsandsolarcells,suchan appliedvoltageisimpracticalandthereforedissociationmustrelyonothermeans. Thisstrongbindingforcecanbeovercomebylocalanisotropyinthemolecular energylevelssurroundinganexciton.Suchanisotropycanbeformedinadevicebya typeIIdonor-acceptorheterojunctionofmolecularmaterialsinwhichtheLUMOand HOMOofthedonormoleculearehigherinenergythanthoseoftheacceptormolecule 9 Figure1-6depictssuchaheterojunctionbetweencopperphthalocyanineCuPcand fullereneC 60 .Notethatinorderfortheheterojunctioninterfacetoovercomethebinding 35

PAGE 36

Figure1-6.Excitondissociationbydonor-acceptorheterojunction.Schematicband diagramforcopperphthalocyaninedonorandfullereneacceptorontheleft, andmolecularviewontheright,depictingthedissociationofabound excitonbytheoffsettransportlevelsofthesematerials. energyoftheexciton,thenalexcitedenergystatecomprisingthefreeelectronandfree holemustbelowerinenergythanthatoftheexciton.Molecularheterojunctionscanbe realizedinavarietyofways,andthesewillbediscussedmorewithrespecttoorganic photodetectordevicesinChapter2andsolarcellsinChapter3. 1.5ElectronicTransportProperties Wenowturnourattentiontothebehavioroffreeelectronandholechargecarriers withinorganicsolids.Theintrinsicfreecarrierconcentrationinorganicsolidsis exceedinglylow,butfreecarriersmaybeinducedbydissociationofphoto-generated excitonsasdescribedinSection1.4.4,chargeinjectionaswillbediscussedinSection 1.5.1,anddoping.Hereweomitdiscussionsondoping,butforanoverviewofits effectsonorganicsemiconductorsseeArkhipov etal. 25 .Inthefollowingsections,we explorechargeinjection,chargetransportfromthemolecularandbulkviews,theeffect 36

PAGE 37

ofmolecularmorphologyontransport,andthetransportenergylevelsforavarietyof typicalorganicsemiconductors. 1.5.1ChargeInjectionatMetal-OrganicandOrganic-OrganicInterfaces Chargeinjectionatametal-semiconductorinterfaceisdependentontheinterfacial barriertoinjection.Injectioncanbedescribedasamixtureoftwomainmechanisms: electrontunnelingthroughthebarrierandthermionicemissionoverthebarrier.In general,tunnelingdominatesatlowtemperatureandverylargeenergybarriers,while thermionicemissiondominatesathightemperatureandlowbarriers.Formanyreal systems,theinjectedcurrentisdeterminedbyamixtureofthetwoandissaidtofollow athermionic-assistedtunnelingprocess.Inbothcases,however,themagnitudeof injectionhasanexponentialdependenceonthebarrierheightand,fortunneling,the barrierwidth 26 Yet,forthecaseofinjectionintoorganicsemiconductors,theexperimentalevidence generallyshowsadeparturefromthetraditionalthermionicandtunnelingtheory.In fact,organicsemiconductorcontactinterfacesarefoundtogeneratesignicantinterface dipoles,andthesedipolesarefoundtostronglyaffectinjectioncharacteristics 27 .While theelectroninjectionefciencyofcathodesisfoundtodecreasewithincreasingwork function 28 ,themagnitudeofthereductionofefciencyislessthanthermionictheory predicts,andtheslopeofinjectedcurrent-voltagecharacteristicsareoftenindependent oftheelectrodeworkfunction 29 .AsBaldoandForrestshow,thisdeparturefromclassic theorycanbeexplainedbythepresenceofthesedipoleinterfaciallayers,whichserve 37

PAGE 38

toinduceinterfacialenergystatesandallowamulti-stepinjectionprocessthatisnot limitedbyasinglehoppingmechanismoverorthroughtheinjectionbarrier 29 Thepresenceofaninduceddipolelayerattheinterfacebetweenametaland organicgenerallyleadstoanegativevacuumlevelshiftwithrespecttothemetalwork function.Thereareseveralpossibleexplanationsforthiseffect,includingelectron transferfromthemetaltoorganic,mirrorimagechargesinthemetalinducedby thedipole,chemicalinteractionsandothers 30 .Interfacialdipoleshaveimportant implicationsforengineeringcontactstoefcientlyinjectcurrent,andconsequently theeldoforganicsemiconductordevicesingeneralusesdifferenttechniquesto formefcientdevicecontactsthanthoseforinorganicsemiconductors.Furthermore, dipolescanalsoarisebetweenorganic-organicheterojunctions 31 andthiscaneffectfor instancethedissociationofexcitonsatadonor-acceptorinterface. 1.5.2HoppingTransportandChargeMobility Inanymolecularmaterial,wecanviewthetransportofchargeasbeingoftwo parts:intra-molecularandinter-molecular.Intra-moleculartransportcaninfactbevery efcientduetothedelocalizationofchargesaboutaconjugatedmolecule.Forexample, considertherecentexperimentalmeasurementofamobilityof2 10 5 cm 2 V )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 in graphene,asheetofcontinuouslysp 2 -hybridizedcarbon 32 .Althoughgraphenedoes notsharethemolecularsub-unitnatureofbulkorganicsemiconductors,itisinessence averylarge,entirelyconjugatedmoleculeandservesasanillustrativeexamplethat thelowercarriermobilitiesoforganicsemiconductorsarenotnecessarilylimitedby conjugatedmoleculesthemselves. 38

PAGE 39

Inorganicsemiconductors,ormoregenerallyinanydisorderedmolecularmaterial, longrangechargetransportoccursbyhoppingoffreecarriersfrommoleculeto molecule.Conductionisthereforephonon-assistedindirectcontrasttoinorganic semiconductorswhosebandtransportislimitedbyphononscattering.Experimentally, themobilityfollowsthegeneraltemperaturedependence: m = m 0 exp )]TJ/F72 11.9552 Tf 10.617 16.863 Td [( T 0 T 1 = a # where a isafactorwhosevaluerangesfrom1 33 Electronhoppingbetweenmoleculesisanalogoustotheinjectionfromametalinto asemiconductorthroughanenergeticbarrier.Bothprocesseshavethermal-assisted, orthermionic,mechanismsaswellastunnelingmechanisms.Organicsolidsexhibit atransitionfromphonon-assistedmobilitywithelevatedtemperaturetoeld-assisted mobilityatelevatedeldstrength.Underanelectriceldexceeding10 5 V/cm,the mobilitytakesonaelddependenceasthetunnelingtransfermechanismbeginsto dominateanditselfiseldactivated: 33 m E = m exp q kT b p E where E isthemagnitudeoftheelectriceld, m themobilityatzeroeld,and b = p e = pe thePoole-Frenkelfactor.Despitethesegeneralizedforms,aconsistenttheory thatexplainselectrontransportinorganicsemiconductorsremainsanactiveresearch goal. 39

PAGE 40

1.5.3EffectofMolecularMorphologyonChargeTransport Fororganicsemiconductorscomprisedofsmallmolecularunits,longrange transportmustovercomeinter-molecularbarriers.Thesebarriersaretiedtothequality andcrystallinityofthematerial.Here,webrieydiscusstheeffectsofmorphology onthetransportinthewell-studiedpentacenemolecule.Thecarriermobilityin pentacene,showninFigure1-1,variesgreatlybythecrystallinityandorientationof thesemiconductor. Thepoly-crystallinemorphologyofpentacenehasbeenfoundtovaryaccording tothesurfacefromwhichitgrows,andthisstronglyeffectsthecarriermobility.In organiceld-effecttransistorsOFETs,ithasbeenfoundthattheuseofself-assembled monolayersSAMsaswellasthesemiconductorgrowthtemperatureaffectsthesizeof thepoly-crystallinedomains,andthatanintermediatesizethatisbelievedtominimize theinter-grainboundaryhoppingdistanceisfoundtoachievethehighestmobility 34 Ultimately,itisfoundthathigherhydrophobicityofthedielectricontowhichpentacene isgrownresultsinhighercarriermobility 35 .Similarly,theuseofahydrophobicgate dielectricforOFETsusingamixedpolymer-fullerenesemiconductorisfoundtoimprove ambipolartransistoroperationandthisisattributedtoachangeinnanoscalephase morphology 36 .Furthermore,thepolycrystallinityofpentaceneeffectsnotonlycarrier transportbutalsoexcitondynamics;forinstancethetripletstateinpentacenesingle crystalsisfoundtohavealengthenedlifetimerelativetothatofpolycrystallinethin lms 22 40

PAGE 41

Theorientationofthesemiconductorisalsoofprimeimportanceinmanyorganic semiconductordevices.ManysmallmoleculessuchaspentaceneandCuPcstack preferentiallyinaparticularwayasgrowninthinlms.Forinstance,thecrystallographic orientationofpentacenethinlmsasgrownoncommonsubstratesplacesits a b stackingplaneparalleltothesubstrate.Coincidentally,thisistheplaneofhighest conductioninpentaceneduetothelargedegreeof p orbitaloverlappingbetween stackedmolecules,andthereforepentaceneproduceshigheldeffectmobilityinlateral thinlmtransistors.Themobilityofpentaceneintheverticalplaneislowerdueto increasedhoppingdistancebetweenmolecules,howeverithasbeenshownthatthe useofcarbonnanotubesassubstratecanreorientthehighmobility a b stackingplane intheverticaldirection 37,38 .Insummary,themolecularcrystallinityandorientationof organicsemiconductorsstronglyaffectscarriermobility,andtheseeffectsaretiedtothe manipulationofhoppingcarriertransport. 1.5.4BulkElectronicTransport Onceweacceptthemannerinwhichchargesaretransportablefrommolecule tomoleculeviahopping,wecanbackawayfromthemacromolecularviewpointtothe abstractedviewofthematerialassimplyabulksolidwithdenedenergylevels,carrier concentrations,andcarriermobilities.Ohmicconductionthroughsuchasolidfollows Ohm'sLaw, J = qE n m n + p m p .Thecarrierconcentrations n and p canbealteredby freecarrierinjection,doping,orphoto-generation.Atcurrentinjectionlevelswherethe injectedcurrentconcentrationislessthanthecarrierconcentration,thecurrent-voltage characteristicsfollowOhm'slaw.However,organicsolidstypicallycannotsupport 41

PAGE 42

theveryhighcarrierconcentrationsfoundininorganicsemiconductors,andtherefore theinjectionoffreechargesquicklybecomesspace-chargelimitedduetolowcarrier concentration. SpacechargelimitedcurrentSCLCcanbeapproximatedbytreatingthe semiconductorasaperfectinsulatorwhereinthechargestoredacrosstheinsulator isasinthecaseofaparallelplatecapacitor, Q 0 = C 0 V ,withcapacitance C 0 = e = L .The transportofchargethroughtheinsulatorisdeterminedbythecarriervelocity, V ,times thechargeanddividedbythelength, L .Themorerigorousanalyticalapproachtothis derivationdeviatesbythefactor9/8,andthecurrentdensityfollows 17 : J = 9 8 em V 2 L 3 BecauseSCLCisnotdependentoncarrierconcentration,theSCLCmobilitycanbe calculatedforasampleofknownthicknesssimplybymeasuringthecurrent-voltage characteristics.Inordertoderivemobility,adevicestructurethatensuresunipolar conductionmustbeusedandandthemobilityiscalculatedfromtheregioninthe current-voltagetracewhereSCLCoccurs,takenasthatregionthatfollowsa V 2 dependence.InadditiontoSCLC,anumberoftechniquesareusedtocharacterize themobilityinorganicsemiconductorsincludingeldeffecttransistorstructure,timeof ightmeasurements,andseveralothers 39 Organicsemiconductorsalsotypicallycontaintrapstatesthatinuencethecapture ofinjectedcarriers.Thesetrapsarelledunderlowbiaswiththeconductionoffree carriersandunderelevatedbiascanbereleased.Trapreleaseisdependentonthe bindingenergyofthetrapsandtheappliedbias.Therefore,asthevoltageisincreased, 42

PAGE 43

thecurrentcanexhibitrstohmicinjection,followedbytrap-freeSCLC,followedby trapreleaseandasharpincreaseincurrent,trap-freeSCLC,andmoresimilarcycles dependingonthedistributionoftrapstates.Foramorecompletetreatmentofthese phenomena,seetheworkbyLampertandMark 17 Thetreatmentoforganicsemiconductorsasinsulatorsinthespace-chargelimited regimeillustratesthesimilaritybetweenthesetypesofmaterials.Bothclassesof materialsexhibitlowcarrierconcentration;thetruedifferentiatorsarebandgapand toalesserextentcarriermobility.Lowbandgapenablesinjectionwithstablemetals andabsorptionofnear-UV,visible,andnear-IRradiation.Relativelyhighmobility enablesmoreefcientconduction.Asanillustrativeexample,considersiliconoxide: withacarriermobilityof29cm 2 V )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.967 0 Td [(140 ,thismaterialout-performsalmostallorganic semiconductorsintermsofcarriervelocity,howeverwithabandgapof8eV 41 injectionisforbiddenandconsequentlysilicaisusedwidelyasastrongelectronic insulator.Conversely,organicsemiconductorsexhibitcarriermobilitiesrangingfrom below10 )]TJ/F20 8.9664 Tf 6.967 0 Td [(5 upto10 0 ,andprovideoptoelectronicfunctionalitybyuseofgenerallyvery thinmateriallayerswithtransportenergylevelsthatallowelectroninjectionandphotonic interaction. 1.5.5TransportEnergyLevels Inorganicsemiconductors,electronsaretransportedintheLUMOwhileholes aretransportedintheHOMO.Thesetransportlevelsareimportantmetricstousein designingdevicestructuresinordertoproperlyselectinjectionmetalsandtoengineer organicsemiconductorinterfacestoforinstanceefcientlydissociatephoto-generated 43

PAGE 44

excitons.However,itisnottrivialtoexperimentallydeterminethesetransportlevelsas absorptioninorganicsisgovernedbyexcitonenergyratherthantransportenergy,due thestrongexcitonbindingforce.Furthermore,duetothepresenceofdipolelayersas discussedinSection1.5.1,theenergeticsofthesesolidsisdependentonthesubstrate andsurroundinginterfaciallayers.Nevertheless,wecanuseHOMOenergiesas determinedbyphotoelectronspectroscopyPESandLUMOenergiesasdeterminedby inversephotoelctronspectroscopyIPEStoguidedeviceengineering 42 Vacuum eV E HOMO -eV E LUMO -eV Refs. 43 5.7 1.7 2.5 Anthracene 43 5.1 1.8 2.5 Tetracene 44 4.7 2.1 P3HT 45 5.3 2.6 Rubrene 46 5.2 2.7 3.0 CuPc 47 5.1 2.8 2.8 Pentacene 42 5.1 3.2 3.5 ZnPc 48 5.3 3.4 Squaraine 49 5.4 3.4 TiOPc Figure1-7.Electronictransportenergiesforseveraldonororganicsemiconductor materials.TheLUMOenergyasdeterminedbyIPESandtheHOMO energyasdeterminedbyPESareplottedschematicallyrelativetothe vacuumlevel,separatedbythetransportenergygap,atthetopandbottom, respectively.Theopticalbandgapisalsoshownastheenergydifference betweentheHOMOandadottedline.Valuesaretakenfromtheliteratureas cited. 44

PAGE 45

Theelectronictransportenergiesofseveralrepresentativedonormaterialsare plottedinFigure1-7,andthoseforseveralacceptormaterialsalongwithafewlarge bandgapmaterialsusedforotherpurposesinorganicsemiconductordevicesare plottedinFigure1-8.BothguresplottheLUMOandHOMOenergiesrelativetothe vacuumlevel.Theboxesrepresentthetransportenergygapofeachmaterial.The opticalbandgapsofthesematerialsarelowerthanthetransportgapsduetotheexciton bindingenergy,andwhereavailablethesearerepresentedintheguresbytheenergy gapbetweentheHOMOandadottedline.ThedonorsshowninFigure1-7areused inbothtransistorsandphotodiodes.ThesematerialsgenerallyexhibitaHOMOenergy ofapproximately5eV,intowhichholescanbeefcientlyinjectedfromanodessuch asindiumtinoxideITO, =4.5.0eVandgold =5.1eV.Theacceptorsshown inFigure1-8canbeusedasn-typematerialsfortransistorsandasacceptorsfor photodiodes.ThesematerialstypicallyexhibitaLUMOenergyofapproximately4eV, intowhichelectronscanbeefcientlyinjectedfromcathodessuchasAl =4.2eV andAg =4.3eV. 45

PAGE 46

Vacuum eV E HOMO -eV E LUMO -eV Refs. 48 6.2 3.5 4.2 C 60 44 5.8 3.8 PCBM 46 6.6 4.0 4.3 PTCBI 43 8.0 4.0 4.4 NTCDA 46 7.0 4.1 4.8 PTCDA 47 6.4 4.3 5.4 C 70 42 6.3 4.4 F 16 -CuPc 43 6.4 1.8 3.1 BCP 43 6.3 1.9 2.6 CBP 43 5.7 2.0 2.9 Alq 3 Figure1-8.Electronictransportenergiesforseveralacceptororganicsemiconductor materials,aswellasafewlargebandgapmaterialsusedashost,injection, andblockingmaterialsBCP,CBP,andAlq 3 .TheLUMOenergyas determinedbyIPESandtheHOMOenergyasdeterminedbyPESare plottedschematicallyrelativetothevacuumlevel,separatedbythetransport energygap,atthetopandbottom,respectively.Theopticalbandgapisalso shownastheenergydifferencebetweentheHOMOandadottedline. Valuesaretakenfromtheliteratureascited. 46

PAGE 47

CHAPTER2 ORGANICPHOTODIODESASPHOTODETECTORS Nowthatwehaveabasicunderstandingofthepropertiesoforganicsemiconductors,wecanbegintoexplorethewaysinwhichtheseinterestingmaterialscanbe usedindevices.Thischapterrstoutlinesthemotivationsforapplieduseoforganic semiconductorphotodiodesasphotodetectors.Second,itdescribestheoperation ofgeneralphotodetectorsandtheircharacterizationmetricsandmethods.Third,it describesworkpreviouslydonetoimproveperformanceoforganicphotodetector devices,focusingondevicestructuresthathavebeenusedtocircumventthediffusion bottleneckoforganicsemiconductorsdescribedinChapter1.Finally,itdiscusses photomultiplicationinorganicsemiconductors,andthedevicestructuresthatcan giverisetothiseffect.Chapters4and5,buildonthesefundamentalunderstandings anddescribestudiesintoneworganicphotodetectorarchitecturesthatexhibit photomultiplication. 2.1OrganicPhotodetectorApplicationsandAdvantages Theeldofphotodetectionisfullofmanyinterestinganduniqueapplications forwhichthereareequallydiverseandnumeroustechnologies.Focusingonlyon detectorsthatmakeuseofthephotoelectriceffect,whereinphotonsdirectlyinteractwith electronsinasolid,detectortechnologiesincludephotoconductorsandphotoresistors, photodiodes,photomultipliertubes,phototransistors,andcharge-coupleddevices CCD.Withinthesebroadcategories,thereexistfurtherdelineationsbasedon materials,devicestructures,andoperatingconditions,forinstancephotodiodes 47

PAGE 48

canbebrokendownintovarioustypesincludingavalanche,heterojunction,metalsemiconductor,p-i-n,p-n,point-contact,Schottky-barrier,andmore.Thediversityof devicestructuresreectstheingenuityofthesemiconductoreld,butalsoisinline withthediversityofuniqueapplicationdemands.Thedetectionoflightndsapplication inalmosteverymoderntechnology,fromanightlightforthebedroomofachildto telescopesthatgatherinformationaboutthedistantuniverse.Moreover,thedetection oflightisoneofthegreatingenuitiesofevolutionoflifeonearth.Fromchlorophyll sensitizationinplantphotosynthesistothephotoreceptorsofthemammalianeye,nature hasproducedaplethoraofsmallandlargemolecularspeciesthatarespecializedfor thedetectionandharvestingoflight. Itisfromthisreferencepointthatwecantrulyappreciatethepoweroforganic semiconductorphotodetection.Organicsemiconductorsaremostsimilarto,and sometimesarederivedfrom,biologicalmolecules.Organicsemiconductordevices, meanwhile,usethesemiconductorphysicsandtheengineereddevicestructuresfrom inorganicsemiconductordevicestousethesematerialstodetectlight.Thismarriage ofnaturalandhumanderivedtechnologiesholdsgreatpromiseforthefutureoflight detection,asitallowsustoaccessmorefunctionalityandmoreapplicationsthanever before. Organicsemiconductorsareapromisingnewmaterialstechnologywithmany potentiallow-costapplicationsintheelectronics,lighting,andenergyindustries.For thisreason,muchoftheworkinthiseldisdedicatedtoimprovingtheperformance oforganictransistors,light-emittingdevicesOLEDs,andsolarcellssothattheymay 48

PAGE 49

ultimatelybringdownthecostofcommercialelectronics.Yet,theseremarkablematerialsoffermorethansolelytheirlowcost.Lightdetectionbyorganicsemiconductors benetsfromanabundanceofexistingmoleculeswithvaryingspectralresponse,and responsecanfurtherbetunedbychemicalmodication.Perhapsmostimportantfor nextgenerationlightdetection,organicsemiconductorsmaybemadeintoverylarge andconformablegeometries,whereitisdifculttoemployforinstancecrystallinesilicon photodiodes. Largeareaphotodetectorsareneededtodetectdiffuselightinapplicationssuch asmedicalimagingandsecurityimaging,whereascreenofscintillatormaterialdown convertsx-rayorgammarayradiationintounfocusedlight 50 .Organic-baseddetectors mayenableverylowcostdetection,especiallywhencombinedwithlikewisesolution processablescintillatingquantumdots 51 .Furthermore,theconformabilityoforganic materialsontonon-planarsubstratesmayallowtheiruseindevicestructuresthat reducethedemandonoptics,mimickingthemammalianeyesuchastheelaborate silicon-basedstructureKo etal. demonstrated 52 .Combiningthesetwoproperties largeareaandconformabilitymayenableyetmoreapplicationssuchasconformable andsimultaneoustomography,andingeneralmoreadaptivedetectionarrangements especiallyforfuturemedicalapplications.Figure2-1schematicallyillustratessomeof thesefunctionaladvantagesoforganicphotodetectors. 2.2OperationandCharacterizationofOrganicPhotodiodesforSignalDetection Anidealphotodiodecanbemodeledbyanequivalentcircuitconsistingofa photocurrentsourceinparallelwithadiode,junctioncapacitor,shuntresistor,andnoise 49

PAGE 50

Figure2-1.Conceptualdrawingoforganicphotodetectoradvantages.Thelarge-area andexibledeviceadvantagesalloworganicphotodetectorstobeemployed inlargeareaapplicationssuchasmedicalimaging,andcurvalineardetector applicationssuchasarticialvision,allatlowercostthanalternatives. currentsourceandinserieswithaseriesresistorandanyexternallyconnectedload resistance.ThiscircuitisshowninFigure2-2.Tocharacterizeaphotodiode,eachof thesecurrent,resistance,andcapacitancesourcesmaybeeitherdirectlymeasuredor extractedbymodelingcurrent-voltagecharacteristics.Thefollowingsectionsexplore thesecharacteristicsinmoredetail. Figure2-2.Equivalentcircuitforanidealphotodiode,showingaphotocurrentsource andnoisecurrentsource,parasiticresistances,junctioncapacitance,and idealdiode. 50

PAGE 51

2.2.1DarkDiodeCurrent Inthedark,aphotodiodeoperatesasasimplediode.Inorganicsemiconductor theorystatesthatsuchadevicefollowsthediodeequationforcurrentdensity, J J d = J 0 exp qV nkT )]TJ/F20 11.9552 Tf 10.949 0 Td [(1 Thereverse-biascurrentisdependentonthesaturationcurrentdensity, J 0 ,assumed tobediffusion-dominated.Theforwardbiascurrentisassumedtobedominated bySchockley-Read-HallSRHrecombination,forwhichidealdiodeswithsinglecarrierandtwo-carrierlimitedrecombinationexhibitidealityfactors, n ,of n =1and n =2,respectively.Forrealorganicdiodes,thepictureisoftenmorecomplex,with injection-limited,trap-limited,andshuntingeffectsleadingtonon-idealbehaviorand ingeneralpoorersaturationunderreversebiasthanforthecaseofinorganicdevices. Nevertheless,organicdiodescanexhibitverystrongrecticationupto10 6 at V = 1 V measuredastheratiooftheforwardbiastoreversebiascurrentataspeciedvoltage. 2.2.2SpectralResponsivity Themagnitudeofphotocurrentresponsegeneratedbyanyphoto-sensitivedevice isdependentonthewavelengthoftheincidentlight.Forinorganicphotodetectors suchassilicon,theresponsivityextendsacrossabroadspectrumforenergiesabove theopticalbandgap,andopticalltersareoftenusedforinstancetocharacterize thered-green-bluecomponentsofanopticalsignalbyusingthreeseparatesilicon detectorelementseachlteredbyasuitableshortpass,longpass,orbandpasslter. Thespecicityofspectralresponseisoneadvantagefororganicphotodetectors,asthe narrowabsorptionbandsoforganicsemiconductorscanbeusedtotunetheresponse 51

PAGE 52

intheUV,blue,green,red,andnearinfrared,oracombinationoftheabovewithout additionallters. Thereareseveralspectraldependenciesthatareimportanttocharacterize forphotodetectordevice,includingtheabsorptionpropertiesofthesemiconductor materials,thedevicequantumefciency,thedeviceresponsivity,andthedependence oftheresponsivityonopticalpower.Forallsuchmeasurements,alock-inamplication approachisbesttoaccuratelymeasuretheresponsetoweakopticalsignals.Alock-in amplicationmeasurementmakesuseofalock-inampliertoisolatethemodulated photocurrentsignalofaphotodetectordeviceinresponsetoamodulatedopticalsignal. Bymodulatingamonochromaticopticalsignal,usuallyproducedbypassingawhitelight sourcethroughagratingmonochromator,themodulatedphotocurrentcanbeisolated fromthebaselinedarkcurrentandthevariousbandwidthnoisecurrentsignalspresent whenthedeviceisunderbiasingcondition.Furthermore,thephotocurrentcomponent thatisindirectresponsetothemodulatedlightcanbeisolatedfromanyphotocurrent signalthatisinresponsetoambientlight. Tomeasureabsorptionofthesemiconductormaterials,suchasystemisusedto passthemonochromaticsignalthroughthetestsample,andtheincident,reected, andtransmittedsignalsaremeasuredbyaphotodiodewithcalibratedresponsivity.To measurequantumefciencyandresponsivity,acalibratedbeamintensityisdirectedat thetestdevice.Theinternalquantumefciency, h int IQE,orthenumberofelectrons thatcyclethroughthetestcircuitforeachphotonabsorbedbythedevice,isthe absorptionefciency, h abs ,multipliedbytheexternalquantumefciency, h ext EQE, whichisdirectlymeasuredasthenumberofelectronsthatcyclethroughthetestcircuit 52

PAGE 53

foreachphotonincidentonthedevice: h int v = h abs v h ext v = h abs v I p q hv P opt where I p isthedevicephotocurrent, q istheelementarycharge, h isPlanck'sconstant, v isthefrequencyofthemonochromaticlightbeam,and P opt istheincidentopticalpower. Thespectralresponsivity, R ,issimplytheratioofphotocurrenttoincidentoptical power,andisrelatedtothequantumefciencyby R v = I p P opt = h ext v q hv Foranidealphotodiodewith h ext =1, R = l 1240 ,where l isexpressedinnanometers. Formanyapplications,itisimportantthatthephotodetectoroutputasignalthatislinear withtheopticalintensity,anditisthereforeusefultocharacterizethelinearityofthe responsivity.Todoso,thesamephotocurrentmeasurementsetupcanbeusedand theopticalpowercanbesteppeddownbyuseofneutraldensityopticallters.Higher intensitylasersourcesmayalsobeusedforthistypeofcharacterization. 2.2.3TemporalResponse Theoutputofaphotodiodetakestimetomodulateinresponsetoachanging opticalsignal.Therisetimeofresponse, t r ,isthetimeittakesforthesignaltoclimb from10%to90%ofthenaloutputuponinitialillumination.Thelowerlimitstothe temporalresponse,inthissense,canbeestimatedbytheRCtimeconstantofthe devicejunctioncapacitanceinserieswiththedeviceseriesresistance, R S ,andforthe caseofasinusoidalmodulationofsignal, t r =2.2 t RC .Organicphotodetectorscanexhibit verylowcapacitanceontheorderofpicofaradsandRCtimeconstantsontheorder100 53

PAGE 54

ps 53 ,butoftentheultimatetemporalresponseislimitedbytheslowreleaseoftrapped carriers. Therearetwomainmethodsusedtocharacterizethetemporalresponseof thesedevices.Therstistosubjectthedevicetoopticalsignalsofvaryingmodulated frequency,byforinstancechoppingalightsourceatlowfrequencyorcontrollingthe outputofadiodelaserbyahighspeedfunctiongenerator.Thelock-intechnique describedpreviouslycanbeusedtodirectlymeasurephotocurrentinthiscase,and monitorthedegradationofphotocurrentwithincreasingmodulationfrequency.The secondmethodistodirectlymeasuretheriseordecayofthephotocurrentsignalin oneperiodofthesignalbyusingahighspeedoscilloscopetomeasurephotocurrent responsetoamodulatedsignal. 2.2.4Noise Allelectricalcomponents,includingphotodiodes,canactasasourceforelectrical noise.Forstrongsignalapplications,suchasthedetectionofsunlight,thenoisecurrent playsaminimalrole,butforweaksignalapplicationsthenoisecurrentcanapproach thephotocurrentsignal.Itisfortheseapplicationsthatthecharacterizationofnoise currentbecomesimportant.Thetotalnoisecurrentofaphotodiodeconsistsofthree components:theJohnsonnoise, I J ,shotnoise, I S ,andickernoise, I F : I N = q I J 2 + I S 2 + I F 2 andistypicallyexpressedastheroot-mean-squarerms I / p B ,where B isthenoise bandwidthmeasuredinHz,i.e.,thenoisesignaldependsonthesamplingfrequency. 54

PAGE 55

TheJohnsonnoise,orthermalnoise,isthatcomponentofthenoisecurrentthat arisesduetothermaluctuationislimitedbytheshuntresistanceofthedevice: I J = p 4 kTB = R SH Theshuntresistance, R SH isameasureofthephotodioderesistancewhenunder reversebias,andforanidealphotodiodeisinnitelylarge.Foraphotodiodewhose photocurrentisentirelyextractedwhenoperatedinphotovoltaicmode V =0,theshunt resistancecanbemeasuredastheslopeofthecurrent-voltagetraceat V =0. Theshotnoisearisesfromuctuationsinthedarkcurrent, I d ,throughthedevice, I S = p 2 qI d B Finally,theickernoiseisalsodependentonthedarkcurrentandhasaninverse dependenceonfrequency: I F = p KI d B = f Whilethethermalandshotnoisecanbecalculated,theickernoisecontainsthe constant K whichdependsonmanyaspectsofthedeviceandmaterial,andsomust bemeasured.Inpractice,thetotalnoisecurrentismeasured,asthisisthemost importantgureofmerit.Thistypeofmeasurementisdelicate,asanymeasurement circuitintroducesadditionalnoisesources,andwirelesssignalscaninuencethe measurement.ItisbesttoenclosethetestdeviceinaFaradaycage,andthedirect measurementcanbedoneusingalock-inamplierthatisolatesnoisecurrentsat speciedbandwidthabovethedarkandphotocurrentsignals.Foramorecomplete discussionofnoiseinelectroniccircuits,consultthetextbyHorowitzandHill 54 55

PAGE 56

2.2.5Sensitivity Theresponsivityofaphotodetectordenesthesignalstrengththatcanbeextracted fromadetector.However,astheprevioussectiondetailed,detectorsalsooutputanoise signalwhichisindependentoftheopticalsignal.Therefore,thetruesensitivityofa photodetectorhastotakeintoaccountboththeresponsivityandthenoise.Thenoise equivalentpower, NEP = I N = S R istheratioofthenoisecurrenttotheresponsivity,andrepresentstheminimaloptical signalpowerthatisnecessaryforthedetectortooutputasignal-to-noiseratioof1.The detectivity, D =1 = NEP istheinverseofthenoiseequivalentpowerandgivesameasureofthesensitivityin unitsof p Hz = W .However,thenoisesignalisdependentonthesquarerootofthe activedevicearea, A D ,andthereforeabetter,morenormalizedanduniversalmeasure ofaphotodetectorsensitivityisthespecicdetectivity,inunitsofcm p Hz = W : D = D p A D Therefore,highsensitivityrequireslownoisecurrentandhighresponsivity.Assection 2.2.4discusses,noisecurrentarisesduetoavarietyofsourcesbutasubstantial amountsimplycomesfromthedarkcurrent.Therefore,typicallylowdarkcurrent andhighresponsivityleadtoahighlysensitivedevice.Theremainingsectionsin 56

PAGE 57

thischapterexplorethegenerationofphotocurrent,theeffectofdevicestructureon sensitivity,andtherealizationofphotocurrentgaininorganicphotodiodes. 2.3PhotocurrentGenerationinOrganicSemiconductorDevices AswasdiscussedinChapter1,thereareanumberofdifferentprocessesassociatedwiththegenerationoffreecarriersinorganicsemiconductorsbyphotoexcitation, andfreecarriersthemselvesmustovercomethetransportlimitationsoforganic materials.Therefore,theentireprocessofgeneratingphotocurrentinthesematerialsis amulti-stepprocess,witheachstephavingpotentialsignicantlossmechanisms.We canmeasuretheefciencyofphotocurrentgenerationbyconsideringtheconceptof quantumefciency,asdiscussedinSection2.2.2: h ext = h abs h int Theelectricallossmechanismsarecontainedintheinternalquantumefciencyterm, andreectsthemultistepphotocurrentgenerationprocessconsistingofexcitondiffusion,excitondissociation,andfreecarriercollection.Therefore,thetotalphotocurrent generationprocessistheproductoffourindependentprocesses: h ext = h abs h diff h diss h coll TheseprocessesareshownschematicallyinFigure2-3.Considerthatfororganic semiconductors,typicalabsorptionlength L abs isontheorderof100nm,whiletypical excitondiffusionlength, L diff ,isontheorderof10nm.Thislengthscaleimbalancegives risetowhatiscommonlyreferredtoasthediffusionbottleneckinthephotoresponse oforganicsemiconductordevices.Aheterojunctiondeviceconsistingofdonorand 57

PAGE 58

acceptormaterialswithenergyleveloffsetsgreaterthantheexcitonbindingenergycan beconstructedtoefcientlydissociateboundFrenkelexcitons.However,excitonsthat aregeneratedfartherthan L D fromthedonor-acceptorD-Ainterfacecannotdiffuseto andbedissociatedbythisstructure.Therefore,devicesusingneatlayerswiththickness greaterthan L abs cannotefcientlydissociategeneratedexcitons.Thus,therearisesa tradeoffbetweenabsorptionanddissociationefciencyinthesematerials,duetotheir shortexcitondiffusionlength. Figure2-3.Multi-stepphotocurrentgenerationprocessinorganicsemiconductor devices,showingtheabsorptionofaphotonandcreationofaboundFrenkel excitoninthedonorofadonor-acceptorheterojunctionstructure,theexciton diffusionanddissociationtoandattheheterojunctioninterface,andnally thecollectionofseparatedelectronsandholesatthecathodeandanode, respectively. Thegenerationofphotocurrentiscriticaltophotodetectorsandsolarcells.As thisdissertationfocusesonthedevelopmentofsuchdevices,theprocessesthatare involvedintheproductionofphotocurrentwillbecontinuouslyrevisitedthroughoutthe comingchapters. 2.4DeviceStructureinOrganicPhotodetectors Therstchallengetoproducingafunctionalorganicphotodetectoristoefciently producephotocurrent.Sections1.4.4and2.3describedtheuseofdonorandacceptor 58

PAGE 59

heterojunctionstoproducephotocurrent.Thesematerialscanbecombinedinto devicesinmanyways,thesimplestamongthembeingtheclassicTangbilayerstructure betweenneatsmallmoleculedonorandacceptorlayers 9 andthebulkheterojunction formedbyisotropicmixingofpolymersinsolution 55 .Muchoftheworkdevotedto overcomingthediffusionbottleneckinorganicphotodiodeshasfocusedonsolarcell application;thesearediscussedinSection3.2.2. Ithasbeenshownthatthediffusionbottleneckmaybeovercomeinorganic photodetectorswhenapplyingareversebiasandusingalternatingverythin < 30 A layersofdonorandacceptor,suchthatnotonlydotheheterojunctioninterfaceslie withintheexcitondiffusionlength,butalsotheperiodicheterojunctionsareata pitchshortenoughtoallowphotogeneratedcarrierstotunnelthroughtheinterface barriers 53,56 .Thisdevicestructurewasalsoshowntoproduceextraordinarybandwidth fororganicdetectors,ontheorderof430MHzowingtotherapidtunnelingprocess involved. Tomaximizesensitivity,thedarkcurrentofsuchdevicesmustbeminimizedto reducethenoisecurrent,asdescribedinSection2.2.4.Thestrengthofrecticationin anorganicphotodiodeisdependentontheworkfunctionoftheanodeandcathode, andasXueandForrestshowed,exposinganITOanodetooxygenplasmaorUV-ozone treatmenttoreduceitsworkfunctionstronglysuppressesdarkcurrentunderreverse bias 56 .WhiletheP3HT:PCBMsystemismostlyexploredforitsuseinsolarcells, Ramuz etal. showthatwithoptimizationforphotodetection,devicesexhibitdetectivity of D =7 10 12 cm p Hz = W ,whichapproachesthedetectivityofsiliconphotodiodes approximately10 13 .TheyshowthatbyremovingthePEDOTanodeinterfaciallayer, 59

PAGE 60

generallyusedtoproducehigheropencircuitvoltageinsolarcells,andbythickeningthe activepolymerlayer,thedarkcurrentisstronglysuppressedandsensitivityincreased 15 Furthermore,Gong etal. demonstratedsimilardetectivity, D > 10 12 cm p Hz = W ,for apolymerfullerenesystemwithphotoresponsedeepintothenearinfrared l < 1400 nm,whichsurpassestheinfraredsensitivityofsilicon 16 .Othereffortshavecombined theuseofpolymerswithinfrared-sensitivequantumdotstoproducehighlysensitive devices 57 alongwitheffortstoomitthepolymerandusepurelyquantumdot-based devices 58 2.5PhotomultiplicationinOrganicSemiconductors Anotherroutetoimprovethesensitivityoforganicphotodetectorsistosurpass 100%quantumefciencyandsubstantiallyimprovetheresponsivityusinganinternal photomultiplicationmechanism.Photomultiplicationcanbeinducedininorganic avalanchephotodiodesbyapplyingareversebiasabovethebreakdownvoltage,and usingcomplexcircuitrythistypeofdeviceisabletoachieveahighenoughsensitivity todetectsinglephotons 59 .Althoughtheavalancheeffectcannotbeinducedinlow mobility,highlydisorderedorganicsemiconductors,Reynaertandcolleaguesargue thatthissameenergeticdisordercangiverisetoanintrinsicphotomultiplication mechanism 60 Photoconductivegainarisesindevicesthatexhibitunbalancedcarriertransport, suchthatminorityphoto-generatedcarriersareextractedorrecombineonalongertime scalethanthetransittimeofsecondarymajorityphotocurrent.Therefore,thegainis 60

PAGE 61

proportionaltotheratioofminoritycarrierlifetimetomajoritycarriertransittime: g t p t n Intrinsiccarrierimbalancecanleadtophotomultiplicationinsomematerials,andinfact bulkphotoconductivegainhasbeendemonstratedinafeworganicdevices 6063 .Yet, itisalsopossibletoengineercarrierimbalanceintoadevicebyintroducingextrinsic trappingsites.Yokoyamaandcolleaguesattributedhighgainintheirsmallmoleculebaseddevicestostructuralvoidsthattrapholesatinjectioninterfaces 6468 .Huang andYangalsoascribegainintheirdevicestotheinjectinginterfacebetweenPEDOT andC 60 69 .Inorganicnanoparticledopantshavealsobeenusedastrappingcenters dispersedwithinpolymer-basedphotodetectorstoachievegain 61,70 .Similarly,some inorganicquantumdotphotodetectorshaveachievedgainowingtoacombination ofbulkphotoconductivityaswellasextrinsictrapstateslocatedinnanostructureencapsulatingligands 58,71,72 Figure2-4.Conceptualdrawingshowingsimplicationofdetectordesignusinginternal gain.Usingdetectorelementswithbuiltingaincanreducetheneedto amplifysignalwithadditionalcomponents. 61

PAGE 62

Notonlycanphotomultiplicationimprovesensitivityofdevices,itmayalsoprovidea simplicationtophotodetectioncircuitry.Theoutputofphotodiodesinresponsetoweak signalsisoftenamplied;bybuildinggainintothephotodetectoritself,thedemandson externalamplicationmaybelifted.Figure2-4showsapartialdetectorarraytoillustrate thisadvantage.Simplerandcheapercircuitsarepossibleusingthisconcept,butonly ifthereplacementofeachpixelelementdoesnotincreasethecomplexityorcostofthe structure.Ingeneral,technologiesthatproduceinternalphotocurrentgainarenoteasily integratedintodetectorarrays,butthepotentialtodosousingorganicdetectorsmay spurfuturedevelopmentinthisdirection. Chapters4and5exploretwodevicestructuresthataredesignedtogenerate internalphotomultiplication.First,Chapter4exploresthepossibilityofusinganorganic phototransistor,asthisnotonlycanproducephotocurrentgainbutalsocanreplace thebackplanetransistorinimagearraysasshowninFigure2-4.Next,Chapter 5describesanovelphotodiodestructurethatusesaconnementmateriallayerto producehighinternalphotocurrentgain.First,Chapter3completestheintroduction portionofthisdissertationwithadiscussiononorganicsolarcells. 62

PAGE 63

CHAPTER3 ORGANICPHOTODIODESASSOLARCELLS 3.1SolarEnergy 3.1.1SolarSpectrum TheSungeneratesheatthroughthefusionofhydrogenintohelium.Asforany objectwiththermalenergy T > 0K,itemitsaportionofthisenergyisotropicallyand withaspectraldependencethatdependsonitstemperature.Accordingtothermal radiationtheory,suchablackbodyobjectcanbethoughtofasanenclosedcavity withinwhichelectromagneticstandingwavesmaybeexcitedbythermalenergy.This simplepredictionalongwiththelawsofthermalphysicsallowsustopredictthespectral dependenceoftheSun'sradiatedenergy.Withintheidealizedsolarcavity,modesof oscillationwithfrequency w =2 p f maybeexcitedindiscreteunits, s ,ofthequantum energy h w .Ifwemultiplythedensityofthesemodeenergystatesbytheirprobabilityof occupancy,wendthePlanckdistributionfunction, h s i ,andmultipliedbytheenergy h w givesthethermalaverageenergyineachmode: h E i = h s i h w = h w e h w = kT )]TJ/F20 11.9552 Tf 10.949 0 Td [(1 Integratingoverallmodesgivesthethermalradiationenergyperunitvolumeforthe blackbodyobject,andtheStefan-Boltzmannlawofradiationexpressesthisasaspectral densityofradiation: 1 u w = h p 2 c 3 w 3 e h w = kT )]TJ/F20 11.9552 Tf 10.949 0 Td [(1 1 seeforinstanceKittelandKroemerforamoredetailedderivation 73 63

PAGE 64

Blackbodythermalradiationtheoryisreadilyusedtodeterminethesurfacetemperature ofobjectsintheuniverse 2 .Thesurfaceofthesunisestimatedtobe5778K,andthe blackbodyradiationpredictedbytheabovetheoryforthattemperatureisplottedin Figure3-1.TheAM0solarspectrumisdenedbyASTMstandardE-490andisthat whichhasbeendirectlymeasuredincidentattheoutsideofEarth'satmosphere;ascan beseeninthegure,thisspectrumissimilartothatpredictedbyblackbodytheory. Figure3-1.Solarspectrumtheoreticallyemittedfroma5778Kblackbodydashedblack line,theASTMAM0spectrumasmeasuredincidentoutsidethe atmosphereofearthlledbluebars,andtheASTMAM1.5spectrum incidentatthesurfaceofearthafterpassingthroughtheatmosphereata 37 tilttonormallledorangebars. However,thespectrumincidentattheouteratmosphereundergoessignicant changeasitpassesthroughtheatmosphere,asgaseousspeciessuchaswater vapor,oxygen,ozoneandothernaturalandanthropogenicaerosolsattenuatespectral 2 Forinstance,thisisthebasicprinciplethathasallowedphysiciststouserecent measurementsofthecosmicmicrowavebackgroundtomeasurethebackground temperatureT=2.7Kandderivetheageoftheuniverse.4 10 10 years 74 64

PAGE 65

regionsespeciallyintheinfraredandparticulatespeciesscatterlightespeciallyinthe visiblespectrum.Theeffectoftheatmosphereonthesolarspectrumisvariableand complex 75 ,andthereforetheeldofphotovoltaicsreliesonanagreeduponstandardfor testingsolarcells.ASTMstandardG-173denestheAM1.5referencespectraforthis purposeastheirradiancespectrumincidentata37 tiltofthesun-facingearthsurface theaveragelatitudeofthe48contiguousUnitedStates,correspondingtoanairmass attenuationlengthof1.5timestheattenuationlengthofnormalincidence.TheASTM denestwoAM1.5referencespectra;theAM1.5Globalspectraisdesignedforstatic atsolarpanelsandintegratestoatotal1000W/m 2 alsoplottedinFigure3-1,while theAM1.5Direct+circumsolarisdesignedforconcentratingsystemsandintegratesto atotalof900W/m 2 .ThisworkisfocusedonstaticplanarPV,andsohenceforthany referencetotheAM1.5spectrumreferstotheASTMglobaltiltstandard. 3.1.2SolarCells Asolarcellissimplyaphotodiodedeviceusedinphotovoltaicmodewherein thesolarcellisplacedinserieswithanexternalload.Undershortcircuit,asolar celloutputsamaximumamountofphotocurrentrelatedtothenumberofphotons absorbedbythedevice,andunderopencircuitoutputsamaximumvoltagerelatedto thebandgapofthesemiconductingmaterialandotherdevicefactors.Awell-suitedload canextractthemaximumpowerintermediatebetweentheseconditions.Thiswillbe discussedinmoredetailinSection3.3.Thebandgapofasemiconductordeterminesin largepartthetradeoffbetweencurrentandvoltage,asalowerbandgapcollectsmore photonsfromthesolarspectrum,butatthesametimealargeportionoftheenergy 65

PAGE 66

fromshortwavelengthlightislosttorapidthermalizationofthephotocarrierstothe bandgap.Figure3-2plotstheultimateefciencyofasolarcellentirelybaseduponits bandgapassumingeveryincidentphotonwithenergygreaterthanthebandgap, E g contributespowerequalto qE g andcomparesthistotheirradianceandphotonux oftheAM1.5spectrum.Thereare,however,additionalthermodynamiclimitstothis efciency,asShockleyandQueisserdemonstrateusingadetailedbalanceapproach thelimittoasiliconsolarcellwithEg=1.1eVisapproximately30%fora6000K blackbodyemitter 76 .Nevertheless,thetrendshowninFigure3-2illustratesthetradeoff toefciencythatarisesinasinglejunctionsolarcell. Figure3-2.AM1.5irradiance,solarux,andultimateefciencyofsinglejunctionsolar cells.Thespectrairradianceanduxaredrawnwithrespecttowavelength oflight,whiletheultimateefciencyplotshowstheultimateefciencyofa cellwithabandgapcorrespondingtoanabsorptioncutoffatthewavelength oflightatwhichitisplotted. Siliconhappenstohaveabandgapthatstrikesanearidealbalancebetween photocurrentandphotovoltage.Itsuseinsolarcellsdatestotherstdemonstrationof 66

PAGE 67

anefcientsolarcellofanytypein1954atBellLaboratories,andsincethattimehas beenthemostwidelyemployedmaterialforterrestrialphotovoltaicapplications.Single crystallinesc-SicellsuseCzochralskiorotherp-dopedingotformationmethodswith adiffusedthinn-dopedlayertoformanasymmetricjunction.Multicrystallinemc-Si ingotformationprocessesareusedaswelltoproduceslightlylowerefciencycellsat slightlylowercost.Thebestsiliconcellefciencyhasreached25% 77 seeFigure3-3, whilethebestcommercialmoduleefciencies 3 approach20%.Astheimprovementto efciencyhasslowedinthepasttwodecades,themaindevelopmentintheeldhas shiftedtoreducecosts.Thecombinationofthereductionofsiliconcostsbroughtabout bythemicroelectronicsindustryandmanycleverprocessimprovementsinthesolar industryhasreducedthemanufacturingcostofsilicon-basedsolarenergyproduced fromapproximately$100/Win1970to$1-3/Win2011. Inadditiontosc-Siandmc-Si,thereareanumberofthin-lmtechnologiesthat attempttodrivedownthecostofsolarbyavoidingtheuseofexpensivewaferfabrication processingandinsteaddepositthinlayersofsemiconductorontoasubstratesuchas glass.Amorphousthinlmsofsiliconareusedonglass,andthistechnologyhasthe advantageofsimplicityandplentifulmaterialsupply,thoughthematerialsuffersfrom lightdegradationandmoduleefciencieshavestruggledtosurpass6% 78 .Anumber 3 Amoduleisapackagedproductthatusesmultiplesolarcellstoproduceadesigned voltageandcurrentrating;duetoareducedllfactorofactivesolarcellareaespecially forcircularcellsandelectricalcharacteristicmismatchingbetweencells,modulepower conversionefcienciesaretypicallylowerbyafactorofapproximately10-20%relativeto thebestindividualcellefciency 67

PAGE 68

ofthin-lmcompoundsemiconductorplatformshavebeendevelopedforthispurpose, butthemostsuccessfulatthistimehavebeenCdTeandCdInGaSeCIGS.The developmentofpowerconversionefciencyoverthepastthreedecadesforavariety ofsolarcelltechnologiesiscatalogedinFigure3-3;forthemostrecentdata,consult MartinGreen'stables 11 Figure3-3.Thetime-dependentchampionsolarcellefcienciesforvarioussolarcell technologies,asofApril2010.CompiledbyLawrenceKazmerskiatthe NationalRenewableEnergyLaboratory,USA. FirstSolar,acompanythatmanufacturesCdTesolarcells,hasledthethin-lm marketbyreducingmanufacturingcostsbelowthelowestcostsiliconcellsseeFigure 3-4.Notethatcost,inthissense,takesintoaccountthemanufacturingcostthat 68

PAGE 69

producesoneWattofoutputfromtheendproduct,andthereforebenetsfrombothhigh powerconversionefciencyaswellaslowmanufacturingcosts. Figure3-4.Recentdeclineintheinstalledcostforsolarpoweraveragesellingprice ASPandcostmanufacturingcost,forcrystallinesiliconcellsproducedby YingliSolarandthinlmCdTethinlmcellsproducedbyFirstSolar.Source: DeutscheBank 79 Asthecostsofsolarenergyhavereducedandgovernmentshaveincentivized installations,theworldhasrecentlyexperiencedarapidexpansionoftheinstalledsolar energycapacityseeFigure3-5.Whilethesecostsareapproachinggridparity 4 ,the recentsurgeinsolarinstallationshasinlargepartbeendrivenbystatesponsored incentiveprogramssuchasthefeed-intariffsystemsinSpain,Germany,andItaly. Thereissomeconcernforthesustainabilityofthin-lmmanufacture,asmany oftheelementsusedinthemostefcientcellsCd,In,etc.areeithertoxicorrare 4 Asthecostsinvolvedinproducingsolarenergyhavereduced,`gridparity'has beenanticipatedasthepointatwhichthelevelizedcostofenergyLCOEforsolaris competitivewiththoseofothermainstreamsourcesofenergysuchascoal,naturalgas, andnuclear. 69

PAGE 70

Figure3-5.Worldwideinstalledphotovoltaiccellenergycapacitybyyear,1990-2010. Source:DeutscheBank 79 ,1990-2009;consensusestimate2010. commodities.Furthermore,whilethemanufactureofthin-lminorganicsolarcells useslessmaterialthanc-Siatlowercost,itstilltypicallymustusehighprocessing temperaturesandoftenvacuumbasedmethodssuchassputtering,whichallinate thecapitalcostofmanufacture.Workinorganicsolarcellsisdrivenbythepotentialto lowercostsevenfurtherbyusingmaterialsthatdonotrequiretheseprocesses.Organic materialsandpolymerscanbedepositedatlowtemperatureusingsolutionprocessing, andthereforepromisetodramaticallyreducethecostofthinlmsolarcells. 3.2OrganicPhotovoltaicCells OrganicphotovoltaicOPVtechnologyhascarriedremarkableinterestinthe researchcommunityoverthepast20years,asitholdsgreatpotentialforfutureenergy generationandatthesametimethetechnologyisripewithchallengestobothscientic understandingaswellaspracticalimplementation 80 .Itspromisecomesmainlyfrom processingadvantagesthatmayallowthetechnologytobedeployedatlowercostthan 70

PAGE 71

othersolartechnologies.Itspracticallimitationsoflowefciencyandlowlifetimeare activelybeingovercomewithimprovedfundamentalunderstandingofthechemistryand physicsoforganicsemiconductors.Asaconsequence,thepresentchampionorganic solarcellsprovidepowerconversionefciencyPCEexceeding8% 11 .Thissection discussesthematerialsanddevicestructuresusedinthesedevicesandthewayin whichdevicesareprocessed. 3.2.1SmallMolecules,Polymers,andProcessing OrganicphotovoltaiccellsOPVsarebroadlybrokendownintotwotypes:those thatuseorganicsmallmoleculesandaretypicallyfabricatedusingvacuumthermal evaporationandthosethatusepolymersandaretypicallyfabricatedusingsolution processingsuchasspincoating.Thefabricationprocessesusedaremoreorless requiredbythematerialtype,assmallmoleculesareveryeasilysublimedatrelatively lowtemperaturebutingeneralhavelowsolubilityinorganicsolvents,andpolymersdue totheirlargemolecularweightaredifculttosublimeandeasilydissolveinsolvents. Thesefabricationmethodsalsogiverisetotherelativeadvantagesanddisadvantages toeach,astheelectronicmaterialpropertiesaresimilarbetweenthetwomaterialtypes. Vacuumprocessingallowsthedepositionofcomplex,multi-layereddevicestructures whilesolutionprocessinginmanycasesforbidssuchstructures.Smallmolecule cellscanbemoreeasilyleveragedfordeviceandmaterialphysicsstudies,butsuffer fromslowerandmorecapital-intensivefabrication.Polymercellsareeasilyfabricated fromsolutionbutaredependentthechemistrythatpredeterminesthewayinwhich 71

PAGE 72

mixedpolymersselfassembleintoalm.Bothmaterialtypescurrentlyproducesimilar efciencychampionsolarcells,PCE=8%citeGreen2011. AsdiscussedinChapter1,oneoftheprincipaladvantagesoforganicsemiconductorsoverinorganicsemiconductorsistheirabilitytobeprocessedinsolutionand atlowtemperature.Whilemostsmallmoleculecellsareprocessedinvacuumusing thermalevaporation,polymersolarcellsaretypicallydepositedusingspincoatingfor smalldeviceareas.Vacuumthermalevaporationisalsotypicallyusedforthedeposition ofthecathodeAgorAlandsometimesusedtodepositanodeandcathodeinterlayers suchasLiFandMoO 3 .Sputteringunderhighvacuum,meanwhile,istypicallyused toproducethetransparentconductingoxidetypicallyITO.Ultimately,toleverage theadvantageoforganicsolarcells,theseprocessingmethodsmustbeviablein alargescalemanufacturingcapacity,mostadvantageouslyusingrolltorollR2R processingofcontinuoussubstratesdrawnthroughsequentialprocessingstepsand prohibitingbatch-typeprocessingsteps.Thevacuumprocessesarescalableinthisway, althoughthecapitalcostsarehighfortheenablingequipment.Spin-coating,however, isnotscalablebeyondbatchprocessing.Nevertheless,thereareahostofalternative solutiondepositiontechniquesthatare,asreviewedbyKrebs etal. 81 .Chapter7ofthis thesisexploresonesuchtechniqueforproductionoforganicsolarcellsusingspray processing.Inaddition,thereareresearcheffortsunderwaythatattempttoreplacethe materiallayers,mostnotablytheelectrodes,withsolutionprocessablematerialssuchas nanoparticlesandconductivepolymers.Ultimately,thegoalistoproduceorganicsolar cellsusingentirelysolutionprocessingsteps,andthishasbeenachievedinsmalland largescalethoughthecellsthusfarproducedsufferfromverylowPCE 82 72

PAGE 73

3.2.2DeviceStructureinOrganicSolarCells Section2.3discussedthemulti-stagephotocurrentgenerationprocessthatis characteristicoforganicsemiconductors.Variousdevicestructuresareutilizedin organicphotovoltaicstomaximizetheefciencyofthisprocess.Thechallengeis similarinorganicphotodetectors,howeverforthecaseofphotodetectionanexternal biasmaybeappliedtocoaxelectronsandholesfromthedevice,ensuringthatthey overcomesmallenergeticbarrierstoextraction.ForOPV,thesesmallenergeticbarriers cansignicantlydegradeextractionefciency.Therefore,devicestructureinorganic photovoltaicsisinlargepartdirectedatovercomingthediffusionbottlenecktoabsorb thesolarspectrumefciently,whilestillallowingextractionofgeneratedphoto-electrons bythesmallbuiltineldofthedevice. Theoriginalinventionoftheorganicsolarcellusedaheterojunctionbetweena neatdonorandaneatacceptor 9 .Duetoshortexcitondiffusionlengthsororganic smallmolecules,donorlayerssuchasthephthalocyaninesaretypicallylimitedto 10nm,whilethelongerexcitondiffusionlengthoffullerenesallowstheuseof 40nmfullereneC 60 ,whichultimatelylimitstheefciencyoflightabsorptionand cellefciency.Thislimitationcanbeovercomebymixingthedonorandacceptorto formabulkheterojunctionaswasrstdoneusingpolymers 83 ,however,thisreduces thecarriermobility,reducingthellfactor,andalsoreducestherecticationenabled byahardheterojunction,reducingtheopencircuitvoltage.Athirdoptionmaybe employed,usingacombinationplanar-bulkheterojunctionthatincludestwohard heterojunctioninterfaces,onewiththeacceptorandonewiththedonor,bothwiththe 73

PAGE 74

centralmixedheterojunctionlayer 84 .Ultimately,theidealdonor-acceptorheterojunction hasamorphologywithphaseperiodicityonthelengthscaleoftheexcitondiffusion length,whileatthesametimeprovidingpercolatedpathsfordissociatedelectrons andholesthroughtheacceptoranddonortothecathodeandanode,respectively.We refertothisastheheterojunctionextractionchallenge,andthisidealizedstructureis representedschematicallyingure3-6alongwiththeplanar,bulk,andplanar-bulk types. Figure3-6.Cross-sectionschematicsforthedonor-acceptorheterojunctiontypesused fororganicphotovoltaics,includingaplanar,bbulk,cplanar-bulk,and didealplanar-columnar-bulk,showingdonororangeandacceptorblack sandwichedbetweenananodeandcathode. Anotherwaytoovercometheexcitondiffusionlimitationistousethemany inorganicmaterialstructuresthathavebeendemonstratedtoexhibitfeaturesizeon theorderof10nm,suchasnanowires,nanorods,orsimplyuselithographictechniques todirectlytemplate.Dye-sensitizedsolarcellsDSSC 5 haveusedavarietyofthese techniques,asthedevicestructureisbasedontheuseofinorganicoxidesaselectronacceptors,yetthehighestperformingstructureusesacolloidaltitaniumoxide 85 5 Dye-sensitizedsolarcellssuspendlight-sensitizingorganicdyemoleculesinaholetransportingelectrolytewhichisdispersedinahigh-surfacearea,electron-accepting inorganicoxide. 74

PAGE 75

Hybridorganiccellsuseaninorganicacceptoralongwithasolidstatepolymerorsmall molecule.Forthesedevices,nanoassemblyofnanobersandnanorodscanmimic theidealinterpenetratingD-Aheterojunction 86,87 ,howeversimilartoDSSCsthebest devicesuseainterpenetratingbulkmixture 88 .Therearepotentialadvantagestousinga hybridstructureoverthemoretypicalpolymer-fullerenemixture,asinorganicmaterials suchasnanoparticlescanbemadetoabsorbdeeperintotheinfraredspectrumand alsomaybemoreenvironmentallyrobust.Hybridcellefciencieshaveremainedlower thanfullyorganiccells,thoughrecentlyZhouetal.havereachednearly5%PCEusing CdSenanorods 89 Polymersolarcellsmostlyarelimitedtothebulkheterojunctiontypeasmulti-layer structurerequiresdifferentmaterialstobedissolvedintoorthogonalsolvents,andin generalthisisdifculttoachieve.Therefore,thesuccessofimplementingpolymers inlargepartisduetotheself-assemblingnatureoftheinterpenetratingphasesof donorandacceptor.Thepolymerbulkheterojunctionwasrstinventedusingthe systemMDMO-PPV:PCBM 83 ,andsincethenhasbeendevelopedusingavariety ofpolymerspairedwithPCBMofvaryingbuckyballsize.Ithasbeenfoundthatthe structureofthepolymer,especiallythearrangementofsidegroupsalongthepolymeric chain,affectstheintercalationofPCBMunits,insomecasesproducingabimolecular crystalinwhichthepolymerandPCBMproduceanorderedbulkheterojunction 90 ThisdoesnotoccurforthehighlyefcientP3HT:PCBMsystem,inwhicheach phasecrystallizesindependently,andithasbeenfoundthatthechoiceofsolvent stronglyaffectstheultimatephasesegragationandphasesize 91 .Furthermore,inthis systemithasbeenfoundthatthedegreeofregioregularitywithintheP3HTpolymer 75

PAGE 76

stronglyeffectsthecrystallizationprocessandtheultimatecellefciency,withhigher regioregularitypromotingimprovedcellefciency 92 .Therefore,itisevidentthatthere areanumberofphysicalpropertiesaswellasprocessingconditionsthataffectthesolar cellperformance,andthesearestronglytiedtotheheterojunctionextractionchallenge. Whilemostoftheefcientorganicsolarcellmaterialstothisdateonlyabsorbout to l =700nm,thesolarspectrumasdiscussedinSection3.1.1providesenergy wellintotheinfraredspectrum.Therefore,manyadditionalmaterialshavebeentested foruseinorganicsolarcellsthatabsorbmorestronglyintothenearinfrared,including leadphthalocyanine 93 ,tinphthalocyanine 94,95 ,chloroaluminumpthlalocyanine 96 andothersamongthesmallmoleculesandalsoPCPDTBT 97 andothersamongthe polymers.Inaddition,oneoftheperceivedadvantagesofhybridsolarcellsistheability toincorporatelowbandgapinorganicnanomaterialssuchasPbSinordertoabsorb furtherintotheinfraredspectrum. Anti-reectivecoatingsareanimportantimplementationforallsolarcells,including bothinorganicandorganic.Fororganicthinlmcellsthatdonototherwiseabsorb allthelightcoupledintothedevice,thereareadditionalpurelyopticalstructuresthat canenhanceabsorptionbyredirectinglightoffofnormalincidence.Theseinclude forinstancemicrolenses 98 ,pyramidalstructures 99 ,andv-shapedsubstrates 100 .In addition,Chapter6exploresanewmethodthatredirectslightbyabsorbingandreemittingusingauorescentabsorber. Theopencircuitvoltageoforganicsolarcellscanalsobeimprovedbyaltering devicestructure,asitisacomplexfunctionoftheelectrodeFermienergies,thedonor andacceptortransportenergylevels,andotherfactors 101,102 .Thevoltageofasingle 76

PAGE 77

organiccellcanbeenhancedusingoneoftwogeneralapproaches:areducedark currentunderforwardbiasandboptimizethedonor-acceptortransportenergies.The darkcurrentcanbereducedbyforinstanceusinginterlayerssuchasMoO 3 betweenthe donorandtheanode 103105 oradopedPINdonor-acceptorstructure 106 .Manyorganic materialshavebeenexploredforoptimaltransportenergiesthatdecreasetheloss ofexcitonenergybydissociation 102 .Furthermore,materialshavebeensynthetically alteredbyforinstanceplacinglanthanidenitrideswithinthecageofabuckyballto reducetheacceptorLUMOenergyandincreaseopencircuitvoltage 107 .Finally,higher voltageandmorecompleteabsorptionofthesolarspectrumcanbeachievedbyusing atandemstructureinwhichtwocellsareconnectedinseriesandplacedontopof oneanother.Theefciencyoftandemincreaseswiththeorthogonalityofthespectral responseofeachofthesubcells 108,109 3.3OperationandCharacterizationofSolarCells Section2.2introducedtheoperationandcharacterizationofphotodiodesusedfor detectionapplications.Whilethefundamentalprinciplesarethesameforphotodiodes usedinphotovoltaics,thesedevicesareoperateddifferentlyandassuchdemand adifferentsuiteofcharacterizationmethods.First,wemusttakeintoaccountthe responseofasolarcelltothesolarspectrum,andsecondweareinterestedinthe operationofphotovoltaicdevicesthatpoweranexternalload. Aswecharacterizethesedevicesitishelpfultoreferencetheequivalentcircuit ofasolarcell,showninFigure3-7.Thiscircuitisthesameasthatforageneralized photodiodeasshowninFigure2-2exceptthatitomitsthenoisecurrentsource 77

PAGE 78

andthejunctioncapacitance,asinthecaseofasolarcellthenoisecurrentisnot asignicantcomponentduetothehighintensityofthesolarsignalandwearenot concernedwiththejunctioncapacitanceasasolarcellacceptsacontinuoussignaland temporalresponseisnotcritical. Figure3-7.Equivalentcircuitforasolarcell,showingaphotocurrentsource,diode, seriesandparallelparasiticresistances,andexternalload. Thissectioncoversrstthespectralconsiderationsofasimulatedsolarspectrum. Next,thecharacterizationofthedominatingequivalentcircuitcomponentsarecovered, includingathephotocurrentsource,bdiodebehavior,andctheparasiticshunt andseriesresistances.Inthenalsection,thedegradationoforganicsolarcells isdiscussedasitrelatestocharacterization,aswellasstabilizingencapsulation fabricationmethods. 3.3.1SolarSimulation Section3.1.1introducedsolarradiationandtheASTMsolarspectrumstandards.In ordertoproperlycharacterizeaphotovoltaiccelltopredictitsresponsetothisradiation whenemployedinvariableatmosphericconditions,itisnecessarytoconstructa morecontrolledenvironmentthatsimulatesthisspectralirradiance.Astandardwayto constructasolarsimulator,andthewayinwhichwedosointhiswork,istouseahighly 78

PAGE 79

stablexenonarclampsource.Thisproducesaspectrumsimilartoa5800Kblackbody source,withlinestructuresdistributedthroughoutwithaconcentrationoflinestructure intherangeof l =750nm.Tomimictheincidenceofsolarradiation,thebeam mustbehomogenizedandcollimatedtoensureuniformityacrossthebeam,andoptical ltersmayalsobeusedtosimulateatmosphericattenuation.Ultimately,inorderforthe simulatortoproduceaccuratedevicemeasurements,itmustoutputauniformbeamwith littlevariationoverthedevicearea.Figure3-8plotstheintensityofthesimulatorusedin thiswork,measuredthrougha0.5mmpinholeaperturesteppedacrossthebeam. Figure3-8.Uniformityofsolarsimulatorbeamatthepositionoftestdevice,as measuredthroughasecondaryapertureof0.5mmdiameter.Thebeamis passedthroughaprimaryapertureofdiameter3mmjustbeforedevice incidence,andsotheintensityfallsoffrapidlyattheedges. Anotherconsiderationforaccuratemeasurementisthestabilityofthesimulator beamwithtime.Xenonarclampsarechosenfortheirrelativestability,butdepending onthepowerelectronicstheoutputmayvaryonshorttimescales.Figure3-9plots theresponseofanorganicsolarcellC 60 :CuPcsampledovertimewithashort currentmeasurementintegrationtime.UsingahighqualityDCpowersupplyand 79

PAGE 80

xenonarclamp,theoutputcanstillvaryupto5%onmstimescales;inthisworka measurementintegrationtimeof100msisusedtoaveragetheseuctuations.Note thatthedegradationoftheaverageresponseintimeisnotduetothesolarsimulator, butrathertotheshortlifetimeoftheorganicsolarcell.Thiswillbediscussedfurtherin Section3.3.5. Figure3-9.Variationoftheoutputofasolarsimulator.Intensityofthebeamassampled usinganorganicsolarcellC 60 :CuPcmeasuredwith1msintegrationtime overthecourseof10minutes. Regardlessoftheparticularsetup,noarticialsolarsimulatorcanprecisely reproducethetruesolarspectrum.Therefore,thespectralmismatchfactor, M ,must becalculatedusingacalibratedreferencesolarcellandusedtocorrecttheresulting measurementerroroftestsolarcells.Themismatchfactorisexpressedas: M = R l 2 l 1 E ref l S R l d l R l 2 l 1 E ref l S T l d l R l 2 l 1 E S l S T l d l R l 2 l 1 E S l S R l d l where E ref l and E S l arethespectralirradianceforthereferenceAM1.5spectrum andthesimulatedspectrum,respectively,and S R l and S T l arethespectral responsivityforthereferencesolarcellandtestsolarcell,respectively.Bymeasuring 80

PAGE 81

thespectralresponsivityofthetestsolarcell,oneisabletointegratethatresponsivity withtheAM1.5solarspectrumtodeterminethetotalcurrentthatthedevicewould produceunderstandardtestconditionsSTC,butherebyusingareferencesolarcell weareabletodeneasimplenumericmismatchfactorthatcanbeusedtocorrect futuremeasurementsunderthesimulatedsolarspectrum. 3.3.2Current-VoltageCharacterization Althoughtheoperationofsolarcellsdoesnotuseanexternalbias,wecanuse appliedbiastosimulatetheoperationofsolarcellsconnectedinserieswithanexternal loadbyapplyingaforwardbias.Furthermore,byapplyingareversebias,wecan examinethedependenceofphotocurrentonbiasinordertodiagnosethesourceoflow efciencycells. Keepinginmindthecomponentsintheequivalentcircuitforasolarcell,theoutput inthedarkfollowstheidealdiodeequationmodiedbytheseriesandshuntresistances, andwiththeadditionofthephotocurrentsource, J p J = J 0 exp q V )]TJ/F58 11.9552 Tf 10.95 0 Td [(JAR S nkT )]TJ/F20 11.9552 Tf 10.949 0 Td [(1 + V )]TJ/F58 11.9552 Tf 10.95 0 Td [(JAR S R SH )]TJ/F58 11.9552 Tf 10.949 0 Td [(J p where A istheareaofthedevice.Typicalcurrentvoltagecharacteristicsforasolar cellinthedarkandunderilluminationareshowninFigure3-10.Tocharacterizethe efciencywithwhichthesolarcellconvertsopticalenergyintoelectricalenergy,we characterizethepowerconversionefciency, h p = P out P in = V OC I SC FF P opt 81

PAGE 82

wherewedenethellfactor,FF, FF = V max I max V OC I SC Figure3-10.Current-voltagecharacteristicsofasolarcellinthedarkandunder illumination. Theshortcircuitcurrent, I SC ,isthetotalphotocurrentgeneratedbythesolarcell at V =0,andisdependentontheabsorptionandphotocurrentextractionefciencies ofthedevice.Theopencircuitvoltage, V OC ,isthemaximumvoltagegeneratedbythe solarcell,andisdependentontheband-gapofthesemiconductors,theworkfunctions oftherectifyingelectrodes,thedarkcurrent,thetransportenergylevelsofthedonor andacceptormaterials,andotherfactors 101,102,110 .Thellfactorisalsodependenton ahostofdeviceproperties,butingeneralhigherllfactorsareachievedformaterials anddevicestructuresthatmoreefcientlytransportandextractphotogeneratedcarriers. Thellfactorisalsostronglydependentontheseriesandshuntresistance,andthese arediscussedinthefollowingsection. 82

PAGE 83

3.3.3EffectofParasiticResistances Anidealsolarcellexhibitshighconductivityunderforwardbias,veryhighresistance underreversebias,andverylittledependenceofthephotocurrentextractiononthe reversebias.Suchadevicewouldhaveallfactornearunitybutisdifculttoachieve inpractice.Theseriesresistance, R S ,reducesconductivityunderforwardbias,while theshuntresistance, R SH ,increasestheconductivityunderreversebias.Theeffectof thebuiltineld, V BI ,onextractinggeneratedelectronsandholesisreducedbyboth parasiticresistances,andthecumulativeeffectofthesethreeeffectsreducesllfactor anddeviceefciency. Seriesresistanceisanissueespeciallyforthinlmsolarcellsduetotheuseof transparentelectrodesthatsufferfromhighsheetresistance.Forinstance,highquality indiumtinoxideITOhasasheetresistanceof15 /square.Thus,thelowestseries resistancesuchadevicecanexhibit,withouttheuseofhigherconductivitybuslinesthat blockaportionoftheincidentlight,is R S =20 .Thisdoesnotstronglyaffecttheoutput fromsmallresearchsizeddeviceswithareaontheorderof0.1cm 2 ,asthemaximum photocurrentsuchadevicewilloutputisapproximately0.2mA,correspondingtoa voltage0.05V.However,thevoltagedroppedscaleswiththearea,andsoadevicejust 1cm 2 insizewilllose0.5V.Ifforsuchadevicethe V OC =0.5V,thecurrentoutputwill looklikeasimpleresistoracrossthefourthquadrant,withastraightlineconnecting I SC to V OC FF=0.25.Moregenerally,asFigure3-11depicts,anincreaseinseries resistancepivotsthecurrent-voltagetraceaboutthevoltageintercept.Theseries 83

PAGE 84

resistancesupressesPCEmorestronglywithhigheropticalintensity,asthevoltage droppedacrossitincreaseswiththephotocurrentgenerated. Figure3-11.Effectofparasiticresistanceonsolarcelloperation.Thesolidtracedepicts anidealizedsolarcelloutputwithlowseriesresistanceandhighshunt resistance.Ontheleft,thedashedtracedepictstheoutputfromasimilar solarcellwithreducedshuntresistance,andontherightthedashedtrace depictstheoutputfromasimilarsolarcellwithincreasedseriesresistance. Lowshuntresistancetypicallyresultsfrompoormaterialorlmquality,with defectsthatallowpartialshortingofcurrentfromanodetocathode.Thisbecomes veryimportantforlargeareadevices,asanysolarcellisaninnitesumoftheinnitely smallsolarcellscontainedwithinconnectedinparallel.Ifjustasmallportionofthe solarcellsuffersfrompoormaterialorlmquality,theshuntresistanceoftheentire solarcellwillbereduced.AsFigure3-11shows,reducingshuntresistancepivotsthe current-voltagetraceabouttheintersectionwiththecurrentaxis,andalsoreduces theopencircuitvoltage.LowshuntresistancemorestronglysuppressesPCEatlow powerintensities,asthevoltagesupplyingcurrentthroughtheshuntincreaseswiththe logarithmoftheintensity,therebyincreasingtheshuntedcurrentlogarithmically,while thephotocurrentincreaseslinearlyandmorerapidlywiththeintensity. 84

PAGE 85

3.3.4SpectralResponse Thespectralresponseofsolarcellsrelativetotheincidentsolarspectrum determinesthetotalphotocurrentthedevicecansupply.AscoveredinSection3.3.1, thespectralmismatchfactorreliesonknowledgeofthespectralresponsivityofthetest solarcell.Thedeterminationoftheresponsivityaswellasthequantumefciencyis doneusingthesamemethodsappliedtophotodetectors,ascoveredinSection2.2.2. However,becausetheresponseofsolarcellsisdependentontheintensityofirradiation, alightbiaswithcontinuousnon-modulatedintensitynearthetotalintensitysuppliedby theAM1.5spectrummustbeappliedtothedevicewhilemeasuringtheresponsetoa weakmonochromaticlightsource.Thislight-biasmethodisdescribedinASTMstandard E1021. Thecharacterizationofspectralresponseisespeciallyimportantfordonor-acceptor organicsolarcellsasitprovidesawaytoevaluatetheindependentcontributionsto photocurrentofthetwodifferentsemiconductingmaterials,inadditiontoanyltering effectscreatedbyothermaterialsusedinthepackageddevice.Furthermore,the externalquantumefciencyspectrumcanbeusedtocrosscheckthesolarsimulator calibration.Theshortcircuitcurrentunderstandardtestconditionscanbeestimatedby integratingthetestcellresponsivitywiththeAM1.5spectrum: J SC calc = Z l 2 l 1 E ref l S T l d l However,thiscalculationreliesonthequalityofthelightbiasandthedegreeof intensity-dependenceofthetestcellfornon-ideallightbias. 85

PAGE 86

3.3.5DegradationofOrganicSolarCells Whileallsolarcellsexhibitsomedegreeofenvironmentalandoperational degradation,organicsolarcellsinparticularareverysensitivetoenvironmentaloxygen andwatervapor.Mostapplicationsofsolarcellsrequirelongtermoperationstability, andthereforethispresentsadisadvantagetotheuseoforganiccells 111 .Nevertheless, devicescanbeencapsulatedfromtheelementstoextendlifetime,andworkisongoing toelucidatethefundamentalmechanismsofdegradationinordertodesignmaterials thatbetterwithstandoperationunderenvironmentalconditions. Figure3-12.OrganicsolarcellswithdevicestructureITO/CuPc20nm/C 60 50nm/BCP 8nm/Al100nmencapsulatedbyCytopuoropolymer.JVcharacteristics leftandEQErightshowingconsistentperformancebetweenvarious devicesmadeinthesamebatchmeasured20hourspostfabrication. Performanceascharacterizedimmediatelyfollowingfabricationwassimilar withinmeasurementerror.. Figure3-9showsthereal-timedegradationofthecurrentoutputofanunencapsulatedCuPc:C 60 solarcell,showingadegradationofnearly5%overthecourse of10minutes.Thesecellsareparticularlypronetorapiddegradation,aselectron trapsforminC 60 duetoadsorbedwatermolecules.Here,weshowthattheuse 86

PAGE 87

ofahighlyhydrophobicencapsulationlayeroftheperuorinatedpolymerCytop, previouslydemonstratedeffectiveforencapsulatingOLEDdevices 112 ,drastically improvestheoperationallifetimeofsuchcells.TheJ-Vcharacteristicsandspectral EQEareshownforsimilarcellsencapsulatedbyCytopinFigure3-12.Measuredone dayafterencapsulation,thesecellsshownomeasurabledegradationcomparedto measurementstakenimmediatelyafterfabrication.Theseplotsalsocomparethevalue ofshortcircuitcurrentpredictedbyintegrationofEQEwiththatmeasuredbythesolar simulator.Theagreementbetweenthevaluesdemonstratesthepropercalibrationofthe solarsimulator. Insummary,whilenewresearchworkseekstodesignorganicsemiconductor materialsthataremoreenvironmentallystable,theuseofencapsulationisapractical andcommerciallyviabletechniquethatcanproduceusefuldevicelifetime.Therefore, themainfocusoftheorganicsolarcellresearcheldcontinuestobeonimproving powerconversionefciency,andincreasinglyonwaystoproducethesecellsinamore cost-effectivemannerforcommercialization.Followingthesemotivations,Chapters6 and7exploreanewdevicestructuretoenhancesolarcellabsorptionefciencyandthe useofspraydepositionasalow-costfabricationoption. 87

PAGE 88

CHAPTER4 ORGANICFIELDEFFECTPHOTOTRANSISTORS 4.1Introduction 4.1.1Motivation Chapter2hasoutlinedmanyofthemotivationsthatdrivethedevelopmentand useoforganicphotodiodesasphotodetectors.Madeinlargeareaonconformable substrates,thesedevicesmayenablenewfunctionality,andatthesametimepromiseto simplifyandreducethecostofphotodetection.Chapter5willpresentanewphotodiode structurededicatedtothisend;butrst,inthischapterweexploretheuseoftheorganic phototransistorstructure.Withbuilt-inphotomultiplication,organicphototransistorsmay enabletheultimateincircuitrysimplication,astheymaybeusedtonotonlyreduce readoutelectronicsbutalsotoreplacethetransistorreadoutmatrixofdetectorpixel arrays. Inorganicphototransistorsaretraditionallyusedtobuildelectronicgatesswitchable bythepresenceorabsenceofambientlight.Theyarenotgenerallyusedtoform imagearraysasarephotodiodes,however,organicphototransistorsmaybeusedin thisfashionduetotheconsiderableeasewithwhichsucharrayscanbefabricated. Nevertheless,therealsoexistdisadvantagestotheuseofphototransistorsinsucha way:thespeedofresponseisgenerallyslower,andtheygenerallyhavepoorlinearity, astheapplicationoflightmimicstheexponentialcurrentmodulationinducedbychanges tothegatevoltage. Thischapterrstprovidesabriefbackgroundongeneralorganictransistor technology,asthedevicestructuresfororganicphototransistorstypicallyusethose 88

PAGE 89

previouslydevelopedfortransistors.Next,wereviewpriorworkintheliterature relatedtoorganicphototransistors.Wethenoutlinetheexperimentaltechniquesfor thefabricationofpentacenephototransistorsusedinthiswork.Finally,wediscuss theperformanceofthesephototransistors,bothincomparisontopriorworkandwith respecttopracticalimplementationasphotodetectors. 4.1.2OrganicFieldEffectTransistors Organiceld-effecttransistorsOFETsusingorganicsmallmoleculesand polymershavebeenwidelystudiedanddevelopedoverthepasttwodecades.During thattime,manychallengesandlimitationstothetechnologyhavebeenovercome, includinglowmobility,largehysteresis,highvoltageoperation,andp-typeonly operation.Thecurrentstateofthetechnologyiscommercializedandcompetitive withinorganictechnologiesforcertainlowswitchingspeedapplications.Themost notableapplicationistheuseoflargeareaactiveorpassivematrixtransistorarrays forthereadoutofdetectorpixelarraysortodrivelightemittingpixelarrays,forwhich hydrogenatedamorphoussilicon a -Si:Htransistorshavetraditionallybeenused. EarlyresearchusedOFETdevicestructureconsistingofathinlmorganic semiconductordepositedonaninorganicdielectricsuchassiliconoxide.Byoptimizing thepurityandthemorphologyofthethinlmpentacenesemiconductorlayer,device eld-effectmobilityupto1cm 2 V )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 andON/OFFratioupto10 8 wasdemonstrated 34,113,114 .Thehighestmobilityfororganicsemiconductorshasbeenachieved usingsinglecrystalsofrubrene m > 10cm 2 V )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 115 ,thoughsinglecrystalline organicmaterialsmaybemoredifculttoimplementinlargearea 116,117 89

PAGE 90

Despitehighmobility,theoperationofOFETsusinginorganicdielectricssuffers fromhysteresisandmemoryeffectsandalsolimitsthepotentialforlowcostmanufacture.Hysteresisandmemoryeffectshavebeenattributedtoelectrontrappingatthe semiconductor-dielectricinterface 118,119 .PolymerssuchabenzocyclobuteneBCBand Cytopaswellasself-assemblingmonolayerssuchasoctadecyltrichlorosilaneODTS andhexamethyldisilazaneHMDShavebeenusedeitherinplaceof,orasasurface treatmentforinorganicdielectricstoovercomethisproblem 34,120,121 .Thesematerials canreduceelectrontrapping,improvesemiconductormorphology,increasemobility, andenablen-typetransistoroperation 122,123 .Infact,theseeffortshaveshownthat manyorganicsemiconductorscanbeoperatedinn-typeaswellasp-typepolarityand canexhibitairstability.Forinstance,fullereneC 60 transistorshavebeendemonstrated witheldeffectmobilityofupto5cm 2 V )]TJ/F20 8.9664 Tf 6.966 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.966 0 Td [(1 usingBCBasthedielectric,whilethose usingaSiO 2 dielectricdonotproduceworkingtransistorsduetostrongelectron trapping. 124,125 Theuseofhighqualityultra-thinSAMsasthesoledielectriclayerhas alsobeenusedtoenableverylowvoltageoperation 126 ThecumulativeeffectofthesebreakthroughsinOFETtechnologyhaveenabled successfulimplementationincomplexcircuits.Forinstance,ambipolarityincomplementarytransistorshasenabledinvertersoperatinginkHzfrequencies 127 .More complexapplicationsarenowpossible,includingforinstanceexibleorganicmemory transistorarrays 128 andlargeareaultrasonicsensorswithOFETreadoutarrays 129 .The technologyholdsgreatpromisefortheseandmanymoreapplications,andisnowbeing commercializedbyseveralcompaniesintheprintedelectronicsindustry. 90

PAGE 91

4.1.3OrganicPhototransistorGain Themotivationforusingaphotodetectorwithbuilt-ingainisultimatelytodetect aweakopticalsignalandprovideanampliedelectricalsignalusingasingledevice. Standardtransistorsoutputcurrentgaininthesensethatasmallsignalappliedto thegateofthetransistorswitchesthedeviceintotheonstate,andthecurrentthat passesfromsourcetodraininresponsetootherelementsinthecircuitcanbefar greaterthanthegate-modulatingsignal.Ifthethresholdvoltageofatransistorcan bemadehighlysensitivetoabsorptionofweakopticalsignals,thenunderaconstant gatebiasthesourcetodraincurrentcanbemodulatedbytheopticalsignalandsucha phototransistorcanoutputalargephoton-to-electrongain. Organicsemiconductorsareespeciallywellsuitedforthistypeofdevice,asthey intrinsicallyexhibitlowcarrierconcentrationandlowconductivity,andatthesame timestronglyabsorblight;thus,theapplicationoflightandgenerationofphoto-excited carriersexertsalargemodulationofcarrierconcentrationandpotentiallyphototransistor gain.Thisphenomenonhasinfactbeendemonstratedinorganicphototransistors;the nextsectionexploresthesepriorworks. 4.1.4OrganicPhototransistorLiteratureReview Theliteraturehasmanyexamplesshowingpromisingperformanceoforganic phototransistorsusingavarietyoforganicsemiconductorsandOFETstructures. Thesedevicesusethesamegeneralstructuresandtechniquesdevelopedforstandard OFETs,ascoveredinSection4.1.2.Themaindifferenceissimplyincharacterization, asthetypicalbottomgate,top-contactOFETgeometryexposesthechannelregionto 91

PAGE 92

incidentlight.Table4-1providesarepresentativesummaryoforganicphototransistor performancepreviouslycharacterized.Alloftheseworkscharacterizedthephotocurrent insomewayandreportedoneormoreofthreemetrics:theresponsivity,thequantum efciency,andthephotosensitivity.ThersttwoaredenedinSection2.2.2,andthe photosensitivityissimplytheON/OFFratioforcurrentoutputinthelightcomparedto thatinthedark: P = I illum d I dark d AsevidencedbytheresponsivityandphotosensitivityvaluesreferencedinTable 4-1,organicphototransistorsappearverypromisingforuseaslightsensors.However, resultshavevariedgreatlypartlyduetodifferencesincharacterizationmethods. Specically,thereisalargediscrepancybetweenthepervasivelyhighvaluesfor responsivityandthelesserreportedlowervaluesforquantumefciency.Thework presentedherefocusesontheaccuratedeterminationofphotocurrentinpentacene OFETsinanefforttoaddressthesediscrepancies. 4.2Experiment 4.2.1DeviceFabrication Organicphototransistorswerefabricatedonp ++ siliconwafers r =0.01 cm with200nmthickthermallygrownsurfaceoxidelayers.Thesesubstratesservedas theglobalgateandgatedielectricforthedevices.Inordertocontactthegate,the oxideontheundersideofthesiliconwaferwasetchedwith10%HFfor10minutes. Substrateswerethensonicatedinsoapanddeionizedwaterfor10minutes,physically scrubbedwithfoamswabsandDIwater,sonicatedinDIwaterthenisopropanoleachfor 92

PAGE 93

Table4-1.Literaturesummaryofreportedorganicphototransistor performance.Mostworkhasusedamorphousorpolycrystalline neatorblendedorganicsemiconductorsineldeffecttransistor geometry;exceptionstothisstructurearenoted.Photoresponse metricssitedaremaximumvaluesreported. SemiconductorDielectricOptical bandwidth Optical intensity mW = cm 2 R A/W Photosensitivity EQERef. Rubrene a Air405nm1.2mW-3 x 10 3 -130 F 16 CuPc a SiO 2 Broadband4.46-10 2 5 -131 SwaOTAD-C 60 SiO 2 370nm0.00140.3--132 P3HTdopedSiO 2 Spectral2.25---133 PentaceneTa 2 O 5 Broadband103.54 x 10 3 -134 PentacenePMMABroadband10-2 x 10 0 -134 MDMOPPV:PCBM BCBBroadband1-1000.1510 1 2 -135 MDMOPPV:PCBM PVABroadband1510 2 3 -135 DT-TTFSiO 2 Broadband2500-10 4 -136 DPASPSiO 2 Broadband1000.20310 2 -137 Spiro-DPSPSiO 2 Broadband0.1270.442 x 10 3 -138 PentaceneSiO 2 540nm0.1-10 1 -139 TetraceneSiO 2 540nm0.1-3 x 10 3 -139 PentaceneSiO 2 365nm510-501.3 x 10 5 0.30%140 CuPcSiO 2 365nm1.550.5-23 x 10 3 0.10%140 BPTTSiO 2 380nm1.55822 x 10 5 141 F8T2BCB/SiNSpectral0.0002510 2 75104%142 Spiro-DPSPSiO 2 370nm0.112715 x 10 2 -143 Td-PTC/TPDnone b 465nm1.8cd--2.90144 a singlecrystallinematerials b diodestructure 10minutes,andblowndrywithnitrogen.Tonishsubstratecleaning,substrateswere boiledintrichloroethylenefor15minutesandsonicatedinacetonethenisopropanolfor 15minuteseach,andagainblowndry. 93

PAGE 94

Inaeld-effecttransistor,thesemiconductor-dielectricinterfacecanintroduce defectsthattrapcarriersandreducemobility.Toreducetheseeffects,theSiO 2 surface wastreatedwithoctadecyltrichlorosilaneODTS.Thistreatmentreducesdangling OH-groupsattheoxidesurfaceandpromotesmoreoptimalpentacenepolycrystalline growthatoptheextendedhydrocarbontails 34 .Substratesweretreatedin15mMODTS intolueneheldat60 Cimmediatelyaftera15minuteUV/ozonetreatmentusedto maximizemonolayerdensity.Aftertreatment,substrateswererinsedsuccessivelyin hexane,acetone,andisopropanolfor3minuteseach,blowndry,andannealedinairfor 30minutesat110 C.Finally,substratepreparationwascompletedbyapplyingalayerof silverpaintormeltedgalliumtotheundersideofthewafertoimproveelectricalcontact atthegate. Figure4-1.Devicestructureusedtoproducepentaceneeldeffecttransistorsnotto scale.Topgoldcontactsareusedonathinpentacenelayergrownontopof aselfassembledmonolayerofODTSonsiliconoxidedielectric,withap ++ Si -Gagatecontact. Aftersubstratepreparation,pentacenewasthermallyevaporatedundervacuum at5x10 )]TJ/F20 8.9664 Tf 6.967 0 Td [(7 Torrontoaheatedsubstrate Ctoformthesemiconductinglayer. Finally,toformtop-contactdrainandsourceelectrodes,goldwasevaporatedontopof 94

PAGE 95

pentacenethroughashadowmaskdeningachannellengthandwidthof0.075and2.0 mm,respectively. 4.2.2TransistorCharacterization Todeterminetransistorpropertiesinthedark,includingmobility,ON/OFFratio, thresholdvoltage,andsub-thresholdslope,deviceswerecharacterizedusinganAgilent 4155Csemiconductorparameteranalyzer.BothoutputcharacteristicsconstantV g variableV ds andtransfercharacteristicsvariableV g ,constantV ds weremeasured. Fieldeffectmobilitywascalculatedinthesaturatedregimefromthefollowingequation forthesaturatedcurrent: I sat ds = WC i 2 L m V g )]TJ/F58 11.9552 Tf 10.95 0 Td [(V t 2 Fromthisequation,themobilityfollowsthesquarerootofdrain-sourcecurrent, m = 2 L WC i d p I ds d V g 2 Toexperimentallyndthesquaredtermontherightside,theslopeofalineartof thesquarerootofdraincurrentplottedversusgatevoltagewasused.Mobilitywas calculatedinthelinearregimeaswell,bythefollowingequation: I lin ds = WC i L m V g )]TJ/F58 11.9552 Tf 10.949 0 Td [(V t )]TJ/F58 11.9552 Tf 12.145 8.094 Td [(V d 2 V d wherethesecondtermwasfoundbyalinearttothelowV d regionoftheoutputcurve andV t wasfoundbytheintersectionofalinearttoalowV d transferscanwiththeV g axis.Thresholdvoltage,V t ,isanimportantquantityinthecharacterizationofOFETs. ThestandardwaytomeasureV t isbylinearextrapolationofaplotofI d versusV gs at lowV ds toensurethatthedeviceisoperatinginthelinearregime.Theextrapolated 95

PAGE 96

intercept,V g0 ,ontheV g axiswhereI d =0isusedtocalculateV t bythefollowing equation: V t = V g 0 )]TJ/F58 11.9552 Tf 12.145 8.093 Td [(V ds 2 TwomoreimportanttransistorguresofmeritaretheON/OFFcurrentratioandthe sub-thresholdslope.TheON/OFFratioistheratiooftheoutputcurrentintheONstate totheoutputcurrentintheOFFstate.Thesubthresholdslope,S,describestherate atwhichatransistorchangesitsstatefromOFFtoON,andismeasuredinvoltsper decadeofchangeinI ds currentinthesubthresholdregime. 4.2.3PhototransistorPhotoresponseCharacterization Ideally,phototransistorphotoresponsecanbecharacterizedinasimilarmanner asthemethodsdescribedinSection2.2.2foratwo-terminalphotodiode.However,the three-terminalnatureandgeometryofthinlmtransistorscanmakesuchcharacterizationmorepracticallydifcult,andasaresultthephotoresponseofphototransistors isoftendoneusingastaticmeasurementtechnique.Thistechniquegenerally repeatsthesamemeasurementofoutputandtransfercharacteristicsasdescribed inSection4.2.2underillumination.Usingthesemeasurements,wecandeneashiftin thresholdvoltage,deriveameasureofphotocurrentbysubtractingthetransistoroutput currentmeasuredinthedarkfromthatmeasuredunderillumination,andderivethe photosensitiviybyusingtheratiooftheoutputcurrentunderilluminationtothatinthe darkataspeciedgatebias. 96

PAGE 97

Thestaticmeasurementmethodassumesthatthedarkcurrentisconstantbetween extendedmeasurements.Asthismaynotnecessarilybetrue,amoretemporallyresolvedphotocurrentmeasurementismoredesirable.Weprobedthismorerapid phototransistorresponsebyusingasamplingdynamicphotocurrentmeasurement technique.Here,weilluminatedthedevicewithamechanically-choppedlightsource, heldthedeviceataconstantgatebiasandsource-drainbias,andsampledthetransient drain-sourcecurrentwithtime.Thismethodcanresolvethedecayofphotocurrent withtime,andwecanderivethephotocurrentbycalculatingthepeak-to-peakoutput current.Wealsomeasuredthespectralexternalquantumefciencyusingamodied lock-inamplierphotocurrentmeasurementsystem.Thetransistordevicewasmounted perpendiculartotheincidentbeam,andaprobesystemwasmountedperpendicularin ordertocontactthetopdrainandsourceelectrodes.Withaconstantappliedgatebias anddrain-sourcebias,thedrain-sourcecurrentwasfedthroughalock-inamplierand thephotocurrentandEQEweremeasuredasdiscussedinSection2.2.2. 4.3ResultsandDiscussion 4.3.1TransistorOperationinDark Thestandardoperationofatransistorismeasuredasthedrain-sourcecurrent undervariedgatevoltagetransfercharacteristicsandthedrain-sourcecurrent undervarieddrain-sourcevoltageoutputcharacteristics.Thesecharacteristicsfor exemplarydevicesareplottedinFigure4-2.Theperformanceofthesepentacene OFETsmatchesthebestdevicesreportedintheliteratureusingsimilarsemiconductordielectricmaterials 34 ,exhibitingeldeffectmobilityupto m p =0.15cm 2 V )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 s )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 ,a 97

PAGE 98

thresholdvoltagenearV=0,andON/OFFratioat V t 20Vof10 6 .However,againin linewithliteraturereports,thesedevicesexhibitdevicetodevicevariationwithreduced mobilityandON/OFFratio,andnotablywiththethresholdvoltagerangingfrom+10Vto -30V. Figure4-2.Typicaloutputleftandtransferrightcharacteristicsforpentaceneeld effecttransistorsusingthestructureshowninFigure4-1.Theoutput characteristicsshowseveralplotsmeasuredatdifferentgatebias,asshown, andthetransfercharacteristicsshowoneplotunder V ds =60. 4.3.2StaticPhotocurrentCharacterization AsdiscussedinSection4.2.3,thesimplestwaytomeasurephotosensitivityand photocurrentinphototransistorsisbythestaticmethod.Figure4-3plotsthetransfer characteristicsofapentaceneOFETmeasuredinthedarkandundera1mW/cm 2 monochromaticbeamcenteredat l =550nm.Themostnotablechangeintransistor outputunderapplicationoftheopticalsignalisalargeshiftinthethresholdvoltagefrom V t =0VtoV t =+30V. Ifwedenethephotocurrentasthedifferencebetweenthetwotransfercurves ateachappliedgatebiasandcalculatetheexternalquantumefciencybasedon 98

PAGE 99

Figure4-3.TransfercharacteristicsofapentaceneOFETinthedarkandunder illumination.Thesedataareusedinthestaticmethodologytodetermine phototransistorphotocurrent. thisphotocurrent,wearriveattheresultplottedinFigure4-4.ThemaximumEQE isfoundtooccurintheaccumulationregimeandreachesnearlyEQE=100atV g = -30V.However,withthisgatebiasthetransistoristurnedONinthedarkandsothe photosensitivityreachesonlyP=3.Inthedepletionregime,theEQEfallsoffrapidly,but at V G =10Vabalanceisstruckwithgain,EQE=10,andphotosensitivity,P=10 6 .This characterizationconcurswiththatofNohetal. 140 ,andifrepeatablemarksaremarkably functionalsignal-to-noiseratio. 4.3.3DynamicPhotocurrentCharacterization Figure4-5plotsthesampledtransistordrain-sourcecurrentusingthesampling dynamicphotocurrentmeasurementtechniqueasafunctionoftimeinresponseto 250 m W/cm 2 monochromaticilluminationcenteredat l =470nm,modulatedto4Hz byamechanicalchopper.Evenatsuchlowmodulationfrequency,thepeaktopeak photocurrentisontheorderofafewnanoamps,nearlytwoordersofmagnitudeless 99

PAGE 100

Figure4-4.PhototransistorEQEasdeterminedbystaticmethodology,using photocurrentdenedasthetotaloutputunderilluminationorangesquares minusthetotalcurrentinthedarkblacksquares. thanthatcalculatedbythestaticmethod.IfweusethisdatatodetermineEQEforthis device,wecalculateaquantumefciencyofEQE=0.4.Itisimportanttonotethatsuch acalculationisbasedontheverysmallareaofthetransistorchannelexposedtolight. Thesetransistorshaveachannellengthandwidthof75 m mand2mm,respectively, andthe250 m W/cm 2 circularbeamhasadiameterof1mm,thereforethetotalpower incidentonthechannelis75nW. Figure4-6plotsthemeasuredEQEusingthelock-indynamicphotocurrent measurementtechniqueforarepresentativepentacenephototransistorwithnear zerothresholdvoltagebothintheaccumulationregimeV g =-20Vandinthedepletion regimeV g =+20V.Thesemeasurementswerealsoperformedwitha4Hzmodulated spectrallyvariedmonochromaticopticalsignal.Thismeasurementisolatesthetransient photocurrentonthetimescaleofthemodulation,andagainweseethatthequantum efciencyiswellbelowthatmeasuredbythestaticmethod.Nevertheless,weseethat 100

PAGE 101

Figure4-5.Transientphototransistoroutputundermodulatedillumination,takeninthe depletionregimewithV g =20Vblacksquares,andalow-passFourier lteredplotofthesamedatatoaidtheeyeorangeline.Usingthedynamic photocurrentmethodology,thephotocurrentisextractedfromthepeakto peakcurrentoutput. thedeviceasmeasuredbythismoreaccuratemethodstillexhibitsasmallgainupto EQE=2. Figure4-6.Pentacenephototransistorquantumefciencyasmeasuredbythedynamic lock-intechnique,foradevicebiasedwithV ds intheaccumulationregime V g =-20V,blacksquaresandinthedepletionregimeV g =+20V,orange squares. 101

PAGE 102

4.3.4PhototransistorPhotoresponseandInstability Acharacteristicofpentacenetransistorsfabricatedonsiliconoxidedielectric, andtoalesserdegreeonODTS-treatedSiO 2 ,isstrongelectrontrappingatthe interfacebetweenthetwomaterials.Electrontrappingleadstotwoproblemsfor transistoroperation:hysteresisandbiasstressdegradationoftransistor output 145,146 .Deepelectrontrapsattheinterfacebetweentheorganicsemiconductor andthedielectricinduceabuilt-inelectriceld,drawingfreeholestotheinterfaceata concentrationabovethatatquasi-equilibrium.Thiseffectshiftsthethresholdvoltagein thepositivedirection,sincethechannelispopulatedbytrapstatesundersmallpositive appliedvoltage.UnderoperationintheONstatewithappliednegativegatevoltage,the adherenceofelectronstodeeptrapcentersdecayswithtime 147 .Thus,thethreshold voltageofthedevicediffersdependingontheinitialstateofthedeviceONorOFF,and thisresultsinoperationalhysteresis. Duetothegatevoltage-dependentresponseoftrappedelectrons,hysteresisoccurs whenthegatevoltageischangedintime.Ifthegatevoltageisheldconstantinthe accumulationregime,however,electrontrapsleadtothebiasstresseffect 146 .Initially, thesetrapsinducefreeholesandamoreconductivechannel;yet,withextendedtime thesetrappedelectronsrecombinewithfreeholesandtheeffectisreduced.Thiseffect ismostoftendescribedasachangeinthresholdvoltage,andcanbemodeledbythe followingequation: V t t = V g )]TJ/F58 11.9552 Tf 10.949 0 Td [(V init t 1 )]TJ/F58 11.9552 Tf 10.95 0 Td [(e )]TJ/F58 6.9738 Tf 8.699 3.532 Td [(t t b 102

PAGE 103

where t isthedecaytimeoftrappedchargestrapstatesassumedtobeexponentially distributedand b isatemperaturecorrectionterm 148 .Theexponentialdecayinduced bythereleaseofelectrontrapstatesalsomanifestsitselfinadecayofdrain-source current,andinfactwefoundthatI ds ofpentaceneOFETspresentedheredecaywitha timeconstant, t 40s. Weattributethelargedifferencebetweenthephotocurrentmeasuredbythestatic measurementandthatmeasuredbythedynamicmeasurementtothepresenceof concurrenthysteresisandbiasstresseffects.Thestrongelectrontrappingpresent inthesedevicesinuencesthetransistorcurrentoutputandthresholdvoltage,and thereforethevaluesderivedforphotocurrentbysubtractingstatictransfercurves measuredinthedarkfromthosemeasuredunderilluminationdoesnotisolatethe photocurrentandleadstofalsephotocurrentvalues.Usedasphotodetectingelements, thetemporallyresolvedphotocurrentistherelevantmetricfordeterminationofthe functionalityofsuchdevices.Infact,Debucquoyetal.foundthatthebiasstresseffect issimplyacceleratedunderilluminationinpentaceneOFETs.Figure4-7reprints theirndings,showingthattheuseofdielectricsthatlimitelectrontrappingslowsthe saturationofthresholdvoltageshiftundertheapplicationoflight 148 .Byapplyinga smallbias, V ds =0.1V,andexposingthedevicestoapproximately10mW/cm 2 l =660 nmmonochromaticlight,theyslowdownthissaturationprocessandshowtheshiftin thresholdvoltagefollowsthesametrendunderilluminationasinthedark.Asaresult, theuseofhydrophobicdielectricssuchaspoly a -methylstyreneP a MSnotonly reducesthebiasstresseffectbyreducingelectrontrapping,butalsolikewisereduces thephotoresponseasdenedbyashiftinthresholdvoltage. 103

PAGE 104

Figure4-7.Correlationofbiasstressinstabilitywithphototransistorphotoresponse.As thegatedielectricbecomesmorehydrophobicandlesslikelytotrap electrons,thebiasstresseffectisreducedandsimultaneouslytheshiftin thresholdvoltagecharacteristicofphototransistorphotoresponseis diminished.ReprintedwithpermissionfromRef. 148 .Copyright2007, AmericanInstituteofPhysics. 4.4ConclusionsandFutureWork Inthiswork,wetestedtheoperationofpentacenetransistorsunderilluminationand foundthatasothershavereportedilluminationstronglyshiftsthethresholdvoltage.A strongthresholdvoltageshiftgivesrisetophotosensitivityupto10 6 andgainupto100, asdenedbythestaticmeasurementmethod.However,wefoundbyusingdynamic measurementmethodsthatthisphotoresponseisnotatrueaccountofthedevice performance,andwecorrelatetheperceivedphotosensitivitywiththebiasstresseffect. Byusinghydrophobicgatedielectrics,thetrappingeffectcanbereducedcreatinga morerobusttransistoroperation,butthisalsodegradesthephotoresponse. 104

PAGE 105

Nevertheless,wedondthatsimplepentacenetransistorscanoutputatrue photocurrentgain,uptoEQE=2.Transistorchanneltransconductanceoffersan intrinsicmechanismforphotocurrentgain,andforfuturework,wemayexplorenew devicestructuresdesignedtoaugmentthisgain.Forinstance,thenextchapterpresents acarrierconnementphotodiodedevicestructurewhosemechanismmayalsobe implementedinorganicphototransistors.Multilayerheterojunctionswithsinglecarrier connementmaybeusedtoblocktheinjectionofholesorelectronsforp-typeorn-type transistors,respectivelyinordertosuppressthedarkONstatecurrentwhileallowing theirinjectionunderthepresenceoflight,leadingtoenhancedphotosensitivity. 105

PAGE 106

CHAPTER5 PHOTOMULTIPLICATIONBYCARRIERCONFINEMENTINORGANIC PHOTODIODES Chapter2describedthemotivationsfortheuseoforganicphotodetectors,and particularlyforthosephotodectorsthatexhibitbuilt-inphotocurrentgain.Chapter3 exploredtheuseoftheeldeffecttransistorstructuretothisend.Inthischapter,we exploreaneworganicphotodiodedevicestructuretoachievehighphotocurrentgain. 5.1OrganicPhotodiodeswithPhotocurrentGain AscoveredinSection2.5,photoconductivegainarisesindevicesthatexhibit unbalancedcarriertransport,suchthatminorityphotogeneratedcarriersareextracted orrecombineonalongertimescalethanthetransittimeofsecondarymajority photocurrent.Severalorganicphotodiodedevicestructureshaveachievedinternal gainthatarisesfromintrinsiccarrierimbalanceandphotoconductivityofthebulk material 6063 .Itisalsopossibletoengineercarrierimbalanceintoadeviceby introducingextrinsictrappingsites.ForinstanceYokoyamaandcolleaguesattribute gainsofupto10 5 tostructuralvoidsthattrapholesattheinterfacebetweenagold anodeandtheorganicsemiconductor 65,66,68 .Inaddition,severalresearchgroupshave producedphotocurrentgainbydispersinginorganicnanostructuresdesignedtotrap chargeswithinsolution-castpolymer-baseddevices 58,7072,149 Inallthesedemonstrationsofphotocurrentgaininorganicdevices,thephotocurrent gainisduetotrappingofminoritycarriers,whetherachievedintrinsicallythroughoutthe bulk,extrinsicallyatheterojunctionorelectrodeinterfaces,orextrinsicallythroughout thebulkintrappingcenters.Thetrappedminoritycarriersproduceaninternalelectric 106

PAGE 107

eldthatleadstoenhancedinjectionofmajoritycarriersfromtheelectrode.Inthis way,thesedevicesfollowthewell-knownphotoconductivegainmodel 26 ,whereinthe magnitudeofthegainisproportionaltotheratiooftheminoritycarrierlifetimetothe majoritycarriertransittime.Fordevicesofthistype,therenaturallyexistsatradeoff betweentheresponsemagnitudeandtheresponsetime,dependingonthelongevity oftrapstates.Forexample,Yokoyamaandcolleaguesachievedfasterresponsetime 20msbyusingabulkheterojunctionstructure 150 ,butthisreducedtheresponse magnitudegain 15.Tocontrolforthisinherenttradeoff,itisthereforeappropriate tousethegain-bandwidthproductGBPgure-of-merit,andfurthermoretospecifyat whatfrequencythemaximumGBPisachievedtoprovideapplicationrelevance. Duetothistradeoff,highgainandimaging-compatiblebandwidth > 60Hzhave forthemostpartremainedmutuallyexclusiveinorganicandhybridorganic-inorganic photodetectors.ThenewdevicestructurepresentedhereseekstoextendtheGBPand reducetheinuenceofthistradeoffbetweengainandbandwidth. 5.2CarrierConnement Thischapterdescribesanew,generalorganicphotodiodedevicearchitecture thatemploysheterojunctionsformedbyavacuum-depositedmultilayerstackof smallmolecularweightorganicmaterials.Thestructureleveragestheconceptof carrierconnementbymarryingblockinglayers,usedtoachievebothfastandstrong secondaryphotocurrentgain,withabulkheterojunctionofdonorandacceptorto produceastrongprimaryphotocurrent. 107

PAGE 108

Ourphotodiodedevicedesignisbasedonastandardorganicphotodiodebulk heterojunctionstructure.Theprincipaldifferencewithourapproachinvolvestheuse ofconnementlayers,designedtoconneholeswithinthephoto-activeregionof thedevice,muchinthesamewaythisconcepthasbeenusedintheeldoforganic light-emittingdiodesforchargeconnementwithinemissivelayers 151 .Figure5-1a showstheenergybanddiagramofthedeviceunderopencircuitcondition.Both naphthalenetetracarboxylicdianhydrideNTCDAandfullereneC 60 areusedasholeblockinglayersHBLs.NTCDAhasadeephighestoccupiedmolecularorbitalHOMO energy,ensuringstrongholeconnement,andfurthermorehasbeenshowntoprovide air-stableelectrontransportinorganiceld-effecttransistors 152 .WealsouseC 60 asan HBLasitsrelativelydeeperHOMOenergycanconneholeswithinCuPc,anditisa convenientchoiceaswealsouseitinthephoto-activelayer. Figure5-1.Generaldesignofourdeviceshowingatheat-bandenergyleveldiagram ofthedeviceshowingtheFermileveloftheelectrodesandtheHOMO energylevelsbottomandLUMOenergylevelstopofthevariousorganic semiconductorlayersinthemultilayerdevice,bthespectralabsorption coefcientsofthetwoopticallyactivematerialsC 60 andCuPcusedinthe bulkheterojunctionlayer,andcthemolecularstructureoftheconstituent materials. 108

PAGE 109

Forthephoto-activesemiconductorlayer,weusethecopperphthalocyanineCuPc -fullereneC 60 donor-acceptorD-Asystemforthepurposeofdemonstrationbecause ithasbeenwidelystudiedfororganicphotovoltaicapplications 153155 .Thismolecular D-Asystemisfurthermorewellsuitedforvisibledetectionasitsspectralresponsespans thevisiblewavelengthrange,astheabsorptioncoefcientspectrainFig.5-1bshows. Thespectralresponseofourdevicecanbeeasilyvaried,however,byuseofdifferent donormoleculeswithC 60 ,suchaslead-phthalocyanineortin-phthalocyaninefor instancetocoverfurtherintothenear-infraredspectrum.Furthermore,wearguethatthe fundamentaldeviceoperationmechanismisnotmaterialdependent,andthereforeall materialsusedmaybesubstitutedtotunethespectralresponse,responsemagnitude, bandwidth,andotherparameters.However,theelectrontransportlevelsoftheblocking layerandacceptormoleculewithinthemixedphoto-activelayermustbesimilar,and hereinwelimitourscopetotheNTCDA-C 60 pairing.WeuseabulkD-Aheterojunction, ratherthantheplanarorplanar-bulktypes 84 ,becauseourdevicemechanismrequires theunobstructedtransportofelectronsfromanodetocathodethroughtheactivelayer. BathocuproineBCPwasusedasapassivationlayertopreventdopingofthe activelayerbydiffusedaluminumfromthecathode,aswellasanexcitonblocking layertopreventexcitonsintheactivelayerfromelectrodequenching 156,157 .Despiteits shallowLUMOenergy,whenusedinthesedevicesBCPdoesnotblockelectrons.Note thatHOMOandLUMOenergiescitedinFig.1aandthroughoutthisarticlearevalues measuredbyUPSandIPESfromRefs 42,47 anddonotaccountfordipoleformation atmolecularinterfaces;thereforetheyshouldberegardedasapproximate.Also,we assignaFermileveltoindiumtinoxideITOof4.5eV,asweusethisanodematerial 109

PAGE 110

withoutUV/ozonetreatmentwhichiscommonlyusedtoincreasetheworkfunction.The chemicalstructuresofthemolecularspeciesusedinthesedevicesareshowninFig. 5-1c. 5.3DeviceDesignandGainMechanism OurgeneraldeviceconceptusesathinholeblockinglayerHBLinsertedbetween theanodeandactivelayerinanorganicphotodiodetoproducephotocurrentgain. Fig.5-2schematicallyillustratesthephotocurrentmechanismforourdevicewithHBL Fig.5-2bcomparedwithastandardorganicphotodiodewithoutHBLFig.5-2a. AstandardphotodiodenoHBLunderreversebiasisdesignedtoproduceastrong primaryphotocurrentbyminimizingthebarriersforextractingphoto-generatedelectrons andholes,whilelimitingdarkcurrentbyimposinglargebarriersforelectronandhole injectionattheanodeandcathode.Underillumination,anabsorbedphotonexcitesan electroninthedonormoleculefromtheHOMOtoaboundexcitonenergystate.This excitonisfreetodiffusefrommoleculetomolecule,andinabulkD-Aheterojunction withnephaseseparation,hasahighprobabilityofdiffusingtoaD-Ainterfacewhich providestheactivationenergytofreetheelectronintotheLUMOoftheacceptorandthe holeintotheHOMOofthedonor.Underreversebias,thesephotogeneratedelectrons andholesaresweptoutofthedevice.Becauseinthestandarddevicethereisonly aprimaryphotocurrentofatmostoneelectronthroughtheexternalcircuitforevery photonabsorbed,theinternalquantumefciencyislimitedtoamaximumof100%. InourdevicewithaHBL,thebarrierstocarrierinjectionunderreversebiasstill existwhenthedeviceisinthedark.Thecathodesideofthedeviceisunchanged, 110

PAGE 111

Figure5-2.Theproposedmechanismresponsibleforphotocurrentgaininourdevice. Underillumination,acontroldevicewithnoblockinglayerageneratesa primaryphotocurrentofelectronsandholesthatarefreelyextractedfrom thedeviceviathecathodeandanoderesultinginamaximumof100% internalquantumefciency.Withtheadditionofthehole-blockinglayer HBLshowninb,theprimaryholephotocurrentisconnedwithinthe activebulkheterojunctionlayer,substantiallyreducingtheenergeticbarrier forholeinjectionfromtheanodeandresultinginaninuxofsecondary electronphotocurrent,leadingtophotocurrentgainwithinternalquantum efciencysignicantlyexceeding100%. whiletheanodesidenowhasaHBLthatimposesapproximatelythesameelectron injectionbarrierasdoestheactivelayer;therefore,darkcurrentislimitedanddiode recticationisretainedasinthecaseofthestandarddevice.However,thesituation changeswhenthedeviceisilluminated.Justasinthecaseofthestandarddevice, photonsareconvertedtoexcitons,whicharethendissociatedatD-Ainterfaces,andthe photogeneratedelectronsaresweptoutofthedevice.However,photogeneratedholes accumulateattheinterfacebetweentheactivelayerandtheHBL.Thisaccumulated chargegeneratesastronglocalizedelectriceldthatscreenstheappliedvoltageand dropsasubstantialportionofthatappliedvoltageacrossthethinHBL.Thismechanism cansubstantiallylowertheinjectionbarrierforelectrons,andthereforeessentiallyacts asaswitchtoopenthedevicetoelectron-onlyphotoconductionunderillumination. 111

PAGE 112

Asaresult,thisdevicestructureactivatesastrongsecondaryphotocurrentthatmay exceed100%internalquantumefciency.Intheory,thisdesigncouldbeinvertedto conneelectronswithinthephoto-activelayerbyuseofanelectron-blockinglayeratthe cathodeinterface,butwefocushereontheanode-HBLdesign. Althoughthephotocurrentgainresultingfromthisdevicedesignisnotdue tobulkphotoconduction,thephotocurrentnonethelesscanbedescribedbythe photoconductivegainmechanism.Inbulkphotoconductivedevices,thesecondary carriermovesthroughthebulksemiconductormoreslowlythantheprimarycarrier,or maybetrappedandpreventedfromtransitingthebulk.Duringthelifetimeofthetrapped secondarycarrier,asecondarycurrentofprimarycarriersmaybeinjectedintoand transportedacrossthebulkduetotheelectriceldproducedbythetrappedsecondary carrier.Thus,thephotocurrentgaininthesephotoconductivedevicesisproportionalto theratioofprimarycarriertransittimetosecondarycarrierlifetime.Inourdevice,holes actasthesecondarycarrierconnedneartheinterfaceoftheactivelayerbytheHBL andelectronsactastheprimarycarrierinjectedintotheactivelayerthroughtheHBL. Thereareanumberofdesignspecicationstoconsiderinordertosuccessfully constructsuchadevice.First,thebasephotodiodewithoutHBLneedssufcient reverse-biasinjectionbarriersatbothelectrodes,tominimizedarkcurrent.Next,the HBLmusthaveadeepenoughhighest-occupiedmolecularorbitalHOMOenergyto ensurestrongconnementofphotogeneratedholes.Furthermore,thelowest-occupied molecularorbitalLUMOenergyoftheHBLmustnearlymatchthatoftheactivelayer, soasnottogreatlyincreaseordecreasetheinjectionbarrierunderdarkconditions, againtominimizedarkcurrent.Next,asthisdevicedesignreliesonthephotoconductive 112

PAGE 113

gainmechanism,weseektominimizethetransittimeofelectronsthroughthedevice byusinganactivelayerwithhighelectronmobility,inordertoincreasesecondary photocurrent.Finally,thedynamicsofconnedholeswithintheactivelayerexercise greatcontroloverboththemagnitudeofthesecondaryphotocurrentandthereforethe gain,aswellasthedetectorbandwidth.Longlifetimeensureslargephotocurrent,while shortlifetimeensuresfastdetectorturn-offandincreasedbandwidth. 5.4Experiment 5.4.1DeviceFabrication Wefabricateddevicesbyvacuumthermalevaporationoftheorganiclayersand thealuminumcathodeinthesamechamberontoglasssubstrateswithcommercially patternedindiumtinoxideITOanodes R S 15 /square.Substrateswererst cleanedbysonicationinsuccessivebathsofTergitoldetergent,DIwater,acetone,and isopropanol.UnlikethetypicalprocedurefororganicoptoelectronicdevicesusingITO anodes,theITOwasnottreatedwithUV/ozonepriortodepositionoftheorganiclayers. C 60 ,copperphthalocyanineCuPc,naphthalenetetracarboxylicdianhydrideNTCDA, andbathocuproineBCPwerepurchasedfromM.E.R.Corp.,SigmaAldrich,AlfaAesar, andTokyoChemicalIndustry,respectively.C 60 andCuPcwerepuriedoncebythermal gradientsublimation,NTCDAwaspuriedtwicebythermalgradientsublimation 158 ,and BCPwasusedasreceived.Depositionwascarriedoutatabasepressureof106 )]TJ/F20 11.9552 Tf 9.289 0 Td [(6 Torr.MaterialdepositionratesweremonitoredusingSigmahardware,software,and quartzcrystals.NTCDAandBCPweredepositedat0.05nm/s,andactivelayerswere depositedat0.2nm/sthedepositionratesoftheindividualmaterialswithinmixedactive 113

PAGE 114

layersrangedfrom0.02.18nm/s.Aluminumcathodeswereevaporatedat0.2.4 nm/sthroughshadowmasks,deninga2mmcross-bardeviceareaof0.04cm 2 .To protectdevicesfromambientconditionsfortheshortdurationofcharacterization,200 nmofmolybdenumoxide.9%,AlfaAesarwasthermallyevaporatedtoencapsulate devices. 5.4.2PhotocurrentandAbsorptionMeasurement Photocurrentmeasurementsforexperimentalaswellascalibratedphotodiodes wereperformedinairrelativehumidity35-50%withasystemconsistingofaNewport 66902Xenonlamp,aNewport74100monochromator,aStanfordResearch540optical chopper,aKeithley428currentamplier,andaStanfordResearchSystemsSR830 lock-inamplier.Thelampandmonochromatoroutputsacontinuousmonochromatic beamwithgratingovertonesremovedbyopticallters.Thechoppermodulatesthe beamandthelock-inampliermeasuresthemodulatedphotocurrentampliedbythe currentamplieratthereferencechopperfrequency.Thissystemyieldsanaverage lightintensityofapproximately40 m W/cm 2 from l =300 )]TJ/F20 11.9552 Tf 11.474 0 Td [(800nm.Thesystemis controlledbyacomputerrunningin-housedataacquisitionsoftware. Forabsorptioncoefcientmeasurements,thesystemwasusedtomeasure thephotocurrentofacalibratedNewport818-UVinresponsetothetotalincident intensityandthetransmittedandreectedopticalintensityfromsamplesmounted8 offthebeamnormal.ThethicknessofthesampleswasmeasuredwithaDektak150 prolometer,andtheabsorptioncoefcientswerecalculatedbytheBeer-LambertLaw. 114

PAGE 115

Forexternalquantumefciencymeasurements,thesystemwasusedtomeasure experimentalphotodiodephotocurrentundervariousconditions.Thecurrentamplier wasusedtoapplyabiasofV=0toV=-4Vacrossthedevice.Frequencyresponse wasmeasuredbyvaryingthefrequencyoftheopticalchopperfrom25Hzto6kHz. Neutraldensitylterswereusedtostepdowntheincidentopticalpowertoperform dynamicrangemeasurements.Inallcases,thetotalpoweronthedevicewasdened asthetotalbeampoweratthedeviceasmeasuredbythecalibratedphotodiode,as theentirebeamdiameter=1mmfallswithinthedeviceareammby2mmafter focusingandcollimating. 5.4.3TransientPhotocurrentMeasurement Asecondphotocurrentsetupwasusedtoprobetime-dependenceofthephotoresponse.ANewport3mW l =635nmdiodelaserwasdrivenbyasquarewaveoutput fromaTektronixfunctiongenerator.Theincidentpowerwasreducedbyaseriesof neutraldensitylters,yieldinga1.6mW/cm 2 lightbeamwithinthedevicearea.Samples werebiasedatV=-3VbyaDCpowersupplyandthedevicecurrentmonitoredby anAgilentoscilloscopewithvaryingsamplingtimedomainstosurveythetemporal responsefrom10nsto10ms. 5.4.4DarkCurrentandNoiseCurrentMeasurements Currentdensityvoltagecharacteristicsweremeasuredinthedarkbyapplyinga variablebiasandmeasuringcurrentusinganAgilent4155Csemiconductoranalyzer. Fornoisecurrentmeasurements,deviceswereisolatedinanaluminumfoilFaraday cageinthedarkandbiasedwithaKeithley428currentamplier.Thenoisecurrentwas 115

PAGE 116

calculatedinternallybyaStanfordResearchSystemsSR830lock-inamplieratvarying internalreferencefrequencies. 5.5ResultsandDiscussion 5.5.1EffectoftheBlockingLayerStructure TotesttheeffectoftheHBL,wemeasuredthespectralphotocurrentofvarious devices,withandwithoutHBLs.ThecontroldevicestructurewithnoHBL,usingITO asanode,a50nmthick2:1byweightofC 60 :CuPcbulkheterojunctionasactivelayer, an8nmthickBCPinterlayer,andAluminumascathode,functionsasanormalorganic photodiode.Asthereversebiasisincreased,thephotocurrentincreasesbutreachesa saturationexternalquantumefciency h EQE ofbelowunity. Figure5-3.Externalquantumefciencyincontrolphotodiodeswithnoblockinglayers fabricatedwiththesamestructureasshowninFigure5-1withtheomission oftheblockinglayersplottedforseveralappliedbiasesalongwiththe deviceabsorptionefciency.Withelevatedbias,thesecontroldevices saturatebelow100%internalquantumefciency. However,wefoundthatphotocurrentgainwith h EQE > 1canbeachievedwiththe introductionofeitheranNTCDAoraC 60 HBLbetweentheITOanodeandtheC 60 :CuPc 116

PAGE 117

blend.Fig.5-4showsseveralspectral h EQE plotsfordevicesusingavarietyofHBLs, allunderareversebiasof V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3V.Wefoundthat,byitself,NTCDAproducesstronger photocurrentgainthandoesC 60 .Also,while5nmNTCDAonitsownsufcientlyblocks holestoproduceagainmechanism,athickerC 60 layerisrequiredtoreachmaximum efcacy. Figure5-4.Spectraldependenceoftheexternalquantumefciencymeasuredat V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3 Vandunder25Hzmodulatedillumination,fordeviceswiththegeneral structureshowninFig.5-1ausingvariousthicknessesandcombinationsof C 60 only,NTCDAonly,andDHBLhole-blockinglayersbetweentheITO anodeandC 60 -CuPcphoto-activelayer.FortheDHBLstructures,the constituentthicknessesarelistedas t C 60 = t NTCDA Furthermore,wefoundthatNTCDAandC 60 maybeusedintandemasadual holeblockinglayerDHBLtoproducethestrongestgainmechanism.Whena20 nmthickNTCDAHBLisaddedtoa10nmC 60 HBL,wendtheresultingdevice increasesfrom h EQE =3to h EQE =12inthespectralrangeofstrongCuPcabsorption l =650nm.KeepingtheC 60 thicknessat10nm,wendthataswereduce theNTCDAthickness,thegainincreasesto h EQE =20for t NTCDA =10nmandto 117

PAGE 118

h EQE =30for t NTCDA =5nm.Thisndingisconsistentwithourhypothesisthatthe gainisproportionaltotheelectriceldinducedbytheconnedcharges,andtherefore inverselyproportionaltothedistanceofthechargeconnementzoneCCZfromthe anodesurface.Furthermore,whentheNTCDAlayerisfurtherreducedinthicknessto 2nm,wendaverylargeincreaseinthephotocurrentgainto h EQE =220,afullorder ofmagnitudelargerthanthedeviceswiththickerNTCDAintheDHBL.Foralldevices, theelectroninjectionfromthehighworkfunctionanodeintotheLUMOoftheorganic semiconductorsarisesfromacombinationofthermionicemissionandFowler-Nordheim tunnelingtoovercometheinjectionbarrier. 159,160 WeexpecttheHBLtoatleastpartially screentheappliedeldbythebuildupofholesattheHBL-activelayerinterface,andso withareductionoftheHBLthickness,provideditstillblockswiththesameeffectiveness, theeldwillbedroppedacrossashorterdistanceandthetunnelingdistancewill likewisedecrease.Forthedevicesusing5nmandthickerNTCDAHBLs,webelieve thetunnelingmechanismisprimarilylimitedtoinjectionintotheNTCDAmolecule,and thoseinjectedelectronsmustthenovercomeanothersmallinjectionbarrierintoC 60 However,whentheNTCDAlayerisreducedto3nm,alargenumberofelectronsmay nowtunneldirectlyintoC 60 andmayexplainaverylargeincreaseinphotocurrent. Furthermore,thetunnelingprocessisexponentiallydependentontheelectriceldand soweexpectanonlinearincreaseinthetunnelingcomponentofinjectedcurrentupon reductionoftheHBLthicknessandthecorrespondingincreaseinelectriceldactingat theinjectinginterface. Therefore,wendbycomparisonofdeviceswithHBLstothosewithoutthatthe gainisindeedduetotheHBLstructure,andfurthermorebycomparisonofdifferent 118

PAGE 119

HBLsthatthisgainmechanismisstronglydependentonthematerialandthicknessof thethinlmHBL.Fortheremainderofthisarticle,wefocusonfurthercharacterization anddiscussionofdevicesusingtheDHBLoptimizedforhighgain,withacombinationof 10nmC 60 and2.5nmofNTCDA,asisshowninFig.5-1b.Moredetailed h EQE data forthisDHBLdevicewithoptimalgainisshowninFig.5-5,withseveralspectral h EQE plotsforvariousappliedvoltagesmeasuredusinga25HzmodulationfrequencyinFig. 5-5andanervoltagedependenceofthe h EQE for l =690nmilluminationmodulated atboth25Hzand1kHzshowninFig.5-5.Theonsetofgainforthisdeviceoccurs underverylowvoltage, V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(150mV,andunder V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(4V h EQE rangesfrom160to380 acrossthevisiblespectrumfromfrom l =400nm. Figure5-5.Voltageandspectraldependenciesofthequantumefciencyforoptimal connementphotodiodestructurewithaDHBLconsistingof2.5nmNTCDA and10nmC 60 .Thespectraldependenceisshownontheleft,measured under25Hzmodulatedmonochromaticilluminationatseveralapplied biasesasshown.Thenervoltagedependenceisshownontherightforthe samedeviceunderilluminationby l =690nmmonochromaticlight modulatedat25Hzlledblacksquaresand1kHzopenbluecircles. Notethatthepeaksinthespectralresponseofthesedevicesarebroadened withrespecttothecontroldevicewithouttheHBL.Thiscanbeexplainedbythe 119

PAGE 120

non-linearityofthephotocurrentgainmechanism.Itiscommoninphotodiodes, especiallyonesexhibitinggain,fortheretobeadrop-offinresponsivitywithhigher opticalintensities.Thisarisesduetosaturationoffreecarriersandreductionofinternal quantumefciency.Inthiscase,at l =800nmillumination,forinstance,theabsorption efciencyoftheCuPcisverylowandthereforethesteadystateofphoto-generated freeholeconnementisfurtherfromsaturationthanthatgeneratedwithmorestrongly absorbed l =700nmillumination.Asaresult,thiscreatesaslightnon-linearityinthe photocurrentspectrumthatdeviatesfromtheabsorptionspectrum. 5.5.2EffectofBulkHeterojunctionMixingRatio Usingabulkheterojunction,thereisfreedomtovarytherelativeconcentrations oftheconstituentmolecules,enablinganadditionalelementofcontroloverdevice performance.TherelativeconcentrationoftheC 60 :CuPclmhasastronginuenceon materialpropertiesthatultimatelydeterminedeviceperformance,suchasspectral absorption,freeelectronandholemobilities,excitondissociationefciency,and recombinationratesofexcitonsandfreecarriers;ultimatelythesealleffectthecharge collectionefciencyandthusthequantumefciency 154,155 .Wetestedtheeffectonour deviceperformanceofvaryingtherelativeconcentration,between10%and65%CuPc, usingtheoptimalDHBLdevicestructuredescribedinSection5.5.1.Fig.5-6plotsthe gain,bandwidthdecibelfrequency, f 3 dB ,andthegain-bandwidth-productGBPfor devicesbiasedat V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3Vandunderilluminationbya6.3Wmonochromaticbeam centeredat l =690nm.Here,weuse h EQE asthegaingure,ratherthantheinternal quantumefciency h IQE ,whichisthemeasureofinternalphotocurrentgain,because 120

PAGE 121

theexternalphotocurrentrelativetotheexternalopticalsignalisthemostrelevant metricforpracticaldeviceapplications. Figure5-6.Effectofthebulkheterojunctionmixingratioonthegainandbandwidth.The gainG,3-decibelfrequency f 3 dB ,andgain-bandwidthproductGBPfor severaldevicesusingthedevicestructureshowninFig.5-1a,aDHBLwith 2.5nmNTCDAand10nmC 60 ,andvaryingphoto-activelayercomposition asplottedonthehorizontalaxisinwt%CuPc. WendthatwhenmoreCuPcisaddedtothelm,gaindecreaseswhilebandwidth increases.Thegainisapproximately100fordeviceswithlessthan35%CuPc,and thisfallsoffrapidlytoagainofonly5fordeviceswith65%CuPc.Thebandwidth, meanwhile,increasesfromabout f 3 dB =100Hzwith10%CuPctoaplateauofabout f 3 dB =1kHzabove30%CuPc.Asthegainandbandwidthfollowopposingtrendsin devicesexhibitingaphotoconductivegainmechanism,theGBPservesasamore comparablemetricthatexpressesmorecompletelythestrengthofthephotocurrentgain mechanism.WendthattheGBPisoptimizedinourdeviceswhenaconcentrationof CuPcrangingfrom2535%isused,wherebyweachieveaGBPofapproximately10 5 121

PAGE 122

Thetrendsseeninthisseriesofexperimentscanbeexplainedwellbythe relativeelectronandholemobilitiesofthemixedlms.InamixedlmofC 60 and CuPc,thesevaluesarehighlydependentonthecompositionofthemixture 154 .Bythe photoconductivegainmodel,weknowthatfastelectrontransittimeleadstohighergain, andsowecancorrelatethegainwiththeelectronmobility.Indeed,Randetal.show thattheelectronmobilityofthemixedlmsaturatesbeyondapproximately70%C 60 andthiscorrespondswellwiththegainsaturationwendatapproximatelythesame concentration.Furthermore,thephotoconductivemodelpredictsfasterbandwidthfor shorterholelifetime.Whiletheholemobilityisnotdirectlyrelatedtothismodel,wecan predictthathigherholemobilityallowstheconnedholestomoveaboutmorefreely, increasingtherecombinationrateanddecreasingthefreeholelifetime.Therefore,we expectthebandwidthtobeproportionaltotheholemobility,whichitindeedis. 5.5.3TemporalResponse AsdiscussedinSection5.1,organicphotodetectorswithgainhavebeenlimitedby slowresponse.Furthermore,thesedeviceshaveallbeenenabledbymechanisms inwhichlong-livedminoritytrapstatesinducegain.Thegainmechanisminour connementdeviceissimilarinitsgeneralbehavior,yetdifferentbecauseholesare notphysicallytrapped.Instead,holesareconnedtothephoto-activelayersinnontrapstates.InthinlmsoforganicsemiconductorssuchasCuPc,thesestatesare delocalizedandholesarefreetomovewithinthebulkandabouttheinterfacebetween theactivelayerandHBL.Consequently,theirrecombinationcross-sectionislargerthan 122

PAGE 123

thoseofholestrappedinlocalizeddefectstates,andwethereforeexpectthesedevices toexhibithigherbandwidththantrapping-typedevices. Ontheotherhand,ifwereducetheholelifetimetoincreasebandwidth,asSection 5.1discusses,wealsoexpectthetradeoffoflowgain.Infact,wendthatourdevice exhibitsveryhighgainatlowbias.Wecanexplainthisapparentcontradictionby focusingonthedynamicsofholeswithintheconnementtypedevice.Whileaholein atrappingtypedevicespendsitsentirelifetimeverynearitsphoto-generatedposition, aholeinaconnementtypedevicemovesfromitsphoto-generatedpositiontowards theHBL-photoactivelayerinterface.Ineithercase,holedynamicsaffectthesteady statedistributionoftrappedorconnedholesunderillumination.Bytheprincipleof superposition,wecansumtheelectrostaticpotentialsofthesedistributedholes,and likewisetheirforceactingonapossiblesecondaryelectronattheanode.Thisforce isthesourceofthedevicegain.Astrongerforcepullsmoreelectronsintothedevice. Theforceisdependentnotonlyonthenumberoffreeholes,butalsoontheiraverage distancefromtheanodeinterface.WiththemovementoffreeholestowardstheHBL inaconnementdevice,iftheHBLismadesubstantiallythinnerthanthephoto-active layer,weassurethattheaveragedistanceofconnedholesundersteadystateisless thanthatforatrappingtypedevice.Furthermore,becausethesteadystatedistribution dependsonboththerecombinationrateandthetransittimeofholes,higherhole mobilitynotonlycanincreasebandwidthbyincreasingrecombinationrate,butalsocan increasegainbydecreasingtheaveragedistancebetweenfreeholesandtheinjecting anode,andthereforeincreasesthelocalizedeldactingonsecondaryelectronsfor injection. 123

PAGE 124

WemeasuredthetemporalresponseofdeviceswiththeoptimizedDHBLanda 2:1C 60 :CuPcphotoactivelayer.Fig5-7showsthenormalizedphotocurrentrelative tothatmeasuredat25Hz,undermonochromaticilluminationcenteredat l =690 nmirradiation,andbiasedat V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3V.Asshown,the )]TJ/F20 11.9552 Tf 9.289 0 Td [(3dBfrequencyoccursat approximately1kHz.Furthermore,thebandwidthofthesedevicesisnotstrongly spectrallydependent,andweconcludethatfreecarriersgeneratedinbothactivelayer materialsCuPcandC 60 areextractedonsimilartimescales.Thesemeasurements weretakenbychangingthechoppermodulationfrequencyofthelock-inamplier measurementtechniquedescribedinSection5.4.2. Figure5-7.Optimalconnementphotodiodestructureresponseasfunctionof bandwidth.Relativephotocurrentinresponseto l =690nmmonochromatic illuminationasmeasuredbythelock-intechniquedescribedinSection5.4.2 isplottedasafunctionofchoppermodulationfrequencyandexpressedasa valuerelativetothatmeasuredat25Hz. Tofurthercorroboratethesendings,weusedadigitaloscilloscopetomeasurethe outputofthesamedeviceunderilluminationbythesquarewaveoutputofa l =635 nmdiodelaser,asexplainedinSection5.4.3.Fig5-8showstheoutputtraceofthis 124

PAGE 125

deviceimmediatelyfollowingtheturnoffofthelaser,normalizedtothemaximumoutput current.ThelargeouterplotinFig.5-8showstheoutputmeasuredwithseveraldiscrete samplingtimedomainsusingtheoscilloscope,from4 m sto4ms,whiletheinsetplot showsa400mssamplingdomainona100Hzsquarewaveinput.Botharenormalized tothemaximumtotalsignal,includingbothdarkcurrentandphotocurrentunderan appliedbiasof V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3V.Inlogarithmicscale,weseethatthedevicebeginstoturn offapproximately1 m safterthelightsourceisremoved.Afterapproximately200 m s, thephotocurrenthasfallentohalfitsfullmaximumvalue,whichisonlypartofthefull signalincludingdarkcurrent,withasignal-to-noiseratioofapproximately1:1at5Hz. Therefore,theoorofthisphotocurrentresidesat0.5intheplot,belowwhichisentirely darkcurrent.Thedarkcurrentanditsimplicationswillbediscussedinthenextsection. Thereforethismeasurementyields f 3 dB =2.5kHz.Becauseweattributetheturn-off ofthedevicetotherecombinationofconnedholes,thisexperimentsuggeststhat connedholesbegintorecombineafter1us,yetsomearestillpresentafter10ms, suggestingtheexistenceofalimitednumberoflong-livedtrapstatesinthephoto-active layerand/oratitsinterfacewiththeHBL. 5.5.4DarkCurrentandOperationalDegradation Inphotodiodes,themostsignicantsourceofnoisecomesfromuctuationsinthe darkcurrent.Therefore,inordertoachievehighsignal-to-noiseratio,itisimportantto minimizedarkcurrent.Oneoftheadvantagesofourconnementdevicestructureis theretainmentofdioderecticationandlowdarkcurrentunderreversebias.Figure5-9 showsthedarkcurrentforacontroldeviceandoptimalconnementdevicemeasured 125

PAGE 126

Figure5-8.Transienttemporalresponseofoptimalphotodiodedevicestructure. Continuouslyrecordedtemporaldecayofafullsignaldarkcurrentand photocurrentasmeasuredbythelasertransientmethoddescribedin Section5.4.3asafunctionoftimeandexpressedasafractionofthefull signalunderillumination.Insetshowslinearscaleandtheapproximateoor ofdecaycorrespondingtothedarkcurrentbeing50%ofthefullsignal. inthedarkandbeforeoperationunderillumination.ThedevicewithHBLshowsaslight decreaseinforward-biascurrent,andweattributethistoareductionofholeinjection throughtheHBLleadingtounipolartransportofelectronsinjectedfromthecathode. ThedevicewithHBLalsoshowsaslightincreaseinthereverse-biascurrent.Thiswe attributetothesamemechanismactingonphoto-generatedholesunderillumination; however,inthedarkthismechanismonlyactsonthesmallconcentrationofholesatthe HBL-activelayerinterfacepresentduetothermalgenerationandleakageinjectionfrom thecathode. WefoundthatwhilethedarkcurrentofthedevicewithHBLexhibitsstrong recticationintially,thisrecticationdegradeswithoperation.Devicesdegrademost stronglywhenoperatedunderareversebiaswhilebeingilluminatedwithultraviolet 126

PAGE 127

Figure5-9.Currentdensity-voltageJ-VcharacteristicsinthedarkforaCuPc:C 60 heterojunctionphotodiodewithadualNTCDA:C 60 holeblockinglayeropen redcirclesandforacontroldevicewithnoHBLbutanotherwiseidentical structurelledblacksquares. radiation.Figure5-10plotsthedarkcurrentmeasuredimmediatelyafterfabrication, afteroperatingunderhighintensitymW/cm 2 reddiodelaser l =635nm illumination,andafteroperatingundersimulatedsolarilluminationwhichincludesa smallfractionofultravioletlight.Thedarkcurrentremainsrelativelyconstantwithtime underilluminationbythereddiodelaser,butrapidlyincreasesunderilluminationwith thesimulatedsolarorotherultravioletsourcesoflight.Thisdegradationmaybedue toabsorptionofultravioletlightbyNTCDA,whichmaycauseNTCDAtocrystallizeand formthephysicalhole-trappingstructuresdescribedbyNakayamaetal. 68 toproduce strongphotocurrentgain. Infact,wendthattheevolutionofdegradationindarkcurrentalsoaffectsthe photoresponsestrengthandspeed.Thephotocurrentgainincreaseswithtime delivereddoseofultravioletradiation.Asthegainincreases,thespeedofresponse 127

PAGE 128

Figure5-10.Darkcurrent-voltagecharacteristicsforadevicebeforeoperationclosed blacksquares,afteroperatingunder75mW/cm 2 monochromatic l =635 nmilluminationopenbluecircles,andafteroperatingunderabroadband Xenonarclampsourcewithanintensityof100mW/cm 2 openred triangles. decreases.Thisbehaviordirectlyfollowsthephotoconductivegainmodel,andissimply amanifestationofthephoto-generatedholesbecomingtrappedwithinthedevice,most likelyattheNTCDAinterface,foralongerlengthoftime.Asthisdegradationmechanism increaseswithtimeandwiththeintensityofultravioletradiation,wecanusethistotune thetradeoffbetweenphotoresponsemagnitudeandspeed.However,withultraviolet treatmentnotonlydothedarkcurrentandthephotocurrentincrease,butthenoise currentincreasesaswell. Inordertoproducedeviceswithhighsignal-to-noiseratio,thisdegradation effectmustbeminimized.Weexploredafewinterfacialengineeringapproaches, includingalteringtheworkfunctionoftheITOelectrodebyUV/ozonetreatment andtheapplicationofverythinlayersofmetalbetweentheITOandNTCDA.These approachesdosubstantiallyreducethedegradationeffect,howevertheyalsoreduce 128

PAGE 129

thephotoresponsemagnitudeofthedevices.Therefore,morefutureworkisstillneeded tocounteractthedegradationinthesedevices. 5.6Conclusions Inconclusion,wehavestudiedtheoperationofaneworganicphotodiodedevice structurethatproduceshighgain.Weattributethegaintotheconnementoffree photo-generatedholesbyhole-blockinglayersbetweentheanodeandthephotoactive layerandtheproductionofastrongsecondaryelectronphotocurrentinjectedfrom theanodeunderreversebias.WefoundthatbothNTCDAandC 60 workwellasHBL, andthatacombinationofathickernmthickC 60 HBLwithathinner.5nmthick NTCDAHBLcreatethestrongestgain.WealsondthatthecompositionofthephotoactivebulkheterojunctionofCuPcandC 60 stronglyinuencesthegainandbandwidth duetothechangeintheelectrontoholemobilityratio,andobtaintheoptimalgainbandwidthproductindeviceswithapproximately30%CuPcinthelm.Furthermore, wendthatthesedeviceshavearelativelyhighbandwidthof f 3 dB =1kHz,which isextraordinarycomparedtopreviousorganicphotodetectorsoperatingwithhigh gain.WeattributetheresultinghighGBPtotheconnement,ratherthantrapping,of holeswithinthephotoactivelayerbytheHBLs.Finally,wendthatthedarkcurrent degradeswithuseandresultsinhighnoisecurrent,ultimatelygivingthesedetectors lowsensitivity.Therefore,thesedevicesshowgreatpromiseforpracticalapplication duetotheirhighgain,bandwidth,dynamicrange,andspectraltunability.However,to realizethispotential,futuredevelopmentisrequiredtodiagnosethephysicalcauseof thedegradationinordertoreducenoisecurrentandimprovesensitivity. 129

PAGE 130

CHAPTER6 DOWN-CONVERSIONFORORGANICSOLARCELLABSORPTIONEFFICIENCY ENHANCEMENT 6.1Introduction 6.1.1Motivation InorganicsolarcellsOSCs,thereexistsatrade-offbetweendeviceinternal quantumefciencyandabsorptionefciency.Thethickeradeviceismade,the moresunlightitcanabsorb;however,thickerlayersreducetheefciencywithwhich adeviceconvertsanabsorbedphotonintodevicephotocurrentduetoshortexciton diffusionlengthandlowmobility-lifetimeproducttheso-calleddiffusionbottleneck. Manyapproacheshavebeendesignedtoovercomethisfundamentallimitationto performance,asdescribedinChapter5.However,themostsuccessfultechnique,the bulk-heterojunction,inherentlyincreasesseriesresistanceandreducesshuntresistance asitremovesthewell-dened,two-dimensionalheterojunctionbetweentwoneatlayers. Abilayerstructuregenerallyexhibitssuperiorcarriertransportandhigherllfactor,but thephotocurrentremainslowerthanthatgeneratedbyabulkheterojunctionduetothe diffusionbottleneck 155,161 Organicsolarcellefciencyisfurtherlimitedbythepooroverlapofdevice absorptionwiththesolarspectrum.P3HTandmanyotherorganicsemiconductors usedinOSCsabsorblightwithwavelengthlessthan700nm,anddonotabsorbinthe nearinfrared.Furthermore,theopencircuitvoltageoforganicsolarcellsistypicallywell belowthebandgapenergyoftheactivelayersduetostrongexcitonbindingenergy andpoorenergyalignmentofdonorandacceptortransportenergylevels 101 .For 130

PAGE 131

instance,whileabulkheterojunctionofPCBMandP3HTexhibitsanopticalbandgap ofabout1.8eV,itproducesantransportbandgapofabout1.4eV 44 ,anddevices employingthisheterojunctionexhibitanopencircuitvoltageofonly0.55.65V.Forthe P3HT:PCBMsystem,theexternalquantumefciencycanapproach80%,butstillthe powerconversionofdevicesislimitedto4%duetotheseenergeticlosses. Thespectralabsorption,chargecollectionefciency,andopencircuitvoltage arethreekeycharacteristicsofanorganicsolarcell,andarealllinkedtothespecic materialpropertiesofthesemiconductorsemployed.Asinglecharacteristiccanbe controlledbysubstitutingdifferentmaterials,forinstancetopushthespectralresponse intothenearinfrared.However,suchasubstitutioncannegativelyaffecttheother performance-governingcharacteristics.Syntheticorganicchemistshavedesigneda varietyoforganicsemiconductorsthatattempttocontrolallthedevicerelevantmaterial properties 162 ,howeverthisisanexceedinglydifculttaskandhasproducedlimited success.Therefore,itwouldbehelpfultoinsomewaydecouplethesepropertiesinto differentmaterialsinordertogainmorecontroloverthenaldeviceefciency.Thework describedinthischapterattemptstodecouplesomeoftheopticalabsorptionnormally requiredoftheorganicsemiconductorsintoanexternalabsorptiveuorescentantenna. 6.1.2Concept:Down-ConversioninAdvancedDeviceArchitecture Weproposehereanewadvanceddevicestructurethatusesaluminescent enhancementlayerLELoutsidethedevicetoabsorbshortwavelengthlightand photoluminescedown-convertedlightintoanorganicsolarcell.Thisstructurehas thepotentialtoimprovespectraloverlapwiththesolarspectrumandalsodecouple 131

PAGE 132

absorptionpropertiesfromtheelectricalpropertiesoftheacceptorsemiconductor,allin abilayerheterojunctioncellthatcanachievehighextractionefciency. BycouplingaLELdirectlytoanorganicsolarcell,theLELlayermayservethree functions:absorblightthatisnototherwiseefcientlyabsorbedbytheOSCand re-emitatawavelengththatismoreefcientlyabsorbedintheOSC,couplealarge fractionofthere-emittedlightintowaveguidingmodesthatincludestheactivedevice layers,ultimatelyincreasingtheopticalpathlengthofweaklyabsorbedlightthroughthe device,andenablethechoiceofatransparentwidebandgapacceptorlayersuchas aninorganicoxidethatmayimprovethecollectionefciencyoftheOSC.Ageneralized multilayerdevicestackisshowninFigure6-1toillustratetheabsorptionandre-emission ofincidentsunlightbyaLELcoupledtoanorganicsolarcell. Figure6-1.Schematicdiagramshowingthedown-conversionabsorptionantenna concept.Onright,thedevicepartiallyabsorbsintheantennaabsorption spectrumandreemitsinalldirections,someofwhicharewaveguidedwithin thedeviceincreasingtheprobabilityofabsorption. Thisconceptisinspiredbytheuseoforganicsemiconductorsinluminescentsolar concentratorsLSCs 163165 .LSCsconsistofuorescentthinlmsthatabsorbincident lightandre-emitred-shiftedlightinalldirectionswithalargefractionpropagatingby waveguidingmodeslaterallythroughtheLSCtoultimatelybecollectedattheedgesof 132

PAGE 133

theLSCbysmallmatedphotovoltaiccells.Thisgeometrycreatesaconcentrationof sunlightonsmallcellswithouttheuseofactivetrackingneededbymirrorconcentrators, andthereforeLSCsarecurrentlyapromisingtechnologyfornextgenerationsolar energygeneration. WhiletheLELlayerusedinthisworkcanemploythesamematerialsasusedin LSCs,therearesubtledifferencesintherelationshipbetweentheluminescentmaterial propertiesanddeviceperformance.First,itisimportanttominimizetheself-absorption SAlossinbothLSCsandourLEL,butbecauselightdoesneednotpropagateon meterlengthscalesasinLSCs,aLELdevicestructurecantolerategreaterSAloss. Second,LSCsmustbedesignedsimilartoopticalberstomaximizeinternalreection, whileforaLELwewanttominimizeemissionawayfromthedevicebutatthesametime maximizeemissionintothedevice.Finally,theLSCconcentrateslightdeliveredtothe PVcellandthereforecellsmustbeselectedthatefcientlyconvertelevatedintensityof opticalenergy,whiletheLELconceptdoesnotproduceconcentratedlight.However, whiletheorganicsolarcellinourconceptreceivesareducedphotonuxduetoPLand otherlosses,thephotonuxismorestronglyabsorbedduetothedown-conversion processandincreaseofopticalpathlength;therefore,thecelldoesreceiveanelevated levelofabsorbedphotonuxandthereforebehavesinasimilarmannerasundersolar intensityelevatedbythelevelofabsorptionenhancement. Inordertotestthisconcept,weemployrayopticsmodelingtodeterminethetotal absorptionefciencyofvariousdevicestructures.BecausetheuseofanexternalLEL doesnoteffecttheelectronicbehavioroftheinteriorcell,weareabletofocusentirely ontheabsorptionpropertiesofaparticularcell,withandwithoutanexternalLEL,and 133

PAGE 134

evaluatetheenhancement,ifany,thatresultsbasedonthepresenceoftheLEL.The nextsectiondescribesthemodelingmethodology,andthesubsequentsectionsdiscuss theresultsofmodelingvariousdevicestructures. 6.2OpticalSimulation 6.2.1ConceptualOverview Inordertotestthisnewconcept,acomputationalsimulationprogramwaswritten usingtheprogramminglanguageJava.Thesimulationprogramwasaheavilyreworked versionofasimulatorbuiltformodelingtheextractionefciencyofOLEDsusing microlensstructures;theOLEDworkiscoveredinAppendixA.Thephotovoltaic absorptionsimulatorusesrayopticstotracelightthroughvariousdevicearchitectures, monitoringreection,refraction,absorption,andemission.Ultimately,theprogram determinesthespectraldistributionofthefractionoflightthatisabsorbedineachdevice layerandthatwhichisemittedfromthedevice.Themodelfocusesentirelyonthe opticsyieldingabsorptiondataanddoesnotattempttomodeltheinternalphotoelectric conversionprocess.However,thedown-conversionstructuredoesnotaffectthe photovoltiacconversionasthestructureisplacedoutsidetheactivedevice;therefore, wecanusethismodeltocomparerelativeabsorptionefciencyandextrapolateto relativephotoconversionefciencyknowingtheexperimentalconversionefciencyofthe activephotovoltaicdevice. FordevicesthatemployaLEL,wemusttakeaccountseveralabsorptionfactors: primaryabsorptionefciencyofallthedevicelayers,re-emissionefciencydetermined bythePLyieldoftheLEL,andsecondaryabsorptionefciencyofthere-emittedlight 134

PAGE 135

inalldevicelayers.Wedenetheeffectiveabsorptionefciencyofeachmateriallayer inthedeviceastheefciencywithwhichthatlayerabsorbstheinitialincidentsolar spectrum,takingintoaccountbothprimaryandsecondaryabsorption.Inotherwords, ifaphotonofwavelength l =500nmisincidentonthedevice,absorbedbytheLEL, re-emittedat l =650nm,andre-absorbedinthedonorlayer,thenthedonorlayer effectivelyabsorbsthe l =500nmphoton.Togaugetheperformanceofasolarcell employingtheLELconcept,weneedtocalculatetheeffectiveabsorptionefciencyof theactivelayersALs, h AL effective A ,whichfollows h AL effective A l = h AL ,1 0 A l + h h AL ,2 0 A l h LEL ,1 0 A l h LEL PL l 1 )]TJ/F60 11.9552 Tf 10.95 0 Td [(h LEL ,2 0 A l i where h PL isthePLyieldoftheLEL,and1 0 and2 0 superscriptsrefertoprimary andsecondaryabsorptioninthenotedlayers.Eachoftheabsorptiontermsis interdependentontheabsorptionofotherlayers,thelossesatelectrodes,the multiplepassesthroughlayersasaresultofwave-guidedmodes,thepossibilityof sequentialabsorptionandre-emissionwithintheLELdependentonthelocationof initialabsorption,etc.Ultimately,themodeloutputsdatathatcanbeusedtodetermine eachterm,andequation6canbeusedtocomputethetotalactivelayereffective absorptionefciency. 6.2.2ModelDesign Theraytracingmodelisbuilttotrackasinglerayintoandthroughoutasolarcellas itisreectedortransmittedthroughinterfacesandabsorbedwithinthevariouslayers inthecell.Allphysicsarecalculatedassumingnon-interferingraybehavior,andtheray fractionspredictedareusedasprobabilitiesthatactonadiscreteray.Manydiscrete 135

PAGE 136

raysaregeneratedandtracked,andtheendbehaviorispredictedbyaveragingall discreteraybehavior.ThisprogramdesignisoftheMonteCarlovarietyandtherefore asimpliesprogrammingbyavoidingrecursiverayopticsandballowstheprogram tobesplitamongmultipleprocessthreads,speedingupcalculationonmulti-core processors. Adiscreterayisrstgeneratedabovethesolarcellwithdirectionnormaltothe cell.Thelocationoftheinitialgenerationisdeterminedbyrandomnumbers, X i and X j generatedfromuniformprobabilitydensity, P i [0 < X i < W ]and P i [0 < X j < L ],where W and L arethewidthandlengthofthesolarcell.Thegeneratedrayisgivenarandom wavelengthwhosevalueisgeneratedfromaprobabilitydensityequaltothenormalized AM1.5spectrum.Therayisfurthermoregivenarandompolarization,eitherentirely s-polarizedorentirelyp-polarized,bythegenerationofrandombooleanvalue.This generateddiscreterayisthentrackedthroughthedevice,whichismadeupofdiscrete materiallayersassumedtohaveperfectlyuniforminterfaceswithnosurfaceroughness. Therefore,weassumethereectionortransmissionofthediscreterayateachinterface isentirelydeterminedbytheFresnelequationsusingwavelength-dependentcomplex indicesofrefractionforeachmateriallayer. Theraymovessequentiallyfrominterfacetointerface,andeachinterfaciallocation isdeterminedbyndingtheintersectionoftheraydirectionvectorwiththeinterfacial plane.Thepropagationlengthwithinthatlayer, d ,isusedtocalculatetheabsorption efciency,bytheBeer-LambertLaw, A l =1 )]TJ/F58 11.9552 Tf 10.949 0 Td [(e )]TJ/F60 8.9664 Tf 6.967 0 Td [(a l d 136

PAGE 137

knowingtheabsorptioncoefcient, a l ,ofthematerial.Arandomnumber, X ,is generatedfromtheprobabilitydensityfunction P [0 < X < 1],andif X < A ,thediscrete rayisentirelyabsorbed.Otherwise,thediscreterayisentirelytransmittedandgoeson tointeractwiththenextinterface. Inordertodetermineifarayisreectedorrefracted,therefractedraydirection mustrstbecalculated.Basedonthedirectionvectorwithintheinitialmateriallayer, ~ v i andtheinterfacialnormalvector, ~ n ,Snell'sLawinvectorformisusedtocalculatethe potentiallytransmittedrefractedraydirectionvector, ~ v t : ~ v t = n i n t ~ v i +cos q t ~ n )]TJ/F58 11.9552 Tf 12.413 8.094 Td [(n i n t cos q i ~ n where cos q i = ~ n ~ v i and cos q t = s 1 )]TJ/F72 11.9552 Tf 10.95 16.863 Td [( n i n t 2 )]TJ/F20 11.9552 Tf 5.475 -9.69 Td [(1 )]TJ/F20 11.9552 Tf 10.949 0 Td [(cos 2 q i Intheequationsabove, n denotestheindexofrefractionand q representspolar propagationanglewithrespecttothesurfacenormal i and t subscriptsrepresent incidentandtransmittingmaterials,respectively. ThedecisiontoreectorrefracttherayateachinterfaceisgovernedbytheFresnel equationforreectivity, R = n t cos q i )]TJ/F58 11.9552 Tf 10.95 0 Td [(n i cos q t n t cos q i + n i cos q t 2 forap-polarizedrayand R = n i cos q i )]TJ/F58 11.9552 Tf 10.95 0 Td [(n t cos q t n i cos q i + n t cos q t 2 137

PAGE 138

forans-polarizedray.Todeterminethebehavioroftheray,arandomnumber, X ,is generatedfromauniformprobabilitydensity, P [0 < X < 1],andif X > R ,therayis transmitted,andtheraytakesonthedirectioncalculatedinEquation6.Otherwise, therayisreectedandtakesonthedirectiongivenby ~ v r = ~ v i )]TJ/F20 11.9552 Tf 10.95 0 Td [( ~ v i ~ n ~ n Forthecalculationof R atthemetalcathodeinterface,complexindicesofrefraction 166 areusedtoaccountfortheextinctionatthemetalsurface.Ifinthiscasethegenerated numberisgreaterthantheFresnel-calculatedreectivity,themodeltreatsthisas absorptionbythemetal. Theprogramiteratesoverasetnumberofraysforthedatapresentedinthis chapter,10 7 discreteraysareusedandrecordsthelayerwithinwhicheachrayis absorbedfordevicelayersorintowhicheachlayerisemittedinthecaseofthe surroundingairenvironment.Eachraypropogatesuntilitiseitherabsorbedoremitted, andnoarticialendpointsareusedsuchasamaximumnumberofreections.Finally, theprogramusestabulatedwavelength-dependentabsorptioncoefcientsandindices ofrefractionforeachmaterial,andthereforethegenerateddataisdependentonthe wavelengthoftherayasdeterminedbytheAM1.5solarspectrum. 6.3LuminescentEnhancementLayer 6.3.1DesignandMaterialSelection AneffectiveLELmustsimplyabsorbanduoresceincidentblueandgreenlight efciently.Traditionally,organicuorescentdyeshavebeenusedforthispurposeina varietyofapplications,includingdyelasers 167 andbiologicaluorescentimaging 168 138

PAGE 139

butquantumdotsarealsonowbeingemployedforsimilarapplications 169 .Theideal materialforourapplicationwouldabsorbacrosstheblueandgreenandemitinthe redandnearinfraredwithnear100%photoluminescentquantumefciencyPLyield. Ideally,wealsowanttominimizetheself-absorptionlossintheLELbymaximizing wavelengthshiftbetweenabsorptionandemission.Fluorescentsmallmoleculessuch astherhodamines 170 canachievehighluminescentefciencyinthered,however theStokesshiftofthesematerialsisgenerallysmallnm.Phosphorescent moleculesexhibitanenhancedStokesshiftandthereforehavelowerselfabsorption loss;however,thesemoleculesalsotypicallyexhibitlowerPLyieldcomparedto uorescentmolecules,especiallythosethatphosphoresceinthenearinfrared. HereweexploretheuseofDCJTB-dicyanomethylene-2t-butyl -6-,1,7,7tetramethyljulolidyl-9-enyl-4 H -pyran,amoleculediscoveredearlyinthedevelopment ofOLEDtechnologybyKodakthatexhibitsnearunityPLyieldandanemissionband centeredatabout l =615nmindichloroethaneandproducesefcientemissiondoped intoAlQ 3 171 .Currieetal. 165 usedthismoleculeforthesepropertiesasadopantin AlQ 3 hosttoproducehighlyefcientorganicluminescentconcentrators.AlQ 3 ,apolar moleculeoftenusedasahostinOLEDwork,producesasolidstatesolvatingeffect similartoaneatlmofDCJTB,andthereforeenhancestheStokesshift,butalso isolatestheelectronicexcitationstatestoisolatedDCJTBmolecules,eliminatingthe concentrationquenchingeffectofaneatlayer 172 .Furthermore,AlQ 3 itselfabsorbsin theblueandefcientlytransfersenergytotheDCJTBmolecules,thereforeadopedlm consistingofbothmoleculesabsorbsacrosstheblueandgreenandemitsinthered. 139

PAGE 140

Thereductionofself-absorptionlossinsolarluminescentconcentratorsis especiallyimportantastheenergyofabsorbedsunlightmustbelaterallytransferred overmeterlengthscalestoareceivingsolarcellmatedperpendiculartothewaveguided andconcentratedlight.Forthiswork,thethereductionofselfabsorptionlossisalso important,butthecriticaldistanceoverwhichself-absorptionplaysaroleisontheorder ofthethicknessoftheLELratherthanthelateralsizeofthesolarcell.Nevertheless,this designalsofacesabsorptionlossesbythetransparentanodeandreectionlossesby thecathode.Theseeffectsbecomemoreimportantforlightthatisonlyweaklyabsorbed bytheactivelayers,aslightmaybewaveguideddownthesolarcellwithrepeated reectionsandtransmissionpathsthroughtheanodeandLEL.Therefore,forthiswork webeginwiththeuorescentlayerdesignofCurrieetal.,andinthenextsectionexplore amorepracticallyachievableLELbysolutiondeposition. 6.3.2Solution-DepositedLuminescentEnhancementLayers Inordertoavoidconcentrationquenching,DCJTBmustbedopedatlowconcentration.Forinstance,Currieetal.used2%DCJTBtoproducehighlyefcientemissionin athermallyevaporated5.7 m mAlQ 3 :DCJTB%lm.Thispresentsapracticaldifculty fortheexperimentalrealizationoflayersofthisthicknessusingthermalevaporation. AlthoughthemainfocusofthisworkcentersonthedemonstrationoftheLELconcept, weultimatelyseektodesignpracticaldevices.Therefore,hereweexplorethedoping ofDCJTBintothepolymerpolymethyl-methacrylatePMMA,whichishighlysoluble inchloroformandsolutionscanbemadewithhighenoughviscositytobespuncoat intomicronthicklayers.Furthermore,PMMAhasanindexofrefractionn=1.49that 140

PAGE 141

closelymatchesglassandthereforetheLELcanbeseparatedfromtheactivedeviceby aglasssubstrate,anadvantagethatwillbediscussedinSection6.4.2. TheuseofPMMAashostinplaceofAlQ 3 hastwomainconsequencesthataffect thefunctionalityoftheLEL.First,PMMAishighlytransparentandthereforedoesnot absorbintheblueregionofthesolarspectrumasdoesAlQ 3 .Therefore,theonlylight thatisdown-convertedbyaLELconsistingofPMMA:DCJTBisthatwhichisabsorbed byDCJTB.Second,PMMAisnon-polaranddoesnotred-shifttheemissionandreduce theselfabsorptionofDCJTBasdoesAlQ 3 .However,wefoundthatusingAlQ 3 asa secondaryhostinadditiontoPMMAnotonlyexpandsabsorptionintothebluebutalso stronglyred-shiftsemission.Essentially,PMMAactssimplyasasolidstatediluenttothe opticallyactiveAlQ 3 andDCJTBmolecules. TooptimizethePLefciencyofthesolution-depositedLEL,severalformulations weretestedvaryingthecontentofPMMA,AlQ 3 ,andDCJTB.Itwasfoundthatasolution of50mg/mLPMMA,20mg/mLAlQ 3 ,and1mg/mLDCJTBgavethehighestrelative PLyield.Figure6-2showsthemeasuredabsorptanceandPLforthisformula.Lower andhigherloadingofDCJTBresultedinreducedPLyield,andtheincorporation of20mg/mLAlQ 3 wasfoundtostronglyred-shiftthePLemissionmaximumfrom l =600nmforPMMA:DCJTBlmsto l =640nmforPMMA:AlQ 3 :DCJTBlms, dependingontheconcentrationofDCJTBlongerwavelengthandlowerPLyieldfor higherconcentrationDCJTB.TocomparethePLyieldofthesesamples,lmswere illuminatedwithabroadbandwhitelightsourceandaportionoftheDCJTBemission thatexitedthesubstratewaveguidedmodewasmeasuredwithaOceanOpticsJAZ spectrometer.Thesemeasurements,whilenotcapableofproducingameasureof 141

PAGE 142

absolutePLyield,arereproducibleandcapableofmeasuringnotonlytheemission spectrumbutalsotherelativeemissionefciencybetweensamples.Theabsorption andphotoluminescenceofthismorepracticallyrealizableLELareusedinthemodeling presentedintheremainderofthischapter,assuminganabsolutePLyieldof90%. Figure6-2.AbsorptionandphotoluminescenceofDCJTB:Alq3inPMMA. 6.4Down-ConversioninOrganicSolarCells Organicsmallmoleculesolarcellsconsistingofmetalphthalocyaninedonorsand fullereneacceptorsarewellstudiedintheliteratureandachieverelativelyhighinternal quantumefciency;howeverefciencyislimitedduetoweakabsorptionbytheverythin materiallayersnecessarytoefcientlyextractphotocurrent.Theabsorptionefciency isfurthermoresubjecttoaspectraldependencearisingfrommolecularabsorption inGaussian-likebands;consequentlytheabsorptionefciencydropsintherangeof l =500nmduetotheenergeticseparationofstrongabsorptionbythefullerene andthephthalocyaninemolecules.Thiscanespeciallybethecaseforsolarcellsusing nearinfraredNIRabsorbingdonormoleculessuchastinphthalocyanine 94,95 orlead 142

PAGE 143

phthalocyanine 93 ,astheredshiftinabsorptionofthedonoropensupalargerspectral regiontoweakabsorptionalthoughthedecreaseinopticalbandgapnevertheless increasesthetotalabsorbedphotonux.Figure6-3plotstheEQEfortwoorganic cells,bothusinganacceptorofC 60 andoneusingadonorofcopperphthalocyanine CuPcandtheotherusingadonorofleadphthalocyaninePbPc,comparingtheir spectralresponsetotheAM1.5spectrum.Ascanbeseen,thesmallerbandgapPbPc materialabsorbsfurtherintotheNIRandconsequentlyopensupalargerspectral regionexhibitingpoorabsorptionbetweentheacceptoranddonorabsorptionbands. Figure6-3.ExternalQuantumEfciencyinPhthalocyanine:FullereneSolarCells comparedtotheAM1.5solarspectrum.TheEQEisplottedinredfor PbPc:C 60 nm:50nmandinblueforCuPc:C 60 nm:50nm,withthe AM1.5spectrumplottedinblack.NotetheportionoftheAM1.5spectrum whosephotonenergyisabovethebandgapforthesematerialsbutisnot stronglyabsorbedandconvertedintophotocurrent. Thedown-conversionconceptmayenhancetheefciencyofthesecellsby convertinglightthatwouldotherwisebeonlyweaklyabsorbedintolightoflonger wavelengththatismorestronglyabsorbedbythesolarcell,andfurthermoreby increasingthepathlengththroughwhichtheuoresceduxpasses.Here,weexamine 143

PAGE 144

theeffectsofthisconceptonsmallmoleculeorganiccellsusingtheraytracingmodel. Toexploretheviabilityofthedown-conversionconcept,werstexamineitseffectona donor-acceptorheterojunctionofleadphthalocyaninePbPcandfullereneC 60 ,andtest afewdevicegeometriesthatvarythelocationoftheLELwithrespecttothesubstrate andotherdevicelayers.Next,weexaminetheeffectofthespectralabsorptionofafew donormolecules,includingPbPc,CuPc,andsquaraine,onthemodeledabsorption. Withtheinformationprovidedbythesesimulations,wecandesigndevicestructuresthat efcientlymakeuseofthedown-conversionprocess. 6.4.1ModeledAbsorptioninaStandardC 60 :PbPcSolarCell WerstexaminethemodelpredictionsforacontrolC 60 :PbPcsolarcellwith noLEL.Figure6-4plotsthemodeledabsorptiondataforthisdeviceaswellasthe calculatedspectralabsorptanceforboththeentirecelldeviceabsorptanceandforthe activeC 60 :PbPclmsonlylmabsorptance.Thesummedtotalfractionoftheincident AM1.5spectrumthatisabsorbedwithineachlayerisalsosummarizedinTable6-1. Therelevantdataforpredictingthedeviceperformanceisthetotaleffectiveactivelayer absorptance,whichforthecaseofthiscontroldeviceis36%oftheincidentspectrum from l =300nm. Byintegratingthemodeledlmabsorptancespectrum,wecanpredictthemaximum photocurrentaparticulardevicestructurecanachieve.Forinstance,themodeled spectrumfrom l =300nmdelivers49.0%oftheintegratedAM1.5photonux; therefore,asolarcellwith100%absorptionefciencyand100%internalquantum efciencyacrossthisspectralregion,withanabsorptionstepcutoffat l =900,can 144

PAGE 145

Figure6-4.ModeledabsorptionefciencyforPbPc:C 60 solarcellswithLELstandard structure. deliveratheoreticalmaximumphotocurrentdensityof33.8mA/cm 2 .Themodelpredicts anabsorptionefciencyof36%foracontrolC 60 :PbPccell;therefore,thiscellcan theoreticallyproduceamaximumof12.2mA/cm 2 ArealcellexhibitsIQElowerthan100%forinstance,seetheEQEplottedinFigure 6-3butthisneverthelessgivesusametricforcomparison.Forinstance,ifarealcell experimentallyachievesashortcircuitcurrentdensityof8.0mA/cm 2 ,andourmodel predictsarelativeabsorptionenhancementbyuseofaLELof15%,thenwecanpredict thattherealcellwithLELwillproduceapproximately9.2mA/cm 2 .Suchapredictionis validasthesolarcellitselfisunchangedbytheexternalopticalLELstructure.However, thedown-conversionoflightfromweaklyabsorbedenergytomorestronglyabsorbed energyeffectivelyincreasestheintensityoflightincidentonthedevice;therefore,such predictionsassumealineardependenceofphotocurrentonincreasedopticalintensityin therelevantintensityrangeinthiscase,1.15sunintensity. 145

PAGE 146

Table6-1.ModeledabsorptiondataforC 60 :PbPcsolarcellsofvaryingLELstructure. Theabsorptance, A ,theverticalloss, VL ,andthelateralloss, LL ,arelisted foreachdevicelayer.Primaryandsecondaryabsorptionandlossesare markedwiththesubscripts0and LEL ,respectively.Thetotaleffective absorptanceoftheactivelayersislistedwithanenhancementfactorrelative tothecontrolabsorptance. ControlSubstrate n =1.5 Substrate n =1.7 ITOon Glass 0.2cm ITOon Glass 1cm ITOon Glass 10cm ITOon Glass 100cm DonorPbPcPbPcPbPcPbPcPbPcPbPcPbPc AcceptorC 60 C 60 C 60 C 60 C 60 C 60 C 60 n LEL -1.51.71.51.51.51.5 DeviceL&W1cm1cm1cm0.2cm1cm10cm100cm A 0 ,glass0%0%0%0%0%0%0% A 0 ,acceptor10%1%1%1%1%1%1% A 0 ,donor26%23%22%23%23%23%23% A 0 ,LEL-34%33%34%34%34%34% A 0 ,ITO9%5%5%5%5%5%5% A 0 ,Ag4%2%2%2%2%2%2% VL 0 51%35%36%35%35%35%35% PLyield,LEL-90%90%90%90%90%90% A LEL ,acceptor-3%3%1%2%3%3% A LEL ,donor-51%56%7%26%46%52% A LEL ,LEL-6%5%3%3%5%6% A LEL ,ITO-15%17%2%8%14%15% A LEL ,Ag-4%3%1%2%4%4% VL LEL -18%13%14%17%18%18% LL LEL -5%4%73%42%11%2% A D )]TJ/F58 8.9664 Tf 6.967 0 Td [(A 36%41%42%26%33%40%42% Enhancement0%16%17%-27%-8%11%17% 6.4.2Down-ConversionDeviceStructures Thinlmsolarcellsbothorganicandinorganiccanbebrokendownintotwo classesofdevicestructure:substrateandsuperstrategeometry.Thesegeometries refertothedirectionfromwhichlightisincidentonthedeviceandtheplacementofthe transparentelectrode.Lightisincidentonthetopandbottomsurfacesforsubstrateand 146

PAGE 147

superstrategeometries,respectively,thatbuildanactivelayermaterialstackontopofa supportingsubstrate.Theimportancetodevicedesignisthatthetransparentelectrode alwaysmustfacetheincidentlight,andthereforelieseitherontopofapredeposited stackofactivematerialsinthecaseofthesubstratemodality,ordirectlyontopof atransparentsubstratesuchasglassinthecaseofthesuperstratemodality.This distinctionisespeciallyimportantfororganicsolarcells,asthesputteringdeposition processusedforITOcanbedestructivetopre-depositedorganiclayers,andtherefore thesuperstratemodalityisthestandardwhereinorganiclayersaredepositedatopa pre-sputteredITOelectrode. Figure6-5.Substrateandsuperstratedown-conversiondevicegeometries. Thereareanumberofwaysinwhichadown-conversionLELcanbematedtoan organicsolarcell.ThesimplestcaseistoplaceaLELdirectlyontopofasubstrate stack,asshowninFigure6-5a.However,thisrequiresthatatransparentelectrode bedepositedontopoftheorganiclayerstackandthereforerequirestheuseof alternativeelectrodesthedepositionofwhichdoesnotdamageorganicmaterial 173 Thesuperstratestructurewithtransparentelectrodebetweenthesubstrateandorganic layerscanbeinsteadbedepositedontopofthetheLEL,asshowninFigure6-5b.In 147

PAGE 148

thiscase,ITOcanpotentiallybeusedastheopticalcharacteristicsoftheorganicLEL arelesssusceptibletodegradationbyITOsputteringthantheelectroniccharacteristics oftheactivelayers.Finally,thesimplestimplementationthatallowsthestandarduseof ITOdepositedonglassisshowninFigure6-5c.ThisstructureplacestheLELonthe othersideoftheglasssubstrateandcompletelydecouplesthedepositionoftheLEL fromdepositionoftheorganicsolarcell.However,theLELindexofrefractionmustbe equaltoorlessthanthesubstrateindexofrefractiontoavoidcreatingawaveguided modewithintheLELinsulatedfromthedevicebythesubstrate. 6.4.3SimulatedDependenceonDeviceStructure Hereweexploretheeffectofthesedevicestructuresaspredictedbytheraytracing modelforC 60 :PbPcsolarcells.Asthepreviousdiscussionintroduced,thesestructures notonlyhaveimplicationsfortherelativeeaseoffabricationofthestructure,butalso ontheeffectivenesswithwhichthestructureincoupleslightemittedfromtheLELinto theactivelayers.AlthoughtheLELonglassstructureispotentiallymoreexperimentally feasiblethanthesubstrategeometry,thetwoareopticallynearlyequivalentandso herewefocusonthecomparisonbetweenthesubstratestructureandtheITOonglass superstratestructure. Figure6-6showsthemodeledabsorptiondatafora1cm 2 C 60 :PbPcsolarcell usingasubstrateLELdevicestructure.Theprimaryabsorptionissimilartothat shownforthecontroldeviceinFigure6-4,withtheexceptionthatwenowseeastrong absorptioncomponentfromtheDCJTBLELandduetoitslteringeffectweaker absoptioninC 60 .ThesecondaryabsorptionoftheLELemissionisalsoshownforeach 148

PAGE 149

Figure6-6.RaytracingmodelresultsforC 60 :PbPcsolarcellswithsubstrateLEL n =1.7structure. materiallayer,andthesumofallsecondaryabsorptionyieldsthetotalLELemission;as programmed,thisemissionfollowsaGaussiandependencewithanemissionmaximum at l =650nmandstandarddeviationof s =20nm.ThesedataaresummarizedinTable 6-1.TheLELtransfersthelightwhichitabsorbstoPbPcwithanefciencyof50%, determinedbyitsPLyield%multipliedbythesecondaryabsorptionefciencyof PbPc%.TheLELabsorbs34%oftheincidentlightprimaryabsorption;therefore, 20%oftheincidentAM1.5spectrumfrom l =300-900nmisabsorbedbytheLEL antennaandtransferredtothePbPcdonor.Amuchsmallercomponentistransferred toC 60 ,andultimatelytheactivelayersinthisLELdevicestructureabsorb42%ofthe solarspectrumenablingamaximumphotocurrentof J SC =14.2mA/cm 2 ,whichisa17% enhancementrelativetothecontroldevicewithoutLEL. ForthesubstrateLELdevicestructure,theLELisdirectlycoupledtothetransparentelectrode.Therefore,lightdoesnottraversefarinthelateraldirectionbeforebeing absorbedasthethicknessofthedevicelayersthroughwhichLEL-emittedlighttraverses 149

PAGE 150

ismuchsmallerthanthelateraldevicesize.IntheITOonglassLELstructure,however, thepresenceofa1mmthickglasssubstrateasaseparatorbetweentheLELandsolar celldramaticallychangesthebehavioroftheLEL-emittedlight.Inthiscase,lightemitted atalargeanglewithrespecttothesubstratecantraveldistancesontheorderof1 mmbetweeneachwave-guidedreection.Therefore,thisstructurecanpotentiallyallow asignicantamountofLEL-emittedlighttobeemittedlaterallyfromthedevice. Figure6-7.Effectofdevicesizeonlateralwaveguidedlossandactivelayerabsorption intheITOonglassLELstructure.TheendpointofLEL-emittedlightis graphedontheleft,showingthestrongeffectofthedevicesizeonthe quantityoflightlaterallylost.Theactivelayerabsorptanceisplottedonthe right,showingthestrongrelationshipbetweenlateralwave-gudiedlossand depresseddeviceperformance. Figure6-7showstheeffectofdevicesizeontheabsorptionpropertiesoftheITOon glassLELstructure.Forverysmall2mmby2mmresearchsizeddevices,73%ofthe LEL-emittedlightisemittedlaterallyfromtheedgeofthedevice.Thiseffectisreduced withincreasingdevicesize,andwitha1mby1mdevice,therelativeenhancement achievedmatchesthatachievedbythesubstrateLELstructure.Forallthesedevices, theincidentlightisuniformlydistributedovertheentireareaofthesolarcell;therefore, 150

PAGE 151

the1mlengthscaleisnotneededtosurpassthelengthoverwhichlightlaterally traverses,butratherisneededtosufcientlyreducetheratiooftheperipherallylocated areafromwhichlighthasahighprobabilityoflateralescapetothetotalarea.The resultsofthesesimulationsarealsosummarizedinTable6-1.Inconclusion,inorderfor themorepracticallyfabricatedITOonglassstructuretoproducesignicantabsorption enhancement,itmustbefabricatedinlargearea. 6.4.4SimulatedDependenceonDonorMaterialAbsorption TherearethreemainfactorsthatdeterminetheefciencyoftheLELstructure. First,thelossbytheabsorptanceofthetransparentelectrode,non-idealPLefciency, andLELselfabsorptionallreducetheenergytransferefciency.Second,highefciency requiressynergybetweentheabsorptionoftheLELandtheactivelayers,i.e.,where theactivelayersabsorbweaklytheLELabsorbsstrongly.Andnally,efcientenergy transferrequiresstrongresonancebetweentheactivelayeraborptionandtheLEL luminescence.StrongresonanceminimizestheeffectoflossyLELandelectrode absorption,asitminimizesthenumberofpasseswave-guidedlightmusttakebefore absorptionbytheactivelayers.Inthissection,weexaminetheeffectofLELstructureon solarcellsusingdonorswithvaryingresonancewithDCJTBemission. CopperphthalocyanineCuPcisawidelyuseddonorinorganicsolarcellswith strongabsorptionintheredcharacteristicofphthalocyanines.CuPcdoesnotabsorb asfarintothenearinfraredasdoesPbPcandconsequentlythespectralresponse ofC 60 :CuPcsolarcellssufferslessfromadepressioninresponsivityintherangeof l =500nmthandoC 60 :PbPccells.However,CuPcabsorptionstronglyresonates 151

PAGE 152

withDCJTBemission.Figure6-8plotsthemodeledabsorptionpropertiesofdevices usingacopperphthalocyanineCuPcdonorwithafullereneacceptor.Asalsolistedin Table6-2,CuPcinasubstrateLELstructureabsorbs57.2%oftheLEL-emittedlight, slightlygreaterthan55.5%absorbedinthesamestructurewithPbPc.Thisstructure enhancesabsorptionrelativetothecontroldevice25%. Squarainesareaclassofsmallmoleculethatexhibitverystrongabsorptioninthin lmandhavebeenusedasdonorsinorganicsolarcells 48,174 .Wang etal. fabricated solarcellsusinga6.5nmthick,4-bis[4-N,N-diisobutylamino-2,6-dihydroxyphenyl] squarainedonorthatproduceapowerconversionefciencyof3.1% 48 .Thismaterial exhibitsnearidealabsorptionforuseinanLELstructure,givenitsstrongabsorption from=600nm.WeexplorethedevicestructureofWang etal. hereasastandard controldeviceaswellasinasubstrateLELstructure;themodeledabsorptiondata isplottedinFigure6-9andlistedinTable6-2.Thesquarainedonorabsorbsan extraordinary67.3%oftheLEL-emittedlight,andthisLELstructureenhancesactive layerabsorptionby27%,correspondingtomaximumshortcircuitcurrentdensities of J SC =10.5mA/cm 2 and J SC =13.5mA/cm 2 ,respectively.Usingtherelative enhancementfactor,wewouldpredictthatthestructureofWang etal. wouldproduce PCE=3.9%,27%greaterthanthecontroldevicewithoutLEL. 6.5ConclusionsandFutureWork Inthischapter,weproposedanewadvanceddevicearchitectureconceptthat usesauorescentantennalayertodown-convertandre-directparttheincidentAM1.5 solarspectrumontotheactivesolarcell.Thedown-conversionprocessisexecuted 152

PAGE 153

Figure6-8.RaytracingmodelresultsforCuPc:C 60 solarcells:athetracedabsorption propertiesofacontrolC 60 nm:CuPcnmdevice,bthoseforthe samedevicewithDCJTBLELsubstratestructure,cthesecondary absorptionpropertiesoftheLEL-emittedlightfromb,anddthepredicted absorptancespectraforthecontroldeviceblackandactivelmsred,and theLELstructuredeviceblueandactivelmsgreen. byaluminescentenhancementlayerLELthatmustefcientlyabsorbthatmaybe otherwiseinefcientlyabsorbedbytheactivelayersALsinthesolarcell.Consisting ofauorescentsmallmoleculeDCJTBdopedintoahostmaterialeitherAlQ 3 or PMMA,theLELisotropicallyre-emitslightwithacharacteristicStokesshift.Someof 153

PAGE 154

Figure6-9.Raytracingmodelresultsforsquaraine:C 60 solarcells:athetraced absorptionpropertiesofacontrolC 60 nm:squaraine.5nmdevice,b thoseforthesamedevicewithDCJTBLELsubstratestructure,cthe secondaryabsorptionpropertiesoftheLEL-emittedlightfromb,andd thepredictedabsorptancespectraforthecontroldeviceblackandactive lmsred,andtheLELstructuredeviceblueandactivelmsgreen. 154

PAGE 155

Table6-2.ModeledabsorptiondataforsolarcellsusingCuPcandsquarainedonors. Theabsorptance, A ,theverticalloss, VL ,andthelateralloss, LL ,arelisted foreachdevicelayer.Primaryandsecondaryabsorptionandlossesare markedwiththesubscripts0and LEL ,respectively.Thetotaleffective absorptanceoftheactivelayersislistedwithanenhancementfactorrelative tothecontrolabsorptance. CuPcSquaraine ControlSubstrateLELControlSubstrateLEL DonorCuPcCuPcSquaraineSquaraine AcceptorC 60 C 60 C 60 C 60 n LEL -1.7-1.7 DeviceL&W1cm1cm1cm1cm A 0 ,glass0%0%0%0% A 0 ,acceptor10%1%11%1% A 0 ,donor15%12%21%18% A 0 ,LEL-33%-33% A 0 ,ITO9%5%9%5% A 0 ,Ag5%3%5%3% VL 0 61%46%55%41% PLyield,LEL-90%90%90% A LEL ,acceptor-2%-1% A LEL ,donor-57%-67% A LEL ,LEL-5%-5% A LEL ,ITO-18%-14% A LEL ,Ag-3%-1% VL LEL -13%-11% LL LEL -1%-1% A D )]TJ/F58 8.9664 Tf 6.966 0 Td [(A 25%31%31%40% Enhancement0%25%0%27% thislightislost,butmostiscoupledintotheALswhereitsabsorptionisdependenton theresonancebetweenLELemissionandALabsorption.Thisconceptcanleadtoa greateramountoftheincidentsolarspectrumultimatelyabsorbedbytheALsduetoa combinationoftwofactors:amoreefcientabsorptionbytheALsoftheLELemission spectrumthanthedown-convertedincidentlightandbthere-directionofalarge 155

PAGE 156

fractionofthere-emittedlightintowaveguidingmodesthatcanundergomultiplepasses throughtheALs. Anopticalray-tracingsimulationwasdesignedtotesttheviabilityofthisconcept asappliedtostandardorganicsolarcells.Usingthissimulator,weshowthattheuseof anLELwithnearunityabsorptionbelowitsabsorptioncutoffofapproximately l =600 nmandaPLyieldof90%canleadtoanrelativeabsorptionenhancementintheALs ofupto17%,25%,and27%forC 60 :PbPc,C 60 :CuPc,andC 60 :squarainesolarcells, respectively.Theserelativeabsorptionenhancementsaccountforsizableincreasesin thetotalabsorbedphotonux,amountingto6%,6%,and9%additionalabsorptionof thetotalAM1.5photonuxfrom l =300nm.BecausetheLELopticalstructure isplacedontheoutsideofthesedevicesanddoesnotinuencethephoto-electrical operationofsuchdevices,weexpectthattheseabsorptionenhancementswilltranslate intosimilarenhancementtothetotalphotocurrentgeneratedbysuchdevices,and thereforeconcludethatthisstructurecantheoreticallyenhancepowerconversion efciencybyupto27%. Forfuturework,wemayattempttoreplicatethesecomputationalresultsinexperimentalphotovoltaicdevices.Asastart,wehaveshownthatamoreexperimentally practicalLELlayerdepositedfromsolutioninahostofPMMAcanreplicatethe enhancedred-shiftandreducedselfabsorptionrealizableusinganAlQ 3 host 165 .To replicatetheseresultsexperimentally,however,itwillbecriticaltodemonstratethata PLyieldof90%ispossibleusingsuchanLELstructure.Furthermore,althoughthe modelprovidesarigoroustreatmenttothebehavioroflightwithinthedevicefromthe perspectiveofrayoptics,thistreatmentisnotentirelyvalidforthemateriallayersin 156

PAGE 157

thesedeviceswhicharemuchthinnerthanthewavelengthofvisibleandinfraredlight; andfurthermorethetruebehaviorwillbestronglyaffectedbywaveinterferencepatterns duetothepresenceofareectingelectrode.Therefore,weconcludethatthisconcept showspromisefortheimprovementoforganicsolarcellperformance,howevermore workisneededtodeterminethetruelevelofpotentialenhancement. 157

PAGE 158

CHAPTER7 SPRAYDEPOSITEDPOLYMERSOLARCELLS:EFFECTOFSOLUTION PROPERTIES 7.1Introduction 7.1.1Motivation PolymersolarcellsPSCsareapromisingenergytechnologybecausetheycan bemadeusingroomtemperaturesolution-basedprocessingmethodsonexiblesubstrates.Suchprocessingiscompatiblewithroll-to-rollR2Rlargescalemanufacture, whichmayallowPSCstobedeployedasalowcostrenewableenergysource 80 Researchersinacademiaandindustryhaveinventednewmaterialsanddevice structures,leadingtheeldtowardshigherefciencies.Consequently,state-of-the-art laboratorycellssmall,non-R2RhavenowreachedpowerconversionefciencyPCE surpassing8% 11 .However,mostsolution-processeddevicesmadeonthelaboratory scaleusethespincoatingmethodtodeposittheactivepolymersemiconductor,andthis processisnotcompatiblewithlarge-scaleR2Rmanufacture.Organicsolarcellshave beenproducedatlargescaleusingentirelyR2Rprocessesusingthepolymer-fullerene semiconductorsystemP3HT:PCBMandothers,butmoreworkisneededtoimprove theseprocessesasthePCEispresentlylimitedtoabout2%ascomparedtotheabout 4%PCEusingspincoatingwiththeP3HT:PCBMmaterialsystem 175 ItisthegoalforworkpresentedinthischaptertodevelopnewprocessingtechnologyonalabscalethatiscompatiblewithandeasilyscaledtoR2Rmanufacturingof organicsolarcells.Thereareavarietyofsolution-basedprintingandcoatingtechniques thataresuccessfullyemployedinthemanufacturingsector,someofwhichhavebeen 158

PAGE 159

exploredforuseindepositingorganicsolarcells 81 .Inthisworkwechoosetoexplore thespraycoatingmethodforthefollowingreasons: asanon-contactmethod,itminimizescontaminationissuesinsensitivesemiconductormaterials becausethenozzleisdecoupledfromthesubstrate,itpotentiallyallowsdeposition onnon-planargeometries theprocesscanbescaledfromonestationarynozzledepositingoveracentimeter lengthscaleinalabtotensorhundredsofnozzlesmountedonameterwidth scalewiththesubstratemovingrapidlyunderneathallowingkilometerlengthscale R2Rmanufacture recentliteratureshowspromisingresultsforspray-depositedorganicsolarcells 7.1.2DenitionofProblem PSCsaremadeupofanactivepolymersemiconductorlayersandwichedbetween ananodeandacathode,oneofwhichisreectiveandtheothertransparent.To produceacompletelyR2Rprocessedcell,allthreelayersmustbedepositedin compatibleprocesses.Allthreelayershavebeensuccessfullyfabricatedusingspray coating,withsomereportsfocusingonuseofsprayforonelayeronly,whileothers attemptingtoproducealllayersusingspraycoating 176183 .Also,spraycoatinghasbeen usedtofabricateorganicphotodetectorsandtransistors 184186 .Inthisproject,wefocus solelyonspraycoatingoftheactivepolymerlayerinPSCs. Thereexistvaryingspraynozzletechnologies,andseveralhavebeenemployed forthefabricationofpolymeractivelayersinPSCsincludingairbrushpressure spray 179,182,183 ,ultrasonic 176,177 ,andelectrohydrodynamic 181 .Inthisproject,we useultrasonicsprayforthefollowingreasons: 159

PAGE 160

ultrasonicsprayusesanultrasonicallypowerednozzletipwhichconsequently operatesinaself-cleaningandclog-freemanner,minimizingsystemdowntime becauseultrasonicspraydoesnotrequirehighpressureowtosheer-atomize liquid,itresultsinlessoverspraythanothermethodsi.e.,theamountofmaterial andsolventthatreectsoffthesubstrateduetomomentum whileinthecaseofpressurespraythedropletcharacteristicsareinuencedbythe owrate,ultrasonicspraydecouplesowcontrolfromdropletcharacteristics ultrasonicspraycanoperateefcientlyatverylowowrates m L/minwithoutloss insprayquality,allowingformoreefcientuseofmaterialforsmallscalelabspray deposition 7.2TheoryandHypothesis 7.2.1FloodSprayandMulti-PassSpray Spraydeposition,asrelevanttoorganicsemiconductors,canbedividedbetween twotypes:oodsprayandmulti-passspray.Inoodsprayorsingle-passspray, enoughliquidissuppliedatthesurfaceinonesinglepassofthenozzleheadtoproduce acontinuouslmofliquid.Multi-passspraysuppliesliquidtothesurfaceatarateslow enoughtoisolatedropletsandalloweachdroplettodrywithoutaggregatingwithother droplets,andrequiresmanypassesofthespraynozzleoverthesubstrate.Therefore, inoodspraythedropletcharacterisminimizedinthenaldriedlmbecausethe dropletscombinebeforedrying,whilethisdropletcharacterdenesthemorphologyof thedriedmulti-passsprayedlm. Simplesprayprocessesasappliedtopolymersolarcellshaveresultedpreviously inveryroughandinhomogeneouslmqualitywhichissubstantiallydifferentthanthe smoothanduniformlmsroutinelystudiedasproducedbythespincoatingmethod. Naturally,themorphologyhasastrongeffectonsolarcelldeviceperformance,andfor 160

PAGE 161

instanceGreenetal.showedthatamongP3HT:PCBMlmssprayedfromavariety ofsolvents,thosesprayedfromchlorobenzeneCBwerethesmoothestandwhen incorporatedintodevicesresultedinthehighestdevicePCEof1.6% 187 .P3HT:PCBM lmssprayedfrompureCBhaveresultedinavarietyofefciencies,thevariance ofwhichisduetoavarietyofprocessingdifferences,butitappearsthattherelative efcienciesyetachievedarestronglytiedtothelmsmoothness.Forinstance,inorder toachieve3.2%PCE,Steireretal.employedanextraspraystepwithpuresolventand nosolutetosmooththetopsurfaceofthelms 177 .Thesmoothnessoflmsistiedtoa varietyofsprayprocessingparametersaswellasthesolutionchemistry,butitappears mostaffectedbytheuseofeithersinglepassoroodspray. Floodspraycanresultinmoreuniformlmqualityduetoareductioninsurface roughnesswithlessdropletcharacter,andthereforethismethodgenerallywouldbe preferredfortheproductionoforganicphotovoltaicactivelayerlms.Infact,whilemost oftheworksofarpublishedusesamulti-passapproach,Girottoetal.havejustrecently demonstratedthehighestPCEforasprayedpolymercellP3HT:PCBM,PCE=3.75% usingoodspray 188 .Essentially,wecanthinkofoodsprayasthebestmethodto mimictheuniformityofpolymerlmsachievedusingspincoating. However,inthiswork,wespecicallydevelopmulti-passsprayforitspotential advantagesasappliedtoformingadvanceddevicestructures.Multi-passspray, becauseitdepositsalmonedropletatatime,isanadditiveprocessthatpotentially maybeleveragedforproductionofmulti-layerdevicestructures,gradientbulkmixture structure,orperhapssomeotherinterestingstructures.Furthermore,webelieveandwill describelaterherethatthemulti-passspraymethod,whensufcientlyoptimized,can 161

PAGE 162

enableareductionofdefectdensityanddarkcurrentforpolymerthinlms,whichisvery importantforlargeareasofverythinlms.Inordertodeveloptheprocessofmulti-pass spray,wesetoutinthisworktoopposetheformationofveryroughmorphologyby alteringthesolutionchemistryusingchemicaladditives.Thefollowingthreesections describeourmotivationandhypothesisfortheuseofalkaneadditivesinthisregard. 7.2.2Multi-PassSprayandtheCoffeeRingEffect Ashasbeendescribedintheprevioussection,multi-passsprayproduceslms whosemacroscopicmorphologyisdominatedbyadropletcharacter.Forlmsofthis nature,theelectronicfunctionalitywillbedeterminedinlargepartbythelargescale inhomogeneitiesthesedropletsproduce.Ultimately,inanymulti-passspraydeposited lmtherewillremaindropletboundarieswhichmaynotonlyaffecttheelectronic transportthroughtheboundaries,butalsomayproducesignicantdeviceshuntingdue totheresultingsurfaceroughness.So,itisimportanttominimizetheroughnessatthese boundaries. Furthermore,eachdrieddropletcanproduceavarietyofinternaldeposition patternsinthenallmdependentonthesolutionproperties.Theuiddynamicsacting duringevaporationcanproducedepositionpatternsrangingfromapointdeposition patterninthecenterofthedroplettoaringdepositionpatternaroundtheedge,aswell asmoreuniformdepositionpatternssomewherebetweenthesetwoextremes. Boththeinter-dropletandintra-dropletdepositionuniformityarestronglyaffected bywhatiscommonlyreferredtoasthecoffeeringeffect.Coffeeringdepositionof colloidalsuspensionsorsolvent-solutesolutionsoccurwhenadropletedgecontact 162

PAGE 163

linebecomespinnedtothesurfaceandpreferentialevaporationoccursatthedroplet edge,asshowninFigure7-1.Thepinningofthedropletisinitiallycausedbysurface imperfectionsorcontamination,andthecurvatureofanynon-sphericaldropletis higherattheedgethantowardsthecenterandthereforeallowsahigherprobabilityfor moleculesintheliquidphasetoescapeintothegasphase.Asahigherproportionof liquidisevaporatedfromtheedge,liquidandsuspendedorsolvatedmaterialisdrawn outtotheedgesconstitutingauidowpromotedbycapillaryforce.Asthisprocess unfolds,theedgeofthedropletbecomesmoresecurelypinnedastheimperfections becomeaugmentedbysolutedeposition,andultimatelythemajorityofmaterialisleft depositedattheedge 189 Figure7-1.Schematicshowingthedropletdryingdynamicsthatproducethecoffeering effect. Therearewaystocounteracttheformationofcoffeeringdropletformations.First, dropletpinningcanbeavoidedbyeliminatingimperfectionsorcontaminationonthe substratesurface,orbyalteringthesurfacechemistry.Next,lowervolatilityuidsmay beusedorgasowconstrictedinordertoreducetheoverallevaporationrateofthe solutioninordertolimitthedifferentialevaporationratebetweenthecenterandedge 163

PAGE 164

ofthedroplettoalevelthatdoesnotgiverisetosignicantcapillaryforce 190 .However, forthepurposesofmulti-passsprayinwhichmanydiffuselayersmustbedepositedon topofeachotheronlyafterthepreviouslayerhasdried,reducingtheevaporationrateis counterproductivetoefcientprocessing.Infact,ifpossiblewewouldideallyapplyheat tothesubstratetoincreasetherateofevaporationandallowmorerapidprocessing, andasSoltmanetal.conrm,thisenhancesthecoffeeringeffectbyincreasingthe evaporativeuxdifferential 191 .Furthermore,thesurfacechemistryisdeterminedby thechoiceofelectrodematerialontowhichthepolymersemiconductorisdeposited, andsurfacetreatmentsaretypicallyinsulatingandthereforenotcompatiblewitha photovoltaicdevice.Therefore,wemustndanothersolutiontothisproblem. 7.2.3MarangoniFluidFlowinDroplets Althoughweareconnedtoaparticularsubstratechemistrybypolymersolarcell devicestructure,wedohavecontroloverthesolutionchemistryandmayinuenceits abilitytowetoutwardandtherebyescapepinningforces.Furthermore,wemayalter theuidowcurrentswithinthedropletinordertocounteractthecapillaryforcesthat drawsolutetotheedgeandproducetheringeffect.Asitturnsout,byintroducingan additionalchemicaltothesolvent-solutesolutionthathasbothlowersurfacetensionas wellashighervolatilitythantheprimarysolvent,bothoftheseeffectscanbeachieved simultaneously. Marangoniowscanariseindropletsbasedonagradientinsurfacetension.A thermalMarangonieffectisseenwhentherearisesathermalgradientacrossthe droplet.Athermalgradientcouldbeexternallyapplied,butmayalsoarisedueto 164

PAGE 165

evaporativecoolingacrossthedroplet.Forasphericaldropletwithuniformevaporative uxacrossthesurface,theentiredropletcoolsatthesamerateduetoevaporation,but theedgeisinclosercontacttothesubstratethermalsink,leadingthisregiontobeof highertemperatureandlowersurfacetension.Ontheotherhand,inanon-spherical dropletwithhigherevaporativeuxattheedge,thethermalgradientandopposing surfacetensiongradientpointintheoppositedirection.Ineithercase,sheeruid owmaycarrysoluteoppositethesurfacetensiongradient.Infact,HuandLarson demonstratedthatthiseffectcancounteractthecoffeeringeffectinpureoctane solutionswithsuspendedPMMAparticles,ultimatelyleadingtoadepositionofparticles mostlyatthecenterpointofthedroplet 192 Figure7-2.Schematicshowingmulti-componentMarangonidropletuidowto counteractthecoffeeringeffectandproduceuniformsolutedeposition. Inmulti-componentuids,asolutalMarangonieffectcantakeplacewherein convectiveowoccursduetoconcentrationgradients.Again,thesegradientscanarise duetopreferentialevaporation,butinthiscaseduetoonespeciesbeingofhigher volatilitythantheother.Consider,forinstance,adropletatrestonaatsurfacethat consistsofanidealmixtureoftwouids,oneofwhichisofhighervolatilityandlower 165

PAGE 166

surfacetensionwithaconcentrationof f ,asshowninFigure7-2.Thehighervolatility speciesevaporatesmorerapidlyfromtheperimeter,settingupaconcentrationand surfacetensiongradient,andbymassconservationaconvectioncurrentwillarisewith thisspeciesmovingoutwardtoreplacetheevaporatedmaterial.ThissolutalMarangoni effectcounteractsthethermaleffect,andAharonandShawshowforthecaseof heptaneandhexadecanethatthiscanleadtodropletinstabilityifthedropletradiusis greaterthanacriticalradiuscharacteristicanddependentonthesystemproperties 193 Fantonetal.furtherdemonstratethatadropletwithdifferentialevaporationrateand surfacetensionwillowoutwardwithacharacteristicMarangonivelocity, v c v 2 c f = A f )]TJ/F60 11.9552 Tf 10.949 0 Td [(f 2 h d s d f where A istheevaporationvelocityofthevolatilecomponent, h f isthedynamic viscosity,and s f isthesurfacetension 194 .ThisMarangonivelocityisameasureof theforcewithwhichthismixtureofuidscanwetasubstrateoutward,andultimately overcomepinningandtheundesirablecoffeeringeffect.Thisvelocityisincreasedunder fourconditions:ahighvolatilityofthemorevolatilespecies,bmixtureconcentration fractionapproaching1:1,cincreaseddifferentialsurfacetension,i.e.,increased differencebetweensurfacetensionofthetwocomponents,andddecreasedsolution viscosity.Thefollowingsectiondiscusseshowtheseprinciplesguideouruseofalkane additivesforuseinspraydepositionofpolymersolarcells. 7.2.4AlkaneAdditivesforSprayDeposition Astheprevioussectionexplained,thecoffeeringeffectcanpotentiallybeovercome bycreatingamulti-componentsolutionwithdifferentialsurfacetensionandevaporation 166

PAGE 167

rate.Alkanesareidealforthisfunction,astheyhavesomeofthelowestsurface tensionsandviscositiesofthecommonlyavailableandknownchemicals.Furthermore, theevaporationratecanbeeasilyvariedtooptimalvalues,asevaporationrateis stronglyaffectedbyalkanemolecularsize.Therefore,thisworkfocusesontheuse ofalkanesaddedtoaprimarysolvent-solutesystemtoimprovelmmorphologyand ultimatelydeviceperformanceinsprayedpolymersolarcells.SeeTable7-1fora summaryoftherelevantchemicalpropertiesofalkanesusedinthisworkaswellas somesolventsthathavetypicallybeenusedforpolymerphotovoltaics.Figure7-3also depictsthechemicalstructuresofthesespecies. Loweringtheeffectivesurfacetensionandviscosityofthesprayedsolutionby addingalkanesmayalsopromotebetteruniformityinthelmbyreducingthesize oftheatomizeddroplets.Dropletdiameterofsprayedsolutionshasbeenstudied foritsapplicationinseveralindustriesincludingforinstancepesticideapplication, foodprocessing,andfuelinjection.Dropletdiameterhasbeenfoundtovarygreatly withexperimentalconditions,andismoststronglydependentonthenozzletypeand setup 197 .Forthiswork,wechosetouseultrasonicsprayforitsindustriallyrelevant lowersystemdowntime,andmaximizedthefrequencyofthenozzletoachievethe minimaldropletsize.However,thedropletsizemaybefurtherreducedbyreducing surfacetensionandviscosityassomestudiessuggest 198,199 .Theexperimental evidenceforthisisweak,butifweconsiderthebasicfunctionofsurfacetensionwe canexpectthistogenerallyholdtrue.Consider,forinstance,adropletsuspended fromalledcapillarytubeofdiameter d .Gravityactsonthedroplet,andthemaximum dropletmassthatcanbeheldupbytheverticalcomponentofsurfacetensionforces 167

PAGE 168

Table7-1.Chemicalpropertiesrelevanttothespraydropletformation processforsolventsandpotentialadditivesrelevanttothe depositionofpolymersolarcells a ChemicalViscosity mPas Surface tension mN = m Vapor pressure mmHg REL b ppm Toluene0.5627.721150 Chlorobenzene0.7533.0975 c o-Dichlorobenzene1.3235.4150 Chloroform0.5426.71602Ca d Mesitylene0.6627.6225 Cyclohexane0.8924.278300 n-Heptane0.3919.74085 iso-Octane0.8818.841300 n-Octane0.5121.11075 a ValuestakenfromRef. 195 andRef. 196 ;measuredat25 C b Recommendedexposurelevelmaximum,byNIOSHforoccupationalsafety c NIOSHrequestedthatOSHAreevaluatethisnumber d Potentiallycarcinogenic follows mg = p d s sin a where s issurfacetensionand a istheangleofthesurfacetensionforcewithrespectto thetube-dropletinterface.Thestabilityofanincreasingdropletmassisproportionalto itssurfacetension. Theuseofchemicaladditivestoaprimarysolventmixturealsotswellintothe constraintsinherenttopolymersemiconductorsolutionprocessing.Whileorganic 168

PAGE 169

b b b b bb "" b b b b bb "" Cl b b b b bb "" Cl Cl b b b b bb "" toluenechlorobenzene1,2-dichlorobenzenemesitylene S S P P S S P P " " " cyclohexanen-heptaneiso-octanen-octane Figure7-3.Chemicalstructureofsolventstopandalkanes bottomusedinsprayedpolymersolarcells semiconductorsaretoutedfortheirsolutionprocessingcompatibility,theytypicallydemandhighlytoxicpolarandoraromaticsolventssuchasthechloroformorchlorinated benzenestobeefcientlydissolved.Furthermore,itiswellknownthattheperformance oforganicsemiconductordevicesishighlydependentonthechoiceofsolventand itsinuenceonthenanoscalemorphologyofthedriedlm 200,201 .Therefore,when solutioncastingpolymersemiconductorlmsweareconstrainedtotheuseofjust afewsolvents,andwethereforecannotgreatlyaffectthesolutionchemistryand dropletdynamicsbysolelyvaryingthiscomponent.Theuseofadiluentadditive,or 169

PAGE 170

potentiallyanadjuvantadditive 1 ,mayprovidetheimprovedlmformationweseek withoutsignicantlyalteringtheimportantinteractionbetweentheprimarysolventand thesemiconductor.Ontheotherhand,theuseofnon-solvatingalkanesmaybring soluteoutofsolutionanddisrupttheinteractionbetweentheprimarysolventandthe semiconductor.ThedevelopmentofthesespraysolutionsisdiscussedinSection7.4. 7.3Experiment 7.3.1CustomSpraySystem:ConstructionandOptimization Acustomspraysystemwasdesignedandbuilttousefortheseexperiments. Thesystemwasdesignedwithasetofsimplegoals:enablethespraydepositionof semiconductorsolutionsona1inchsquaresubstrateofadjustabletemperatureina controlledandrepeatableprocesswithinacontrolledatmosphere.Thesystemuses a130kHzultrasonicspraynozzle.Theadvantagesofultrasonicsprayareoutlinedin Section7.1.2,and130kHzisthehighestfrequencycommerciallyavailable,allowingfor thesmallestdropletsize d < 20 m m forH 2 O.Acarriergasnozzleassemblyisusedto redirectthesprayuxtowardsthesubstrate.Asyringepumpisusedtopumpsolutionat asetowratethroughthenozzle.Ahotplateisusedtoheadthesubstratestage,which isbuiltintoanaluminumframethatxesallcomponents.Theultrasonicspraynozzle andcarriergasnozzlearemountedonanassemblythatactuatesacrossthesubstrate 1 Adiluentisachemicaladditivethathasminimaleffectonthesolutionexcept throughdilution,whileanadjuvantadditivemodiestheeffectofanotherspecieswithin themixtureorofthemixtureasawhole 170

PAGE 171

byamotorandballscrewsystemtoallowtranslationinatwo-dimensionalhorizontal plane.Table7-2liststhevariouscomponentsandmaterialsinvolvedinthesystem. Table7-2.Spraysystemcomponentsandsuppliers. ComponentSupplier&ModelNotes UltrasonicnozzleSonaer130K50TNarrowspraypattern,highest frequencyavailableforsmallest dropletsize Ultrasonicpower source Sonaerultrasonic generator Suppliescontrolledfrequencyand amplitudepowertonozzlehead SyringepumpNewEraPump SystemsNE-300 Feedssolutionthroughspray nozzle;controlsowrate HotplateFisherScientic 11-200-49SH Suppliesheattothermallycoupled substratestage Framing80/20extruded aluminum Framessystemincompactshape totintostandard15inchor 390mminnerdiameterglovebox antechambers Ballscrew& linearbearing Scavengedfrom PhillipsAnalytical ellipsometer Translatesmotorrotational motionintolinearmotionofspray assembly DCmotorand controlswitch RadioShack components Driveslinearmotion;manualmotor switchmovesnozzleassembly acrosssubstrateandback TubingHamilton19gauge teontubingwithleur assembly Deliversuidfromsyringeinto spraynozzle SyringesB&Dpolypropylene1 &3mL Ifsyringepumpisprogrammedfor innerdiameterofsyringe,delivers preciseowratetonozzle ThespraysystemisdepictedschematicallyinFigure7-4.Theschematicshows thegeometryofthespraynozzleandcarriergasnozzlesrelativetothesubstratestage 171

PAGE 172

below.Notpicturedistheunderlyinghotplate,thesyringepump,andtheultrasonic powersource. Figure7-4.Schematicofthespraysystem.Thenozzleassemblymovesleftandright actuatedbytheballscrew,andtheentiresystemsitsontopofahotplateto controltemperature. Overthecourseofitsuse,thesystemhasgonethroughafewiterationsofretooling inordertomorecompletelyachievethesegoals.Here,wewillbrieygooverthese iterationstodocumentthepurposeandmotivationforthevariouschangestothe system.Thesystemwasrstconstructedwiththeultrasonicnozzlemountedvertically whoseuxwasdirecteddownwardandperpendiculartothesubstrateapproximately 2cmbelow.Itwasfoundthatuctuationsintheoutputofthespraynozzleweretoo greattoproduceuniformlmsinthisgeometry.Furthermore,thesprayuxthatexits thenozzlehasverylittleforwardmomentumandiseasilydisruptedbylocalaircurrents. Therefore,acarriergassystemwasinstalledtodirecttheoutputofthespraymore forcefullytowardsthesubstrate.Thiscarriergaswasdirectedatthesubstrate,and thespraynozzlewasturnedperpendiculartothecarriergasow.Inthisway,sprayed solutionisatomizedatthespraynozzletipandmovesperpendiculartothesubstrate 172

PAGE 173

momentarilybeforebeingswepttowardsthesubstratebythecarriergaswithout anycarriergasow,dependingonthesolutionowratethisuxwilltravel5to10 cmperpendiculartothesubstratestagebeforefallingtowardsit.Thecarriergas wasinstalledwithaowmetertoallow0-20scfhowofnitrogen.Itwasfoundthat about10scfhworkswellwiththesystemgeometry.Thissetuphighlightsoneof theadvantagesofultrasonicspray-theowofthesolutionisdecoupledfromthe pressurewithwhichitisthrownatthesubstrate.Figure7-5showstransmissionoptical microscopyimagesofP3HTlmssprayedusingthesystembeforeandafteraddinga carriergas,using5mg/mLP3HTintoluenesprayedat0.3mL/minona20 Csubstrate with10scfhgasow. Next,thesystemwasreconguredtouseamotorandballscrewtoactuatethe nozzleassembly.Theactuationisdonebymanuallyippingaswitchbackandforth betweenleft,right,andneutralpositions.Whilethetimingisstilldonemanuallyin thisconguration,thespeedofeachthrowisconstantanddeterminedbythevoltage suppliedtotheDCmotorcurrently12V;themotorcansafelyreceiveupto24V. Also,duringthisrecongurationamorerobustthreenozzlecarriergasassemblywas installed,onenozzleactingdirectlyperpendicularlytothesprayux,andtheothertwo surroundingeithersideandtiltedinapproximately30degreestocontroltheuxspread. Inaddition,thespraynozzletosubstratedistancewasincreasedfrom2cmto5cm.At 2cm,theuxspreadwasnotwideenoughtoprovidetoleranceincoveringall4device locationsonastandardsubstrateelectrodeconguration.At5cm,theuxcoatsan entire1inchsquaresubstratemoreevenly,andthedevicetodevicevariationoneach substrateismuchlower. 173

PAGE 174

Figure7-5.Theeffectofcarriergasonlmuniformity.Opticalmicroscopyimage showingP3HTlmsprayedfromtolueneusinginitialsprayconguration left,andafteraddingacarriergasnozzleright.Scalebarrepresents1 mm. Theworkdescribedlaterinthischapterwasdonewiththeabovedescribed geometry.Analimprovementwasmadetothesystemtoallow2axisactuationby addinganadditionalballscrewassemblyinordertoallowmorecontrolleddeposition overlargerareaforfuturedirectionsofthesystem.However,noneoftheworkdescribed inthischapterwasdoneusingthe2-axisconguration. Thesystemasbuilthasanumberofexperimentalparametersthatcanbeadjusted. Thenozzle-substratedistance,orthethrowdistance,isakeyparameterofthesystem. Thefurtherthesubstrateisfromthenozzle,themoredropletsspreadoutfromone anotherandevaporate.Spreadingcanimproveuniformityoverlargerdistances,butcan increasetheamountofwastedspray.Excessiveevaporationcanallowsolidparticulates toformbeforedropletsarriveatthegrowinglm.Therefore,anoptimumcompromise isnecessaryandforthissystemandforthesolventsusedthisdistancewasfoundto be5-8cm.Otherfactorsthataffecttheevaporationofthesolventalsoplayprominent roles,suchasthesubstratetemperature,thesolutionowrate,thenozzlethrowrate, 174

PAGE 175

andthenozzleresttime.Theseparameterscometogethertodeterminetherateat whichsolutionisdepositedonthesubstrate,theintegratedtimeoverwhichsolutionis sprayedinonepass,therateofevaporation,andthetimewaitedtoallowsolutiontodry beforedepositinganotherlayerofdroplets.Inmulti-passspray,itisessentialtoallow thedropletstodrybeforedepositingadditionaldropletstoavoidnon-uniformpuddling ofsolution.Theadjustableparametersmustbeadjustedtoaccountforsolutionswith differentevaporationrates,andaresummarizedinTable7-3. Table7-3.Spraysystemadjustableparameters.Specicsetpointvaluesarealsolisted foreachparameter,boththeachievablerangeofsetpointvalueaswellasthe optimizedvalueconsideredbestpracticesforthissystem. ParametersAchievable setpointrange Bestpracticessetpoint Nozzle-substratedistance2-20cm5cm Nozzlefrequency130kHz130kHz Nozzlepower1-5W3.2Wfor0.2mL/min solutionow; solution ow Nozzlethrowrate10cm/s10cm/s Throwresttime0 )]TJ/F69 11.9552 Tf 10.95 0 Td [( Sufcientfordroplet drying,0-1s Carriergasowrate0.2-20scfh10scfh Substratetemperature20-100 C50 C Solutionconcentration0.1-20mg/mL2mg/mL Solutionowrate0.01-10mL/min0.2mL/min 7.3.2FilmandDeviceFabrication Filmswerefabricatedonglasssubstratesforthepurposeofcharacterizingthe uniformity,thickness,andabsorptionoflmcoatingprocesses.Thespraysystemwas 175

PAGE 176

usedtocoatmulti-passlmsontosubstratesusingthebestpracticesparametersettings listedinTable7-3.Filmsofthicknessusefulfordeviceswerefabricatedbyusingatotal numberofspraycoatpassesintherangeof100to250.Filmswerealsospuncoat forcomparisonbyusingaLaurell150mmspincoater.Bothdepositionmethodswere performedincontrolledenvironmentwithlessthan0.1ppmwaterandunknownoxygen levelsprovidedbyanMBraunglovebox.Someoftheearlyspraysystemstudieswere alsoperformedinambientconditionsinafumehoodandalsoinaowthroughnitrogen boxwithawaterlevelofapproximately100ppm. ForlmsanddevicesusingP3HT:PCBM,solutionswerepreparedwitha1.0:0.8 weightratioofP3HT:PCBMinvarioussolvents,typicallytolueneorCB.Solutionsused forspincoatingwerepreparedanddepositedwithconcentrationof12,24,and36 mg/mL.Solutionsusedforspraycoatingwerepreparedwithconcentrationtypicallyof 12mg/mLandthendilutedwitheitherpuresolventoramixtureofsolventandalkane additivestotheirnalsolutionconcentration.Concentrationsof0.5to5mg/mLwere tested,and2mg/mLtypicallygavethebestresultsandwasthereafterusedasthe standarddepositionconcentration. DeviceswerefabricatedontocommerciallypatternedindiumtinoxideITO substrateswithsheetresistanceof15 /square.TheITOwasmodiedbyaMoO 3 interfaciallayerdepositedbyvacuumthermalevaporationVTEwhichhasbeen showntoincreasetheworkfunctionoftheanodeandimprovesolarcellperformance byreducingthereversebiasdiodesaturationcurrentandincreasingthellfactor 104 MoO 3 coatingthicknessesfrom3-5nmwerefoundtoprovideequalbenettodevice 176

PAGE 177

performance,and5nmwasthestandardthicknessusedthereafter.Next,polymerlms weredepositedaspreviouslydescribed. Variousannealingconditionsweretestedforthesedevices,includingaprocess referredtoaslmannealingwhereinthelmswereannealedafterpolymerdeposition butbeforecathodedeposition,andaprocessreferredtoasdeviceannealingwherein thelmswereallowedtodryatroomtemperatureafterpolymerdeposition,cathodes deposited,andthecompleteddeviceannealedpostcathode-deposition.Ineither case,thelmsweretransferredfromastandalonegloveboxintoaVTEchamber integratedgloveboxandsubsequentlyintotheVTEchamberwhereina100nmthick aluminumcathodewasdepositedat0.2nm/sec.Toimprovetheenergylevelalignment ofthecathodeandLUMOofPCBM,somedevicesusea1nmthicklayerofthermally depositedLiFbetweentheactivelayerandthealuminum.Thedeviceannealprocess useda30minute150 Cannealinnitrogenatmosphere,whereasthelmanneal processwasa10minute110 Cannealfollowedbya30minuteannealatdecreasing temperatureafterthehotplatewasturnedoffandallowedtocool.Forbothanneal processes,thelmswereallowedtocoolfor10minutesbeforeremovingfrominert environmentfortesting. 7.3.3FilmAbsorptionandDeviceQuantumEfciencyCharacterization Filmabsorptionanddevicequantumefciencymeasurementswereperformedin thelaboratoryambientcentralairconditioned,22 C,35-50%relativehumidityusinga systemconsistingofaNewport66902Xe-arclamp,aNewport74100monochromator,a StanfordResearch540opticalchopper,aKeithley428currentamplier,andaStanford 177

PAGE 178

ResearchSystemsSR830lock-inamplier.Thelampandmonochromatoroutputs acontinuousmonochromaticbeamwithgratingovertonesremovedbyopticallters. Thechoppermodulatesthebeamandthelock-inampliermeasuresthemodulated photocurrentampliedbythecurrentamplieratthereferencechopperfrequency. Thissystemyieldsanaveragelightintensityofapproximately40 W = cm 2 from l =350 nmto l =800nm.Thesystemiscontrolledbyacomputerrunningin-housedata acquisitionsoftware. Forabsorptionefciencymeasurements,thesystemwasusedtomeasure thephotocurrentofacalibratedNewport818-UVSiphotodiodeinresponsetothe transmittedandreectedopticalintensitiesfromsamplesmountedwithan8 tilt tothebeamnormal.Forabsorptioncalculations,weareinterestedinisolatingthe componentofthebeamthatisabsorbedwithinthelmbyindirectmeasurementof thattransmittedthroughandreectedfromtheanalyzedlmaswellasreference glasssubstrate.Therefore,weusethefollowingformulatocalculateabsorptance,or absorptionefciency: A = I glass T )]TJ/F58 11.9552 Tf 10.95 0 Td [(I lm T )]TJ/F58 11.9552 Tf 10.95 0 Td [(I lm R I glass T whereIdenotesopticalintensity, R and T subscriptsdenotereectedandtransmitted components,respectively,and glass and lm superscriptsdenoteglassreferenceand experimentallmsamples,respectively. Thissamesystemwasalsousedtomeasuresolarcellexternalquantumefciency. Thetotalpoweronthedevicewasdenedasthetotalbeampoweratthedeviceas measuredbythecalibratedphotodiode,astheentirebeamdiameter=1mmfalls 178

PAGE 179

withinthedeviceareammby2mmafterfocusingandcollimating.Thereection lossfromthesubstratewasnotconsideredasinthecaseofabsorptionmeasurements, asthisisgenerallytreatedasadevicelossmechanismthatcanbeovercomebyantireectiontechniques.ExternalquantumefciencyEQE, h ext ,wascalculatedbythe equation h ext = #incidentphotons #extractedelectrons = I p q hv P opt where I p isthedevicephotocurrent, q istheelementarycharge, h isPlanck'sconstant, v isthefrequencyofthemonochromaticlightbeam,and P opt istheincidentopticalpower. 7.3.4ThicknessMappingUsingTransmissionOpticalMicroscopy Theinhomogeneitypresentinourspray-coatedlmsoccursonalateralscaleof10 to1000 m.Whileatomicforcemicroscopyisgenerallyapowerfultooltodeterminethe surfaceroughnessandmorphologyofpolymersemiconductorlms,theseinstruments donotprovidealargeenoughscanareatoderiveusefulinformationonthescale necessaryforthesesprayedlms.Inordertomapthethicknessandroughnessofour lmsonthescalenecessary,wedevelopedatechniquethatusestransmittanceimage datafromadigitalopticaltransmissionmicroscopetoproduceamappedthickness.The generalstrategyofthistechniqueisasfollows:ailluminateusingmonochromaticlight ofwavelength l m thatisabsorbedbythelmtobeanalyzed;bdenetheabsorption coefcientofthelm, a f ,at l m bystudyingtheabsorptionandthicknessofareference lmofthesamematerialwithauniform,well-denedthickness, t ref ;crecordimages ofthesprayedlm,referencelm,andbaresubstratesusingthemicroscope;d calculateatransmittancemapofthesprayedlmusingthesemicroscopeimages; 179

PAGE 180

andecalculateathicknessmapofthesprayedlmusingthetransmittancemapand absorptioncoefcient. Toproducemonochromaticilluminationwithinthemicroscope,wesimplyaltered thestockbroadbandhalogenilluminationsourceofthemicroscope,aMoticBA310, bylteringwithaThorlabsFB500-10bandpassopticallter.Thisproducedasample illuminationcenteredat498nmwithaFWHMof9.4nm,asmeasuredbyanOcean OpticsJazspectrometer.Itisimportantforthesecalculationsthattheabsorption coefcientofthesampledoesnotvarygreatlywithintheilluminationspectrum,so itisbesttouseasnarrowaspectrumaspossibleinaspectralregioninwhichthe absorptioncoefcientofthelmisasataspossible.ForP3HT:PCBM, l m =498works well. Wedened a f and t ref bystudyingsamplesspuncoatfromchlorobenzeneusing thespectroscopicabsorptionmethoddescribedinSection7.3.3andprolometry.We analyzedseveralspuncoatlmswiththicknessesrangingfrom t ref : lm =100nmto t ref : lm =350nmasmeasuredbyprolometryandcalculatedtheabsorptioncoefcient tobe a l =498nm f =0.0086nm )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 0.0002,or.6 0.2 10 4 cm )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 Next,werecordedimagesofsprayedlms,spuncoatlmsofknownthickness, andreferenceglassslidesusingonlythegreenlight-sensitivesensorsoftheCMOS cameraataxedintegrationtimewhileensuringthatthedynamicrangeofthesensors weremaximizedwithoutsaturation.Wecalculatedthetransmittance, T ref : glass i ,for eachpixelintheimagebydividingthetransmissionintensityrecordedbythegreensensitiveCMOSpixelsforexperimentalsamplesbytheincidentintensity,measured asthetransmittedintensitythroughatransparentglassslide.Forverythickportionsof 180

PAGE 181

sprayedlms,itwasnotpossibletoaccuratelycomparethetransmissiontotheincident illuminationataconstantintegrationtimeduetolimiteddetectorpixeldynamicrange. Forthesepixels,wecalculatedtherelativetransmittance, T ref : lm i ,ortheratioofthe transmittedintensitythroughthesprayedlmtothatthroughthereferencespuncoat lm.AllimagecalculationsweredonewithintheopensourcesoftwareImageJ. Next,wecalculatedthicknessmapsbasedonthesetransmittanceandrelative transmittancemapsbyapplyingthefollowingequationstoeachpixelintheimages: t i = )]TJ/F20 11.9552 Tf 10.485 12.427 Td [(ln T ref : glass i a m fortransmittance,and t i = t ref : lm )]TJ/F20 11.9552 Tf 12.145 8.861 Td [(ln )]TJ/F58 11.9552 Tf 5.475 -9.689 Td [(T ref : lm i a m forrelativetransmittance. Applyingtheseformulaetoeachpixelinanimageproducesanimagewitheachpixel displayingthelmthicknessofthelminthatlocation.Inordertojointhedifferent imagesintooneuniedthicknessmap,thesaturationlevelsofthetransmittancemaps weredeterminedbythehistogramdistributionofintensitiesforeachimage,andthese imageswerethenthresholdedbeloworabovethesesaturationlevels.Forinstance,the glass-referencedthicknessmapmighthaveaccuratedataasdeterminedbydynamic rangesaturationforthicknessesbelow300nm,andthespuncoatlmreferenced thicknessmapisthenusedforthicknessesgreaterthan300nm.Asimpleplugin programforImageJwaswrittentojoinimagesaccordingtothismethodology. Inordertovalidatethetechnique,andensureaccuratecalculations,eachtime thetechniquewasemployedacalibratedspuncoatsamplewasalsomeasuredand 181

PAGE 182

processedinthesamewaytocross-checkthattheprocessyieldedthesamethickness determinedbyprolometry. 7.3.5DeviceCurrent-VoltageCharacterization Devicesweretestedundera100mW/cm 2 simulatedAM1.5solarspectrumusing anOrielsolarsimulatorwitha150WXe-arclampandAM1.5lterwhoseintensitywas characterizedwithacalibratedsingle-crystallinesiliconreferencesolarcell.Thespectral responseofthereferencecellwasalteredbyuseofaKG1shortpasslterinorder tominimizethespectralmismatchfactorbetweentheP3HT:PCBMsolarcellsandthe referencecell.Thismismatchfactorwascalculatedtobe1.01,thismismatchwastaken intoaccounttoilluminatedevicewithatrue1-sunspectrum,accordingtotheASTM StandardE973 202 .Currentdensityundersimulatedsolarilluminationaswellasinthe darkwasmeasuredusinganAgilent4155Csemiconductorparameteranalyzer. 7.4ResultsandDiscussion 7.4.1FilmsSprayedfromPureSolvent Section7.2laidoutourhypothesisforusingalkanestoimprovethelmmorphology anddeviceperformanceforpolymerphotovoltaics.Beforeembarkingonadiscussionof theeffectofalkanes,rstwediscussthelmmorphologythatpuresolventsproduce. WepreparedlmsofP3HT:PCBMbysprayingfromchloroform,toluene,chlorobenzene,anddichlorobenzene.Allpuresolventstestedproducedveryroughlmswitha substantialcoffeeringeffectasanalyzedwithanopticalmicroscope.Ofthesesolvents, chloroformproducedthemostexaggeratedcoffeeringeffect,whereinalmostallofthe polymerandfullerenearedepositedattheedgeofdroplets,asshowninFigure7-6a. 182

PAGE 183

Purechlorobenzeneandtolueneproducealessexaggeratedringeffectwhereinmost ofthesoluteisdepositedattheedgeofthedroplets,thoughaportionisalsodeposited throughoutthecenter,asshowninFigure7-6bandc.Itisevidentthatthesolvent choicestronglydetermineslmquality.Wechosetostudyinmoredetaillmssprayed fromtolueneandchlorobenzene,aschlorobenzeneproduceshighperformancecells castbyspincoatingandwhiletolueneproducespoorerperformingdevicesascast byspincoating,ithastheadvantageofbeingmoreenvironmentallytolerablethan chlorinatedsolvents. Figure7-6.TheeffectofsolventchoiceonsprayedP3HT:PCBMlmquality.Optical microscopyimages,withscalebarsdepictingof100 m,showing P3HT:PCBMlmssprayedfromachloroform,bchlorobenzene,andc toluene.Allsprayprocessingparametersareidentical.1mL/minliquid ow,20scfhgasow,50throwswiththeexceptionofsolvent. Althoughlmssprayedfromtolueneandchlorobenzeneproducemoreuniform morphologythansomeothersolventssuchaschloroform,thesurfaceroughness canneverthelessbesevere.Figure7-7ashowsasurfaceheightmapasproduced byaDektak150prolometerinmappingmodeforaP3HT:PCBMlmsprayedfrom puretoluene.Ascanbeseen,thelocalvariationsoverthe250 m mby750 m mscan areareachesupto4 m m.Furthermore,thehistogramshowninFigure7-7bgives 183

PAGE 184

Figure7-7.Severesurfaceroughnessofsprayedlmsasmeasuredbyprolometry.A surfacemaptopshowsregionsofverythick-4 mtowardstheedgesof dropletsaswellasregionsofverythin < 100nmmaterialtowardsthe centerofdroplets,foralmsprayedfrompuretoluene.Thiswiderangeof thicknessandsevereroughnessisshownbyahistogrambottomofthe mappedlmthickness,showingaskewedrightdistributionofthickness. aquantitativelookatthedistributionofthicknessoverthescannedarea.Weseea skewedrightdistributionwithamedianthicknessofapproximately1 m.Thislmistoo thicktoproduceanefcientsolarcell,butthedistributionofthicknessischaracteristic oftheprocess-inordertofabricatealmwithamedianoraveragethicknessnearly optimalforthematerialsystemnmforP3HT:PCBM,theroughnessinherentinthe processensurestherewillbemuchthinnerandmuchthickerregions,whichmaygive 184

PAGE 185

risetoshuntinginthinregionsandreducedphotocurrentcollectioninthickregionsof photovoltaicdevices. Figures7-6and7-7demonstratetheseverenon-uniformitythatmulti-passspray coatingfrompuresolventcanproduce.Forelectronicallyusefullms,smoother morphologyisrequired.Thisisthechallengeweseektoaddresswiththiswork, andthenextsectiondiscussestherangeofsolutionsusedtopromotemoreuniform morphology. 7.4.2Solvent-AlkaneSolutionsandDropletBehavior Inordertoquicklytestthevalidityofouralkanesolutionhypothesiswitharangeof solventsandalkanes,werstqualitativelyevaluatedabatteryofsolutionsbydropping largedropletsoftestsolutionsontoaheatedglassslidescoatedwithPEDOT:PSS.For solvents,wetestedtoluene,chlorobenzene,dichlorobenzene,andtrichlorobenzene, butagainnotedsimilarbehaviorforthechlorobenzenesandreducedourteststosolely tolueneandchlorobenzene.Incombinationwiththesetwo,wetestedtheeffectof addingvariousamountsofn-hexane,cyclohexane,n-heptane,iso-octane,n-octane,and n-decane.Thesealkanes,asshowninTable7-1,allhavelowsurfacetensionbuthave varyingvolatility.Fortheseexperiments,thehotplatewaskeptat50 Cand50 lof2 mg/mLP3HT:PCBMsolutionwasdropped.Whilethetemperatureofthehotplatewas foundlatertoaffectdeviceperformance,forthepurposesoftheseexperimentsitmade littledifferenceasthekeymetricisthedifferencebetweentheevaporationrateofthe solventandalkane. 185

PAGE 186

Ingeneral,wefoundthatthemorevolatilethealkane,themorerapidlydropletswet outwardawayfromtheirinitialdroppedlocation.Nevertheless,thehigherthevolatilityof thealkanethemorequicklythealkaneevaporatedoutofsolution,leavingpuresolvent behind.Ifpuresolventisleftbehind,inthecaseofbothchlorobenzeneandtoluene, thedropletwillcollapsebacktoasmalldropletwithhighcontactangleanddepositthe remainderofsoluteinaringstructureinasimilarmannertodropletsinitiallyconsisting ofpuresolvent.Therefore,highervolatilityalkanesrequirehigherloadingtoavoid thiseffect.However,higherloadingofnon-solvatingalkaneprecipitatessoluteoutof solution.Therefore,thereisanoptimaltrade-offofalkanevolatilityandloading,andwe foundthatiso-octaneandheptaneworkwellforbothtypesofsolventintherangeof 20-50%loading. Furthermore,wefoundthatthepresenceoflowervolatilityalkanes,n-octaneand n-decane,haveastrongtendencytoprecipitatesoluteduringdropletdrying.Giventhe theorythatdifferentialevaporationratebetweensolventandalkaneinpartdetermines wettingvelocity,andalsoconsideringourexperimentalndingthatthereexistsan optimaltrade-offbetweenvolatilityandloading,theselongerchainalkanescanbe expectedtobesuitableforuseasadditiveswithverylowvolatilitysolventssuchas dichlorobenzeneandtrichlorobenzene. Finally,wefoundthatcyclohexanecategoricallybehavesdifferentlythanthenalkanesinthatdrieddropletsexhibitanoticeablymoreuniformdeposition.Inaddition,it isfoundthatsolutionsusingcyclohexanecantolerateamuchhigherloadingofalkane thanthen-alkanesandiso-octane,withupto85%volumecyclohexaneand15% 186

PAGE 187

chlorobenzenekeepingsolutewelldissolvedasopposedtoamaximumof50%isooctaneinchlorobenzeneand40%iso-octaneintoluenebeforesoluteprecipitation occurs.Also,wefoundthatevenforsolutionsusingloadinglevelsofalkanethatdonot initiallyprecipitatesolute,extendedstoragetimeyieldsprecipitation,andsosolutions weredilutedwithalkaneimmediatelypriortospraydeposition.Forlmanddevice tests,aswillbedescribedinthenextsections,wefocusedmainlyoniso-octaneand cyclohexane,andforiso-octanefound1:2alkane:solventmixturestogivethebest deviceresults. 7.4.3FilmMorphologyStudies Themorphologyofpolymerphotovoltaiclmsistypicallystudiedusingatomic forcemicroscopyAFMforitsabilitytoresolvesurfaceroughnesslengthscales typicalofspuncoatpolymers-10nmandbothAFMandTEMtoresolvepolymer phasesegregationonlengthscalesof10-100nmtypicalofP3HT:PCBMlms 203205 However,forthepurposesofthiswork,ourprimarymotivationistocharacterizethe largescaledropletmorphologyofsprayedlms.Becausethesedrieddropletlm residuesrangefrom10-100 m,andtheirsurfaceroughnesscanbeontheorderof 100nm,opticaltransmissionmicroscopyOTMisamoresuitablechoicethanAFMand servedasthefocusofourlmmorphologystudies. Figure7-8comparesOTMimagesoflmssprayedfrompuretolueneand2:1 toluene:iso-octaneontosubstratesheldat50 C.Filmssprayedfrompuretolueneshow apronouncedringeffectwithevidenceofdropletpinning.Toluene:iso-octanelms showawavydropletperimeter,suggestingthesedropletsarenotpinned.Rather,these 187

PAGE 188

dropletswetoutwardatacharacteristicvelocityuntilthedropletisdriedbyelevated substratetemperature.Furthermore,notetheinnerpatternstructurewithindroplets:the boundariesofunderlyingdropletsarevisible,suggestingthatthedepositionofadditional dropletsdoesnotentirelydissolveunderlying,previouslydrieddroplets. Figure7-8.Effectofisooctaneonmacroscopiclmmorphologyinlmssprayedfrom toluene.Opticalmicroscopyimagesof1.0:0.8P3HT:PCBMsprayedlms withapproximately0.125mg/cm 2 depositedsolute,sprayedfroma2mg/mL solutionconsistingof:apuretolueneandb2:1toluene:iso-octaneby volume.Scalebarsare100 m. Figure7-9providesasimilarcomparisonoflmmorphologyforlmssprayedfrom chlorobenzenesolutions.ThepureCBlmalsoshowsapinnedringeffectdeposition pattern.Dropletsinbothlmsdepositedfrompuresolventappeartocompletely dissolveunderlyingdroplets,asnonunderlyingstructureisseenwithineachdroplet. ForlmssprayedformCB:iso-octane,themorphologydoesnotdramaticallychange ascomparedtopureCB.Thedropletsstillseemtobemostlypinned,andthoughthe depositionpatternseemstobeingeneralslightlymoreuniform,theeffectissmall. Wedo,however,seeahigherprevalenceofcrystallitesthatformattheboundaries ofthedrieddropletswheniso-octaneorheptaneareloadedintoCB.Nevertheless, itseemsthatthisloadingofiso-octaneintoCBdoesnothavethesameeffectasit 188

PAGE 189

doesintoluene,wherepinningisovercomebyoutwardwetting.However,wendthat whenahigherloadingofcyclohexaneisusedinCB:5CB:cyclohexane,anoutward wettingeffectcanbeseenindropletresidues,asshowninFigure7-9c.Theselms arenoticeablymoreuniform. Figure7-9.Effectofisooctaneandcyclohexaneonmacroscopiclmmorphologyin lmssprayedfromchlorobenzene.Opticalmicroscopyimagesoflms sprayedfroma2mg/mLsolutionof1.0:0.8P3HT:PCBMdissolvedina purechlorobenzene,b2:1CB:iso-octanebyvol,andc1:5 CB:cyclohexane.Scalebarsare100 m. Thus,wendthatalthoughsolutionssuchas2:1CB:iso-octanerapidlywet outwardswhendepositedwithlargedroplets-5mmindiameteronPEDOT:PSS lms,thesameeffectisnotseeninlmsmulti-passsprayedfromsuchsolutions.We nd,however,thatsolutionssuchas2:1toluene:iso-octaneand1:5CB:cyclohexanedo exhibitstrongwettingasdepositedbymulti-passspray.Wepostulatethatthisbehavior isstronglytiedtothesolvatingefciencyofthesolution.Asolutionwhichisnearits solubilitylimithasadiminishedabilitytosolvateadditionalsolute,andsodroplets dissolvedriedundercoateddropletswithlessefciency.Inorderforadroplettowet rapidlyoutwardontopofapredepositedlm,itmustnotdissolvetheunderlayer.Our experimentalndingscorroboratethisview,asboththe2:1toluene:iso-octaneand 189

PAGE 190

the1:5CB:cyclohexanesolutionswith2mg/mLP3HT:PCBMweredevelopedasthe highestloadingofalkanethatcouldbeachievedwithouthittingthesolubilitylimitof thedilutedsolvent.Therefore,thesesolutionswhendepositedhavelittleeffectonthe undercoatedlm,andmaywetoutward.However,thiswettingopposesthetendency ofthedropletstopinonhighsurfaceirregularitiesandtheneteffectisabalanceof thetwo.Conversely,solvatingdropletssuchasthosedepositedfrompuresolventthe solventareassuredtopinasthedropletedgesdissolvetheundercoatedlayerandare thenconnedbythesurroundinglm. Tofurtherunderstandthemorphologyoftheselms,wedevelopedamethodto useOTMtomapsurfacethicknessplotsoverlargeareas.Thetechniqueisdescribed inSection7.3.4;essentially,itusesanopticalmicroscopetomaptransmittancethrough asample,andbycomparisontoacalibrationstandard,ultimatelymaplmthickness. Becauseitmakesuseofanopticalmicroscope,thiscanbedoneatvariouslength scales,butwechosetomapoverapproximately3mmby3mmareas,whichareslightly largerthanourdevicesizeof2mmby2mm,andstillgivessufcientresolutionfor dropletmorphology.Figure7-10showsthicknessmapsforlmssprayedfromapure chlorobenzene,b2:1CB:iso-octane,c1:5CB:cyclohexaneandfordalmspun coatfromCB.Forallfourmaps,thefullverticalscaleshownis800nm.Weclearlyseea progressionofmoreuniformlmmorphologyaswemovefromalmsprayedfrompure solvent,throughlmssprayedwithprogressivelymorealkane,tonallyaspuncoatlm. Weconcludethatthemoreuniformmulti-passsprayedlmsareproducedby solutionswhosedropletswetoutward.Byourtheoryandexperimentalanalysis,we proposethattheconditionsforidealdepositionuniformityrelyonsurfacetension 190

PAGE 191

Figure7-10.Filmthicknessplotsforlmssprayedfroma2mg/mL1.0:0.8P3HT:PCBM solutionofapurechlorobenzene,b2:1CB:iso-octanebyvolume,c 1:5CB:cyclohexaneandda200nmthicklmspuncoatP3HT:PCBM lm.Foreach,the2-Dmapontheleftandthe3-Dprojectionontheright showthesamedata,andallhavefullscaleof800nm. 191

PAGE 192

differential,evaporationdifferential,andsolvatingefciency.Idealsolutionsaremade upofacombinationofsolventandalkaneinwhichthetwospeciesevaporatein approximatelythesameamountoftime,theloadingofthealkaneislowenoughnot toprecipitatesolute,butnallyishighenoughtoreducesolvatingefciency.Inother words,alkaneshouldbeaddedtothesolventinaconcentrationthatbringsthemixed solutiontonearitssolubilitylimit,andthatcombinationofsolventandalkaneshould beevaporateonthesametimescaleasdeterminedbytherelativeconcentrationsand relativeevaporationrates. However,notethatthisndingissolelybasedonthemacroscopicuniformityofthe sprayedmulti-passlm.Inthenextsection,weaddresstheaffectofthesesolutionson deviceperformance. 7.4.4EffectofAlkaneAdditivesonDevicePerformance Theprevioussectiondemonstratesthattheuseofalkaneadditivesinthesolution canhaveasubstantialeffectonthelmformationprocessofsprayedP3HT:PCBM lms.Whileweexpectamoreuniformlmtoproduceahigherperformancesolar cell,theadditionofalkanestothesolventsolutionmayalsohaveadditionalconsequencesforthesemiconductingbehaviorofthelmwhicharenotapparentinthe macroscopiclmstudypreviouslyshown.Becauseofthediffusionbottleneckinorganic semiconductors 84,200 ,therealizationofhighperformanceorganicsolarcellsrequires optimalnanostructureinwhichthedonorandacceptormoleculesphasesegregateinto nanoscopicphasesontheorderof20nmandalsoformpercolatingnetworkstocollect holesandelectronsfromthedonorandacceptor,respectively.P3HT:PCBMiswidely 192

PAGE 193

usedinorganicsolarcellspreciselybecauseitcansatisfythisconditionwithsimple processingtechniques,althoughitiswelldocumentedthatperformanceissensitive toprocessingconditionssuchassolventused,additivesused,annealingconditions, solutionconcentration,anddryingtime 206 .Forexample,theuseoftolueneallowslarge crystallitesofP3HTandPCBMtoformduringthedryingprocessandthereforedue tothediffusionbottleneckleadstolowphotocurrent,whiletheuseofchlorobenzene enablesanerphasesegregationandenableshigherphotocurrent 91 .Therefore,aswe dilutethesolvencyofchlorobenzeneandtoluenewithalkanesinthisstudy,wemay affectthecapacityofthesolventtoenableoptimalnanostructure,therebydegrading deviceperformance. Wersttestedtheeffectofusingalkanesalongwithtolueneastheprimarysolvent. Tolueneisalesstoxicandenvironmentallydamagingchemicalthanthechlorinated benzenes,buttypicallyhasresultedinsubstantiallyinferiordeviceperformancerelative totheuseofchlorobenzene.Usingpuretoluene,weproducedspraycoateddevices withamaximumof1.5%PCE.However,withtheadditionofiso-octaneinasolution of67%tolueneand33%iso-octane,weproducedsprayeddeviceswithPCEof2.5%. Figure7-11showstheJ-Vcharacteristicsinthedarkandunder1sunilluminationfor sprayeddeviceswithandwithouttheuseofiso-octane.Themostdramaticdifference betweendevicessprayedwithandwithoutalkaneisseeninthedarkcurrent.Using iso-octane,thedarkcurrentunderV=-1Vissuppressedby3ordersofmagnitudein comparisontothedevicewithoutoctane.Thissuppressionofdarkcurrentisaresult ofincreasedshuntresistanceandconsequentlythellfactorandopencircuitvoltage 193

PAGE 194

areimprovedcomparedtothatwithoutalkane.SeeTable7-4forasummaryofthese results. Figure7-11.Sprayeddeviceperformance:toluenevs.toluene:iso-octane.J-V characteristicsfor1.0:0.8P3HT:PCBMsprayedlmssprayedfroma2 mg/mLsolutionconsistingofpuretolueneopenblacksquaresand2:1 toluene:iso-octanelledredsquaresinthedarkleftplotandunder simulatedAM1.5illuminationrightplot. Althoughwedidnotfullyoptimizetheconditions,thebestdevicesweproducedas spuncoatfromtoluenereached1.5%PCE,andthisissimilartothelimitedprevious reportsthathaveusedtoluene.This,therefore,istherstreportofamaterial-solution systemwhereinthesprayeddevicegreatlyoutperformsthespuncoatdevice,asspin coatingtypicallyproducesoptimaldeviceperformance.Thisresultdemonstratesthe abilityofanon-solventadditivetonotonlydrasticallyalterthelmmorphology,but alsoimprovedeviceperformance.Thisisaninterestingresult,becausebyadding alkanewereducethesolvencyofanalreadypoorsolvent,toluene.Weproposethat thisimprovementarisesfromtwoeffects.First,theprocessofmulti-passsprayonto anelevatedsubstratetemperature T =50 Cdecreasesthesettingtimeofthelm, 194

PAGE 195

andthereforereducestheabilityofthepolymertoaggregateintolargedomainsduring lmdrying,whichhasbeenshowntoberesponsibleforlowdevicephotocurrent 91 Therefore,thissprayprocesstheoreticallyenableshigherphotocurrentextraction efciencyoftoluene-castlms.Second,whiletheaggregatedcrystallitesintoluene-cast P3HT:PCBMcancreatepinholesinthelm,theprocessofmulti-passspray,ifoptimized totheextentthatdropletscanwetoutwardwithoutdissolvingtheundercoatedlayers, promotesanadditivestackingoflayers.Thisadditivelmformationprocessallows pinholescreatedduringpreviouspassestobewettedandcoatedagain,ultimately reducingthepinholedefectdensityintheselms.Therefore,weseereduceddark currentandanimprovedllfactor,asshowninFigure7-11. Table7-4.Solarcellperformancecharacteristicsofpolymersolarcellssprayedfroma varietyofsolutionmixtures. SolventAlkaneSolvent vol% Spray passes J SC mA/cm 2 V OC mV FFPCE % TolueneNone1002006.075820.4161.47 TolueneIso-octane672006.736370.5752.46 ChlorobenzeneNone1002007.936110.5432.63 ChlorobenzeneIso-octane672009.566190.5693.37 ChlorobenzeneCyclohexane402007.016270.5942.62 ChlorobenzeneCyclohexane172005.986350.6402.43 ChlorobenzeneCyclohexane172607.076310.6042.69 Next,wetestedtheuseofchlorobenzeneasprimarysolvent.Usingpurechlorobenzene,weachievedatbestaPCEof2.6%.Usingthesamevolumeratioofiso-octane asfortoluene%vol,weachievedatbestaPCEof3.4%.Figure7-12showsthe J-Vcharacteristicsinthedarkandunder1sunilluminationforchlorobenzene-sprayed 195

PAGE 196

deviceswithandwithouttheuseofiso-octane.Again,weseeadramaticdecrease inthedarkcurrent,andweagainexplainthiseffectasaresultofareducedpinhole defectrateinthelm.Itisinteresting,though,thatwhileweachievesubstantially improveddeviceperformanceusing33%iso-octaneinCB,wedidnotseeadramatic differenceinthelmmorphologyaswedidusingtoluene.However,wewereableto achievesimilaruniformityeffectswhenusinghigherloadingofcyclohexane,asshown inFigure7-9.Yet,wefoundthatdevicesusinghighloadingofcyclohexaneproduced lessphotocurrent.AssummarizedinTable7-4,astheconcentrationofCBinsolution isreducedfroma67%CBsolutionwithiso-octaneto40%and17%CBsolutionswith cyclohexane,theshortcircuitcurrentdensityisreducedfrom9.5to7.0and6.0mA/cm 2 respectively.However,thellfactorisincreasedwiththesemoreuniformcyclohexane solutions.Thislikelyisduetothicknesseffects,whichwillbediscussedinsection7.4.5. Figure7-12.Sprayeddeviceperformance:chlorobenzenevs. chlorobenzene:iso-octane.J-Vcharacteristicsfor1.0:0.8P3HT:PCBM sprayedlmssprayedfroma2mg/mLsolutionconsistingofpure chlorobenzeneopenblacksquaresand2:1chlorobenzene:iso-octane lledbluesquaresinthedarkleftplotandundersimulatedAM1.5 illuminationrightplot. 196

PAGE 197

Wecanexplainthistrendagainbyconsideringtheroleofthesolventinthelm formationprocess.WhenchlorobenzeneispresentinsolutionwithP3HTandPCBM, thesolventrestrictstheabilityofeachsolutetoformaggregatedcrystallites.Therefore, aswereducetheamountofCBinsolution,wereduceitsabilitytoensurenephase segregationandhighphotocurrent.Infact,wealsotestedhigherloadingofiso-octane insolutionwithCB,asthesolvatingstrengthofCBallowsthis,andfoundthatdevice photocurrentwasreducedwithincreasingalkane. Consider,then,ourhypothesisthatthepresenceofalkaneessentiallyserves threepotentialfunctions:awetting/solvatingeffectsincreaselmuniformity,b wetting/solvatingeffectsdecreasedefectdensity,andcloadingofalkanedilutesthe activityofthesolvent.ForbothtolueneandCB,thefunctionsaandbaredesiredfor improvingdeviceperformance.Functioncisdesirableforuseoftoluene,toreduce itsactivityinpromotingcrystalliteformationduringdrying,butisundesirablefortheuse ofCB,aswewanttomaximizetheactivityofCBinpromotingnephasesegregation. Inlinewiththistheory,weachievedthebesttoluene-sprayeddeviceswithsolutions inwhichthetoluenecontentwasminimized%iso-octaneintolueneapproaches solubilitylimitswhileweachievedthebestchlorobenzene-sprayeddevicesusing solutionswithaminimalloadingofalkanethatstillproducedthewetting/solvating effectCBislessvolatilethantolueneandthereforerequiresmorealkaneloadingto ensuresimilarevaporationtimes.InthecaseofCB,webelievethatalthoughthis optimalsolutionof33%iso-octanedoesnotproduceastrongwettingeffect,itreduces thesolvatingefciencyenoughtoensurethattopcoatsdonotcompletelydissolve undercoats. 197

PAGE 198

7.4.5ActiveLayerThicknessinSpunandSprayedDevices Activelayerthicknessisacriticalparameterfororganicsolarcells.Notonlydoes thethicknessdeterminethedevicephysicsunderlyingabsorption,excitondiffusion, excitondissociation,andfreecarriercollectionefciencies,butitalsodeterminesthe opticalelddistributionwithintheactivelayerduetoconstructiveanddeconstructive interferencefromlightreectedfromthebackcathode.Thus,foranyorganicorpolymer solarcell,theretypicallyexistslargevariationsindeviceperformancethatarisedueto smalldifferencesinactivelayerthickness.ForP3HT:PCBM,thethicknessdependence hasbeenstudiedbothexperimentallyaswellasanalytically,andingeneralathickness ofabout220nmisconsideredtogiveoptimalphotocurrentandpowerconversion efciency 207210 Ourspraydepositedsolarcells,however,donotexhibitawell-denedthickness. Instead,theyaremadeupofarangeofmanythicknesses,fromlessthan100nmto greaterthan500nm,dependingonthesolutionsused.Anysolarcellcantheoretically betreatedasanynumberofsub-cellsdivisiblebyareainparallelwitheachother. Thetotalcurrentdensityisdeterminedbytheaveragecurrentdensityweightedbythe portionofthetotalareathateachsubcelllls.Forourspraydepositeddevices,the thicknessfunctioniscontinuousandgreatlyvaried,butwecanstillgaininsightinto theoriginsofdeviceperformancebycorrelatingtheirthicknessprolewiththicknessdependentperformancefunctions.Inordertoquantifytheseperformancefunctions, wecanusespincoatingtoproducedeviceswithwelldenedthickness.Althoughthe literaturedoescontainstudiesofthisnature,deviceperformanceisalsowellknown 198

PAGE 199

tovarygreatlybetweenlabsandprocessingmethods,andsowesetouttodetermine thisdependenceinourlab.Inadditiontoitsuseincorrelatingthicknessdistribution andultimateperformance,thisalsogivesusamorecontrolledbenchmarktowhichto compareoursprayeddeviceperformance. Wefabricatedandtesteddevicesofvaryingthicknessbyspincoatingandused thesamepost-processingmethodsasourspraydepositeddevices.Wevariedthe lmthicknessfrom60nmto350nmbyspincastingfromvariouschlorobenzene solutionconcentrationsandusingvaryingspinspeeds.Table7-5liststhebestdevice performancefromeachthicknessstudied,andFigure7-13summarizesthephotocurrent andPCEdependenceonthicknessandplotstheJ-Vcharacteristicsunder1sun illuminationforseveralofthesethicknesses.Overall,wefoundthatthedevicePCE varieslessthan15%overthethicknessrange110 < t < 350nmandofthosetested, t =260nmproducedthehighestPCEof3.6%. Theseresultsshowasurprisingsimilaritybetweentheperformanceofhighly uniformspuncoatlmsandhighlyirregularspraycoatedlms.Itisatoncesurprising andalsoinformativethatwhilethebestspraycoateddeviceexhibitedPCE=3.4%,the bestspuncoatdeviceonlyexhibitedPCE=3.6%.First,becausethephotocurrents aresimilar,thissuggeststhatthenanostructureofthesprayedlmsisnotdegraded signicantlyduetotherapiddryingprocessorthepresenceoflargeamountsofdiluting alkane.Second,thebestsprayeddevicesachievehigherperformancethanwewould expectforadistributionofsprayedlmthicknessthatisuniformacrossthethicknesses examinedbythespincoatingstudy.Admittedly,manyofthespraycoateddevices usingtheoptimalCB:iso-octane2:1mixturedidresultinefcienciesinthe2.7%199

PAGE 200

Table7-5.Deviceperformancecharacteristicsfordeviceswithdifferentspun coatactivelayerthicknessasachievedbyalteringsolution concentrationandspinspeedandasmeasuredbyprolometry Solution conc. mg/mL Spin speed rpm Thickness nm J SC mA/cm 2 V OC mV FFPCE % J EQE SC a mA/cm 2 121000603.525820.5871.203.92 12800774.876020.6211.825.31 12500905.886030.6152.186.28 123001007.775800.5952.688.34 2410001108.456030.6033.078.17 248001408.575930.5632.867.85 245001758.215910.5912.867.81 243002058.015780.5802.697.91 3610001858.076130.6093.018.33 368002108.726130.6203.319.44 365002609.446080.6203.569.77 363003509.466010.5973.399.70 a ShortcircuitcurrentdensityobtainedbyintegrationofEQEwithstandardAM1.5solar spectrum 2.9%rangewhichismoreinlinewithwhatwewouldexpectfromthespuncoatdevice performancegivensuchadistribution.However,wealsoseeingeneralareduction ofreverse-biasdarkcurrentaswellasanincreasedopencircuitvoltageforspray coateddevicescomparedtospuncoatdevices.Theseeffectsperhapscounteract someofthesub-optimaleffectofthicknessvariance.Intheleast,wecanconcludethat whiletheremayberoomforimprovementinprocessing,theuseofalkanesdoesnot signicantlydegradephotocurrentgenerationandextraction,andbythemulti-layer 200

PAGE 201

Figure7-13.Thicknessdependenceofspuncoatsolarcellperformance.J-V characteristicsundersimulatedAM1.5illuminationleftandperformance characteristicsrightincludingshort-circuitcurrentdensityopentriangles andpowerconversionefciencyclosedsquaresfor1.0:0.8P3HT:PCBM lmsspunfromvaryingconcentrationsolutionsatvaryingspeedsresulting invariedlmthickness. stackingphenomenonreducesdarkcurrentandthereforehasaninherentadvantage overotherlmcoatingmethodssuchasspincoating. 7.4.6EffectofAnnealingConditions Inanefforttooptimizespraydepositedpolymersolarcellperformance,we experimentedwithdevicestructuresandannealingconditionspreviouslyshownto improvespuncastdevices.Electrodeinterfaciallayershavebeenshowntoimprove 201

PAGE 202

energylevelalignmentbetweentheactivelayersandelectrodesbyFermilevelpinning andconsequentlyimprovephotocurrentextractioninorganicsolarcells 110 .Tothisend, weempoloyeda5nmthickMoO 3 anodeinterfaciallayer 104 betweentheITOanodeand P3HT:PCBMactivelayerinalldevicesreportedinthischapter. Inaddition,weexploredtheuseofaLiFcathodeinterfaciallayer 211 forsprayed devicesinordertoraisethedevicellfactor,whichisslightlylowerthanourspun coatdevicesusingthesamepostprocessing.Wefoundthatevaporating1nmof LiFbetweentheactivelayerandaluminumachievedincreasedllfactorinspuncast devicesusingalmannealprocess,uptoFF=0.66,whereasinourthicknessstudy ofspuncoatdevicesusingadeviceannealprocessseeSection7.3.2thehighestll factorachievedwasFF=0.62.Annealingprocessesingeneralhavebeenshownto promoteimprovedlateralphasesegregationofP3HTandPCBM 212,213 ,anddevice annealinghasbeenfoundtopromoteamorefavorableverticalgradientdistribution ofP3HTandPCBM 214 .However,wefoundthatdeviceannealingafterdepositingLiF degradesperformancewhenusingaLiFinterfaciallayer,mostlikelyduetodiffusionof Li + intotheactivelayer.Figure7-14summarizestheseresults,showingthatwhileusing LiFcanachievehighllfactorandpowerconversionefciency,itrequirestheuseofa lmannealprocessratherthanadeviceannealprocess. Wenextappliedthesendingstospraycoateddevices,butfoundthatwhiletheuse oflmannealingandaLiFcathodeinterlayerimprovesspuncoatdeviceperformance,it doesnothavethesameeffectforspraycoateddevices.Itseemsthatthelmannealing processthatworksforspuncastdevicesisnotsufcientforspraycoateddevices. 202

PAGE 203

Figure7-14.EffectofannealingondeviceswithspuncoatactivelayersandLiF/Al cathodes.J-Vcharacteristicsfor1.0:0.8P3HT:PCBMlmsspuncoatat 800rpmfrom24mg/mLchlorobenzenesolutionsinthedarkleftplotand undersimulatedAM1.5illuminationrightplot.Deviceannealingwith LiF/Alcathodesopenblacksquaresiscomparedtolmannealingwith LiF/Alcathodesllednavysquares,FF=0.66,PCE=3.3%. Figure7-15comparesthebestperformanceweachievedforspraycoateddevicesusing almannealwithLiFtothatofdeviceannealingandwithnoLiF. Theinadequacyofthelmannealprocessforspraycoateddevicescouldarise duetoseveralfactors.Forinstance,thehighsurfaceroughnessofthelmmayreduce theeffectivenessofsuchathininterlayer.Ontheotherhand,amoredisordereddonoracceptornanostructureinsprayeddevicesmayrequireamoreenergeticanneal.The disordermaybeincreasedinthebulkduetoamorerapiddryingprocessatelevated temperature,orsimplyatthedroplet-dropletboundariesthatdonotexistinspun coatlms.Forthepurposesofthiswork,weconcludedthatthiswasaninteresting phenomenonbutoutofthescopeofthisworkwhichfocusesontheoptimizationofthe sprayprocessitselfratherthanpostproductionprocessing.However,theuseofLiFor otherinterfaciallayersmaybeaninterestingavenuetoexploreforfuturework. 203

PAGE 204

Figure7-15.Effectofannealingondeviceswithsprayedactivelayersandcathodes.J-V characteristicsfor1.0:0.8P3HT:PCBMlmssprayedthrowsfrom2 mg/mL2:1CB:iso-octanesolutionsinthedarkleftplotandunder simulatedAM1.5illuminationrightplot.FilmannealingwithLiF/Al cathodesopenblacksquaresandtrianglesiscomparedtodevice annealingwithAlcathodeslledredsquaresandtriangles.Twodevices areshownforeachtoshowvariabilityofdarkcurrent,butconsistent differenceinJ-Vunderillumination. 7.5Conclusions Inthisstudy,wehavedevelopedtheprocessofspraycoatingasacandidate processforlarge-scale,low-costR2Rmanufactureofpolymersolarcells.Weproposed thatmulti-passspray,whileitinherentlyproducesrougherlmsthanoodspray,is neverthelessaworthwhileprocesstopursue.Ourhypothesiswasthattheaddition ofalkanestopolymer-solventsolutionscanimprovethemorphologyofspray-coated lmswithimprovedthicknessuniformity.Wesuccessfullycorroboratedourhypothesis bystudyingvarioussolutionmixturesandtheireffectsonlmmorphology.Wethen correlateuniformityimprovementswiththerelativeconcentrations,surfacetensions, andevaporationratesofthemixedsolventandalkane,andprovideageneralformula forhowtooptimizethelmuniformitybysuchchemicalparameters.Wendthatthis 204

PAGE 205

uniformitycanresultinamarkedimprovementindeviceperformance,ndingthat sprayedsolutionsoftoluene:iso-octaneandchlorobenzene:iso-octanesubstantially improvedeviceefciencyascomparedtosprayedpuresolventsolutions,andultimately matchorexceedthedeviceperformanceofhighlyuniformspuncoatlms.Finally,we supportourhypothesisthatmulti-passsprayisanadvantageousdepositionprocess duetoitsabilitytobuildlmsinanadditivemanner,bydemonstratingthiseffectusing optimizedalkanemixtures,andshowthattheadditivelmformationprocesscan leadtoreduceddarkcurrent,andultimatelyhighperformancepolymersolarcells.In conclusion,wendfurtherevidencethatspraycoatingisapromisingprocessforthe futureoforganicsolarcells. 205

PAGE 206

CHAPTER8 CONCLUSIONSANDFUTUREWORK Organicsemiconductorsexhibitadvantageouscharacteristics,suchmechanical exibilityandlow-temperatureprocessing,thatcanbereadilyleveragedforfunctional electronicandoptoelectronicapplications.Thisthesisexploredthewidescopeof realizabledevicestructuresthatareenabledbytheadaptabilityofthesematerials.In therstpart,twophotodetectordevicestructureswereexploredfortheirabilitytooutput aphotocurrentgain:atheeldeffectorganicphototransistorandbanovelmultilayer connementheterojunctionorganicphotodiode.Inthesecondpart,twosolarcelldevice structureswereexploredfortheirpotentialtoimprovetheefciencyandmanufacturing scalabilityoforganicsolarcells:aanexternaldown-conversionantennafororganic solarcellsandbbulk-heterojunctionpolymersolarcellspreparedbyspraydeposition. 8.1OrganicFieldEffectPhototransistors Thecombinationoftransistorandphotodetectingelementwithinanorganic phototransistormayenablethesimplicationofcircuitry,expeciallyifthesedevices canbemadetooutputaphotocurrentgain.InChapter4,wereviewedtheliterature inwhichseveralreportsndhighgaininorganicphototransistors.Wetestedthe operationofpentacenetransistorsunderilluminationandfoundthat,asothershave reported,illuminationstronglyshiftsthethresholdvoltage.Astrongthresholdvoltage shiftgivesrisetophotosensitivityupto10 6 andgainupto100,asdenedbythestatic measurementmethod.However,wefoundbyusingdynamicmeasurementmethods thatthisphotoresponseisnotatrueaccountofthedeviceperformance,andwe correlatetheperceivedphotosensitivitywiththebiasstresseffect.Byusinghydrophobic 206

PAGE 207

gatedielectrics,thetrappingeffectcanbereducedcreatingamorerobusttransistor operation,butthisalsodegradesthephotoresponse. Nevertheless,wedondthatsimplepentacenetransistorscanoutputatrue photocurrentgain,uptoG=2.Transistorchanneltransconductanceoffersanintrinsic mechanismforphotocurrentgain,andforfuturework,wemayexplorenewdevice structuresdesignedtoaugmentthisgain.Newdevicestructures,however,mustensure thatthedeviceexhibitsstableoperationinthedark,withoutstrongbiasstresseffectsor hysteresis. 8.2CarrierConnementinOrganicPhotodiodesforPhotocurrentGain Next,anewmultilayerorganicphotodiodestructurewasdesignedandstudied thatusesacarrier-selectiveconnementmaterialtoproduceanextraordinarilyefcient photoconductivegainmechanism.Weattributethegaintotheconnementoffree photo-generatedholesbyhole-blockinglayersHBLsbetweentheanodeandthe photoactivelayerandtheproductionofastrongsecondaryelectronphotocurrent injectedfromtheanodeunderreversebias. WefoundthatbothNTCDAandC 60 workwellasHBL,andthatacombinationof athickernmthickC 60 HBLwithathinner.5nmthickNTCDAHBLcreatethe strongestgain,achievingG=100acrosstheentirevisiblespectrumunder V = )]TJ/F20 11.9552 Tf 9.289 0 Td [(3 V.Weexploredtheeffectofthecompositionofthephoto-activebulkheterojunction ofCuPcandC 60 ,andfoundthatitstronglyinuencesboththegainandbandwidth duetothechangeintheelectrontoholemobilityratio.Theoptimalgain-bandwidth productGBPwasobtainedindeviceswithapproximately30%CuPcinthelm.These 207

PAGE 208

optimizedphotodiodesexhibitabandwidthof f 3 dB =1kHzandGBP=10 5 ,whichis extraordinaryamongorganicphotodetectorsoperatingaboveimaging-compatible bandwidth > 60Hz.WeattributetheresultinghighGBPtotheconnement,rather thantrapping,ofholeswithinthephotoactivelayerbytheHBLstructure. Finally,wendthatthedarkcurrentdegradeswithuseandresultsinhighnoise current,ultimatelygivingthesedetectorslowsensitivity.Thesedevicesshowgreat promiseforpracticalapplicationduetotheirhighgainandbandwidth.However,to realizethispotential,futuredevelopmentisrequiredtodiagnosethephysicalcauseof thedegradationinordertoreducenoisecurrentandimprovesensitivity. 8.3Down-ConversionforEnhancedOrganicSolarCellAbsorptionEfciency Shiftingtoorganicsolarcells,weproposedanewadvanceddevicearchitecture conceptthatusesauorescentantennatodown-convertandre-directparttheincident AM1.5solarspectrumontotheactivesolarcell.Thedown-conversionprocessis executedbyaluminescentenhancementlayerLELthatmustefcientlyabsorbthat maybeotherwiseinefcientlyabsorbedbytheactivelayersALsinthesolarcell. ConsistingofauorescentsmallmoleculeDCJTBdopedintoahostmaterialeither AlQ 3 orPMMA,theLELisotropicallyre-emitslightwithacharacteristicStokesshiftat highPLyield%.Someofthisre-emittedlightislost,butmostiscoupledintothe ALswhereitsabsorptionisdependentontheresonancebetweenLELemissionandAL absorption. Wedesignedanopticalray-tracingsimulationprogramtotesttheviabilityofthis conceptasappliedtostandardorganicsolarcells.Usingthissimulator,weshowed 208

PAGE 209

thattheuseofaLELcanenhanceabsorptionintheALsupto17%,25%,and27% forC 60 :PbPc,C 60 :CuPc,andC 60 :squarainesolarcells,respectively.BecausetheLEL opticalstructureisplacedontheoutsideofthesedevicesanddoesnotinuence thephoto-electricaloperationofsuchdevices,weexpectthattheseabsorption enhancementsmaytranslateintosimilarenhancementtothetotalphotocurrent generatedbysuchdevices,andthereforeconcludethatthisstructurecantheoretically enhancepowerconversionefciencybyupto27%. Forfuturework,experimentalphotovoltaicdevicesmayusetheLELstructurein anattempttoreplicateourcomputationalresults.Furthermore,althoughthemodel providesarigoroustreatmenttothebehavioroflightwithinthedevicefromthe perspectiveofrayoptics,futureworkmayextendthiscomputationalworkbytaking intoaccounttheopticalwaveinterference.Waveopticsisimportantinthesedevicesdue tothereectingelectrode,andthistreatmentmaydetermineamoreaccuratelevelof potentialsolarcellefciencyenhancement. 8.4AlkaneDiluentsforImprovedPerformanceofSprayedPolymerSolarCells Finally,astheultimateadvantageoforganicsolarcellsmaylieintheirprocessabilityandcommercialscalability,thenaltopicstudiedinthisthesisfocusedonthe processingoforganicsolarcellsbyspraydeposition.Spraydepositionmayenablelow costmanufacture,howeveritinherentlyproducesroughandnon-ideallmmorphology. Hereitwasfoundthatbydissolvingpolymersemiconductorsinsolventsolutions formulatedwithalkaneadditives,theuniformityofthesprayedorganicthinlmswas signicantlyimproved. 209

PAGE 210

Weexploredtheuseofiso-octaneandcyclohexaneamongotheralkanesasadditivestotolueneandchlorobenzenesolventsfortheP3HT:PCBMbulkheterojunction. Wefoundthatthesediluentscanimprovethemorphologyofspray-coatedlmswith improvedthicknessuniformity.Wecorrelatedtheseuniformityimprovementswiththe relativeconcentrations,surfacetensions,andevaporationratesofthemixedsolventand alkane,andprovideageneralformulaforhowtooptimizethelmuniformitybysuch chemicalparameterstakingintoaccounttheuiddynamicsinthesesystems. Wefoundthattheimproveduniformityoflmssprayedusingalkanediluents canresultinamarkedimprovementindeviceperformance.Solarcellsprayedfrom solutionsoftoluene:iso-octaneandchlorobenzene:iso-octaneexhibitpowerconversion efciencyPCEof2.5%and3.4%,respectively,whilethosesprayedfrompuretoluene andpurechlorobenzeneexhibitPCEof1.5%and2.6%,respectively,corresponding toanenhancementinefciencyofupto40%.Furthermore,theefciencyofthese devicesprayedfromsolventswithalkanediluentultimatelymatchorexceedthedevice performanceofhighlyuniformspuncoatlms.Weattributedthisenhancementpartially totheabilityofmulti-passspray,usingsolutionsnearthesolubilitylimitbydilutionwith alkanes,tobuildlmsinanadditivemanner.Thisadditivelmformationwasseen inopticalmicroscopystudies,andwebelievethisleadstotheverylowdarkcurrent realizedindevicessprayedfromalkanesolutions. Forfuturework,weareintheprocessofscalingthecustom-builtspraysystemto largerareainordertostudytheinherentscalabilityofthesprayprocess.Ultimately, spraydepositionisapromisingfabricationprocessforitslow-cost;however,herewe haveshownthatusingslowadditivelmformation,multi-passspraymayenablethe 210

PAGE 211

realizationofmoreadvanceddevicestructuressuchasmultilayerheterojunctionsusing polymersemiconductors. 211

PAGE 212

APPENDIXA MODELINGOLEDOUTCOUPLINGEFFICIENCY A.1Introduction Thesuccessoforganiclight-emittingdeviceOLEDtechnologyforgenerallighting applicationswillbedecidedbytheefciencywithwhichdevicesconvertelectrical energytoforward-directedlight.Incurrentdevices,asubstantialamountoflight generatedwithintheorganicactivelayersislostandneveremittedasusefullight. Mostofthislossisduetoabsorptionafterrepeatedinternalreectionoflightbelow thecriticalescapeangle, q c .Typicaldevicesexhibitlightoutcouplingefcienciesof about20% 215 .Therefore,thereisasubstantialamountofroomforimprovementin thesepracticaldevicesbydevisingwaystodirectmoreofthegeneratedlightoutofthe device.Asimplewaytoenhanceoutcouplingefciencyistousealensstructureon theoutsideoftheOLEDdevice.However,suchastructuremustbecompatiblewiththe manufactureoflarge-areaOLEDdevicesinordertobeuseful.Arraysofmicrolenses mayservethisend,astheyhavebeenproducedusingavarietyofmethodspotentially compatiblewithlargeareamanufactureandtheirmicrometerthicknessminimizesthe useofcostlymaterial. Herewedevelopacomputermodeltoenablerapidoptimizationofmicrolens fabricationparameters.Theseparametersincludelensrefractionindex,n lens ,lens contactangle, g ,lenspackingarrangementsquarelatticeorclosepacked,andlens arrayspacing. Apriori ,weexpectcertainoptimalvaluesfortheseparameters.We expectmicrolensarraysconstructedwithn lens =n org and g =90 inclosepacked arrangementwithnospacingtoyieldthehighestoutcoupling.Yet,thesevaluesin 212

PAGE 213

tandemwillbedifculttoachievepractically.Therefore,thismodelnotonlyprovides avericationofouroptimizationexpectations,butalsoallowsustoquantifypractical fabricationtradeoffswhennecessary. A.2OLEDOutcouplingRayTracing Usingrayoptics,emissionofaphotonoriginatingatarandomrecombinationevent withintheOLEDactiveorganiclayerisdependentonvevariables:positionofemission x,y,zanddirectionofemissionpolarangle, q ,relativetothedevicesurfacenormal andazimuthalangle, f ,inthedeviceplane.Forplanardevicestructureswherethe devicelengthandwidtharemuchgreaterthanthedevicethicknessminimaledge effects,thepositionaldependenceandazimuthalangledependencedropout.So,with thisgeometry,rayopticscalculationsaresinglevariablefunctionsofthepolarangleand theproblemcaneasilybesolvedanalytically. Forthesimplestcalculation,weassumethefollowing:zeroabsorption,perfect reectionatthemetalcontact,perfecttransmissionatinterfacesforincidentangles belowthecriticalangle,andzerotransmissionforincidentanglesabovethecritical angle,wheretotalinternalreectionoccurs.Thecriticalangle, q c ,iscalculatedbythe followingequation: q c =sin )]TJ/F20 8.9664 Tf 6.967 0 Td [(1 n o n org .A Theoutcouplingefciencyiscalculatedbyintegratingtheemissionprobabilityfunction T q f ,x,y,zinpolarcoordinates: h out = R t 0 R L 0 R L 0 R 2 p 0 R p 0 T q f x y z sin q d q d f d x d y d z R t 0 R L 0 R L 0 R 2 p 0 R p 0 sin q d q d f d x d y d z .A 213

PAGE 214

Fortheplanargeometryandsimplifyingassumptionsdescribedabove,T=1for q < q c andT=0for q > q c ,and h out reducesto h out = R q c 0 sin q d q R p 2 0 sin q d q =1 )]TJ/F20 11.9552 Tf 10.95 0 Td [(cos q c .A Theproblembecomesconsiderablymoredifculttosolveanalyticallywhenthe devicesurfaceexhibitsacurvaturethatispositionallydependent,asinthecasewhere microlensesareaddedtoincreaselightoutput.Inthiscase, T isafunctionofallve variables.Furthermore,topredicttheperformanceofrealdevices,wemusttakeinto accountabsorptionwithinthedeviceandatthemetalsurface. WedesignedacustomcomputermodelwritteninthecomputerlanguageJava specicallydesignedtocalculatethelightoutcouplingefciencyinOLEDdevices employingmicrolensarrays.Thismodelservedasthebasisonwhichaphotovoltaic absorptionmodelwasdesigned,presentedinChapter6.AlthoughthePVabsorption andOLEDoutcouplingmodelsdescribeverydifferentdevicefunctions,thecoreray opticsisequallyapplicableandmostlyreused.Foradescriptionoftherayoptics treatmentanddesignofthemodelitself,seethedescriptioninSection6.2.2.Both modelsusediscretelayerstructuresthoughwhichraysaretracked,andthemost prominentdifferenceintheOLEDmodelisthepresenceofanon-planarlayerstructure forthemicrolensarray.Inthecomputermodeling,thisstructureistreatedinthesame way,buttheinterfaceisdescribedbyspheresrecessedinaplanarsurface,thedepth andseparationofwhichdeterminethecontactangleandlensspacing. 214

PAGE 215

A.3VericationofModel Herewecomparemodelcalculationstoanalyticallyderivedoutcouplingefciencies inordertovalidatethemodel.Aswediscussed,theanalyticalapproachdoesnotgetus fardeterminingtheeffectsofmicrolensesonoutcouplingefciency.Nevertheless,the simplecalculationforaplanardeviceservesastheonlyclearbenchmarkfortestingthe validityofourmodel,asfewexperimentaldataexistatthepresenttimetocorroborate predictions.Themodel,then,mustpredictthesamevalueascalculatedbyEquation Aforthefollowingthreeconditions:lensabsent,lenspresentwhosecurvature ismathematicallymodeledyetstretchedtonearinnitescaleleavinganearlyplanar lens,andlenspresentinallcurvatureswiththelensindexequaltothatofair.All threeofthesebenchmarkstestthemodelindifferentwaysandwasusedtovalidatethe modelcoding.For n org =1.75,EquationAgives h out =17.93%.Thisvalue+/-0.3% errorwasreachedforallthreeconditionsoutlinedaboveusingtheraytracingmodel. A.4ResultsandConclusions Therayopticsmodelwasrunusingavarietyofparameters,toprobetheeffects oflensindexofrefraction,wavelengthofemission,encapsulationthickness,andlens spacing.FigureA-1plotsthelensindexofrefractiondependenceusinga1mmthick encapsulationlayerandcomparesthedifferenceineffectsbetweenuseofanaluminum cathodetotheuseofasilvercathode.Silverhashigherreectivity,anditisseenthatits useasacathodeproducesahigheroutcouplingefciency.Furthermore,theextraction efciencypeakswiththelensindexofrefractionequalstheindexofrefractionofthe organiclayers. 215

PAGE 216

FigureA-1.Extractionefciencyasafunctionofmicrolensindexofrefractioncomparing devicesusingaluminumandsilvercathodesand1000 m mencapsulation layerthickness. FigureA-2plotsthewavelengthofemissiondependence.Thesedependences mostlyfollowthereectivityofthemetals,asitisseenthatinthecaseofgold cathodes,theextractionefciencyfallsoffatshortwavelengthsduetoabsorptionof thewaveguidedmodesbythegoldcathode. FigureA-2.Extractionefciencyasafunctionofemissionwavelengthforseverallens packingarrangements. 216

PAGE 217

FigureA-3plotstheextractionefciencyasafunctionofmicrolenscontactangle usingseveraldifferentencapsulationthicknesses.Weexpectedthistobegreatestat g =90 ,andthemodeldoesinfactpredictthismaximum.However,wealsoseethatfor thinnerencapsulatethickness,theextractionefciencyplateausabove g > 30 )]TJ/F20 11.9552 Tf 10.95 0 Td [(40 FigureA-3.Extractionefciencyasafunctionofmicrolenscontactangle,comparing severalencapsulationlayerthicknessesusingbothaluminumcathodeleft andsilvercathodesright. Wealsoexploredtheeffectofthearrangementofthelenseswithrespecttoeach otherandtheinter-lensdistance.Wedenedtwolensstructures,squarelatticeand close-packed.Inthesquarelatticearrangement,thelensesarearrangedinasquare arrayequidistantfrom4neighborsinperpendicularrows.Theclose-packedarrayis showninFigureA-4.Forbothtypes,wealsointroducedaspacingparameterthatvaries thedistancebetweenlenses.FiguresA-5andA-6showthecombinedeffectsoflattice typeandspacingalongwithcomparisonsbetweenaluminumandsilvercathodesand encapsulationthickness. 217

PAGE 218

FigureA-4.Microlensspacingarrangementsusedinopticalsimulations.Topviewof lensspacingmethodologyforclose-packedarrangement.Thesimulationis connedtooneunitcell.Whenarayexitsthecell,itisre-mappedtothe equivalentpositioninthesamecell. FigureA-5.Extractionefciencyasafunctionofencapsulationlayerthicknessfor severallenspackingarrangementsusingbothaluminumcathodesleftand silvercathodesright.SLandCPdenotesquarelatticeandclose-packed, respectively.1 m mmarksthosesampleswithaspacerlayerof1 m m. Inconclusion,therayopticsmodelpredictsthatOLEDoutcouplingefciencycan beoptimizedtovaluesgreaterthan90%byusingasilvercathode,alensmaterialwith n lens = n org andindividualmicrolenseswith g =90 inclosepackedarrangementwith nospacing.Oftheseparameters,itismostimportanttoensurethatthelensindexof 218

PAGE 219

FigureA-6.Extractionefciencyasafunctionofmicrolensspacingforclosepacked structurewithencapsulationlayerthicknessof0.1 m mleftand10 m m rightforAlandAgcathodeswith45 and90 lenscontactangles. refractionisgreaterthanorequaltotheindexoftheactiveorganiclayers.Also,forthe top-emittinggeometryitisveryimportanttoincludeaspacinglayerwhichcandoublein functionasanencapsulationlayerwiththicknessonthesameorderofthediameterof theindividuallenses.Next,itisimportanttoachievelenscontactanglesofatleast45 Lessthanthis,theoutcouplingefciencyfallsoffquickly.Finally,outcouplingefciency willbegreatestwiththeleastamountofspacingbetweenlenses.Yet,withsufcient encapsulationspacing,smalllensspacinghasrelativelylittleeffectonoutcoupling efciency,sothereissometoleranceforthemicrolensfabricationprocess. 219

PAGE 220

APPENDIXB LISTOFPUBLICATIONSANDPRESENTATIONS Publications 1.W.T.HammondandJ.Xue.Spray-coatedorganicsolarcellefciencyenhancementbyincorporationofnon-solvatingdiluentinsolution.Inpreparation. 2.W.T.HammondandJ.Xue.Heterostructureorganicphotodiodeswithphotomultiplicationbyuseofconnementlayers.Inpreparation. 3.W.Cao,J.D.Myers,Y.Zheng,W.T.Hammond,andJ.Xue.Enhancedlight harvestinginorganicsolarcellsusingpyramidalrearreectors. Appl.Phys.Lett. Underreview. 4.E.Wrzesniewski,S.-H.Eom,W.T.Hammond,W.Cao,andJ.Xue.Transparent oxide/metal/oxidetrilayerelectrodeforuseintop-emittingorganiclight-emitting diodes. J.Photon.Energy .Underreview. 5.W.T.HammondandJ.Xue.Anorganicphotodiodedevicestructureexhibitinglow voltage,imaging-speedphotocurrentgain. Appl.Phys.Lett. 97:073302. 6.J.E.McKisson,W.Hammond,J.Proftt,A.G.Weisenberger.AJavadistributed acquisitionsystemforPETandSPECTimaging. Nucl.Sci.Symp.Conf.Rec. 3591-3593. 7.W.T.Hammond,E.L.Bradley,R.E.Welsh,J.Qian,A.G.Weisenberger,M.F. Smith,S.Majewski,M.S.Saha.Agammacamerare-evaluationofpotassium iodideblockingefciencyinmice. HealthPhysics 92:396-406. 8.J.Proftt,W.Hammond,S.Majewski,V.Popov,R.R.Raylman,A.G.Weisenberger.Implementationofahigh-rateUSBdataacquisitionsystemforPETand SPECTimaging. Nucl.Sci.Symp.Conf.Rec. 3063-3067. Presentations 1.W.T.HammondandJ.Xue.Ultrasonicspray-depositedpolymersolarcellswith improvedefciencybyuseofdiluentadditives.Presentedat2010MRSFall Meeting,Boston,MA,Nov2010. 2.W.T.HammondandJ.Xue.Vacuum-deposited,multilayerorganicphotodiodes exhibitingstrongandfastphotomultiplication.PresentedatFLAVS2010,Orlando, FL,Apr2010. 3.W.T.HammondandJ.Xue.Tunnelingorganicphotodiodesexhibitinglarge photomultiplicationunderlowbias.Presentedat2008MRSFallMeeting,Boston, MA,Dec2008. 220

PAGE 221

4.W.T.HammondandJ.Xue.Quantumefciencyinorganicphototransistors. Presentedat2008APSMarchMeeting,NewOrleans,LA,March2008. 5.W.Hammond,Y.Tekabe,L.Johnson,S.Majewski,V.Popov,B.Kross,R.Wojcik, A.G.Weisenberger,J.Proftt.Developmentofhighperformanceminigamma camerasbasedonLaBr3scintillatorandH8500andH9500PSPMTsandtheiruse insmallanimalstudies.Presentedat2006IEEEMedicalImagingConference,San Diego,CA,Nov2006. 6.W.Hammond,J.Cella,C.McLoughlin,K.Smith,R.Welsh,E.Bradley,M.Saha,J. Qian,S.Majewski,V.Popov,M.Smith,A.Weisenberger.Nuclearimagingofiodine uptakeinmousetissues.Presentedat2005APSAprilMeeting,Tampa,FL,Apr 2005. 221

PAGE 222

REFERENCES 1.Berberan-Santos,M.,editor. Fluorescenceofsupermolecules,polymers,and nanosystems Springer,2008. 2.Stokes,G.G.Onthechangeofrefrangibilityoflight. Phil.Trans.RoyalSoc. 142 463. 3.Wade,Jr.,L.G. OrganicChemistry ,volume6PearsonPrenticeHall,NJ,2006. 4.Spanggaard,H.&Krebs,F.C.Abriefhistoryofthedevelopmentoforganicand polymericphotovoltaics. Sol.EnergyMater.Sol.Cells 83 ,125. 5.Weiss,D.S.&Abkowitz,M.Advancesinorganicphotoconductortechnology. Chem.Rev. 110 ,479. 6.Kearns,D.&Calvin,M.Photovoltaiceffectandphotoconductivityinlaminated organicsystems. J.Chem.Phys. 29 ,950. 7.Pope,M.,Kallmann,H.P.,Chen,A.,&Gordon,P.Chargeinjectionintoorganic crystals:Inuenceofelectrodesondark-andphotoconductivity. J.Chem.Phys. 36 ,2486. 8.Helfrich,W.&Schneider,W.G.Recombinationradiationinanthracenecrystals. Phys.Rev.Lett. 14 ,229. 9.Tang,C.W.Two-layerorganicphotovoltaiccell. Appl.Phys.Lett. 48 ,183 10.Tang,C.W.&VanSlyke,S.A.Organicelectroluminescentdiodes. Appl.Phys.Lett. 51 ,913. 11.Green,M.A.,Emery,K.,Hishikawa,Y.,&Warta,W.Solarcellefciencytables version37. Prog.Photovolt:Res.Appl. 19 ,84. 12.Eom,S.-H. etal. Whitephosphorescentorganiclight-emittingdeviceswithdual triple-dopedemissivelayers. Appl.Phys.Lett. 94 ,153303. 13.Chiang,C.K. etal. Electricalconductivityindopedpolyacetylene. Phys.Rev.Lett. 39 ,1098. 14.Shaheen,S.E. etal. 2.5 Appl.Phys.Lett. 78 ,841. 15.Ramuz,M.,Brgi,L.,Winnewisser,C.,&Seitz,P.Highsensitivityorganic photodiodeswithlowdarkcurrentsandincreasedlifetimes. Org.Electron. 9 369. 16.Gong,X. etal. High-detectivitypolymerphotodetectorswithspectralresponse from300nmto1450nm. Science 325 ,1665. 222

PAGE 223

17.Lampert,M.A.&Mark,P. CurrentInjectioninSolids AcademicPress,NewYork, 1970. 18.Baldo,M.A.,O'Brien,D.F.,Thompson,M.E.,&Forrest,S.R.Excitonicsinglettripletratioinasemiconductingorganicthinlm. Phys.Rev.B 60 ,14422 19.Horvath,A.,Weiser,G.,Lapersonne-Meyer,C.,Schott,M.,&Spagnoli,S.Wannier excitonsandFranz-Keldysheffectofpolydiacetylenechainsdilutedintheirsingle crystalmonomermatrix. Phys.Rev.B 53 ,13507. 20.Pope,M.&Swenberg,C.E. ElectronicProcessesinOrganicCrystalsand Polymers ,2ndeditionOxfordUniversityPress,1999. 21.Lamansky,S. etal. Highlyphosphorescentbis-cyclometalatediridiumcomplexes: synthesis,photophysicalcharacterization,anduseinorganiclightemittingdiodes. J.Am.Chem.Soc. 123 ,4304. 22.Thorsmolle,V.K. etal. Morphologyeffectivelycontrolssinglet-tripletexciton relaxationandchargetransportinorganicsemiconductors. Phys.Rev.Lett. 102 017401. 23.Murphy,C.B. etal. ProbingF orsterandDexterenergy-transfermechanismsin uorescentconjugatedpolymerchemosensors. J.Phys.Chem.B 108 ,1537 24.Scully,S.R.&McGehee,M.D.Effectsofopticalinterferenceandenergytransfer onexcitondiffusionlengthmeasurementsinorganicsemiconductors. J.Appl. Phys. 100 ,034907. 25.Arkhipov,V.I.,Heremans,P.,Emelianova,E.V.,&B assler,H.Effectofdoping onthedensity-of-statesdistributionandcarrierhoppingindisorderedorganic semiconductors. Phys.Rev.B 71 ,045214. 26.Sze,S.&Ng,K. Physicsofsemiconductordevices ,3rdeditionJohnWileyand Sons,Inc.,2007. 27.Crispin,X. etal. Characterizationoftheinterfacedipoleatorganic/metal interfaces. J.Am.Chem.Soc. 124 ,8131. 28.Parker,I.D.Carriertunnelinganddevicecharacteristicsinpolymerlight-emitting diodes. J.Appl.Phys. 75 ,1656. 29.Baldo,M.A.&Forrest,S.R.Interface-limitedinjectioninamorphousorganic semiconductors. Phys.Rev.B 64 ,085201. 30.Ishii,H.,Sugiyama,K.,Ito,E.,&Seki,K.Energylevelalignmentandinterfacial electronicstructuresatorganic/metalandorganic/organicinterfaces. Adv.Mater. 11 ,625. 223

PAGE 224

31.Braun,S.,Salaneck,W.R.,&Fahlman,M.Energy-levelalignmentatorganic/metal andorganic/organicinterfaces. Adv.Mater. 21 ,1450. 32.Bolotin,K. etal. Ultrahighelectronmobilityinsuspendedgraphene. SolidState Commun. 146 ,351. 33.Horowitz,G.Organiceld-effecttransistors. Adv.Mater. 10 ,377. 34.Shtein,M.,Mapel,J.,Benziger,J.B.,&Forrest,S.R.Effectsoflmmorphology andgatedielectricsurfacepreparationontheelectricalcharacteristicsoforganicvapor-phase-depositedpentacenethin-lmtransistors. Appl.Phys.Lett. 81 268. 35.Walser,M.P.,Kalb,W.L.,Mathis,T.,Brenner,T.J.,&Batlogg,B.Stable complementaryinverterswithorganiceld-effecttransistorsoncytop uoropolymergatedielectric. Appl.Phys.Lett. 94 ,053303. 36.Singh,T.B.,Gunes,S.,Marjanovic,N.,Sariciftci,N.S.,&Menon,R.Correlation betweenmorphologyandambipolartransportinorganiceld-effecttransistors. J. Appl.Phys. 97 ,114508. 37.McCarthy,M.A. etal. Reorientationofthehighmobilityplaneinpentacene-based carbonnanotubeenabledverticaleldeffecttransistors. ACSNano 5 ,291 38.McCarthy,M.A.,Liu,B.,&Rinzler,A.G.Highcurrent,lowvoltagecarbon nanotubeenabledverticalorganiceldeffecttransistors. NanoLett. 10 ,3467 3472. 39.Karl,N.Chargecarriertransportinorganicsemiconductors. Synth.Met. 133-134 649. 40.Goodman,A.M.Electronhalleffectinsilicondioxide. Phys.Rev. 164 ,1145 41.Schneider,P.M.&Fowler,W.B.Bandstructureandopticalpropertiesofsilicon dioxide. Phys.Rev.Lett. 36 ,425. 42.Kahn,A.,Koch,N.,&Gao,W.Electronicstructureandelectricalpropertiesof interfacesbetweenmetalsandpi-conjugatedmolecularlms. J.Polym.Sci.PartB: Polym.Phys. 41 ,2529. 43.Djurovich,P.I.,Mayo,E.I.,Forrest,S.R.,&Thompson,M.E.Measurement ofthelowestunoccupiedmolecularorbitalenergiesofmolecularorganic semiconductors. Org.Electron. 10 ,515. 44.Guan,Z.-L. etal. Directdeterminationoftheelectronicstructureofthepolyhexylthiophene:phenyl-[6,6]-C 61 butyricacidmethylesterblend. Org.Electron. 11 1779. 224

PAGE 225

45.Ding,H.&Gao,Y.Electronicstructureatrubrenemetalinterfaces. Appl.Phys.A: Mater.Sci.Process. 95 ,89. 46.Zahn,D.R.,Gavrila,G.N.,&Gorgoi,M.Thetransportgapoforganicsemiconductorsstudiedusingthecombinationofdirectandinversephotoemission. Chem. Phys. 325 ,99. 47.Rand,B.P.,Burk,D.P.,&Forrest,S.R.Offsetenergiesatorganicsemiconductor heterojunctionsandtheirinuenceontheopen-circuitvoltageofthin-lmsolar cells. Phys.Rev.B 75 ,115327. 48.Wang,S. etal. Highefciencyorganicphotovoltaiccellsbasedonavapor depositedsquarainedonor. Appl.Phys.Lett. 94 ,233304. 49.Placencia,D. etal. Organicphotovoltaiccellsbasedonsolvent-annealed,textured titanylphthalocyanine/c60heterojunctions. Adv.Funct.Mater. 19 ,1913 50.vanEijk,C.W.E.Inorganicscintillatorsinmedicalimagingdetectors. Nucl.Instr. Meth.Phys.Res.A 509 ,17. 51.Campbell,I.&Crone,B.Quantum-dot/organicsemiconductorcompositesfor radiationdetection. Adv.Mater. 18 ,77. 52.Ko,H.C. etal. Ahemisphericalelectroniceyecamerabasedoncompressible siliconoptoelectronics. Nature 454 ,748. 53.Peumans,P.,Bulovic,V.,&Forrest,S.R.Efcient,high-bandwidthorganic multilayerphotodetectors. Appl.Phys.Lett. 76 ,3855. 54.Horowitz,P.&Hill,W. TheArtofElectronics CambridgeUniversityPress,1989. 55.Halls,J.J.M. etal. Efcientphotodiodesfrominterpenetratingpolymernetworks. Nature 376 ,498. 56.Xue,J.&Forrest,S.R.Carriertransportinmultilayerorganicphotodetectors:II. effectsofanodepreparation. J.Appl.Phys. 95 ,1869. 57.Rauch,T. etal. Near-infraredimagingwithquantum-dot-sensitizedorganic photodiodes. Nat.Photon. 3 ,332. 58.Konstantatos,G. etal. Ultrasensitivesolution-castquantumdotphotodetectors. Nature 442 ,180. 59.Cova,S.,Ghioni,M.,Lacaita,A.,Samori,C.,&Zappa,F.Avalanchephotodiodes andquenchingcircuitsforsingle-photondetection. Appl.Opt. 35 ,1956 60.Reynaert,J.,Arkhipov,V.,Heremans,P.,&Poortmans,J.Photomultiplicationin disorderedunipolarorganicmaterials. Adv.Funct.Mater. 16 ,784. 225

PAGE 226

61.Campbell,I.H.&Crone,B.K.Bulkphotoconductivegaininpolyphenylene vinylenebaseddiodes. J.Appl.Phys. 101 ,024502. 62.Gao,J.&Hegmann,F.A.Bulkphotoconductivegaininpentacenethinlms. Appl. Phys.Lett. 93 ,223306. 63.Campbell,I.H.&Crone,B.K.Anearinfraredorganicphotodiodewithgainatlow biasvoltage. Appl.Phys.Lett. 95 ,263302. 64.Hiramoto,M.,Imahigashi,T.,&Yokoyama,M.Photocurrentmultiplicationin organicpigmentlms. Appl.Phys.Lett. 64 ,187. 65.Hiramoto,M.,Nakayama,K.,Sato,I.,Kumaoka,H.,&Yokoyama,M.Photocurrent multiplicationphenomenaatorganic/metalandorganic/organicinterfaces. Thin SolidFilms 331 ,71. 66.Katsume,T.,Hiramoto,M.,&Yokoyama,M.Photocurrentmultiplicationin naphthalenetetracarboxylicanhydridelmatroomtemperature. Appl.Phys. Lett. 69 ,3722. 67.Nakayama,K.,Hiramoto,M.,&Yokoyama,M.Directtracingofthephotocurrent multiplicationprocessinanorganicpigmentlm. J.Appl.Phys. 84 ,6154 68.Nakayama,K.,Hiramoto,M.,&Yokoyama,M.Photocurrentmultiplicationat organic/metalinterfaceandsurfacemorphologyoforganiclms. J.Appl.Phys. 87 3365. 69.Huang,J.&Yang,Y.OriginofphotomultiplicationinC 60 baseddevices. Appl. Phys.Lett. 91 ,203505. 70.Chen,H.-Y.,F.,L.K.,Yang,G.,Monbouquette,H.G.,&Yang,Y.Nanoparticleassistedhighphotoconductivegainincompositesofpolymerandfullerene. Nat. Nano. 3 ,543. 71.Konstantatos,G.,Clifford,J.,Levina,L.,&Sargent,E.H.Sensitivesolutionprocessedvisible-wavelengthphotodetectors. Nat.Photon. 1 ,531. 72.Konstantatos,G.,Levina,L.,Tang,J.,&Sargent,E.H.SensitivesolutionprocessedBi 2 S 3 nanocrystallinephotodetectors. NanoLett. 8 ,4002. 73.Kittel,C.&Kroemer,H. ThermalPhysics ,2ndeditionW.H.Freeman,1980. 74.Dunkley,J. etal. Five-yearwilkinsonmicrowaveanisotropyprobeWMAP observations:Bayesianestimationofcosmicmicrowavebackgroundpolarization maps. Astrophys.J. 701 ,1804. 75.Lee,R.L.Mietheory,airytheory,andthenaturalrainbow. Appl.Opt. 37 ,1506 1519. 226

PAGE 227

76.Shockley,W.&Queisser,H.J.Detailedbalancelimitofefciencyofp-njunction solarcells. J.Appl.Phys. 32 ,510. 77.Green,M.A.Thepathto25%siliconsolarcellefciency:Historyofsiliconcell evolution. Prog.Photovolt:Res.Appl. 17 ,183. 78.Green,M.A.Thin-lmsolarcells:reviewofmaterials,technologiesand commercialstatus. J.Mater.Sci.:Mater.Electron. 18 ,15. 79.O'Rourke,S.,Kim,P.,&Polavarapu,H. SolarPhotovoltaicIndustry2010Global Outlook:Dejavu? DeutscheBank,2010. 80.Xue,J.Perspectivesonorganicphotovoltaics. Polym.Rev. 50 ,411. 81.Krebs,F.C.Fabricationandprocessingofpolymersolarcells:Areviewofprinting andcoatingtechniques. Sol.EnergyMater.Sol.Cells 93 ,394412. 82.Krebs,F.C.Allsolutionroll-to-rollprocessedpolymersolarcellsfreefromindiumtin-oxideandvacuumcoatingsteps. Org.Electron. 10 ,761. 83.Yu,G.,Gao,J.,Hummelen,J.C.,Wudl,F.,&Heeger,A.J.Polymer photovoltaiccells:Enhancedefcienciesviaanetworkofinternaldonor-acceptor heterojunctions. Science 270 ,1789. 84.Xue,J.,Rand,B.,Uchida,S.,&Forrest,S.Ahybridplanar-mixedmolecular heterojunctionphotovoltaiccell. Adv.Mater. 17 ,66. 85.O'Regan,B.&Gratzel,M.Alow-cost,high-efciencysolarcellbasedondyesensitizedcolloidalTiO 2 lms. Nature 353 ,737. 86.Ravirajan,P. etal. Hybridpolymer/zincoxidephotovoltaicdeviceswithvertically orientedZnOnanorodsandanamphiphilicmolecularinterfacelayer. J.Phys. Chem.B 110 ,7635. 87.Olson,D.C.,Piris,J.,Collins,R.T.,Shaheen,S.E.,&Ginley,D.S.Hybrid photovoltaicdevicesofpolymerandZnOnanobercomposites. ThinSolidFilms 496 ,26. 88.Oosterhout,S.D. etal. Theeffectofthree-dimensionalmorphologyonthe efciencyofhybridpolymersolarcells. Nat.Mater. 8 ,818. 89.Zhou,R.,Yang,Y.,Zheng,Y.,&Xue,J.unpublishedresults. 90.Mayer,A.C. etal. Bimolecularcrystalsoffullerenesinconjugatedpolymers andtheimplicationsofmolecularmixingforsolarcells. Adv.Funct.Mater. 19 1173. 91.Zhao,J. etal. Phasediagramofp3ht/pcbmblendsanditsimplicationforthe stabilityofmorphology. J.Phys.Chem.B 113 ,1587. 227

PAGE 228

92.Kim,Y. etal. Astrongregioregularityeffectinself-organizingconjugatedpolymer lmsandhigh-efciencypolythiophene:fullerenesolarcells. Nat.Mater. 5 ,197 203. 93.Dai,J.,Jiang,X.,Wang,H.,&Yan,D.Organicphotovoltaiccellswithnearinfrared absorptionspectrum. Appl.Phys.Lett. 91 ,253503. 94.Rand,B.P.,Xue,J.,Yang,F.,&Forrest,S.R.Organicsolarcellswithsensitivity extendingintothenearinfrared. Appl.Phys.Lett. 87 ,233508. 95.Yang,F.,Lunt,R.R.,&Forrest,S.R.Simultaneousheterojunctionorganicsolar cellswithbroadspectralsensitivity. Appl.Phys.Lett. 92 ,053310. 96.Bailey-Salzman,R.F.,Rand,B.P.,&Forrest,S.R.Near-infraredsensitivesmall moleculeorganicphotovoltaiccellsbasedonchloroaluminumphthalocyanine. Appl.Phys.Lett. 91 ,013508. 97.Peet,J. etal. Efciencyenhancementinlow-bandgappolymersolarcellsby processingwithalkanedithiols. Nat.Mater. 6 ,497. 98.Myers,J.D.,Eom,S.-H.,Cassidy,V.,&Xue,J.Enhancedorganicphotovoltaiccell performanceusingtransparentmicrolensarrays.in MRSFallMeeting ,number G5.6,2010. 99.Cao,W.,Myers,J.D.,&Xue,J.Lighttrappinginorganicsolarcellsusing pyramidalrearreectors.in MRSFallMeeting ,numberG5.10,2010. 100.Rim,S.-B.,Zhao,S.,Scully,S.R.,McGehee,M.D.,&Peumans,P.Aneffective lighttrappingcongurationforthin-lmsolarcells. Appl.Phys.Lett. 91 ,243501 101.Brabec,C.J. etal. Originoftheopencircuitvoltageofplasticsolarcells. Adv. Funct.Mater. 11 ,374. 102.Vandewal,K.,Tvingstedt,K.,Gadisa,A.,Inganas,O.,&Manca,J.V.Ontheorigin oftheopen-circuitvoltageofpolymer-fullerenesolarcells. Nat.Mater. 8 ,904 103.Kinoshita,Y.,Takenaka,R.,&Murata,H.Independentcontrolofopen-circuit voltageoforganicsolarcellsbychanginglmthicknessofMoO 3 bufferlayer. Appl. Phys.Lett. 92 ,243309. 104.Kim,D.Y. etal. Theeffectofmolybdenumoxideinterlayeronorganicphotovoltaic cells. Appl.Phys.Lett. 95 ,093304. 105.Hancox,I. etal. Theeffectofamooxhole-extractinglayerontheperformanceof organicphotovoltaiccellsbasedonsmallmoleculeplanarheterojunctions. Org. Electron. 11 ,2019. 228

PAGE 229

106.Gebeyehu,D. etal. Highlyefcientp-i-ntypeorganicphotovoltaicdevices. Thin SolidFilms 451-452 ,29. 107.Ross,R.B. etal. Endohedralfullerenesfororganicphotovoltaicdevices. Nat. Mater. 8 ,208. 108.Xue,J.,Uchida,S.,Rand,B.P.,&Forrest,S.R.Asymmetrictandemorganic photovoltaiccellswithhybridplanar-mixedmolecularheterojunctions. Appl.Phys. Lett. 85 ,5757. 109.Dennler,G. etal. Designofefcientorganictandemcells:Ontheinterplay betweenmolecularabsorptionandlayersequence. J.Appl.Phys. 102 ,123109 110.Steim,R.,Kogler,F.R.,&Brabec,C.J.Interfacematerialsfororganicsolarcells. J.Mater.Chem. 20 ,2499. 111.Jrgensen,M.,Norrman,K.,&Krebs,F.C.Stability/degradationofpolymersolar cells. Sol.EnergyMater.Sol.Cells 92 ,686. 112.Granstrom,J. etal. Encapsulationoforganiclight-emittingdevicesusinga peruorinatedpolymer. Appl.Phys.Lett. 93 ,193304. 113.Lin,Y.-Y.,Gundlach,D.,Nelson,S.,&Jackson,T.Pentacene-basedorganic thin-lmtransistors. IEEET.Electron.Dev. 44 ,1325. 114.Dimitrakopoulos,C.D.&Mascaro,D.J.Organicthin-lmtransistors:Areviewof recentadvances. IBMJ.Res.Dev. 45 ,11. 115.Takeya,J. etal. Veryhigh-mobilityorganicsingle-crystaltransistorswithin-crystal conductionchannels. Appl.Phys.Lett. 90 ,102120. 116.Briseno,A.L. etal. Patterningorganicsingle-crystaltransistorarrays. Nature 444 913. 117.Liu,S.,Wang,W.M.,Briseno,A.L.,Mannsfeld,S.C.B.,&Bao,Z.Controlled depositionofcrystallineorganicsemiconductorsforeld-effect-transistor applications. Adv.Mater. 21 ,1217. 118.V olkel,A.R.,Street,R.A.,&Knipp,D.Carriertransportanddensityofstate distributionsinpentacenetransistors. Phys.Rev.B 66 ,195336. 119.Gu,G.,Kane,M.G.,Doty,J.E.,&Firester,A.H.Electrontrapsandhysteresisin pentacene-basedorganicthin-lmtransistors. Appl.Phys.Lett. 87 ,243512. 120.Chua,L.-L. etal. Generalobservationofn-typeeld-effectbehaviourinorganic semiconductors. Nature 434 ,194. 229

PAGE 230

121.Kalb,W.L.,Mathis,T.,Haas,S.,Stassen,A.F.,&Batlogg,B.Organicsmall moleculeeld-effecttransistorswithCytopgatedielectric:Eliminatinggatebias stresseffects. Appl.Phys.Lett. 90 ,092104. 122.Facchetti,A.,Yoon,M.H.,&Marks,T.J.Gatedielectricsfororganiceld-effect transistors:Newopportunitiesfororganicelectronics. Adv.Mater. 17 ,1705 123.Zhou,J.,Zhang,F.,Lan,L.,Wen,S.,&Peng,J.Inuenceofpolymerdielectricson C 60 -basedeld-effecttransistors. Appl.Phys.Lett. 91 ,253507. 124.Zhang,X.-H.,Domercq,B.,&Kippelen,B.High-performanceandelectrically stableC 60 organiceld-effecttransistors. Appl.Phys.Lett. 91 ,092114. 125.Jang,J.,Kim,J.W.,Park,N.,&Kim,J.-J.AirstableC 60 basedn-typeorganic eldeffecttransistorusingaperuoropolymerinsulator. Org.Electron. 9 ,481 126.Novak,M. etal. Low-voltagep-andn-typeorganicself-assembledmonolayereld effecttransistors. NanoLett. 11 ,156. 127.Kitamura,M.&Arakawa,Y.Low-voltage-operatingcomplementaryinverterswith C 60 andpentacenetransistorsonglasssubstrates. Appl.Phys.Lett. 91 ,053505 128.Kim,S.-J.&Lee,J.-S.Flexibleorganictransistormemorydevices. NanoLett. 10 2884. 129.Kato,Y. etal. Large-areaexibleultrasonicimagingsystemwithanorganic transistoractivematrix. IEEET.Electron.Dev. 57 ,995. 130.Mathews,N.,Fichou,D.,Menard,E.,Podzorov,V.,&Mhaisalkar,S.G.Steadystateandtransientphotocurrentsinrubrenesinglecrystalfree-spacedielectric transistors. Appl.Phys.Lett. 91 ,212108. 131.Tang,Q. etal. Photoswitchesandphototransistorsfromorganicsingle-crystalline sub-micro/nanometerribbons. Adv.Mater. 19 ,2624. 132.Saragi,T.,Fetten,M.,&Salbeck,J.Solution-processedorganicthin-lm phototransistorsbasedondonor/acceptordyad. Appl.Phys.Lett. 90 ,253506 133.Meixner,R.M.,Gobel,H.,Yildirim,F.A.,Bauhofer,W.,&Krautschneider,W. Wavelength-selectiveorganiceld-effectphototransistorsbasedondye-doped poly-3-hexylthiophene. Appl.Phys.Lett. 89 ,092110. 134.Hu,Y.,Dong,G.,Liu,C.,Wang,L.,&Qiu,Y.Dependencyoforganic phototransistorpropertiesonthedielectriclayers. Appl.Phys.Lett. 89 ,072108 230

PAGE 231

135.Marjanovic,N. etal. Photoresponseoforganiceld-effecttransistorsbasedon conjugatedpolymer/fullereneblends. Org.Electron. 7 ,188. 136.Mas-Torrent,M.,Hadley,P.,Crivillers,N.,Veciana,J.,&Rovira,C.Largephotoresponsivityinhigh-mobilitysingle-crystalorganiceld-effectphototransistors. ChemPhysChem 7 ,86. 137.Saragi,T.,Onken,K.,Suske,I.,Fuhrmann-Lieker,T.,&Salbeck,J.Ambipolar organicphototransistor. Opt.Mater. 29 ,1332. 138.Saragi,T.,Pudzich,R.,Fuhrmann-Lieker,T.,&Salbeck,J.Lightresponsive amorphousorganiceld-effecttransistorbasedonspiro-linkedcompound. Opt. Mater. 29 ,879. 139.Choi,J.-M. etal. Comparativestudyofthephotoresponsefromtetracene-based andpentacene-basedthin-lmtransistors. Appl.Phys.Lett. 88 ,043508. 140.Noh,Y.-Y.,Kim,D.-Y.,&Yase,K.Highlysensitivethin-lmorganicphototransistors: Effectofwavelengthoflightsourceondeviceperformance. J.Appl.Phys. 98 074505. 141.Noh,Y.-Y. etal. High-photosensitivityp-channelorganicphototransistorsbasedon abiphenylend-cappedfusedbithiopheneoligomer. Appl.Phys.Lett. 86 ,043501 142.Hamilton,M.&Kanicki,J.Organicpolymerthin-lmtransistorphotosensors. IEEE J.Select.TopicsQuan.Electron. 10 ,840. 143.Saragi,T.,Pudzich,R.,Fuhrmann,T.,&Salbeck,J.Organicphototransistorbased onintramolecularchargetransferinabifunctionalspirocompound. Appl.Phys. Lett. 84 ,2334. 144.Zukawa,T.,Naka,S.,Okada,H.,&Onnagawa,H.Organicheterojunction phototransistor. J.Appl.Phys. 91 ,1171. 145.Gu,G.&Kane,M.G.Moistureinducedelectrontrapsandhysteresisin pentacene-basedorganicthin-lmtransistors. Appl.Phys.Lett. 92 ,053305 146.Wang,S.D.,Minari,T.,Miyadera,T.,Aoyagi,Y.,&Tsukagoshi,K.Biasstress instabilityinpentacenethinlmtransistors:Contactresistancechangeand channelthresholdvoltageshift. Appl.Phys.Lett. 92 ,063305. 147.Gu,G.,Kane,M.G.,&Mau,S.-C.Reversiblememoryeffectsandacceptorstates inpentacene-basedorganicthin-lmtransistors. J.Appl.Phys. 101 ,014504 148.Debucquoy,M. etal. Correlationbetweenbiasstressinstabilityandphototransistor operationofpentacenethin-lmtransistors. Appl.Phys.Lett. 91 ,103508. 231

PAGE 232

149.Campbell,I.H.&Crone,B.K.Efcientplasticscintillatorsutilizingphosphorescent dopants. Appl.Phys.Lett. 90 ,012117. 150.Matsunobu,G.,Oishi,Y.,Yokoyama,M.,&Hiramoto,M.High-speedmultiplicationtypephotodetectingdeviceusingorganiccodepositedlms. Appl.Phys.Lett. 81 1321. 151.Zheng,Y.,Bekele,R.,Ouyang,J.,&Xue,J.Organicphotovoltaiccellswith verticallyalignedcrystallinemolecularnanorods. Org.Electron. 10 ,16211625 152.Tanida,S.,Noda,K.,Kawabata,H.,&Matsushige,K.N-channelthin-lm transistorsbasedon1,4,5,8-naphthalenetetracarboxylicdianhydridewithultrathin polymergatebufferlayer. ThinSolidFilms 518 ,571. 153.Peumans,P.&Forrest,S.R.Very-high-efciencydouble-heterostructurecopper phthalocyanine/C 60 photovoltaiccells. Appl.Phys.Lett. 79 ,126. 154.Rand,B.P.,Xue,J.,Uchida,S.,&Forrest,S.R.Mixeddonor-acceptormolecular heterojunctionsforphotovoltaicapplications.I.Materialproperties. J.Appl.Phys. 98 ,124902. 155.Xue,J.,Rand,B.P.,Uchida,S.,&Forrest,S.R.Mixeddonor-acceptormolecular heterojunctionsforphotovoltaicapplications.II.Deviceperformance. J.Appl.Phys. 98 ,124903. 156.Peumans,P.,Yakimov,A.,&Forrest,S.R.Smallmolecularweightorganicthin-lm photodetectorsandsolarcells. J.Appl.Phys. 93 ,3693. 157.Gommans,H. etal. Ontheroleofbathocuproineinorganicphotovoltaiccells. Adv. Funct.Mater. 18 ,3686. 158.Forrest,S.R.Ultrathinorganiclmsgrownbyorganicmolecularbeamdeposition andrelatedtechniques. Chem.Rev. 97 ,1793. 159.Brewer,P.,Lane,P.,deMello,A.,Bradley,D.,&deMello,J.Internaleldscreening inpolymerlight-emittingdiodes. Adv.Funct.Mater. 14 ,562. 160.Hoven,C.V. etal. Electroninjectionintoorganicsemiconductordevicesfromhigh workfunctioncathodes. Proc.Natl.Acad.Sci. 105 ,12730. 161.Blom,P.W.M.,Mihailetchi,V.D.,Koster,L.J.A.,&Markov,D.E.Devicephysics ofpolymer:fullerenebulkheterojunctionsolarcells. Adv.Mater. 19 ,1551 162.Cheng,Y.,Yang,S.,&Hsu,C.Synthesisofconjugatedpolymersfororganicsolar cellapplications. Chem.Rev. 232

PAGE 233

163.Batchelder,J.S.,Zewail,A.H.,&Cole,T.Luminescentsolarconcentrators.1: Theoryofoperationandtechniquesforperformanceevaluation. Appl.Opt. 18 3090. 164.Sark,W.G.V. etal. Luminescentsolarconcentratorsareviewofrecentresults. Opt.Express 16 ,21773. 165.Currie,M.J.,Mapel,J.K.,Heidel,T.D.,Goffri,S.,&Baldo,M.A.High-efciency organicsolarconcentratorsforphotovoltaics. Science 321 ,226. 166.Palik,E.D.,editor. HandbookofOpticalConstantsofSolids ,volume2Academic Press,1991. 167.Sch afer,F.,editor. DyeLasers ,volume1SpringerBerlin/Heidelberg,1973. 168.Johnson,I.Review:Fluorescentprobesforlivingcells. Histochem.J. 30 ,123 169.Resch-Genger,U.,Grabolle,M.,Cavaliere-Jaricot,S.,Nitschke,R.,&Nann,T. Quantumdotsversusorganicdyesasuorescentlabels. Nat.Meth. 5 ,763 170.Kubin,R.F.&Fletcher,A.N.Fluorescencequantumyieldsofsomerhodamine dyes. J.Lumin. 27 ,455. 171.Chen,C.H.,Tang,C.W.,Shi,J.,&Klubek,K.P.Improvedreddopantsfororganic electroluminescentdevices. Macromol.Symp. 125 ,49. 172.Bulovi c,V. etal. Bright,saturated,red-to-yelloworganiclight-emittingdevices basedonpolarization-inducedspectralshifts. Chem.Phys.Lett. 287 ,455 173.Schmidt,H. etal. Efcientsemitransparentinvertedorganicsolarcellswithindium tinoxidetopelectrode. Appl.Phys.Lett. 94 ,243302. 174.Silvestri,F. etal. Efcientsquaraine-basedsolutionprocessablebulkheterojunctionsolarcells. J.Am.Chem.Soc. 130 ,17640. 175.Krebs,F.C. etal. Aroundrobinstudyofexiblelarge-arearoll-to-rollprocessed polymersolarcellmodules. Sol.EnergyMater.Sol.Cells 93 ,1968. 176.Steirer,K.X. etal. Ultrasonicallysprayedandinkjetprintedthinlmelectrodesfor organicsolarcells. ThinSolidFilms 517 ,2781. 177.Steirer,K.X. etal. Ultrasonicspraydepositionforproductionoforganicsolarcells. Sol.EnergyMater.Sol.Cells 93 ,447453. 178.Girotto,C.,Rand,B.P.,Steudel,S.,Genoe,J.,&Heremans,P.Nanoparticlebased,spray-coatedsilvertopcontactsforefcientpolymersolarcells. Org. Electron. 10 ,735. 233

PAGE 234

179.Girotto,C.,Rand,B.P.,Genoe,J.,&Heremans,P.Exploringspraycoatingas adepositiontechniqueforthefabricationofsolution-processedsolarcells. Sol. EnergyMater.Sol.Cells 93 ,454. 180.Kim,S. etal. Spin-andspray-depositedsingle-walledcarbon-nanotubeelectrodes fororganicsolarcells. Adv.Funct.Mater. 20 ,2310. 181.Park,S.-E. etal. Spraydepositionofelectrohydrodynamicallyatomizedpolymer mixtureforactivelayerfabricationinorganicphotovoltaics. Sol.EnergyMater.Sol. Cells 95 ,352. 182.Kim,K.-J. etal. Inspectionofsubstrate-heatedmodiedpedot:pssmorphology forallspraydepositedorganicphotovoltaics. Sol.EnergyMater.Sol.Cells 94 1303. 183.Na,S.-I. etal. Fullyspray-coatedito-freeorganicsolarcellsforlow-costpower generation. Sol.EnergyMater.Sol.Cells 94 ,1333. 184.Tedde,S.F. etal. Fullyspraycoatedorganicphotodiodes. NanoLett. 9 ,980 185.Abdellah,A.,Fabel,B.,Lugli,P.,&Scarpa,G.Spraydepositionoforganic semiconductingthin-lms:Towardsthefabricationofarbitraryshapedorganic electronicdevices. Org.Electron. 11 ,1031. 186.Chan,C.K. etal. Highperformanceairbrushedorganicthinlmtransistors. Appl. Phys.Lett. 96 ,133304. 187.Green,R. etal. Performanceofbulkheterojunctionphotovoltaicdevicesprepared byairbrushspraydeposition. Appl.Phys.Lett. 92 ,033301. 188.Girotto,C.,Moia,D.,Rand,B.P.,&Heremans,P.High-performanceorganicsolar cellswithspraycoatedhole-transportandactivelayers. Adv.Funct.Mater. 21 64. 189.Deegan,R.D. etal. Capillaryowasthecauseofringstainsfromdriedliquid drops. Nature 389 ,827. 190.Deegan,R.D. etal. Contactlinedepositsinanevaporatingdrop. Phys.Rev.E 62 756. 191.Soltman,D.&Subramanian,V.Inkjet-printedlinemorphologiesandtemperature controlofthecoffeeringeffect. Langmuir 24 ,2224. 192.Hu,H.&Larson,R.G.Marangonieffectreversescoffee-ringdepositions. J.Phys. Chem.B 110 ,7090. 193.Aharon,I.&Shaw,B.D.Marangoniinstabilityofbi-componentdropletgasication. Phys.Fluids 8 ,1820. 234

PAGE 235

194.Fanton,X.&Cazabat,A.M.Spreadingandinstabilitiesinducedbyasolutal marangonieffect. Langmuir 14 ,2554. 195.Haynes,W.M.&Lide,D.R.,editors. CRCHandbookofChemistryandPhysics CRCPress,2011. 196. NIOSHPocketGuidetoChemicalHazards DepartmentofHealthandHuman Services,CenterforDiseaseControlandPrevention,NationalInstitutefor OccupationalSafetyandHealth,2007. 197.Bouse,L.F.Effectofnozzletypeandoperationonspraydropletsize. Trans.ASAE 37 ,1389. 198.Bouse,L.F.,Kirk,I.W.,&Bode,L.E.Effectofspraymixtureondropletsize. Trans.ASAE 33 ,0783. 199.Hoffmann,W.C. etal. Sprayadjuvanteffectsondropletsizespectrameasured bythreelaser-basedsystemsinahigh-speedwindtunnel. J.ASTMInternat. 5 JAI1012333. 200.Hoppe,H.&Sariciftci,N.S.Morphologyofpolymer/fullerenebulkheterojunction solarcells. J.Mater.Chem. 16 ,45. 201.Nilsson,S.,Bernasik,A.,Budkowski,A.,&Moons,E.Morphologyandphase segregationofspin-castedlmsofpolyuorene/pcbmblends. Macromolecules 40 8291. 202.ASTME973. StandardTestMethodforDeterminationoftheSpectralMismatch ParameterbetweenaPhotovoltaicDeviceandaPhotovoltaicReferenceCell AmericanSocietyforTestingandMaterials,WestConshocken,PA,2002. 203.Ma,W.,Yang,C.,Gong,X.,Lee,K.,&Heeger,A.Thermallystable,efcient polymersolarcellswithnanoscalecontroloftheinterpenetratingnetwork morphology. Adv.Funct.Mater. 15 ,1617. 204.Yang,X. etal. Nanoscalemorphologyofhigh-performancepolymersolarcells. NanoLett. 5 ,579. 205.Park,S.H. etal. Bulkheterojunctionsolarcellswithinternalquantumefciency approaching100 Nat.Photon. 3 ,297. 206.Chen,L.-M.,Hong,Z.,Li,G.,&Yang,Y.Recentprogressinpolymersolarcells: Manipulationofpolymer:fullerenemorphologyandtheformationofefcient invertedpolymersolarcells. Adv.Mater. 21 ,1434. 207.Li,G.,Shrotriya,V.,Yao,Y.,&Yang,Y.Investigationofannealingeffectsandlm thicknessdependenceofpolymersolarcellsbasedonpoly-hexylthiophene. J. Appl.Phys. 98 ,043704. 235

PAGE 236

208.Moule,A.J.,Bonekamp,J.B.,&Meerholz,K.Theeffectofactivelayerthickness andcompositionontheperformanceofbulk-heterojunctionsolarcells. J.Appl. Phys. 100 ,094503. 209.Monestier,F. etal. Modelingtheshort-circuitcurrentdensityofpolymersolarcells basedonp3ht:pcbmblend. Sol.EnergyMater.Sol.Cells 91 ,405. 210.Zeng,L.,Tang,C.W.,&Chen,S.H.Effectsofactivelayerthicknessandthermal annealingonpolythiophene:Fullerenebulkheterojunctionphotovoltaicdevices. Appl.Phys.Lett. 97 ,053305. 211.Brabec,C.J.,Shaheen,S.E.,Winder,C.,Sariciftci,N.S.,&Denk,P.Effectof LiF/metalelectrodesontheperformanceofplasticsolarcells. Appl.Phys.Lett. 80 1288. 212.Bavel,S.S.v.,Sourty,E.,With,G.d.,&Loos,J.Three-dimensionalnanoscale organizationofbulkheterojunctionpolymersolarcells. NanoLett. 9 ,507 213.Campoy-Quiles,M. etal. Morphologyevolutionviaself-organizationandlateral andverticaldiffusioninpolymer:fullerenesolarcellblends. Nat.Mater. 7 ,158 214.Beal,R.M. etal. Themolecularstructureofpolymerfullerenecompositesolarcells anditsinuenceondeviceperformance. Macromolecules 43 ,2343. 215.Sun,Y.&Forrest,S.R.Organiclightemittingdeviceswithenhancedoutcoupling viamicrolensesfabricatedbyimprintlithography. J.Appl.Phys. 100 ,073106 236

PAGE 237

BIOGRAPHICALSKETCH WilliamThomasHammondgrewupinHampstead,Marylandwithhisparents BarbaraandTomandhisbrotherKevin.Hespenthischildhoodcollectingbaseball cards,buildingforts,andearningbaseballandbasketballtrophies.Whilehepreferred sportstoacademics,BilltookhisstudiesseriouslyatSacredHeartSchoolinGlyndon andLoyolaBlakeeldHighSchoolinTowson.HelefthometoattendtheCollegeof WilliamandMaryinVirginia,andgraduatedin2005withaBachelorofSciencein physicsandadesiretoapplyscienceinhislifeandcareer. Aftercollege,Billextendedhisseniorthesisworkinhealthphysicsandjoined theDetectorandImagingGroupattheJeffersonNationalAcceleratorFacilityin NewportNews,Virginia.AtJLab,heworkedwithateamonprototypepositronemission tomographyPETimagersdedicatedtothedetectionofbreastcancerandheart disease.Billthendecidedtoattendgraduateschooltoextendhisphysicseducation andpursuehispassionforsolarenergy.InAugustof2006,heenrolledattheUniversity ofFloridatostudyelectronicmaterialsintheDepartmentofMaterialsScienceand Engineering,andjoinedProf.JiangengXue'sorganicelectronicsresearchgroup.His researchworkatUFfocusedonthedesignofnewdevicestructuresandprocessing methodsfororganicsemiconductorphotodiodes.HereceivedaMasterofSciencein Decemberof2007andDoctorofPhilosophyinMayof2011. 237