On Interface Modification for Improved Organic Optoelectronic Device Performance

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On Interface Modification for Improved Organic Optoelectronic Device Performance
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english
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Hartel, Michael J
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Doctorate ( Ph.D.)
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University of Florida
Degree Disciplines:
Materials Science and Engineering
Committee Chair:
So, Franky Fat Kei
Committee Members:
Norton, David P
Xue, Jiangeng
Hummel, Rolf E
Rinzler, Andrew Gabriel

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Subjects / Keywords:
oled -- opv -- organic
Materials Science and Engineering -- Dissertations, Academic -- UF
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Materials Science and Engineering thesis, Ph.D.
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Abstract:
Interfacial effects play a large role in the device characteristics of organic optoelectronic devices. The understanding of these various interfaces is vital to obtaining improved device performance. In this work,interface engineering was employed to improve the efficiency of organic photovoltaics (OPV) and organics light-emitting diodes (OLEDs). First, through the passivation of the ZnO defects created during synthesis,the efficiency of OPV devices was improved by 2.5 times. Two loss mechanisms in these polymer/ZnO planar heterojunction solar cells were studied. This improvement comes from a large increase in the short circuit current (Jsc)and fill factor (FF). Transient photocurrent measurements (TPV) showed that the passivation of ZnO defects leads to less surface recombination of photocarriers, reducing photocurrent losses. Time-resolved photoluminescence (TRPL)measurements showed shorter PL lifetime upon defect passivation. This indicated better charge transfer, a key process in OPV devices. Next, polymer-based OLEDs were modified by incorporating a novel interfacial layer between the cathode and light-emitting layer. Electroabsorption (EA) spectroscopy results indicated that the interfacial layer forms an interface dipole which decreases the electron injection barrier.When doped with Cs2CO3 to further decrease the injection barrier, Polymer LEDs were fabricated with a current efficiency over 2 times greater than devices employing the standard Cs2CO3 cathode. Finally, the light emission of small molecule/fullerene bilayer OLEDs at below bandgap threshold voltages was studied. It was found with capacitance-voltage (C-V) and EA spectroscopy measurements that the accumulation of charges at the heterojunction strongly modifies the electric field distribution. It was also found that the accumulation of both holes and electrons at an abrupt interface is important for the energy up-conversion process needed to obtain the half band-gap electroluminescence (EL) threshold. The findings in this dissertation provide insight into interface engineering for improved OPV and OLED device performance. This is possible through understanding the chemistry and structure of interfaces.
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by Michael J Hartel.
Thesis:
Thesis (Ph.D.)--University of Florida, 2013.
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Adviser: So, Franky Fat Kei.
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1 ON INTERFACE MODIFICATION FOR IMPROVED ORGANIC OPTOELECTR ON IC DEVICE PERFORMANCE By M ICHAEL J OSEPH HARTEL A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREME NTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013

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2 2013 M ichael J oseph H artel

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3 To my loving parents

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4 ACKNOWLEDGMENTS thank, first and foremost, my parents for always being loving and supportive though out my ac ademic career. You have g uided me with unwavering encouragement and priceless words something no one can e will always keep me driven I need to express my gratitude to my advisor, Dr. Franky So, for sharing his deep knowledge in the field of organic electronics and for sharpening my mind with his high standards for academic research. I thank my other committee members, Rolf Hummel, Jiangeng Xue, Andrew Rinzler and David Norton. I also want to acknowledge a ll of my lab mates for insightful and thought provoking conversations throughout the years. During my early years as a graduate stude nt I learned an immense amount from the more senior students including Galileo Sarasqueta, Cephas Small, Jaewon Lee, Neetu Chopra, Alok Gupta and Do Young Kim. More recently, I have had the chance to work closely with several lab members whom, as a team pr oduced successful research. These lab mates include Erik Klump, Dania Constantinou, Tzung Han Lai, Jesse Manders, Ben Swerdlow, and Cass Xiang. A special thank goes to my lab mate Song Chen who sat behind me in the office and always managed to have an answ er for my never ending questions. outside with whom great memories were made during my grad school career.

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5 TABLE OF CONTENTS P age ACKNOWLEDGMEN TS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST OF ABBREVIATIONS ................................ ................................ ........................... 10 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 FUNDAMENTALS OF ORGANIC SEMICONDUCTORS ................................ ........ 14 1.1 Overview ................................ ................................ ................................ ........... 14 1.2 Bonding and Electronic States ................................ ................................ .......... 14 1.3 Transport ................................ ................................ ................................ .......... 15 1.4 Excitons ................................ ................................ ................................ ............ 17 2 INTRODUCTION TO ORGANIC ELECTRONIC DEVICES ................................ .... 20 2.1 Overview ................................ ................................ ................................ ........... 20 2.2 Organic Photovoltaics ................................ ................................ ....................... 20 2.2.1 Materials and processing ................................ ................................ ......... 20 2.2.2 Device structure and operating mechanism ................................ ............ 20 2.2.3 Planar heterojunction vs bulk heterojunction ................................ ........... 22 2.3 Organic Light Emitting Diodes (OLEDs) ................................ ........................... 23 3 CHARACTERIZATION OF ORGANIC ELECTRONIC DEVICES ............................ 28 3.1 Characterization of Organic Photovoltaic Devices ................................ ............ 28 3.1.1 Standard spectra ................................ ................................ ..................... 28 3.1.2 Photo J V ................................ ................................ ................................ 28 3.1.3 External quantum efficiency (EQE) measurement s ................................ 29 3.2 Characterization of OLEDs ................................ ................................ ............... 30 3.3 Electroabsorption (EA) Spectroscopy ................................ ............................... 31 4 ENHANCED EFFICIENCY OF ORGANIC/INORGANIC PLANAR SOLAR CELLS VIA DEFECT PASSIVATION ................................ ................................ ...... 39 4.1 Introduction and Background ................................ ................................ ............ 39 4.1.1 ZnO properties and applications ................................ .............................. 40 4.1.2 Molybdenum oxide (MoO x ) ................................ ................................ ...... 40 4.2 Experimental ................................ ................................ ................................ ..... 41

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6 4.2.1 ZnO nanoparticle synthesis and characterization ................................ .... 41 4.2.2 Solar cell device fabrication and measurement ................................ ....... 43 4.3 Results and Discussion ................................ ................................ ..................... 44 4.3.1 Photo (J V) and EQE measurements ................................ ...................... 44 4.3.2 Transient photovoltag e measurements ................................ ................... 45 4.3.3 Time resolved fluorescence measurements ................................ ............ 46 4.4 Summary ................................ ................................ ................................ .......... 47 5 NOVEL CATHODE INTERFACE LAYER FOR IMPROVED ELECTRON INJECTION ................................ ................................ ................................ ............. 55 5.1 Introduction and Background ................................ ................................ ............ 55 5.2 D evice Fabrication and Measurement ................................ .............................. 61 5.3 Results and Discussion ................................ ................................ ..................... 63 5.4 Summary ................................ ................................ ................................ .......... 66 6 INVESTIGATION OF THE HALF GAP ELECTROLUMINENCE THRESHOLD OF RUBRENE/FULLERENE HETEROSTRUCTURES ................................ .......... 72 6.1 Introduction and Background ................................ ................................ ............ 72 6.2 Device Fabrication and Measurement ................................ .............................. 73 6.3 Results and Discussion ................................ ................................ ..................... 74 6.4 Summary ................................ ................................ ................................ .......... 82 7 CONCLUSIONS AND FUTURE WORK ................................ ................................ .. 91 APPENDIX; LIST OF PUBLICATIONS ................................ ................................ ......... 94 LIS T OF REFERENCES ................................ ................................ ............................... 95 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 108

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7 LIST OF TABLES Table 1 1 The Structure and optical properties of polyacenes. ................................ ........... 19 4 I Photovoltaic Parameters of PCDTBT/ZnO Hybrid Solar Cells ............................ 54

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8 LIST OF FIGURES Figure page 1 1 Structure of ethene possessing a double bond between carbon atoms. ............ 18 1 2 Scheme of the orbitals and bonds for two sp 2 hybridized carbon ato ms. ........... 18 1 3 Hopping transport mechanism described by the Gaussian Disorder Model. ...... 18 2 1 Operating mechanism of a bilayer organic photovoltaic device .......................... 26 2 2 Cross section schematics of various donor/acceptor heterojuction configurations for organic photovoltaic devices.. ................................ ................ 26 2 3 General operating mechanism of an OLED. ................................ ....................... 27 3 1 Extraterrestrial and AM 1.5G spectrum. ................................ ............................. 35 3 2 Characteristi c plot of current density voltage (JV) for an organic solar cell ........ 35 3 3 Illustration of the Stark effect.. ................................ ................................ ............ 36 3 4 Electroabsorpt ion (EA) spectroscopy setup. ................................ ....................... 36 3 5 Electroabsorption (EA) spectrum of a polymer OLED.. ................................ ...... 37 3 6 Electroabsorption (EA) spectra of an MDMO PPV OPV device with various DC biases. ................................ ................................ ................................ .......... 37 3 7 Electroabsorption (EA) spectroscopy signal at 550 nm for OPV device.. ........... 38 3 8 Photo J V characteristics of MDMO PPV OPV devices. ................................ ..... 38 4 1 Transmission electron microscope images of ZnO nanoparticles. ...................... 48 4 2 X ray diffraction pattern of ZnO NPs revealing the Wurtzite crystal structure. .... 49 4 3 Steady state PL spectra of ZnO NP films. The band edge emission is located around 365nm while the defec t emission is between 425 to 575nm. .................. 49 4 4 The XPS spectra O1s peak for ZnO films.. ................................ ......................... 50 4 5 AFM images of untreated ZnO surface.. ................................ ............................. 51 4 6 Structure and materials of hybrid OPV devices ................................ ................. 51 4 7 Solar cell characteristics of PCDTBT/ZnO devices. ................................ ............ 52

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9 4 8 The photo J V characteristics of ITO/PEDOT:PSS/MDMO:PPV/ZnO/Al devices with varying UVO treatment times. ................................ ........................ 53 4 9 Transient photovoltage de cay for devices employing untreated and UVO treated films ................................ ................................ ................................ ...... 53 4 10 Fluorescence lifetime curves of 10 nm films of PCDTBT on various surfaces. The PL signal at 700nm was monitored after excita tion at 375nm. .................... 54 5 1 Chemical structures of polymers used for EML layers in OLED devices.. .......... 66 5 2 Characteristics of ITO/PEDOT/ MEH PPV/PF NR 2 /Al OLEDs.. .......................... 67 5 3 The structure and materials used in PLED devices. ................................ ........... 68 5 4 Device characteristics of MEH PPV OLED s.. ................................ ..................... 69 5 5 J V plot of electron only devices fabricated with the structure Al/MEH PPV(100nm)/Interlayer/Al ................................ ................................ ................... 70 5 6 Electroabsorpt ion spectroscopy measurements for the device with structure ITO/PEDOT:PSS(35nm)/MEH PPV(85nm)/Cs 2 CO 3 +PVPy(10nm)/Al(100nm). .. 70 5 7 EA spectroscopy bias scans for MEH PPV devices employing vario us cathode configurations. ................................ ................................ ...................... 71 6 1 The mo lecular structure and enerfy level digars for rubrene/C60 devices. ........ 83 6 2 Device charac teristics of rubrene OLEDs. ................................ .......................... 84 6 3 C V and I V characterisics of rubrene/C60 devices .. ................................ .......... 85 6 4 EA spectra for the individual mate rials used in the rubrene/C60 OLED ............ 86 6 5 Bias dependence of the 546 nm EA signal for rubrene OLEDs with and without a LiF buffer layer. ................................ ................................ ................... 87 6 6 Bias dependence of the electric field modulated photocurrent using the EA setup without incident monochromatic light. ................................ ....................... 87 6 7 EA spectra for the ITO/PEDOT:PSS/rubrene/C 60 /Al de vice as various bi ases. ................................ ................................ ................................ ............... 88 6 8 EA data for rubrene/C60 devices.. ................................ ................................ ...... 89 6 9 OLED device data for device with neat and mixed interfa cecs.. ......................... 90

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10 LIST OF ABBREVIATION S AFM Atomic Force Microscope ASTM American Society for Testing and Materials CT Charge Transfer EA Electroabsorption EIL Electron Injection Layer EL Electrolumin escence EQE External Quant um Efficiency FF Fill Factor GDM Gaussian Disorder Model HOMO Highest Occupied Molecular Orbital IPES Inverse Photoemission Spectroscopy Jsc Short Circuit Current LUMO Lowest Unoccupied Molecular Orbital MDMO PPV Poly(2 methoxy 5 dimethyloctyloxy) 1,4 phenylenevinylene) MEH PPV Poly[2 methoxy 5 (2 ethylhexyloxy) 1,4 phenylenevinylene] MoOx Molybdenum Oxide OLED Organic Light Emitting Diode OPV Organic Photovoltaic P3HT Poly(3 hexylthiophene) PCDTBT P oly[N hepta decanyl 2,7 carbazole alt 5,5 di 2 thienyl benzothiadiazole)] PCBM [6,6] phenyl C61 butyric acid methyl ester PCE Power Conversion Efficiency

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11 PEDOT:PSS P oly(3,4 ethylenedioxythiophene):polystyrenesulfonate PL Photoluminescence PPV P oly(p phenylenevinylene) TEM Transmission Electron Microscopy TPV Transient Photovoltage TRPL Time Resolved Photoluminance UVO UV ozone V OC Open Circuit Voltage UPS Ultraviolet Photoemission Spectroscopy VTE Vacuum thermal evaporation XPS X Ray Photoemission Spectroscopy ZnO Zinc Oxide

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12 A bstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ON INTERFACE MODIFICATION FOR IMPROVED ORGANIC OPTOELECTR ON IC DEVICE PE RFORMANCE By Michael J. Hartel May 2013 Chair: Franky So Major: Materials Science and Engineering Interfacial effects play a large role in the device characteristics of organic optoelectronic devices. The u nderstanding of these various interfaces is vi tal to obtain ing improved device performance. In this work, interface engineering was employed to improve the efficiency of organic photovoltaics (OPV) and organics light emitting diodes (OLEDs). First, through the passivation of the ZnO defects created d uring synthesis, the efficiency of OPV devices was improved by 2.5 times T wo loss mechanisms in these polymer/ZnO planar heterojunction solar cells were studied This improvement comes from a large increase in the short circuit current (J sc ) and fill fact or (FF). Transient photocurrent measurements (TPV) showed that the passivation of ZnO defects leads to less surface recombination of photocarriers, reducing photocurrent lo sses. Time resolved photoluminescence (TRPL) measurements showed shorter PL lifetime upon defect passivation. This indicated better charge transfer, a key process in OPV devices Next, polymer base d OLEDs were modified by incorporating a novel interfacial layer between the cathode and light emitting layer Electroabsorption (EA) spectros copy results indicate d that the interfacial layer forms an interface dipole which decreases the

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13 electron injection barrier. When doped with Cs 2 CO 3 to further decrease the injection barrier, P olymer LEDs were fabricated with a current efficiency over 2 time s greater than devices employing the standard Cs 2 CO 3 cathode Finally, the light emission of small molecule/fullerene bilayer OLEDs at below bandgap threshold voltages was studied. It was found with capacitance voltage (C V) and EA spectroscopy measuremen ts that the accumulation of charges at the heterojunction strongly modifies the electric field distribution. It was also found that the accumulation of both holes and electrons at an abrupt interface is important for the energy up conversion process needed to obtain the half band gap electroluminescence (EL) threshold The findings in this dissertation provide insight into interface engineering for improved OPV and OLED device performance This is possible through understanding the chemistry an d structure of interfaces.

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14 CHAPTER 1 FUNDAMENTALS OF ORGA NIC SEMICONDUCTORS 1.1 Overview Due to the nature of bonding in organic materials, the electrical and physical properties are intrinsically different than traditional semiconductor materials such as Si a nd GaAs. This chapter will introduce the reader to several key concepts governing the processes in organic electronic devices including the nature of bonding and electronic states, charge carrier transport, and excitons. 1.2 Bonding and Electronic States To understand the optical and electronic properties of organic semiconductors, it is important to first understand the intramolecular bonding. Organic semiconductors are based on the unique property of the carbon atom to hybridize its atomic orbitals. Thi s is shown in t he molecule ethane in Figure 1 1 In ethane, there are three bonds surrounding each carbon atom. Since three atomic orbitals are mixed, the s orbital and 2 p orbitals, it is called sp 2 hybridization. T hree bonds are formed that have large binding energies an d whose electrons localized between the atoms The remaining p orbitals form bond whose electrons are more loosely bound and d elocalized within the molecule. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) are analogous to the conduction and valance bands in inorganic semiconductors. The energetic diagram for these orbital is shown in Figure 1 2 bonds bonds are the covalent intramolecular forces that hold the atom togeth er. In an organic solid, there are also interactions between neighboring molecules who are held together by van der Waals forces. If these forces are too weak,

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15 the molecular solid is disordered with various local environments including molecular geometries and orientations. The more conjugation in the molecular system, the more charges are delocalized and free to move. This increases the charge mobility of the material. The conjugation length also affects the absorption spectra as seen in Table 1 1 for the archetypal group of organic semiconductors, polyacenes. Increasing the conjugation length will red shift the absorption and emission spectra due to a decrease in the HOMO LUMO separation 1 Therefore, modification of chemical structure of organic molecules can greatly change the ir electronic and optical properties. 1.3 Transport In inorganic semiconductors, there is long range atomic order and strong interatomic coupling which delocalizes the electronic states 2 The states in the conduction band and valance band are separated by a forbidden bandgap. Thermalized or photoexcited carriers are free to be transported within these bands. If a defect is present, a spatially localized defect is formed within the bandgap 3 For organics, highly ordered crystal structures can show band transport similar to their inorganic counterparts. One example is highly pure single crystals of 5,6,11,12 Tetraphenylnaphthacene more commonly known as rubrene. The charge mobility in rubrene has been measure d up to 40 cm 2 / ( V s ) and is among the highest reported for organic materials 4 8 However, most organic semiconductor materials currently used in devices such as OLEDs and OPV have mobilities that are orders of magnitude lower (10 5 10 2 cm 2 /(V*s)) 9 13 The lower mobility for organic semiconductors is d ue to the fact that intermolecular interactions in organi c electronic materials are weak causing electron ic states to be

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16 localized on individual molecules or polymer chains This causes narrower transport bands that are easily disrupted by disorder. The variation of local environments, including molecular orientation and chemical defects, result s in energetic ally disordered electronic states. In order to fully understand charge conduction, we need to understand that adding or removing a charge from a molecule will result in structural relaxation of the molecule and its surrounding environment including changin g the bond lengths. Therefore, as a charge moves through a disordered organic solid, it hops between molecules since it is easily localized by defects and disorder. This hopping mechanism was proposed by L.D Landau in 1933 14 The charge transp ort model, called the Gaussian Disorder M odel (GDM) is also based on this hopping mechanism where the energies and separations are in a Gaussian shape. Carrier motion is field assisted and thermally activated and hence th e mobility ( is a function of electric field (F) and temperature (T) as described by the following equation s 15 : w here k b is Boltz s contant, while are related to the disorder parameters. The energetic disorder ( ) describes the width of the Gausian distribution of energy states for transport sites. The positional disorder ( ) is the geometric randomness arising from structural or chemical defects. The Gaussian Disorder Model of charge transport is shown in Figure 1 3.

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17 1.4 Excitons Excitons are coulombica l l y bound electron hole pairs In inorganic semiconductors, these electron hole pairs are called Wannier excitons. Wannier excitons in ino rganic semiconductors can be delocalized over several crystallographic units having a radius of ~10nm and a binding energy of only ~10meV. The refore, excitons created by light absorption in inorganic photovoltaics will be dissociated by thermal energy and not considered excitonic cells There are two types of excitons formed in organic devices. Frenkel excitons are localized on a single conjugated segment with a radius of separation of ~10 16,17 Charge transfer (CT) excitons are electron hole pairs that are delocalized over adjacent molecules. Both of these types of excitons can be treated as a neutral particle with a binding energy between 0.1 1eV 18 For OLEDs, the injection of oppositely charged carriers can form an exciton which will rediatively decay to emit light equal to the bandgap of the emissive material. In OPV, the absorption of light directly creates excitons. If these excitons diffuse to a heterojunction with offsetting electron affinities and ionization potentials within its lifetime it can dissociate into free charge carrier. The processes in OPVs and OLEDs w ill be discussed in Chapter 2

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18 Figure 1 1. Structure of ethene possessing a double bond between carbon atoms. Figure 1 2. Scheme of the orbitals and bonds for two sp 2 hybridized carbon atoms. Figure 1 3. Hopping transport mechanism described by the Gaussian Disorder Model.

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19 Table 1 1. The Structure and optical properties of polyacenes. Molecule Structure Absorption (max) Emission (max) Benzene 205 nm 278 nm Naphthalene 286 nm 321 nm Anthracene 365 nm 400 nm Tetracene 390 nm 480 nm Pentacene 580 nm 640 nm

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20 CHAPTER 2 INTRODUCTION TO ORGA NIC ELECTRONIC DEVIC ES 2.1 Overview Interest in the use of organic semiconductors has been widespread due to the possibility of light weight a nd flexible optoelectronic devices along with inexpensive and rob ust fabrication processes. However, this chapter will only cover OPVs and OLEDs. 2.2 Organic Photovoltaics Currently, the solar cell market is dominated by devices fabricated using silicon. H owever, these modules are still not cheap enough for widespread use. Based on the ease of proces sing, organic photovoltaics is a promising technology As the understanding of the device physics and development of new materials moves forward, organic solar cells are improving in both power conversion efficiency and lifetime. It is possible that in the near future, this budding technology will become as cheap, if not cheaper than fossil fuel generated electricity 19,20 This chapter will discuss the processing of different types of organic photo voltaics as well as their device structures and operating mechanisms. 2.2.1 Materials and p rocessing Both small mole cules and polymers are employed to make organic photovoltaic devices. Small molecule devices are usually processed using a vacuum thermal evaporation process where polymer solar cells are processed using solution deposition techniques such as spin casting or vapor spraying. 2.2.2 Device structure and operating m echanism The diagram of the energetic structure and operating mechanism of org anic solar cells is shown in Figure 2 1 The basic bilayer structure involves an electron donating

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21 layer and an electron accepting layer. These are sandwiched between two electrical contacts that are the connections for charge extraction. The four fundamen tal processes are light absorption, exciton diffusion, exciton dissociation and transport and collection. There is an efficiency associated with each of these processes and the total photo conversion efficiency is the product of each of these. This can be written as the following: The first process of absorption is determined by the bandgaps of the donor and acceptor layers. The energetic levels of the donor and acceptor can be tuned by controlling the chemical structure which researchers call bandgap eng ineering 19 There is a push to reduce th e bandgap of the donor polymer to allow for more efficient matching of the absorption with the solar spectrum 21 24 Upon light absorption, a coulombically bound electron hole pair is formed called an exciton. Once an exciton is formed, it will randomly diffuse a length (L D ) given by the formula: Where D is the diffusion constant which is temperature dependent and which is the charge carrier lifetime. After generation, it is important for efficient devices that the exciton diffuse s to the donor/ acceptor interface before its lifetime. The diffusion length can be determined by measuring the fluorescence lifetime of films of varying thickness on quenching surfaces 25 29 The diffusion length has been measured t o be around 5 20nm and varies with crystalline order 26 .Once the exciton diff uses to the interface between the donor and acceptor it can be dissociated into free carriers if the energy

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22 offset is enough to overcome the coulombic attraction. In well designed systems, the ED can be taken to be unity. Charge dissociation is the proces s of transferring the electron from the donor to the LUMO of the acceptor, leaving a free hole in the HOMO of the donor. After charge dissociation, the free carriers are able to move though the device to be collected at the contacts. Charge transport occur s by a hopping mechanism and is subject to losses due to defects that act as traps. Charge carriers that reach the contacts contribute to photocurrent. 2.2.3 Planar heterojunction vs bulk h eterojunction Early device structures utilized a bilayer design o f neat donor and acceptor films sandwiched between the carrier extraction contacts 30 As discussed earlier, organic mater ials have short exciton diffusion lengths Only excitons that are generated within the diffusion length of the donor/acceptor interface are able to be effectively dissociated. Therefore, exciton diffusion length is the limiting factor for bilayer heterostr uctures. Once excitons are dissociated at the donor/ acceptor interface, the free carriers can then be transported to the contacts for collection. The structures of bulk vs. planar hetero junctions are shown in Figure 2 2 Transport and collection is effici ent in these bilayer structures since bimolecular recombination that leads to a reduction in photocurrent is a product of the number of hole and the number of electrons (np). Away from the donor/acceptor interface, this is not an issue since only one typ e of charge carrier is present. Bulk heterojunction solar cells were invented to overcome the issue of short exciton diffusion lengths. In this type of structure, the donor and acceptor materials are intimately mixed and self assemble to form interpenetrat ing nanodomains. These nanodomains are ideally on the length scale of the exciton diffusion length. Therefore,

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23 excitons ge nerated in the donor or acceptor domains have a better chance of reaching a donor/acceptor interface where charge transf er can take pl ace. The loss mechanisms involving carrier generation, transport and extraction are still not completely understood 31 This is due to the complex interplay of the individual chemical structure, blend morphology and photophysics that finally determines the device efficiency 31 40 Th e understanding of this phase separation has allowed for higher efficiencies. An example of this is work that dealt with P3HT:PC 60 BM morphology. Upon mild heat treatments, the morphology was optimized leading to a 3 fold improvement in short circuit curren t and power conversion efficiency 38 40 A power conversion efficiency of 5% was reached by annealing the P3HT:PC 60 BM bul k heterojunction film at 150 C for 30 minutes 38 2.3 Organic Light Emitting Diodes (OLEDs) OLEDs are the most mature technology based on organic semiconductors. Research effort s in both academia and industry have paved the way for products that have reached the market for lighting applications a s well as passive and active matrix displays. Lighting accounts for approximately 22% of the electricity consumption in the United States with 40% of that amount coming from the use of inefficient (10 lm/W) incandescent bulbs 41 Active matrix OLED displays are now competing with liquid crystal displays (LCD) and plasma due to their ease of fabrication and low power consumption 42 The first report of electroluminescence from an organic material was in the 1960s when hundreds of volts were applied to a single crystal of anthracene 43 A major milestone was the first report of an OLED by Tang and VanSlyke 44 They employed a dual layer structure with a hole transporting layer and emissive layer of aluminum

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24 quinolinolate (Alq 3 ), a luminance of 100cd/m 2 at a voltage bias of 10V was demonstrated 4 4 This has been the archetypal structure for OLEDs still researche d today and their Applied Physics Letters publication from 1987 has b een cited over 4300 times to date Operating m echanism : Progress in understanding the processes of carrier injection, charge transport, interfaces and energy transfer, OLEDs have evolved into a mature technology. The basic structure of an OL ED device is shown in Figure 2 3 For simplicity, only two layers are shown besides the layers used as contacts. Light emission from OLEDs is generated by the following processes: 1) charge injection, 2) charge transport, 3) exciton formation, 4) exciton recombination yielding light emission. The first step towards light emission involves the injection of electrons from the cathode int o the LUMO of the ETL and injection of holes into the HOMO of the HTL. The nature of the metal/organic semiconductor interface is fundamental to charge injection and hence controlling the amount of charge carriers present in the device. It has been shown t hat strong interface dipoles The difference in work function of the contacts relative to the appropriate transport levels of the organic layers present inje ction barriers that mu st be overcome The conductive polymer, poly(ethylenedioxythiophene)/p olystyre nesulphonic acid (PEDOT: PSS), is a commonly used material for efficient hole injection due to its high ionization potential (IP) of around 5.2eV 45 49 This has been shown to form Ohmic contact with a polyfluorene based emissive layer 49 Formation of a stable, low wo rk function cathode for efficient electron injection is more difficult since the use of low work function metals such as Ca, Ba, Cs, and Mg are highly reactive with the organic materials they are evaporated on

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25 and have been shown to quench luminance 50 53 Evaporating a thin insulating layer of a metal halide, such as LiF or CsF, before a higher work function such as Al has been shown to help form an efficient electron injection contact 54 58 Once charges are inject ed into the OLED, excitons are formed in the emissive layer. With the recombination of an exciton, light is emitted with energy equal to the ba n dgap minus the exciton binding energy. Based on the spin of the electron and hole, the exciton, it either a sing let or a triplet. Singlet excitons have a total spin of zero while triplet excitons have a total spin of one 59 62 Triplet excitons cannot recombine directly because the transition is forbidden by spin conservation, giving the triplet a lifetime on the order of 10 6 s. This process is called phosphorescence. Singlet exci tons can directly recombine yielding a shorter lifetime (10 9 s) 63 by a fluorescence process Since the ratio of triplets to singlets is 3:1, the harvesting o f triplet excitons is important for high efficiency OLEDs 64 It has been shown in electrophosphorescent OLEDs th at it is possible to harvest nearly 100 % of the excitons created by electrical injection 60 After light is emitted by the recombination of an exciton, it may be trapped in the ractive indices. There is currently extensive research in this area, although beyond the scope of this dissertation.

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26 Figure 2 1 Operating mechanism of a bilayer organic photovoltaic devic e Figure 2 2 Cross section schematics of various donor/accep tor heterojuction configurations for organi c photovoltaic devices. A) Planar heterojunction. B) Bulk heterojunction. C) P lanar columnar heterojunction sandwiched between an anode and cathode.

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27 Figure 2 3. General operating mechanism of an OLED.

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28 CHAPTER 3 CHARACTERIZATION OF ORGANIC ELECTRONIC DEVICES 3 .1 Characterization of Organic Photovoltaic Devices In this section, the measurement of organic solar cells will be discussed along with the important figures of merit for device characteristi cs and the experimental setups It will cover the measurements of photo J V and external quantum efficiency ( EQE ). This will include a discussion about the standard solar spectra used for the measurement of photo J V 3.1 .1 Standard s pectra The OPV devi ces are tested under white light from a solar simulator whose spectrum mimics that of the actual solar spectrum. The extraterrestrial and air mass (AM) 1. 5G spectra 65 are shown in Figure 3 1 The AM 1.5G spectra was chosen to be the standard for solar cell characterization because it is the most representative of the solar spectrum under average atmospheric conditions. The differences in the spectra arise from light absorption and scattering due to atmosph eric gasses. When these spectra are integrated, it can be seen that the irradiance of the sun before passing through the atmosphere is 136 mW/cm 2 and only 100 mW/cm 2 after wards Therefore, when testing OPV devices, a solar simulator is used that mimics the AM 1.5G condition with the irradiance set to 100 mW/cm 2 3.1 .2 Photo J V In organic photovoltaic (OPV) devices, photocarriers are g enerated in the active in potential (V BI ) to be collected by the contacts The cu rrent density voltage characteristics (J V) with and without illu mination are shown in Figure 3 2 The J V characteristics without illumination f ollow typical diode

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29 characteristics. The current density under zero bias is called the short circuit current (J sc ). The voltage where the summation of the dark current and photocurrent equals zero is called the open circuit voltage (V oc ). The origin of V oc has been under constant research and has been found to be influenced by a number of factors. These factors inc lude nanoscale phase separation of the active layer 66 the shunt resistance 67 electric field dependent geminate recombination 68 and bimolecular recombination 69 The maximum power output of the cell occurs at P max =|J max V max |. The fill factor (FF) is a : The power conversion efficiency (PCE) is the most important solar cell parameter and is given by : where P o is the incident power intensity While the electrical power can be calculated through the J V curves, th e incident power needs to be standardized as discussed in the next section. 3.1.3 External quantum efficiency (EQE) m easurements Another quantity associated with solar cell characterization is t he external quantum efficiency (EQE) which is the ratio of gen erated electrons to the number of incident photons at a particular wavelength. Recently, an EQE approaching 100% was demonstrated using a donor acceptor copolymer:fullerene blend (PCDTBT:PC 70 BM) 70 The EQE is given by :

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30 w here c is the speed of light, h T ( ) is the measured photocurrent from the lock in amplifier, I D is the photocurrent from the Si photodetector, and R D is the responsivity o f the Si photodetector. For the EQE measurement, w hite light from a quartz halogen (QH) lamp is fed into a monochromat o r. The monochromatic light is focused with a series of lenses onto the test de vice. The signal coming from the device is fed into a current amplifier and then into the lock in amplifier. F or the incident power measurement, the beam of monochromatic light is focused onto a calibrated Si photo dio de. The signal from the photodiode is fed into the current amplifier and then into the lock in amplifier. The incident power is then the photocurrent from the Si photodiode divided by the pho todiode responsivity. The J sc can be calculated from the EQE data by: w here S( sc found from the photo J V measurements should be the same as from integrating the EQE spectra is the system is correct ly calibrated. 3 .2 Characterization of OLEDs The relevant OLED characterization for this dissertation includes the measurement of current density voltage (J V) and luminance voltage (L V). The voltage was sourced from a Keithley 2400 SourceMeter controlle d by a homemade Labview program while it

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31 silicon photodiode fed into a Keithley 6485 Picoammeter. To convert the diode measured at incremental device current densities with a Konaka LS 100 luminance meter while the photocurrent at the same current density was recorded. These two values were recorded in 0.1 mA steps and a conversion factor is found using the slope of lumina nce plotted versus photocurrent. L ) can then be calculated by: Where L is the luminance, A is the device area, and I D is the device current. 3 .3 Electroabsorption (EA) Spectroscopy A crucial parameter influencing the operation of organic light emitting diode (OLED) and organic photovoltaics (OPV) devices is the electric field present in the 71 73 as well as the charge carrier mobility 15,74 are field dependent. In organic photovoltaics, the electric field is the driving force for charge carrier extraction. Electroabsorption (EA) spectroscopy is a non invasive method to probe the internal electric fields in organic semiconductor devices 75 81 EA spectroscopy is based on e xternally applied electric fields influencing the interaction of molecules with light. The change in absorption of a molecule with an applied electric field can be explained by the Stark Effect w hich is illustrated in Figure 3 3 Under an applied electric field, dipo les will be present in materials which are dependent on its polarization. This caused a splitting in energy levels and previously forbidden tran sitions become allowed as well as a red shift in the 1A g 1B u transition. The excitonic Stark shif t follows:

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32 w here F is the externally applied electric field and material The polarization is the dipole unit per volume and is described as: w here is the dielectric constant of the vacuum and is the polarizability of the material. The field dependent susceptibility is nonlinear and for mo lecules and polymers with inversion symmetry, all even terms of disappear. Since the imaginary part of the susceptibility is directly proportional to the absorption constant where E is the energy of the photon t he chance in transmittance can be described as: If the applied electric field F is a superposition of both AC and DC biases (V), as well as an internal electric field V bi the chang e in transmittance is: Therefore, when the V DC is of equal magnitude but opposite direction as the V bi the EA response vanishes. Experimentally, the V AC amplitude is kept constant while changing the applied V DC until V DC exactly cancels out V bi EA measurement s etup : The measurement setup is operated in a reflection type geometry which means that the light is passed through a semitransparent electrode, bounces off the back contact and passes through the film again before being detected. The experim ental setup is shown in Figure 3 4 A white light sour ce is fed through a monochromato r and using a series of lenses, is focused on the active device

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33 area. After bouncing off the back contact, the light is then focus ed onto a silicon photodetector. The signal is fed into a preamplifier and then into a lock in amplifier (Stan ford Research SRS 830). The AC bias superimposed on a DC bias is applied to the device using a function generator. The reference signal from the f unction generator is sent to the lock in amplifier. The EA signal as a function of wavelength can then be scanned at a fixed DC bias. For every wavelength measured, the EA response ( T) must be normalized to the transmission (T) to get rid of the spectral response of the system which is caused by the intensity distribution of the lamp, performance of the monochromato r. This is performed by using the same experimental setup as the T m easurement except there is no bias applied to the device and the signal referenced to the lock in amplifier comes from a chopper wheel that is set in front of the exit slit of the monochromato r. EA spectroscopy can be used to study both OLED and OPV devi ces. Lane et al. showed that EA spectroscopy is a tool that can be used to determine the injection barriers from the contacts to the transporting layer. The results from these EA measurements are sho wn in Figure 3 5. Polymer LEDs with identical structures besides the choice of anode material were studied. Th e EA spectra for the polymer LEDs employing either an ITO or PEDOT:PSS anode both have the same line shape as seen in Figure 3 .5. However, when the EA signal for the 414 nm peak is scanned as a function of DC bias, the device with a PEDOT:PSS anode has a much higher V bi (2.9 V) than the device with an ITO anode (1.9 V). The increase in V bi is a direct measurement of lowered injection barriers and explains the increase in performance for the device with th e PEDOT:PSS anode.

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34 The V bi of organic solar cells is the driving force for charge extraction and hence an important characteristic. We have studied the effect of increasing the V bi on OPV devices. The ac tive layer of the OPV was a film form by spincastin g from a solution Poly(2 methoxy 5 dimethyloctyloxy) 1,4 phenylenevinylene) (MDMO PPV) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) in a weight ratio of 1:4 Figure 3 6 shows the EA spectra for an OPV device with the structure ITO/PEDOT:PSS /MDMO PPV:PCBM/Al. As discussed earlier, the excitonic peak varies linearly with V DC Figure 3 7 shows the V DC dependence of the 550 nm peak for devices employing either a MoO x or PEDOT:PSS anode. The built in potentials measured for the MoO x and PEDOT:PSS devices are 1.3 V and 0.85 V, respectively. The increase in V bi can explain the increase in short circuit current (J sc ) and fill factor (FF) for the MoO x device compared to the structure with PEDOT:PSS. These results are shown in Figure 3 8. In summary, EA spectroscopy is a useful tool to study both the carrier injection barriers in OLEDs and the potential for photocarrier extraction in OPV devices.

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35 Figure 3 1. Extraterrestrial and AM 1.5G spectrum. Reproduced from data obtained from the American Soc iety for Testing and Materials (ASTM) 65 Figure 3 2 Characteristic plot of current density voltage (JV) for an organic solar cell

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36 Figure 3 3 I ll ustration of the Stark effect. A ) Energetic leve l in field free case. B ) Energy level splitting due to an electric field. Figure 3 4 Electroabsorption (EA) spectroscopy setup

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37 Figure 3 5. Electroabsorption ( EA ) spectrum of a polymer OLED. Inset: Bias scans of the EA signal at 414nm for LEDs wit h ITO (closed circles) and PEDOT:PSS (open circles) anodes.( Adapted from Lane et al. 76 ) Figure 3 6. Electroabsorption (EA) spectra of an MDMO PPV OPV device with various DC biases.

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38 Figure 3 7. Electroabsorption ( EA ) spectroscopy signal at 550 nm for OPV device A ) Device emp loying MoO x anode. B ) Device employing PEDOT:PSS anode Figure 3 8. Photo J V characteristics of MDMO PPV OPV devices with PEDOT:PSS (blue triangles) and MoO x (black squares) anodes.

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39 CHAPTER 4 ENHANCED EFFICIENCY OF ORGANIC/INORGANIC PLANAR SOLAR CE LLS VIA DEFECT PASSIVATION 4 .1 Introduction and Background Progress in the field of organic photovoltaics (OPVs) has been rapid due to the synthesis of new materials and development of new device architectures 19 21,82 For example, inverted device structures and metal oxides, such as ZnO, may reduce manufacturi ng costs by enabling roll to roll processing 83 85 Additionally, ZnO has also been used as an electron acceptor to replace costly fullerene based materials 86 88 The drive to use metal oxides as an acceptor is also due to their high dielectric constant which facilitates exciton dissociation by reducing the Coulombic attraction between the bound electron hole pair. However, the efficiency in the bilayer type hybrid structures utilizing ZnO as the acceptor has been limited to around 1% 89 Recently, we showed UV ozone (UVO) treatment of t he ZnO charge collection contact improved device performance in inverted, low bandgap polymer/fullerene bulk heterojunction solar cells 85 ZnO is known to have radiative, sub bandgap defects, which may affect the efficiency of charge carrier generation and collection. In this work, UVO treatment is employed to passivate the ZnO defects. This allows for an understanding of the effect of ZnO defects on exciton dissociation and ch arge recombination at the donor/acceptor interface which have not been previously studied in detail. In this chapter, donor acceptor copolymer /ZnO nanoparticle bilayer heterojunction devices were fabricated. The effect of sub bandgap defects in ZnO film s on device performance was studied and two photocurrent loss mechanisms were identified. T ransient photovoltage measurements (TPV) showed reduced interface recombination of photogenerated carriers in devices with UVO treatment. Also, time resolved

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40 photolu minescence (TRPL) measurements suggest increased photoinduced charge transfer after defect passivation by UVO treatment. Therefore, the population of ZnO defects plays a significant role in both bimolecular recombination and exciton dissociation. 4 .1.1 ZnO properties and applications ZnO is a wide bandgap (~3.3eV) 90 semiconductor with a variety of short wavelength photonic applications s uch as electrodes for dye sensitized solar cells 91,92 and the emitting material in LEDs 93 95 ZnO nanostructures are widely studied for sensing applications due to their high sensitivity to their chemical environment 96 99 A variety of nanostructures have been synthesized such as spheres, tetrapods, ribbons and pill ars. These structures are processed in a variety of ways such as a hydrothermal chemical synthesis and thermal evaporation in a tube furnace 100 102 Intrinsic defects can occur during processing and have been related to a variety of point defects such as vacancies, interstitials and substitutional defects. Understand ing these defects is extremely important since they affect the optical and electronic properties. Defects formed within the ZnO bandgap have ionization energies that range from 0.05 2.8eV. The predominate defect types have been found to be from zinc inters titials and oxygen vacancies 90 Deep level emission from ZnO films has been reported although these transitions have been controversial. One of the most highly reported sub bandgap emissions is green/yellow and has been attributed to oxygen vacancies 100,101,103 108 4 .1.2 Molybdenum o xide (MoO x ) The use of MoO x as a hole injection material in OLEDs and hole extraction material in OPVs has been widely studied. It was tho ught to be a p type material until n type characteristics were proven directly with ultraviolet/inverse photoemission

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41 spectroscopy (UPS/IPES). The work function of MoO x has been measured to be 6.7 eV below the vacuum level with an electron affinity of 6.5 eV and ionization potential of 7.2 eV. The stoichiometry and surface contamination affect the energetic levels and it has been shown that oxygen or air exposure can reduce the work function to between 5.3 5.7 eV. Such a high work function is suitable for m aking Ohmic contact to many organic hole transporting materials and is explained through Fermi level pinning. It has been examined with UPS measurements, that upon deposition of organic molecules, strong electron transfer from the HOMO of the organic to th e 4d band of the Mo occurs. This formation of Fermi level pinning for Ohmic contact is not unique and has been explained by the fact that if the work function of an n type or p type oxide is larger than the HOMO of the organic, electron transfer occurs. Th e offset between the HOMO level and Fermi energy will approach a minimum fixed value even upon further increases of the 4 .2 Experimental 4 .2.1 ZnO nanoparticle synthesis and characterization In our work, ZnO nanoparticles were sy nthesized according to a method previously reported 100 102 T he synthesis was performed by dropwise addition of a stoichiometric amount of tetramethylammonium hydroxide dissolved in ethanol (0.55 M) to 30 mL of 0.1 M zinc acetate dehydrate dissolved in dimethyl sulfoxide (DMSO) under continuous stirring. The soluti on was stirred for an additional hour and then centrifuged to precipitate the nanoparticles. After washing twice with a 1:3 volume ratio of heptane/ethanol, the ZnO nanoparticles were dis persed in pure ethanol. Figure 4 1 shows transmission electron micros cope (TEM) images of the ZnO nanoparticles. The average diameter of the nanoparticles is ~5 nm. The crystal structure of the

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42 nanoparticles was determined to be the Wurtzite structure from the X ray diffract ion (XRD) data shown in Figure 4 2. In order to pa ssivate the ZnO defects, a 5 minute UVO treatment was carried out on the ZnO films deposited on a blank glass substrate after a 30 minute heat treatment at 100C. A treatment time of 5 minutes was found to be the optimum UVO exposure time for the highest d evice performance. In order to verify the ZnO defect passivation, photoluminescence (PL) measurements were performed. Figure 4 3 shows the effect of UVO treatment on the PL spectra. The band edge in the absorption matches the 360 nm excitonic peak shown in the PL spectra. The strong broad band peak centered at 515 nm has been attributed to sub gap defects which act as radiative recombination centers 105,108 111 Before the treatment, the ratio of the intensity of the excitonic peak to that of the defect peak was 1:0.6. After the treatment, the ratio is significantly increased to 1: 0.2 due to the passivation of oxygen vacancies and dangling surface bonds that are created during synthesis 105 In order to identify the source of the ZnO defects, X ray Photoemission Spectroscopy (XPS) was carried out. Figures 4 4a and 4 4b show the O1s region of the XPS spectra of untreated and UVO treated films, respectively. The lower binding energy peak (O1) at 530 eV is attributed to O 2 ions present in a stoichiometric wurtzite ZnO structure, whereas the higher binding energy peak (O2) at 531.5 eV is associated with O 2 ions in oxygen 112 Before UVO treatment, the ratio of peak area of O1 to O2 is 1:1.39. After treatment, the peak area ratio changes to 1:1.04. Therefore, the XPS results indicate that UVO passivates oxygen vacancies and dangling bonds whi ch are mid gap defect states and act as recombination centers.

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43 In order to visualize the effect of UVO treatment on the ZnO surface morphology, atomic force microscopy (AFM) images were taken in tapping mode. Figures 4 5a and 4 5b show the height images of a ZnO film before and after UVO treatment, respectfully. The root mean squared (RMS) roughness of both films is ~4.1 nm and it can be concluded that there is no change in film morphology upon U VO treatment. Figure 4 5c and 4 5d show the phase images of t he untreated and treated ZnO film, respectfully. With UVO treatment, the phase roughness RMS decreases from 6.6 to 4.8. The AFM tip sample interaction influences the phase signal and detects changes in surface chemistry. Therefore, changes in the van der Waals forces, dipole dipole interactions or electrostatic forces can be observed between samples. It is apparent from these results that while there are no changes in surface morphology, there are changes in surface chemistry. 4 .2.2 Solar cell device fab rication and measurement OPV devices were fabricated employing UVO treated films and compared to reference devices without treatment. The devic e structure is shown in Figure 4 6a. Devices were fabricated on pre patterned ITO coated glass substrates with a sheet ultrasonication in acetone, isopropanol and de ionized water for 15 min each, followed by a rinsing in de ionized water The ZnO NP films were spin cast from a solution usi ng chloroform as the solvent and dried at 100C in air yielding a thickness of 35 nm. A UVO treatment of 5 minutes was then carried out. A 75 nm film of poly[N hepta decanyl 2,7 carbazole alt 5,5 di 2 thienyl benzothiadiazole)] (PCDTBT ) was spin cast from a solution using chlorobenzene as the solvent and annealed at 70C for 30 minutes in a nitrogen glovebox. The chemical structur e of PCDTBT is shown in

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44 Figure 4 6b.The anode was thermally evaporated and consisted of a 5 nm layer of MoOx and a 100 nm layer of Ag. The current density voltage (J V) characteristics of the devices were measured under a 100 mW cm 2 AM 1.5G solar simulator. The photocarrier lifetimes were determined from TPA measurements. The TPA setup used in this study has b een previously described 113 For this measurement, the devices were photo biased to the o pen circuit voltage (V oc ) using a 100 mW cm 2 AM 1.5G white light. A voltage perturbation (~25 mV) was generated by a 5 27 nm optical pulse with a width of 8 n s. The charge transfer characteristics in the device were studied using TRPL. The TRPL measurement s were performed with a time correlated single photon counting (TCSPC) spectrometer (Picoquant, Inc.). A pulsed laser (375 nm) with an average power of 1 mW, operating at 40 MHz, with duration of 70 ps was used to excite the PCDTBT. The active area of each device was 4.6 mm 2 4 .3 Results and Discussion 4 .3.1 Photo (J V) and EQE m easurements The photo JV characteristics of devices with and without UVO treatment of the ZnO are shown in Figure 4 7a. Devices with and without the UVO treatment both had the same V oc of 0.70 V, indicating no change in energy level offset between the conduction band of ZnO and highest occupied molecular orbital (HOMO) of PCDTBT. There is however, a large improvement in J sc and fill factor (FF) when using the treated ZnO. The devic e without UVO treatment has a J sc of 0.60 mA/cm 2 and a FF of 39% yielding a power conversion efficiency (PCE) of 0.19%. The devices employing the UVO treated ZnO layer were over 2.5 times as efficient with a PCE of 0.52 %, J sc of 1.30 mA/cm 2 and FF of 55%. The average results from 8 devices are summarized in Table 4

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45 I. The external quantum efficiency (EQE) spectra measured for the PCDTBT/Z nO devices are shown in Figure 4 7b. For the devices employing the UVO treated ZnO, the EQE reaches 13.7% compared to ju st 7.8% for the device with untreated ZnO. Bilayer devices were also fabricated using another common donor polymer, (Poly[2 methoxy 5 dimethyloctyloxy) 1,4 phenylenevinylene])(MDMO PPV). The photo J V MDMO PPV as the donor polymer and the res ults ar e shown in Figure 4 8. Similar to the results with the PCDTBT devices, the J sc and FF were both improved leading to a significant PCE improvement. For devices using untreated ZnO films, the J sc and FF were 0.35 mA/cm 2 and 41 %, respectively. The most effic ient devices employing the treated ZnO films had J sc of of 0.75 mA/cm 2 and 51% FF. This increase in J sc and FF lead to a PCE increase of the MDMO PPV/ZnO devices from 0.12% to 0.28%. Unlike the devices made with PCDTBT, there is a reduction in the V oc for devices employing the treated ZnO films. This is most likely due to the oxidation of MDMO PPV by the treated ZnO. The HOMO energy of PCDTBT is 5.5 eV 114 which is higher than the HOMO of MDMO PPV (~5.2 eV 115 ). This makes PCDTBT more stable a nd less prone to oxidation. Through better understanding of the loss mechanisms that reduce the device J sc and FF, we can more effectively optimize the materials selection and processing for these organic/ZnO hybrid structures. 4 .3.2 Transient p hotovoltag e m easurements One possible consequence of defects in ZnO is the recombination of photogenerated free charge carriers at the donor/acceptor interface. This was investigated by performing transient photovoltage (TPV) measurements and the results are plotted in Figure 4 9. Studying the photo carrier decay dynamics at the flat band condition allows us to understand the effect of ZnO defects on interface recombination

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46 since device with and without surface treatment have the same active and hole transporting lay ers. In a bilayer structured polymer/ZnO solar cell, bulk recombination is dominated by bimolecular processes that occur at the polymer/ZnO interface. Thus, the defect density could directly impact the bulk recombination rate of the solar cell. Any differe nces in carrier lifetime can therefore be attributed to defects on the surface of ZnO. This low level perturbation ( ~25 mV) allows the photo voltage decay to follow a single exponential behavior through which the carrier lifetime can be easily extracted. to be 365 ns compared to only 186 ns for devices with untreated ZnO. Our data indicate the recombination at the PCDTBT/ZnO interface is one of the mechanisms of photocur rent loss mechanism and an enhancement in the photogenerated carrier lifetime is due to reduction in recombination resulting from UVO treatment. 4.3.3 Time resolved fluorescence m easurements Another loss mechanism studied involves photo induced electron t ransfer from the PCDTBT donor to the ZnO acceptor. Ineffective forward charge transfer will limit the number of photogenerated charge carriers, resulting in photocurrent loss. In the bilayer cell, charge generation mainly occurs at the polymer/ZnO interfac e. Therefore, we expect modification of the ZnO defect density will affect the dynamics of exciton dissociation. Photoinduced electron transfer can be studied by studying the fluorescence decay of the singlet stat e of the donor polymer. Figure 4 10 shows t he 700nm PCDTBT emission fluorescence decay curves of a PCDTBT/ZnO bilayer film sample. The PL lifetimes of a 25nm PCDTBT film on a non quenching glass substrate, with as processed ZnO and treated ZnO were measured. The exciton quenching ability of ZnO was also compared to a thermally evaporated film of C 60 which is a common

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47 acceptor used in small molecule OPV devices. Since the wavelength of 375 nm does not excite the ZnO NP, the PL decay from PCDTBT can be studied independently. The measured PL lifetime ( PL ) is related to the radiative (k R ) and non radiative rate constant (k NR ) by (1) w PL is the radiative lifetime. Upon UVO treatment of ZnO, passivat ion of ZnO defects leads to an enhancement in separation of photogenerated electron hole pairs, resulting in an increase in the non radiative decay constant and a reduction in fluorescence lifetime. Therefore, a shorter PL lifetime indicates better photo i nduced electron transfer for the device employing a UVO treated ZnO film. By fitting the decay curves, the measured PL lifetimes for PCDTBT on the glass substrate, as processed ZnO, UVO treated ZnO and C 60 were found to be1.39, 1.24, 0.80 and 0.88ns respec tively. With the passivation of defects, the ZnO film shows enhanced charge transfer. With UVO treatment, the ZnO film even surpasses the ability of C 60 to effectively dissociate excitons which is expected since ZnO has a higher dielectric constant while i ts conduction band energy is sufficient offset from the LUMO of PCDTBT to induce electron transfer. 4.4 Summary In this chapter, two loss mechanisms in polymer/ZnO planar heterojunction solar cells were studied. By passivation of defects at the PCDTBT/Zn O interface, devices showed a power conversion efficiency improvement of over 250% compared to the devices using untreated films. This improvement comes from a large increase in the short circuit current (J sc ) and fill factor (FF). Transient photocurrent m easurements

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48 showed that the passivation to ZnO defects leads to less surface recombination of photocarriers, reducing photocurrent losses. Time resolved fluorescence measurements showed a shorter PL lifetime upon UVO treatment of ZnO indicating better forw ard photoinduced charge transfer. This work allows us to better understand the loss mechanisms in solar cells where careful control of defects allows for control of photocarrier recombination kinetics and efficient exciton dissociation and which are both vital to achieving higher efficiency. Figure 4 1. Transmission electron microscope images of ZnO nanoparticles.

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49 Figure 4 2. X ray diffraction pattern of ZnO NPs revealing the Wurtzite crystal structure. Figure 4 3. Steady state PL spectra of Z nO NP films. The band edge emission is located around 365nm while the defect emission is between 425 to 575nm.

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50 Figure 4 4. The XPS spectra O1s peak for ZnO films. A) U ntreated ZnO film spectra. B) UVO treated ZnO film spectra

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51 Figure 4 5. AFM images of untreated ZnO surface A) Height image of untreated film. B) Height image of treated film. C) Phase image of untreated film. D) Phase image of treated film. Figure 4 6. a)Structure of bilayer hybrid devices: ITO/ZnO NP/PCDTBT/MoOx/Ag. b) chemical s tructure of PCDTBT.

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52 Figure 4 7. Solar cell characteristics of PCDTBT/ZnO devices. A) Photo J V characteristics. B) EQE spectra

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53 Figure 4 8. The photo J V characteristics of ITO/PEDOT:PSS/MDMO:PPV/ZnO/Al devices with varying UVO treatment times. Figure 4 9. Transient photovoltage decay for devices employing untreated (black squares) and UVO treated (red circles).

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54 Figure 4 10. Fluorescence lifetime curves of 10 nm films of PCDTBT on various surfaces. The PL signal at 700nm was monitored after excitation at 375nm. Table 4 I. Photovoltaic Parameters of PCDTBT/ZnO Hybrid Solar Cells Donor Polymer ZnO Treatment V OC (V) J SC (mA/cm 2 ) FF (%) PCE (%) PCDTBT none 0.70 0.600.03 391 0.190.02 PCDTBT 5 min UVO 0.70 1.300.04 552 0.520.02 MDMO PP V none 0.83 0.350.03 411 0.120.03 MDMO PPV 5 min UVO 0.77 0.760.02 462 0.260.02

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55 CHAPTER 5 NOVEL CATHODE INTERF ACE LAYER FOR IMPROV ED ELECTRON INJECTIO N 5 .1 Introduction and B ackground It is well known that one of the most critical requiremen ts for efficient OLEDs is good charge injection as well charge balance of holes and electrons. Typically OLEDs consist of an indium tin oxide (ITO) anode, a hole injection/transporting layer (HTL), an emissive layer (EML), an electron transporting layer (E TL), and a metal cathode. These multilayer structures are easily fabricated using vacuum thermal deposition. In many organic electronic devices, low work function metals are commonly used as the cathode material for efficient electron injection. However, l ow work function metals such as Ca or Ba, are highly susceptible to environmental degradation and formation of quenching sites at the cathode interface. Considerable efforts have been devoted to realize efficient electron injection through interfacial engi neering such as modification of electrodes using thin films of alkaline metal fluorides such LiF 116 and CsF 117 Self assembled monolayers (SAMs) have also been utilized to tun e the injection barriers and improve the device performance. These SAMs form interfacial dipoles that realign the energy barriers between the metal contact and the underlying polymer layers to enhance charge injection 117 120 Polymer OLEDs (PLEDs) have th e advantage of simple and low cost processing using robust fabrication methods such as spin casting and ink jet printing. However, many light emitting polymers, such as poly(phenylenevinylene), are p type where hole transport dominates. These devices often have a recombination zone near the emitting layer (EML)/cathode interface resulting in poor device performance due to quenching of excitons by the metal cathode. In order to circumvent this problem, an electron

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56 injection/transporting layer (ETL) can be in troduced between the EML and cathode. The purpose of the ETL is to not only block holes, but to also improve electron injection which moves the exciton recombination zone away from the cathode interface and improves device performance. The requirements for an ideal ETL include: (i) Ability to process thin, uniform films from solvents that are orthogonal to the ones used to form the emitting layer. This approach allows for fabrication of multilayer stacks without interlayer mixing and dissolving the underlyi ng layers. (ii) Appropriate highest occupied molecular orbital (HOMO)/ lowest unoccupied molecular orbital (LUMO) levels to enable efficient electron injection while blocking holes. (iii) Good chemical stability. Oxadiazole small molecules and polymers are among the most studied materials for efficient electron injection in polymer LEDs. These polymers have been shown to be electron deficient in nature and chemically stable. The oxadiazole molecule 2 (4 biphenyl) 5 (4 tert butylphenyl) 1,3,4 oxadiazole (PBD) was first used in 1998 by Adachi et al. as an electron transport material in a bilayer small molecule OLED based on a triphenylamine derivative as the emissive material 118 Although enhanced performance was found using PBD, amorphous films formed by thermal evaporation lead to crystallization due to its low glass transition temperature of 60C. This low Tg greatly decreased the de vice lifetime. To overcome this problem of low Tg, oxadiazole containing polymers were synthesized which were more thermally stable. However, the drawback to using poly(1,2,4 oxadiazole) is that it is only processable in strong acids such as sulfuric acid and methanesulfonic acid 119 In order to improve the solubility of

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57 polyoxadiazole derivatives, alkyl or alkoxy side chains were add ed. Wang et al. 120 fabricated P LEDs using alkoxy solubilized oxadiazole containing polymer as an ETL and its chemical structure is show n in Figure 5 1a The conjugated polymer contains oxadiazole rings in the main chain and bearing long side groups attached to phenylene moieties. Devi ces are fabricated with poly[2 methoxy 5 (2 ethylhexyloxy) 1,4 phenylene vinylene] (MEH PPV) as the emitting layer. The device having a structure of ITO/PEDOT/MEH PPV/interlayer/Al showed a brightness of 800 cd/m2 with external quantum efficiency (EQE) of 0.26%. With the incorporation of the interlayer, there was an improvement in brightness and EQE by 50% and 25%, respectively, compared to control devices without the interlayer. Polypyridine (PPY) is a blue luminescent polymer which has been used as an emi tting layer in PLEDs 121,122 PPY and its derivatives have electron withdrawing electron deficient pyridine ring. Films of PPY have been shown to be resistant to both photo and electrochemical oxidation. Its chemical structure is shown in Figure 3 1b. Dailey et al. first explored the use of PPY as an ETL in poly(p phenylenevinylene) (PPV) devices 123 Devices with the PPY interlayer rea ched 900 cd/m2 with a maximum EQE of 0.25%. This performance was significantly better than the device without the interlayer. This enhancement in device performance was said to be due to enhancement in blocking of holes as well as enhancement in electron i njection due to its low electron affinity (EA) of 3.4 eV which provides a reduction in the electron injection barrier when using Al as the cathode metal. Another approach is to use interfacial polymers which can be spin casted from alcohol or water solvent s. This approach is particularly attractive because it allows for

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58 fabrication of multilayer devices without interlayer mixing. Spin casting ETLs from polar solvents prevents dissolving the underlying polymer EML which is usually processed using organic sol vents. Although many published works have focused on using electron deficient polymers such as derivatives of 1,3,4 oxadiazole and pyridine, electron rich polymers such as polyfluorene derivatives have been shown to be efficient ETLs in PLED devices 124 128 One of the first demonstrations of a bilayer cathode incorporating an alcohol soluble polyfluorene copolymer with a high work function metal was published by Wu et al. 129 Devices fabricated using the ETL and high work function cathode matched the performance of devices using a low work function metal cathode. The interfacial alternating copolymer used for the ETL was poly[9,9 (N,N dimethylamino )propyl) 2,7 fluorene) alt 2,7 (9,9 dioctylfluorene)] (PF NR 2 ). When inserting this conjugated amino alkyl substituted polyfluorene at the Au polymer interface, the amino group strongly with the Au surface leading to formation of interface dipole and reduc tion in electron injection barrier. Its chemical structure is shown in Figure 5 1c. The device characteristics for MEH PPV PLEDs employing Al, Ba/Al and PF NR2 cathodes are shown in Figure 5 2. PLED devices fabricated with the structure ITO/PEDOT:PSS/MEH P PV/PF NR 2 /Al showed a luminance of 454 cd/m2 and an EQE of 1.54 % as shown in Figure 5 2a and 5 2b, respectively. These results are comparable to devices fabricated with a low function cathode although the efficiencies are lower at low operating voltages. These results are also a drastic improvement from the device without an interlayer which showed a luminance of 6 cd/m 2 and an EQE of 0.002%. Figure 5 2c shows the I V characteristics for the PLED devices with different cathode configurations.

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59 It is apparen t that the addition of the interlayer layer leads to enhanced current compared to devices with low work function metals. Since the anode remains the same for all devices studied, it can be inferred that the increase in current density with the incorporatio n of PF NR 2 is due to improved electron injection allowing the device efficiency to reach values comparable to the device with a low work function cathode. Similar improvements in device efficiency using PF NR 2 were also found in green emitting poly(2 (4 ( 3',7' dimethyloctyloxyphenyl) 1,4 phenylene vinylene) (P PPV) and blue emitting polyfluorene (PFO) devices 129 By using the conjugated polymer electrolyte, PF NR3+I the electron injection is further improved and th e device performance reaches an EQE of 2.85 % and luminance of 773 cd/m 2 This is better than the device with a Ba/Al cathode which showed an EQ E of 2.46 % and luminance of 749 cd/m 2 Photovoltaic measurements of these devices revealed that the addition of the interlayer increases the built in potential (V BI ) of the devices. The V BI increases from 1.1 V, which is the difference in work function between PEDOT:PSS and Al, to 1.6V. X ray diffraction results show that PF NR 2 form a self assembled layer at the Au polymer interface with the amino group interacts strong with the Au surface forming an interface dipole. The formation of the interface dipole was also confirmed by measurements of electroabsorption. Wu et al. found that the insertion of the PF NR 2 layer results in an abrupt increase in the built in potential determined from electroabsorption spectroscopy. This interface dipole causes an abrupt shift in the vacuum level and reduces the barrier for electron injection. More recently, Huang et al. published work on polymer ETLs using alcohol soluble polyfluorenes 130 This studied aimed at better understanding of the effect of the polymer

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60 main chain on electron injection and device performance. Three neutral conjugated polyme rs, poly[9,9 bis(2 (2 (2 diethanolaminoethoxy)ethoxy)ethyl) fluorene] (PF OH), poly[9,9 bis(2 (2 (2 diethanolaminoethoxy)ethoxy) ethyl)fluorine alt 4,40 phenylether] (PFPE OH) and poly[9,9 bis(2 (2 (2 diethanolamino ethoxy) ethoxy)ethyl)fluorene alt benzo thiadizole] (PFBT OH) were developed as ETLs. PF OH and PFPE OH contain electron rich main chains while PFBT OH has an electron deficient main chain. Contradictory to previously published results for efficient electron injection/transport, PLEDs employing PFBT OH showed the worst performance. The devices employing PF OH and PFPE OH were around 40 times more efficient than the PFBT OH devices. This is despite the conventional design rule that electron deficient molecules are more effective for electron injec tion 131 The best performing devices incorporated PFPE OH, which is shown in Figure 5 1d. 130 Devices were fabricated with the structure ITO/PEDOT:PSS/poly(N vinylcarbazole)(PVK)/PFDBT02/ETL/Al where PFDBT02 is used as a red emitting fluorescent polymer. Devices with the polymer ETL were over a fac tor of three better than devices employing a low work function Ba/Al catho de. The EQE and luminance at 25 mA/cm 2 for the device incorporating the EI L were 3.48 % and 1040 cd/m2, respectively, compared to only 0.91% and 317 cd/m 2 for the device with a Ba/A l cathode. In order to gain insight for the better performance of devices containing the PF OH and PFPE OH ETLs compared to devices with PFBT OH, ph otovoltaic measurements were carried out to measure the V BI of the devices. The measure V BI of the device wi th an Al cathode increase from 1.1 V to 2.1 V and 2.2 V for devices containing PF OH and PFPE OH ETLs, respectively. Consistent with device performance, the V BI for the device with the PFBT OH ETL was only 1.3 V. Huang et al.

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61 indicated that this is most li kely due to the electron deficient backbone of PFBT OH interfering with the formation of an interface dipole between the hydroxyl groups on the sidechain and Al cathode. The n doping of ETL layers in OLEDs has been proven to enhance performance previously in vacuum deposited devices by increasing the conductivity and reducing the electron injection barriers 132 138 The doping of ETL layers allowed for the formation of thicker films to avoid pinhole formation while avoiding the increase in driving voltage which leads to higher power consumption. Alcohol soluble polymers, that have been proven to improve PLED device performance, can be doped with alkali metal salts due to their unique solubility 139,140 In this work, we explore a solution processed Cs 2 CO 3 doped poly(4 vinylpyridine) (P VPy ) polymer layer as a cathode interface layer to improve charge balance in Poly[2 methoxy 5 (2 ethylhexyloxy) 1,4 phenylenevinylene] ( MEH P PV ) based polymer light emitting diodes (PLED). Th rough L I V measurements as well as electron only devices we show improved electron injection into MEH PPV. With the composite interface layer, a luminescence ef ficiency of 3.94 cd/A is demonstrated which is a 214 % increase over the control device using a LiF/Al cathode. Electroabsorption spectroscopy and single carrier devices results indicate this is due to the low work function Cs 2 CO 3 and favorable interface dipole which reduces the barrier for electron injection. 5 .2 Device F abrication and M easuremen t The structure of the fabrica ted devices is shown in Figure 5 3 a Pre patterned ITO coated glass substrates wit were cleaned in the following sequence; ultrasonication in acetone, isopropanol and de ionized water for 15 min each, followed by a rinsing in de ionized water and a 15 minute UV Ozone

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62 treatment. The cleaned ITO substrates were spincast with a 35 nm layer of poly(3,4 ethylenedioxythiophene):polystyrenesulfonate ( PEDOT:PSS Bayer Corp. 4083) as the hole transporting l ayer and subsequently annealed at 180 C for 15 minutes. The MEH PPV polymer layer whose structure is shown in Figure 5 3b was spin casted and ann ealed for 30 minutes at 70 C. The Cs 2 CO 3 and P VPy layers were then spin casted from a 0.3 wt % solution in 2 methox yethanol at a spin speed of 600 rpm. The composite layer was also spin casted from a 0. 3wt% solution with a Cs 2 CO 3 :P VPy ratio of 1:1. Finally, a 100 nm thick aluminum layer was deposited as the cathode. All of the device processing was carried out under inert conditions inside a glovebox besides the deposition of PEDOT:PSS. The active pixel area of the device wa s 0.046 cm 2 Immediately after device fabrication, the devices were transferred out of the glovebox and t he brightness current voltage (L I V) measurements were taken under ambient conditions. The voltage was sourced from a Keithley 2400 SourceMeter while it simultaneously measured current. The light output was detected with a silicon photodiode fed into a Keithley 6485 Picoammeter. To con vert the diode photocurrent to densities with a Konaka LS 100 luminance met er while the photocurrent w as simultaneously recorded These two values were recorded in 0.1mA steps and the conversion factor of 583x10 6 (cd/m 2 )/A was found using the slope of luminance plotted versus photocurrent. The EA spectroscopy measurements were carried out using an AC amplitude of 0.3V with a frequency of 1 kHz.

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63 5 .3 Results and Discussion The lumina nce current voltage (L I V) measurements were carried out for device employing various electron injection layers (EILs), wi th the results shown in Figure 5 4 a The closed symbols show the I V characteristics and the open symbols are for the L V characteris tics. Given the fact that the barrier height for hole injection from PEDOT:PSS into MEH PPV is small (0.1 0.2 eV) and MEH PPV is a hole dominant polymer due to its higher mobility of holes than electrons, carrier recombination occurs near the cathode. With the addition of the P VPy layer, holes are blocked at the MEH PPV/P VPy interface which yields a reduction in current density compared to the Al only cathode as shown in Figure 5 4 a While the device with Cs 2 CO 3 shows higher current density due to its effic ient electron injection abilities 141 143 the current density is further increased in the device employi ng the composite Cs 2 CO 3 +P VPy layer. The enhancement in electron injection is due to the formation of an interface dipole The dipole is formed by the nitrogen atom of the pyridine side group of the P VPy donating its electrons to t he metal atoms in the ele ctrode. This interface dipole lifts the work function of the metal with respect to the LUMO level of MEH PPV and reduces the barrier for charge injection. As shown in Figure 5 4 a the luminance value at 4.5V for devices with the PVPy /Al cathode is 57 cd/m 2 compared to just 27 cd/m 2 for devices employing only Al and no interfacial layer This is d ue to better charge balance since blocked holes at the M EH PPV/P VPy interface recombine with injected electrons and more facile electron injection from the formatio n of the interface dipole. When Cs 2 CO 3 is added to PVPy as an interlayer, electron injection is further enhanced, r esulting in a luminance of 8400 cd/m 2 at 4.5 V. This luminance is over four times higher than that of the device using only a Cs 2 CO 3 interlay er.

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64 The current efficiencies of the devices as a function of current density are shown in Figure 5 4 b Due to the blocked holes in the emission layer at the cathode interface as well as the presence of the interface dipole improving electron injection, th e devic e efficiency improves from 0.06 cd/A for devices utilizing the Al only catho de to 0.4 cd/A for devices with the P VPy /Al cathode. As expected, the Cs 2 CO 3 /Al cathode shows even better device performance owing to the formation of a low work function co ntact and reaches a value of 1.8 cd/A. The current efficiency of the device with Cs 2 CO 3 + P VPy interlayer reaches 3.9 cd/A due to the further improvement in charge balance and is among the highest reported efficiencies for MEH PPV PLED devices. To unde rstand the improved electron inject tion of the composite layers, electron only devices were fabricated. These devices were fabricated using an ITO/ aluminum anode instead of ITO/PEDOT:PSS to intentionally create a large hole injection barrier of about 1eV Figure 5 5 shows the higher current density due to better electron injection using the Cs 2 CO 3 + PPY composite cathode compare d to Cs 2 CO 3 cathode. To understand the effect of the interlayers, electroabsorption spectroscopy was utilized in this study. EA s pectroscopy measures the changes in optical constants of materials under applied electric fields and has been used to probe the electric fields within organic electronic devices 75 78,144 146 Through this method, we investigated the effect of the various interlayers on the built in potential (V BI ) of the devices which directly relates the barrier for electron injection. The EA response is proportional to the (3) square of the electric field E 145 (1)

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65 is the absorption coefficient and d is the film thickness. The electric field within the device is a result of both the built in field of the device along with the externally applied electric field: (2) Substituting Eq. (2) into Eq. (1), it is expected that the EA signal varies linearly with the applied DC bias and goes to zero when the applied DC bias is equal to the V BI Figure 5 6 shows the EA spectra for the PLED device with Cs 2 CO 3 + PPY interlayer. There is a strong EA peak at 565 nm which disappears when the forward V DC applied is 1.75 V. It is at this applied external voltage that the built in potential is compensated and flat band condition is met. Fi gure 5 7 shows the bias scans of the EA signal for devices with varying cathode interface layers. The zero level crossing indicates the built in potentials for devices employing various interface layers. The V BI for the device structure with the Al only ca thode is 1.05 V which closely matches the work function difference of Al (4.2eV) and PEDOT:PSS (5.2eV). With the addition of P VPy the V BI increases to 1.25V. This can be explained by the formation of an interfacial dipole due to nitrogen donor atom on the side chain of the P VPy The nitrogen atom of the pyridine side group of P VPy donates its electrons to the aluminum atoms in the electrode forming an interfacial dipole. This lifts the work function of the metal with respect to the LUMO level of the MEH PP V and reduces the barrier for charge injection. When the Cs 2 CO 3 +PVPy layer is used, the V BI increases to 1.75V from 1.55V when just the Cs 2 CO 3 is incorporated. The increase in V BI using P VPy clearly shows the barrier for electron injection is lowered yiel ding better device performance.

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66 5 .4 Summary MEH PPV based PLEDs were fabricated using an n doped insulating polymer interfacial layer. Electroabsorption spectroscopy results indicate the polymer forms an interface dipole which decreases the electron inje ction barrier. When doped with Cs 2 CO 3 to further decrease the injection barrier, MEH PPV polymer LEDs were fabricated with a current efficiency of 3.9 cd/A. The findings here provide insight into the elimination of electron injection barriers for improved device performance. Figure 5 1. Chemical structures of polymers used f or EML layers in OLED devices. A ) poly[2,5 bis((2 ethylhexyl)oxy) 1,1' biphenyl 4,4' diyl 1,3,4 oxadiazole 2, 5 diyl 1,4 phenylene](P PBD) B ) Polypyridine (PPY) c) poly[9,9 (N,N dimethylamino)propyl) 2,7 fluorene) alt 2,7 (9,9 dioctylfluorene)] (PF NR 2 ). C ) poly[9,9 bis(2 (2 (2 diethanolaminoethoxy)ethoxy) ethyl)fluorine alt 4,40 phenylether] (PFPE OH). D ) poly[9,9 bis(2 (2 (2 diethanolaminoethoxy)ethoxy) ethyl)fluorine alt 4,40 phenylether] (PFPE OH). E ) poly[9,9 bis(2 (2 (2 diethanolaminoethoxy)et hoxy)ethyl) fluorene] (PF OH). F ) poly[9,9 bis(60 diethoxylphosphorylhexyl)fluorene] (PF EP).

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67 Figure 5 2. C haracteristics of ITO/PEDOT/MEH PPV/PF NR 2 /Al OLEDs. A ) J L V charact eristics. B)L V and Q V characteristics. C ) I V characteristics of MEH PPV devices with Al, Ba/Al and PF NR 2 cathodes. (Reproduced from Wu. et al. 129 )

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68 Figure 5 3 A ) Schematic of the fabri cated PLED device structure. B ) Chemical structure of MEH PPV

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69 Figure 5 4 Device characteristics of MEH PPV OLEDs. A ) Luminance Current Voltage (L I V) characteristics for devices with varying interlayers. B ) Luminous efficiencies (cd/A) for devices with various inter layers.

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70 Figure 5 5 J V plot of electron only devices fabricated with the structure Al/MEH PPV(100nm)/Interlayer/Al Figure 5 6 Electroabsorption spectroscopy measurements for the device with structure ITO/PEDOT:PSS(35nm)/MEH PPV(85nm)/Cs 2 CO 3 +P VPy (10n m)/Al(100nm).

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71 Figure 5 7 EA spectroscopy bias scans for MEH PPV devices employing various cathode configurations

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72 CHAPTER 6 INVESTIGATION OF THE HALF GAP ELECTROLUMI NENCE THRESHOLD OF RUBRENE/FULLERENE HE TEROSTRUCTURES 6 1 Introduction and Background The interest in organic semiconductors has been driven in recent years by the promise of light weight and flexible displays which is afforded by cheap and robust fabrication methods 147,148 The injection of both holes and electrons is required for light emission. The lowest potential at which this o ccurs is usually approximately equal to the bandgap energy of the emissive material so that the flat band condition is met when Ohmic contacts are used. This is a requirement of both inorganic and organic LEDs and breaking this limit must requires up conve rsion processes, such as a radiative Auger recombination process. This up conversion process has been well studied in inorganic LEDs and was recently reported in organic devices 149,150 The small molecule, 5,6,11,12 Tetraphenylnaphthacene (Figure 6 1a) also known as rubrene, and C 60 (Figure6 1b) were used to form a bilayer OLED s tructure as shown in Figure 6 1c The device showed electroluminescence (EL) at <1 V and ha d the characteristic emission spectrum of rubrene 151 This is interesting since the EL turn on is less than half the b andg ap of rubrene (2.2 eV). The phenomen on was explained by proposing the Auger up conversion mechanism 151,152 In this chapter, results are reported to help better understand the half gap EL phenomenon. This is accomplished by understanding the importance of the rubrene/C 60 interface; especially the accumulation of injected charge carriers which is found to modify the internal electric fields inside the device. OLED devices were fabricated and studied by measuring luminance current voltage (L I V) characteristics The capacitance voltage (C V) measurements of devices with and without a C 60 layer allowed fo r the understanding of charge injection and accumulation

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73 at the rubrene/ C 60 interface. The effect of charge accumulation at the rubrene/C 60 interface on the internal electric field is studied with elec troabsorption (EA) spectroscopy after the careful ident ification of the spectral features. Auger up conversion process : The Auger up conversion mechanism is a process where an electron hole pair recombines and the released energy is transferred to a third particle. This transfer of energy allows for an electro n (or hole) to overcome large transport barriers and exhibit light emission at voltages potentials less than the The Auger fountain mechanism has been observed in inorganic semiconductor systems but not studied as much for their organic counterparts 151 6 2 Device Fabrication and M easurement In this study, d evices were fabricated on pre patt erned ITO coated glass following sequence; ultrasonication in acetone, isopropanol and de ionized water for 15 min each, followed by a rinsing in de ionized water and a 15 mi nute UV Ozone treatment. The cleaned ITO substrates were spincast with a 35 nm layer of poly(3,4 ethylenedioxythiophene):polystyrenesulfonate ( PEDOT:PSS Bayer Corp. 4083) as the hole transporting layer and subsequently annealed at 180 C for 15 minutes. A 40nm later of rubrene and 25nm layer of C 60 were thermally evaporated at 0.5/s. The device s without a C 60 layer contained a 70nm thick layer of rubrene both with and without a 1nm layers of LiF for comparison Finally, a 100 nm thick aluminum layer was de posited as the cathode. All of the device processing was carried out under inert conditions inside a glovebox besides the deposition of PEDOT:PSS. The active pi xel area of the device was 0.04 cm 2

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74 After device fabrication, the L I V measurements were mad e in ambient conditions The voltage was sourced from a Keithley 2400 SourceMeter while it simultaneously measured current. The light output was also simultaneously detected with a silicon photodiode fed into a Keithley 6485 Picoammeter. The differential c apacitance voltage measurements were done using a Hioki 3552 50 LCR meter ran using a LABVIEW program. The devices were biased from 0 3 V superimposed on an AC drive voltage of 30mV and constant frequency of 1kHz. EA spectroscopy was performed using the m easure ment setup described in Figure 2 4. The AC amplitude was fixed at 0.3 V and the frequency used in obtaining the EA spectra was 1000 Hz. In all measurements, the ITO electrode was bias as positive and the Al contact as negative. 6 .3 Results and D is cussion T he I V and L V characteristics are shown in Figure 6 2 a an d 6 2 b, respectively At 2V, the current densities are 0.2, 3.4 and 132 mA /cm 2 for the ITO/PEDOT:PSS/r ubrene/Al, ITO/PEDOT:PSS/ rubrene/LiF/Al and ITO/PEDOT:PSS/ rubrene/C 60 /Al devices, resp ectively. The current turn on is lower for the ITO/PEDOT:PSS/ rubrene/LiF device compared to the ITO/PEDOT:PSS/ rubrene/Al device which is expected with due to lower barrier for electron injection. The current turn on for the ITO/PEDOT:PSS/ rubrene/C 60 device s is even lower. Figure 6 2 b shows the LV characteristics of the three device configurations. The luminance turn on (1 cd/m 2 ) voltages for the ITO/PEDOT:PSS/ rubrene/Al, ITO/PEDOT:PSS/ rubrene/LiF/Al and ITO/PEDOT:PSS/ rubrene/C 60 /Al devices, are 1.1, 2.25 an d 2.75V, respectively. To understand the device operating mechanism, we must first start with charge carrier injection from the contacts. Capacitance voltage (C V) measurements have been

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75 performed by numerous groups in order to study the injection onset o f majority and minority charge carriers 153 155 Campbell et al. reported that the increase in capacitance in polymer light emitting diodes (PLEDs) was caused by traps at the organic metal interface 153 Shrotriya et al. extended this model and explained that the change in capacitance is d ue to minority and majority injection from the contacts 154 Berleb et al. studied the NPB/Alq system and determined that negative charges at the junction are responsible for the bias and frequency dependent capacitance 155 They also found that charge injection into NPB will occur above a threshold vo ltage (V T ) at which point flat band condition is reached in the NPB layer while a built in field still exists in the Alq layer. For our work, we take a similar approach using C V measurements to study the injection, charge accumulation and recombination of charges in the OLED devices while relating these findings to direc t measurements of built in voltage (V BI ) Figure 6 3 a shows the C V results along with the I V characteristics for the ITO/PEDOT:PSS/r ubrene (70nm)/Al device At 0 V, the capacitance is 0.9 2 nF and is approximately equal to the geometric capacitance 154 The capacitance remains constant until a forward bias of 1.9 V where majority c arrier injection occurs and a large increase in capacitance in observed. It is expected that holes will be injected into rubrene first and will be the majority charge ca rrier This is assumed since it is well known that PEDOT:PSS form s Ohmic with rubrene due to the small hole injection barrier of around 0.2 eV while the barrier for electron injection from Al is around 0.8V. As the bias increases, t he capacitance eventually reaches a value of 2.1 nF at 2.45 V before decreasing due to mi nority carrier injection This coincides with the current turn on for the device plotted as the open circles in Figure 6 3 a. Figure 6 3 b shows the C V and I V

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76 charact eristics for the device structure ITO/PEDOT:PSS/r ubrene/C 60 /Al Again, we assumed holes ar e the majority carrier since it has been found that a strong potential barrier exists at the C 60 /Al interface 156 This was attributed to hot Al atoms reacting with C 60 to form carbide species. From the C V plot, we observe that majority carrier (hole) injection starts to occur at a very low voltage of around 0.25 V. The capacitance continues to increase to 5.45 nF at 0.9 V at which point dual carrier injection causes the capacitance to decrease due to recombination leading to light emission It is obvious that the device structure of ITO/PEDOT:PSS/r ubrene/C 60 /Al sh ows not only a lower V T for majority injection but a explained by the addition of the capacitance values for the individual rubrene and C 60 layers. Upon injection of holes after the V T of 0.25 V, the increased carrier concentratio n in the rubrene laye r will increase resistance. It is after the V T that the electric field in rubrene is screened by accumulated charge carriers and bias increases will be dropped across the C 60 layer. Due to the offset in en ergetic levels, holes will accumulate at the rubrene/C 60 interface in forward bias after at V T of 0.25V and will lead to unequal electric fields within the rubrene and C 60 layers. In order to understand the effect of the charge carriers on the internal ele ctric field s electroabsorption (EA) spectroscopy was employed. EA spectroscopy measures the changes in optical constants of materials under applied electric fields. The modulation of transmission by an applied electric field is possible due to the pertu rbation of energy levels of the organic films and has been used to probe the electric fields within organic electronic devices 75 78,144 146

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77 Several groups have used EA spectroscopy to understand the field distribution in bilayer OLEDs such as the archetypal NPB/Alq system. Martin et al. studied the individual electric fields in the NPB/Alq system experimentally as well as through simulation modeling 78 It was found that when the barrier for electron injection is high er than the ba rrier for hole injection, a large hole density is accumulated at the heterojunction due to the 0.6 eV LU MO offset between NPB and Alq. T his distribution of charges results in the average ele ctric field in NPB being less than that in Alq. The rubrene/C 60 sy stem can be modeled in a similar way. The offset in energetic levels of rubrene and C 60 provides a barrier for transport and will confine both charge carriers to the organic organic heterojunction Also, there is almost no barrier for hole injection from P EDOT:PSS into rubrene while a potential barrier exists for electron injection from Al into C 60 The EA spectra were collected for the individual films of rubrene and the results are shown in Figure 6 4. It should be noted that unlike previous work that st udied electric field distributions in multilayer OLEDs, the individual materials studied here have excitonic peaks which are nearly the same and located at 546nm. This presents a challenge when trying to determine the electric fields of the rubrene and C 6 0 layers of the bilayer structure spectroscopically. How ever, it can be seen in Figure 6 4b that the EA spectrum of C 60 has shoulder of its exciton peak that extends to around 700nm. This feature is absent in the sp ectrum of neat rubrene (Figure 6 4a) and can be used to monitor the electric field of the C 60 layer in the bilayer configuration. The EA signal from the excitonic peak of rubrene at 546 nm was measured as a function of voltage so that the V bi of both device without a C 60 layer could be measured.

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78 These results are plotted in Fi gure 6 5. The V bi of the rubrene/Al and rubrene/LiF/Al were found to b e 1.65 V and 2.15 V respectively The larger built in potential for the device employing the LiF/Al cathode shows that the lower work functio n cathode f orms a lower barrier for electron injection. It should be also be noted that one issue that arises when measuring the EA spectra of OLEDs is that the field modulated electroluminescence signal that is detected with the Si photodiode can be ordered of magn itude higher than the signal coming from the change in transmission ( T). This effect can be compensated by measuring the field modulated EL signal as a function of bias without the incident monochromatic light. Below the luminance turn on voltage, no modi fication to the collected T/T is needed. However, above the turn on voltage, the value of field modulated EL must be subtracted from the measured T before normalization and presented as T/T. The bias dependence of field modulate d EL signal is shown in F igure 6 6 The EA spectra for the rubrene/C 60 bilayer device was measured a t various DC bias es and the results are shown in Figure 6 7 As expected, the excitonic feature of the bilayer structur e was found to be located at 546 nm which is the same value as the individual materials. In reverse bias, the EA signal for the rubrene/C 60 bilayer structure has a line shape similar to that of the rubrene/Al device. This can be explained by looking at the spectra of t he individual layers in Figure 6 4. Even though t he film thicknesses of the neat layers were the same (100 nm) and the same conditions were used for the EA measurement, the EA signal at 546 nm for the rubrene device was 3x10 5 compared to only 7.2x10 6 for the C 60 device. Not only is the EA response over 4 times greater for the rubrene film, the rubrene thickness used in the bilayer structure is

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79 40% larger than the C 60 layer This will also yield a stronger response from the rubrene layer compared with the C 60 layer in the bilayer configuration In forwar d bias, the lineshape is different than in reverse bias. The shoulder feature of the neat C 60 layer is now very apparent indicating that the field is now being dropped more across the C 60 layer than the rubrene. Also, a t forward bias of only 1.25 V, the si gn of the EA signal is reversed and the absolute magnitude of the signal ( 5.8x10 5 ) is already over 3 times larger than at a bias of 3 V (1.8x10 5 ). Therefore, this indicates that in forward bias, the electric field for the bilayer device is mostly droppe d across the C 60 layer and that additional sources (i.e accumulated charge carriers) may be contributing to the electric field. Although its detailed analysis is beyond the scope of this work, a brief discussion will be presented about the sub bandgap fea tures measured for the bilayer device that were not measured in the devices wi th only rubrene and C 60 materials individually In Figure 6 7 b, there is a feature located from 650 1050 nm with its peak at 845 nm I ts magnitude is V DC independent from rever se biases up to a mild forward bias of 0.25 V. As mentioned earlier, the features of the first harmonic EA spectra due to the Stark effect will vary linearly with V DC 81,135,141 Therefore, the feature at 850 nm of the bilayer is intrinsically different than the excitonic feature found at 546 nm for both the neat rubrene and C 60 spectra. T his feature is also coincidental with the broadband emission reported from the exiplex formed at the rubrene/C 60 interface 157 The EL emission from exciplex states results from the accumulation of electrons and hole at an interface resulting in an interfacial charge transfer (CT) exciton. These CT excitons formed between two different materials can recombine e missively with energy equal to the

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80 energy offset of the HOMO and LUMO values of the respective materials 150,157,158 In Figure 6 7 b, it can be seen that above bias values where charge injection is present, the subbandgap EA features change as the feature at 850 nm seems to evolve into a positive peak at 800 nm and a negative feature at 900 nm. The identification of all the sub bandgap features is beyond the scope of the work and will require intense spectroscopic analysis and deep understanding of how free charge carriers interact with CT states. In order to det ermine the built in potential of the bilayer device, t he magnitude of the 546 nm excitonic peak was measured as a function of bias and the results are shown in Figure 4 8 a. In reverse bias, the field across the layers is most likely homogenous since virtua lly no charges are injected and the device is fully depleted. The EA signal varies linearly with applied DC bias for all three AC frequencies used. For the 1kHz and 20kHz frequencies, a drastic shift in the slope occurs around a l ow forward bias of 0.22V. This also the same voltage where hole injection occurs as measured using CV (Figure 6 3b) The AC frequency was then set to 40 kHz in order to reduce the possibility of charge distribution. This can be attributed to the fact that at higher frequencies, c harge carriers do not have enough time to accumulate at the rubrene/C 60 interface. At a frequency of 40 kHz, the slope of the plo t is almost linear indicating only a small shift in the electric field distribution with the applied AC bias The measured buil t in potential is 0.9 V, which is the difference in energy between the LUMO of C 60 and the workfunction of PEDOT:PSS. To corroborate these finding s the EA signal at 546 nm was measured as a function of frequency for various DC biases and the r esults are p lotted in Figure 6 8 b Below injection condition at 0.5 V and 0 V, the EA signal is

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81 independent of frequency. This is expected since the Stark Effect, which is responsible for the change in absorption coefficient, happens instantaneously upon application of an electric field. At a forward bias of 0.5 V, the EA signal is negative; indicating the electric field in the OLED is reversed past the flat band condition. The EA signal is also now frequency dependent which is due to charge carriers influencing the e le ctric field distribution Upon higher injection conditions at 0.7 V, the electric field is even more frequency dependent based on the slope of the plot from 1kHz to 40kHz. This is expected since at the higher bias more charges are present to a ffect the electric field distribution. From the data presented, the following mechanism is proposed: With the injection of holes at very low biases, the increase in conductivity of the rubrene layer and the accumulation of holes at the rubrene/C 60 interface allows for the drop in electric field to be enhanced in the C 60 layer. At around 0.9V, both holes and electrons are present at the rubrene/C 60 interface. Electrons in the C 60 can then recombine wi th holes in the rubrene layer. According to the Auger fountain mec hanism, t he energy from this recombination is then transferred to an electron in the LUMO of C 60 which then can overcome the transport barrier into the LUMO of rubrene The free electron and hole can form an exciton which will recombine to emit light equal to the band gap energy of rubrene One of the requirements of the Auger up conversion mechanism is the accumulation of both holes and electrons in close proximity 152 In order to test this in our device structures, a 10 nm mixed layer of rubrene doped with C 60 in a 1:1 ra tio was inserted in between the rubrene and C 60 layers to disrupt the neat interface The device

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82 structu res fabricated were ITO/PEDOT:PSS/Rubrene(35nm)/Rubrene:C 60 (10nm)/C 60 (20nm)/Al(100nm) and ITO/PEDOT:PSS/Rubrene(40nm)/C 60 (25nm)/Al(100nm). The L I V results are shown in Figure 6 9 Without a neat interface for charge accumulation, the device with a mixed layer shows a lower current density (Figure 6 9a) as well as a higher luminance turn on voltage (Figure 6 9b) The lower current density is most likely due lower recombination current. The luminance turn on increased from 1.05V with a neat interface to 1.4 V for the device including the mixed interlayer elucidating the importance of a neat interface in the device structure. 6.4 Summary In this chapter, result s were reported to help better unders tand the half gap EL phenomenon in rubrene/C 60 heterojunction OL EDs. This was accomplished through the understanding of the rubrene/C 60 interface; especially the accumulation of injected charge carriers. The injection of majority carriers (holes) accumulate at the interface between rubrene and C 60 which help facilitate electron injection. Electrons from C 60 can then recombine with the accumulated holes in the rubrene layer to release energy which another electron can use to overcome the transport gap due to the large LUMO offset. The Auger up conversion mechanism was al so found to be more efficient for a structure with a neat heterojunction a s opposed to a mixed interface.

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83 Figure 6 1. The molecular st ructure for A)rubrene and B ) for fullerene C 60 C )The energy level diagram of ITO/PEDOT:PSS/rubrene/C 60 /Al device

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84 Figure 6 2 Device characteristics of rubrene OLEDs. A ) Cu rrent Density Voltage (J V). B ) Luminance Voltage (L V) plots of OLEDs with varying device configurations

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85 Figure 6 3 C V (closed squa red) and I V (open circles) characterisics. A ) ITO/PEDOT:PSS/Rubrene/Al B ) ITO/PEDOT:PSS/Rubrene/C60/Al devices.

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86 Figure 6 4 EA spectra for A ) ITO/C 60 (100nm)/Al and B ) ITO/rubrene(100nm)/Al at 3 V V DC

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87 Figure 6 5 Bias dependence of the 546 nm EA signal for rubrene OLEDs with and without a Li F buffer layer. Figu re 6 6. Bias dependence of the electric field modulated photocurrent using the EA setup without incident monochromatic light.

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88 Figure 6 7 EA spectra for the ITO/PEDOT:PSS/rubrene/C 60 /Al device as various biases. A) Data from 450nm to 1100nm. B) Data from 600nm to 1100nm.

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89 Figure 6 8 EA data for rubrene/C60 devices. A ) Bias sweep of the 550nm EA signal with various AC bias frequencies. B ) Frequency sweep of the 550nm EA signal at various DC biases.

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90 Figure 6 9 OLED device da ta for device with neat and mixed interfacecs. A ) Curr ent Density Voltage (J V). B ) Luminance Voltage (L V) for OLED devices.

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91 CHAPTER 7 CONCLUSIONS AND FUTU RE WORK Chapters 1 provided readers with a background of the fundamentals of organic elec tronic material properties. Chapter 2 introduced two devices based on organic electronic materials, organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). Chapter 3 discussed the relevant characterization concepts and experimental setups o f organic electronic devices. In C hapter 4 two loss mechanisms in polymer/ZnO plana r heterojunction solar cells were studied. By passivation of defects at the PCDTBT/ZnO interface, devices showed a power conversion efficiency improvement of over 250% co mpared to the devices using untreated films. This improvement comes from a large increase in the short circuit current (Jsc) and fill factor (FF). Transient photocurrent measurements showed that the passivation to ZnO defects leads to less surface recombin ation of photocarriers, reducing photocurrent losses. Time resolved fluorescence measurements showed shorter PL lifetime upon UVO treatment of ZnO indicating better forward photoinduced electron transfer. This work allows us to better understand the loss m echanisms in solar cells where careful control of defects allows for control of photocarrier recombination kinetics and efficient exciton dissociation and which are both vital to achieving higher efficiency. Future work may involve the incorporation of de fect free ZnO NP into bulk heterojunction (BHJ) solar cells which are known to have higher efficiencies than planar heterojunction devices. It may also be possible to increase the light absorption in ZnO based hybrid solar cells by functionalizing the ZnO nanoparticles with self assembled monolayers that can act to harvest more of the solar spectrum.

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92 In Chapter 5 MEH PPV based PLEDs were fabricated using an n doped insulating polymer interfacial layer. Electroabsorption spectroscopy re sults indicate that t he polymer interfacial layer decreases the electron injection barrier by forming an interface dipole With the doping of the interfacial layer with Cs 2 CO 3 to further decrease the injection ba rrier, MEH PPV polymer LEDs were fabricated with a current effic iency of 3.9cd/A which was more than 2x higher than the devices employing the standard Cs 2 CO 3 cathode layer The findings here provide insight into the elimination of electron injection barriers for improved device performance. Future work still needs to b e performed to find optimal interface materials for improved performance and lifetime while minimizing cost. Chapter 6 studied the injection and accumulation of charge carriers in the recently reported rubrene/C 60 OLED. It was found that injection of ch arge carriers will greatly affect the internal electric fields and that accumulation of both holes and electrons at a neat heterojunction is necessary for the Auger energy up conversion process. There remain several questions about Auger upconversion in or ganic devices. It is still not well understood what all of the material requirements are for the upconversion mechanism to take place. As of today, the only examples for EL upconversion in organic devices use either C 60 as shown in this work, or ZnO as the ETL 151,152 It is interesting that for systems employing materials with proper energy level alignment for accumulation have been studied (i.e Alq/NPB structures), the sub bandgap luminance turn on has not been seen. It is very possible that the answer lie s in the nature of charge transfer at the interface of the heterostructure. This stimulates the use of advanced spectroscopic measurements, such as EA, for the investigation of charge transfer states and this

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93 dynamics in relation to the mechanism of Auger up conversion. Also, it may be possible to lower the luminance turn on voltage even more by improving the injection of electrons. As stated earlier, the C 60 different combination of materials, this may be possible. This work has laid some ground work for understanding the importance of interface engineering for organic optoelectronic devices

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94 APPENDIX LIST OF PUBLICATIONS 1. Michael J. Hartel Song Chen, Ben Swerdlow, he Half Gap Electroluminescence Threshold of Rubrene/Fullere ne in preparation) 2. Michael J. Hartel Song Chen, Hsien Yi Hsu, Benjamin Swerdlow, Jesse Manders, Kirk Schanze b and Franky So Defect Induced Loss Mechanisms in Polymer Inorgan ic Planar Heterojunction Solar Cells ACS Applied Materials and Interfaces (submitted). 3. Michael J. Hartel Jegadesan Subbiah, Franky So "Interlayers for Efficient Electron Injection in Polymer LEDs," IEEE Journal of Display Technology vol.PP, no.99, p p.1 7, doi: 10.1109/JDT.2013.2247738 (2013) 4. Jesse R. Manders, Sai Wing Tsang, Michael J. Hartel Tzung han Lai, Song Chen Solution Processed Nickel Oxide Hole Transport Layer in High Efficiency Polymer Photo voltaic Cells Advanced Functional Materials (2013) DOI: 10.1002/adfm.201202269 5. Kenneth R. Graham Patrick M. Wieruszewski, Romain Stalder Michael J. Hartel Jianguo Mei, Franky So, John R. Reynolds, Improved Performance of Molecular Bulk Heterojun ction Photovoltaic Cells through Predictable Selection of Solvent Advanced Functional Materials 22, (2012), doi: 10.1002/adfm.201102456 6. Jegadesan Subbiah Do Young Kim Michael J. Hartel Franky So, MoO 3 /poly(9,9 dioctylfluorene co N [4 (3 methylpropyl)] diphenylamine) double interlayer effect on Applied Phys ics Lett ers 96 063303 (2010)

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108 BIOGRAPHICAL SKETCH Michael Hartel was born in 1984 in Durham, NC He moved to Syracuse, NY at the age of 3. After graduat ing from Cicero North Syracuse H igh School, he attended Alfred Uni versity for his B.S degree in ceramic e ngineering where he was a member of several student organizations and played Division III lacrosse. He graduated in May 2007 and started gradua te school at the University for Florida in the Fall of 2007. He obtained his M.S in Materials Science in 2009 and continued on for his Ph.D. under the supervision of Dr. Franky So. Upon graduating, he will join the In tel Corporation in Portland, OR.