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The Effects of Materials and Processing on Organic Photovoltaics

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Title:
The Effects of Materials and Processing on Organic Photovoltaics
Creator:
Constantinou, Iordania
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[Gainesville, Fla.]
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University of Florida
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english
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1 online resource (138 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Materials Science and Engineering
Committee Chair:
SO,FRANKY FAT KEI
Committee Co-Chair:
SINGH,RAJIV K
Committee Members:
ANDREW,JENNIFER
JONES,KEVIN S
RINZLER,ANDREW GABRIEL
Graduation Date:
4/30/2016

Subjects

Subjects / Keywords:
Annealing ( jstor )
Charge carriers ( jstor )
Electric current ( jstor )
Electrons ( jstor )
Excitons ( jstor )
Fullerenes ( jstor )
Heterojunctions ( jstor )
Narrative devices ( jstor )
Photovoltaic cells ( jstor )
Polymers ( jstor )
Materials Science and Engineering -- Dissertations, Academic -- UF
charge-transfer -- organic -- photogeneration -- photostability
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Electronic Thesis or Dissertation
bibliography ( marcgt )
theses ( marcgt )
Materials Science and Engineering thesis, Ph.D.

Notes

Abstract:
Organic semiconductors (OSCs) are a relatively new class of materials gaining attention as a possible replacement for traditional semiconductors due to their unique electronic and mechanical properties such as low-temperature processability, low cost and device flexibility. While some organic electronic devices, like organic light emitting diodes (OLEDs) have reached commercial maturity, organic photovoltaics (OPVs) are still a long way from commercialization mostly due to their low operational stability. First, the polymer photovoltaic stability in the cases of exposure to ambient air during processing and extended light exposure was investigated. It was found that ambient processing induces fast initial device degradation due to a reduction in hole mobilities combined with a small reduction in device absorption. The impact of air exposure on device performance was shown to only be significant in the first hour of exposure and efficiencies for devices exposed for up to three hours plateaued at about 20% the efficiency of devices made in nitrogen atmosphere. Next, the impact of 1 sun light exposure on device performance was examined for devices made using the same active layer materials. It was found that light exposure for 24 hours leads to a significant reduction in device performance, negatively impacting all device parameters. Ultraviolet (UV) light induced defects were proven to be the primary reason for device degradation and device stability was significantly improved with the use of UV filters. Next, the effects of thermal annealing and polymer side-chains on charge carrier generation and recombination were investigated. It was shown that both thermal annealing and the polymer side-chains can have a strong impact on the formation of charge transfer (CT) states at the polymer-fullerene interface and significantly influence the charge generation process. Additionally, carrier transport and recombination were also shown to be affected through changes in pi-pi stacking and defect density in the active layers. Thermal annealing was found to improve carrier generation due to improved CT state delocalization but an increase in the concentration of defects caused the device efficiency to be lower overall. Similarly, brunched side-chains on the polymer acceptor moiety were found to enhance carrier generation and carrier transport. ( en )
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In the series University of Florida Digital Collections.
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Includes vita.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (Ph.D.)--University of Florida, 2016.
General Note:
Adviser: SO,FRANKY FAT KEI.
General Note:
Co-adviser: SINGH,RAJIV K.
General Note:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2016-11-30
Statement of Responsibility:
by Iordania Constantinou.

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UFRGP
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Copyright Iordania Constantinou. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Embargo Date:
11/30/2016
Classification:
LD1780 2016 ( lcc )

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THE EFFECTS OF MATERIALS AND PROCESSING ON ORGANIC PHOTOVO TAICS By IORDANIA CONSTANTINOU A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2016

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2016 Iordania Constantinou

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To my family

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4 ACKNOWLEDGMENTS I would first like to thank my advisor Prof. Franky So for giving me the opportunity to be a member of his group and for encouraging me to think big and pushing me to be better. I would also like to thank Prof. Jennifer Andrew, Prof. Kevin Jones, Prof. Rajiv Singh and Prof. Andrew Rinzler for serving on my committee and providing feedback regarding my research. I would also like to acknowledge the help and support of several of the past and present members of OEMDLab. First, I want to thank Prof. Sai-Wing Tsang not only for his help and guidance in research but also for welcoming me into the group and being my first friend in Florida. I would also like to thank Dr. Tzung-Han and Erik Klump for the good times in the office and the great collaboration. I thank my labmates Dr. Chao-Yu Xiang, Dr. Dewei Zhao, Dr. Hye-Yun Park, Xiangyu Fu and Mike Sexton for the meaningful research related conversations but mostly for all the crazy times around Gainesville and all our amazing trips and barbecues. I would also would like to express my gratitude for Prof. Stelios Choulis and Dr. Marios Neophytou for believing in my capabilities and convincing me to go to graduate school. I want to thank James Deininger, Chi Kin Lo and Prof. John R. Reynolds from Georgia Institute of Technology, Dr. Hsien-Yi Hsu, Dr. Subhadip Goswami and Prof. Kirk S. Schanze from the Department of Chemistry at UF and Sin-Hang Cheung and Prof. Shu-Kong So for Hong Kong Baptist University for the collaboration on a number of projects. Finally I would like to thank my mom and dad for their love and support during this experience and for always believing in me and encouraging me. I want to thank my sister for always being available to talk, day and night and for never letting me miss her. I also want to thank the rest of my family for always being proud of me and always waiting for me to come home. I especially want to thank Nate for his love, patience and support, and for always feeding me

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................8 LIST OF FIGURES .........................................................................................................................9 ABSTRACT ...................................................................................................................................13 CHAPTER 1 INTRODUCTION TO ORGANIC SEMICONDUCTORS ...................................................15 1.1 Introduction .......................................................................................................................15 1.2 Comparison between Organic and Inorganic Semiconductors .........................................15 1.3 Molecular Structure and Bonding .....................................................................................16 1.4 Optical Absorption in Organic Semiconductors ...............................................................17 1.5 Excitons ............................................................................................................................18 1.5.1 Exciton Formation ..................................................................................................18 1.5.2 Exciton Diffusion and Dissociation ........................................................................18 1.6 Charge Carrier Transport ..................................................................................................19 1.6.1 Stacking ................................................................ ................................ ............19 1.6.2 Hopping Transport Mechanism ..............................................................................20 1.7 Charge Carrier Recombination .........................................................................................21 1.7.1 Monomolecular Recombination .............................................................................21 1.7.2 Bimolecular Recombination ...................................................................................22 1.8 Device Architectures ........................................................................................................23 1.8.1 Standard versus Inverted Device Structure ............................................................23 1.8.2 Heterojunctions .......................................................................................................23 1.9 Charge Transfer States in Bulk Heterojunctions ..............................................................25 1.10 Film Formation ...............................................................................................................26 1.10.1 Thermal Evaporation ............................................................................................26 1.10.2 Spin Coating .........................................................................................................27 1.10.3 Rollto -Roll Processing ........................................................................................27 2 POLYMER SOLAR CELL AND MATERIALS CHARACTERIZATION .........................32 2.1 Device Characterization ....................................................................................................32 2.1.1 Current-Voltage Measurements ..............................................................................32 2.1.2 External Quantum Efficiency .................................................................................33 2.1.3 Device Absorption ..................................................................................................34 2.1.4 Internal Quantum Efficiency ..................................................................................35 2.2 Materials Characterization ................................................................................................35 2.2.1 Photothermal Deflection Spectroscopy ..................................................................35 2.2.2 Photoluminescence Spectroscopy ..........................................................................36

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6 2.2.3 Atomic Force Microscopy ................................ ................................ ...................... 37 2.3 Charge Transport Characterization ................................ ................................ ................... 37 2.3.1 Space Charge Limited Current ................................ ................................ ............... 37 2.3.2 Energetic Disorder ................................ ................................ ................................ .. 38 2.3. 3 Recombination ................................ ................................ ................................ ........ 39 2.3.4 Transient Photocurrent ................................ ................................ ........................... 41 3 HIGH EFFICIENCY AIR PROCESSED DITHIENOGERMOLE BASED POLYMER SOLAR CELLS ................................ ................................ ................................ ...................... 46 3.1 Introduction and Motivation ................................ ................................ ............................. 46 3.2 Air Processed Organic Solar Cells in the Lit erature ................................ ........................ 47 3.3 Results and Discussion ................................ ................................ ................................ ..... 48 3.3.1 Current Voltage Measurements ................................ ................................ .............. 48 3.3.2 Film Absorption ................................ ................................ ................................ ...... 49 3.3.3 External Quantum Efficiency ................................ ................................ ................. 49 3.3.4 Total Device Absorption ................................ ................................ ........................ 50 3.3.5 Reverse Bias External Quantum Efficiency ................................ ........................... 51 3.3.6 Internal Quantum Efficiency ................................ ................................ .................. 52 3.3.7 Space Charge Limited Current Mobility ................................ ................................ 52 3.3.8 Atomic Force Microscopy ................................ ................................ ...................... 53 3.4 Transfer Matrix Optical Simulations ................................ ................................ ................ 53 3.5 Prolonged Device Air Exposure ................................ ................................ ....................... 54 3.6 Summary and Conclusions ................................ ................................ ............................... 55 3.7 Experimental Section ................................ ................................ ................................ ........ 56 3.7.1 Device Fabrication ................................ ................................ ................................ .. 56 3.7.2 Device Characterization ................................ ................................ ......................... 56 4 PHOTODEGRADATION IN HIGH EFFICIENCY INVERTED POLYMER SOLAR CELLS ................................ ................................ ................................ ................................ .... 65 4.1 Introduction and Motiv ation ................................ ................................ ............................. 65 4.2 Results and Discussion ................................ ................................ ................................ ..... 66 4.2.1 Basic Device Characterization Before and After 24 Hour Light Exposure ........... 66 4.2.2 Effect of Elevated Temperature on Device Performance ................................ ....... 67 4.2.3 Effect of 24 H our Light Exposure on Charge Generation ................................ ...... 68 4.2.4 Effect of 24 Hour Light Exposure on Charge Carrier Recombination ................... 68 4.2.5 Effect of 24 Hour Light Exposure on Charge Carrier Extraction .......................... 69 4.2.6 Effects of Light Exposure on the Electronic Properties of ZnO ............................ 70 4.3 Summary and Conclusions ................................ ................................ ............................... 72 4.4 Experimental Section ................................ ................................ ................................ ........ 73 4.4.1 Device Fabrication. ................................ ................................ ................................ 73 4. 4.2 Device Characterization ................................ ................................ ......................... 74 5 EFFECT OF THERMAL ANNEALING ON THE ELECTRONIC STATES OF POLYMER SOLAR CELLS ................................ ................................ ................................ .. 84

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7 5.1 Introduction and Motivation .............................................................................................84 5.2 Results and Discussion .....................................................................................................85 5.2.1 Basic Device Characterization Before and After Thermal Annealing ...................85 5.2.2 Effect of Thermal Annealing on Film Morphology ...............................................86 5.2.3 Effect of Thermal Annealing on Dielectric Constant .............................................87 5.2.4 Effect of Thermal Annealing on Charge Generation .............................................88 5.2.5 Effect of Thermal Annealing on Charge Transport and Energetic Disorder ..........89 5.2.6 Effect of Thermal Annealing on Charge Carrier Recombination...........................90 5.3 Temperature Induced Deep Trap Formation ....................................................................92 5.4 Summary and Conclusions ...............................................................................................92 5.5 Experimental Section ........................................................................................................93 5.5.1 Device Fabrication ..................................................................................................93 5.5.2 Device Characterization .........................................................................................94 6 EFFECT OF SIDE-CHAINS ON CHARGE GENERATION AND DISORDER IN POLYMER SOLAR CELLS ................................................................................................102 6.1 Introduction and Motivation ...........................................................................................102 6.2 Results and Discussion ...................................................................................................104 6.2.1 Basic Solar Cell Device Characterization ............................................................104 6.2.2 Effect of Polymer Side Chains on Film Morphology ...........................................105 6.2.3 Effect of Polymer Side Chains on Charge Carrier Generation .............................105 6.2.4 Effect of Polymer Side Chains on Charge Carrier Recombination ......................107 6.2.5 Effect of Polymer Side Chains on the Device Open Circuit Voltage ...................108 6.2.6 Effect of Polymer Side Chains on Stacking and Energetic Disorder .............109 6.3 Summary and Conclusions .............................................................................................111 6.4 Experimental Section ......................................................................................................112 6.4.1 Device Fabrication ................................................................................................112 6.4.2 Device Characterization .......................................................................................112 7 CONCLUSIONS AND FUTURE WORK ...........................................................................121 7.1 Summary .........................................................................................................................121 7.2 Future Work ....................................................................................................................123 LIST OF REFERENCES .............................................................................................................124 BIOGRAPHICAL SKETCH .......................................................................................................138

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8 LIST OF TABLES Table page 3-1Summary of average device characteristics for the OPVs fabricated in this study. ..........64 4-1Average device parameters for p( OPVs fabricated in this study. ....76 4-2Average device parameters for p( OPVs fabricated in this study. ............81 4-3Average device performance for devices aged and measured under 400 nm LP filter. ....83 5-1Average device characteristics for the OPVs fabricated in this study. ..............................96 5-2Summary of mobilities and energetic disorder for holes and electrons in annealed and as-prepared PCDTBT:PC 70 BM devices ......................................................................99 6-1Summary of average device characteristics for the OPVs fabricated in this study. ........115 6-2Zero field mobility at room temperature and energetic disorder data for hole transport in the Oct and EtHex devices. ...........................................................................120

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9 LIST OF FIGURES Figure page 1-1The conjugated structure of ethene. ...................................................................................28 1-2Schematic representation of the p-orbitals on a benzene ring. ..........................................28 1-3Schematic representation of the molecular orbitals and chemical structure of polyethyne. .........................................................................................................................28 1-4Simplified schematic of photoconversion at a donor/acceptor heterojunction. .................29 1-5Illustration of molecular orientations in organic electronic devices. .................................30 1-6Schematic representation of OPV device architectures. ....................................................30 1-7Donor-acceptor heterojunctions for OPV devices. ............................................................30 1-8Energy diagram illustrating the two routes for CT state dissociation. ...............................31 2-1Current-Voltage curves for a typical OPV showing the maximum power generated by the cell along with V OC and J SC. ....................................................................................43 2-2Device stack consisted of a glass substrate, a transparent electrode, the photoactive layer and a reflective electrode. .........................................................................................43 2-3Schematic representation of photothermal deflection spectroscopy. .................................44 2-4Schematic representation of a photoluminescence spectroscopy setup. ............................44 2-5J-V curve fitted using using the Poole-Frenkel modified Mott-Gurney equation for the calculation of hole mobility. ........................................................................................45 2-6 A diagram of the transient photocurrent measurement setup. ...........................................45 3-1Chemical structures active layer materials. .......................................................................58 3-2 J V curves for p(DTG-TPD):PC 70 BM devices processed in nitrogen and air. ...............58 3-3 UV-Vis absorption spectra for p(DTG-TPD):PC 70 BM films processed in nitrogen and air.................................................................................................................................59 3-4EQE spectra of p(DTG-TPD):PC 70 BM devices processed in nitrogen and air. ................59 3-5Device absorption measured in reflection for devices processed in nitrogen and air. .......60 3-6EQE as a function of reverse bias at two constant excitation wavelengths, 500 nm and 683 nm for devices made in nitrogen and air. .............................................................60

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10 3 7 Normalized IQE for devices made in nitrogen and devices made in air. .......................... 61 3 8 TPD):PC 71 BM films. ........................... 61 3 9 Transfer matrix simulations for electric field distribution within a p(DTG TPD):PC 71 BM OPV device ................................ ................................ .............................. 62 3 10 Device characterization for devices exposed to air for up to 3 hours during processing. ................................ ................................ ................................ ......................... 63 4 1 J V characteristics for p(DTG:TPD):PC 70 BM devices before and after 24 hour light exposure at 1 sun intensity. ................................ ................................ ................................ 75 4 2 EQE spectra for p(DTG:TPD):PC 70 BM devices before and after 24 hour light exposure at 1 sun intensity. ................................ ................................ ................................ 75 4 3 Device absorption efficiency for p(DTG:TPD):PC 70 BM before and after 24 hour light exposure at 1 sun intensity. ................................ ................................ ........................ 76 4 4 J V characteristics for p(DTG:TPD):PC 70 BM devices before and after 24 hours thermal treatment at 50 o C. ................................ ................................ ................................ 77 4 5 AFM topography images for p(DTG:TPD):PC 70 BM films before and after 24 hour 1 sun illumination. ................................ ................................ ................................ ................ 77 4 6 Sub bandgap EQE spectra show no significant changes in carrier generation after 1 sun illumination for 24 hours. ................................ ................................ ............................ 78 4 7 Log log Jsc v s Light intensity plot. The slopes resulting from the linear fit are shown in parenthesis. ................................ ................................ ................................ ......... 78 4 8 Responsivity v s Light intensity. The zero intercepts and slopes are depicted in the figure. ................................ ................................ ................................ ................................ 79 4 9 Photocurrent transients for devices before and after 24 hour, 1 sun light exposure. ......... 80 4 10 Steady state PL spectra of ZnO sol gel films. ................................ ................................ ... 80 4 11 Chemical structure of poly terthiophene co isoindigo (T3 iI). ................................ ......... 81 4 12 J V characteristics for p(T3 iI):PC 70 BM before and after 24 hour 1 sun light exposure and 24 hour aging in the dark. ................................ ................................ ............ 81 4 1 3 Photocurrent transients at different DC biases for devices exposed to 1 sun illumination for 24 hours. ................................ ................................ ................................ .. 82 4 1 4 J V characteristics for p(DTG TPD):PC 70 BM d evices before and after light exposure using 400 nm LP filters. ................................ ................................ ................................ ..... 82

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11 4-15 Optical transmittance of PET substrates. ...........................................................................83 4-16 Steady-state PL spectra of ZnO sol-gel aged under 1 sun light filtered using 400 nm LP filters.............................................................................................................................83 5-1Chemical structure of the PCDTBT polymer. ...................................................................95 5-2J-V characteristics for PCDTBT:PC 70 BM devices with and without annealing. ..............95 5-3EQEs measured at 540 nm as a function of reverse bias for annealed and not annealed devices. ...............................................................................................................96 5-4AFM topography images of PCDTBT: PC 70 BM films......................................................96 5-5Dielectric constants measured at 1 KHz, 10 KHz and 100KHz for as-prepared and annealed PCDTBT:PC 70 BM devices. ................................................................................97 5-6Sub-bandgap EQE spectra for annealed and aspr epared devices. ....................................97 5-7PL transients for as-prepared and annealed PCDTBT:PC 70 BM films. ..............................98 5-8PL transients for as-prepared and annealed PCDTBT films. .............................................99 5-9Zero-field mobilities vs. square of reciprocal temperature for PCDTBT:PC 70 BM annealed and as-prepared devices ....................................................................................100 5-10 J SC vs. I on log-log scale fitted for bimolecular recombination. ......................................100 5-11 Voc vs. I fitted for monomolecular recombination.. ........................................................101 5-12 PDS spectra for PCDTBT:PC 70 BM annealed and as-prepared devices ..........................101 6-1Chemical structures of the polymers used in this study. ..................................................114 6-2J-V characteristics for Oct and EtHex devices. ...............................................................114 6-3External quantum efficiencies for Oct and EtHex devices. .............................................115 6-4 AF M topography images for polymer-fullerene films, 1:1.5 weight ratio, spin coated on ITO/PEDOT substrates. ..............................................................................................115 6-5Sub-bandgap EQE spectra for the Oct and EtHex devices. .............................................116 6-6Transient PL decays for Oct and EtHex devices. ............................................................116 6-7Transient PL decays for pristine polymer films. ..............................................................117 6-8J SC vs. I fitted for recombination for the EtHex and Oct devices. ...................................118

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12 6-9Reverse bias EQE at 535 nm wavelength for the EtHex device and the Oct device. ......118 6-10 Sub-bandgap EQE data fitted for Equation 6-3. ..............................................................119 6-11 Zerofi eld mobilities vs square of reciprocal temperature for EtHex and Oct devices ...119 6-12 stacking distance, disorder and transport. ........................................................................120

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13 Abstract of Dissertation Presented to the Graduate S chool of the Unive rsity of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy THE EFFECTS OF MATERIALS AND PROCESSING ON ORGANIC PHOTOVO TAICS By Iordania Constantinou May 2016 Chair: Franky S o Ma jor: Materials S cience and Engineering Organic semiconduc tors (OSCs) are a re latively n ew class of m ater ials gaining attention as a possi ble replacement for tradit ional semiconductors due to their unique electronic and mechanical properties such as low-temperature proc essability, low cost a nd device flexibility. While som e o rganic electronic de vices, li ke orga nic lig ht emitting diodes (OLEDs) have re ached commercial matur ity, organic photovoltaics (OPVs) are sti ll a long wa y from commercialization mostly due to their low operational stability. First, the polymer photovoltaic stability in the cases of exposure to ambient air during pro cessing a nd extende d light exposure was investigated It was found that ambient processing induce s fast initial device de gradati on due to a reduction in hole mobi lities combined with a small re duction in device a bsorption. The im pact of a ir exposure on device pe rformance was shown to only b e signi ficant in the first hour of exposure a nd efficiencies for devices exposed for up to three h ours plateaue d at about 20% th e e fficiency o f devices made in nitrogen atmosphere. Next, the im pact of 1 sun lig ht exposure on device pe rformance wa s examined for devices made usi ng the same active la yer material s. It was found that lig ht exposure for 24 hours leads to a sig nificant reduction in device pe rformance, negatively im pacting a ll de vice

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14 parameters. Ultra violet (UV) light induced defects were proven to be the primary reason for device degradation and device stability was significantly improved with the use of UV filters. Next, t he effects of thermal annealing and polymer side chains on charge carrier generation and recombination were investigated It was shown that both thermal annealing and the polymer side chains can have a strong impact on the formation of charge transfer (C T) states at the polymer fullerene interface and significantly influence the charge generation process. Additionally, carrier transport and recombination were also shown to be affected th r ough Thermal annealing was found to improve carrier generation due to improved CT state delocalization but an increase in the concentration of defects ca u sed the device efficiency to be lower overall. Similarly, brunched side moiety were found to enhance carrier generation and carrier transport.

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15 CHAPTER 1 INTRODUCTION TO ORGANIC SEMICONDUCTORS 1.1 Introduction Organic semiconductors are a relatively new class of materials gaining attention as a possible replacement for traditional semiconductors in specific applications due to their unique electronic and mechanical properties. Unlike traditional elemental semiconductors, such as Si and Ge, OSCs are the result of synthetic chemistry allowing for a large variety of mol ecules to be designed with properties tailored to the requirements of specific applications. Some of the unique properties of OSCs include low temperature processability, low material consumption for large scale applications, low cost and device flexibilit y. Some promising applications for OSCs include OPVs, 1 thin film transistors (TFTs) 2 and organic photodetectors (OP Ds) 3 while organic light emitting diodes (OLEDs) can already be found in cell phone and TV displays. 4,5 The effect of the chemical structure on the electronic properties and devices will be analyzed in the following sections. 1.2 Comparison between Organic and Inorganic S emiconductors The main differences b etween organic and inorganic semiconductors arise from the difference in bond types present. In OSCs electronic coupling is weak due to the rather weak van der Waals interactions between molecules compared to the covalent bonds present in inorganic semicon ductors. As a result of the weak electronic coupling, electronic states in OSCs are highly localized resulting in discrete molecular orbitals compared to the continuous bands present in OSCs. Due to the lack of delocalization, dielectric constants in orga nic materials are significantly lower compared to their inorganic counterparts, 3 5 compared to 10 15. 6 As a consequence of the low dielectric constant, the Coulomb interaction between electrons and holes is much greater, resulting in higher exciton bin ding energies in OSCs (0.1 0.3 eV). 7 Due to the high binding

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16 energies, thermal dissociation of excitons at room temperature is very unlikely. Instead, an external driving force is required for the dissociation of tightly bound excitons, typically in the form of a heterojunction interface or applied electric field. In addition to exciton dissociation, the localized electronic stated in OSCs also dictate charge transport since charges have to move between discrete molecular orbitals throug h hopping resulting in lower carrier mobilities in OSCs. 8 Finally, a benefit of the discrete mole cular orbitals is the high absorption coefficient for OSCs relative to inorganic semiconductors. 9 1.3 Molecula r S tructure and Bonding The bonding of sp 2 hybridi z ed carbon is shown in Figure 1 1 for eth e ne. Ethene is the simplest example of a double bond between two carbons and will be used in the section to explain bonding and conjugation in small molecules and polymers. Carbon's ground state configuration is 1s 2 2s 2 2p 2 or 1s 2 2s 2 2p x 1 2p y 1 In this configuration, the carbon does not have enough unpaired electrons to form the required bonds so one of the 2s 2 electrons is promoted to 2p z 1 changing the configuration to 1s 2 2s 1 2p x 1 2p y 1 2p z 1 The carbon is now in an excited state with four un paired electrons. In sp 2 hybridiz ation the 2s orbital is mixed with only two of the three available 2p orbitals, creating three sp 2 orbitals and one p orbital. The three sp 2 orbitals eventually bond with two hydrogen atoms and the second carbon atom formi ng bonds. The p orbitals between two carbons also overlap creating bonds. 10 Molecular bonding orbitals are commonly referred to as highest occupied molecular orbitals (HOMOs) whereas anti bonding orbitals are commonly referred to as lowest unoccupied molecular orbitals (LUMOs). 11 The difference between the HOMO and LUMO energies is known as the optical bandgap. In planar conjugated molecu les, the optoelectronic properties are mostly dictated by the orbitals. The example of a benzene molecule is shown in Figure 1 2 A Benzene has six electrons which are delocalized over the entire benzene ring due to the p orbital overlap. Since

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17 the electrons do not belong to one particular bond, a delocalized ring of electron density bond plane in the z direction is shown in Figure 1 2B Due to the directionality of the electron ring density, charge transport is more favorable when molecules are stacked on top of each other instead of side by side. The configuration during which molecules are stacked and adjacent orbitals overlap is referred to as stacking. stacking distance is an important parameter in the evaluation of efficient charge transport and minimization of charge carrier recombination. In the case of polymer semiconductors where a large chain of carbons is formed, electrons become delocalized along the entire chain. Polymer semiconductors posses s an e conjugated organic backbone and, unlike inorganic semiconductors, transport is not band like but is instead thermally activated hopping between chains. An example of a polymer semiconductor is shown in Figure 1 3 The p orbitals of individu al carbons in the polyethyne chain are shown in Figure 1 3A. The respective conjugated polymer chain for polyethyne is shown in Figure 1 3B. 1.4 Optical Absorption in Organic Semiconductors As mentioned above, the lowest energy electronic excitation in conjugated OSCs is the transition With typical optical bandgaps between 1.5 and 3 eV, light absorption or emission across the entire visible range is possible. In addition, due to the weak electronic delocalization, OSCs have relatively large absorpt ion coefficients, typically on the order of 10 5 cm 1 which are dominated by the properties of the individual molecules. 12 Absorption occurs when a molecule interacts with a photon and undergoes a t ransition from an initial lower energy state to a final higher energy state th rough the coupling of the electromagnetic field to the transition dipole moment. The transition dipole moment associated with absorption is related to the change in the molecule charge distribution upon excitation. 13

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18 1.5 Excitons 1.5.1 Exciton F ormation Upon absorption, electron hole pair s called excitons are formed, and are typically localized on individual molecules. Excitons are electrically neutral and are formed when an electron is excited from the HOMO level to the LUMO level leaving behind a hole. Depending on the total spin of the two particles that make up the exciton, it can be classified as a singlet or triplet with net spin of zero or one, respectively. While photogenerated excitons must be singlets, excitons formed by positive and negative polarons meeting and becoming bound ca n be either singlets or triplets. In this case, 25% of excitons formed are singlet excitons and 75% of excitons are triplet excitons. 14 Due to the low dielectric constant of organic semiconductors, excitons typically have large bi nding energies. In the case of organic photovoltaics, which are the focus of this dissertation, th e large exciton binding energy has to be overcome in order for free charge carriers to be generated. 1.5.2 Exciton Diffusion and Dissociation Upon photogenera tion on a conjugated segment of a molecule, excitons start migrating to lower energy conjugated segments within the excitonic density of states There are two commonly observed exciton transport processes namely F rster and Dexter energy transfer F rster energy transfer is a non radiative energy transfer that takes place b etween two neighboring molecules or materials, a n electron donor and an electron acceptor. 15 T his process can be thought of as the emission and subsequent absorption of a virtual photon that occurs within a relatively long length scale typically in the order of a few nanometers. 16 Dexter energy transfer involves t he direct exchange of electrons and therefore requires good overlap between the electron densities of both the donor and acceptor molecules. Limited by the requirement of good e lectron density overlap, Dexter energy transfer is a r adiationless nearest neighbor process

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19 with a typical length sca le of 0.1 1 nm 17 Due to the inherent disorder in OSCs, the second step of exciton diffusion is a thermally activated h opping process that can be fully described in terms of a random walk 18 In organic semiconductors, typical exciton diffusion lengths are in the order of 10 nm and exciton lifetimes do not exceed 1 ns. 19 In the case of organic photovoltaics and photodetectors, excitons must be dissociated in order for current to be generated. In order to overcome the large exciton binding energy for efficient exciton dissociation, a common strategy is the use of a heterojunction between two different organic semiconductors, an electron donor material and an electron acceptor material. An energetic offset between the LUMO levels of the two materials provides the driving force for electrons to move to the lower LUMO e nergy, whereas an energetic offset between the HOMO levels acts as an energy barrier that keeps the hole on the donor material. 20 After the electron transfer, an initial separation between the electron and hole is achieved leaving the exciton loosely bound across the interface. This loosely bound state is called a CT state, and it is common at heteroju nction interfaces 21 The CT exciton can easily subsequently dissociate into free charges, commo nly referred to as polarons. The exciton generation, diffusion and transfer processes are illustrated in Figure 1 4 A more in depth analysis of the importance of CT states in organic photovoltaics can be found in Section 1 9. 1.6 Charge Carrier Transport 1.6.1 Stacking Semiconducting small molecules and polymer backbones are often mostly made up of conjugated aromatic hydrocarbons It is typical for planar a romatic molecules stacked assemblies in which the molecul es align closely in a parallel, cofacial manner. 22 As mentioned above, interaction is an important parameter for charge transport. It has previously been shown that close stacking distances are partially responsible for high hole mobilities

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20 observed in some OSCs. 23 In order to facilitate transport in an optoelectronic device, the planar backbones have to be stacked perpendicular to the electrodes. In the case of organic field effect transistors (OFETs), transport is optimized when the backbones are aligned in an edge on orientation relative to the substrate so that charges more efficiently move between the source and the drain. S imilar, in the case of OPVs it is preferable for backbones to align in a face on orientation relative to the substrate so that charges can efficiently move toward the anode and cathode. 24 An illustration of edge on orientation in an OFET and face on orientation in an OPV are shown in Figure 1 5 A and Figure 1 5 B respectively. 1.6.2 Hopping Transport Mechanism In disordered systems with no long range order like OSCs, where charge carriers are localized on different molecular sites, charge transport occurs through a hopping transport mechanism A complete description for the hopping transport in OSCs includ es both polaronic contributions as well as energetic disorder contributions The polaronic hopping mechanism is typically described within the framework of Marcus theory for small polaron transport. 25 The equation proposed by Rudolph A. Marcus for the hopping rate from site i to site j across the distance r ij is : (1 1) Here, I ij is the transfer integral, i.e. the wavefunction overlap between sites i and j, is the reorganisation energy related to the polaron relaxation kT is the thermal energy and G ij is due to the energy difference between the two sites. As it is clear from the equation, an increase in the wavefunction overlap between the initial and final hopping sites would increase the hopping rate. Additionally, an increase in the polaron relaxati on energy would reduce the hopping rate by increasing the hopping energy barrier. The optimum hopping rate can only be achieved when

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21 the energy difference between the two sites ij ) equals the reorganiz ation energy due to polaron relaxation ( ). In the cases where the hopping rate is limited by molecular reorganization polaron models like the Marcus theory are valid. In the cases where the hopping rate is limited by structural disorder, polaron models like the Marcus theory are less useful and i t is typical for transport to be characterized within the energetic disorder framework. 26 M ore details about energetic disorder and how it is related to charge carrier transport can be found in Section 2.3.2 1.7 Charge Carrier Recombination 1.7.1 Monomolecular Recombination M onomolecular recombination is a first order process where charge recom bination occurs either between an electron and hole generated by the same photon known as geminate recombination, or through a recombinatio n center, known as Shockley Read Hall recombination Geminate recombination in OPVs occurs when an electron hole pai r recombines prior to dissociation. Geminate losses become significant in cases where the polymer fullerene intermixing is poor and the domain size of each component is greater than the exciton diffusion length. Excitons that fail to reach the polymer full erene interface recombine geminately. 27 Another route of geminate recombination is through relaxed CT states acting as traps at the polymer fullerene interface. 28 Photogenerated excitons trapped at those states typically end up recombining. Geminate recombination is not a radiative rec ombination process and only a small fraction of excitons emit light upon recombination. 29 Generally, geminate recombination is minimum in the case of fully optimized polymer solar cells with optimum morphology. Shockley Read Hall (SRH) recombinatio n is medi ated through trap states. The two steps of SRH recombination are: 1) An electron (or hole) gets trapped at an energy state with in the forbidden band which is introduced through defects in the materials or at the interfaces 2) A

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22 hole (or electron) gets cap tured by the same defect state before the electron escapes t o the conduction band The two charges eventually recombine. 30 What makes SRH recombination monomolecular is the time difference between the initial capture of the first carrier and the subsequent capture of the second carrier. Shallow traps near the band edge are less likely to contribute to SRH recombination since captured charges are likely to be thermally re emitted into the transport band before the opposite charge gets captured. Therefore, deep traps like b and tail states dominate SRH recombination. 31 1.7.2 Bimolecular Recombination Bimolecular recombination is a second order recombination which occurs between charges that were not generated from the separation of the same exciton. After exciton splitting, opposite carriers in the bulk heterojunction ( BHJ ) blend attract each other dur ing transport and can form a CT exciton at the polymer fullerene interface. The recombination of that CT exciton is called bimolecular recombination. 32 Bimol ecular recombination is the most prominent mechanism of recombination in organic solar cells and it greatly affects the device photocurrent and fill factor (FF) (s ee S ection 1.7.2 ). The most commonly used theory for the description of bimolecular recombina tion is the Langevin theory. According to the Langevin theory the rate of bimolecular recombination is proportional to the sum of the charge carrier mobilities and also proportional to the carrier concentration in the active layer. 33 Indeed bimolecular recombination increases with increasing light intensity due to the increased carrier concentration in the active layer. 34 The predicted increase in bimolecular recombination for increased carrier mobilities originates from the assumption that higher mobility would enable opposite carriers to find each other easier. That is often found not to be the true since more mobile carr iers manage to reach the electrodes and get extracted from the device faster before recombination occurs.

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23 1.8 Device Architectures 1.8.1 Standard versus Inverted Device Structure Lab scale OPVs are typically built on a transparent substrate usually glass, coated with a transparent conductive electrode. I ndium tin oxide ( ITO ) is classically used as the bottom electrode due to its high transparency in the visible range and low sheet resistance The standard OPV c onfiguration is as follows: Glass/ITO/Hole transport layer (HTL)/Active layer/Electron transport layer (ETL)/Aluminum (Al). In the standard architecture, upon exciton dissociation photogenerated holes move towards the anode, in this case ITO/HTL, and electrons move to wards the ETL/Aluminum cathode. In the case of inverted OPVs, the device configuration changes to: Glass/ITO/ETL/Active layer/HTL/Silver (Ag). In this case, the electron collecting cathode (ITO/ETL) is at the bottom of the device whereas holes are collecte d at the top. Common HTL materials include poly(3,4 ethylenedioxythiophene) polystyrene sulfonate ( PEDOT:PS S), molybdenum oxide (MoOx) and nickel oxide (NiO). The most widely used ETL materials include zinc oxide (ZnO) and titanium oxide (TiO 2 ). 35 Even though highly efficient devic es have been reported in the literature using both architectures, inverted devices offer processing advantages since it is possible to use all solution processed electrodes and show improved ambient stability due to the absence of a low work function elect rode and the replacement of acidic PEDOT:PSS 36 The normal and inverted device architectures are illustrated in Figure 1 6A and Figure 1 6 B respectively. 1.8.2 Heterojunction s The exciton dissociation processes in OPVs described in Section 1.5.2 was based on a donor acceptor material interface. One of the first attempts to create a donor acceptor material interface was the planar heterojunction (PHJ) shown in Figure 1 7A In this configuration, an electron donor material, typically a conjugated polymer or small molecule, was deposited

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24 directly over the device anode, followed by the electron acceptor. Fullerenes such as C 60 and its derivatives are the most widely used acceptor materials in organic solar cells. 37 In the case of solution processed OPVs, photogenerated excitons diffuse to the polymer fullerene heterojunction where they dissociate. The short exciton diffusion lengths in most amorphous organic semicondu ctors entails that excitons generated further from the interface recombine geminately before reaching the interface. Despite the high geminate recombination in PHJ, bimolecular recombination is minimized since electrons and holes are spatially separated af ter dissociation which prevents them from finding each other. The random BHJ was an attempt to maximize the polymer fullerene interface without having to sacrifice the spatial separation between electrons and holes. A BHJ is a donor acceptor mixture in the solid state with nanostructured morphology formed by spontaneous phase separation ( Figure 1 7B ) In order for charge transport to be efficient, the donor and acceptor components must self assemble to form interpenetrating networks with connected pathways to the electrodes while at the same time maintaining the s elf assembly length scale in the order of 10 20 nm 38 Achieving th e optimum de gree of phase segregation is critical and mostly out of our control. Fine phase separation can lead to increased bimolecular recombination whereas big domains of individual materials can lead to increased geminate recombination, especially if the domain si zes exceed the exciton diffusion length. Various processing methods such as thermal and vapor annealing, mixing solvents, vacuum drying and solvent additives have been used in order to control or change the domain size for minimum recombination and optimum device performance. The most common technique used for the approximation of the domain size in OPVs is atomic force microscopy (AFM). 39 For more details about AFM see S ection 2.2.3

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25 1.9 Charge Transfer States in Bulk Heterojunctions The exciton dissociation process in OPVs is often a limiting factor determining the device performance due to the low dielectric constants and hence the large exciton binding energies in most polymers (See Section 1.2) Most polymer fuller ene blends exhibit additional sub bandgap absorption and emission features. These features were explained to originate from CT states radiative ly coupled to the ground state. 40 In BHJ OPVs, CT excitons are formed at the polymer fullerene interface upon photo excitation. These CT s tates have been shown to significantly impact the device photocurrent and open circuit voltage. The maximum achievable value for the open circuit voltage (V OC ) is determined by the effective bandgap at the heterojunction which is in turn determined by the energy of the charge transfer state energy manifold. 41 In addition to V OC CT states mediate the exciton dissociation process and determine the device photocurrent. 42 Excitons at these CT manifolds undergo a fast photoinduced charge transfer dissociation process in the sub picosecond timescale. 43 Even though there is no doubt that CT states mediate exciting dissociation, the CT exciton dissociation mechanism is still under debate. There are two main contradicting theories explaining charge separation at a donor/acceptor interfac e. The first theory suggests that the probability of charge dissociation is higher for CT excitons with energy greater than the absorbing material bandgap compared to lower energy relaxed CT excitons. The high energy CT excitons are often referred to as ho t CT excitons whereas the lower energy relaxed CT excitons are typically referred to as cold CT excitons. 44 In this case, hot CT excitons possess a higher degree of delocalization and are populating the bound, relaxed CT state. 45 Cold CT excitons are considered to be less delocalized and dissociation into free charge carriers has been shown to occur equally efficiently on a 100 fs

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26 to ns time scale. It is believed that most dissociation events proceed via cold CT states and that the photocurrent generation in organic solar cells is primarily controlled by the recombination dynamics of these states. Th e second theory suggests that regardless of the excitation energy, an ultrafast kinetic component exist in charge separation and charges separate in similar rates independent of the excitation energy. 46 CT state dissociation is shown in Figure 1 8 1.10 Film Formation 1.10.1 Thermal Evaporation Thermal evaporation is a common method of thin film deposition traditionally used for the deposition of metal electrodes and interlayers in organic photovoltaics. Materials such as Al, Ag, Calcium (Ca), Lithium fluoride (LiF) and MoOx are typically thermally evaporated on top of the active layer in order to form the finished device. The basic principles behind thermal evaporation is the resistive heating of a typically t ungsten or t a ntalum boat or filament filled with the material to be deposited, and the subsequent melting and evaporation of that material. Material deposition happens under high vacuum which allows for the evaporated particles to travel directly towards the substrate where they get deposited without reacting with or scatter against other background gases. Additionally, material particles have a long mean free path under high vacuum which minimizes material waste and allows for bigger deposition chambers. For accurate thickness control and in situ monitoring of deposition rates, quartz crystal microbalance s (QCM s ) are used. The QCM contains a quartz piezoelectric crystal vibrating at its resonance frequency and it is typically placed over the evaporation source, close t o the substrate 47 As ma terial is deposited d uring evaporation, the QCM measures the mass variation per unit area based on the change in its resonance frequency The thickness of the deposited film account s for any

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27 difference s in the radial distance be tween the substrate and the QCM. Thermal evaporation was used for the deposition of top electrodes for all devices made for this dissertation. 1.10.2 Spin Coating All active layers as well as all anode interlayers menti oned in Chapters 3, 4, 5, and 6 were prepared from solution and were deposited using spin coating. Spin coating allows for the deposition of uniform films on flat substrates. During spin coating, the substrate is placed on a rotating block where it is secu red using low vacuum and spun at high speeds after a small amount of solution is dropped at the center of it. The solution, driven by centrifugal force moves away from the center of the substrate and forms a flat film. As expected, the film thickness is in versely proportional to the angular velocity. Spin speeds range between 200 8000 rpm allowing for a large range of film thicknesses. The disadvantage of this technique is the large amount of material waste during spinning compare to other deposition techni ques like doctor blading and the limited scalability for industrial applications. 1.10.3 Roll to Roll Processing The main advantage of organic electronics lays in the potential for roll to roll (R2R) processing. R2R processing refers to a family of manufa cturing techniques used to create electronic devices on a roll of flexible plastic typically polyethylene terephthalate (PET), or metal foil. 48 During R2R, the flexible substrate is continuously transferred betw een two moving rolls that deliver material similar to the printing of a newspaper. This substrate transport principle opens up the possibility for the incorporation o f several printing processes on the same production line. Some of these printing technique s include gravure printing, flexographic printing, screen printing and inkjet printing. High throughput, low cost and speed are the main factors that differentiate R2R manufacturing from conventional manufacturing 49

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28 Figure 1 1. The conjugated structure of ethene. Schematic representation of sp 2 hybridized carbon atoms forming bonds and non hybridized p z orbitals overlapping and forming bonds. Figure 1 2. Schematic representation of the p orbitals on a benzene ring. A) The p orbitals of individual carbons on the benzene ring. B) The delocalization of adjacent p orbitals forming a r ing of electron density The black lines represent bonds. Figure 1 3. Schematic representation of the molecular orbitals and c hemical structure of polyethyne. A) The p orbitals of individual carbons in the chain. B) The respective conjugated polymer chain.

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29 Figure 1 4 Simplified schematic of photoconversion at a donor/acceptor heterojunction A) Photon absorption, exciton formation. B) Exciton diffusion to the donor/acceptor interface. C) Electron transfer from the donor to the acceptor, CT exciton.

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30 Figure 1 5 Illustration of molecular orientations in organic electronic devices. A ) T he edge on molecule arrangement in an OFET B ) T he face on molecule orientation in an OPV. Figure 1 6 Schematic representation of OPV device architectures. A ) Standard device configuration. B ) Inverted device configuration. Figure 1 7. Donor acceptor heterojunctions for OP V devices. A) Planar donor/acceptor heterojunction. B) Random donor/acceptor bulk heterojunction

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31 Figure 1 8 Energy diagram illustrating the two routes for CT state dissociation. Blend absorption and exciton photogeneration are shown on the left. In the case of cold CT states dissociation, the excitons first thermalizes in the CT manifold and then get separated. In the case of hot CT states (dashed arrows) the excitation energy leads to separated charges directly.

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32 CHAPTER 2 P OLYMER SOLAR CELL AND MATERIALS CHARACTERIZATION 2.1 Device Characterization 2.1.1 Current Voltage Measurement s The most fundamental solar cell characterization technique is the current density (J) voltage (V) measurement. For terrestrial cells the standard J V measurement conditions are A ir Mass 1.5 light spectrum (AM1.5) at 1 kW/m 2 irradiance, usually referred to as the one sun conditio n. 50 Under one sun incident light, the voltage is swept and the solar cell output current is recorded. A typical J V curve is shown in Figure 2 1 along with all the figures of merit that can be extracted from it. The most important device parameters extracted from J V curves are the short circuit current (J SC ), V OC FF, and power conversion efficiency (PCE). The J SC is due to generation and collection of photo generated carriers. It is the maximum current collected from a solar cell and occurs when the voltage across the device is zero. The V OC corresponds to the maximum voltage available from a solar cell and occurs at the open circuit condition when the net current flowing through t he device is zero. The PCE of a solar cell is defined as the ratio between the maximum power generated by the solar cell, P MAX and the one sun incident power, P INC (2 1) As shown in Figure 2 1 P MAX is simply the power produced by the solar cell at the maximum power point (MPP) and it is represented on the J V curve by the small dotted rectangle. The product between J SC and V OC represents the ideal power generation that is indicated on the J V curve by the big dotted rectangle. Finally, the FF of the solar cell is defined as the ratio between the maximum power generated by the solar cell and the ideal power output. 51

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33 (2 2) 2.1.2 External Quantum Efficiency The External quantum efficiency (EQE) of a solar cell is defined as the ratio of the number of photo generated electrons collected at the electrodes to the number of incident photons of a given energy. (2 3) Ph is the device photocurrent, q is the Inc is the power of the incident light. EQE can also be broken down into four component efficiencies namely absorption efficiency (n A ), exciton diffusion efficiency (n ED ), charge transfer efficiency (n CT ) and charge collection efficiency (n CC ). 9 (2 4) Absorption efficiency is the percentage of optical absorption that leads to the generation of tightly bound exciton in the photoactive layer. Exciton diffusion efficiency is the percentage of photogenerated excitons that successfully diffuse to the donor acceptor interface. In well optimized solar cells, EQE loss due to n ED is typically expected to be negligible. Charge transfer efficiency is the percentage of excitons at the donor acceptor interface that successfully dissociate into free charges. Finally, charge collection efficiency is the percentage of free charges that successfully travel through the active layer and get collected at the electrodes. The voltage dependence of EQE can be u sed as an analytical tool for the investigation of the EQE component processes of photocurrent generation mentioned above. Monochromatic EQE measurements under reverse bias can give us an approximation of n A Under large reverse

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34 bias all excitons are assum ed to be dissociated (n CT =100%) and all carriers are assumed to be collected by the electrodes (n CC =100%). The EQE plateau at large reverse bias can be used as an approximation of n A. 52 In addition to EQE measurements for energies over the absorption edge, sub bandgap EQE is also used to measure weak but significant photocurrent at energies below the bulk absorption. Sub bandgap EQE measures the optical excitation acro ss the heterojunction, directly from the polymer HOMO to the acceptor LUMO. In the sub bandgap energy regime, charge transfer (CT) states are responsible for photocurrent generation. This makes sub bandgap EQE a useful tool for probing charge transfer effi ciency. 53 2.1.3 Device Absorption In organic photovoltaic cells, light enters the device through the glass substrate and transparent electrode and reaches the photoactive layer where a fraction of it gets absorbed. Any light that is not absorbed by the photoactive layer reaches the back re flective contact, typically a reflective metal, where it gets reflected back for a second pass through the device. An example of such a device is shown in Figure 2 2 This double pass absorption, in the absence of interference effects, is governed by Beer Lambert law: (2 5) the absorption coefficient and t is the optical path length. In the case of organic photovoltaics, the optical path l ength is double the photoactive layer thickness. Transfer matrix simulations can be used in order to predict the absorption behavior of multi layer stacks with known thicknesses and optical constants such as refractive indices and extinction coefficients. Transfer matrix simulations also take into account the interference

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35 between incident and reflected light which realistically happens in all devices and results in more complicated absorption spectra. 3 2.1.4 Internal Quantum Efficiency Internal quantum efficiency (IQE) refers to the efficiency with which absorbed photons can generate free carriers that are collected by the electrodes. IQE can be obtained from EQE simply by not taking into account the contribution of absorption efficiency IQE only accounts for the electrical processes than contribute to EQE. To measure IQE, the EQE of the solar cell is measured and then divided by the device absorption typically calculated from transmission and reflection measurements. This relationship i s shown in Equation 2 6 54 (2 6) 2.2 Materials Characterization 2.2.1 Photothermal D eflection S pectroscopy Photothermal deflection spectroscopy (PDS) is a highly sensitive, pump probe technique that measures the bending of light due to optical absorption. The operation principle relies on a chopped, monochromatic pump light with energy lower than the optical band gap of the material creating a periodic temperature rise in the fluid surrounding the sample upon light absorption by the sample and subsequent non radiative recombination. The surrounding fluid is carefully selected so that it does not dissolve the layer under investigation and also has an ind ex of refraction that is sensitive to small temperature fluctuations. Fluids frequently used for PDS include carbon tetrachloride (CCl 4 ) and Perfluorohexane ( C 6 F 14 ) The probe laser beam is directed parallel to the sample and perpendicular to the pump beam only probing the changes in the index of refraction of the liquid. The pump beam is finally detected using a position detector.

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36 The amount of deflection of the pump beam can be shown to scale with the absorption of the material of interest. The principle of operation is shown in Figure 2 3 55,56 2.2.2 Photoluminescence Spectroscopy Photoluminescence (PL) spectroscopy is a contactless, non destructive method to investigate the electronic structure of materials. In a typical steady state photoluminescence (PL) experiment the semiconductor is excit ed using light with energy greater than the bandgap energy. Once the photons are absorbed, the excitons relax to the minimum energy available and recombine. In the case of radiative recombination, the PL emission spectra are recorded using a monochromator and a detector. The general experimental setup for the measurement of a typical PL spectrum is shown in Figure 2 4 The emitted PL spectra are a direct way to study various material properties such as the optical bandgap, defect levels and recombination me chanisms. 57 Transient PL is a powerful tool for the characterization of the mechanisms that determine charge carrier dynamics in semiconductors. Transient PL via Time Correlated Single Photon Counting (TCSPC) is particularly suitable for the study of fast charge carrier decay dynamics in the nanosecond and sub nanosecond time scale. The TCSPS method is used to measure the photons emitted at different times following a pulsed laser excitation. For emission detection, a single photon sensitive detector i s used. An exponential decay curve is typically generated on an intensity versus time graph and must be thought of as having a certain rate rather than occurring at a specific time after excitation. Therefore different processes that happen in different ti mescales appear on the same decay curve as variations in the slope. Fundamental material processes such as exciton dissociation, charge transfer between molecules and recombination can be studied in depth using this technique. 58

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37 2.2.3 Atomic Force Microscopy Atomic force microscopy i s a type of scanning probe microscopy (SPM) with vertical resolution down to fractions of a nanometer and lateral resolution limited by the size of the tip, typically in the order of tens of nanometers. Samples are scanned using a probe tip mounted on a cantilever. The working principle relies on the deflection of the cantilever due to forces between the tip and the sample when the tip approaches the sample. T he lateral and vertical deflection is measured using a light spot reflecte d from the top of the cantilever into a position sensitive photodetector. A feedback loop is used to adjust for the cantilever deflection and map the surface to atomic precision. Based on this principle, topography images are generated that map out the hei ght variation on the sample. 59 In the case of soft materials like organic semiconductors, the forces measured in AFM are mostly van der Waals forc es. For that reason, the samples are measured in tapping mode, as opposed to contact mode, keeping the tip close enough to the sample for detection while preventing the tip from sticking to the soft surface. 2.3 Charge Transport Characterization 2.3.1 Spa ce C harge Limited C urrent Transport in organic semiconductors is more frequently analyzed using the Mott Gurney law of space charge limited current (SCLC) in a solid. For charge transport through a semiconductor of thickness d, the current density J is: (2 7) r 0 is the vacuum permittivity. In order for the Mott Gurney equation to accurately describe transport in organic semiconductors the following assumptions are necessary: (i) Conduction of only one

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38 carrier type, (ii) Trap free material, (iii) Negligible intrin sic carrier concentration, and (iv) Parallel plate electrodes. 60 The m ost important requirement for accurate mobility results is contact selectivity. An Ohmic contact is necessary for carrier injection since transport cannot be injection limited. A single carrier device (hole only or electron only device) can be made by sand wiching the semiconductor layer between two carrier selective contacts of the same type. For electron only devices, low workfunction metals such as aluminum are typical used for carrier injection. Similarly, in hole only devices, high workfunction metals s uch as gold are suitable for hole injection. 61 2.3.2 Energetic Disorder In reality, organic semiconductors are highly amorphous and rarely trap free and therefore the use of a model for transport based on the trap free assumption is only good as a first approximation. As described above, transport in organic semiconductors occurs by hopping through a manifold of localized states that are energetically and positionally disordered (See S ection 1 .6.2 ) The width of these energetic fluctuations is commonly r eferred to as energetic disorder. 62 In other words, energetic disorder is defi ned as the broadening of density of states (DOS) distribution or the Gaussian distribution of the HOMO and LUMO energies. In amorphous organic semiconductors where traps significantly influence charge carrier transport, charge carrier mobilities show a hi gher order dependence on the applied electric field. The Poole Frenkel equation is used to describing the effect of energetic disorder on the charge carrier mobility. (2 8)

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39 0 Poole Frenkel coefficient. 63 T aking into account the field and temperature dependence of the mobility the Mott Gurney equation can be re written as: (2 9) The field and temperature dependent SCLC equation is the most comm only used model to determine electron and hole mobilities in organic semiconductors. An example of a current density voltage curve for a hole only device fitted for zero field mobility using the Poole Frenkel modified Mott Gurney equation is shown in Figur e 2 5 The resulting zero field mobilities can be further analyzed using the Gaussian disorder model mentioned above. The temperature dependent zero field mobilities given by: (2 10) is the carrier mobility as the temperature approaches infinity, k is the Boltzmann constant and is the energetic disorder. 64 2.3.3 Recombination Light intensity dependent J V measurements are powerful tools for the study of charge carrier recombination in organic solar cells. At the short circuit condition the internal electric field in the device is high and carriers are being swept out of the device quickly. Due to the low carrier concentration in the device at short circuit first order recombination is dominant. In the absence of recombination J SC is proportional to light intensity. The J SC can be correlated to illumination intensity (I) by: (2 11)

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40 where represents the slope of the log log J sc vs I line. In the absence of recombination J SC is linearly proportional to light intensity and 65 Beyond the MPP, the pro bability of charge collection becomes dependent on the incident light intensity due to the decreasing electric field in the device. The reduced carrier sweep out and therefore increased charge carrier density in the device is responsible for the transition from monomolecular recombination at short circuit to bimolecular recombination. At V OC the internal device electric field is zero and the photogenerated carrier sweep out is minimum forcing all carriers in the device to recombine and driving current to ze ro. The forced recombination of carriers at open circuit makes the studies at V OC useful in providing detailed information about different recombination mechanisms. Voc can be correlated to illumination intensity by: (2 12) where k is the Boltzmann constant, T is temperature, N C is the density of states and n e and n h are the electron and hole concentration respectively. For bimolecular recombination that involves electron hole pairs, n e n h in the Equation 2 12 is proportional to the exciton generation which is in turn proportional to light intensity. In the case of monomolecular recombination n e and n h would each be proportional to light intensity making the product n e n h proportional to I 2 Equation 2 13 can then be modified as follows: (2 13) (2 14) In the case of bimolecular recombination, the slope of V OC versus ln(I) is equal to kT/q whereas for monomolecular recombination the slope is equal to 2kT/q. Thus the investigation of the

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41 effect of light intensity on V OC provides a method to directly distinguish between monomolecular and bimolecular recombination. 66 Similar to J SC and V OC the study of the device responsivity (R) as a function of light intensity can also provide valuable information regarding bimolecular recombination. Responsivity in organic solar cells is defined as the current produced by the solar cell divided by the power of the incident light and can simply be calculated by dividing the measured short circuit current in Amps by the know power of the incident light in Watts. The J SC can be gener ally considered equal to the generation current (J G ) minus the monomolecular recombination current (J MM ) and bimolecular recombination current (J BM ). While J G and J MM are proportional to I, J BM is proportional to I 2 as explained above in Section 1.7 Thi s relationship is summarized below in Equation 2 15 (2 15) where Ro is the responsivity in the absence of bimolecular recombination when I = 0 and is a constant. Based on Equation 2 15 for negligible bimolecular recombination the linear fits to R versus I data should have a slope of 0. For > 0 the device is to some extent limited by bimolecular recombination. 67 2.3.4 Transient Photocurrent Transient photocurrent (TPC) is an optoelectronic measurement in which the sample is held at short circuit under steady state conditions wh en a small transient signal is created by a laser. The laser is typically driven using a function generator. The current signal is recorded using an oscilloscope with low input impedance to establish a configuration close to short circuit. TPC measurements are widely used in organic solar cell studies since they provide information about a wide range of processes depending on the timescales probed. Such processes include

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42 carrier density, carrier transport, recombination, and extraction efficiencies and even fundamental properties such as the density of states. 68 70 The experimental setup for the TPC measurement is shown in Figure 2 6

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43 Figure 2 1 Current Voltage curves for a typical OPV showing the maximum power generated by the cell along with V OC and J SC. Figure 2 2 Device stack consisted of a glass substrate, a transparent electrode, the photoactive layer and a reflective electrode. Incident light (I IN ) travels through glass and the transparent electrode and ends up in the photoactive layer where it gets absorbed. Any light that does not get absorbed on the first pass is reflected back into the device by the reflective electrode.

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44 Figure 2 3 Schematic representation of phototherma l deflection spectroscopy. The pump beam perpendicular to the sample is used for excitation and the amount of deflection of the parallel probe beam determines the sample absorption. Figure 2 4 Schematic representation of a photoluminescence spectroscopy setup. Monochromatic light is illuminated on the sample, it gets absorbed and eventually gets re emitted. The PL spectrum is recorded using a monochromator and a detector.

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45 Figure 2 5 J V curve fitted using using the Poole Frenkel modifie d Mott Gurney equation for the calculation of hole mobility. Figure 2 6 A diagram of the transient photocurrent measurement setup. A short laser pulse excites a biased device held at short circuit. The transient is recorded on an oscilloscope.

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46 CH APTER 3 H IGH E FFICIENCY A IR P ROCESSED D ITHIENOGERMOLE B ASED P OLYMER SOLAR C ELLS 3.1 Introduction and Motivation Organic photovoltaics have attracted an increased interest over the past decade since they potentially offer a less expensive and more sustainable alternative for solar energy harvesting 71 At the same time they offer additional features such as ambient temperature solution processability, m echanical flexibility and lightweight. 72 Laboratory scale PCEs for BHJ OPV s based on conjugated polymers and fullerene derivatives have been steadily increasing and are currently exceeding 10%, 73,74 w conjugated molecule:fullerene cells have also reached 10%, 75 and polymer:polymer cells have reached 6%. 76 In order for polymer solar cells (PSCs) and organic solar cells in general, to reach their full potential, device processing must become compatible with high volume manufacturing processes with interfacial layers, acti ve layers and electrodes all deposited under air ambient conditions. Since polymer solar cells can be processed from solution, they present versatility in their production methods. The most attractive advantage for PSC s is the possibility for high throughp ut production on flexible substrates using deposition techniques compatible with R2R processing ( see S ection 1.10.3 ) 77 Compared to devices fabricated on the laboratory scale (ca. 0.1 1 cm2 in area), PSC s fabricated by R2R processing usually ha ve lower PCEs. 78 The lower efficiency is mainly due to the difference in device structures and deposition processes required for R2R processing which include large area devices, thick active layers and air ambient processing 79,80 Despite the fact that air ambient processing is necessary for high volume Reprinted with permission from Constantinou, I. et al. High Efficie ncy Air Processed Dithienogermole Based Polymer Solar Cells. ACS Appl. Mater. Interfaces 7, 4826 4832 (2015) DOI :10.1021/am5087566 Copyright 2015 American Chemical Society.

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47 manufacturing, m ost laboratory devices reported are still fabricated in inert environments. 74 This is mainly due to the fact that expo sure to oxygen and water has been shown to damage the organic semiconductors used in the devices, hence degrading performance. 81,82 For the eventual commercialization of OPV s, it will be highly advantageous to use all ambient processing. It is therefore important to study the effects of ambient processing on high performance OPV s in order to understand the associated degradation mechanisms. 3.2 Air P rocessed Organic Solar Cells in the Literature To date, most studies on air processed solar cells found in the literature are focused on devices based on blends of poly(3 hexylthiophene) (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PC 60 BM). 83 Recently, Wu et al. studied the effect of air processing on P3HT:PC 60 BM device performance. They foun d that P3HT:PC 60 BM devices processed in ambient air conditions can exhibit reversible degradation upon annealing therefore exhibiting good air processability. 84 This is in agreement with other reports on the formation of a reversible charge transfer complex in P3HT due to oxygen. 85,86 While P 3HT:PC 60 BM has served as the standard upon which the field was built; this material system has become less interesting due to the relatively low PCEs (~4%) when compared to most newly developed high performance polymer s Donor acceptor polymer:PC 71 BM blend s are now better candidates for air processing studies since they exhibit PCEs in the range of 8 10%. U ntil now there have been very few reports on air processed devices based on high efficiency low bandgap polymers. 87 89 While the study of air processed devices is important for commercialization, preliminary results on highly efficient polymers such as PTB7 show significant degradation upon exposure to oxygen. 90 92 Previously, we reported on BHJ OPVs based on poly(dithienogermole alt thienopyrrolodione) p(DTG TPD):PC 71 BM blends with a power conversion efficiency up to 8%. 93 We have al so demonstrated that high efficiencies can be achieved with a p(DTG

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48 TPD):PC 70 BM active layer thickness > 200 nm, a thickness that is compatible with R2R processing. 94 In this chapter, we studied the effect of air processing on the properties of these donor/acceptor materials and the resulting device performance. Our results show that air processing and exposure for a period of time up to 10 minutes leads to less than 10% degradation in power conversion efficiency, with the PCE decreasing from 8.5 0.25 % for devices made in nitrogen atmosphere to 7.7 0.18 % for the devices made in ambient air. To the best of our knowledge, t his is the highest efficiency reported for devices processed in air. The chemical structures of PC 70 BM and p(DTG TPD) are shown in Figure 3 1A and Figure 3 1 B respectively. 3. 3 Results and Discussion The objective of this work was the comparison between devices with active layers proce ssed in air and nitrogen, and the study of air ambient on the device parameters. Throughout this report, the active layers processed in nitrogen and the active layers processed in ambient air 2 In our main set of experiments, the active To further demonstrate the air processability of these devices, the po lymer films we re intentionally exposed to air for up to three hours before cathode deposition, which resulted in a further 8% reduction in PCE to 7. 04 %. In order to explain the difference in device performance due to air exposure, the effects of ambient processing on devi ce parameters were studied through electrical and optical characterizations with the aid of optical simulation s 3.3 .1 Current Voltage Measurements Figure 3 2 shows the J V characteristics for a reference device processed in nitrogen and an Air device ex posed to ambient air conditions for 10 minutes during processing. The corresponding performance parameters are summarized in Table 3 1 It is interesting to note that processing the devices in air did not significantly affect the different device parameter s. Air

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49 processed d evices showed a less than 10% decrease in J SC from 15.9 mA / cm 2 to 14.6 mA / cm 2 no change in V OC of 0.85 V and no statistical change in FF, resulting in a decrease in PCE close to 10%. Thus the reduction in PCE can be attributed to the de crease in J SC alone. A similar reduction in current was also observed by Nam et al. during the investigat ion of the effects of air processing on the performance of P3HT:PC 60 BM BHJ solar cells A 15% reduction in PCE due to air exposure was observed which was almost exclusively due to a reduction in J SC 95 3.3 .2 Film A bsorption The decrease in J SC due to air exposure might be attributed to degradation of either the optical absorption or the transport properties of the active layer. Figure 3 3 shows the ultraviolet visibl e (UV Vis) absorption spectra of both the "N 2 "and "Air" processed active layers coated on PEDOT:PSS. As shown in Figure 3 3 the absorption spectra for both films are identical, indicating that the optical properties of the polymer fullerene blend do not c hange due to air exposure. Further, based on our film thickness measurements, no difference was observed in thickness due to air processing as the active layer thicknesses were determined to be 90 3 nm for both devices. 3.3 .3 External Quantum Efficiency In addition to film absorption, EQE measurements were perform ed in order to determine the spectral response difference for these devices T he EQE data are shown in Figure 3 4 Interestingly, the EQE spectra exhibit obvious differences in the wavelength range between 400 and 570 nm In the 400 to 570 nm wavelength range the Air device showed a slightly lower EQE than the N 2 device, which is consisted with the slight decrease in Jsc. However, in the longer wavelength range between 570 and 800 nm, the t wo EQE curves are identical.

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50 3.3 .4 Total Device Absorption In order to understand the origin of the decrease in EQE, the device 2 w ere measured and the data are shown in Figure 3 5 Total device absorption refers to the optical absorption of the device measured in reflection, which takes into account the effects of optical interference between the incident light and light reflected off the back electrode. Light enters the device through the transparent electrode and trave ls through each layer to the back aluminum electrode where it is reflected. It is assumed, that the reflected light from the back aluminum electrode then travels back through to the front of the solar cell, where it enters an integrating sphere and is coll ected by a silicon photodetector. 96 More information about device absorption measurements can be found in S ection 2.1 .3 As expected, for wavelengths between 400 570 nm, the device absorption for the "Air" device is slightly lower than that of the "N 2 device, but the difference is not as pronounced as that in EQE. Since the film absorption spectra for both the "N 2 and "Air" devices measured in transmission were the same, this difference in device absorption is not attributed to a change in the active layer. Instead, it is attributed to changes in one of the interlayers that provide the electrode contacts. In order to verify our assumption that the difference in device absorption is due to a change in one of the interlayers, the series resistance (R s ) for both devices was calculated using J V measurements performed in the dark. An increase in Rs going from 6 3 c m 2 to 9 3 cm 2 was observed when devices were processed in air. Even though this increase is small, it could be enough to explain the observed subtle changes in EQE and device absorption with air processing without having an obvious impact on other devi ce parameters such as the FF. A lthough the devices used to measure the device absorption were encapsulated, and the LiF/Al contact was never directly exposed to air, we believe that this small change in device absorption is because of the LiF deposition d irectly on top of the air exposed active layer. It has

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51 been shown that exposure of LiF to air ambient leads to chemical changes in the interlayer. 97 Based on a study from Glowa cki et al., Al atoms deposited on top of LiF can cause LiF ions to diffuse into the underlying organic layer. 98 In the case of the Air devices, where the active layer was exposed to, and may retain a small amount of residual, oxygen and wate r, the reaction with LiF ions could cause a slight change in the total absorption of the device This slight change was reflected in the device absorption spectra. It should also be noted that the change in device absorption is unlikely to have originated from degradation of PEDOT:PSS since, for both the Air device and the N 2 device, the PEDOT:PSS was directly exposed to air for the same period of time. 3.3 .5 Reverse Bias External Quantum Efficiency In order to verify the accuracy of our data and ensure that the absorption efficiency is actually lower for the Air devices, the EQEs were measured at 500 nm and 683 nm as a function of reverse bias. The wavelengths chosen for these measurements correspo nd to the wavelengths shown in Figure 3 6 was in good agreement with the device absorption data. As anticipated, the data for the EQE as a function of voltage at 683 nm overlap and saturate at the same value under reverse bias. In the saturation region where the EQE value levels out, the charge collection efficiency is assu med to be 100% since all charge carriers are collected at the respective electrodes ( see S ection 2.1.2 ) 99 In contrast, at 500 nm, the two EQE curves are separated even at a large rever se bias, indicating that the absorption efficiency at 500 nm is indeed lower for the device made in air.

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52 3.3.6 Internal Quantum Efficiency To further evaluate whether the difference in EQE is solely due to the difference in the device absorption, the IQE s for the two devices were determined using the EQE data and the device absorption data for both devices. 100 The normalized IQE spectra for the two devices are shown in Figure 3 7 The IQE spectrum for the N 2 device is almost flat at all wavelengths, and the IQE for the Air device is indeed lower at wavelengths below 550 nm. Since IQE only reflects the electrical properties in the device and does not take into account any difference in the total device absorption, this reduction in IQE at short wavelengths is solely due to e lectrical effects. This reduction in IQE at short wavelengths for the Air device confirms that the overall reduction in EQE is not only due to a reduction in the device absorption but also due to other electrical effects. 3.3 .7 Space Charge Limited Current Mobility In order to verify whether the changes in IQE ca me from electrical effects associated with charge transport, the electron and hole mobilities were measured using single carrier devices assuming the space charge limite d current (SCLC) model ( see S ection 2.3.1 ) 101 A detailed description of this method can be found in the literature. 102 As expected, the measured average electron mobilities for both devices are similar, i.e ., 50.2x10 4 cm 2 / Vs fo r the "N 2 device and 20.35x10 4 cm 2 / Vs for the Air device. This is in good agreement with what has recently been reported by Nicolai et al. on photo oxidation defects as the products formed in the presence of water and oxygen in organic semiconductors 103 Their findings suggest that for st able trap free materials the target electron affinity of organic molecules should be larger than 3.6 eV. These numbers are consistent with multiple reports showing that organic molecules with electron affinities close to 4 eV or larger are less susceptible to reduction due to the presence of water and thus exhibit a higher level of air stability. 104,105 Since the electron affinity for PC 7 1 BM is around

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53 4.3 eV, no change in charge transport was expected for electrons. One the other hand, the average hole mobilities for the "N 2 device ( 1.60.3 x10 4 cm 2 /Vs) were almost one order of magnitude larger than that for the "Air" device (30.7 x 10 5 cm 2 /V s). 3.3 .8 Atomic Force Microscopy To determine whether the difference in hole mobilities is due to morphology, tappin g mode AFM measurements were performed on both active layers Details about this technique can be found in the literature as well as S ection 2.2.3 106 Figures 3 8 A and Figure 3 8 B show 5 p(DTG TPD):PC 71 BM films for the "N 2 and "Air" devices After active layer exposure to air there was no observable change in the domain size, no obvious phase reconstruction and no distinct features. Even though AFM only probes the interfacial structure, it is clear that the effect of air processing on the morphology of the films is not dramatic. Even though subtle cha nges in morphology such as differences in intermixing on the molecular scale could be responsible for a small change in device performance, we think it is more likely that the difference in hole mobilities is du e to a higher trap density in the active lay er upon air exposure as previously reported. 27 3.4 Transfer Matrix Optical Simulations I n order to gain a deeper understanding of the effect of hole transport on EQE, transfer matrix formalism (TMF) simulations were used to calculate the electric field distribution inside the device. 107,108 The optical modeling was based on the following stack of materials: ITO(90nm)/PEDOT:PSS(30nm)/p(DTG TPD):PC 71 BM(90nm)/LiF(1nm)/Al(100nm). The optical model assumes that all surfaces are planar and all layers are isotropic. 109 The complex refractive indices for all materials were acquired using vari able angle spectroscopic ellip sometry (VASE). 110 To acquire t he electric field distribution inside the device, a constant energy radiator is applied to the optical model. The normalized electric field distribution can be seen in Figure 3

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54 9 The normalized electric field intensity in the device |E| 2 corresponds to absorption in the active layer, and indicates that the distribution of the electric field inside the device is wavelength dependent. The figure shows that there are two electric field intensity maxima within the active layer. The first maximum is located a t short wavele n gths (450 600 nm), where the IQE spectra for the devices are different. This maximum is located closer to the center of the active layer, further away from the anode compared to the longer wavelength maximum (650 750 nm) which is located clo ser to the anode as shown in Figure 3 9 A For clarity, Figure 3 9 B shows the electric field distribution inside the device for the two wavelengths of interest (500 nm and 700 nm). Even though the positional shift in the peak electric field intensity is not large, i.e. ~12 nm, a difference of a few nanometers is large enough to cause a change in device performance due to the low mobility in organic semiconductors. 111 This observation, in combination with the degraded hole mobilities, suggests that the difference in photocurrent between the two devices is d ue to the change in the number of holes reaching the electrodes. 112 Since holes from photons with wavelengths between 400 to 600 nm are generated farther away from the anode and ha ve lower mobilities in the Air devices, they are less likely to be extracted, leading to a reduction in photocurrent. Hence, lower hole mobilities due to air exposure accounts for most of the decrease in the EQE in the short wavele n gth region resulting i n a reduction in Jsc. It is worth highlighting that the reduction in J SC is less than 10% and part of it is also due to a small decrease in device absorption as mentioned above. 3.5 Prolonged Device Air Exposure The data above show the effects of air expo sure for 10 minutes on device performance. While this exposure time is sufficient for processing in laboratory environments, a longer air exposure might be required in manufacturing environments. In order to study the effect of

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55 prolonged air exposure, devi ces were fabricated from films intentio nally exposed to air for up to 3 hours. Figure 3 10 A shows the J V characteristics for devices fabricated in air and kept in ambient conditions for 1, 2 and 3 hours, compared to a device made in air and kept in air for 10 minutes. Once again, the device parameter most affected by the long air exposure is J SC A 10% decrease in current was observed as the air exposure time increases from 10 minutes to 1 hour, whereas no changes in FF or V OC were observed. Surprisingly almost no further degradation was observed for devices kept in air for two additional hours A summary of the average device parameters is given in Table 3 1 Further, the EQE data for th ese devices depicted in Figure 3 10 B showed the same trend as befor e where the device degradation was only obvious in EQE at shorter wavelengths. Based on the air exposure time data, it is reasonable to assume that most of the degradation in the active layer is due to the first hour of air exposure. 3.6 Summary and Concl usions In conclusion, we have presented high efficiency, air stable p( DTG TPD ) :PC 71 BM OPV s processed in air with PCEs up to 7.7% compared to 8.5% for devices processed in nitrogen. It was found that upon exposure to air, devices exhibited a lower J SC compa red to devices processed in nitrogen, while V OC and FF were unaffected. Optical and electrical characterization, as well as optical simulations indicate that the reduction in photocurrent originates partly from a decrease in the device efficiency of the de vice due to changes in the cathode, while the remaining current decrease is attributed to a decrease in hole mobility Finally, our data show that with a high level of air stability and power conversion efficiency, p( DTG TPD ) :PC 71 BM is a promising candidat e for roll to roll processing and the commercialization of the organic photovoltaic technology

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56 3.7 Experimental Section 3.7.1 Device Fabrication The devices used in this study have a standard architecture with the structure: ITO /PEDOT:PSS/p(DTG TPD): P C 71 BM/LiF / Al, where PEDOT:PSS was used as the hole extraction layer and LiF/Al was used as the cathode. ITO coated glass substrates were UV ozone treated for 15 min between solvent cleaning and spin casting of the hole transport layer. PEDOT:PSS PVP AL 4083 ( Heraeus Precious Metals GmbH & Co. KG) was spin coated on top of the ITO coated glass substrates in ambient air conditions and annealed at 140 o C for 20 minutes. The average relative humidity in the laboratory during deposition was approximately 40%. Coate d with PEDOT:PSS, half of the substrates were then transferred into a glove box filled with N 2 where the active layer was spin coated from solution in C hlorobenzene (CB) : 1,8 diiodooctane ( DIO ) (5 vol %). P( DTG TPD ) was synthesized and purified as previou sly reported 113 The rest of the substrates were kept in ambient air conditions and the active layer was spin coated on top of PEDOT:PSS from the same solution. All substrates we re then transferred to a ther mal evaporator where 1 nm of LiF and 100 nm of Al were deposited on top of the active layer. 3.7 .2 Device Characterization Current voltage characteristics were measured using a Keithley 4200 semiconductor parameter analyzer sys tem with a Newport Thermal Oriel 94021 1000 W solar simulator, using the AM1.5 G solar spectrum at 100 mW/cm 2 incident power. Hole only devices with a structure ITO/MoOx(8 nm)/Active Layer(90 nm) /MoOx(8 nm) /Ag(100 nm) were used for hole mobility measurem ents and electron only devices with a structure of ITO/ZnO(40nm)/Active Layer(90 nm)/LiF(1nm)/Al(100 nm) were used for electron mobilities measurements. EQE measurements were conducted using an in house setup consisting of a Xenon DC arc lamp, an ORIEL 741 25

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57 monochromator, a Keithley 428 current amplifier, an SR 540 chopper system and an SR830 DSP lock in amplifier from Stanford Research Systems (SRS). To measure the device absorption samples were tilted at 7 angle relative to beam normal in order to allow space for the detector without blocking the incident beam. The same lock in setup was used for both EQE and device absorption measurements. All thicknesses of the active layers were determined using a Dektak surface profiler.

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58 Figure 3 1. Chemic al stru ctures active layer materials. A ) PC 70 BM. B ) p(DTG TPD) (Material provided by James J. Deininger ) Figure 3 2. J V curves for p(DTG TPD):PC 7 0 BM devices processed in nitrogen and air.

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59 Figure 3 3. UV Vis absorption spectra for p(DTG TPD):PC 7 0 BM films processed in nitrogen and air Figure 3 4 EQE spectra of p(DTG TPD):PC 7 0 BM devices processed in nitrogen and air.

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60 Figure 3 5 Device absorption measured in reflection for devices processed in nitrogen and air. Figure 3 6 EQE as a function of reverse bias at two constant excitation wavelengths, 500 nm and 683 nm for devices made in nitrogen and air.

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61 Figure 3 7 Normalized IQE for devices made in nitrogen and devices made in air. Figure 3 8 topography images of p(DTG TPD):PC 71 BM films. A) Processed in Nitrogen. B ) Processed in Air.

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62 Figure 3 9 Transfer matrix simulations for electric field distribution within a p(DTG TPD):PC 71 BM OPV device. A ) El ectric field intensity profile |E| 2 for t he studied device structure. B ) Electric field distribution for 500 nm and 700 nm illumination.

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63 Figure 3 10 Device characterization for devices exposed to air for up to 3 hours during processing. A ) J V curves of the OPVs made in air for different exposure times. B ) EQE spectra for devices made in air for different exposure times.

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64 Table 3 1. Summary of average device characteristics for the OPV s fabricated in this study Atmosphere / Exposure time J SC [ mA/cm 2 ] V OC [ V ] FF [%] PCE [%] Nitrogen 15.90 0.86 63 8.50 Air / 10 m inutes 14.6 0 ( 0.85 62 ( 7.7 0 Air / 1 hour Air / 2 hours Air / 3 hours

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65 CHAPTER 4 P HOTODEGRADATION IN H IGH E FFICIENCY I NVERTED P OLYMER S OLAR C ELLS 4.1 Introduction and Motivation With multi junction photovoltaic devices efficiencies now reaching 12%, the idea of OPV commercialization becomes viable. 114,115 For OPVs to reach the market, more emphasis should be given to the development of devices that combine processability, high efficiency and good operating stability. 116 Eve n though considerable work has been done in order to understand the degradation of BHJ OPVs, record operational device stabilities still remain under 10 years. 117 Before large scale production can become realistic, it is necessary to take a deeper look into the mechanisms responsible for OPV degradation. It has been established that the main causes of device degradation are oxygen, moisture, high temperatu res, and light. 118 Further, the different degradation mechanisms in OPV can be grouped into three general categories: Light induced burn in, long term deg radation, and thermal burn in degradation. 119 In order to counter some of the mentioned degradation causes such as air sensitivity and therm al degradation, the inverted device architecture was introduced. More details about the standard and inverted device architectures can be found in Section 1.8 In Chapter 2 we demonstrated inverted p(DTG TPD):PC 70 BM devices with good ambient air stability during processing with efficiencies reaching 7.7%. 120 These findings along with high efficiencies for active layer thickness > 200 nm previously reported, suggest that the p(DTG TPD):PC 70 BM system makes a promising candidate for the study of the degradation mechanisms in OPVs. 94 In this chapter we investigate the effect of light on the device performance of high efficiency, inverted solar cells with p(DTG TPD):PC 70 BM as the active layer and ZnO sol gel as the electron transport layer. Our results show a rapid decrease in all device parameters upon 1 sun light exposure of encapsulated devices. The light induced

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66 degradation was found to be due to charge extraction pro blems caused mainly due to photodegradation of the ZnO layer. We found that ZnO photodegradation was caused primarily due to high energy light with wavelengths below 400 nm. The use of a 400 nm longpass filter significantly reduced the light induced burn i n degradation and it is proposed as a strategy for extended device lifetimes. 4.2 Results and Discussion 4.2.1 Basic Device Characterization Before and After 24 Hour Light Exposure The J V characteristics for p( DTG TPD):PC 70 BM devices are presented in Figu re 4 1 As shown in the figure, n o device degradation is observed when encapsulated devices are kept in the dark for 24 hours. This suggests that there is no obvious degradation due to residual oxygen in the device or due to a failing encapsulation. In con trast, significant overall device degradation occurred when devices were kept under 1 sun light for 24 hours. The decrease in device performance after 1 sun light exposure was primarily due to a 45% drop in the device FF as well as a 40% drop in V OC A sum mary of the device parameters is given in Table 4 1 A resistor like behavior was also observed after 24 hour light exposure, where the photocurrent was highly dependent on the applied electric field. This could suggest the deterioration of charge transpor t or the creation of a barrier to charge extraction afte r 1 sun light exposure. The EQEs for the same devices are shown in Figure 4 2 An overall decrease in EQE is observed for devices exposed to 1 sun light for 24 hours, whereas no significant change in EQE is observed for devices kept in the dark for 24 hours. This overall decrease in EQE could suggest a decrease in any of the EQE component processes, namely absorption, exciton diffusion, charge transfer, and charge collection (See S ection 2.1.2 ).

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67 Figu re 4 3 shows the absorption efficiency for device before and after exposure to 1 sun illumination for 24 hours. As explained before, for the measurement of device absorption efficiency, light enters through the transparent electrode, passes through the abs orbing active layer where part of it gets absorbed and eventually reaches the cathode where it gets reflected back for a second pass (Section 2.1.3 ). Thus, the total optical path length is equal to two times the absorber layer thickness. Figure 4 3 shows no change in device absorptance after light exposure for 24 hours. These data suggest that no changes occur in the absorbing active layer that could account for the decrease in d evice EQE after light exposure. 4.2.2 Effect of Elevated Temperature on Device Performance During light exposure, the sample temperature was monitored with the use of an infrared temperature gun. The maximum temperature reached by the sample during 1 sun light exposure was found to be approximately 50 o C. In order to check wh ether this temperature could provide enough energy for film reorganization we checked the effect of 50 o C on a device kept in the dark. Figure 4 4 shows the J V characteristics for p(DTG:TPD):PC 70 BM devices before and after 24 hours of 50 o C heating on a h ot plate in the dark. No changes in device parameters were observed after heating, suggesting that elevated temperature did not cause any changes in device morphology such as a n increase in the domain size. In order to confirm that no significant change i n domain size occurred after 1 sun light exposure, we examined the effects of light on the morphological properties of the photoactive layer using AFM. The AFM images for a fresh device and a device kept under 1 sun for 24 hours are shown in Figure 4 5A an d Figure 4 5B respectively As expected, after light exposure, no large scale changes in morphology were observed. It is therefore reasonable to conclude that photodegradation is not due to changes in domain size and exciton diffusion efficiency.

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68 4.2.3 E ffect of 24 Hour Light Exposure on Charge Generation Sub bandgap EQE measurements were performed in order to understand the difference in the CT states between fresh and photodegraded devices. By monitoring the response at energies below the bandgap (<1.66 eV), we were able to compare the effectiveness of the CT states for the two devices based on the amount of carriers collected ( See S ection 2.1.2 ). 53,121 As shown in Figure 4 6 sub bandgap EQE remains the same af ter light exposure. There are therefore no obvious changes in the electronic states at the donor acceptor interface after 1 sun illumination for 24 hours. This suggests that charge transfer efficiency is not affected by light exposure and therefore carrier generation is not the reason for the decrease in EQE. Finally, the effect of light on charge collection efficiency was investigated. Charge collection efficiency is defined as the fraction of dissociated carriers that are collected at the electrodes. Mul tiple processes go into charge collection efficiency, namely charge carrier recombination, charge transport, and carrier extraction. 52 4.2.4 Effect of 24 Hour Light Exposure on Charge Carrier Recombination The effect of 24 hour, 1 sun light exposure on the charge carrier recombination kinetics was studied using light intensity depended J V measurements as described in Section 2.3.3 The device short circuit current wa s measured for a range of illumination intensities and bimolecular recombination was approximated using the power law in Equation 2 11 Based on Equation 2 11 at the short circuit condition, for negligible bimolecular recombination the slope should be clo se to 1. 122 Any deviation from unity implies bimolecular recombination is dominant Figure 4 7 shows J SC versus light intensity on the log log scale fitted using the power law described above. As seen in the figure, the slope decreased from 0.96 to 0.81 after 1 sun light exposure for 24 hours signifying an important increase in bimolecular recombination.

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69 In order to understand the degree to what bimolecular recombination affected the device performance, we investigated the device responsivity as a function of light intensity. The responsivities of devices before and after exposure to 1 sun for 24 hours a re compared in Figure 4 8 For both devices R exhibits a sublinear dependence on light intensity. For fresh devic es, the decrease in R with increasing light intensity is less sharp compared to the device exposed to 1 sun for 24 hours. According to bimolecu lar recombination theory, the higher slope represents a higher degree of bimolecular recombination for the photodegraded devices which is in agreement with our previous data. 123 In addition to that, the extrapolated difference in r esponsivities at the zero light intensity intercepts where bimolecular recombination is zero, suggests that bimolecular recombination is not the only limiting factor in device performance after photodegradation. More details about bimolecular recombination theory and responsivity can be found in Section 2.3.3 4.2.5 Effect of 24 Hour Light Exposure on Charge Carrier Extraction Transient photocurrent TPC measurements were employed in order to evaluate the effect of light on charge carrier extraction efficie ncies. The current response on square pulses of light is See Se ction 2.3.4 ). 67,124 Figure 4 9 A shows measurements repeated at three applied DC voltages for fresh devices. The data indicate that the shape o f the current transients are independent of the applied voltage. Even for voltages close to V OC photocurrent is extracted in the same direction. In contrast to the response of the fresh devices, photodegraded devices exhibit current in reverse direction u pon switching off the light when applying voltages close to V OC The photocurrent undershoots are shown in Figure 4 9 B It has recently been established that this kind of undershoot is a fingerprint of an extraction barrier. The reverse current was explain ed to be due to charges stored at an extraction barrier diffusing back towards the bulk where they recombine. This would imply a reverse current compared to the photocurrent. 125 It is therefore evident tha t along with bimolecular recombination, a decrease in

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70 charge extraction efficiency due to an energy barrier at one of the electrodes are the main reasons for device degradation upon extended light ex posure. 4.2.6 Effects of Light Exposure on the Electronic Properties of ZnO Multiple reports exist in the literature discussing the effect of UV radiation on the electronic properties of ZnO films. 126,127 It is well know that the electronic properties of ZnO are complex and depend highly on the processing conditions. An array of conductivity responses upon exposure to visible and UV radiation have been previou sly reported. While some observed a ZnO conductivity increase upon exposure to UV light, others report device degradation even for short illumination times. 128,129 In order to investigate whether the device photodegradation observed in this study is due to an extraction barrier formed at the ZnO cathode, we used steady state photoluminescence to track changes in the ZnO film when exposed to 1 sun illumination for up to 24 hours. The PL spectra for ZnO films are shown in Figure 4 10 As seen in the figure, in addition to the main band to band PL peak at 371 nm, a broader band in the 425 525 nm wavelength range is also present. This lower energy emission band is well kn own as evidence of the presence of defect states in ZnO. 130 Figure 4 10 shows the evolution of the defect peak during 1 sun illumination of ZnO films for up to 24 hours. In the first few minutes of illumination, there are no noticeabl e changes in the film PL spectra. After about 20 minutes of 1 sun illumination, an increase in the green photoluminescence peak relative to the main peak becomes clearly distinguishable. After 24 hours of illumination the defect band becomes much more pron ounced. It is believed that the origin of green luminescence in ZnO is Zn vacancies acting as deep acceptors, p doping the ZnO film. 131,132 We believe that these defects are responsible for the creation of the energy barrier in our photodegraded devices that results in decreased charge efficiency.

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71 In ord er to determine whether degradation is solely due to an increase in the concentration of defects in the ZnO film or whether it is related to the ZnO/p(DTG TPD):PC 70 BM interface we repeated the photostability experiment for ITO/ZnO/poly terthiophene co isoi ndigo p(T3 iI):PC 70 BM/MoOx/Ag devices. 133 The chemical structure of p(T3 iI) is show n in Figure 4 11 The J V characteristics for p(T3 iI):PC 70 BM devices before and after light exposure are shown in Figure 4 12 After 1 sun illumination for 24 hours, similar degradation signatures were observed for p(T3 iI):PC 70 BM devices compared to p(DT G TPD):PC 70 BM devices. More specifically, a 40% decrease in FF was observed, accompanied by a dramatic decrease in V OC and J SC ( Table 4 2 ). In addition to the significant drop in Jsc, a resistor like behavior was also observed where the device photocurrent was highly depended on the bias. Further, the PC transients for photo aged p(T3 iI):PC 70 BM devices in Figure 4 13 showed similar photocurrent undershoots as p(DTG TPD):PC 70 BM devices. We believe that a similar extraction barrier was created for these devices after 1 sun light exposure for 24 hours. This is evidence that ZnO photo degradation in ITO/ZnO/Active Layer/MoOx/Ag inverted devices is in dependent of the active layer. In order to improve device photostability, we studied the origin of photoinduced defects in ZnO films with the use of optical filters. We found that device photostability was dramatically improved when devices were aged under 1 sun illumination, filtered using a 400 nm long pass (LP) filter. The J V characteristics for de vices aged and measured under filtered 1 sun illumination are shown in Figure 4 14 Despite the fact that filtering light with wavelengths lower than 400 nm reduced J SC device photostability was significantly improved. As shown in Figure 4 1 4 after 48 ho urs of continuous exposure to 1 sun filtered light both the V OC and J SC of

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72 the device remain mostly unaffected. A less than 15% overall device performance drop was observed mostly due to a small decrease in the device FF. A summary of all device parameters before and after light exposure can be found in Table 4 3 We can therefore conclude that UV light induced ZnO defects are responsible for device degradation and severe photodegradation can be prevented using UV filters. In the case of flexible OPVs, the most commonly used substrate is polyethylene terephthalate (PET). Apart from being mechanically flexible PET has been shown to also work as a UV filter since it does not transmit light with wavelengths below 380 nm ( Figure 4 15 ). 129 Burn in photodegradation for inverted devices made on PET substrates is therefore expected to be minimal. Finally, the effect of UV light on ZnO was confirmed using steady state PL. Similar to the data presented above, PL measurements were performed on ZnO films kept under filtered and unfiltered 1 sun illumination for 24 hours. The PL spectra are shown in Figure 4 1 6 In contrast to ZnO films aged under unfiltered 1 sun illumination, films aged under 400 nm LP filters showed no increase in the green ph otoluminescence peak. This is further proof that high energy UV light is responsible for the creation of defects in ZnO that in turn affect the electronic properties of the film and cause an extraction barrier for electrons. 4.3 Summary and Conclusions The effect of light on device stability was investigated in this report for solar cells made in the inverted architecture. Significant decrease was observed in all device parameters along with an overall decrease in EQE upon 1 sun light exposure for 24 hours. The decrease in EQE was shown to be unrelated to device absorption since no changes were observed in the device absorption spectra after photodegradation. Additionally, elevated temperature due to extended light exposure was also not found to cause any ch anges in device performance. A tomic force

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73 microscop y images showed insignificant changes in domain size after photodegradation suggesting that exciton diffusion efficiency was not affected by light exposure. Sub bandgap EQE measurements for devices before and after photodegradation revealed no difference in carrier generation that might contribute to the overall decrease in EQE and device performance. One the other hand, bimolecular recombination was found to become more significant after device exposu re to 1 sun illumination for 24 hours. Light intensity depended device responsivity measurements showed that bimolecular recombination was not the only reason for device degradation. Indeed, photocurrent transients revealed the formation of a charge extrac tion barrier at one of the electrodes. Steady state PL measurements confirmed the formation of a defect band in ZnO upon 1 sun light exposure for 24 hours, hindering the collection of electrons. Devices made using p(T3 iI):PC 70 BM as active layers deposited on top of ZnO showed similar degradation upon light exposure proving that photodegradation was solely due to the ZnO film and not the ZnO/Active Layer interface. ZnO degradation was shown to be due to exposure to UV light and can be prevented with the use of UV filters. Stability for devices kept under filtered 1 sun illumination for 24 hours was significantly increased, and burn in photodegradation was mostly eliminated. Finally, our data show that with relatively high efficiency, good air stability and p romising photostability, p(DTG TPD) is a suitable candidate for roll to roll processing. Additionally, our data show that the photostability of inverted devices made using ZnO as a hole transport layer can be significantly improved with the use of UV filte rs. 4.4 Experimental Section 4.4.1 Device Fabrication. Devices used in this study had the following structure: ITO/ZnO/p(DTG TPD):PC 70 BM/MoOx/Ag. ZnO was used as the electron transport layer and MoOx/Ag was used as the anode. 134 ITO coated glass substrates were cleaned in acetone and isopropanol before

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74 spin casting the ZnO layer. ZnO sol gel was synthesized and processed based on the recipe previously reported by A. J. Heeger. 135 P(DTG TPD):PC 70 BM films were pre pared as described in Section 3.7.1 P(T3 iI):PC 70 BM active layer was spin coated from 1 ,2 Dichlorobenzene (oDCB) : DIO (2.5 vol %) solution, at a 20mg/ml concentration to form 100 nm thick films. P(T3 iI):PC 70 BM was synthesized and purified as previously reported. 133,136 In a thermal evaporator 5 nm of MoOx and 100 nm of Ag we re deposited on top of the active layer at pressures < 10 6 torr. 4.4.2 Device Characterization Current voltage characteristics were measured using a Keithley 4200 semiconductor parameter analyzer system with a Newport Thermal Oriel 94021 1000 W solar sim ulator, using the AM1.5 G solar spectrum at 100 mW/cm 2 incident power. External quantum efficiency (EQE) measurements were conducted using an in house setup consisting of a Xenon DC arc lamp, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, an SR 540 chopper system and an SR830 DSP lock in amplifier from SRS. For device absorptance measurements, samples were tilted at 7 angle relative to beam normal in order to allow space for the detector without blocking the incident beam. The same lock in setup was used for both EQE and device absorptance measurements. All thicknesses were determined using a Dektak surface profiler. Transient photocurrent measurements were performed using a Tektronix AFG 3101 function generator driving a Newport 635 nm las er. Device photocurrent was measured using a Tektronix DPO 3054 digital oscilloscope with 50 ohm input impedance. A Keithley 428 programmable current ampli fier was used to apply the bia s.

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75 Figure 4 1. J V characteristics for p(DTG:TPD):PC 70 BM devices before and after 24 hour light exposure at 1 sun intensity. Figure 4 2. EQE spectra for p(DTG:TPD):PC 70 BM devices before and after 24 hour light exposure at 1 sun intensity.

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76 Table 4 1. Average device parameters for p( OPVs fabricated in this study. p(DTG TPD):PC 70 BM J SC [mA/cm 2 ] V OC [V] FF [%] PCE [%] Fresh 14.80 ( 0.6 ) 0.84 ( 0.02 ) 66 ( 0.5 ) 8.3 ( 0.1 ) 24 hours Dark 14.60 ( 0.2 ) 0.85 ( 0.00 ) 66 ( 0.7 ) 8.2 ( 0.2 ) 24 hours 1 sun 9.82 ( 0.4 ) 0.66 ( 0.02 ) 43 ( 0.9 ) 2.8 ( 0.3 ) Figure 4 3. Device absorption efficiency for p(DTG:TPD):PC 70 BM before and after 24 hour light exposure at 1 sun intensity.

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77 Figure 4 4. J V characteristics for p(DTG:TPD):PC 70 BM devices before and after 24 hours thermal treatment at 50 o C. Figure 4 5 AFM topography images for p(DTG:TPD):PC 70 BM films before and after 24 hour 1 sun illumination. A) Fresh p(DTG:TPD):PC 70 BM films B ) After 24 hour of 1 sun illumination.

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78 Figure 4 6. Sub bandgap EQE spectra show no significant changes in carrier generation after 1 sun illumination for 24 hours. Figure 4 7. Log log Jsc v s Light intensity plot. The slopes resulting from the linear fit are shown in parenthesis.

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79 Figure 4 8 Responsivity v s Light intensity. The zero intercepts and slopes are depicted in the figure.

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80 Figure 4 9 Photocurrent transients for devices before and after 24 hour, 1 sun light exposure. A ) TPC transients for devices before exposure to light. B ) TPC transients for devices exposed to 1 sun for 24 hours. Figure 4 10. Steady state PL spectra of ZnO sol gel films. Clear evolution of a defect band in the 425 525 nm wavelength range with 1 sun light exposure for up to 24 hours. The main band to band PL peak is seen at 371 nm.

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81 Figure 4 11 Chemical structure of poly terthiophene co isoindigo (T3 iI) (Material provided by Chi Kin Lo ) Figure 4 12. J V characteristics for p(T3 iI):PC 70 BM before and after 24 hour 1 sun light exposure and 24 hour aging in the dark. Table 4 2 Average device parameters for p( OPVs fabricated in this study. P(T3 iI):PC 70 BM J SC [mA / cm 2 ] V OC [V] FF [%] PCE [%] Fresh 15.52 ( 0.6 ) 0.71 ( 0.0 ) 67 ( 0.6 ) 7.33 ( 0.3 ) In the dark, 24 hours 14.73 ( 0.3 ) 0.71 ( 0.0 ) 69 ( 0.4 ) 7.17 ( 0.2 ) Under 1 sun, 24 hours 10.88 ( 0.4 ) 0.61 ( 0.0 ) 43 ( 0.9 ) 2.87 ( 0.2 )

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82 Figure 4 13 Photocurrent transients at different DC biases for devices exposed to 1 sun illumination for 24 hours. Figure 4 1 4 J V characteristics for p(DTG TPD):PC 70 BM devices before and after light exposure using 400 nm LP filters.

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83 Table 4 3 Average device performance for devices aged and measured under 400 nm LP filter. p(DTG TPD):PC 70 BM J SC [mA/ cm 2 ] V OC [V] FF [%] PCE [%] Fresh 400 LP 12.4 ( 0.2 ) 0.84 ( 0.01 ) 65 ( 0.4 ) 6.8 ( 0.2 ) 1 sun, 24 hours 400 LP 12.4 ( 0.3 ) 0.83 ( 0.02 ) 63 ( 0.7 ) 6.5 ( 0.3 ) 1 sun, 48 hours 400 LP 12.3 ( 0.3 ) 0.82 ( 0.02 ) 59 ( 0.6 ) 6.0 ( 0.3 ) Figure 4 15 Optical transmittance of PET substrates. Figure 4 1 6 Steady state PL spectra of ZnO sol gel aged under 1 sun light filtered using 400 nm LP filters. Film spectra show no changes when kept under 1 sun illumination for 24 hours.

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84 CHAPTER 5 EFFECT OF THERMAL ANNEALING ON THE ELECTRONIC STATES OF POLYMER SOLAR CELLS 5.1 Introduction and Motivation In order to achieve high performance OPVs, optimizing the blend morphology is considered to be critical. It is widely accepted that the morphology and molecular packing of both the polymer donor and the fullerene acceptor must be optimized for optimum charge extraction. 137 It is also important for the domain size to be smaller than the exciton diffusion length in order for efficient exciton dissociation. 138 One of the most commonly used methods for morphology and domain size optimization is t hermal annealing. 139,140 The effect of thermal annealing on polymer fullerene blends based on P3HT and PC 60 BM has been extensively studied over the past decade due to the polymer's interesting semi crystalline nature. 134,141 Even though P3HT has served as the benchmark for organic solar cell research for many years, it has recently been replaced by other more efficient, low bandgap polymers due to the relatively low PCEs. 142 Many high ly efficient polymers used today are not semi crystalline like P3HT and the effect of thermal annealing on the film morphology is less dramatic. One of these high efficiency polymers is poly [N hepta decanyl 2,7 carbazole alt 5,5 (4,7 di 2 thienyl 2,1,3 benzothiadiazole)] (PCDTBT). PCDTBT is amorphous and thermal annealing does not have a significant effect on film morphology. 100,143 The chemical structure for PCDTBT co polymer is shown in Figure 5 1 A variety of experimental techniques such as atomic force and electron microscopy as well as X ray scattering have been used in the past to determine the physical changes in the Constantinou, I. et al. Effect of Thermal Annealing on Charge Transfer States and Charge Trapping in PCDTBT:PC 70 BM Solar Cells. Adv. Electron. Mater. 1, (2015). DOI: 10.1002/aelm.201500167. Reproduced with permission from John Wiley and Sons.

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85 polymer fullerene blends upon thermal annealing. 106 While these techniques have been used to study the film mo rphology and molecular packing, they do not provide much information about changes in the electronic structure of the materials as well as changes in the polymer fullerene interaction that might also result from thermal annealing Since thermal annealing c an have a significant impact on blend morphology, it can be expected that it can also affect the dielectric environment in the blend and hence the formation of CT states. As discussed in Section 1.9 a change in the CT state manifold can subsequently have an effect on the effective bandgap at the heterojunction and the exciton dissociation process. 144 In this report, we studied the effect of thermal annealing on the electronic properties of PCDTBT:PC 70 BM blends and the resulting device performance. Our result s show that along with an increase in dielectric constant, thermal annealing also has a positive impact on the effectiveness of the CT states for more efficient exciton dissociation. Even though the resulting photo generated current in annealed devices is slightly higher, a significant dec rease in the device FF was observed, resulting in an overall decrease in device power conversion efficiency. Energetic disorder measurements along with PDS measurements confirmed that the decrease in FF was not because of an increase in bimolecular recombination but because of an increase in monomolecular recombination due to an increase in the trap states within the bandgap 5.2 Results and Discussion 5.2.1 Basic Device Characterization Before and After Thermal Annealing The J V characteristics for PCDTBT:PC 70 BM devices with and without annealing are presented in Figure 5 2 As shown in the figure, annealing at 150 o C results in a decrease in the overall device performance as has previously been reported. 145 The decrease in device performance after annealing is caused primarily by a decrease in the device FF, whereas the J SC and V OC remain mostly unch anged. A summary of the devic e parameters is given in Table 5 1

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86 Interestingly, in reverse bias the current output for the annealed PCDTBT:PC 70 BM device is higher than for the as prepared device. In order to determine whether the current output for the ann ealed PCDTBT:PC 70 BM device at higher reverse bias is due to an increase in free carrier generation, the EQEs were measured at 540 nm as a function of reverse bias. Under a large field, the charge separation efficiency is assumed to be 100% and all photo ge nerated excitons are completely dissociated, resulting in a saturation of EQE as the reverse bias increases. 52 The wavelength chosen for these measurements correspond s to a regime where the EQEs are the same for both devices under short circuit conditions (Figure 5 3 inset). Figure 5 3 shows the EQE under reverse bias for both devices. At relatively low reverse bias (~ 2 V), the EQEs for both devices appear to have rea ched a plateau, which represents the effective carrier generation rate. At 5V all available charge carriers are effectively collected by the electrodes. It is apparent from Figure 5 3 that the effective carrier generation rate is higher for the annealed P CDTBT:PC 70 BM device and there are overall more available carriers to be collected compared to the device that was not annealed. The higher EQE at high reverse bias for the annealed device is in agreement with our J V data confirming that the charge generat ion rate is higher for the annealed devices. Even though the difference in performance observed in reverse bias is relatively small, it was consistently observed during the measurement of tens of devices and has previously been reported in the literature. 146 We believe that this increase in charge generation after annealing is an indication of reduced Coulomb exciton binding energy and the formation of more effective CT states after thermal annealing. 5.2. 2 Effect of Thermal Annealing on Film Morphology In order to confirm that the increase in charge generation after annealing is not due to a change in the morphology of the blend, we examined the effects of thermal annealing on the

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87 morphological properties o f the photoactive layer using AFM It has recently been shown that coarse phase separation in PCDTBT:PC 70 BM blends occurs at annealing temperatures above 155 C due to cold crystallization of PC 70 BM. 147 In order to avoid coarse phase separation the annealing temperature for this study was kept at 150 C. Fi gures 5 4A and Figures 5 4B show the topographical images of PCDTBT:PC 70 BM films after and before thermal annealing. Before thermal annealing, the surface of PCDTBT:PC 70 BM films was smooth, with a root mean square (RMS) roughness of 0.6 nm and no distinct features. These results are in agreement with what has been shown in the past for this material system. 148 As expecte d, after thermal annealing, no apparent changes in phase morphology were observed. Since thermal annealing does not cause any macroscopic changes in the film morphology such as an increase in the domain size, we believe that the difference observed in the device performance might be due to changes in the electron ic structure of the materials. 5.2.3 Effect of Thermal Annealing on Dielectric Constant From the AFM data, there is no obvious indication of any changes in the donor acceptor interaction after ther mal annealing. One way to qualitatively probe for changes in the polymer fullere ne interaction is the measurement of the dielectric constant of the polymer fullerene blend. In the case of PCDTBT:PC 70 BM, capacitance voltage (C V) measurements reveal that th e dielectric constant increases by 14% after thermal annealing, indicating a change in the blend dielectric environment and the polymer fullerene interactions (Figure 5 5 ) This increase in the dielectric constant could also signify a reduction in the Coulomb binding energy of the CT states which would in turn lead to the formation of more delocalized CT excitons The formation of more delocalized CT excitons accounts for a n increase in the photocurrent generation for the annealed device, seen in reverse bias in Figure 5 2 and Figure 5 3 as previously argued 7

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88 5.2.4 Effect of Thermal Annealing on Charge Generation Sub bandgap EQE measurements were pe rformed in order to understand the difference in the CT states for both annealed and as prepared devices (Figure 5 6 ). By monitoring the response at energies below the band gap (<1.7 eV), we were able to compare the effectiveness of the CT states for the t wo devices based on the amount of carriers collected. 121 Figure 5 6 shows the photocurrent response due to the CT exponential absorption tail (1.2 eV 1.7 eV). 149 As shown in Figure 5 6 the sub bandgap EQE for the annealed device is higher than that of the as prepared device. The results confirm our hypothesis for the existence of more effective CT states below the bandgap facilitating the dissociation of excitons into polarons in the ann ealed device. To understand the exciton dynamics and the annealing effect on the delocalization of the CT excitons, transient photoluminescence m easurements were carried out on both the pristine PCDTBT films and PCDTBT:PC 70 BM blends. Bernardo et al. have p reviously shown that higher delocalization of CT excitons lead s to a fast decay component in photoluminescence. The fast decay component in transient PL was explained to be due to a fast dissociation mediated by the delocalized CT states. 150 The transients for the PCDTBT:PC 70 BM blends are shown in Figure 5 7 In the case of the sample with no annealing, transient PL shows a single exponential decay componen t with a lifetime of 9883 ps indicating a dissociation route that is similar to that of the pristine polymer see Figure 5 8 For the annealed sample, an additional initial fast decay component with a lifetime of 16221 ps is present, which accounts for a bout 65% of the integrated PL intensity, followed by a slower comp onent with a lifetime of 9259 p s. This is an indication of an additional dissociation route in the blend films where CT states exist. It is worth mentioning that the fast PL decay component in the annealed blend films is not present in any of the pristine polymer sample Figure 5 8A and Figure 5 8 B This fast decay component in the annealed blends indicates that most of the excitons at t he CT states go through a fast dissociation

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89 process, leading to a higher charge generation efficiency which is in agreement with our EQE and sub bandgap EQE data With these data, we can conclude that CT excitons in PCDTBT:PC 70 BM devices are indeed more de localized after thermal annealing which makes the exciton dissociation process more efficient. This can be a result of more effective CT states as well as smaller Coulombic interactions due to the increase in dielectric constant after thermal annealing. D espite the higher CT exciton delocalization in the annealed device and the reduced Coulomb exciton binding energy, the fill factor of the annealed device actually decreases from 56% to 46%. 5.2.5 Effect of Thermal Annealing on Charge Transport and Energeti c Disorder In order to explain the difference in FF, the carrier transport properties for both devices were investigated by measuring the temperature dependent space charge limited current. Single carrier blends devices were used to measure both hole and electron mobilities. The zero field 0,e 0,h were determined at each temperature based on the SCLC model given by the Mott Gurney equation ( See Section 2.3.2 ) The SCLC model used is valid for unipolar transport in trap free semiconductors with an Ohmic injecting contact for field dependent mobilities. 101 The resulting zero field mobilities were further analyzed using the Gaussian disorder model (GDM) 151 In organic BHJ devices, electrons and holes transport through the fullerene and polymer domains, respectively. Since each of these domains are energetically fluctuating, the energetic disor Equation 2 10 can be interpreted as the width of the energetic fluctuations in the BHJ. As explained in S ection 2.3.2 in conjugated polymer fullerene blends the energetic disorder can also be described as the Gaussian distribution of the HOMO/ LUMO energies, and its value can be extracted from the temperature dependent mobility data and Equation 2 10 26

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90 The results of the GDM analysis for electron and hole transport in annealed and as prepared PCDTBT:PC 70 BM devices are summarized in Figure 5 9 A summary of the high temperature mobilities and energetic disorder parameters is shown in Table 5 2 For both electrons and holes, after thermal annealing transport was slightly improved. Annealed devices h along with a slight increase in mobility. It is worth mentioning that the values for energetic disorder are comparable to what has been reported in the literature. 152,153 Based on the transport data, it is obvious that thermal annealing provides an effective driving force for film reorganization and morphology optimization for optimized transport. 154 Even though the change in energetic disorder is statistically significant, especially in the case of electron transport, the improved transport properties do not dominate the devi ce performance and cannot explain the decrease in FF in the annealed device. 155 This slight improvement in transport properties after annealing could also suggest that these devices are not limited by bimolecular recombination. 156 5.2.6 Effect of Thermal Annealing on Charge Carrier Recombination In an effort to study the charge carrier recombination kinetics at the short cir cuit condition, J SC was measured for a range of illumination intensities from 1 sun down to 0.1 suns and the results were fitted to the power law described in Section 2.3.3 Based on Equation 2 11 at the short circuit condition, for negligible bimolecular recombination the slope should be close to 1. Any deviation from unity implies bimolecular recombination is dominant 122,157 Figure 5 10 shows J SC versus light intensity on a log log scale. For annealed devices the fitting of the data gave a slope of 0.985 (0.01) and for as prepared devices the fitting of the data gave a slope of 0.979 (0.01). The fact that both slopes are close to 1 implies that the devices are not severely limited by bimolecular recombination. This is also in agreement with the energetic disorder data that showed slight improvement in transport after thermal annealing. In addition, the fact that the

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91 slopes for these devices are very similar proves that bimolecular recombination is not responsible for the difference in the FF. Next, the influence of light intensity on V OC was also measured for a range of illumination intensities. At V OC all photo generated carriers are forced to recombine in the device driving the photocurrent to zero. 66 The forced recombination of carriers at open circuit m akes the studies at V OC useful in providing detailed information about different recombination mechanisms. Voc can be correlated to illumination intensity by Equation 2 12 Based on th e equation, for bimolecular recombination the slope of V OC vs. ln(I) sho uld be equal to kT/q. 158 Figure 5 11 shows the relationship between V OC and ln(I) for the annealed and as prepared PCDTBT:PC 70 BM devices. For both devices the slopes of V OC vs. ln(I) are greater than kT/q, further confirming that bimolecular recombination is mostly negligible for these devices. The V OC vs ln(I) plot also provides information on monomolecular Shockley Read Hall recombination. 66,159,160 As shown in Figure 5 11 the V OC for the annealed device shows a stronger dependence on light intensity. The slope increases from 1.870.18 kT/q before annealing to 2.470.35 kT/q after annealing, indicating a stronger SRH trap assisted recombination. Based on the data and analysis descr ibed above we believe that the annealed PCDTBT:PC 70 BM devices are more limited by monomolecular, trap assisted recombination mainly due to an increase in the number of traps after annealing. This is in agreement with what has previously been reported for t his system. 24 25, 30 Further, since both the electron and hole mobili ties were enhanced after thermal annealing, our data suggest the SRH recombination is due to the formation of deep traps.

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92 5.3 Temperature Induced D eep Trap Formation To determine whether deep traps are present in the annealed device, PDS was used to probe the mid gap optical absorption for annealed and as prepared PCDTBT:PC 70 BM films. 161,162 It should be noted that while PDS measurements have been performed for abs orption near the band edge of PCDTBT, absorption of the mid gap states in PCDTBT:PC 70 BM has not previously been investigated. 146 Figure 5 12 shows the PDS data for photon energies between 0.6 eV and 1.6 eV. For both samples, ab sorption drops sharply when the photon energy is lower than 1.7 eV. In the sub bandgap region, the annealed sample shows a featureless absorption with absorption coefficients in the range of 2 3102 cm 1 In contrast, the as prepared sample exhibited lowe r absorption coefficients (< 2102 cm 1 ) in the sub bandgap region below 1.3 eV. The absorption peak centered at about 0.9 eV is attributed to the C H vibrational overtones, indicating the sensiti vi ty of the PDS measurement 55 The slightly larger subgap ab sorption of the annealed sample indicates the presence of deep traps induced by thermal annealing, and is consistant wth the observed SRH recombination mechanism suggested by Figure 5 11 5.4 Summary and Conclusions The photo physical properties and the effect of the dielectric constant on the delocalization of CT excitons for annealed and as prepared PCDTBT:PC 70 BM devices were investigated An increase in the dielectric constant was observed upon thermal annealing promoting the delocalization of CT excitons. Using sub bandgap EQE and transient PL measurements, we confirmed that annealing results in a more effective carrier generation process due to a higher degree of CT exciton delocalization. Even though no obvious difference in the macroscopic film morphology was observed the solar cell performance changed significantly after thermal treatment mostly due to a decrease in the device FF It was shown that even though the photocurrent generated for the annealed PCDTBT:PC 70 BM device is slightly higher due to a

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93 higher degree of CT exciton delocalization a larger amount of generated photo carriers end up recombining resulting in a lower FF. Energetic disorder measurements were employed in order to understand the decrease in FF after annealing but showed no significant changes in the device transport properties. Recombination measurements at short circuit and open circuit conditions revealed negligible bimolecular recombination for both devices but a higher SRH recombination for the annealed PCDTBT:PC 70 BM devices leading to a lower FF. Finally, the increase in SRH recombination was shown to be due to an increase in the concentration of deep traps after thermal annealing. Our findings suggest that thermal annealing can cause significant changes in the electr onic structure of organic semiconductors and can subsequently play an important role in the photo generation process in organic BHJ PVs. 5.5 Experimental Section 5.5.1 Device Fabrication Active layer solutions were prepared by dissolving a 1:4 weight rati o of PCDTBT (1 Material) and PC 70 BM (Nano C) in CB:oDCB mixed solvent solution. The solutions were stirred overnight at 50 C. B ulk heterojunction OPVs were fabricated in the conventional architecture (glass/ITO/PEDOT:PSS/PCDTBT:PC 70 BM/LiF/Al). ITO coated glass substrates were cleaned in acetone and isopropanol and subsequently treated under UV ozone for 15 min before the deposition of HTL. PEDOT:PSS PVP AL 4083 (Heraeus Precious Metals GmbH & Co. KG) was spin coated on top of ITO coated glass substrates i n ambient air conditions and annealed at 140 o C for 20 minutes. The substrates were then transferred into a glove box filled with N 2 where the active layer was spin coated. Annealing was performed on a hot plate in the glove box after active layer deposit ion at 150 o C for 20 minutes. Thermal evaporation was used for the deposition of 1 nm LiF and 100 nm of Al at a pressure of 1 10 6 torr. Hole only devices with a structure of ITO/MoOx/active layer/MoOx/Ag were used for hole mobility measurements

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94 and e lectron only devices with a structure of ITO/ZnO/active layer/LiF/Al were used for electron mobilities measurements. 5.5.2 Device Characterization Current voltage characteristics were acquired using a Keithley 4200 semiconductor parameter analyzer along wi th a Newport Thermal Oriel 94021 10 00 W solar simulator, at 100 mW/ cm 2 incident power. EQE measurements were conducted using an in house setup consisting of a Xenon DC arc lamp, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, an SR 540 chopper system and an SR830 DSP lock in amplifier from SRS. The same lock in setup was used for both EQE and PSR measurements. All thicknesses of the active layers were determined using a Dektak surface profiler. For the temperature dependent measurements, liquid nitrogen was used for device cooling in an evacuated cryostat. For accurate temperature monitoring a silicon diode was attached to the back of the tested device. Transient PL measurements were performed using a TCSPC spectrometer (Picoquant, Inc.). A pulsed laser (375 nm) with an average power of 1 mW, operating at 40 MH z, with duration of 70 ps was used for excitation. PDS measurements were carried out using a standard setup consisting of 1 kW Xe arc lamp and a 1/4 m grating monochromator (Oriel) as the tunable light source. 44 The pump beam was modulated by a mechanical chopper at 13 Hz Perfluoroheaxane was used as the deflecting fluid. A Uniphase HeNe laser was used for probing. For phase sensitive measurements a lock in amplifier (Stanford Research, Model SR830) was used. All PDS spectra were normalized to the incident power of the pump beam.

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95 Figure 5 1 Chemical structure of the PCDTBT polymer. Figure 5 2 J V characteristics for PCDTBT:PC 70 BM devices with and without annealing.

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96 Figure 5 3 EQEs measured at 540 nm as a function of reverse bias for annealed and not annealed devices. Inset: EQEs for annealed and not annealed devices. Table 5 1. Average device characteristics for the OPVs fabricated in this study. PCDTBT:PC 70 BM Jsc [mA / cm 2 ] V OC [V] FF [%] PCE [%] Annealed ( ( As prepared ( Figure 5 4. AFM topography images of PCDTBT: PC 70 BM films A ) After thermal annealing. B ) B efore thermal annealing.

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97 Figure 5 5 Dielectric constants measure d at 1 KHz, 10 KHz and 100KHz for as prepared and annealed PCDTBT:PC 70 BM devices (Data from T. H. Lai) Figure 5 6 Sub bandgap EQE spectra for annealed and as prepared devices. The spectra confirm that more states below the bandgap are capable of exciton dissociation in the annealed device (Data from T. H. Lai)

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98 Figure 5 7. PL transients for as prepared and annealed PCDTBT:PC 70 BM films. The transients show a single exponential decay for the as prepared device and a bi exponential decay for the annealed device, indicating a fast exciton dissociation process in the annealed f ilms (Data from H. Y. Hsu )

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99 Figure 5 8 PL transients for as prepared and annealed PCDTBT films. A) As prepared and annealed PCDTBT film. B) Annealed PCDTBT film. The transients show a single exponential decay for both films indicating the same route of exciton dissociation (Data from H. Y. Hsu ) Table 5 2 Summary of mobilities and energeti c disorder for holes and electrons in a nnealed and a s prepared PCDTBT:PC 70 BM devices PCDTBT:PC 70 BM (cm 2 /Vs) h (meV) (cm 2 /Vs) e (meV) As prepared 9 9 ( 1.7) *10 4 2.6 ( 0.4) *10 3 Annealed 3. 0 ( 0.3) *10 3 9 .0 ( 0.7) *10 3 56.9 ( 2.5)

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100 Figure 5 9. Zero field mobilities vs. square of reciprocal temperature for PCDTBT:PC 70 BM annealed and as prepared devices. Energetic disorder can be determined by the slope. The high temperature limit can be determined by the y intercepts. Figure 5 10 J SC v s. I on log log scale fitted for bimolecular recombination Simialar slopes show that bimolecular recombination is not limiting.

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101 Figure 5 11. Voc v s. I fitted for recombination Greater slope for annealed devices reveals SRH recombination. Figure 5 12 PDS spectra for PCDTBT:PC 70 BM annealed and as prepared devices. Higher long wavelength absorption for the annealed films reveals the presence of traps (Data from S. H. Cheung ).

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102 CHAPTER 6 E FFECT OF S IDE C HAINS ON C HARGE G ENERATION AND D ISORDER IN POLYMER S OLAR CELLS 6.1 Introduction and Motivation As explained in Chapter 5 the nature of the polymer fullerene interaction is critical to the performance of organic photovoltaic devices. 163 It is widely accepted that the molecular arrangement at the polymer fullerene interface and the resulting interfacial energetics must be optimum in order for photogenerated excitons to be efficiently dissoc iated into free charge carriers. Even though the polymer fullerene interface plays a major role in exciton dissociation, 164 charge separation, 165 and charge recombination, 166 a clear strategy towards the optimization of this interface for the most favorable polymer fullerene interaction has not been established to date. In solution processed polymer solar cells, side chain engineering is considered to be critical to control the polymer solubility and achieve optimum device performance. 167,168 Some of the most high performing conjugated polymers are based on donor acceptor s ystems and have branched alkyl groups on the donor moieties and either no side chains or linear alkyl groups on the acceptor moieties, although notable exceptions excist. 169 171 In polymer fullerene systems, the donor acceptor conjugated polymer is responsible for most of the light absorption. Upon exciton photogen eration in donor acceptor conjugated polymers there is partial charge transfer within the polymer, with a higher electron density located on the acceptor moiety. In order for excitons to be fully dissociated, electron s within the polymer have to be transfe rred Reprinted with permission from Constantinou, I. et al. Effect of Polymer Side Chains on Charge Generation and Disorder in PBDTTPD Solar Cells. ACS Appl. Mater. Interfaces 7, 26999 27005 (2015). DOI: 10.1021/acsami.5b09497 Copyright 2015 American Chemical Society.

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103 to the fullerene. It is thus believed that adding bulky side chains to the acceptor moiety affects the polymer fullerene interaction through steric hindrance, keeping the fullerene away, and consequently decreasing the electronic coupling between the polymer chains and the fullerene molecules thereby limiting the charge transfer rate. 172 Bulky polymer side chains are also believed to affect the concentration of PCBM in the mixed polymer fullerene r egions which disrupts the long range connectivity and efficient charge transport. 173 Recently, it has been suggested that a preferred intermolecular arrangement exists in high performing polymer fullerene systems, where the fullerene molecule is docked on the acceptor moiety of the polymer for more efficient charge transfer It was proposed that OPV m aterials should be designed with the acceptor moiety of the polymer being more sterically accessible and the donor moiety more sterically hindered. In the case of poly(di(2 ethylhexyloxy)benzo[1,2 b:4,5 b']dithiophene co octylthieno[3,4 c]pyrrole 4,6 dione ) (PBDTTPD), it has been suggested that having a linear n octyl side chain on the acceptor moiety enables a stronger electronic coupling with fullerene. In contrast, a bulkier ethylhexyl side chain on the acceptor moiety results in a weaker interaction bet ween the polymer and the fullerene molecule. 172 In this work, we studied the effect of polymer side chains on the electronic properties of PBDTTPD:PC 71 BM blends and the resulting device performance. Our results show that going from a linear side chain on the polymer's acceptor moiety to a branched side chain has a negative impact on the overall device performance due to a significant reduction in J SC and FF. Sub bandgap EQE measurements as well as tra nsient PL measurements showed that the reduction in device performance was partially due to a less effective carrier generation process for the polymer with the branched side chain due to reduced CT state delocalization. Additionally, using

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104 light intensity dependent J SC measurements and temperature dependent mobility measurements we showed that the decrease in the device FF for the polymer with the branched side chain resulted from an increase in bimolecular recombination as well as an increase in energetic disorder. While the effects of CT states and chemical structure on device performance have been reported, a good understanding of the relationship between these parameters is still lacking. 21,174 In an effort to correlate the nature of CT states to the poly mer chemical structure and polymer fullerene interactions we studied the effect of different side chains on the performance of the PBDTTPD :PC 70 BM OPV devices. 6.2 Results and Discussion 6.2.1 Basic Solar Cell Device Characterization The molecular stru ctures for PBDT(EtHex) TPD(Oct) and PBDT(EtHex) TPD(EtHex) are presented in Figure 1. 175 177 As shown in the figure, the side chains on the electron donating benzodithiophene (BDT) moieti es are the same for both polymers whereas the side chains on the electron withdrawing thieno[3,4 c]pyrrole 4,6 dione (TPD) moieties change from a linear n octyl chain ( Figure 6 1A ) to a bulky ethylhexyl chain ( Figure 6 1B ). Throughout this report, devices made using PBDT(EtHex) TPD(Oct):PC 70 BM and devices made from PBDT(EtHex) TPD(EtHex):PC 70 BM will be denoted as Oct and EtHex respectively. The JV characteristics for devices incorporating the two polymer fullerene systems are depicted in Figure 6 2 As show n in the figure, going from the linear n octyl side chain to the more bulky ethylhexyl side chain resulted in a decrease in device performance as has previously been reported. 175 The decrease in device performance is caused primarily by a decrease in J SC for the EtHex devices, going from 11.1 mA/cm 2 to 3.9 mA/cm 2 A smaller decrease was also

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105 observed in the device FF, while the V OC increased from 890 mV to 980 mV. A summary of the average device characteristics for the devices fabricated in this study is given in Table 6 1 In order to understand the origin of the large reduction in J SC we measured the EQE spectra for the two syst ems. As expected, the EQE for the Oct devices was overall much higher than the EQE for the EtHex devices at all wavelengths. The EQE spectra for both devices are shown in Figure 6 3 The higher overall EQE for the Oct devices could be an indication of more favorable polymer fullerene interaction promoting exciton dissociation for the polymer with the linear side chain on the acceptor moiety. 177 6.2.2 Effect of Polymer Side Chains on Film Morphology In order to verify whether the decrease in photocurrent for the EtHex device is due to dramatic differences in the morphology of the blend, we examined the influence of the side chains on the morphological properties of the photoactive layer using AFM. Figure 6 4 shows the topographical images of PBDT(EtHex) TPD(Oct):PC 70 BM films ( Figure 6 4A ) and PBDT(EtHex) TPD(EtHex):PC 70 BM ( Figure 6 4B ) films. For both the Oct blend and the EtHex blend, the films appeared to be featureless and modera tely smooth, with a RMS roughness of 8 nm and 7 nm respectively. From the AFM results, it is clear that the difference in the side chain does not have a dramatic impact on film morphology with respect to phase separation and domain size. Since the differen ce in the side chain does not cause any obvious changes in the film morphology, we believe that the difference observed in device performance is most likely due to changes in the electronic structure of the blend as well as changes in polymer fullerene int eractions. 6.2.3 Effect of Polymer Side Chains on Charge Carrier Generation Sub bandgap EQE measurements were performed in order to understand the effect of the side chains on the polymer fullerene interactions and exciton dissociation for the two blends. By

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106 monitoring the response at energies below the band gap (<1.8 eV), we were able to compare the effectiveness of the CT states for the two devices based on the amount of carriers collected. 53,121 Figure 6 5 shows the photocurrent response for the CT exponential absorption tail (1.2 eV 1.75 eV). As shown in Figure 6 5 the sub bandgap EQE for the Oct device is higher than that of the EtHex device. The data indicate that for the Oct device, the CT states belo w the bandgap lead to more efficient CT exciton dissociation and polaron generation compared to the EtHex device. T his could be the result of more favorable polymer fullerene interaction due to reduced steric hindrance in the case of the n octyl side chain on the acceptor moiety. Transient photoluminescence PL measurements were carried out on both pristine PBDTTPD films and PBDTTPD:PC 70 BM films in order to understand the exciton dynamics and the side chain effects on the delocalization of the CT excitons. The fast decay component in transient PL can be explained to be due to a decrease in the number of CT excitons as a result of fast dissociation mediated by the CT states. The transient PL data for the PBDT(EtHex) TPD(Oct):PC 70 BM and PBDT(EtHex) TPD(EtHex): PC 70 BM blends are shown in Figure 6 6 The data for both samples show an initial fast decay component followed by a slower component. 178 For the Oct sample, the initial fast decay component accounts for 92% of the signal intensity and has a lifetime of 17419 ps. The slow decay component accounts for the remaining 8% of the signal and has a lifetime of 73328 ps. In the case of the EtHex blend, the initial fast decay component only accounts for 80% of the signal strength and has a lifetime of 21511 ps. The slow decay component accounts for the remaining 20% of the signal and has a lifetime o f 81122 ps. The fast decay for both the Oct and EtHex samples indicates that an additional dissociation route exists in the blend films, possibly originating from the CT states and thereby reducing the observed radiative recombination signal. It is import ant to note that the

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107 initial fast decay component present in the blend films is not present in any of the pristine polymer PL transients ( Figure 6 7 ). In the case of the Oct device, this fast decay component accounts for 92% of the excitons at the CT state s going through fast dissociation. I n contrast, 80% of the excitons go through fast dissociation in the samples made with the polymer with bulkier, branched ethylhexyl side chains. Even though the difference in the PL decays is subtle for the two polymers due to their identical backbone, the faster exciton dissociation rate for the Oct device suggests that the CT states are indeed more delocalized resulting in more efficient exciton dissociation. This is also in agreement with our EQE and sub bandgap EQE da ta. The difference in PL lifetimes for the two systems could be the result of less favorable polymer fullerene interaction and reduced CT state delocalization due to steric hindrance caused by th e bulky ethylhexyl side chain. 6.2. 4 Effect of Polymer Side Chains on Charge Carrier Recombination In addition to the reduced CT state delocalization for the EtHex device leading to a major difference in J SC a 16% reduction in FF was also observed for the EtHex device which could be the result of an increase in n on geminate recombination when going from linear side chains to bulkier side chains. 179 181 In order to identify whether and what type of non geminate recombination played a role in device performance, we investigated the dependence of the J SC on a range of illumination intensities from 1 sun down to 0.2 suns. As explained in Section 2.3.3 and based on Equation 2 11 for bimolecular recombination to be considered negligible t he slope should be very close to 1. Any deviation from unity implies that bimolecular recombination is a limiting factor for these devices. 66 Figure 6 8 shows the dependence of J SC on light intensity on a log log scale fitted using the power law described above. For the Oct device the slope was 0 .99 (R 2 =0.999) indicating that bimo lecular recombination is not a limiting factor for device performance On the other hand, the slope for the EtHex device was 0.95 (R 2 =0.999). While the

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108 difference is small, the results suggest that the device is more limited by bimolecular recombination. To further investigate the charge generation process in these devices we measured EQE under reverse bias, as shown in Figure 6 9 While the EQE under 535 nm illumination for the Oct device increased by 5% when the reverse bias was increased from 0 V to 4. 5 V, the EQE for the EtHex device increased by 7% for the same conditions. The slightly larger increase in EQE for the EtHex device further proves that recombination is slightly higher for that device. However, the EQE for the EtHex device remains much low er than the EQE for the Oct device even at high reverse bias. This is consistent with the sub bandgap EQE and transient PL data presented above, indicating that charge generation is significantly mo re efficient in the Oct device. 6 .2.5 Effect of Polymer S ide Chains on the Device Open Circuit Voltage The higher V OC for the EtHex device compared to the Oct device can also be partially explained using the bimolecular recombination data presented above It is well known that V OC originates from the Fermi energy level splitting triggered by illumination and is proportional to the carrier concentration within the device as shown in Equation 6 1 182 (6 1) where n and p are the electron and hole concentrations respectively, n i is the intrinsic carrier concentration, k is the Boltzmann constant and T is temperature. Thus, as the carrier concentration in a device increases, so does the V OC One way to show that the carrier concentration within the EtHex device is higher is through recombination. Langevin recombination is desc ribed in the following equation: 32

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109 (6 2) w here recombination prefactor and R is the recombination rate. We can assume TPD(Oct) since the mobility is higher (see below), while the data in Figure 6 8 indicates that the recombination rate is higher for PBDT(EtHex) TPD(EtHex). Both of these factors point toward a higher carrier concentration in the PBDT(EtHex) TPD(EtHex) device, leading to an increased V OC Further, CT band fits (Figure 6 10 ) show a 0.0 5 eV difference in the effective energy of the CT states when fitted with Equation 6 3 which could also be partially responsible for the difference in V OC 29 (6 3) Where k is the Boltzmann constant, f accounts for the internal quantum efficiency, number of CT states, and electronic coupling, E CT width of the CT band. The fit revealed an E CT of 1.51 eV for the EtHex device and an E CT of 1. 46 eV for the Oct device. 6.2.6 Effect of Polymer Side Chains on Stacking and Energetic Disorder Even though the impact of higher bimolecular recombination on the device performance is apparent, the impact of a change in the polymer side chains on bimolecular recombination is not intuitive. It is well known that in organic semiconductors the recombination rate decreases with improved charge transport. 124 We believe that the increase in bimolecular recombination for the EtHex device can be a result of a change in charge transport induced by the bulkier side chains It has been previously shown th at a change in the polymer side chains can result in a change in the stacking distance. 183 Using 2D grazing incidence X ray scattering (G IXS),

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110 changes from 3.6 for the n octyl side chains to 3.8 for the ethylhexyl side chains. 175 Since longer molecular stacking distances lead to a weaker inter chai n electronic coupling, we can expect a decrease in hole mobility for the EtHex material. 184,185 In addition, a broad er distribution the 175 T his structural disorder can be translated into energetic disorder in the polymer's transport band which would result in lower hole mobilit ies and higher energetic disorder. 152,153 I n order to investigate any changes in charge transport induced by the difference in the side chains, carrier transport for both devices was investigated using temperature dependent SCLC measurements. Zero 0,h were calculated as explained before. 101 Similar to Chapter 5 the resulting zero field mobilities were further analyzed using the Gaussian disorder model. 151 The temperature dependent zero field mo bilities were calculated using Equation 2 10 The GDM analyses for hole transport in the Oct and EtHex devices are shown in Figure 6 11 As it is obvious from the slope of the fit lines in Figure 6 11 the temperature dependence of the mobility is stronger for the EtHex device indicating a higher energetic disorder. A summary of the zero field mobilities at room temperature and energetic disorder parameters is presented in Table 6 2 As expected, t he zero field mobility at room temperature for the EtHex dev ice was one order of magnitude lower than that of the Oct device It is worth mentioning that the values for mobility measured for the purpose of this study are similar to what has previously been reported for the same system. 179 The decrease in mobility for the polymer with the bulkier side chains was also accompanied by an increase in the energetic disorder 81 2.2 meV for the EtHex devices and 65 1.8 meV for the Oct device

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111 Th e schematic shown in Figure 6 12 is used to summarize the impact of the bulkier side disorder, transport and recombination. Replacing the linear n octyl side chain on PBDTTPD (Figure 6 12A ) with a br a nched ethylhexyl side chain (Figure 6 12B ) causes an increase in the contributing to a reduced JSC for the EtHex device. Further, the increase in structural disorder has been shown to cause disorder in the polymer's transport band as revealed by the energetic disorder data. This can be pictured as energetic tails of shallow trapping states entering the effective gap, causing disorder and affecting recombination. 186 The band tail of the Gaussian distribution of the density of states for the HOMOs are also shown in Figure 6 12A and Figur e 6 12 B for the Oct and EtHex device respectively. Th e existence of an increased number of traps near the transport band was verified by the l ower mobilit y as well as the higher bimolecular recombination measured for the EtHex devices. 6.3 Summary and Conc lusions In conclusion, the effect of polymer side chains on device performance was investigated for PBDT(EtHex) TPD(Oct):PC 70 BM and PBDT(EtHex) TPD(EtHex):PC 70 BM BHJ solar cells. It was shown that going from a linear side chain on the polymer's acceptor mo iety to a branched side chain had a negative impact on the overall device efficiency due to a significant reduction in J SC and FF. Using sub bandgap EQE and transient PL measurements, we showed that the carrier generation process is more effective for the polymer with the linear side chain due to a higher degree of CT state delocalization leading to more efficient exciton dissociation and charge carrier generation. Further, the increase in stacking distance and disorder previously reported for the bulki er ethylhexyl side chain, were shown to result in a lower hole mobility, a higher bimolecular recombination and a higher energetic disorder. Our findings support what has

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112 previously been suggested about reduced polymer fullerene interaction for bulky polym er side chains. We have concluded that the use of linear side chains on the polymer's acceptor moiety can not only promote the photo generation process in organic BHJ OPVs due to more effective CT states but can also favor carrier transport resulting in im proved solar cell performance. 6.4 Experimental Section 6.4.1 Device Fabrication Active layer solutions were prepared by dissolving a 1:1.5 weight ratio of PBDT TPD (Sigma Aldrich) and PC 70 BM (Nano C) in CB:DIO (1 vol %) mixed solvent solution. The solutions were stirred overnight at 100 C. B ulk heterojunction OPVs were fabricated in the conventional architecture (glass/ITO/PEDOT:PSS/ PBDT TPD:PC 70 BM/LiF/Al). ITO coated glass substrates were cleaned in acetone and isopropanol and subsequently treate d under UV ozone for 15 min before the deposition of HTL. PEDOT:PSS PVP AL 4083 (Heraeus Precious Metals GmbH & Co. KG) was spin coated on top of ITO coated glass substrates in ambient air conditions and annealed at 140 o C for 20 minutes. The substrates w ere then transferred into a glove box filled with N 2 where the active layer was spin coated. Thermal evaporation was used for the deposition of 1 nm LiF and 100 nm of Al at a pressure of 1 10 6 torr. Hole only devices with a structure of ITO/MoOx/acti ve layer/MoOx/Ag were used for hole mobility measurements. 6.4.2 Device Characterization Current volt age characteristics were acquired using a Keithley 4200 semiconductor parameter analyzer along with a Newport Thermal Oriel 94021 1000 W solar simulator, at 100 mW/cm 2 incident power. EQE measurements were conducted using an in house setup consisting of a Xenon DC arc lamp, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, an SR 540 chopper system and an SR830 DSP lock in amplifier from SRS. L ong wavelength EQE

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113 measurements were made using the same setup, with additional 700 nm and 1000 nm long pass filters as needed. All thicknesses of the active layers were determined using a Dektak surface profiler. For the temperature dependent measurements liquid nitrogen was used for device cooling in an evacuated cryostat. Transient PL measurements were performed using a TCSPC spectrometer (Picoquant, Inc.). A pulsed laser (375 nm) with an average power of 1 mW, operating at 40 MHz, with duration of 70 p s was used for excitation. The emission was monitored using a 700 nm long pass filter.

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114 Figure 6 1 Chemical structures of the polymers used in this study. A ) PBDT(EtHex) TPD(Oct) B ) PBDT(EtHex) TPD(EtHex). Figure 6 2 J V characteristics for Oct and EtHex devices.

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115 Table 6 1. Summary of average device characteristics for the OPVs fabricated in this study. Polymer:PC 70 BM Jsc [mA/cm 2 ] Voc [V] FF [%] PCE [%] PBDT(EtHex) TPD(Oct) 11.10 ( 0.5 ) 0.89 ( 0.01 ) 60 ( 2.0 ) 5.9 0 ( 0.6 ) PBDT(EtHex) TPD(EtHex) 3.90 ( 0.6 ) 0.98 ( 0.01 ) 51 ( 3.0 ) 1.95 ( 0.4 ) Figure 6 3 External quantum efficiencies for Oct and EtHex devices. Figure 6 4 AF M topography images for polymer fullerene films, 1:1.5 weight ratio, spin co ated on ITO/PEDOT substrates. A ) Oct device films B ) EtHex device films.

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116 Figure 6 5 Sub bandgap EQE spectra for the Oct and EtHex devices. Figure 6 6 Transient PL decays for Oct and EtHex devices (Data from Subhadip Goswami )

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117 Figure 6 7 Transient PL decays for pristine polym er films. A) P ristine PBDT(EtHex) TPD(Oct). B ) PBDT(EtHex) TPD(EtHex) (Data from Subhadip Goswami )

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118 Figure 6 8. J SC vs I fitted for recombination for the EtHex and Oct devices. Figure 6 9 Reverse bias EQE at 535 nm wavelength for the EtHex device and the Oct device.

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119 Figure 6 10 Sub bandgap EQE data fitted for Equation 6 3 Figure 6 11 Zero field mobilities v s square of reciprocal temperature for EtHex and Oct devices. Energetic disorder can be d etermined by the slope.

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120 Table 6 2 Zero field mobility at room temperature and energetic disorder data for hole transport in the Oct and EtHex devices. Polymer:PC 70 BM o,h [cm 2 /Vs] at RT h [meV] PBDT(EtHex) TPD(Oct) 1.4 ( 0.6 ) 10 4 65 ( 1.8 ) PBDT(EtHex) TPD(EtHex) 1.6 ( 0.9 ) 10 5 81 ( 2.2 ) Figure 6 12 stacking dist ance, disorder and transport. A ) PBDT(EtHex) TPD(Oct):PC 70 BM B ) PBDT(EtHex) TPD(EtHex):PC 70 BM.

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121 CHAPTER 7 CONCLUSIONS AND FUTURE WORK 7.1 Summary The scope of this dissertation was the investigation of the effect of materials processing and structure on device performance and charge carrier kinetics in polymer solar c ells. A variety of electrical and optical measurements were used in order to study the effect of ambient air processing, extended light exposure, thermal annealing and polymer side chains on device stability as well as charge generation and recombination. The main goal of this dissertation was to identify materials properties and processes that lead to better device performance and long operational lifetimes. Chapters 3 and 4 were focused on the effect of processing on device stability. In Chapter 3 we show ed that processing dithienogermole based polymer fullerene devices in ambient air causes a fast initial degradation in device performance in as little as 10 minutes. Degradation was determined to be due to a decrease in hole mobility as a result of recombi nation and a decrease in devi ce absorption as a result of changes in the LiF interlayer Even though the initial device performance degradation was rapid, it was found that device performance plateaus after the first hour of air exposure. This study points out the importance of both air stable active layer materials as well as air stable interlayers. In order to establish a clear polymer design strategy for good air stability further studies are necessary. Additionally, a comprehensive study on the air stab ility of devices using a variety of interlayers is also necessary for longer operational lifetimes to be a possible. As a continuation of Chapter 3, Chapter 4 focuses on the impact of extended light exposure on device performance. It was found that 1 sun light exposure for up to 24 hour s can dramatically decrease device performance by negatively affecting all device parameters. Once

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122 again the primary reason for device degradation was found to be the degradation of the cathode interlayer along with an increase in bimolecular recombination UV light induced defects were identified in the ZnO interlayer and were found to create a carrier extraction barrier at the ITO/ZnO interface. Device degradation due to extended light exposure was found to be independent of the active layer materials for devices made on ZnO. Finally, ZnO photodegradation was shown to be significantly suppressed with the use of UV filters and device operational stability was considerably increased. The tradeoff to using U V filters was the reduced device photocurrent due to the fact that the light never reaches the device active layer. Once more, the poor photostability of the ZnO interlayer was found to be detrimental to the device performance. Similar to the previous chap ter, further investigation of the photostability of active layer materials and contact materials is necessary for longer OPV device lifetimes. The second part of this dissertation was focused on the effects of active layer processing and materials str ucture on device performance, charge carrier generation and charge carrier recombination. In Chapter 5 the effect of thermal annealing on device performance was investigated. It was shown that apart from the well established changes in active layer morpho logy induced by thermal annealing charge carrier dynamics can also be influenced. Despite the fact that the device performance was slightly lower, charge carrier generation was more efficient after thermal annealing. The more efficient carrier generation was shown to be due to more effective CT states after thermal annealing seen in sub bandgap EQE and transient PL measurements Even though thermal annealing improved charge carrier generation, the overall device performance was not improved due a decrease in FF. The decrease in the device FF was explained to be due to an increase in the concentration of deep traps in the active layer causing an increase in SRH recombination. In summary, our findings suggest that in addition to

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123 the macroscopic morphology ch anges induced by thermal annealing, the dielectric environment at the donor acceptor interface can also be manipulated with thermal annealing for the optimization of charge carrier generation in OPVs Finally, Chapter 6 was focused on the effect of the po lymer chemical structure on device performance as well as charge carrier generation and recombination. Two polymers with ident ical backbones but different side chains were investigated The polymer with the linear side chain on the acceptor moiety was foun d to have superior performance due to more efficient CT exciton dissociation and reduced structural disorder. The motivation behind this work was the identification of polymer structural characteristics that can positively impact device performance and therefore be used to propose a polymer design strategy. We found that bra nched side chains on the polymer acceptor moiety inhibit fullerene molecules from approaching and therefore hinder charge transfer and exciton dissociation. We also found that bra nche d side chains can cause structural disorder and increase the stacking distance, hindering charge transport. In order for universal polymer design rules to be derived, further studies comparing a larger number of polymers would be necessary. 7.2 Future Work Despite the low power conversion efficiencies and relatively short operational stabilities the field of o rganic photovoltaics applications such a wearables and automotives For the t echnology to reach its full potential, more research should go into improving the module scale solar cell device power conversion efficiency and stability A better understanding of fundamental device physics and degradation mechanisms is necessary for the realization of this goals, complimented by a universal polymer design strategy

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124 L IST OF REFERENCES 1. Dou, L. et al. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 25, 6642 6671 (2013). 2. Sirringhaus, H. 25th Anniversary Article: Organic Field Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 26, 1319 1335 (2014). 3. Peumans, P., Yakimov, A. & Forrest, S. R. Small molecular weight organic thin film photodetectors and solar cells. J. Appl. Phys. 104, 3693 3723 (2003). 4. Tang, C. W. & Vanslyke, S. a. Organic electroluminescent diodes. Appl. Phys. Lett. 51, 913 915 (1987). 5. Burroughes, J. H. et al. Light emitting diodes based on conjugated polymers. Nature 347, 539 541 (1990). 6. Torabi, S. et al. Strategy for Enhancing the Dielectric Constant of Organic Semiconductors Without Sacrificing Charge Carrier Mobility and Solubility. Adv. Funct. Mater. 25, 150 157 (2015). 7. Clarke, T. M. & Durrant, J. R. C harge photogeneration in organic solar cells. Chem. Rev. 110, 6736 6767 (2010). 8. Vissenberg, M. C. J. M. & Matters, M. Theory of the field effect mobility in amorphous organic transistors. Phys. Rev. B 57, 12964 12967 (1998). 9. Xue, J. Perspectives on O rganic Photovoltaics. Polym. Rev. 50, 411 419 (2010). 10. Cacialli, F. Organic semiconductors for the new millennium. Phil. Trans. R. Soc. Lond. A 358, 173 192 (2000). 11. Brdas, J. L., Calbert, J. P., da Silva Filho, D. a & Cornil, J. Organic semiconduct ors: a theoretical characterization of the basic parameters governing charge transport. Proc. Natl. Acad. Sci. U. S. A. 99, 5804 5809 (2002). 12. Brtting, W. Organic Semiconductors. Semiconductors 6, 1 11 (2005). 13. Yokoyama, D. Molecular orientation in small molecule organic light emitting diodes. J. Mater. Chem. 21, 19187 19202 (2011). 14. Adachi, C., Baldo, M. a., Forrest, S. R. & Thompson, M. E. High efficiency organic electrophosphorescent devices with tris(2 phenylpyridine)iridium doped into electron transporting materials. Appl. Phys. Lett. 77, 904 906 (2000). 15. Forster, T. 10th Spiers Memorial Lecture. Transfer mechanisms of electronic excitation. Discuss. Faraday Soc. 27, 7 17 (1959).

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138 BIOGRAPHICAL SKETCH Iordania Constantinou was born in Nicosia, Cyprus in 1989. She received her Bachelor of Science from the D epartment of Mechanical Engineering and Materials Science and Engineering at the Technical University of Cyprus in 2007. After being awarded a Fulbright scholarship, d engineering at the University of Florida. She joined Prof essor and graduated with her PhD in the spring of 2016