Tailor-Made Additives in Molecular Bulk-Heterojunction Organic Photovoltaics and Chromophore Spacer Control in Polymer L...

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Title:
Tailor-Made Additives in Molecular Bulk-Heterojunction Organic Photovoltaics and Chromophore Spacer Control in Polymer Light-Emitting Diodes
Physical Description:
Honors thesis
Creator:
Wieruszewski, Patrick M.
Publisher:
Department of Chemistry, University of Florida
Honors Program, University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:
Copyright Date:
2012

Subjects

Subjects / Keywords:
bulk-heterojunction
organic
photovoltaic
solar
morphology
light-emitting
oled
pled

Notes

Abstract:
Organic electronics are quickly growing in popularity with promising applications in transistors, light-emitting devices, and renewable solar energy (photovoltaics). Organic electronics are highly researched due to their solution-processability, use of non-toxic materials, and ability to produce thin, lightweight, and flexible devices. To compete with their inorganic counterparts in the consumer and industrial markets, however, power conversion efficiencies (PCEs) must be increased. In the first portion of this work, the effects of tailor-made additives in molecular bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are explored. A tailor-made additive is structurally similar to its parent molecule, with the exception of a minor defect; and when incorporated into BHJ OPVs, affords fine tuned morphologies and increased PCEs. In the second portion of this work, a new class of electroactive polyolefins for use in polymer light-emitting diodes (PLEDs) is explored. Through acyclic diene metathesis (ADMET) polymerization, polyolefins with precisely spaced chromophores were afforded. When incorporated into PLEDs, systematic control of external quantum efficiency (EQE) and hole mobility is achieved through varying the number of carbon atoms between chromophores on the electrically insulating backbone. Through the use of novel synthetic materials and innovative device design, performance in OPVs and PLEDs may be increased to meet industrialization requirements.
Additional Physical Form:
Document formatted into pages; contains 141 pages.
Creation/Production Credits:
Video of a P-9,9-TF-S pixel at a constant bias of 13 V.
Thesis:
Thesis (B.S.)—University of Florida, 2012.
Biographical:
Includes biographical references.
Issuing Body:
Adviser: Reynolds, John R.

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University of Florida Institutional Repository
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University of Florida
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All rights reserved by the source institution and holding location.
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AA00010693:00001

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1 T AILOR M ADE ADDITIVES IN MOLECULAR BULK HETEROJUNCTION ORGANIC PHOTOVOLTAICS AND CHROMOPHORE SPACER CONTROL IN POLYMER LIGHT EMITTING DIODES By PATRICK M. WIERUSZEWSKI A THESIS PRESENTED TO THE DEPARTMENT OF CHEMISTRY OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF BACHELOR OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Patrick M. Wieruszewski

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3 To my family and Megan.

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4 ACKNOWLEDGMENTS I would like to start off by thanking my family for their emotional and financial support throughout my undergraduate studies. dedication, I would not be where I am today. I am also very grateful for having a scholarly older sister. Graduating college with a dual undergraduate degree, she has always been there for me, providing tips to help me t hrough the tough times. I am very thankful for the continued support and encouragement from my girlfriend Megan. I am appreciative for her patience and helpfulness on nights I spent late in the lab. Her support has helped me stay on track mentally duri for anything more. I am very thankful of my advisor, Professor John Reynolds, for taking me on as an undergraduate, as I had just barely over a year of college experience under my belt It is his support and encourag ement that gave me an appreciation for research in the sciences, and has led to my success as a researcher. I also thank my mentor, Ken Graham, for teaching me theory and techniques regarding organic electronics and glovebox use. It was a pleasure having Ken as a colleague and a friend throughout this experience. I am also grateful for his continuous support while thousands of miles away at KAUST in Saudi Arabia. I would like to thank the entire Reynolds group for their support and help throughout my r esearch experience. They were very encouraging and overall a great group of people to be around. I thank Frank Arroyave for teaching me the freeze pump thaw process. I thank nsal Koldemir for the occasional delightful conversation s we shared I thank Emily Thompson for her assistance with obtaining high quality pictures

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5 and videos of illuminated pixels presented in the PLED section of this thesis I also I thank Dr. Kenneth Wagener for allowing me to collaborate with Brian Aitken to produce the PLED portion of this work. I also thank Brian for his support and suggestions throughout our collaboration. I am very grateful for Dr. Kirk Schanze allowing me to keep my office spac e and bringing me into his research group upon the departure of the Reynolds group from UF. I also thank Romain Stalder and Dr. Aubrey Dyer for their continued support as mentors in my research and thesis writing after Dr. Graham I a m very thankful for Dr. Ben Killian, Dr. Anna Brajter Toth, Dr. John Mitchell, and Dr. Jeff Gower for giving me the opportunity to serve as a teaching assistant for their courses. Teaching both lower division and upper division chemistry courses has h elpe d me grow as a professional. The experience I gained by instructing and managing courses is invaluable, and I am grateful for that. Finally, I would like to thank Dr. Michael Kessler of the Major Analytical Instrumentation Center, Department of Materials Science and Engineering for his patience and persistence in obtaining many of the TEM images presented in this work. I thank Cheryl Googins, Annyetta Douglas and Sara Klossner of the chemistry department for t heir help throughout my studies at UF.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURE S ................................ ................................ ................................ ........ 10 LIST OF OBJECTS ................................ ................................ ................................ ....... 15 LIST OF ABBREVIATIONS ................................ ................................ ........................... 16 ABSTRACT ................................ ................................ ................................ ................... 17 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 19 1.1 Or ganic Solid State Electronics ................................ ................................ ...... 19 1.1.1 Band structure Theory ................................ ................................ ......... 19 1.1.2 Electrical Conduction and Transport ................................ .................... 23 1.1.3 Semiconductor Physics ................................ ................................ ....... 25 1.1.4 Space Charge Limited Current ................................ ............................ 28 1.2 Organic Photovoltaics ................................ ................................ .................... 30 1.2.1 Background and Motivation ................................ ................................ 30 1.2.2 Organic Photovoltaic Devices ................................ .............................. 32 1.2.2.1 The Bulk Heterojunction ................................ ......................... 32 1.2.2.2 Device Operation ................................ ................................ .... 34 1.2.2.3 Quantification of OPV Parameters ................................ .......... 36 1.3 Organic Light Emitting Diodes ................................ ................................ ........ 39 1.3.1 Background and Motivation ................................ ................................ 39 1.3.2 Organic Light Emitting Devices ................................ ........................... 40 1.3.2.1 Materials ................................ ................................ ................. 40 1.3.2.2 De vice Operation ................................ ................................ .... 42 2 EXPERIMENTAL TECHNIQUES AND INSTRUMENTATION ................................ 44 2.1 Introduction ................................ ................................ ................................ .... 44 2.2 Material s & Purification ................................ ................................ ................... 44 2.2.1 Materials for Glovebox Use ................................ ................................ 44 2.2.1.1 Synthetic Materials for OPVs and PLEDs ............................... 44 2.2.1. 2 Freeze Pump Thawed Solvents ................................ .............. 45 2.2.1.3 Materials for Thermal Deposition ................................ ............ 46 2.2.2 Solvent Distillation ................................ ................................ ............... 47 2.2.3 Stir Bar Cleaning ................................ ................................ ................. 48 2.2.4 Glass Syringe Cleaning ................................ ................................ ....... 49

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7 2.3 Device Fabrication ................................ ................................ .......................... 50 2.3.1 Indium Tin Oxide Cleaning ................................ ................................ .. 50 2.3.2 Photovoltaic Devices ................................ ................................ ........... 52 2.3.3 Light Emitting Diode Devices ................................ ............................... 55 2.3.4 Space Charge Limited Current Devices ................................ .............. 55 2.3.4.1 Hole only Devices ................................ ................................ ... 55 2.3.4.2 Electron only Devices ................................ ............................. 56 2.4 Device Measurement ................................ ................................ ..................... 58 2.4.1 Organic Photovoltaics ................................ ................................ .......... 58 2.4.1.1 Solar Simulation ................................ ................................ ...... 58 2.4.1.2 Power Conversion Efficiency ................................ .................. 60 2.4.2 Polymer Light Emitting Diodes ................................ ............................ 61 2.4.2.1 Electroluminescence ................................ ............................... 63 2.4.2.2 Luminance ................................ ................................ .............. 64 2.4.2.3 PLED Performance ................................ ................................ 66 2.4.3 Charge carrier Mobility Devices ................................ ........................... 68 2.5 Morphology Characterization ................................ ................................ ......... 68 2.5.1 Atomic Force Microscopy ................................ ................................ .... 69 2.5.1.1 Tapping Mode ................................ ................................ ......... 69 2.5.1.2 Position dependent Morphology ................................ ............. 70 2.5.1.3 Thickness Measurements ................................ ....................... 74 2.5.2 Transmission Electron Microscopy ................................ ...................... 7 5 2.5.2.1 Sample Preparation ................................ ................................ 76 2.5.2.2 Top down Imaging ................................ ................................ .. 77 3 ASYMMETRIC AND SYMMETRIC OLIGOMERS AS TAILOR MADE ADDITIVES IN ORGANIC PHOTOVOLTAICS ................................ ....................... 78 3.1 Introduction ................................ ................................ ................................ .... 78 3.2 Isoindigo as an Electron Donor ................................ ................................ ...... 79 3.3 iI(TT) 2 i Bu 3 Si ................................ ................................ ................................ .. 80 3.3.1 Photovoltaic Performance ................................ ................................ ... 80 3.3.2 Charge Mobility ................................ ................................ .................... 82 3.3.3 Morphology ................................ ................................ .......................... 84 3.3.4 Crystal Growth ................................ ................................ ..................... 89 3.4 iI(TT i Bu 3 Si) 2 ................................ ................................ ................................ .. 90 3.4.1 Photovoltaic Performance ................................ ................................ ... 91 3.4.2 Charge Mobility ................................ ................................ .................... 92 3.4.3 Morphology ................................ ................................ .......................... 94 3.5 iI(TT) 2 H ................................ ................................ ................................ ......... 99 3.5.1 Photovoltaic Performance ................................ ................................ 100 3.5.2 Charge Mobility ................................ ................................ .................. 101 3.5.3 Morphology ................................ ................................ ........................ 103 3.6 Chapter Summary ................................ ................................ ........................ 105

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8 4 ELECT ROACTIVE POLYOLEFINS WITH PRECISELY CONTROLLED CHROMOPHORE SPACING ................................ ................................ ................ 108 4.1 Electroactive Polyolefins ................................ ................................ .............. 108 4.2 Perfectly Regioregular Terfluorenylidene Polymers ................................ ..... 110 4.2.1 Electroluminescence ................................ ................................ ......... 111 4.2.2 Luminance ................................ ................................ ......................... 112 4.2.3 PLED Performance ................................ ................................ ............ 114 4.2.4 Pixel Anomaly ................................ ................................ .................... 115 4.3 Sequ enced Triarylamine Functionalized Polyolefins ................................ .... 119 4.4 Chapter Summary ................................ ................................ ........................ 123 5 PERSPECTIVES AND OUTLOOK ................................ ................................ ....... 125 5.1 Bulk Heterojunction Organic Photovoltaics ................................ .................. 125 5.2 Polymer Light Emitting Diodes ................................ ................................ ..... 126 APPENDIX: DERIVATION OF THE FIELD INDEPENDENT AND DEPENDENT MOTT GURNEY LA W ................................ ................................ .......................... 128 LIST OF REFERENCES ................................ ................................ ............................. 131 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 141

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9 LIST OF TABLES Table page 3 1 Summary of OPV parameters for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ............................ 81 3 2 Summary of OPV parameters for increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ............................ 92 3 3 Summary of OPV parameters for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM so lar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ................................ ........... 101

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10 LIST OF FIGURES Figure page 1 1 Exaggerated diagram of energy states with increasing degrees of ................................ ................................ ................................ ................. 20 1 2 Band diagrams of a metal (a), insulator (b), and semiconductor (c). .................. 21 1 3 Band diagrams of a monovalent (a) and bivalent (b) metal. ............................... 22 1 4 Band diagrams of p type (a) and n type (b) semiconductors. ............................. 26 1 5 Energy band diagrams of a metal and n type semiconductor before (a) and after contact (b). ................................ ................................ ................................ 27 1 6 Work functions of materials used in this work ................................ ..................... 27 1 7 Band diagrams of electron only (a) and hole only (b) devices to employ the SCLC method. ................................ ................................ ................................ .... 30 1 8 An exaggerated view of a BHJ (a) and a schematic of a simple BHJ OPV device (b). ................................ ................................ ................................ ........... 32 1 9 Selected donor (a) and acceptor (b) units commonly used in organic and polymeric materials for OPVs. ................................ ................................ ............ 33 1 10 Various fullerene derivatives that have been used as acceptors in BHJ OPVs recently ................................ ................................ ................................ ............... 34 1 11 Mechanism of the conversion of photoenergy to a usable current in a photovoltaic device light absorption and formation of an exciton (a), exciton diffusion to the donor acceptor interface (b), charge transfer (c), charge separation and transport (d), charge collection at the electrodes (e), and the resulting usable current (f). ................................ ................................ ................. 36 1 12 Sample J V curve of illuminated and dark current, with all parameters of interest labeled for the calculation of the PCE. ................................ ................... 37 1 13 Schematic of a simple OLED device. ................................ ................................ 40 1 14 A variety of hole transport (a) and electron transport (b) layers found in th e literature. ................................ ................................ ................................ ............ 41 1 15 Mechanism of the emission of light from an applied bias in an OLED charge injection (a), charge transport (b), charge recombination and formation of an exciton (c), and the resulting emission of light (d). ................................ .............. 43

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11 2 1 Solvent distillation c olumn and cooling system setup. ................................ ........ 47 2 2 ITO pattern (a) and shadow mask pattern (b). ................................ .................... 51 2 3 SolidWorks model of the device holder used for thermal deposition. ................. 54 2 4 Schematic of patterned mask (a) and SolidWorks model of the evaporation device holder (b) used for anode deposition of electron only devices. ............... 57 2 5 Schematic of an electron only device. ................................ ................................ 58 2 6 Schematic of the air mass conditions used for photovoltaic measurements (a) and their standard spectra (b). ................................ ................................ ............ 60 2 7 SolidWorks schematic of the top (a,b) and bottom (c,d) of the PLED device holder. ................................ ................................ ................................ ................ 62 2 8 An illuminated PLED pixel under constant bias, viewed from the outside of the device holder. ................................ ................................ ............................... 63 2 9 Luminous flux leaving the source, labeled with the parameters used to calculate luminance. ................................ ................................ ........................... 64 2 10 Block diagram of the Konica Minolta CS 100 luminance meter objective lens (O), aperture stop (S 1 ), chopper (C), field stop (S 2 ), spectral response correction filter (F), silicon photocell (P), current to voltage con verter (I/V), and analog to digital converter (A/D). ................................ ................................ 65 2 11 CIE 1931 color space chromaticity diagram, showing a deep red color. ................... 66 2 12 Schematic of an AFM. ................................ ................................ ........................ 70 2 13 Structure of APFO DOT. ................................ ................................ .................... 71 2 14 AFM images of APFO DOT:PC 71 BM (1:4) in chlorobenzene at the center (a), between the center and the edge (b) and towards the edge (c) of the device. The device ITO and cathode layout with the labeled A, B, and C positions (d). 72 2 15 Mechanical process of spin coating clean device secured to the spin chuck by vacuum (a), solution deposited on device (b) and spin speed ramped up (c). ................................ ................................ ................................ ...................... 73 2 16 Use of SpmLabAnalysis software to calculate the stepheight for an AFM image containing a scratch (left) and the film (right) (a), height distribution (b), and stepheight calculation (c). ................................ ................................ ............ 75 2 17 Schematic of a TEM. ................................ ................................ .......................... 76

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12 2 18 Area of a device from which a TEM sample was prepared. ................................ 77 3 1 made ................................ ................................ ................................ ........ 78 3 2 Chemical structures of trans isoindigo (a) and iI(TT) 2 the parent oligomer used in the experiments presented in this work (b). ................................ ........... 79 3 3 Chemical structure of the asymmetric tailor made additive, iI(TT) 2 i Bu 3 Si. ......... 80 3 4 Representative J V curves of 0 to 100 % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ............................ 82 3 5 Summary of hole mobilities for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM SCLC devices. ................................ ................... 83 3 6 Representative J V plots of 0, 10, 20 (a), 30, 40 (b), 50 and 100 (c) mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 S i]:PC 61 BM SCLC devices. .............. 84 3 7 AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices. All images shown are 5 5 m. ................................ 86 3 8 AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si ]:PC 61 BM devices. All images shown are 1 1 m. ................................ 86 3 9 TEM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices. ................................ ................................ ..................... 87 3 10 Cross sectional TEM images of 20 (a) and 100 (b) mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices as well as a 3 nm Gaussian blur (c) applied to image b. ................................ ................................ 88 3 11 Crystal length dependence on mole % iI(TT) 2 i Bu 3 Si as a function of iI(TT) 2 (a) and representative polarized microscope i mages of increasing mole % iI(TT) 2 i Bu 3 Si in iI(TT) 2 :iI(TT) 2 i Bu 3 Si. ................................ ................................ 90 3 12 Chemical structure of the symmetric tailor made additive, iI(TT i Bu 3 Si) 2 ........... 91 3 13 Representative J V curves of 0 to 100 % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM s olar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ................................ ............. 92 3 14 Summary of hole mobilities for increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM SCLC devices. ................................ ................... 93 3 15 Representative J V plots of 0, 10, 20 (a), 30, 40 (b), 50 and 1 00 (c) mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM SCLC devices. ................ 94

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13 3 16 A FM images of increasing mole % symmet ric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices, with the typical color scale (a) and in gray scale (b). All images shown are 5 5 m. ................................ ................................ .. 96 3 17 A FM images of increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices. All images shown are 1 1 m. .............................. 97 3 18 TEM images of increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices. ................................ ................................ .................. 98 3 19 Chemical structure of an asymmetric tailor made additive, iI(TT) 2 H. ............... 100 3 20 Representative J V curves of 0 to 100 % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. ................................ ................................ ................................ ........... 101 3 21 Summary of hole mobilities for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM SCLC devices. ................................ ........................ 102 3 22 AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. All images shown are 5 5 m. ................................ ...... 104 3 23 AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. All images shown are 1 1 m. ................................ ...... 104 3 24 TEM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. ................................ ................................ .......................... 105 4 1 Typical electroactive polymer system structures conjugated polymers (a), main chain funtional polymers (b) and side chain functional polymers (c). 109 4 2 Family of electroactive terfluorenylidene polymers regioregular unsaturated (a), regioregular saturated (b), regiorandom unsaturated (c), and regiorando m saturated (d). ................................ ................................ ............... 111 4 3 Electroluminescence spectra of saturated polymers at a constant driving bias. ................................ ................................ ................................ .................. 112 4 4 Luminance and current density as a function of driving voltage for unsaturated (a) and saturated (b) polymers. ................................ .................... 113 4 5 Calculated EQEs for the entire family of terfluorenylidene polymers. ............... 115 4 6 AFM images of P 3,3 TF S (a), P 6,6 TF S (b), and P 9,9 TF S (c). ................ 116 4 7 Optica l microscope images of the aluminum electrode on devices with P 3,3 TF S (a), and P 9,9 TF S (b). ................................ ................................ ........... 116

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14 4 8 Time lapsed images taken every 2.5 s of a P 9,9 TF S pixel at a constant driving bias of 13 V. ................................ ................................ .......................... 117 4 9 Family of regioregular sequenced triarylamine functionalized polyolefins unsaturated (a) and saturated (b) hole transporters. ................................ ........ 120 4 10 Summary of average hole mobility data (a) and current voltage characteristics (b) for SCLC devices. ................................ ............................... 121 4 11 Representative J V plots of unsaturated (a) and saturated (b) TPF polymer containing SCLC devices. ................................ ................................ ................ 122 A 1 Schematic of an organic semiconducting material sandwiched by two electrodes, with an applied voltage (V) and diffusion length (L) of the current flow in the positive x direction. ................................ ................................ .......... 128

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15 LIST OF OBJECTS Object page 4 1 Video of a P 9,9 TF S pixel at a constant bias of 13 V ................................ ..... 117

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16 LIST OF ABBREVIATION S ADMET Acyclic diene metathesis AFM Atomic force microscopy BHJ Bulk heterojunctio n CB Conduction band EL Electroluminescence EQE External quantum efficiency FF Fill factor HOMO Highest occupied molecular orbital i Bu 3 Si Triisobutylsilyl ITO Indium tin oxide J V Current density voltage J SC Short circuit current density LUMO Lowest unoccupied molecular orbital OLED Organic light emitting diode OPV Organic photovoltaic PC 61 BM [6,6] phenyl C61 butyric acid methyl ester PCE Power conversion efficiency PEDOT:PSS Poly(3,4 ethylenedioxythiophene):poly(styrene sulfonic acid) PLED Polymer li ght emitting diode SCLC Space charge limited current TEM Transmission electron microscopy TPF bis(phenyl m toluylamino)biphenyl VB Valence band V OC Open circuit voltage

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17 Abstract of Thesis Presented to the Department of Chemistry of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Bachelor of Science TAILOR MADE ADDITIVES IN MOLECULAR BULK HETEROJUNCTION ORGANIC PHOTOVOLTAICS AND CHROMOPHORE SPACER CONTROL IN POLYMER LIGHT EMITTING DIODES By Patric k M. Wieruszewski May 2012 Chair: John R. Reynolds Major: Chemistry Organic electronics are quickly growing in popularity with promising applications in transistors, light emitting devices, and renewable solar energy (photovoltaics). Organic electronics are highly researched due to their solution processability, use of non toxic materials, and ability to produce thin, lightweight, and flexible devices. To compete with their inorganic counterparts in the consumer and industrial markets, howeve r, power conversion efficiencies (PCEs) must be increased. In the first portion of this work, the effects of tailor made additives in molecular bulk heterojunction (BHJ) organic photovoltaics (OPVs) are explored. A tailor made additive is structurally sim ilar to its parent molecule, with the exception of a minor defect; and when incorporated into BHJ OPVs, affords fine tuned morphologies and increased PCEs. In the second portion of this work, a new class of electroactive polyolefins for use in polymer lig ht emitting diodes (PLEDs) is explored. Through acyclic diene metathesis (ADMET) polymerization, polyolefins with precisely spaced chromophores were afforded. When incorporated into PLEDs, systematic control of external quantum efficiency (EQE) and hole mobility is achieved through varying the

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18 number of carbon atoms between chromophores on the electrically insulating backbone. Through the use of novel synthetic materials and innovative device design, performance in OPVs and PLEDs may be increased to meet industrialization requirements.

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19 CHAPTER 1 INTRODUCTION Today, electronics span a variety of applications ranging from industrial setti ngs to the consumer household. In 1821, Thomas Johann Seebeck discovered the thermoelectric effect when studying the temperature dependent thermopower of PbS and ZnSb. 1 This nigsberger and Weiss. 2 In 1947, the first working transistor was fabricated at AT&T Bell Telephone Laboratories; 3 thereafter giving rise to a booming interest in semiconducting devices. 1.1 Organic Solid State Electronics The focus of this work is the study of organic electronics, which is relatively new to the field of electronics. In 1977, Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa discovered and synthesized the first conducting polymer; f or which they received The Nobel Prize in Chemistry 2000. 4 interests in studying the electronic properties of organic materials. Since then, organic materials have been featured in a wide variety of electronic device applications electrochromic windows and displays, 5,6 field effect transistors, 7 10 thin film transistors, 11,12 light emitting diodes, 13 20 semiconductor memory devices, 21 photodetectors, 22 and photovoltaics. 23 28 1.1.1 Band structure Theory electrons along the conjugated double bonds of the system. In simple molecular orbital

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20 theory, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have distinct energy states. As the degree of conjugation in the system increases, the number of energy states increases and the energy gap between these states decreases With a conjugated polymeric system, or any organic system with a higher order of conjugation, these energy states begin to approach each other with 1). Figure 1 1. Exaggerated diagram of energy states with increasing degrees of In Figure 1 1, there is an energy difference between the HOMO and LUMO levels gap energy (E g ) in this work. In solid state physics, this gap energy can be used to define various classes of materials. If the gap energy is negligible (~ 0 eV), the material is referred to as a conductor (Figure 1 2a); these are typically metals. If the gap energy is rather large (> 3 eV), the material is referred to as an insulator (Figure 1 2b); these

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21 are typically ceramics. Finally, if the gap energy is somewhere between the above mentioned materials (< 3 eV), the material is referred to as a semiconductor (Figure 1 2c). These gap energy values are estimates and should not be interpreted as the limits for conductors, insulators, and semiconductors. Figure 1 2. Band diagrams of a metal (a), insulat or (b), and semiconductor (c). Here, CB is the conduction band, VB is the valence band, and E F is the Fermi energy. The black shaded regions show occupied energy states. The gap energy is also slightly dependent on temperature. The gap energy at a 29 (1 1) Where are empirically determined material constants. However, there were some complications with this calculation as is related to the Debye temperature, D which can sometimes be negative. 29 There have been computational analyses o f this problem to overcome it; 30 however, thorough examination of these calculations is beyond the scope of this work. It can be seen from Equation 1 1 that as temperature decreases, the gap energy increases, due to decr eased atomic vibrations. Having introduced the valence band (VB) and conduction band (CB) in Figures 1 1, and 1 2, it is appropriate to discuss them now. The valence band is essentially a

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22 ple, a spin up and spin down electron may occupy each available energy state. In a metal monovalent (Figure 1 3a), bivalent (Figure 1 3b), etc. the valence band is partially filled with electrons, giving rise to electrical conduction. One may ask at this bivalent metal have two valence electrons, thus a completely filled valence band and is consequently an insulator? In principle, this should be correct; however, bivalent metals have slightly overlapping bands of higher energy. Due to w eak binding interactions between the valence electrons and the nucleus, some electrons at the top of the lower energy band spill into the bottom of the higher energy band, as electrons tend to assume the lowest energy state possible (Figure 1 3b). Figu re 1 3. Band diagrams of a monovalent (a) and bivalent (b) metal. Here, s and p are atomic orbitals. The grey shaded regions show occupied energy states In an insulator, the valence band is completely filled with electrons, and thus does not electrical ly conduct. In an intrinsic semiconductor, the valence band is completely filled with electrons; however, the gap energy is small in these materials (E g < 3 eV). Thus, in the presence of an energetic excitation thermal for example sufficient energy may b the conduction band, leaving behind a positive charge in the valence band. This

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23 + for short. In the band diagrams of Figures 1 2 and 1 3, E F is the Fermi energy (also referred to as the chemical potential, ), which is extremely important in the discussion of semiconductors. The Fermi energy is the highest energy elec trons assume at absolute zero (T = 0 K). Furthermore, the probability at which an electron assumes a specific energy (E) can be calculated using Fermi Dirac statistics. This probability is called the F (E)), and is defined by, (1 2) Where k B is the Boltzmann constant, 8.6173 10 5 eV K 1 and T is the absolute temperature. Thus, if the band is completely fi lled with electrons, the probability of finding an electron at any given energy state, F (E), is equal to 1 ; and it follows that an empty band has F (E) = 0. This can be visualized using the band diagram presented for the intrinsic semiconductor in Figure 1 2. At absolute zero, the valence band is completely filled with electrons, thus F (E) = 1; similarly, the conduction band is empty, thus F (E) = 0. From Equation 1 2 it can be realized that when T 0 K, and E = E F then F (E) = which is a more useful definition of the Fermi energy than stated before. 1.1.2 Electrical Conduction and Transport Our discussion leading up to the semiconductor devices discussed in this work begins with a foundation in electrical conduction and transport. In semiconducting devices, we are interested in studying the movement of charge carriers, both electrons and holes. We begin with a few fundamental principles of electrical conduction.

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24 (1 3) Where I is the current flow measured in amperes (A), and R is the resistance of the material measured in ohms ( density (J), (1 4) Where 1 cm 1 and is commonly expressed as its reciprocal, the resistivity, and current density is the current flow per unit surface area, (1 5) The external electric field, E is defined by, (1 6) The current density can also be related to the velocity of carriers through the material, (1 7) is the elementary charge, 1.6022 10 19 C. This leads us to the carrier mobility, a highly desired term to characterize the performance of an electronic device. The carrier (1 8) d is the drift velocity of the carriers under the influence of an applied external electric field, E. Combining Equations 1 4, 1 7, and 1 8, we can relate the mobility of carriers to the condu ctivity, (1 9)

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25 Equation 1 9 allows us to calculate the mobility of carriers using intrinsic values. However, as mentioned in section 1.1.1, when sufficient energy is added to a semiconductor, an electron is excited into the conduction band lea ving a hole in the valence band. Since both electrons and holes contribute to electrical conduction, both have a drift velocity, and thus have different mobilities. We are interested in the mobility of both the electrons and holes, and thus Equation 1 9 is rewritten considering both charge carriers, (1 10) Where the subscripts e and h refer to electrons and holes, respectively. 1.1.3 Semiconductor Physics At this point, we should bring our attention back to the band diagrams, and describe typical semiconductors. In sections 1.1.1 and 1.1.2, we discussed intrinsic semiconductors, that is, materials with no impurities. However, extrinsic semiconductors are of more interest for devices. Let us consider silicon, which has a valence of IV, for the purposes of this discussion. If we add a nominal amount of a dopant boron for example, which has a valence of III we introduce an electron deficient impurity, as boron has one electron less than silicon. This results in an increased number of positive charges (holes). These types of dopants are referred to (Figure 1 4a). Contrarily, if we dope the silicon with a dopant of an extra electron phosphorous fo r example, which has a valence of V we introduce an electron rich impurity. These extra electrons are referred to as donors, and the resulting semiconductor is referred to 4b).

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26 Figure 1 4. Band diagrams of p type (a) and n type (b) semiconductors. With semiconducting devices, we want to make metal semiconductor contacts. The following discussion about contacts is important for the selection of metals and materials when fabricating semiconducting devices. Let us cons ider a metal semiconductor contact with an n type material. Prior to contact, the Fermi energy of the n type semiconductor is larger than the Fermi energy of the metal (Figure 1 5a). Here, we have introduced a new term, nction is defined as the amount of energy required to remove an electron from the Fermi surface into vacuum. Once the metal and semiconductor are brought into contact and an external electric field is applied, electrons are driven from the semiconductor i nto the metal until also referred to as the depletion region, which has a low charge carrier density that is observed, resulting in an energy barrier (Figure 1 5b). As mentioned, work functions of materials are important for proper selection of materials to fabricate a quality electronic device. Figure 1 6 summarizes work functions of materials that were used in this work.

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27 Figure 1 5. Energy band diagrams of a metal and n type semiconductor before (a) and after contact (b). Here, is the work function of a metal (m) and semi conductor (s), is the electron Black shaded regions shown occupied states. Figure 1 6. Work functions of materials used in this work 1 nm lithium fluoride on aluminum, 31 calcium, 32 aluminum, 31 indium tin oxide (ITO), 33 gold, 34 poly(3,4 ethylenedioxythiophene:poly(styrene sulfonic acid) (PEDOT:PSS), 35 and molybdenum (VI) oxide. 36

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28 1.1.4 Space Charge Limited Current In section 1.1.2 we defined an expression for the conductivity allowing us to quantify electron and hole mobilities in a semiconducting device (Equation 1 10). Many method s exist to quantify mobilities in organic devices use of field effect transistors, time of flight measurements, and the space charge limited current (SCLC) method. It is generally accepted that the SCLC method serves as an accurate technique to measure mo bilities of charge carriers in organic semiconducting devices. This is because organic systems, unlike metals, are disordered systems in the sense that they are composed of many intertwined/packed molecules. Thus charge carriers in an organic system tran sport within a molecule along delocalized conjugation, but must also Consequently, organic systems suffer from lower conductivities and thus mobilities compared to classic meta l conductors. Luckily, organic materials satisfy crucial requirements to employ the SCLC method low charge carrier mobilities, and trap free charge transport. In the absence of trap states, the space charge limited current (J SCL ) is defined by the Mott Gurney law, 37 (1 11) 0 is the vacuum permittivity, 8.8542 10 12 F m 1 r is the field independent mobility, L is the length, or in the case of organic devices, the thickness of the film, and V i is th e dropped bias across the device: (1 12)

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29 Where V is the applied bias, V rs is the dropped bias resulting from series resistance of the ohmic contacts, and V bi depletion region (see section 1.1.3 and Figure 1 5b). In organic materials, however, shallow traps are often present, thus the mobility is expected to be field dependent, (1 13) 0 is the zero dependent mobility described by the Poole Frenkel effect can be calculated using the modified Mott Gurney law, 38 (1 14) Derivations of the field independent and dependent SCLC mo dels can be viewed in detail in the appendix Now, a very important complication arises when considering th e SCLC method. The field dependent Mott Gurney law described by Equation 1 14 is useful for measuring the zero field mobility of a single charge carrier and is widely accepted as the method of choice for organic semiconducting devices. 39 43 However, as discussed in section 1.1.1, there are two charge carriers in organic materials which exhibit a mobility electrons and holes. Thus, in order to accurately quantify the charge carrier mobility of electrons or holes, the device must be fabricated to isolate one of the charge carriers. To achieve a device of only electron or hole carriers, metals must be selected with specific work functions to make ohmic contacts in order to inject only one charge carrier t o satisfy the SCLC model. To measure only the mobility of electrons, two low work

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30 function metals must be chosen for the electrodes (Figure 1 7a). This ensures that no holes are injected or collected by either electrode. Similarly, to measure only the m obility of holes, two high work function metals must be used as the electrodes (Figure 1 7b). Accordingly, no electrons are injected or collected by the high work function metals. Figure 1 7. Band diagrams of electron only (a) and hole only (b) devices to employ the SCLC method. 1.2 Organic Photovoltaics Now that we have established the basic principles of electrical properties of materials, conduction, and transport, we can begin our discussion of semiconducting devices, starting with organic p hotovoltaics (OPVs). 1.2.1 Background and Motivation As discussed in the section 1.1, electronics are the driving forces for the day to every day, bringing new gadgets to the consumer and industrial markets. However, with these increasing amounts of technological advances and increased usage of these

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31 gadgets, energy demands to power these devices are higher than ever, and are continuing to rise. Currently, fossil fue ls dominate the energy production market in the world. These mainly petroleum atmosphere. It is estimated that by 2050, the world demand of energy will be approximately 30 terawatts, making the e nergy market a 30 trillion dollar industry. 44 It is estimated that the United States crude oil production will rise over 20% in the next decade. 45 Furthermore, the Energy Information Administration (EIA) e stimates energy related carbon dioxide emissions to increase 3% by 2035. 45 Given such a high demand in energy, renewable sources are being investigated to limit the amount of harmful atmospheric emissions produced by petroleum based materials. T o minimize these numbers, research efforts towards renewable solar energy are increasing, giving rise to the motivation for this work. Currently, the highest performing photovoltaic cells consist of inorganic based materials. Solar Junction currently hold s the world record for PV efficiency of a silicon based cell at 43.5% under concentrated 1,000 suns radiation. 46 Inorganic cells are however brittle, expensive to manufacture, and use toxic materials during fabrication. Organic solar cells, on the other hand, are of interest in thi s work. These cells are relatively inexpensive to produce and use non toxic materials in fabrication. Additionally, organic solar cells are solution processable, and with recent technologies, organic layers can be deposited on flexible substrates through slot die coating, 47 49 ink jet printing, 50 53 and flexographic printing. 54 57 These processes allow for lightweight, flexible cells that are not restricted to a solar field or roof top as are inorganic cells. To

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32 compete with their inorganic counterparts in the consumer market, however, power conversion efficiencies (PCEs) must be increased. Currently, organic solar cells are shy of the 10% mark thought to be required for industrialization ; however, s olar cells based on polymeric 58 60 and small molecular 61,62 systems have reached PCEs of 8 and 6%, respectively. 1.2.2 Organic Photovoltaic Devices A simple OPV has a vertical architecture consisting of a conductive organic layer sandwiched between two electrodes on top o f a transparent substrate (Figure 1 8b). The organic layer is a mixture of an electron donor (will be referred to as donor), and an electron acceptor (will be referred to as acceptor), which is termed as a bulk heterojunction (BHJ, Figure 1 8a). Figur e 1 8. An exaggerated view of a BHJ (a) and a schematic of a simple BHJ OPV device (b). 1.2.2.1 The Bulk Heterojunction The number of approaches to designing molecules for use in OPVs is endless. Over the past several years, a plethora of donors and acce ptors have been used in both

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33 small molecular and polymeric solar cells. Some popular donor and acceptor entities are summarized in Figures 1 9a and 1 9b, respectively. This summary of electron rich and electron deficient entities should not be interprete d as complete, but rather a sample of those one can find in the literature. Figure 1 9. Selected donor (a) and acceptor (b) units commonly used in organic and polymeric materials for OPVs. The donor and acceptor units presented in Figure 1 9 are used t o design molecules for high performing OPVs. As mentioned above, the organic layer is a mixture of donor and acceptor (BHJ). The materials presented in Figure 1 9 are typically used to synthesize a donor molecule for a BHJ.

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34 The acceptor in a BHJ is typ ically a fullerene derivative, most notably PC 61 BM 63 or PC 71 BM 23 (Figure 1 10). These fullerene derivatives have been used in high performance BHJ OPVs and can be purchased with high purity. However, recently many groups have been investigating other fullerene derivatives as acceptors in OPVs dihydronaphthyl, 24,64 bis adduct, 65,66 cyclopropano, 67 and 1,4 fullerene derivatives 68 (Figure 1 10). Figure 1 10. Various fullerene derivatives that have been used as acceptors in BHJ OPVs recently PC 61 BM ; 63 PC 71 BM ; 23 and dihydronaphthyl, 24,64 bis adduct, 65,66 cyclopropano, 67 and 1,4 fullerene derivatives. 68 1.2.2.2 Device Operation The discussion of electrical properties, transport, and choice of materials for use in optoelectronics brings us to the operation of an OPV device. As shown in Figure 1 8b, a simple OPV device consists of an organic layer sandwiched between two electrodes.

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35 The bottom electrode (cathode) must be transparent and lie on a transparent s ubstrate for adequate photon absorption from the sun s light. The choice of materials to compose the organic layer is crucial for device performance. The metal electrodes can be precisely chosen to obtain specific energy levels. The organic layer is a b it trickier, as the energy levels depend on the HOMO and LUMO levels of the donor and acceptor in the BHJ, respectively. These HOMO and LUMO levels play a crucial role in charge transport in the operation of a photovoltaic device. The mechanism for the operation of an OPV device and the generation of a current from photoenergy is summarized in Figure 1 11. First, light absorption of a photon causes excitation of an electron in the donor, promoting it from the HOMO to the LUMO (Figure 1 11a). The excita tion of an electron leaves a hole in the HOMO, and this bound electron donor acceptor interface occurs (Figure 1 11b). Exciton lifetime is about 5 10 nm, thus crucial choice of donor and acceptor is necessary for fabricating devices with ideal mixing thermodynamics and thus morphologies in the solid state. 69,70 Once the exciton is at th e donor acceptor interface, charge transfer occurs, with an electron transferred from the LUMO of the donor to the LUMO of the acceptor (Figure 1 11c). Next, the charges separate and transport to opposite ends of the device (Figure 1 11d). Finally, the c harge collection of the holes and electrons occurs at the cathode and anode, respectively (Figure 1 11e), resulting in a current (Figure 1 11f).

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36 Figure 1 11. Mechanism of the conversion of photoenergy to a usable current in a photovoltaic device light a bsorption and formation of an exciton (a), exciton diffusion to the donor acceptor interface (b), charge transfer (c), charge separation and transport (d), charge collection at the electrodes (e), and the resulting usable current (f). 1.2.2.3 Quantification of OPV Parameters In order for OPVs to compete with their inorganic counterparts in the industrial and consumer markets, PCEs must be increased. As mentioned in section 1.2.2.2, the conversion of photoenergy into a usable current is a mecha nism of the electrons and holes in the BHJ. Essentially, an OPV is exposed to a simulated light source mimicking the photon flux of the sun s radiation. Next, a bias is applied and the current passing through the device is measured. This process is disc ussed in greater depth in chapter 2. Once a pixel is measured, we can plot a current density voltage curve to quantify the PCE. A sample current density voltage curve is shown in Figure 1 12, with the parameters of interest labeled on the curve for the p urposes of this discussion.

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37 Figure 1 12. Sample J V curve of illuminated and dark current, with all parameters of interest labeled for the calculation of the PCE in Equation 1 19. We begin the quantification of the PCE by referring back to the mechani sm by which an OPV device operates (Figure 1 11). In Figure 1 11d, the charges separate and the holes travel to the cathode and the electrons travel to the anode. The energy difference between the two is estimated as the energy difference between the HOM O of the donor and the LUMO of the acceptor, and is what contributes to the photovoltage or open circuit voltage (V OC ). Next, in Figure 1 11e, charge collection occurs at the electrodes. The density of holes and electrons collected contributes to the pho tocurrent, which accordingly is the product of the current density and voltage at any given point on the curve. Thus, the maximum power generated (P m ) must be the product of the maximum voltage and maximum current density, (1 15) This brings us to the fill factor (FF), which is the ratio of the actual maximum power to the theoretical maximum power. The theoretical maximum power (P th ) is the product

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38 of the short circuit current density (J SC ) and the open circuit voltage (V OC ), where the short circuit current density is defined as (1 16) 34 J s, c is the speed of light, 2.9979 10 8 m s 1 q is the electronic charge, P 1.5G is the power of the AM1.5G filtered simulated light source, which is typically used in a laboratory setting to test PV devices (see section 2.4.1.1), the external quantum efficiency (EQE) is the ratio of the collected electrons contributing to the generat ed current to the incident photons of the power source, and can be calculated by (1 17) the ratio of the power produced (P out ) by the cell to the incident power (P in ): (1 18) Combining Equations 1 15, 1 17 and 1 18, we arrive at a useful expression for the calculation of the PCE of an OPV device using easily obtainable par ameters from the characteristic J V curve (Figure 1 12): (1 19) To increase PCE, many optimization efforts have been made to maximize FF, J SC and V OC in devices. However, as with the majority of science, there is a trade off w hen increasing a parameter, resulting in increased PCEs being more difficult to obtain.

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39 1.3 Organic Light Emitting Diodes To continue the discussion of organic electronic devices, we will shift gears towards the other portion of this work, organic light em itting diodes (OLEDs). 1.3.1 Background and Motivation Lighting displays have been playing crucial roles in communication and widely used today liquid crystal display s (LCD), 71 plasma display panels (PDP), 72 cathode ray tubes (CRT), 73 digital light processing projectors (DLP), 74 and light emitting diode dis plays (LED). 75 Many of these technologies, however, have disadvantages to name a few PDPs produce copious amounts of heat, DLPs are difficult to view in well lit areas, CRTs are bulky and heavy, and LC Ds have limited viewing angles. Recently, LEDs have been making more and more appearances in the consumer market in the form of TVs, computer monitors, and large area displays. LEDs are advantageous as they may be used to fabricate thin and consequently lightweight displays. The focus of this work is the study of OLEDs, a class of light emitting diodes based upon organic materials. The realization of electroluminescence the phenomenon of light emission under an induced electric field in organic materi als was by Bernanose 1953 when studying gonacrin, acridine orange, and carbazole. 76 This work is what initially gave rise to the fabrication of multilayer OLEDs studied today. 77,78 Displays based on OLEDs are exciting compared to their count erparts discussed above as they may have lower power consumption, materials can span the entire visible spectrum, thinner and lightweight displays are possible, and they have a larger viewing angle.

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40 1.3.2 Organic Light Emitting Devices A basic OLED has a s imple vertical architecture consisting of an electron transport emissive layer and a hole transport layer sandwiched between two metal electrodes on a transparent substrate (Figure 1 13). To obtain full light emission, the bottom electrode must be a trans parent metal oxide, typically ITO. Multilayer structures are highly researched, however are not the focus of this work. Figure 1 13. Schematic of a simple OLED device. 1.3.2.1 Materials Choice of electroluminescent organic materials for use in LEDs is crucial for fabricating bright, high performing devices. Most organic materials used for LEDs are conjugated systems. As will be discussed in the proceeding section, balanced electron an d hole mobilities must be achieved for proper charge recombination and light emission. Some selected hole transport and electron transport layers that can be found in the literature are summarized in Figures 1 14a and 1 14b, respectively.

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41 Figure 1 14. A variety of hole transport (a) CuPc, 79 TPD, 80 NPB, 81 PEDOT: P SS, 15 and electron transport (b) Alq 3 82 TAZ, 83 layers found in the literature. It can be seen from Figure 1 14a that aromatic amines are excellent choices for hole transporting materials. In chapter 4 we will explore a new class of aromatic amines as hole transporting layers for controllable charge mobility. The list of hole and electron transporters presented in Figure 1 14 should not be interpreted as complete, as new molecules and derivatives of existing transporters are investig ated daily. The possibilities for hole and electron transport materials are endless, perhaps the reason why the field of OLEDs is so exciting.

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42 1.3.2.2 Device Operation The operation of a basic OLED is summarized in Figure 1 15. First, a bias is applied injecting electrons and holes at the cathode and anode, respectively (Figure 1 15a). Choice of electrodes is crucial for efficient charge injection. A low work function metal 1 nm lithium fluoride on 100 nm aluminum was used in this work is needed for the cathode to inject electrons. For the anode, a high work function electrode typically ITO is needed to inject holes. ITO was used in this work as it is widely used and known as a good hole injector and it can be purchased pre patterned on substrates r elatively cheap. 33,84,85 However, any other transparent metal oxide will do just fine as long as the work function is properly lined up with the HOMO of the emissive layer for proper hole injection. Once in the emissive layer, c harge transport occurs (Figure 1 15b), followed by charge recombination to form an exciton (Figure 1 15c), resulting in light emission (Figure 1 15d). When testing OLEDs in a laboratory setting, many parameters are of interest electroluminescence, luminance, and EQE. These parameters and their calculations are discussed in great detail in section 2.4.2.

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43 Figure 1 15. Mechanism of the emission of light from an applied bias in an OLED charge injection (a), charge transport (b), charge recombination and formation of an exciton (c), and the resulting emission of light (d).

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44 CHAPTER 2 EXPERIMENTAL TECHNIQ UES AND INSTRUMENTAT ION 2.1 Introduction Voids and impurities can be detrimental to electronic device performance and stability. Careful and sterile processing techniques and materials are crucial to fabricating quality devices. This chapter will discuss in detail the materials and methods used in device fabrication and characterization. All procedures discussed in this chapter were carried o ut in the Materials Chemistry Characterization Laboratory at the University of Florida, unless specifically noted otherwise. 2.2 Materials & Purification 2.2.1 Materials for Glovebox Use Much of the device fabrication and characterization process took plac e in a custom built MBraun glovebox (project # 05 245) containing an Argon (Airgas South, cat. # ARHP 300) atmosphere. Oxygen and water levels in the glovebox were maintained at <0.1 ppm throughout experimentation. The glovebox catalyst used for removal of residual oxygen and water was regenerated approximately every three months using nitrogen gas balanced with 4% hydrogen (Airgas South, cat. # NI HY4200). 2.2.1.1 Synthetic Materials for OPVs and PLEDs For photovoltaics, the modified fullerene derivative used as an electron acceptor, [6,6] phenyl C 61 butyric acid methyl ester (PC 61 BM), was purchased from nano C (>99.5% purity, cat. # Nano PCBM BF). Isoindigo based oligomers u sed as electron donors in OPVs iI(TT) 2 and asymmetric derivatives were synthesiz ed by Romain 86 Brian Aitken of the Wagener Group synthesiz ed all polymers used to prepare PLEDs in this work. 87,88 Both small molecules

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45 and polymers were synthesized in the George and Josephine Butler Polymer Research Laboratories at the University of Florida. Once purified and characterized, small molecule organics and polymers were immediately introduced into the Ar glovebox, where they were stored until further use in device fabrication. Polyfluorene based polymer containing vials were wrapped in aluminum foil to prevent photo oxidation and the formation of fluorenon e defects. 18 17 2.2.1.2 Freeze Pump Thawed Solvents All devices OPVs and PLEDs presented in this work, were solution processed from chlorobenzene. To ensure there was no dissolved water or oxygen in prepared solutions, chlorobenzene was purchased anyhydrous and freeze pump thawed to remove residual oxygen content. Freeze pump thaw procedures were performed in a synthetic lab part of the UF Butler Polymer Research Laboratories, with the assistance of Frank Arroyave. Anhydrous chlorobenzene was purchased from Sigma Aldrich in 1L sure sealed bottles (99.8%, cat. # 284513). To freeze pump thaw, a Schlenk flask was rinsed sequentially with toluene, acetone, and isopropanol; and baked in an oven overnight. A cannula (stainless steel tube and needle) was used to transfer chlorobenzene from the sure sealed bottle to the Schlenk flask to e nsure the solvent was not exposed to ambient air. Such removal of chlorobenzene was only performed twice per purchased bottle to ensure the sure seal remained effective in preventing exposure to atmosphere. The Schlenk flask was never filled over half of its volume, as this may cause the flask to shatter during the freeze pump thaw process. Once the flask was filled with solvent, any residual oxygen in the flask was removed to prevent condensing liquid oxygen during the freeze pump thaw process. The fla sk was pulled under vacuum and a liquid nitrogen bath was used to freeze the solvent. Once frozen,

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46 the solid thawed under vacuum using a lukewarm water bath. Thawing of the solvent resulted in deoxygenation, which was visually noticed by bubbling. This freeze thaw cycle was performed approximately five times, or until the aforementioned bubbling ceased. After performing the freeze thaw cycles and allowing the solvent to return to a manageable temperature, the Schlenk was backfilled with Ar and transferr ed into the Ar glovebox. The solvent was then transferred into smaller amber storage bottles, which were cleaned using the same procedure as the Schlenk flask described above. Caps of the amber bottles were sealed with Teflon tape and stored until furthe r use. 2.2.1.3 Materials for Thermal Deposition Tungsten evaporator boats coated in Alumina were purchased from R.D. Mathis (cat. # S35B AO W). These evaporator boats allow for thermal evaporation up to ca. 1200 o C, which is sufficient for all of the mat erials used in this work. Gold was purchased from a local supplier (National Coins, 2007 NW 43 rd St., Gainesville, FL 32605, ph: 352 378 3983) in the form of coins (Candian Maple Leaf, 99.99% purity). achine shop cut the coins into ca. 2 mm pieces. Gold pieces were sequentially sonicated in hexanes, toluene, acetone, and 2 propanol; and were allowed to thoroughly dry prior to introduction to the Ar glovebox. Aluminum slugs, ca. 8.0 mm in length (99.99 %, cat. # 40417) and redistilled calcium shots, <1 cm size (99.5%, cat. # 10127) were purchased from Alfa Aesar. Lithium fluoride powder (97%, cat. # 21827) was purchased from Acros Organics. Molybdenum (VI) oxide powder (99.99%, cat. # 203815) was purch ased from Sigma Aldrich. All thermal evaporation materials were used as received without any further modifications, with the exception of gold.

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47 2.2.2 Solvent Distillation With an increasing stress being put on environmentally friendly chemistry, solvents were distilled to recycle their use. Since electronic device fabrication must be rid of all impurities, such solvent distillation was only performed for acetone and 2 propanol used in the cleaning of ITO coated substrates. This section will discuss the distillation process of selected solvents, which will be explicitly recognized in the Indium Tin Oxide Cleaning section of this chapter. Figure 2 1. Solvent distillation column and cooling system setup. To distill solvents, a 5 liter round bottom flask with a handful of boiling chips was Cleaning section). A heating mantle (Glas Col LLC.) coupled with a POWERSTAT Variable Transformer (The Superior Electric Co.) was set to a setting of 60 for acetone (ca. 56 o C) or to 90 for 2 propanol (ca. 83 o C). To cool the system, an external water

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48 source is needed. To limit the amount of wastewater, which can be quite significant as t ypical distillations run upwards of four hours, water was recycled. A 5 gallon cooler was filled with tap water, which was treated with Chloramine T Algicide (Ted Pella Inc, cat. # 18625) to prevent algal growth. A small fish pump used for commercial aqu ariums was connected to the intake tubing on the column and was submerged in the external cooler. The outtake tubing on the column was redirected into the cooler to allow for the water to infinitely cycle through the system. The distillation setup is sho wn in Figure 2 1. Once a reflux was obtained, the first approximate 20 mL of solvent was discarded to remove lower boiling point impurities. Distilled solvent was collected in 4 liter amber storage bottles, which were explicitly labeled. Distillation wa s ceased when approximately 50 mL solvent remained in the round bottom flask to leave behind any solid or higher boiling point impurities. It should be stressed that solvents used for distillation came only from ITO cleaning procedures, and no other sourc es. It is not recommended to recycle solvents from other laboratory usages for use in electronic devices. 2.2.3 Stir Bar Cleaning Due to high cost and to limit waste, stir bars used for solution preparation were recycled, but nonetheless thoroughly clean ed. After being used in solution preparation, stir bars were immediately rinsed with toluene until visibly clean of residual organics. After rinsing, the stir bars were stored in an amber storage bottle filled with chloroform. Once an adequate number of stir bars was collected in the bottle, the chloroform was discarded and stir bars were rinsed again with chloroform, which was then discarded. The bottle was filled with chlorobenzene, heated to 140 o C, and stirred rigorously for two hours. The solvent was allowed to cool prior to being discarded. Once cool, the stir

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49 bars were rinsed twice, sequentially in chloroform, toluene and 2 propanol. The stir bars were laid out on KimWipes and allowed to thoroughly dry for several hours to ensure evaporation of residual 2 propanol prior to introduction back into the Ar glovebox. Through conducting this work and reusing stir bars it was observed that they do not have an infinite lifetime. After extended use, it was realized that the magnetic material inside the stir bar was more visually evident, but had no apparent effect on device performance. Though the effects or cause of this was not investigated, such stir bars were discarded. 2.2.4 Glass Syringe Cleaning With all electronic devices presented in this wor k being solution processed, with the majority of solutions needing filtration, a syringe is necessary to deposit the solution onto substrates. Prior to the work presented here, plastic syringes (Becton Dickenson Tuberculin) were used to deposit solutions onto substrates. Our group recognized an increase in OPV performance from using plastic syringes compared to glass syringes. Polydimethylsiloxane (PDMS) was acknowledged as a lubricant used in plastic syringes and was postulated as the reason for an incr eased PCE. This suspicion was confirmed by Matrix assisted Laser Desorption/Ionization Mass Spectrometry (MALDI MS) and Infrared (IR) spectroscopy and PDMS was further investigated as a method to increase tion. 89 Thus the work presented here utilized only glass syringes to ensure no impurities or enhancement s during device fabrication. After being used to deposit solutions in the glovebox, glass syringes were removed and immediately rinsed with toluene until visibly clean of residual organics. Rinsed glass syringes were stored in a beaker of clean toluene. Once the beaker was

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50 full of syringes, the toluene was discarded and the syringes were rinsed with fresh toluene. The syringes were stacked upright in a new beaker, covered with methanol and sonicated for 15 minutes. The solvent was discarded and the soni cation process was repeated sequentially with chloroform, toluene and 2 propanol. Syringes were blown dry of residual 2 propanol with compressed air and stored in a container lined with aluminum foil to be introduced back into the Ar glovebox. 2.3 Devic e Fabrication The device fabrication techniques presented in this section were collectively optimized by group members and carried out with crucial detail to ensure sterile and reproducible procedures. It should be stressed that the times presen ted here are quite exact and slight deviations in device performance/quality were observed when straying from procedure. 2.3.1 Indium Tin Oxide Cleaning Pre patterned Indium Tin Oxide (ITO) coated glass slides (25 mm 25 mm) were purchased from Kintec (w ww.kintech.hk) or Tinwell Technology Ltd. (www.tinwell.b2s.com) with a 1 (Figure 2 2a).

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51 Figure 2 2. ITO pattern (a) and shadow mask pattern (b). For (a), black shows ITO and white shows glass. For (b), the black area shows the openings (where metals are deposited through) and white is the mask composed of brass. All measurements are in millimeters. Repr inted with permission from Ref 95 A glass scribe was used to etch a device label on the non ITO side of the substrat e for easier identification. ITO coated substrates were scrubbed using a KimWipe dipped in sodium dodecyl sulfate (SDS) (Sigma Aldrich >99%, cat. # L6026) solution. Substrates were stacked in a custom device holder assembled by the UF Department of Chemi stry machine shop, which can hold up to sixteen 25 mm 25 mm substrates. Once in the holder, the devices were submerged in SDS solution and sonicated for 15 minutes. After sonication, SDS solution was discarded and the substrates were rinsed seven times minutes. Substrates were then rinsed and covered with acetone and sonicated again for 15 minutes. After sonication, the acetone was stored in a 4 liter amber bottle labeled was repeated exactly with 2 propanol. During this work, it was noticed that discarding

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52 the 2 propanol immedi ately following sonication caused residual solvent to dry on the surface of the substrates. The effect of this was not investigated, but rather substrates were blown dry with filtered nitrogen gas immediately after removal from 2 propanol. The substrates were placed in a vacuum chamber (Harrick model # PDC 32G) under a particle free hood assembled by Dr. Nate Heston, and evacuated using a vacuum pump coupled with a dry ice/2 propanol cold trap. Detailed experimental procedures and specifications of the p article dissertation. 90 Once under vacuum, the cham ber was backfilled with pure oxygen and evacuated. This was repeated three times to ensure only pure oxygen in the chamber. The RF coil was turned to high, exposing substrates to oxygen plasma for two 10 minute intervals, with a 10 second pure oxygen pur ge between the two. Treating the ITO surface with oxygen is known to enhance surface wetting, improve stoichiometry, overtime. 91 92 To ensure proper energy levels and oxygen rich ITO, substrates were further processed immediately. Procedures following oxygen exposure are specific to the devic e being fabricated. The procedure will continue in the following three subsections for photovoltaic devices, light emitting diodes, and space charge limited current devices. 2.3.2 Photovoltaic Devices Following oxygen exposure, substrates were immediately transferred to the stage of a spin coater (Laurell Technologies Co. model # WS 400BZ 6NPP/LITE), which was under the particle free hood described before. The device was secured by vacuum and a nitrogen purge gas valve was turned on. Poly(3,4 ethylenedio xythiophene) poly(styrene sulfonic acid) (PEDOT:PSS, Baytron P VP Al 4083) was filtered through

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53 substrates were spun cast for 40 seconds with a 3 second ramp at 5000 rpm. A cotton PEDOT:PSS is in an aqueous solution, the water dissolves the film, exposing the conducting ITO surface on the corners which was later used as an electrode contact fo r the negative bias during characterization. PEDOT:PSS coated substrates were immediately introduced into the Ar glovebox and baked on a hotplate at 130 o C for 20 minutes. Substrates were allowed to cool on aluminum blocks prior to storage in individual plastic containers until further processing. PEDOT:PSS treated substrates were used within one week of fabrication. All active layer solutions for photovoltaic devices were prepared in the Ar glovebox. For all photovoltaic cells presented in this work, a cceptor (PC 61 BM) and donor (isoindigo based oligomers) solutions were prepared separately at 20 mg mL 1 in chlorobenzene solution and allowed to stir overnight. Three hours prior to spin casting, acceptor and donor solutions were heated to 60 o C and stirred for an hour and a half. Solutions were combined for the desired fullerene:isoindigo ratio and heated to 60 o C and stirred for an hour and a half to ensure complete dissolution. Active layer solutions were deposited onto PEDOT:PSS coated su bstrates using a a 3 second ramp at 1000 rpm. Active layer coated substrates were thermally annealed on a hotplate at 100 o C for 20 minutes. After cooling on aluminum bl ocks, substrates were transferred to a thermal deposition chamber using a custom built device holder (Figure 2 3). The chamber was evacuated using a roughing pump and a

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54 turbomolecular pump. The chamber was allowed to pump down for at least four hours, bu t no longer than twelve hours prior to top electrode deposition to ensure evaporation of any residual solvent in the film. Once the deposition chamber reached a pressure of 1 10 6 mbar, metal electrodes were deposited. All photovoltaic cells presented in this work received 10 nm of calcium followed by 100 nm of aluminum as the cathode. Calcium was deposited at a rate of 1.0 s 1 and aluminum was deposited at a rate of 2.0 s 1 Once evaporation begun, shutters were not opened immediately, but rather a few nanometers of metal were allowed to evaporate to remove any impurities that might have been present. Controlled deposition was achieved through shadow masks defining 8 pixels per device with active areas of 0.0707 cm 2 each (Figure 2 2b). After dep osition, the chamber was refilled with Ar and devices were removed and stored in individual plastic containers, wrapped in aluminum foil until characterization. Figure 2 3. SolidWorks model of the device holder used for thermal deposition. Inset groves measure 25.2 mm to house shadow masks (Figure 2 2b). Repr inted with permission from Ref 90

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55 2.3.3 Light Emitting Diode Devices The procedure of PEDOT:PSS deposition and annealing for photovoltaic cells was identical for light emitting diode devices. Act ive layer solutions of polymers were prepared at 15 mg mL 1 chlorobenzene and allowed to stir overnight. Solutions were PTFE filter (Whatman). Substrates spun cast for 60 second s with a 3 second ramp at 1000 rpm. Substrates were then transferred to the thermal deposition chamber discussed before and received a layer of 1 nm lithium fluoride followed by 100 nm aluminum. Lithium fluoride was deposited at a rate of 0.1 s 1 and aluminum was deposited at a rate of 2.0 s 1 The same shadow masks were used for LEDs that were used for OPVs (Figure 2 2b). Devices were removed from the deposition chamber and stored in individual plastic containers wrapped in aluminum foil until cha racterization. Since all instruments used for light emitting diode characterization were located outside of the glovebox, devices were removed one at a time and tested immediately to ensure minimal exposure to ambient air. 2.3.4 Space Charge Limited Curre nt Devices As discussed in chapter 1, the SCLC method is very useful in measuring charge carrier mobilities in organic electronic devices. However, the model cannot measure charge carriers in tandem, thus the fabrication of hole only and electron only dev ices is necessary. 2.3.4.1 Hole only Devices For hole only devices, either PEDOT:PSS or MoO 3 was used as the hole injection layer, and gold as the hole collecting layer. For PEDOT:PSS containing devices, the same procedure was followed as that for OPVs an d LEDs for PEDOT:PSS deposition

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56 and annealing. For MoO 3 containing devices, substrates were brought into the glovebox immediately after oxygen plasma exposure (see ITO Cleaning section). Substrates were loaded into the evaporation chamber without any sha dow masks to deposit the MoO 3 received any organic layers yet, the evaporator was only evacuated for approximately 45 minutes to a pressure of 1 10 5 mbar. A layer of 7 nm MoO 3 was deposited at a rate of 0.3 s 1 Upon removal of MoO 3 coated substrates from the evaporation chamber, the active layer was immediately spun cast to ensure proper energy levels, with no shifting. 36 Solutions for SCLC devices were prepared in the sa me fashion as for OPVs and LEDs, however at a concentration of 25 mg mL 1 Active layer solutions were spun cast on PEDOT:PSS or MoO 3 coated substrates for 60 seconds with a 3 second ramp at 1000 rpm. Devices containing isoindigo based oligomers were the rmally annealed at 100 o C for 20 minutes after spin casting. Substrates were transferred into the evaporation chamber, which was evacuated for a minimum of four hours. Once a pressure of 1 10 6 mbar was achieved, 70 nm of gold was deposited through the same shadow masks that were used for OPVs and LEDs (Figure 2 2b). Gold was deposited at a rate of 1.0 s 1 Devices were stored in individual plastic containers and wrapped in aluminum foil until characterization. 2.3.4.2 Electron only Devices For elec tron only devices, Al was used as the anode and LiF/Al was used as the cathode. Substrates were brought into the Ar glovebox immediately after oxygen plasma exposure. Substrates were loaded into the evaporator using a different holder to deposit a patter ned anode (Figure 2 4).

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57 Figure 2 4. Schematic of patterned mask (a) and SolidWorks model of the evaporation device holder (b) used for anode deposition of electron only devices. Repr inted with permission from Ref 90 Opposed to hole only devices, w here the anode (MoO 3 ) was deposited on the entire surface, a patterned device holder was used to controllably deposit the aluminum anode for electron only devices. Depositing a patterned aluminum anode ensures no vertical overlap of aluminum anode and alu minum cathode in the resulting electron only

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58 device (Figure 2 5). Electron only devices with such an overlap were observed to have the majority of the pixels short when testing. Once the evaporation chamber reached a pressure of 1 10 6 mbar, substrates received 100 nm of Al at a rate of 2.0 s 1 as the anode. Active area solutions were prepared and deposited using the same procedure as hole only devices. Electron only devices received 1 nm LiF and 100 nm Al as the cathode. LiF and Al were deposited at the same rates as for LEDs. The same shadow masks were used as before for cathode deposition (Figure 2 2b). Devices were stored in individual plastic containers, wrapped in aluminum foil until characterization. Figure 2 5. Schematic of an electron only device. Black shows the aluminum anode, gray shows the LiF/Al cathode, and red shows the active area (pixels). 2.4 Device Measurement 2.4.1 Organic Photovoltaics 2.4.1.1 Solar Simulation In order for organic photovoltaics to compete with their inorganic counterparts, power conversion efficiencies must be improved. Quantifying the efficiency of a solar cell may not be as easy as going outside on a sunny day and simply applying a bias and m easuring a photocurrent. Though this is possible and how devices would be

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59 applied in the real world, simulated light may provide more accurate and reproducible can drastically change based on location and a variety of environmental factors modified from the instance it leaves the sun to the point it reaches the cell. Such modi fications may include scattering, and absorption or reflection, which alter the profile of the radiation. This results in light at an angle further away from zenith, a point directly above the opposition of gravitational force, appearing less bright. So lar simulators have been manufactured to more accurately represent the qualities of sunlight for laboratory measurements, which follow air mass coefficient (AM) classifications. Numerous tables of air masses can be found in the literature, from which the AM factor is calculated. 93 Many different AM classifications can be calculated, though for the purpose of photovoltaic characterization, only the three most popular will be discussed: AM0, AM1.0 and AM1.5 (Figure 2 6). The AM0 classification is an ex traterrestrial spectrum, mimicking the sunlight prior AM1.0 and AM1.5. The AM1.0 spectrum is that of light passing through the Earth directly orthogonal to the atmosphere, or 0 o to the zenith angle. However, this would only accurately represent sunlight around the equator. The AM1.5 filter provides the profile of the sun passing through one and a half atmospheres, which is the equivalent of penetrating the atmosphere at a n angle of 48.2 o to the zenith. The AM1.5 spectra can be further characterized as direct or global, AM1.5D and AM1.5G, respectively. For AM1.5D, the radiation neglects any diffused radiation as described above, while

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60 AM1.5G represents total ground radiat ion considering diffused and reflected factors. It is presumed that the AM1.5 filters most accurately represent sunlight in the continental United States and is thus used in this work to characterize all photovoltaic cells. For all experiments conducted for this work, a 150 watt xenon arc lamp (Newport 66902 lamp coupled with Newport 66907 power supply), filtered through AM1.5G was used to illuminate cells. Figure 2 6. Schematic of the air mass conditions used for photovoltaic measurements (a) and thei r standard spectra (b). Image (b) repr inted with permission from Ref 95 2.4.1.2 Power Conversion Efficiency To measure photocurrent, individual pixels were irradiated with 1001 mW cm 2 solar simulated AM1.5G light (specifications described in the previo us section) and a bias was applied from 1.0 V to +1.5 V (1 point per 0.02 V) using a Keithley 2400 Sourcemeter. Applying the same bias and covering the pixels from any light exposure measured the dark current. Current voltage characteristics were collec ted using a custom written LabVIEW program. A custom excel template was used to plot current

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61 density voltage curves and photovoltaic parameters were extracted using the equations discussed in chapter 1. 2.4.2 Polymer Light Emitting Diodes To quantify polymer light emitting diode performance, many characterization techniques were utilized. All instruments used for light emitting diode characterization were outside of the Ar glovebox. Thus, devices remained inside individual plastic containers wrapped in aluminum foil inside the Ar glovebox until testing. Inside the Ar glovebox, a device was put inside a custom device holder fabricated by the UF machine shop (Figure 2 7). The device holder was used for all device testing, so there was no need to remov e the device once inside the holder. The device holder containing the device was removed from the Ar glovebox and immediately tested, to limit the exposure to ambient air.

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62 Figure 2 7. SolidWorks schematic of the top (a,b) and bottom (c,d) of the PLED d evice holder. All measurements are in inches. Reprint ed with permission from Ref 95

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63 2.4.2.1 Electroluminescence Light emitting diodes exhibit an optical/electrical phenomenon called electroluminescence (EL), where light is generated upon induction of a n electrical current. To record electroluminescence spectra, the PLED was placed inside the device holder (Figure 2 7), which was screwed into a stand on a workbench. The device holder was positioned so that the pixel of interest was directly orthogonal to the aperture opening on the spectrograph. A constant current was applied using a Keithley 2400 Sourcemeter, and spectra were obtained using an ISA SPEX Triax 180 spectrograph equipped with a silicon charge coupled device (CCD) detector, cooled to ~140 K with liquid nitrogen. The device was turned on for just enough time for spectra to be obtained, ca. 3 seconds (Figure 2 8). The maximum wavelength of emission was recorded, which was later used for performance testing (see PLED Performance section). F or all reported EL spectra in this work, a correction file was applied after obtaining spectra. Figure 2 8. An illuminated PLED pixel under constant bias, viewed from the outside of the device holder.

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64 2.4.2.2 Luminance ified by measuring its luminance (L v ), the total photometric flux on an element of surface per unit area. Luminance is a desired characterization quantity of optoelectronics as it effectively quantifies the way a human eye perceives the brightness of a lig ht. To arrive at the luminance, we begin with the v (2 1) Where K m is a constant determined empirically (683 lm W 1 is the radiant flux with spectral luminous efficiency for photopic vision a to 830 nm. 94 The luminous intensity (I v ), the total photometric flux on an element of surface measured in candelas (cd), is defined by (2 2) 9). Figure 2 9. Luminous flux leaving the source, labeled with the parameters used to calculate luminance in Equation 2 3. Finally, we arrive at the desired calculation of luminance, the luminous intensity (Equation 2 2) per unit area (cd m 2 ). (2 3)

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65 measurement direction (Figure 2 9). To probe PLED brightness and true color, a calibrated Konica Minolta CS 100 luminance meter equipped with a No. 110 lens was used (Figure 2 10). The device holder containing a fresh PLED (Figure 2 7) was screwed into a stand mounted onto a workbench. Mounted on a tripod, the Minolt a luminance meter was aligned directly orthogonal to the face of the device using a level. Using the eyepiece on the Minolta, the circular crosshair was aligned directly in the center of a pixel. Figure 2 10. Block diagram of the Konica Minolta CS 10 0 luminance meter objective lens (O), aperture stop (S 1 ), chopper (C), field stop (S 2 ), spectral response correction filter (F), silicon photocell (P), current to voltage converter (I/V), and analog to digital converter (A/D). To measure luminance, a const ant bias was applied using a Keithley 2400 Sourcemeter, while simultaneously opening the aperture of the Minolta. The device was only powered on long enough for the Minolta to obtain a reading, ca. 4 seconds to prevent pixels from burning out or shorting. This process was done at 0.5 V steps and luminance values were recorded manually. As light emits from the PLED, it travels through the objective lens and the aperture stop. The light then passes through the chopper, field stop and spectral response

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66 co rrection filter. Corrected light strikes the silicon photocell, where current is converted to voltage, transforming the analog signal to a digital output. The signal is processed by the integrated microcomputer, which carries out the necessary calculatio ns to display on the screen. In addition to luminance, the Minolta also provides color coordinates x and y, which follow CIE 1931 color space standards. Color coordinates for each 0.5 V step were manually recorded and plotted using the standard CIE 1931 color space chromaticity diagram (Figure 2 11). Such color coordinates allow for a way to determine the true color of an emitted light. Figure 2 1931 color space chromaticity diagram, showing a deep red color. 2.4.2.3 PLED Performance Thus far we have looked at the optoelectronic phenomenon of the PLED as well as how to measure its brightness and true color. To conclude our discussion on PLED characterization, we now examine the external quantum efficiency (EQE), the ratio of

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67 the number of photons the LED emits onto the surface to the total number of electrons injected in the device. However, to arrive at the EQE, we must first discuss the components. First we calculate the radiant emittance (R v ), the total flux incident on the surface per unit area (W m 2 ) by (2 4) Where R D is the radiant flux detected, A is the active area, which was 0.0707 cm 2 (Figure 2 as empirically determined to be 24.5 %. 95 E) is inversely proportional to its wavelength emitted: (2 5) 34 J s, c is the speed of light, 2.9979 10 8 m s 1 5 by Equation 2 4 to obtain (2 6) Here, we have the energy of the photon per total power. Additionally, we have (2 7) Where I is the current injected and e is the elementary charge, 1.6022 10 19 C. Since we defined the EQE as the ratio of the number of photons produced per number of injected electrons, we can combine Equation 2 6 and Equation 2 7 to arrive at the EQE: (2 8) The EQ E is typically reported as a percentage, where Equation 2 8 is simply multiplied by 100.

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68 To probe PLED EQE experimentally, a fresh PLED was loaded into the device holder (Figure 2 7). Th e device holder was placed face down on a custom stage designed by Ken Graham and assembled by the UF machine shop, which housed a silicon photodiode. The silicon photodiode was attached to a UDT optometer (S471), which was connected to a computer and a sourcemeter. A bias was applied using a Kiethley 2400 Sourcemeter, and data was collected using a custom LabVIEW program written by Ken Graham. To calibrate the UDT, the emission peak obtained from the EL spectra was used. After testing, the LabVIEW program outputs a .txt file with th e 2 ), current density (mA cm 2 ), and EQE (%). 2.4.3 Charge carrier Mobility Devices As discussed, the SCLC model does an excellent job at probing electron and hole mobilities in organic thin films, though not simultaneously. To measure either electron or hole mobilities, a fresh device was loaded into the same holder used for OPV testing. A bias was applied using a Kiethley 2400 Sourcemeter and current voltage characteristics were collected using a custom written LabVIEW program. For hole only devices, a bias from 0 to +10 V (1 point per 0.025 V) was typically found to provide better current voltage curves, and for electron only devices, a bias from 10 to 0 V was used (same point per voltage as the positive bias). Current density voltage data was plotted using IGOR Pro WaveMetrics programming software, which was used to extract charge carrier mobilities using the field dependent SCLC model (Equation 1 14). 2. 5 Morphology Characterization Morphology is extremely important in organic thin films, especially in photovoltaics when trying to gain a fundamental understanding of the correlation between

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69 performance and nanostructure. Different processing techniques ca n contribute to an altered morphology, including solvent choice, spin rate, and annealing. 96 97 Additionally, it was found that morphology varied based on the location on the device from which an image was taken, which will be discussed later. Many techniques are available to probe morphology of a thin film. The most common me thod in the field of organic electronics is Atomic Force Microscopy (AFM), which is a scanning probe microscope. Additionally, there are various electron microscopes and X ray Photoelectron Spectroscopy (XPS) available for surface imaging. Those seeking a true fundamental understanding of nanostructure should be cautious when simply accepting the results from one morphology characterization technique. It will be shown in chapter 3 that Transmission Electron Microscopy (TEM) revealed features that were no t visible from AFM imaging in certain devices. 2.5.1 Atomic Force Microscopy 2.5.1.1 Tapping Mode Atomic Force Microscopy (AFM) is an excellent tool for probing surface profiles of thin films. AFM relies on the attractive or repulsive forces of a nanome ter scale tip scanning just above the surface of a solid. The tip oscillates up and down without touching the surface and adjusts accordingly with the surface features with its flexible cantilever (Figure 2 12). As a result, AFM produces a surface topolo gy image.

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70 Figure 2 12. Schematic of an AFM. To obtain AFM images for this work, a Veeco Innova scanning probe microscope in tapping mode was used. Cantilevers with a tip frequency of ~325 kHz and a force constant of ~40 N m 1 (MikroMasch NSC15) were used. Representative images were taken within 2 mm of a pixel, where the active layer overlapped the ITO (Figure 2 2). 2.5.1.2 Position dependent Morphology In my earlier work with solar cells, a different ITO pattern and shadow mask was used, resulting in a different pixel layout (Figure 2 14d). AFM studies on these devices revealed a change in morphology based on the position of the device where the image was taken. This was recognized and further investigated when studying po lymeric solar cells made with APFO (Figure 2 13). 98

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71 Figure 2 13. Structure of APFO DOT synthesized by nsal Koldemir. Positions A, B and C were assigned to areas on the device, with A being closes t to the center of the device, and C being further towards the edge of the device (Figure 2 14). AFM images were taken at positions A, B and C on the same device, and a change in morphology was observed. It can be seen that the domains in the center of t he device (A) are quite circular and very well defined. However, once we move towards the outside of the device (B then C), we see that the domains are spreading and distorting.

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72 Figure 2 14. AFM images of APFO DOT:PC 71 BM (1:4) in chlorobenzene at the c enter (a), between the center and the edge (b) and towards the edge (c) of the device. The device ITO and cathode layout with the labeled A, B, and C ITO, gray shows the cathode, an d the red box shows the active area (pixel). Such a distortion in domain shape can be attributed to the classical mechanics of spin coating. As described in the Device Fabrication section, active layer solutions are deposited on devices using glass syring es. Once the solution covers the face of the substrate, spin coating commences. As the spin coater ramps up, centrifugal force pulls the solution away from the center of the device, and towards the edges of the device (Figure 2 15).

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73 Figure 2 15. Mec hanical process of spin coating clean device secured to the spin chuck by vacuum (a), solution deposited on device (b), and spin speed ramped up (c). Image (a) shows the parameters for solving Equation 2 13, To q uantify the centrifugal force (F c inertial frame described by (2 9) Where F is the force, m is the mass and the differential, d 2 r/dt 2 is the acceleration. However, we want the force of the ro tating frame, where the time derivative of the acceleration is defined by (2 10) (2 11)

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74 Now, transforming d 2 r/dt 2 in Equation 2 9 using Equation 2 10, we arrive at a new equation of motion: (2 12) Here, the first term is known as the Coriolis force, which contains the term dr/dt and is thus dependent on the velocity of the rota ting frame. Extracting the second term, we arrive at the centrifugal force: (2 13) Equation 2 13 is a rough approximation of what is actually occurring during the spin coating process. To proceed further we would require tedious calculations involving fluid mechanics, which was not investigated in this work. Should the reader be interested in such studies, he or she may consult the literature. 99 100 Since morphology is position dependent, all AFM images were obtained fr om similar areas on the device within 2 mm of a pixel where the active area overlaps the underlying ITO. Typically 3 5 AFM images were taken per device to ensure uniform morphology and accurate representation. 2.5.1.3 Thickness Measureme nts To gauge thickness of active layer films, AFM tapping mode was used to measure surface lifting a portion of the organic layer and leaving behind the PEDOT:PSS film intact. Using the optical microscope on the AFM, the scratch was found by adjusting the x y parameters of the AFM stage. To measure stepheights, images were taken with the scratch and the film in the scanning range. Typically, 1 If a large r surface roughness was apparent, a larger image was taken to obtain a more accurate

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75 stepheight. SpmLabAnalysis software was used to take an average of the film height and the scratch height (Figure 2 16). Taking the difference of these two average heigh ts resulted in the film thickness. Typically 3 5 stepheight images were taken per device, and the average of these was accepted as an accurate representation of the active layer thickness. Figure 2 16. Use of SpmLabAnalysis software to calculate the stepheight for an AFM image containing a scratch (left) and the film (right) (a), height distribution (b), and stepheight calculation (c). 2.5.2 Transmission Electron Microscopy To gain a better understanding of morphology, AFM should not be the only techn ique used. Techniques involving electron microscopy provide a more in depth analysis of film structure in OPVs. Transmission Electron Microscopy (TEM) basically shoots electrons at a sample and the interaction of the electrons with the sample is then pro jected on a film (Figure 2 17). Since the organic layers used in this work were approximately 70 nm, the thickness is sufficient to pass electrons.

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76 Figure 2 17. Schematic of a TEM. 2.5.2.1 Sample Preparation To obtain top down TEM images, samples of or ganic films were prepared on carbon or lacey carbon coated copper grids (Electron Microscopy Sciences cat. # CF150 Cu and CL 200 Cu, respectively). To ensure accurate representation of morphology and performance, the same device of which OPV parameters an d AFM images were reported was used for TEM. A glass scribe was used to cut a square (2 mm 2 mm) out of the device ~1 mm away from the pixel where the active layer overlapped the underlying ITO (Figure 2 18). A razor blade was used to scratch grids on PEDOT:PSS is an aqueous solution, the water dissolved this layer and the glass sunk to the bottom of the dish leaving the thin organic film floating on top of the water. Du mont N7 tweezers (Electron Microscopy Science cat. # 72864 D) were used to pick

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77 up the TEM grid. The grid was then slowly lowered to the surface of the water to lightly touch the floating organic film. The film adhered to the grid and excess water was re moved from the grid by lightly touching the edge with a KimWipe. Figure 2 18. Area of a device from which a TEM sample was prepared. 2.5.2.2 Top down Imaging TEM images were obtained using a JEOL 200CX TEM at the Major Analytical Instrumentation Cente r (MAIC) of the Department of Materials Science and Engineering at the University of Florida with the assistance of Dr. Michael Kesler. A driving voltage of 200 kV was used with a spot size of 1 or 2. A small objective aperture (4) was inserted, which bl ocked a portion of the electrons, reducing the beam intensity. Images were obtained under focus at a magnification of 100 or 200 kX zoom, unless otherwise noted.

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78 CHAPTER 3 ASYMMETRIC AND SYMMETRIC OLIGOMERS AS TAILOR MADE ADDITIVES IN ORGANIC PHOT OVOLTAICS 3.1 Introduction In order for organic photovoltaics to compete with their inorganic counterparts in the consumer market, power conversion efficiencies (PCEs) must be increased. As discussed in chapter 1, many different aspects of device physics, design, and processing techniques contribute to the film morphology, and thus PCE. Recent techniques include additives, 101 103 solvent annealing, 104 use of block copolymers, 105 108 thermal annealing, 27,109 and slow film drying. 110 Such techniques typically have unfavorable mixing thermodynamics, resulting in decreased PCEs, however. Chen et al proposed that P3HT crystallization at the P3HT/PCBM interfa ce in a bulk heterojunction significantly influences film morphology. 111 Thus, if crystallinity can be controlled, a fundamental understanding of morphological effects on device kinetics of crystal growth. 112 114 Such an additive has the same basic structure as the parent molecule (Figure 3 1a), with the exception of a minor defect ( Figure 3 1b). Figure 3 made

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79 This chapter will discuss the use of tailor made additives to obtain more favorable morphologies, resulting in increased PCEs of molecular bulk heterojunction organic photovoltaics. 3.2 Isoindigo as an Electron Donor Isoindigo (Figure 3 2a), a structural isomer of indigo, is a pigment that has been used in dyes since the early 1900s. 115 In 2010, our group synthesized the first electroactive isoindigo based oligomer, iI(TT) 2 (Figure 3 2b), as an electron donor for molecular bulk heterojunction organic photovoltaics. 116 Initial experiments of iI(TT) 2 :PC 61 BM containing solar cells afforded promising power conversion efficiencies thereafter. Within two years of initial publication, other groups synthesized and applied isoindigo based molecules to a variety of electronics organic field effect transistors; 7 thin fi lm transistors; 11 semiconductor memory devices; 21 and small molecule, 89,116 and polymer solar cells. 117 125 Currently, power conversion efficiencies of solar cells containing isoindigo based materials have reached 6.3%. 123 For all solar cells prepared in this ch apter, the parent oligomer used was iI(TT) 2 (Figure 3 2b). Figure 3 2. Chemical structures of trans isoindigo (a) and iI(TT) 2 the parent oligomer used in the experiments presented in this work (b).

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80 3.3 iI(TT) 2 i Bu 3 Si T o probe the effects of a tailor made additive on the morphology of a bulk heterojunction cell, an asymmetric oligomer was synthesized. The resulting oligomer, iI(TT) 2 i Bu 3 Si (Figure 3 3), is structurally similar to its parent molecule, iI(TT) 2 (Figure 3 2b ). Whereas iI(TT) 2 is symmetrical with linear hexyl chains as end groups, iI(TT) 2 i Bu 3 Si is asymmetrical with a linear hexyl chain as one end group and a bulky triisobutylsilyl ( i Bu 3 Si) group as the other end group. The idea here is that the bulky size o f the triisobutylsilyl group would disrupt crystal growth when incorporated in the iI(TT) 2 :fullerene heterojunction. Since iI(TT) 2 i Bu 3 Si still contains one similar end group as the parent molecule, crystal growth should not be inhibited, but rather slowe d down. As mentioned in chapter 1, with typical exciton diffusion lengths ca. 5 15 nm, an ideal morphology may be able to be tuned with an optimized concentration of the tailor made additive. Figure 3 3. Chemical structure of the asymmetric tailor ma de additive, iI(TT) 2 i Bu 3 Si. 3.3.1 Photovoltaic Performance Organic photovoltaic devices were prepared by the same fashion described in chapter 2. Active layers of [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM were prepared, where the concentration of [donor]:acceptor was kept at [1]:1 by weight. To probe the effects of the triisobutylsilyl containing tailor made additive, the concentration of [iI(TT) 2 :iI(TT) 2

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81 i Bu 3 Si] was varied with increasing mole percent iI(TT) 2 i Bu 3 mole % iI(TT) 2 i Bu 3 is mentioned, the OPV cell has an active layer of [40% iI(TT) 2 :10% iI(TT) 2 i Bu 3 Si]:50% PC 61 BM, hence [1]:1 by weight. Solar cell device performance data for increasing mole % iI(TT) 2 i Bu 3 Si is shown in Table 3 1. Immediately there is a noticeable increa se in PCE from 1.30 to 2.24 % with the addition of 20 mole % iI(TT) 2 i Bu 3 Si. This almost double increase in PCE can be attributed to the increases in V OC J SC and FF: 0.72 to 0.82 V, 4.58 to 6.14 mA cm 2 and 0.39 to 0.45, respectively. With the addition of asymmetric oligomer after 20 mole % iI(TT) 2 i Bu 3 Si, there is a gradual decrease in V OC J SC FF, and consequently PCE. Such a decrease can be attributed to the formation of a more amorphous film as the bulky triisobutylsilyl group is disrupting the iI (TT) 2 :PC 61 BM interface crystallization. Figure 3 4 shows representative J V curves of 0 through 100 mole % iI(TT) 2 i Bu 3 Si. The shapes of the J V curves help obtain a visual of the performance, and show a significant difference between a iI(TT) 2 :PC 61 BM (0 % iI(TT) 2 i Bu 3 Si) and a iI(TT) 2 i Bu 3 Si:PC 61 BM (100 % iI(TT) 2 i Bu 3 Si) solar cell. Table 3 1. Summary of OPV parameters for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. mole % V OC (V) J SC (mA cm 2 ) FF PCE (%) 0 0.72 4.58 0.39 1.30 10 0.75 5.79 0.39 1.69 20 0.82 6.14 0.45 2.24 30 0.81 5.28 0.49 2.09 40 0.75 4.54 0.49 1.65 50 0.69 2.55 0.40 0.71 100 0.77 1.17 0.24 0.22

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82 Figure 3 4. Representative J V curves of 0 to 100 % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. 3.3.2 Charge Mobility To probe the effects of the triisobutylsilyl containing tailor made additive on charge carrier mobilities, spa ce charge limited current (SCLC) devices were prepared. Devices were prepared by the same fashion described in chapter 2, with a device architecture of glass/MoO 3 /[iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM/gold, with increasing mole percent of asymmetric oligomer as with OPVs. Calculated hole mobilities using the SCLC equation (Equation 1 14) of measured dark current (see chapter 2) are summarized in Figure 3 5. With increasing mole % of iI(TT) 2 i Bu 3 Si, the hole mobility gradually decreases. From 0 to 20 mole % th ere is a decrease from 1.6 10 5 to 1.0 10 5 cm 2 V 1 s 1 The hole mobility of a device without iI(TT) 2 (100 mole % iI(TT) 2 i Bu 3 Si) dropped by three orders of magnitude to 2.3 10 8 cm 2 V 1 s 1 Figure 3 6 provides representative J V curves of 0, 10, 20 (3 6a), 30, 40 (3 6b), 50 and 100 % (3 6c) iI(TT) 2

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83 i Bu 3 Si fitted with the SCLC equation. Such a continual and drastic decrease in hole mobility with the addition of iI(TT) 2 i Bu 3 Si can be attributed to the bulkiness of the triisobutylsilyl group. As there is an increasing amount of i Bu 3 Si, and thus less linear hexyl chains, the crystallographic packing is less ordered, leading to a more amorphous film. The bulkiness of the triis obutylsilyl group increases the intermolecular hopping distance, and consequently decreas es the charge carrier mobility. This will be further realized when considering the morphology data presented in section 3.3.3. Figure 3 5. Summary of hole mobiliti es for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM SCLC devices. Values reported are averages of 6 8 pixels, with error bars showing +/ one standard deviation.

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84 Figure 3 6. Representative J V plots of 0, 10, 20 (a), 30, 40 (b), 50 and 100 (c) mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM SCLC devices. Data presented are from one pixel and the hole mobility ( ) and field dependent term ( are from the SCLC fit for that pixel, which was used in the calcul ation of the averages in Figure 3 5. 3.3.3 Morphology As described in sections 3.3.1 and 3.3.2, many performance parameters were a result of morphological changes with the addition of iI(TT) 2 i Bu 3 Si. To probe the morphology of a solid state device, atomic force microscopy (AFM) and bright field transmission electron microscopy (TEM) were used. Films for AFM characterization were prepared in the same manner as the photovoltaic cells with [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM at [1]:1 by weight with increasing mo le % iI(TT) 2 i Bu 3 Si. AFM images of

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85 these films are presented in Figures 3 7 and 3 8. Immediately it can be seen that the domain sizes gradually decrease upon initial addition of 10 mole % iI(TT) 2 i Bu 3 Si. Upon addition of 20 mole % iI(TT) 2 i Bu 3 Si, domain sizes are quite small, allowing for better exciton diffusion and thus increased PCE. In addition to a decreasing domain size, devices with 0 to 30 mole % iI(TT) 2 i Bu 3 Si have well defined domains, indicating high crystallinity. When additional iI(TT) 2 i B u 3 Si is added after 30 mole %, the features become distorted and their shapes are less discernable. This distortion may be attributed to the formation of a more amorphous film, which confirms the suspicion that iI(TT) 2 i Bu 3 Si disrupts iI(TT) 2 crystal grow th in the bulk heterojunction. This is confirmed when examining the 100 mole % image, as the surface appears extremely smooth and flat. Using Nanoscope 7.30 software, a surface roughness value of 0.374 nm was calculated for the 100 mole % image, indicati ng virtually no features, and thus a completely amorphous film. In Figure 3 8, the 1 1 m images further confirm the decrease in domain size and crystallinity, expressed by the formation of a more amorphous film as mole % iI(TT) 2 i Bu 3 Si increases.

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86 Fi gure 3 7. AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices. All images shown are 5 5 m, and have the same scale (1 m) and height bar (20 nm). Figure 3 8. AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices. All images shown are 1 1 m, and have the same scale (200 nm) and height bar (10 nm). As mentioned in chapter 2, the results of one method of morphology characterization should not be immediately acc epted as what is truly going on in the

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87 solid state device. In addition to AFM, top down TEM images were taken to further investigate the morphological changes with addition of iI(TT) 2 i Bu 3 Si (Figure 3 9). TEM samples were prepared using the procedure des cribed in chapter 2, and the same film from which AFM images were represented. As with AFM, TEM shows a decrease in domain size with increasing mole % iI(TT) 2 i Bu 3 Si. Also, the domains become distorted and less discernable upon addition of 40 mole % iI(TT) 2 i Bu 3 Si. Figure 3 9. TEM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices. All images have the same 200 nm scale bar. Despite the apparent decrease in domain sizes from 0 to 40 mole % iI(TT) 2 i Bu 3 Si, t he 50 and 100 mole % images are the most striking. Unlike AFM, which showed very flat films for the 50 and 100 mole % devices (Figures 3 7 and 3 8), TEM reveals features. Here there are two different distinct phases, a light and a dark phase. Cross sect ional TEM samples were prepared using a focused ion beam (FIB) for the 20 and 100 mole % iI(TT) 2 i Bu 3 Si devices (Figure 3 10). The full procedure of sample 95

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88 Figure 3 10. Cross sectional TEM images of 20 (a) and 100 (b) mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 i Bu 3 Si]:PC 61 BM devices as well as a 3 nm Gaussian blur (c) applied to image b. Adapted with permission from R ef 95 sectional TEM images prepared using a FIB, and performed x ray photoelectron spectroscopy (XPS) exp eriments to determine the light phase is PC 61 BM and the dark phase is the isoindigo oligomer. 89 Thus, th e white phase crystals in the 100 mole % TEM image are those of PC 61 BM. The cross sectional image of the 100 mole % device suggests that the PC 61 BM rich crystals are concentrated near the PEDOT:PSS interface (Figure 3 10b,c). This explains why the featur es were not visible with AFM, as AFM takes a surface topology image, rather than the bulk of the film (see chapter 2).

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89 It should be stressed, and evident after viewing AFM and TEM images presented in this subsection, that a single morphology technique sh ould not be taken for granted. Performing at least two techniques is a necessity to obtain a full understanding of the fundamental nanostructure in a solid state device. 3.3.4 Crystal Growth To further asses the crystallization occurring in the bulk heter ojunction with the addition of iI(TT) 2 i Bu 3 Si, crystal growth experiments were carried out by Danielle Salazar. A complete and detailed procedure of the experiments can be viewed in Ken 95 An optical microscope was used to view crystals under crossed polars at different mole % iI(TT) 2 i Bu 3 Si, as a function of iI(TT) 2 (Figure 3 11). Crystal size decreased from 172 to 46 m with 0 to 10 mole % iI(TT) 2 i Bu 3 Si, respectively. When 20 mole % iI(TT) 2 i Bu 3 Si was added, no crystals were observed. With only iI(TT) 2 i Bu 3 Si and no iI(TT) 2 the polarized microscope shows very s mall crystals, indicating iI(TT) 2 i Bu 3 experiments further confirm that a tailor made additive, in this case iI(TT) 2 i Bu 3 Si, can indeed slow down nucleation and crystallization growth kinetics, in the iI(TT) 2 :PC 61 BM bulk heterojunction. Thus, with increasing mole % of the additive, morphology can be finely tuned to achieve desired PCEs.

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90 Figure 3 11. Crystal length dependence on mole % iI(TT) 2 i Bu 3 Si as a function of iI(TT) 2 (a) and representa tive polarized microscope images of increasing mole % iI(TT) 2 i Bu 3 Si in iI(TT) 2 :iI(TT) 2 i Bu 3 Si. Crystal lengths are averages of 32 to 72 crystals and the error bars show +/ one standard deviation Adapted with permission from Ref 95 3.4 iI(TT i Bu 3 Si) 2 Section 3.3 demonstrated the use of a tailor made additive to finely tune morphology in molecular bulk heterojunction organic photovoltaics. The oligomer used, iI(TT) 2 i Bu 3 Si, was an asymmetric derivative of the parent molecule iI(TT) 2 To further probe if the asymmetry alone of iI(TT) 2 i Bu 3 Si led to morphology control, an analogous symmetrical derivative was synthesized, iI(TT i Bu 3 Si) 2 (Figure 3 12). A detailed dissertation 86 In this symmetric derivative, both linear hexyl chains in the parent molecule, iI(TT) 2 were replace d with bulky triisobutylsilyl groups.

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91 Figure 3 12. Chemical structure of the symmetric tailor made additive, iI(TT i Bu 3 Si) 2 3.4.1 Photovoltaic Performance Organic photovoltaics were prepared in the same manner as those with the asymmetric oligomer, iI(TT) 2 i Bu 3 Si. Devices had active layers of [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM, and varied with increasing mole % iI(TT i Bu 3 Si) 2 while maintaining a [1]:1 by weight ratio. A summary of OPV parameters from devices containing the symmetrical triisobutylsily l additive are summarized in Table 3 2, with representative J V curves of 0 through 100 mole % iI(TT i Bu 3 Si) 2 in Figure 3 13. Unlike devices containing the asymmetrical additive, iI(TT) 2 i Bu 3 Si, there is no apparent increase in PCE with the symmetrical ad ditive, iI(TT i Bu 3 Si) 2 With the addition of 0 to 30 mole % iI(TT i Bu 3 Si) 2 there is a decrease in V OC J SC and consequently PCE. When more iI(TT i Bu 3 Si) 2 is added past 30 mole %, J SC FF and consequently PCE further decrease by a larger amount than at lower concentrations. Such decreases may be attributed to charge carrier mobilities and morphology, which will be discussed in sections 3.4.2 and 3.4.3, respectively.

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92 Table 3 2. Su mmary of OPV parameters for increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. mole % V OC (V) J SC (mA cm 2 ) FF PCE (%) 0 0.72 4.58 0.39 1.30 10 0.65 4.48 0.43 1.27 20 0.66 3.71 0.43 1.07 30 0.62 1.98 0.43 0.52 40 0.76 1.53 0.36 0.42 50 0.78 1.33 0.33 0.33 100 0.72 1.31 0.28 0.28 Figure 3 13. Representative J V curves of 0 to 100 % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. 3.4.2 Charge Mobility To determine the effect of the symmetrical triisobutylsilyl additive on charge carrier mobility, SCLC devices were prepared by the same manner as discussed in chapter 2. Devices received MoO 3 as the h ole injection layer and gold as the hole collecting layer, and varied by increasing mole % iI(TT i Bu 3 Si) 2 Hole mobility data are summarized in

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93 Figure 3 14, with averages of 6 8 pixels, and the error bar indicating +/ one standard deviation. Figure 3 14. Summary of hole mobilities for increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM SCLC devices. Values reported are averages of 6 8 pixels, with error bars showing +/ one standard deviation. Similar to the asymmetric derivative iI(TT) 2 i Bu 3 Si, there is negligible change in hole mobility upon addition of 10 and 20 mole % iI(TT i Bu 3 Si) 2 Once more iI(TT i Bu 3 Si) 2 is added past 20 mole %, there is a decrease in hole mobility, similar to the asymmetric derivative. However, the decrease in hole mobility is much sharper with the addition of iI(TT i Bu 3 Si) 2 than it was for iI(TT) 2 i Bu 3 Si. From 20 to 40 mole % iI(TT i Bu 3 Si) 2 the hole mobility decreases 3 orders of magnitude from 1.5 10 5 to 1.9 10 8 cm 2 V 1 s 1 An even furthe r order of magnitude decrease to 3 10 9 cm 2 V 1 s 1 occurs with the pure iI(TT i Bu 3 Si) 2 :PC 61 BM device (Figure 3 15c). Such abrupt and sharp decreases in hole mobilities of the iI(TT i Bu 3 Si) 2 devices can be attributed to the bulky nature of the triisobut ylsilyl group. As the asymmetric oligomer (Figure 3 3) contained only one triisobutylsilyl group, the symmetric derivate contains two (Figure 3 12). Thus it is expected that the steric bulkiness of the end groups in iI(TT i Bu 3 Si) 2 prevent

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94 ordered crystal lographic packing to a further extent than iI(TT) 2 i Bu 3 Si did. With the presence of two bulky triisobutylsilyl groups, the intermolecular hopping distance is further increased, resulting in the observed decrease in hole carrier mobility. Figure 3 15. Representative J V plots of 0, 10, 20 (a), 30, 40 (b), 50 and 100 (c) mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM SCLC devices. Data presented are from one pixel and the hole mobility ( ) and field dependent term ( are from the S CLC fit for that pixel, which was used in the calculation of the averages in Figure 3 14. 3.4.3 Morphology To obtain a fundamental understanding of the cause of the observed OPV and charge carrier parameters, AFM and TEM images were taken to probe film mor phology.

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95 AFM samples were prepared in the same manner as for OPVs, and images are presented in Figure 3 16 and Figure 3 17 for 25 m 2 and 1 m 2 areas, respectively. Upon initial addition of 10 and 20 mole % iI(TT i Bu 3 Si) 2 the feature sizes decrease sig nificantly; however, these features remain sharp. Similar to iI(TT) 2 i Bu 3 Si these sharp features suggest crystallinity in the film. With the addition of more than 20 mole % iI(TT i Bu 3 Si) 2 the features become distorted, similar to what was seen for iI(T T) 2 i Bu 3 Si (Figure 3 7). This distortion can be attributed to the formation of a more amorphous morphology. Unlike iI(TT) 2 i Bu 3 Si when 50 and 100 mole % iI(TT i Bu 3 Si) 2 is added to the BHJ, AFM images indicate the formation of much larger crystalline domains. Upon addition of 100 mole % iI(TT) 2 i Bu 3 Si, the surface was virtually feature free, with a very low calculated surface roughness. Moreover, upon addition of 100 m ole % iI(TT i Bu 3 Si) 2 large crystal like features on the scale of several microns are visible. With such large features present, it is apparent that the hole mobility experiences such a drastic decrease.

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96 Figure 3 16. AFM images of increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices, with the typical color scale (a) and in gray scale (b). All images shown are 5 5 m, and have the same scale (1 m). All images in (a) have a 20 nm height bar, and all images in (b) have a 10 nm height bar, except for 50 and 100 %, which have a 20 nm height bar.

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97 Figure 3 17. A FM images of increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices. All images shown are 1 1 m, and have the same scale (200 nm) an d height bar (10 nm). To further examine the features in the AFM images, Nanoscope 7.30 software was used to convert raw 5 m 5 m AFM images into gray scale figures (Figure 3 16b). These gray scale figures allow us to see more easily the changes in fe ature size and crystallinity. It can be seen in Figure 3 16b that the surface of the film begins to become more amorphous with the addition of 30 mole % iI(TT i Bu 3 Si) 2 as realized with the typical color scale images (Figure 3 16a). However, the gray sca le images allow us to see that the larger features begin forming in the 30 mole % device. These features are maxed out on the 10 nm height scale bar indicating their larger size than the rest of the surface. Such formation of larger crystallites already at the 30 mole % device can correlate to the sharp decrease in charge carrier mobility observed in the SCLC devices.

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98 Figure 3 18. TEM images of increasing mole % symmetric oligomer in [iI(TT) 2 :iI(TT i Bu 3 Si) 2 ]:PC 61 BM devices. All images have the same 200 nm scale bar, with the exception of the 50 and 100 mole % devices, which were taken at a lower magnification. Prior to making any conclusions about the relationship between morphology and device performance for iI(TT i Bu 3 Si) 2 TEM images were taken. T EM samples were prepared in the same manner as described in chapter 2. Samples were imaged in an identical fashion to iI(TT) 2 i Bu 3 Si. TEM images for increasing mole % iI(TT i Bu 3 Si) 2 are shown in Figure 3 18. As expected from the AFM images, the features drastically decrease upon initial addition of 10 mole % symmetric tailor made additive. As with the asymmetric additive, these features are very distinct and appear crystalline. However, with addition of more than 20 mole % iI(TT i Bu 3 Si) 2 the features become larger, but yet less distinct. This confirms the suspicion that arose from viewing the gray scale AFM images that larger crystallites begin to form after the addition of 30 mole % iI(TT i Bu 3 Si) 2 The TEM images for 50 and 100 mole % iI(TT i Bu 3 Si) 2 reveal very large features. The 50 mole % image was very difficult to image and it can be seen that there

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99 is a large amorphous phase on the size of microns. These larger domains, containing features that are indistinguishable, match what was observed wi th AFM. Once the pure symmetric device, 100 mole % iI(TT i Bu 3 Si) 2 is viewed, very large crystals are visible. These crystals are on the scale of several microns, and have somewhat discernable shapes. It is evident from viewing AFM and TEM images, that wi th increasing mole % iI(TT i Bu 3 Si) 2 in the iI(TT) 2 :PC 61 BM heterojunction, feature sizes become larger. Such larger features may be due to the bulkiness of the triisobutylsilyl side groups on both ends of the isoindigo containing oligomer. These bulky gro ups inhibit planar packing in the solid state, leading to more amorphous films. Additionally, the bulky groups increase the intermolecular hoping distance, which in turn reduces the charge carrier mobility, as was expressed in Figure 3 14. 3.5 iI(TT) 2 H T hus far we have examined two tailor made additives and their effects on morphology, OPV parameters, and charge carrier mobilities in the iI(TT) 2 :PC 61 BM heterojunction. However, both of the additives contained bulky triisobutylsilyl groups. To further inv estigate the effects of tailor made additives, we now seek to determine if the bulky size of the end group alone leads to the morphological, performance, and mobility changes discussed. To probe the effects of end group size in tailor made additives, anoth er asymmetric derivative was synthesized. iI(TT) 2 H is based on the same parent oligomer, iI(TT) 2 (Figure 3 2b), but has one linear hexyl side chain replaced with a small hydrogen atom (Figure 3 19).

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100 Figure 3 19. Chemical structure of an asymmetric t ailor made additive, iI(TT) 2 H. 3.5.1 Photovoltaic Performance Photovoltaic cells containing the hydrogen end capped tailor made additive were prepared in the same manner as described in chapter 2, with increasing mole % iI(TT) 2 H. Photovoltaic parameters are summarized in Table 3 3. Similar to the asymmetric triisobutylsilyl additive, iI(TT) 2 i Bu 3 Si, there is a increase in V OC with the addition of 10 and 20 mole % iI(TT) 2 H from 0.72 to 0.81 and 0.80 V, respectively. These devices also experienced an in crease in J SC from 4.58 to 5.53 and 5.51 mA cm 2 and FF from 0.39 to 0.41 and 0.41, respectively. However, the increase in J SC was not as significant as that which resulted from 10 and 20 mole % iI(TT) 2 i Bu 3 Si. Consequently, the increase in PCE was not as significant as it was for iI(TT) 2 i Bu 3 Si; with the addition of 10 and 20 mole % iI(TT) 2 H, PCE increased from 1.30 to 1.82 and 1.81 %, respectively. Similar to the asymmetric and symmetric triisobutylsilyl additives, there is a decrease in V OC J SC FF and PCE with addition of 30 to 100 mole % iI(TT) 2 H. The 100 mole % iI(TT) 2 H device exhibited a very low J SC and PCE of 1.79 mA cm 2 and 0.33 %, respectively. Such a low PCE in the pure iI(TT) 2 H:PC 61 BM device may be attributed to an amorphous film as was observed with the asymmetric triisobutylsilyl derivative. This will be further investigated in section 3.5.3.

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101 Table 3 3. Summary of OPV parameters for increasing mole % asymmetric oligomer in [iI( TT) 2 :iI(TT) 2 H]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. mole % V OC (V) J SC (mA cm 2 ) FF PCE (%) 0 0.72 4.58 0.39 1.30 10 0.81 5.53 0.41 1.82 20 0.80 5.51 0.41 1.81 30 0.68 4.95 0.34 1.16 40 0.73 4.37 0.30 0.95 50 0.74 3.99 0.29 0.87 100 0.64 1.79 0.29 0.33 Figure 3 20. Representative J V curves of 0 to 100 % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM solar cells under AM1.5G 100 mW cm 2 simulated radiation. 3.5.2 Charge Mobility To probe the effects of the hydrogen end capped asymmetric tailor made additive on charge carrier mobility, SCLC devices were prepared as described in chapter 2 with increasing mole % iI(TT) 2 H. A summary of average hole mobilities and respective standard deviations is given in Figu re 3 21.

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1 02 Figure 3 21. Summary of hole mobilities for increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM SCLC devices. Values reported are averages of 6 8 pixels, with error bars showing +/ one standard deviation. With the addition of 10 mole % iI(TT) 2 H there is a slight increase in hole mobility from 1.6 10 5 to 3.9 10 5 cm 2 V 1 s 1 When additional iI(TT) 2 H is added past 10 mole % there is a subsequent decrease in hole mobility, as was observed with the asymmetric and symmetri c triisobutylsilyl additives. For iI(TT) 2 H, however, the decrease in hole mobility is very slight; as 10 to 50 mole % is added there is only a decrease from 3.9 10 5 to 2.1 10 5 cm 2 V 1 s 1 respectively. This is very different than the triisobutylsilyl oligomers, which experienced orders of magnitude decreases in hole mobility. Such a steady hole mobility with increasing mole % iI(TT) 2 H may be attributed to the small size of the hydrogen atom. Whereas the bulky triisobutylsilyl groups disrupted crystallographic packing and increased intermolecular hopping distances, the small hydrogen atom decreases the intermolecular hopping distance, and thus preserves the high er hole mobility. Finally, the pure iI(TT) 2 H:PC 61 BM device (100 mole %) experiences a decrease in hole mobility of 6 orders of magnitude to 5.7 10 11

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103 cm 2 V 1 s 1 Such a drastic decrease in hole mobility may be a result of the formation of an amorphous film, as will be discussed in section 3.5.3. 3.5.3 Morpholog y To obtain a fundamental understanding of device performance and nanostructure in the solid state film, AFM and TEM were performed. AFM samples were prepared in the same manner as for OPVs, and images for increasing mole % iI(TT) 2 H are shown in Figures 3 22 and 3 2 2 2 areas, respectively. Similar to iI(TT) 2 i Bu 3 Si, there is a decrease in domain size with the addition of 10 through 50 mole % iI(TT) 2 H. Unlike the iI(TT) 2 i Bu 3 Si devices which showed sharp, discernable domains at concentrations of 10 to 30 mole % asymmetric oligomer, the iI(TT) 2 H show such features all the way through the addition of 50 mole % asymmetric oligomer. Rather than the formation of an amorphous film early on, this may be attributed to a decreased rate of crystallinity. The pure iI(TT) 2 H:PC 61 BM image is similar to that of the pure iI(TT) 2 i Bu 3 Si:PC 61 BM image, indicating an amorphous film. However, some aggregate type features are visible in the 100 mole % iI(TT) 2 H image. Such aggregates may be the result of a decreas e in solubility. Compared to the previous two tailor made additives, iI(TT) 2 H lacks the alkyl solubilizing groups of the hexyl chains and triisobutyl groups, resulting in a lower solubility.

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104 Figure 3 22. AFM images of increasing mole % asymmetric ol igomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. All images shown are 5 5 m, and have the same scale (1 m) and height bar (20 nm). Figure 3 23. AFM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. All images shown are 1 1 m, and have the same scale (200 nm) and height bar (10 nm). As stressed before, to be certain of solid state nanostructure, additional morphology characterization was conducted for iI(TT) 2 H devices. TEM samples w ere

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105 prepared using the procedure described in chapter 2 and the films used for AFM characterization. TEM images are summarized in Figure 3 24, and overall show similarities with the AFM images obtained. TEM reveals a decrease in feature size as increasin g mole % iI(TT) 2 H is added. The features remain sharp and their shapes are discernable. Finally, the pure iI(TT) 2 H device (100 mole %) shows an amorphous film, resulting in a 6 order of magnitude decrease in hole carrier mobility. Figure 3 24. TEM images of increasing mole % asymmetric oligomer in [iI(TT) 2 :iI(TT) 2 H]:PC 61 BM devices. All images have the same 200 nm scale bar, with the exception of the 100 mole % device, which was taken at a higher magnification. 3.6 Chapter Summary The use of tailor made additives to fine tune morphology, and consequently PCE has been demonstrated. Asymmetric and symmetric additives were prepared and incorporated into BHJ OPVs. Additionally, the end group size of the additive was also varied to determine the effect s on morphology and OPV performance.

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106 The triisobutylsilyl containing asymmetric oligomer, iI(TT) 2 i Bu 3 Si was shown to decrease domain sizes with the addition of 20 mole %, resulting in an almost two fold increase in PCE. Charge carrier mobilities were m easured and found to decrease with increasing mole % asymmetric oligomer due to inhibited crystallographic packing because of the bulky triisobutylsilyl group. Increased intermolecular hopping distances due to the size of triisobutylsilyl compared to the linear hexyl chain resulted in significantly lower hole carrier mobilties. Crystallization studies were performed to show iI(TT) 2 i Bu 3 Si decreases the rate of crystallization in the iI(TT) 2 :PC 61 BM heterojunction. However, crystals were visible in the pur e iI(TT) 2 i Bu 3 Si microscope images indicating A symmetric analogue tailor made additive containing two triisobutylsilyl groups was synthesized to determine if the asymmetry alone afforded morphol ogical changes. iI(TT i Bu 3 Si) 2 showed decreases in domain sizes and PCE. Charge carrier mobilities decreased even more compared to the asymmetric derivative. TEM and AFM images revealed the formation of large crystallites on the order of several microns with the addition of 50 and 100 mole % symmetric additive. Such large crystallites and decreases in OPV and charge carrier performance indicate inhibition of crystallographic packing to a larger extent than was observed with the asymmetric derivative du e to the reduced amount of linear hexyl chains and increasing amount of bulky triisobutylsilyl groups. An asymmetric tailor made additive containing a small hydrogen end group was synthesized to determine if the size of end group contributed to morphologic al changes. iI(TT) 2 H showed a constant decrease in domain size leading to a completely

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107 amorphous film. OPV devices showed an initial increase in PCE with small concentrations of iI(TT) 2 H. The small size of the hydrogen atom led to decreased intermolec ular hopping distances resulting in a rather steady charge carrier mobility with increasing iI(TT) 2 H. As evidenced with the three tailor made additives presented in this work, the asymmetry alone leads to morphological changes and an increase in PCE. How ever, careful selection of the end group is necessary to achieve higher PCEs, as was demonstrated with triisobutylsilyl and hydrogen end capped oligomers. This work shows that tailor made additives have promise in the field of optoelectronics for optimizi ng device performance. We believe tailor made additives may be synthesized for a variety of electroactive materials to tune morphology and achieve reproducibly high PCEs in molecular BHJ OPVs.

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108 CHAPTER 4 ELECTROACTIVE POLYOL EFINS WITH PRECISELY CONTROLLED CHROMOPHORE SPACING conjugated organic materials for optoelectronic applications has become an art. As described in chapter 1, there are a variety of electroactive entities to choose from when designing molecular and polymeric materials for light emitting devices. When it comes to molecular materials, proper chromopho re choice is of important ance for fabricating quality devices. A material must be designed so that when incorporated into a solid state device, there is a balance between the electron and hole mobilities for efficient exciton generation and charge recombination. 13,15,26,83,126 Additionally, to fabricate high performance devices, materials with high fluorescence quantum yields in the solid state are necessary. 127 129 Compared to their molecular counterparts, polymeric materials may be advantageous when rationally designi conjugated systems for light emitting devices. As polymers contain a system of repeat units, the design of the material is not limited as it is in small molecules. This chapter will explore a class of polymeric materials, which are not ordinary in t he field of optoelectronics, for systematic control of efficiency and hole mobility. 4.1 Electroactive Polyolefins Polymeric materials are advantageous over their molecular counterparts as their ordered structure may be specifically tailored. Typical conj ugated polymers have delocalized electrons along the backbone, not localized on a single chromophore; and functionalization is branched off the main chain (Figure 4 1a). 130 Such polymers typically differ i n electrical and optical properties than their monomers, but nonetheless can be tuned to emit particular wavelengths achieving a desired color of light. 131

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109 Anoth er class of polymers are main chain functional systems. These polymers contain well defined chromophores, which are connected along an electrically insulated backbone (Figure 4 1b). Such polymers may contain various electroactive entities with varying emissive properties to produce a polymer with desired characteristics. 13,15,20,132 The last class of polymeric materials are side chain functional systems, which will be the focus of this work. These materials contain an electrically insulating aliphatic backbone, w ith conjugated electroactive entities pendant to the backbone (Figure 4 1c). 133 136 Such a class of materials may be referred to as electroactive polyolefins, whereas traditional polyolefins are desired for cable enclosures due to their electrically insulating properties. 137,138 Figure 4 1. Typical electroactive polymer system structures conjugated poly mers (a), main chain funtional polymers (b) and side chain functional polymers (c). Adapted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 Ameri can Chemical Society. Electroactive polyolefins are of interest in this work as they combine electrical and optical properties from pendant chromophore groups with a solubilizing aliphatic backbone. Various electroactive entities may be combined and place d pendant to the backbone for fine tuning of properties. These characteristics provide for a world of

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110 possibilities in designing optoelectronic materials for highly efficient, solution processable devices. To synthesize side chain functional polymers, ring opening metathesis polymerization (ROMP) has been use extensively. 136,139 141 However, ROMP typically affords polymers with variable spacing, and is limited to several carbons. Such polymers have been applied to hole transporting materials, 140 light emitting diodes, 139 and conductive polymers. 136 ROMP, however, does not produce precisely functional polymers, but rather polymers with variable s pacer length. For this work, we seek to obtain a fundamental understanding of electroactive entity spacing on an aliphatic backbone on device performance. functional polyolefins, acyclic diene metathesis (ADMET) p olymerization can be used. First proposed by Dr. Kenneth B. Wagener at the University of Florida, ADMET polymerization allows for the synthesis of diene functionalized electroactive chromophores. 142 In this work, ADMET polymerization was used to prepare regioregular terfluorenylidenes for PLEDs and regioregular triarylamine containing polyfluorenes for hole transporting materials. We s eek to systematically control device performance with chromophore spacer length along the polymer backbone. 4.2 Perfectly Regioregular Terfluorenylidene Polymers To probe the effects of chromophore spacing on device performance, a family of electroactive t erfluorenylidene containing polymers was synthesized. ADMET polymerization was used to synthesize polymers with a terfluorenylidene chromophore on each 8 th 14 th and 20 th carbon of an unsaturated aliphatic backbone (Figure 4 2a). The backbone was furthe r hydrogenated to afford another series of polymers with

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111 identical chromophores and spacing on a saturated backbone (Figure 4 2b). In addition to the regioregular terfluorenylidene polyolefins, regiorandom unsaturated (Figure 4 2c) and saturated (Figure 4 2d) analogues were synthesized. For full synthetic procedure and characterization discussion, the reader is directed to the appropriate publication. 87 For further discussions regarding this set of polymers, the acronyms presented in Figure 4 2 wil l be used. Figure 4 2. Family of electroactive terfluorenylidene polymers regioregular unsaturated (a), regioregular saturated (b), regiorandom unsaturated (c), and regiorandom saturated (d). Polymer light emitting diodes were fabricated with each of th e polymers in the family, using the procedure described in chapter 2. Devices were prepared with an architecture of substrate/ITO/PEDOT:PSS/active layer/LiF/Al. An interesting pixel phenomenon was observed with the P 9,9 TF S device, which will be discus sed later. 4.2.1 Electroluminescence To probe electroluminescence (EL), a fresh device was placed inside the device holder, and the method discussed in chapter 2 was employed. EL spectra of the saturated polymers at a constant driving bias are shown in Figure 4 3. Since the

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112 chromophores are identical in each polymer, their basic EL spectra are the same. However, there are some slight differences between them, most realized with the regiorandom polymer, P R21 TF S. This can be explained due to varied interaction between chromophores in the non ordered polymer (regiorandom) as opposed to the precisely ordered polymers (regioregular). The variations in EL spectra between the regioregular polymers, P 3,3 TF S, P 6,6 TF S, and P 9,9 TF S can again be explained by the different interactions as t he amount of spacing between chromophores changes. Figure 4 3. Electroluminescence spectra of saturated polymers at a constant driving bias. Reprinted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. 4.2.2 Luminance To probe the brightness of devices fabricated with the family of terfluorenylidene polymers, luminance was measured. Figure 4 4 shows luminance and current de nsity as a function of driving voltage for unsaturated and saturated polymers. Unlike the EL spectra (Figure 4 3), the devices exhibit very different J V characteristics. It can be seen that as chromophore spacer length increases, current density drops ( at a constant

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113 bias). Though not studied in this work, the decrease in current density may be the result of decrease charge carrier mobilities. Since the chromophores are pendant to the main chain, increasing the distance between them increases the interm olecular hopping distance, resulting in lower charge carrier mobilities. Figure 4 4. Luminance and current density as a function of driving voltage for unsaturated (a) and saturated (b) polymers. Data presented is for individual pixels. Reprinted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. Luminance, on the other hand, exhibits a much less ordered trend than the curre nt density, though it is still somewhat systematic. With low chromophore spacing, P 3,3 TF, P 3,3 TF S, P 6,6 TF and P 6,6 TF S, exhibit low luminance values of <35 cd m 2 The regiorandom polymers, P R21 TF and P R21 TF S, follow with slightly higher maximum luminance values of 43 and 45 cd m 2 respectively. Finally the polymers with the largest chromophore spacing, P 9,9 TF and P 9,9 TF S, exhibit the largest maximum luminance values of 93 and 366 cd m 2 respectively. Though the data presented in Figure 4 4 is of just a single pixel, it will be shown in section 4.2.3 that these results can be confidently accepted as uniform throughout the

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114 device. Thus, for this class of polymers, a lower chromophore density (increased chromophore spacer length) i s advantageous for increased brightness. 4.2.3 PLED Performance After inves tigating the brightness, which is important when applying materials in an industrial setting, we investigated the performance of the devices. The performance of PLEDs was probed us ing a silicon photodiode and the procedure described in chapter 2. As with the EL, luminance, and current density characteristics, a trend with the external quantum efficiency (EQE) is also noticed. Calculated EQEs for the entire family of terfluorenylid ene polymers are presented in Figure 4 5. Average and max EQEs are presented for at least 8 pixels; with the error bars indicating one standard deviation. Indicated by the relatively low standard deviations, there is little pixel to pixel variation withi n a device. Thus, the current density and luminance data presented in Figure 4 4 can be confidently accepted as accurately representing any pixel on the device. PLED EQE data presented in Figure 4 5 show similar trends as luminance and current density dat a. There is an increase in EQE as the spacing between chromophores increases (decreasing chromophore density). Additionally, saturating the backbone is beneficial to PLED performance, as shown by increasing EQEs. This increase is most evident in P 9,9 T F S, which exhibits a two fold increase in EQE upon saturation. As shown before, P 9,9 TF S also shows a three fold increase in luminance upon saturation. This odd behavior will be described in section 4.2.4. It can be concluded from this data that devi ce performance increases as spacer length between chromophores increases, or chromophore density decreases.

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115 Figure 4 5. Calculated EQEs for the entire family of terfluorenylidene polymers. Data is represented as: average EQE (max EQE), with error bars giving one standard deviation. Reprinted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. 4.2.4 Pixel Anomaly As shown thr ough luminance and PLED EQE calculations, P 9,9 TF S is an outlier when considering the general trend of spacer length. AFM images were taken of P 3,3 TF S, P 6,6 TF S, and P 9,9 TF S using the procedure described in chapter 2 and are presented in Figure 4 6. Calculated surface roughnesses of these images were 0.308, 0.200, and 0.235 nm, respectively. Such roughness data indicate very flat films with no features or aggregation. Upon closer investigation, it was noticed that the top aluminum electrode had a different appearance on the P 9,9 TF S device. Immediately upon removal from the thermal evaporation chamber, inside the gloveblox, a surface coloration was visually apparent. As described in chapter 2, thermal evaporation vapor deposits metals at ver y high temperatures. Since the device holder has room for 8 devices (Figure 2 3), all

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116 devices received the top aluminum electrode during the same deposition. Thus, the variation in electrode color may be a result of a temperature dependent property of th e P 9,9 TF S polymer, rather than experimental error or the presence of impurities. Figure 4 6. AFM images of P 3,3 TF S (a), P 6,6 TF S (b), and P 9,9 TF S (c). All images are 5 m 5 m and have a same 2 nm height scale bar. Adapted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. Figure 4 7. Optical microscope images of the aluminum electrode on devices with P 3,3 T F S (a), and P 9,9 TF S (b). Arrows point to the Al/film edge. Adapted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. Fur ther inspection under an optical microscope revealed indeed a surface coloration as well as a surface roughness of the aluminum electrode on the P 9,9 TF S device (Figure 4 7b). Typically, the aluminum electrode appears smooth and has a

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117 shiny metallic sur face, as was observed for P 3,3 TF S (Figure 4 7a). Analysis of surface features of the aluminum electrodes via AFM was difficult and not achieved. The variation in electrode appearance may be a result of the thermal characteristics of P 9,9 TF S. Differential scanning calorimetry (DSC) was used to calculate a T g of 34 o C for P 9,9 TF S, which is near room temperature. We are led to believe that the coloration upon device removal from the thermal evaporation chamber was a result of the high tempera tures during deposition. Since the temperature required to vaporize metals is high, there could be sufficient energy to surpass the T g This would result in rearrangement of polymer chains underneath the electrode during thermal deposition, altering the morphology of the aluminum electrode. Upon testing the devices, a fluctuation in pixel light output was visible for P 9,9 TF S. Typically when a bias is applied to a PLED, light is uniformly emitted from the pixel (Figure 2 8). However, when P 9,9 TF S w as turned on, luminosity was visually changing. A video was taken of a P 9,9 TF S pixel at a constant driving bias of 13 V, and can be viewed in Object 4 1. Figure 4 8 shows time lapsed images of a pixel from the video. Figure 4 8. Time lapsed images taken every 2.5 s of a P 9,9 TF S pixel at a constant driving bias of 13 V, starting from the top left and ending at the bottom right. Reprinted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynol ds, J. R.; Wagener, K. B. Macromolecules 2012 45 (2), 705. Copyright 2012 American Chemical Society. Object 4 1. Video of a P 9,9 TF S pixel at a constant bias of 13 V (.avi file 20.6 MB)

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118 The fluctuation i n PLED luminosity during testing is less questionable than the electrode appearance. Since the fluctuation is observed when a bias is applied and the device is turned on, it may also be a result of the low T g of P 9,9 TF S. An explanation for this may be the presence of Joule heating, an electro thermal phenomenon typically experienced in transistors and electronic devices, first proposed by James Joule in 1841. 143 Also referred to as resistive heating, Joule heating describes the generation of heat due to a current passing, and can be quantified by: (4 1) Where P is the power dissipated, I is the current flow and R is the resistance of the material. Though it would be very difficult to calculate the exact temperature at which the device is raised to during testing, Joule heating has been known to raise the temperature of PLEDs several tens of degrees. 144 146 Such a rise in temperature may provide sufficie nt energy to surpass the T g and cause polymer chain rearrangement underneath the electrode. This movement would in turn result in the temporal and spatial morphological changes observed in the video of the pixel. To completely confirm the speculation that the fluctuation in pixel luminosity is a result of Joule heating, the device would need to be tested at extremely low temperatures. A possible method to do this would be to devise an experimental apparatus that c an be fully submerged in liquid nitrogen. This would keep the device at liquid nitrogen temperature during testing, causing the heat generated by Joule heating to be negligible. Due to time constraints and experimental limitations, however, this was not carried out in this work.

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119 4.3 Sequenced Triarylamine Functionalized Polyolefins For PLEDs to be competitive with their molecular and inorganic counterparts, charge carrier mobilities must be balanced for effective charge recombination. 79,83,126 Highly efficient organic light emitting devices operate with a multilayer structure consisting of an active layer sandwiched by an electron transport layer and a hole transport layer. 77,78,82,147 149 To date, there has been much research effort geared towards synthesizing and optimizing electron 78,82,150 153 and hole 16,36,40,81,134,147,154 156 transport layers for high performance multilayer organic electronic devices. In some cases, reduced hole mobilities are in fact beneficial to device performance as hole mobiliti es typically exceed electron mobilities. 83 To achieve efficient charge transport charge carrier mobilities must be balanced. In fact, studies where hole mobilities were purposefully reduced to fabricate highly efficient blue light em itting devices have been conducted. 157 To our knowledge, however, polymeric materials with precisely spaced chromophores along an aliphatic backbone hav e not been applied as charge carrying layers in light emitting devices. Having demonstrated systematic control of PLED EQE via inter chromophore distance on pendant functional polymers, 87 we were compelled to study hole injection materials. A fa mily of pendant functional polyolefins with triarylamine flanked fluorenes were synthesized via ADMET. 88 Synthesis afforded polymers with chromophores on every 9 th 15 th and 21 st carbon on an unsaturated backbone (Figure 4 9a). The backbone was f urther hydrogenated to afford identically spaced chromophores on a saturated backbone (Figure 4 9b). The design of chromophores was based on prior advances in light emitting devices. Triarylamines have been demonstrated as efficient

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120 hole injectors in OLE Ds. 80,158 163 A triarylamine flanked polyfluorene, or 2,7 bis(phenyl m toluylamino)fluorene (TPF) has been studied as a hole transporter that can be modified into polymer structures. 80 Additionally TPF is structurally similar to a more bis(phenyl m toluylamino)biphenyl) (Figure 1 14a) 159 Combining the hole transporting properties of TPF and a solubilizing aliphatic backbone, we believe this class of hole transporting materials can sy stematically control charge carrier mobilities for highly efficient PLEDs. Figure 4 9. Family of regioregular sequenced triarylamine functionalized polyolefins unsaturated (a) and saturated (b) hole transporters. To probe the effects of spacer length in the TPF polymers, space charge limited current (SCLC) devices were prepared as described in chapter 2. Devices were fabricated with an architecture of glass/ITO/PEDOT:PSS/active layer/gold. Unlike SCLC devices made with OPV materials, which used MoO 3 as the hole injecting layer, these devices received PEDOT:PSS instead. Though MoO 3 is known to be an efficient hole transporting layer, 36,164 J V characteristics better fit the SCLC model with PEDOT:PSS containing devices with these TPF polymers. This may be advantageous from an industrial point of view as PEDOT:PSS is spun cast from aqueous solution, whereas MoO 3 requires high vacuum vapor deposition, which is quite costly.

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121 Hole mobilities were calculated using the field dependent SCLC model (Equation 1 14), with a thickness (L) of 70 nm, determined by tapping mode AFM. Average hole mobility data and current voltage characteristics are sum marized in Figures 4 10a and 4 10b, respectively. Immediately, it is noticed that the current, and consequently hole mobility, decreases with increasing space between chromophores, with the exception of PE9TPF, which will be discussed later. Figure 4 10. Summary of average hole mobility data (a) and current voltage characteristics (b) for SCLC devices. For image a, the error bars indicate one standard deviation for a minimum of 8 pixels. Adapted (a) and reprinted (b) with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. ACS Macro Letters 2012 1 (2), 324. Copyright 2012 American Chemical Society. Representative current density voltage curves with SCLC fits are presented in Figure 4 11a and 4 11b for unsaturated and saturated polymers, respectively. These SCLC fits represent a single pixel, however, average hole mobilities presented in Figure 4 10a show a very small standard deviation allowing us to accept these J V curves as accurately representing any given pixel on a device. The decrease in current, and consequently hole mobility can be attributed to the structure of the materials. As spacer length between chromophores on the aliphatic, a nd thus electrically insulating backbone

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122 increases, the int ermolecular hopping distance also increases. Since the intermolecular hopping distance is increased, hole mobility is thus decreased. Figure 4 11. Representative J V plots of unsaturated (a) and saturated (b) TPF polymer containing SCLC devices. Data and SCLC fits represent individual pixels. Adapted with permission from Aitken, B. S.; Wieruszewski, P. M.; Graham, K. R.; Reynolds, J. R.; Wagener, K. B. ACS Macro Letters 2012 1 (2), 324. Copyright 2012 American Chemical Society. There is however an exception to the general trend of spacer length and hole mobility PE9TPF. After hydrogenating the backbone of P33TPF, the afforded polymer, PE9TPF, had a compromised solubility. This was possibly a result of the necessity to run the hydrogenation dilute, with a larger amount of catalyst; and thus the presence of residual catalyst in the resulting saturated polymer. Consequently, when SCLC devices were fabricated, extra measures were taken with solution preparation. All polymer solutions were prepared at 15 mg mL 1 in chlorobenzene, and were allowed to stir overnight at room temperature. PE9TPF, however, did not go into solution as easily as the rest of the family. Consequently, the solution of PE9TPF was heated to 50 o C and stirred overnight. This sti ll did not fully dissolve the polymer. The solution was raised to 100 o C just prior to spin casting, which allowed the polymer to completely dissolute. Subsequently, this extra heating may have led to polymer degradation and

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123 thus the formation of traps i n the solid state device, resulting in a lower charge carrier mobility. 4.4 Chapter Summary The use of pendant functional electroactive polyolefins to controllably tune PLED EQE and hole mobility has been demonstrated. Regioregular and regiorandom terfl uorenylidene polyolefins were synthesized and incorporated into PLEDs. Additionally, regioregular triarylamine functionalized polyolefins were synthesized and incorporated into SCLC devices. Finally, the effects on PLED EQE and hole mobility by backbone saturation were studied for the terfluorenylidene and triarylamine polymers, respectively. The terfluorenylidene polymers were shown to have increased luminance and PLED EQE with increasing spacer length, and thus lower chromophore density. Saturating the backbone of the polyolefins showed minor, but nonetheless, increases in EQE. Upon satura tion of P 9,9 TF, luminance increased three fold, and EQE doubled. Due to a low T g P 9,9 TF S experienced temporal and spatial morphological changes underneath the top electrode, presumably a result of Joule heating. A possible instrumental apparatus was proposed to fully confirm the occurrence of Joule heating during PLED testing. Sequenced triarylamine functionalized polyolefins were shown to control hole mobilities over 3 orders of magnitude from ~ 7 10 7 to ~ 3 10 10 cm 2 V 1 s 1 Increasing th e spacer length of chromophores on the aliphatic backbone resulted in a decreased hole mobility due to increasing intermolecular hopping distances. Upon saturation of P33TPF, solubility was compromised due to the presence of residual

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124 catalyst. This resul ted in possible polymer degradation and the formation of trap states due to the necessity of heating the solution prior to spin casting. As evidenced by the results presented in this chapter, pendant functional polymers with chromophores precisely spaced on an electrically insulating backbone, or electroactive polyolefins, are promising materials for light emitting devices. An ideal spacer length may be optimized for electroactive materials to achieve reproducibly high EQEs. Additionally, hole mobilitie s may be strategically tuned to match those of electron mobilities for efficient charge recombination, and thus increased performance. We believe that the synthesis of functional electroactive polyolefins is not limited to the materials presente d in this work. These polymers have immense promise in the field of optoelectronics and can be prepared with a wide variety of chromophores to produce highly efficient light emitting devices.

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125 CHAPTER 5 PERSPECTIVES AND OUT LOOK Over t he past several decades, the field of organic electronics has grown immensely. Many research efforts are being made towards novel synthetic materials for optoelectronic applications in the industrial and consumer markets. Compared to petroleum based mate rials, solar energy may provide a form of cleaner and cheaper power. Additionally, larger, thinner, more lightweight displays with lower power consumption are being sought after. However, for organic optoelectronics to transition from the classic spin co ating process of the laboratory setting to roll to roll printing in the industrial setting, research efforts must be maximized in optimizing synthetic protocols and device design. 5.1 Bulk Heterojunction Organic Photovoltaics The first portion of this work examined the use of tailor made additives for morphological control and increased device performance in molecular BHJ OPVs. Novel synthesis of asymmetric and symmetric derivatives, with bulky and small end groups was achieved. At optimized tailor made a dditive concentrations, critical OPV parameters such as the FF, J SC V OC and consequently PCE were increased. This work, however, was only applied to an isoindigo oligothiophene system. Tailor made additives can be synthesized over a variety of organic materials for electronic applications. The use of tailor made additives in conjunction with solvent additives may lead to breakthroughs in device performance of molecular BHJ OPVs pushing them over their polymeric counterparts. It is without a doubt that organic and polymeric photovoltaics lag behind their inorganic counterparts in terms of PCE. For industrialization, these PCEs must be

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126 increased. However, a researcher working with OPVs should not be discouraged by low device results. OPVs have many adv antages over their inorganic counterparts in a practical industrial and consumer setting they can be cheaply fabricated using non toxic materials. Additionally, OPVs may be solution processed onto thin flexible substrates giving rise to innovative applic ations t shirts, backpacks, wrappings for buildings, etc.; whereas inorganic cells are heavy and brittle, thus limited to a rooftop or solar field. Furthermore, organic materials pose many advantages over their polymeric counterparts as well simpler purif ication, less end group contamination, no molecular weight variation, easier functionalization, and more reproducible syntheses. 5.2 Polymer Light Emitting Diodes The second portion of this work examined the synthesis and application of unique electroacti ve polymers. Typically, polyolefins are sought after industrially for their insulating properties. However, through ADMET, polyolefins can be synthesized with polyolef A family of terfluorenylidene polymers with precisely spaced chromophores was synthesized and incorporated into simple PLEDs. The systematic increase in EQE with increasing chromophore distance is very exciting. The systems observed in this work us ed 9,9 dioctylfluorene as the active fluorophore. However, as discussed in chapter 1, new emissive molecules are being synthesized and applied to optoelectronics daily. ADMET synthesis can be applied to many flavors of chromophores, and carbon spacing ma y be precisely tuned to achieve ideal EQEs. Additionally, such materials may be applied to multilayer devices utilizing electron and hole transport layers for increased performance.

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127 When designing synthetic materials for PLED application, one must consider charge carrier mobilities in the solid state device. As electrons and holes are injected at the electrodes in a PLED, they recombine in the emissive layer forming an exciton, which decays to give of f the characteri stic light the human eye recognize s Balanced hole and electron mobilities are crucial for efficient charge transfer, recombination, and exciton formation. This work examined the synthesis and application of triarylamine functionalized polyolefins for sy stematic control of hole mobility. Though the hole mobilities observed were lower than their inorganic counterparts, they exceeded many electron mobilities in comparable organic systems. ADMET may become a gateway to synthesizing polymers with precisely spaced chromophores for both electron and hole transporting layers. Using similar chromophores with lined up charge mobilities may lead to higher performing PLEDs. Though polymeric systems lag behind their organic counterparts, molecular design has infini te possibilities. A number of companies of already producing OLED televisions available to the consumer, and the results are may become ch eaper and more widely available to the consumer.

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128 APPENDIX DERIVATION OF THE FI ELD INDEPENDENT AND DEPENDENT MOTT GURNEY LAW Consider a p type semiconducting material sandwiched between two high work function metals to create ohmic contacts (Figure A 1), where only holes are injected, no electrons. Figure A 1. Schematic of an organic semiconducting material sandwiched by tw o electrodes, with an applied voltage (V) and diffusion length (L) of the current flow in the positive x direction. ) is given by (A 1) Considering the positive x direction as the flow of electrical current we have (A 2) Where E is the electric field, and is the permittivity (see section 1.1.4). At steady states, the continuity equation holds, indicating the current density, J, is independent of the x directi on, (A 3) The current density may be expressed as, (A 4) Where is the charge mobility. Substitution of A 2 into A 4 gives

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129 (A 5) Rearrangement of A 5 into a usable form, ( A 6) Let us allow E(x) = E and define the variable for ease of derivation, (A 7) Applying a boundary condition so that E(0) = 0 and integrating gives (A 8) Given the length of diffusion in the positive x direction as L (Figure A 1), and the voltage across the active layer (V) given by (A 9) Then rearranging, integrating both sides of A 8, and applying A 9, gives (A 10) Substituting A 7 into A 10 and solving for the current density gives us the Mott Gurney law of the space charge limited current, (A 11)

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130 Equation A 10 provides the field independent space charge limited current model. However, the field dependent mobility is of interest in this work as it most accurately fits the data obtained. To begin the derivation of the field dependent space charge limited current we combine Equations 1 13 (field dependence of the charge mobility) and A 4, (A 12) Again, rearrangement, allowing E(x) = E, and defining a variable, for ease of derivation, (A 13) Substituting A 13 gives (A 14) Integrating both sides of A 14 and applying A 9 gives (A 15) Substitution of A 13 into A 15 gives (A 16) Integration and expansion of A 16 is beyond the scope of this work, however, a detailed approximate solution has been demonstrated by Murgatroyd, 38 and yields the widely used field dependent space charge limited current model, (A 17)

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141 BIOGRAPHICAL SKETCH Patrick Michael Wieruszewski was born April 30, 1990 in Brandon, Florida to Marzenna and Maciej Wieruszewski. He grew up with an older sister and spent his childhood in Riverview, Florida, For high school, he attended King High for two years followed by Riverview High for the last two years. In the fall of 2008, Patrick bega n his undergraduate career at the University of Florida in Gainesville, FL with a major in Chemistry. In the spring of 2010, he joined the research group of Dr. John R. Reynolds with Ken Graham serv ing as his mentor. He initially worked on obtaining images of thin films using an Atomic Force Microscope, but was quickly interested in optoelectronics He began fabricating and characterizing OPVs and PLEDs leading to the work presented in this thesis Patrick earned his Bachelor of Science degree in Chemistry with a minor in Materials Science and Engineering from UF in the spring of 2012 His girlfriend Megan earned her Bachelor of Science in Nursing, also from UF in the spring of 2012 After gradu ation, Patrick and Megan will move to Jacksonville, FL, where Patrick will be beginning the Doctor of Pharmacy program at the University of Florida, College of Pharmacy, and Megan will be working as a registered nurse.


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