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MORPHOLOGY CONTROL IN POLYMER LIGHT EMITTING DIODES AND MOLECULAR BULK-HETEROJUNCTION PHOTOVOLTAICS

By

KENNETH R. GRAHAM

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2011

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© 2011 Kenneth R. Graham

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To Mom, Dad, and my wife, Katherine

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ACKNOWLEDGMENTS

I would like to start by thanking my parents, my wife, my family, and my friends.

Without this group of people I would not be where I am today, nor would I have enjoyed

my graduate school experience as much as I have. I thank my parents for always

allowing me to choose my own direction, while encouraging and supporting me in

whatever direction that may have been. It was this freedom from pressure that allowed

me to approach both undergraduate and graduate school with the idea of pursuing my

interests, which ultimately led me to the University of Florida and the research

presented in this dissertation.

I thank my wife Katherine for being with me throughout undergraduate and

graduate school. She has helped me to maintain a good balance in life, which

ultimately makes every aspect more enjoyable. I especially thank Katherine for

everything she has done over the last few months, without her help I would undoubtedly

not have been able to finish writing this dissertation on time. I thank my friends and

former roommates David Liu, Jared Lynch, Mike Hyman, and Jeff Carter for helping

graduate school to be a very enjoyable time. I also thank the rest of my friends I have

met here at UF for the same.

I thank my research advisor, Professor John Reynolds, for the invaluable guidance

and encouragement he has given me both in research and in life. I thank Dr. Reynolds

for guiding my research in such a way as to provide me with the freedom to pursue my

own ideas while at the same time helping me to stay focused. I also thank Dr. Reynolds

for realizing that work or school is not the only part of life, and for always supporting

other group members and myself in pursuing the other parts of life. I also thank Dr.

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Reynolds and Dianne Reynolds for the social environment they helped to encourage

outside of work through their lakehouse and Christmas parties.

I am grateful for the whole Reynolds group. This group of people has made the

Reynolds lab an enjoyable and productive place to work. Additionally, I have enjoyed

the social events we have had outside of lab as well as traveling to conferences with

other group members. I also thank Dr. Reynolds for encouraging and supporting me

and other group members to travel to these conferences.

I owe a lot of thanks to the synthetic chemists who made the work reported in this

dissertation possible. These include Stefan Ellinger, Tim Steckler, and Dan Patel for

the synthesis of the fluorescent emitting PLED materials, Jonathan Sommer for the Pt-

porphyrin compounds, Jianguo Mei for the original isoindigo materials, and Romain

Stalder, Dan Patel, and Frank Arroyave for the isoindigo materials and asymmetric

oligomers. I owe additional thanks to Dan Patel for his assistance in helping me to

understand and teach organic chemistry, working with me on the XLS project, and

organizing group social events. I also owe additional thanks to Romain Stalder for the

large quantities of isoindigo oligomers, both symmetric and asymmetric, he has

synthesized.

I am extremely grateful for all of the people I have worked with in the MCCL.

These include Richard Farley, Aaron Eshbaugh, Quentin Bricaud, Nathan Heston, Evan

Donoghue, Sridhar Rajam, Aubrey Dyer, Danielle Salazar, Patrick Wieruszewski, and

Caroline Grand. I appreciate Aubrey helping me to get started in the lab and Nate for

helping to teach me solar cells and glovebox maintenance. I also thank Evan for his

help in maintaining the glovebox throughout the years. I owe a lot of thanks to Patrick

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and Danielle for working with me and helping to carry out some of the experiments

reported in this dissertation. I am also grateful to Dr. Reynolds for hiring Patrick and

Danielle to help out with the MCCL and the solid-state projects in the group. I also

thank Caroline for joining the group and for her willingness to take over many of my

responsibilities in the MCCL.

I thank my other committee members Dr. Andrew Rinzler, Dr. Charles Cao, Dr.

Nicolo Omenetto, and Dr. Kirk Schanze for serving on my committee. I also thank Dr.

Kirk Schanze for his guidance on the XLS project. I would also like to thank everyone

who has helped with all of the various parts of graduate school, including Lori Clark,

Sara Klossner, Todd Prox, Gena Borrero, Bob Johnson, Annyetta Douglas, and Dr. Ben

Smith.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES .......................................................................................................... 11

LIST OF FIGURES ........................................................................................................ 13

LIST OF ABBREVIATIONS ........................................................................................... 19

ABSTRACT ................................................................................................................... 21

CHAPTER

1 INTRODUCTION .................................................................................................... 23

Organic Electronics Background ............................................................................. 23 Material Fundamentals ..................................................................................... 24 Definitions ......................................................................................................... 26 Charge Transport ............................................................................................. 28

Organic Light Emitting Diodes (OLEDs).................................................................. 29 Device Operating Principles ............................................................................. 30 Energy Transfer ................................................................................................ 33 Morphology ....................................................................................................... 35 Near-Infrared (Near-IR) Emitting OLEDs .......................................................... 36

Organic Photovoltaics (OPVs) ................................................................................ 41 Device Operating Principles ............................................................................. 42 The Bulk-Heterojunction (BHJ) ......................................................................... 48 Morphology Control .......................................................................................... 50 Small Molecule BHJ OPVs ............................................................................... 54 Space-Charge-Limited Current (SCLC) Mobility ............................................... 57

Morphology Fundamentals ..................................................................................... 59 Thermodynamics of Mixing ............................................................................... 59 Solubility Parameters ....................................................................................... 63 Morphology and Solubility ................................................................................ 66

Overview of Dissertation ......................................................................................... 67 Chapter 2- Experimental Techniques ............................................................... 67 Chapter 3- Fluorescent Near-IR Emitting Polymer Light Emitting Diodes

(PLEDs)......................................................................................................... 68 Chapter 4- Pt-Porphyrin Based Near-IR Emitting PLEDs ................................. 69 Chapter 5- Poly(dimethylsiloxane) (PDMS) as an Additive in Molecular BHJ

OPVs ............................................................................................................. 69 Chapter 6- Solvent Additives (SAs) for Morphology Control in BHJ OPVs ....... 70 Chapter 7- Controlled Morphology Through Tailor-Made Additives .................. 70

2 EXPERIMENTAL TECHNIQUES ............................................................................ 72

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Materials ................................................................................................................. 72 Solvent Purification ........................................................................................... 72 Materials for Device Processing ....................................................................... 73 Thermal Evaporator Materials .......................................................................... 74 PLED Materials ................................................................................................ 74 Solar Cell Materials .......................................................................................... 75 Solvent Additives for OPV Cells ....................................................................... 75

Device Fabrication .................................................................................................. 77 Indium Tin Oxide (ITO) Preparation ................................................................. 77 Polymer Light Emitting Diodes ......................................................................... 79 Photovoltaic Cells ............................................................................................. 81 SCLC Devices .................................................................................................. 82

Measurement Techniques ...................................................................................... 84 PLEDs .............................................................................................................. 84

Silicon photodiode measurement setup ..................................................... 84 Calibration .................................................................................................. 87 Performance calculations ........................................................................... 92 PLED measurement ................................................................................... 93

Photovoltaic Cells ............................................................................................. 99 Power conversion efficiency ....................................................................... 99 Incident photon to current efficiency ........................................................ 101

Spectroscopic Characterization ............................................................................ 103 Solubility Measurements ....................................................................................... 104 Morphology Characterization ................................................................................ 105

Atomic Force Microscopy (AFM) .................................................................... 105 Tapping mode .......................................................................................... 106 Thickness measurements ........................................................................ 108 Phase imaging ......................................................................................... 108 Conductive AFM and electrostatic force microscopy (EFM)..................... 109

Transmission Electron Microscopy (TEM) ...................................................... 110 Top-down sample preparation ................................................................. 110 Bright-field TEM ....................................................................................... 111 Focal-length dependence ........................................................................ 113

Focused Ion Beam for TEM Sample Preparation ........................................... 114 X-Ray Photoelectron Spectroscopy ................................................................ 119 Solution Crystallization Studies ...................................................................... 120

3 FLUORESCENT NEAR-IR EMITTING PLEDS ..................................................... 122

Materials ............................................................................................................... 122 EE-BTD-EE(THP) Based PLEDs .......................................................................... 124

Electro and Photoluminescence ..................................................................... 125 PLED Performance ........................................................................................ 128 Film Morphology ............................................................................................. 130

Donor-Acceptor-Donor (D-A-D) Oligomers with Bulky End-Groups ...................... 132 Electro and Photoluminescence ..................................................................... 133 PLED Performance ........................................................................................ 136

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Cross-linkable PLEDs ........................................................................................... 139 Device Fabrication .......................................................................................... 140 Single-Layer Device Results .......................................................................... 141 Multi-Layer Devices ........................................................................................ 143

Chapter Summary ................................................................................................. 146

4 PT-PORPHYRIN BASED NEAR-IR EMITTING PLEDS ....................................... 148

Materials ............................................................................................................... 148 π-Extended Pt-Porphyrins .................................................................................... 149 Pt-Tetrabenzoporphyrins ...................................................................................... 153

Photophysical Characterization ...................................................................... 154 PLED Performance ........................................................................................ 158

Chapter Summary ................................................................................................. 162

5 PDMS AS AN ADDITIVE IN MOLECULAR BHJ OPVS ........................................ 164

Materials ............................................................................................................... 164 PDMS as an Additive ............................................................................................ 166

Glass vs. Plastic Syringes .............................................................................. 166 PDMS Concentration ...................................................................................... 168

OPV device results .................................................................................. 169 Inter-laboratory reproducibility .................................................................. 171 Film morphologies .................................................................................... 173

Thermal Annealing ......................................................................................... 177 OPV device results .................................................................................. 177 Film morphologies .................................................................................... 178

PDMS Molecular Weight ................................................................................ 180 OPV device results .................................................................................. 180 Film morphologies .................................................................................... 182

PDMS as a Buffer Layer ................................................................................. 183 Hole and Electron Mobilities ........................................................................... 186

Chapter Summary ................................................................................................. 189

6 SOLVENT ADDITIVES FOR MORPHOLOGY CONTROL IN BHJ OPVS ............ 191

Solubility ............................................................................................................... 192 Hansen Solubility Parameters ........................................................................ 192 Solubility Measurements ................................................................................ 194

Device Results ...................................................................................................... 196 BHJ OPV Results ........................................................................................... 196 SCLC Mobilities .............................................................................................. 199

Film Morphologies ................................................................................................ 201 AFM Characterization ..................................................................................... 201 Top-Down TEM Characterization ................................................................... 202 Cross-Sectional TEM Characterization .......................................................... 204

Mechanism ........................................................................................................... 207

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Combined Additives and the Open Circuit Voltage (VOC) ...................................... 209 Chapter Summary ................................................................................................. 213

7 CONTROLLED MORPHOLOGY THROUGH TAILOR-MADE ADDITIVES .......... 215

Background ........................................................................................................... 216 Materials and Strategy .......................................................................................... 218 iI(TT)2-iBu3Si ......................................................................................................... 219

Crystallization ................................................................................................. 219 Morphology ..................................................................................................... 221 BHJ OPV Device Results ............................................................................... 226

iI(TT)2-H ................................................................................................................ 228 Film Morphology ............................................................................................. 228 BHJ OPV Device Results ............................................................................... 230

Chapter Summary ................................................................................................. 232

8 PERSPECTIVES AND OUTLOOK ....................................................................... 234

Near-IR Emitting PLEDs ....................................................................................... 234 Molecular BHJ OPVs ............................................................................................ 237

APPENDIX: CALCULATION OF SURFACE COMPOSITION FROM XPS MEASUREMENTS ............................................................................................... 242

LIST OF REFERENCES ............................................................................................. 244

BIOGRAPHICAL SKETCH .......................................................................................... 260

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LIST OF TABLES

Table page 1-1 Work functions, highest occupied molecular orbitat (HOMO), lowest

unoccupied molecular orbital (LUMO), and redox energies vs. vacuum. ............ 27

1-2 Definitions of terms used in characterizing organic light emitting diodes (OLEDs) and OLED materials. ........................................................................... 30

2-1 Details of poly(dimethylsiloxane) (PDMS) materials used in organic photovoltaic (OPV) cells. .................................................................................... 76

3-1 Solution absorbance maxima, fluorescence maxima, and quantum yields for the near-IR emitting donor-acceptor-donor (D-A-D) oligomers. ........................ 133

3-2 Polymer light emitting diode (PLED) performance characteristics for T-BBT-T(EtHx) and T-BBT-T(iBu3Si) in PVK:PBD at varying doping concentrations. .. 138

4-1 Pt-TPTBP, Pt-TPTNP, and Pt-Ar4TAP solution emission maxima, quantum yields, and PLED performance characteristics. ................................................ 150

4-2 Photophysical properties of the Pt-tetrabenzoporphyrins (Pt-TBPs) in solution and doped in polymer films at

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A-2 Elemental composition as a function of the exit angle for the device containing 0.2 mg/mL 14,000 MW PDMS. ........................................................ 243

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LIST OF FIGURES

Figure page 1-1 Energy levels as a function of the number of aromatic rings in a typical

conjugated polymer and the development of band structure as the number of rings is increased. ............................................................................................... 25

1-2 Energy diagram of isolated materials. ................................................................ 26

1-3 Schematic of organic light emitting diode (OLED) operation .............................. 31

1-4 Schematic of a multilayer OLED or polymer light emitting diode (PLED) whereby the electron transport layer (ETL) and hole transport layer (HTL) help to confine the charges to the emissive layer. .............................................. 32

1-5 Exciton formation on the emissive dopant .......................................................... 34

1-6 Structures of example near-infrared (near-IR) emitting polymers ....................... 37

1-7 Schematic illustration of the processes occurring in an organic photovoltaic (OPV) device. ..................................................................................................... 43

1-8 Example J-V curves for an OPV under illuminatation and in the dark ................ 47

1-9 Schematic cross-sections of bulk-heterojunction (BHJ) morphologies ............... 49

1-10 Selected electron acceptor and electron donor materials used in OPVs. ........... 56

1-11 Energy level schematic of space-charge-limited current (SCLC) devices .......... 58

2-1 Indium tin oxide (ITO) etch pattern. .................................................................... 77

2-2 Solidworks computer animated design (CAD) drawing of silicon photodiode holder ................................................................................................................. 85

2-3 Solidworks CAD drawing of device holder. ......................................................... 86

2-4 Photographs of completed silicon photodiode holder ......................................... 87

2-5 Spherical coordinate system where the dark gray circle represents the photodiode and the PLED pixel is located at the origin. ..................................... 88

2-6 Experimental setup for the pixel to photodiode distance dependent measurements .................................................................................................... 90

2-7 Luminance vs. photodiode detector current ........................................................ 92

2-8 Block diagram showing the experimental setup for measuring PLEDs. .............. 95

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2-9 Screenshot of the PLED test program with red boxes highlighting where the above described information must be entered .................................................... 98

2-10 Schematic representation of the three standard conditions for measuring photovoltaic (PV) cells. ..................................................................................... 100

2-11 Radiant power of the monochromatic light incident on the masked photodiode as a function of the wavelength ..................................................... 102

2-12 Schematic of the Veeco Innova atomic force microscope (AFM) setup. ........... 107

2-13 Schematic showing the phase shift between driving piezo and cantilever. ....... 109

2-14 Schematic showing how a 3-dimensional sample would appear as a 2-dimensional projection through bright field transmission electron microscope (TEM) imaging. ................................................................................................. 112

2-15 TEM images of an oligomer-fullerene blend film at under-focus, focus, and severe over-focus lengths. ............................................................................... 114

2-16 Ion-beam and electron-beam images taken during cross-section preparation . 118

3-1 Structures and acronyms of materials used in fluorescent emitting PLEDs. ..... 123

3-2 Solution photoluminescence (PL) spectra MEH-PPV, PBD, PVK, EE-BTD-EE(THP) (solid), and absorbance of EE-BTD-EE(THP) (dashed). ................... 125

3-3 Film spectra of EE-BTD-EE(THP) .................................................................... 126

3-4 Energy diagram showing the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PVK, PBD, EE-BTD-EE(THP), and MEH-PPV. ................................................................................. 127

3-5 Relative intensity of the EL spectra for varying doping concentrations of EE-BTD-EE(THP) ................................................................................................... 129

3-6 Current density and radiant emittance as a function of the applied bias and the external quantum efficiency (EQE) as a function of current density for EE-BTD-EE(THP) in MEH-PPV PLEDs. ................................................................. 129

3-7 Tapping mode AFM height images of EE-BTD-EE(THP) in MEH-PPV films. . 131

3-8 Bright field TEM images of EE-BTD-EE(THP) in MEH-PPV films. .................... 131

3-9 Solution absorbance (dashed) and PL emission (solid) spectra for E-BBT-E(EtHx) ............................................................................................................. 133

3-10 PL spectra of T-BBT-T(EtHx). ........................................................................... 136

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3-11 Absorbance of PVK and PBD in chlorobenzene with doped film excitation wavelength range (λexc) indicated by dotted lines. ............................................ 136

3-12 Electroluminescence (EL) spectra of donor-acceptor-donor (D-A-D) trimers in PVK:PBD at 1% (by wt.) at 20 V applied bias. .................................................. 137

3-13 Absorbance (dashed) and PL (solid) spectra for PFOX and E-BTD-E(MI) in chloroform......................................................................................................... 141

3-14 EL spectra of varying concentrations of E-BTD-E(MI) in PFOX........................ 142

3-15 Radiant Emittance (a) and EQE (b) of control E-BTD-E(MI) doped PFOX PLEDs without PC and without thermal cross-linking. ...................................... 143

3-16 Radiant Emittance (a) and EQE (b) of E-BTD-E(MI) doped PFOX PLEDs with PC and thermal cross-linking. ........................................................................... 143

3-17 EL spectra of E-BTD-E(MI) in PFOX with TAZ:PFO as an ETL at 100 mA/cm2. ............................................................................................................ 145

3-18 Radiant emittance (a) and EQE (b) of E-BTD-E(MI) in PFOX with TAZ:PFO as an electron transport layer (ETL) after removal of light below 500 nm with a long-pass filter. .............................................................................................. 146

4-1 Chemical structures for the series of π-extended Pt-porphyrins and Pt-tetrabenzoporphyrin derivatives. ....................................................................... 149

4-2 Solution absorbance (a) and solution PL (solid) and PLED EL (dashed) (b) for series of π-extended Pt-porphyrins. ............................................................ 151

4-3 Visible EL spectra of Pt-Ar4TAP in PVK:PBD recorded with the ISA SPEX Triax 180 spectrograph with silicon charge coupled device (CCD) detector. .... 152

4-4 Solution PL spectra of Pt-TBPs in deoxygenated toluene. ............................... 154

4-5 EL spectra of Pt-TBPs at 2% (wt.) in PVK:PBD at 50 mA/cm2. ........................ 158

4-6 Current density and radiant emittance as a function of the applied bias (a) and EQE as a function of current density (b) for Pt-TPTBP, Pt-Ar4TBP and Pt-Ar2TBP. ........................................................................................................ 160

5-1 Structures of iI(TT)2, PC61BM, and poly(dimethylsiloxane) (PDMS) ................. 165

5-2 HOMO-LUMO energies for iI(TT)2 and PC61BM (a) and absorbance of iI(TT)2 in chloroform, in film, and blended with PC61BM. ............................................. 165

5-3 J-V curves for iI(TT)2:PC61BM blend devices processed with plastic or glass syringes under AM1.5 solar simulated illumination. .......................................... 167

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5-4 Infrared (IR) spectra of commercially purchased PDMS and the material extracted from the plastic syringes with prominent peaks labeled. ................... 168

5-5 Power conversion efficiency (PCE) vs. PDMS concentration and J-V plot for a device with no PDMS and with 0.1 mg/mL PDMS. ........................................ 169

5-6 Absorbance (a) and incident photon conversion efficiency (IPCE) (b) spectra of iI(TT)2:PC61BM films and organic photovoltaic (OPV) cells with no PDMS and with 0.3 mg/mL PDMS. .............................................................................. 171

5-7 AFM height images of iI(TT)2:PC61BM blend cells with varying concentrations of PDMS after a 20 minute 100°C thermal anneal. ........................................... 175

5-8 Bright field TEM images of of iI(TT)2:PC61BM blend cells with varying concentrations of PDMS after a 20 minute 100°C thermal anneal ................... 175

5-9 PCE as a function of annealing temperature for devices with no PDMS and with 0.3 mg/mL PDMS. ..................................................................................... 178

5-10 AFM images with no PDMS (top) and with 0.3 mg/mL PDMS (bottom) at increasing annealing temperatures.. ................................................................. 179

5-11 AFM images with no PDMS (top) and with 0.3 mg/mL PDMS (bottom) at increasing annealing temperatures ................................................................... 180

5-12 JSC and PCE as a function of PDMS MW. ........................................................ 182

5-13 AFM height images of iI(TT)2:PC61BM devices with 0.2 mg/mL of varying molecular weights (MWs) of PDMS. ................................................................. 183

5-14 Current density vs. voltage plots with logarithmic scales showing the fit to the field dependent SCLC model for hole only devices .......................................... 187

5-15 Current density vs. voltage plots with logarithmic scales showing the fit to the field dependent SCLC model for electron only devices .................................... 188

6-1 Family of solvent additives selected for application to iI(TT)2:PC61BM blend BHJ OPV cells. ................................................................................................. 192

6-2 Calibration curves of absorbance at 359 and 587 nm vs. iI(TT)2 concentration (a) and at 330 and 432 nm vs. PC61BM concentration (b). ............................... 194

6-3 PCE (a), Jsc (b), FF (c), and VOC (d) obtained from incorporation of the SAs into iI(TT)2:PC61BM BHJ OPVs as a function of SA concentration ................... 197

6-4 PCEs for iI(TT)2:PC61BM blend BHJ OPVs with 0.3 mg/mL SA and no thermal annealing. ............................................................................................ 198

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6-5 Illuminated (a) and dark (b) J-V curves for iI(TT)2:PC61BM blend BHJ OPVs with 0.3 mg/mL SA and no thermal annealing. ................................................. 199

6-6 Hole (a) and electron (b) mobility values measured in iI(TT)2:PC61BM blends with 1.0 mg/mL of SA after thermal annealing. ................................................. 200

6-7 AFM height images of iI(TT)2:PC61BM blend films with additives after 100°C thermal annealing. ............................................................................................ 201

6-8 Top down TEM images of iI(TT)2:PC61BM cells with 1.0 mg/mL SA after 100°C thermal annealing. ................................................................................. 203

6-9 Cross-sectional TEM image of a control sample consisting of a layered structure of ITO/PEDOT:PSS/PC61BM/Al/iI(TT)2/Al. ......................................... 204

6-10 Cross-sectional TEM images of iI(TT)2:PC61BM blend devices after thermal annealing. ......................................................................................................... 206

6-11 Representative J-V curves under simulated AM1.5 illumination (solid lines) and in the dark (dashed lines) of iI(TT)2:PC61BM devices ................................ 210

6-12 Representative J-V curves under simulated AM1.5 illumination (solid lines) and in the dark (dashed lines) of iI(TT)2:PC61BM devices ................................ 212

6-12 AFM height image (a), top-down (b) and cross-sectional (c) TEM images of iI(TT)2:PC61BM blend films with 0.2 mg/mL PDMS and 0.5 mg/mL triethylene glycol (TEG). ..................................................................................................... 213

7-1 Chemical structures of L-alanine, L-leucine, L-phenylalanine, and anthrodithiophene derivatives. .......................................................................... 217

7-2 Chemical structure of symmetric and asymmetric oligomers ............................ 218

7-3 Crystal length dependence on the mole % of iI(TT)2-iBu3Si in solution. ........... 220

7-4 Representative polarized light microscope images showing iI(TT)2 crystals as a function of added iI(TT)2-iBu3Si in solution. ................................................... 220

7-5 AFM height images of [iI(TT)2:iI(TT)2-iBu3Si]:PC61BM (1:1) blend films with varying mole % of iI(TT)2-iBu3Si after 100°C thermal annealing, 5 × 5 μm images. ............................................................................................................. 222

7-6 AFM height images of [iI(TT)2:iI(TT)2-iBu3Si]:PC61BM (1:1) blend films with varying mole % of iI(TT)2-iBu3Si after 100°C thermal annealing, 1 × 1 μm images .............................................................................................................. 222

7-7 Top down TEM images of [iI(TT)2:iI(TT)2-iBu3Si]:PC61BM (1:1) blend films with varying mole % of iI(TT)2-iBu3Si. ............................................................... 224

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7-8 Cross-sectional TEM images of [iI(TT)2:iI(TT)2-iBu3Si]:PC61BM (1:1) blend films with 20% (a) and 100% (b) iI(TT)2-iBu3Si ................................................. 225

7-9 Normalized PCE (normalized to PCE at t=20 min) as a function of the time annealed at 100°C. ........................................................................................... 228

7-10 AFM height images of [iI(TT)2:iI(TT)2-H]:PC61BM (1:1) blend films with varying mole % of iI(TT)2-H after 100°C thermal annealing, 5 × 5 μm images. 229

7-11 AFM height images of [iI(TT)2:iI(TT)2-H]:PC61BM (1:1) blend films with varying mole % of iI(TT)2-H after 100°C thermal annealing, 1 × 1 μm images . 230

7-12 PCE of [iI(TT)2:X]:PC61BM cells with varying mole % X, where X = iI(TT)2-iBu3Si or iI(TT)2-H. ............................................................................................ 231

A-1 X-ray photoelectron spectroscopy (XPS) data showing the 2p region of silicon for the device containing 0.2 mg/mL14,000 MW PDMS at varying exit angles. .............................................................................................................. 243

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LIST OF ABBREVIATIONS

A electron acceptor

BHJ bulk-heterojunction

CB chlorobenzene

CN 1-chloronaphthalene

D electron donor

D-A-D donor-acceptor-donor

DEG-DBE diethylene glycol dibutyl ether

DIO 1,8-diiodooctane

ETL electron transport layer

EQE external quantum efficiency

FF fill factor

HOMO highest occupied molecular orbital

HTL hole transport layer

HD hexadecane

IPCE incident photon to current efficiency

IQE internal quantum efficiency

ITO indium tin oxide

JSC short circuit current density

LP long-pass

LUMO lowest unoccupied molecular orbital

Near-IR near-infrared

NMP N-methyl-2-pyrolidone

ODT 1,8-octanedithiol

OFET organic field effect transistor

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OLED organic light emitting diode (general or vapor deposited)

OPV organic photovoltaic

PCE power conversion efficiency

PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)

PLED polymer based light emitting diode

PTFE poly(tetrafluoroethylene)

PDMS poly(dimethylsiloxane)

Ra distance between two materials or solvents in Hansen space

SA solvent additive

SCLC space-charge-limited current

TEG triethylene glycol

VOC open circuit voltage

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

MORPHOLOGY CONTROL IN POLYMER LIGHT EMITTING DIODES AND

MOLECULAR BULK-HETEROJUNCTION PHOTOVOLTAICS

By

Kenneth R. Graham

August 2011

Chair: John R. Reynolds Major: Chemistry

Organic electronics is a rapidly developing field of technology with promising

applications in light emitting diodes, field effect transistors, and photovoltaic devices. In

all device applications, the morphology of the active layer plays a vital role in

determining the device output characteristics. For example, in organic light emitting

diodes (OLEDs) a homogenous blend, free of aggregates and crystalline domains, is

necessary for efficient operation, whereas in organic photovoltaics, specifically bulk-

heterojunction devices, a nanoscale phase-separated morphology with semi-crystalline

domains is desirable for efficient devices. Given the need for specific morphologies,

this dissertation focuses on control of film morphology in polymer based light emitting

diodes (PLEDs) and molecular bulk-heterojunction (BHJ) photovoltaic cells through both

structural modification of materials and controlled processing conditions.

The first portion of the dissertation focuses on near-infrared (near-IR) emitting

PLEDs, with an emphasis on reducing aggregation and increasing device performance

through structural modification of emitters. Specifically, PLEDs with near-IR emission

are developed through the use of fluorescent emitting donor-acceptor-donor (D-A-D)

oligomers or phosphorescent emitting Pt-porphyrins. The D-A-D based oligomers are

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observed to readily form aggregates that lead to decreases in device performance. To

combat this problem, oligomers with bulky or cross-linkable end-groups are

incorporated into PLEDs. Although neither strategy is shown to reduce aggregation, the

cross-linkable systems allow for solution-processed multilayer PLEDs to be obtained.

The family of Pt-porphyrins incorporated into PLEDs was designed with two primary

purposes; extending the emission wavelength further into the near-IR and optimizing the

molecular structure to both decrease aggregation and increase quantum yield.

The second portion of this dissertation focuses on controlling morphology in

solution-processed molecular BHJ photovoltaic cells through two different methods.

The first method involves the incorporation of solvent additives with known solubility

parameters to develop a predictable method of morphology control. The second

method involves the use of asymmetric oligomers to more predictably control the

crystallization kinetics and thus provide the ability to finely tune the film morphology.

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CHAPTER 1 INTRODUCTION

Organic Electronics Background

The idea and realization of semi-conducting devices based on π-conjugated

organic materials provides the possibility for fabrication through solution processing with

a near infinite degree of control over material, and thereby, device properties. This

control is realized through synthesis of novel designed materials coupled with

appropriate film morphologies and interfaces in suitably engineered device

architectures. As such, the field has expanded to involve the collaborative efforts of

physicists, chemists, materials scientists, and electrical engineers all focusing on

complementary aspects contributing to the development of organic electronics. These

efforts have led to the development of light emitting diodes,1,2,3 electrochromic materials

with applications to windows and displays,4,5 field effect transistors,6-8 resistive

sensors,9,10 memory devices,11,12 and photovoltaics13-15 based on organic

semiconductors with constantly increasing performance.

A key aspect of organic electronics is that they provide the opportunity for non-

energy intensive, inexpensive, and high throughput manufacturing through solution

processing techniques such as slot-die coating,16 screen printing,16 ink-jet printing,17

and spray casting.5,18 Another, and perhaps the greatest, advantage of organic

electronics is that the properties of the organic materials depend on the molecular

structure of the materials involved; thereby, the properties of the materials can be tuned

and controlled through the synthesis of novel materials. This synthetic control over the

molecular structure allows a nearly endless amount of possible materials that could be

synthesized to better match the demands of a particular application. Additionally, the

24

structure can be tailored to alter both the electronic properties and film morphologies

through modification of the π-conjugated backbone and substituent groups respectively.

Material Fundamentals

Prior to understanding organic electronic devices, a background of the organic

materials that drive the devices is useful. The introduction presented herein is intended

only as a brief background and the interested reader is directed towards more thorough

reviews and books.19-22 Briefly, the development of organic electronics is rooted in the

discovery of metallic conductivity in doped polyacetylene by Alan MacDiarmid, Hideki

Shirakawa, and Alan Heeger in the 1970s, for which work they received the 2000 Nobel

Prize in Chemistry.23-25 Additionally, prior to the substantial body of work in conjugated

polymers conducted by the three aforementioned Nobel laureates, D.E. Weiss and co-

workers published a series of papers on electronic conduction in polypyrrole where

metallic conductivities were measured upon doping with iodine.26-28 Although the

majority of the current developments in conjugated polymers utilize their semi-

conducting properties, this initial work helped to establish the basic theory for

conductivity in π-conjugated polymers.

The semi-conducting and conducting properties of π-conjugated materials

originate from the delocalization of π-electrons across an extended number of atoms.

As has been demonstrated both experimentally and theoretically, as the delocalization

of electrons in a π-system increases, the gap between the highest occupied molecular

orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases and the

number of energetic states increases.29 This increase in the number of energetic states

gives rise to an effective band like distribution of states that is commonly depicted to

parallel the band model of inorganic semi-conductors as shown in Figure 1-1.29,30 Once

25

the polymer reaches a specific length the HOMO-LUMO levels no longer change,

thereby the absorption and emission properties become constant. This length in which

the optical and electronic properties saturate is known as the effective conjugation

length and can be viewed as the number of repeat units over which the exciton is

delocalized, where an exciton is defined as an electrically neutral excited state or a

bound electron-hole pair. This extended delocalization of electrons is what gives rise to

the electronic conductivities in organic electronics and allows for positive or negative

charges to be effectively stabilized.

Figure 1-1. Energy levels as a function of the number of aromatic rings in a typical conjugated polymer and the development of band structure as the number of rings is increased.

The absorption and emission properties of the conjugated materials can also be

systematically adjusted through tuning the HOMO-LUMO levels in a controlled manner.

For example, the effective conjugation length can be increased or decreased to red or

blueshift emission respectively,31-33 and monomers that display electron donating and

26

electron accepting properties can be coupled to form oligomers or polymers with

controlled HOMO-LUMO levels.22,34,35 Through utilizing these methods of bandgap

control the absorbance of the materials can be controlled for applications such as

electrochromics and photovoltaics, and correspondingly the emission controlled for

application in organic light emitting diodes (OLEDs). Organic materials also can display

high extinction coefficients and quantum yields, making them promising materials for

photovoltaic cells, electrochromic windows, and OLEDs. For a more thorough

understanding of how the bandgap and HOMO-LUMO levels can be controlled the

reader is directed towards the above listed references.

Definitions

An understanding of the basic terms and definitions used in discussing electronic

energy levels of devices and materials are shown in Figure 1-2.

Figure 1-2. Energy diagram of isolated materials showing the work function of the commonly used electrode materials indium tin oxide (ITO) and Ca (ΦITO and ΦCa), and a typical conjugated polymer with HOMO, LUMO, HOMO-LUMO gap (Eg), electron affinity (EA), and ionization energy (IE) indicated.

27

It should be mentioned that the formal definition of the work function is the energy

required to remove an electron from the Fermi level to vacuum. The oversimplified

definition of the Fermi level is the energy level at which an electronic state has a 50%

probability of being populated, but the interested reader is directed to the more

mathematical definition in Sze and Ng.36 The Fermi level of a semiconductor will lie

somewhere between the HOMO-LUMO levels and therefore the term “work function” is

generally not used when referring to organic semiconducting materials. Work functions

of materials used in this dissertation are listed in Table 1-1.

Table 1-1. Work functions, HOMO, LUMO, and redox energies vs. vacuum.

Molecule Work Function or Redox Potential (eV) HOMO (eV) LUMO (eV)

Normal Hydrogen Electrode (NHE)38

-4.5 - -

Saturated Calomel Electrode (SCE)38

-4.7 - -

Ferrocene/Ferrocenium (Fc/Fc+)37,38

-5.1 - -

Al39 -4.2 - - ~ 1 nm LiF on Al39 -2.8 - - Ca40 -2.9 - - ITO41 -4.7 - - MoO342 -5.7 -5.5 -8.6 PEDOT:PSS43 -5.0 to -5.2 - - PC61BM44 - -4.2 -6.0

Generally the HOMO-LUMO levels of conjugated materials are determined

electrochemically through either cyclic voltammetry (CV) or differential pulse

voltammetry (DPV), with the onsets of oxidation and reduction used to determine the

HOMO and LUMO levels respectively. Since these oxidation and reduction onsets are

determined with reference to a standard electrode or standard redox couple, the energy

of these standards relative to the vacuum level is necessary to convert to HOMO-LUMO

28

energies. The inconsistency in the values of these standards relative to the vacuum

level has resulted in large discrepancies between reported HOMO-LUMO values in the

literature.37,38 Table 1-1 lists what appear in the literature to be the correct values for

these redox processes. Also included in Table 1-1 are work functions and HOMO-

LUMO levels for some selected materials used in this dissertation.

Charge Transport

Charge transport in organic films generally takes place through a hopping

mechanism, whereby the charge “hops” from one conjugated unit to the next.45 The

conjugated units the charge “hops” between could be two units on the same polymer

chain separated by a defect, units on adjacent polymer chains, or in the case of small

molecules the units would be individual molecules. Additionally, intrachain transport

can occur several orders of magnitude faster than interchain transport, resulting in

significantly higher carrier mobilities along a polymer chain.46 Hopping is favored

between states that are close in energy, thereby leading to higher conductivities in more

uniform and ordered materials. The fact that organic materials are generally disordered

and contain lower energy trap states is an important implication for modeling the current

behavior of these materials as will be addressed later.

Related to the conductivity are the electron and hole carrier mobility as shown by

equation 1-1.36

(1-1) Where σ is the conductivity, q is the elementary charge, μe and μh are the carrier

mobilities of electrons and holes respectively, and ne and nh are the densities of

electrons and holes. In an organic semiconducting device, whereby the device has a

σ = q(neµe + nhµh )

29

negligible concentration of free charges, ne and nh, and the device operates based on

the injection or generation of free charges, it is more meaningful to refer to the charge

carrier mobilities rather than conductivities. The charge carrier mobility is independent

of carrier concentration and is related to the drift velocity of an electron or hole in an

electric field through equation 1-2.

µ = vE

(1-2)

Where v is the drift velocity in an electric field, E.

Generally, a material will show significantly higher mobilities when it is in a more

crystalline state as compared to an amorphous state. Part of this higher mobility in

crystalline materials likely originates from the more energetically similar states for

charge transport as discussed in the review by Tessler, et al.45 Additionally, in

crystalline regions of organic materials there can be more electronic coupling between

materials resulting in enhanced conductivities.47 Alternatively, if conductivity is viewed

in terms of the band like model of classical solid state physics then the conductivity is

going to depend largely on the number of scattering sites. In a crystalline material the

number of scattering sites is significantly reduced relative to an amorphous material

leading to significantly higher conductivities.

Organic Light Emitting Diodes (OLEDs)

The concept of generating fluorescence in organic materials through the

application of a voltage bias was first explored with anthracene single crystals in 1965.48

This concept was later expanded to the deposition of thin organic films between two

metal or metal-oxide electrodes, leading to the creation of the modern organic light

emitting diode (OLED).1,2 Within a decade of the realization of vapor deposited

30

molecular OLEDs, the first polymer light emitting diode (PLED) was demonstrated.3

The efficiencies of both devices gradually increased with a wide range of materials and

architectures being explored in the following decades. Throughout this section the

acronym OLED will be used to refer to organic light emitting diodes in general (i.e.

PLEDs and vapor deposited OLEDs) and PLED will be used specifically in referring to

LEDs in which the emissive layer is polymer based and solution processed.

Table 1-2. Definitions of terms used in characterizing OLEDs and OLED materials. Term Definition Quantum yield (ϕ) Ratio of emitted photons to absorbed photons Internal quantum

efficiency (IQE) Total ratio of all photons emitted to electrons injected

External quantum efficiency (EQE)

Ratio of photons emitted in the forward viewing direction to electrons injected

Radiant Emittance Total radiant power emitted in the forward viewing direction divided by the area of the emissive region

Device Operating Principles

The operating mechanism of LEDs, both inorganic and organic based, involves the

injection of electrons from a negatively biased electrode and simultaneous injection of

holes from a positively biased electrode. The electrons and holes then move across the

device before combining somewhere in the active region to form an exciton. The

exciton can then decay radiatively with the emission of a photon of light. This general

mechanism is illustrated in Figure 1-3, where the blue region represents the energy gap

with HOMO and LUMO states above and below the gap.

In an LED with 100% internal quantum efficiency every injected charge would form

an exciton in the emissive layer that would radiatively decay with the emission of a

photon. However, in a real device there are several other processes that may occur

which do not lead to the emission of a photon. These include charge recombination at

31

the electrodes and non-radiative decay of the exciton. The problem of charge

recombination at the electrodes can be reduced through the use of an active material

with balanced electron and hole mobilities or through the use of electron and hole

blocking layers sandwiching the emissive layer.49-51

Figure 1-3. Schematic of OLED operation showing charge injection (a), charge transport (b), exciton formation (c), and exciton decay with light emission (d).

The use of electron and hole blocking layers prevents charge recombination at the

electrodes by confining the charges to the active layer as shown in Figure 1-4. By

selecting a material with a high quantum yield the rate of non-radiative decay can be

reduced; however, generally quantum yields are measured in solution and these

quantum yields may differ significantly from quantum yields in the solid state.47,52-54 As a

fairly extreme example, a family of three fluorenevinylene based trimers showed

quantum yields of 60-70% in dilute solutions and only 2-10% in films.53

32

Figure 1-4. Schematic of a multilayer OLED or PLED whereby the electron transport layer (ETL) and hole transport layer (HTL) help to confine the charges to the emissive layer.

In a typical OLED the statistically predicted ratio of generated singlet to triplet

states is 1:3 (singlet:triplet).55 This ratio is predicted by quantum mechanical

considerations based on the presence of one singlet state and three energetically

equivalent triplet states and has also been determined experimentally.55 If the emissive

material is fluorescent (i.e. it emits primarily from the singlet state) then the maximum

percentage of generated excitons that may result in the emission of a photon is 25%,

since 75% of excitons generated will be triplet states which will not result in emission of

a photon. On the other hand, if the emissive material is a phosphorescent emitter (i.e. it

emits primarily from the triplet state) then a maximum of 100% of the generated

excitons may result in the emission of a photon. In fact, phosphorescent OLEDs have

been constructed which show nearly 100% internal quantum efficiency.50

There are several strategies for obtaining emission from a material in an OLED.

The first and most straightforward is through the use of a single emissive material which

comprises the entire active layer. The second is through doping a small amount of an

emissive organic material, generally a small molecule, into a conductive host material.

33

The first route provides for the most straightforward fabrication, however the

requirements for an effective material in this device architecture are relatively high and

few materials meet all the requirements. For example, the material must have fairly

balanced hole and electron mobilities, a high solid state quantum yield, form good films,

and display minimal crystallinity which generally leads to emission quenching. The use

of an emissive small molecule doped into a conductive organic host matrix eases many

of the demands on the emissive material and provides a more versatile method for

fabricating OLEDs. In these small molecule doped OLEDs the requirements on the

emitter are that it has a high quantum yield and is evenly distributed throughout the film

with minimal to no aggregation.

Energy Transfer

In the case of small molecule doped OLEDs, the energy must be transferred from

the conductive host to the emissive dopant. Generally this occurs through either charge

trapping and recombination on the emissive dopant or through charge recombination in

the host with Förster resonant energy transfer from the host to the dopant as illustrated

in Figure 1-5.56 As evident in Figure 1-5a, charge trapping requires that the HOMO and

LUMO levels of the emissive dopant lie within the HOMO and LUMO levels of the

host.56 In this situation as the electrons and holes move through the device they are

effectively trapped on the lower energy sites of the emissive dopant. When both an

electron and hole become trapped on the same molecule an exciton is formed which

can radiatively decay to emit a photon. Common systems for charge trapping involve a

high bandgap electron transporting material blended with a high bandgap hole

transporting material. The use of separate electron and hole transporting materials as

34

opposed to a single electron and hole transporting material helps to ensure that

recombination only occurs at the emissive dopants and not in the host.

Figure 1-5. Exciton formation on the emissive dopant through charge trapping (a) and through Förster energy transfer (b) from host (blue) to emissive dopant (red). In b the white dashed arrow indicates dipole-dipole coupling.

The process of Förster energy transfer is depicted in Figure 1-5b. In this

mechanism the exciton is formed on the host and through a nonradiative dipole-dipole

coupling the energy is transferred to the emissive dopant.57 Generally Förster energy

transfer can occur on the range of ~5 nm, therefore this requires that emissive dopants

must be distributed evenly throughout the film such that there is a dopant molecule

within 5 nm of any location in the film. Förster energy transfer is slightly less versatile

than charge trapping, as it requires that the emission of the host overlaps with the

absorption of the emissive dopant. In devices where a single material host is used and

an emissive dopant is included which has HOMO and LUMO levels which lie within the

HOMO and LUMO levels of the host, both charge recombination at the emissive dopant

as well as Förster energy transfer may take place.

A strategy for increasing the EQE and radiant emittance of fluorescent emitting

OLEDs is through the use of a phosphorescent sensitizer as demonstrated by Baldo, et

al.58 This concept relies on a host material doped with both a phosphorescent emitter

35

and a fluorescent emitter. The phosphorescent emitter is present at a higher

concentration than the fluorescent emitter, which leads to increased charge

recombination at the phosphorescent emitter relative to the fluorescent emitter. When

the charges recombine at the phosphorescent emitter, Förster energy transfer to the

fluorescent emitter results. Thereby, triplet excitons are effectively utilized by the

phosphorescent emitters to generate singlet excitons on the fluorescent emitters.

Morphology

Polymer based light emitting diodes generally demand an amorphous morphology

with limited crystalline or aggregated domains, as these can lead to the formation of

interchain excited states which generally have longer lifetimes and lower quantum

yields.54,59 In this situation, aggregation is referring to states where two or more chain

segments come together and share their π-electrons for at least the length of an exciton

(~4 repeat units).54 The presence of these interchain excited states have been verified

spectroscopically through a bathochromic shift in absorption and emission, increased

radiative lifetimes, and decreased photoluminescence quantum yields.54,59

For single component PLEDs it has been demonstrated that the casting solvent,

solution concentration, and thermal annealing can have a significant influence on the

film morphology and thus the interchain interactions.54,60 For example, it has been

demonstrated that MEH-PPV films cast from THF show reduced interchain excited

states as compared to films cast from chlorobenzene.54 Consequently, the films cast

from chlorobenzene show an increase in exciton-exciton annihilation, increased

radiative lifetime, and reduced photoluminescent quantum yields as compared to those

cast from THF. The influence of crystallinitty on film quantum yield can also be

observed in the case of regiorandom vs. regioregular P3HT films, where quantum yields

36

of 8% were measured for the less crystalline regiorandom film as compared to 0.5% for

the more crystalline regioregular film.61

In the case of small molecule doped PLEDs, a uniform and aggregate free

distribution of the emissive small molecule is normally necessary to achieve high

efficiency devices. This is primarily due to the decreased quantum yields generally

observed in aggregated species.53,54,59,62 In the case of small molecule phosphorescent

emitters, even aggregates as small as dimers and trimers can lead to triplet-triplet

annihilation processes which significantly reduce the quantum yield.63,64 Triplet-triplet

annihilation is the process by which two molecules with excited triplet states interact to

create one molecule in an excited singlet state and one molecule in the singlet ground

state. This triplet-triplet annihilation process reduces the quantum yield by 50%

assuming that both triplet and singlet states display the same luminescence quantum

yield. Based on the above discussion of quenching processes in OLEDs, it is clear that

a non-aggregated morphology is necessary for a PLED to operate with optimal

efficiency. Strategies for reducing aggregation will be presented in Chapters 3 and 4.

Near-Infrared (Near-IR) Emitting OLEDs

Polymeric light emitting diodes that emit in the near-infrared (near-IR) region of the

electromagnetic spectrum are of particular interest for several applications including

wound healing, night vision illumination sources, invisible signaling,

telecommunications, and consumer electronics.65-67 Most notably the ability to produce

inexpensive, large area, and potentially flexible devices may allow for disposable

bandages which can be worn to accelerate wound healing or as patches on military

uniforms which would serve as both a friend/foe identification source and a night vision

illumination source. Currently, research efforts and progress in near-IR emitting PLEDs

37

has lagged significantly behind those in visible emitting PLEDs. As such, significantly

lower performance values have been achieved in the near-IR as compared to the

visible.

As mentioned earlier, there are two main strategies to obtaining PLEDs with

desired emission wavelengths, which in this case is in the near-IR. These strategies

rely on the use of a near-IR emitting polymer or the use of a host matrix doped with a

near-IR emitting small molecules. The use of near-IR emitting conjugated polymers has

been demonstrated with several polymer structures displayed in Figure 1-6a.68-70 These

polymers all show broad emission in the 700-1000 nm range with EQEs less than 0.4%.

Figure 1-6. Structures of example near-IR emitting polymers (a),68-70 a lanthanide complex (b),71 a D-A-D oligomer (c),72 a phosphorescent metal-organic complex (d),73 and a metalloporphyrin (e)74,75 that have been used in near-IR emitting OLEDs.

38

The use of small molecule near-IR emitters doped into polymer or molecular host

matrices has received much more attention with significantly higher EQEs and emission

wavelengths extending further into the near-IR than polymer emitters.71,75 Several

different classes of near-IR small molecule emitters have been explored including

lanthanide complexes,71,76-80 donor-acceptor-donor oligomers,72,81-83 phosphorescent

metal-organic complexes,73,84-86 and metalloporphyrins.74,75 Representative structures

from each of these molecule classes are displayed in Figure 1-6(b-e). Through the use

of these various small molecule emitters, OLEDs with emission maxima extending out

to 1.6 μm and an EQE of >8% at an emission maximum of 772 nm have been

obtained.71,75 It is worth mentioning that the higher efficiency devices were all vapor

deposited multi-layer devices and not single layer solution processed devices.

An advantage of the lanthanide complexes is narrow emission bandwidths with

wavelengths ranging from 800 to 1600 nm. However, due to the low quantum yield of

the metal centered F states the EQEs of devices based on these emitters are less than

0.5%.71,76-80 Donor-acceptor-donor based oligomers display broad emission spectra with

readily tunable emission maxima ranging from the visible throughout the near-IR. In the

visible region these DAD oligomers have shown quantum yields approaching 80%;87,88

however, when the wavelength is extended into the near-IR a decrease in quantum

yield to ≤20% is observed.81,82 Additionally, these DAD oligomers are fluorescent

emitters which limits the maximum theoretical internal quantum efficiency to 25%

assuming a 100% photoluminescence quantum yield. This results in DAD oligomer

based near-IR emitting OLEDs with EQEs in the 1% range for emission maxima in the

700-900 nm range and

39

of a phosphorescent sensitizer.72,83,89 Applying the phosphorescent sensitizer strategy

mentioned previously to a DAD containing OLED, an EQE of 1.5% and radiant

emittance (R) of 4.0 mW/cm2 at an emission maximum of 815 nm was obtained vs. an

EQE of 0.5% and R of 2.1 mW/cm2 without the phosphorescent sensitizer.83

Phosphorescent emitting metal-organic complexes are a much more promising

class of materials due to their relatively high quantum yields and ability to emit efficiently

from both the singlet and triplet states. As such, phosphorescent emitters containing

iridium are the basis for the majority of high efficiency and high power output visible

emitting OLEDs.51,90,91 The high performance obtained from these Ir complexes is

attributed to the short phosphorescent lifetime (

40

metalloporphyrins. Pt-porphyrins, specifically the red emitting Pt-octaethylporphyrin (Pt-

OEP), were some of the first phosphorescent emitters used in OLEDs.93 Although Pt-

OEP shows red emission, this emission can be shifted into the near-IR region by

expanding the π-conjugated system to Pt-tetraphenyltetrabenzoporphyrin (Pt-TPTBP)

as shown in Figure 1-5e.74,75 Currently, Pt-TPTBP has the highest demonstrated

performance in the 750-800 nm wavelength range with an EQE of 8.5% and a

maximum R of 1.21 mW/cm2 at an emission wavelength maximum of 772 nm.75

Inorganic devices emitting in the near-IR region have already reached

commercialization and display much larger radiant emittance values as well as

increased EQEs relative to OLEDs. For example, inorganic LEDs emitting at 880 and

940 nm have been reported with power outputs of 12-18 mW and 16-26 mW at currents

of 100 mA, resulting in EQEs of 11.3-18.4% and 9-13.6% respectively.94 These

inorganic LEDs consist of a chip with an emission area in the hundreds of microns

range, which is then encapsulated into a rounded cylindrical housing with a diameter of

~3-5 mm.95 Assuming these encapsulated LEDs were packed side-by-side in a square

arrangement, radiant emittance values of the square array would be 50 to 100 mW/cm2.

Compared to their inorganic counterparts the EQEs and radiant emittance values of

OLEDs may seem relatively low; however, as in general with organic electronics this

low efficiency may potentially be offset by the low cost of solution processing or the

prospect of flexible devices. Furthermore, these OLEDs are large area emitters

whereas inorganic LEDs are generally small area emitters that are individually

encapsulated.

41

Organic Photovoltaics (OPVs)

Organic photovoltaics (OPVs) are, in my opinion, one of the most promising and

exciting areas of organic electronics. When one considers the progress in organic

photovoltaics over the past decade, the further potential for development, and the idea

of roll-to-roll printing huge areas of these OPVs it is no wonder that it has attracted

significant research attention both in academics and industry.

There are actually two distinct types of photovoltaics that rely on organic materials

as the absorptive material. One type is the dye sensitized solar cell (DSSC), which

relies on an absorptive organic material bound to TiO2 in an electrolyte containing

solution.96,97 Although these cells have reached power conversion efficiencies of 11%,

they are not the focus of this work and the interested reader is directed towards the

previous references. The other major type of solar cell that relies on organic materials

as the absorptive species is the organic photovoltaic cell (OPV). This cell differs

significantly from the DSSC in that exciton dissociation occurs at the interface between

two organic materials followed by charge collection at the electrodes. Therefore no

electrolyte or TiO2 is necessary, which enables the potential for fabrication using

inexpensive solution processing techniques.

The first OPV which utilized two different organic materials as the electron

donating and electron accepting materials was reported by Tang in 1986.98 Prior to this

report, OPVs had relied on a single organic material sandwiched between low and high

work function electrodes.99 Tang’s initial report was based on a vapor deposited bilayer

structure in which a layer of copper phthalocyanine (CuPC) served as the electron

donating material with a layer of a perylene tetracarboxylic acid derivative as the

42

electron accepting material. This bilayer structure resulted in a PCE of ~1% with the

use of ITO and Ag as the hole and electron collecting electrodes respectively.

Spurred by the initial report of photoinduced electron transfer from a conjugated

polymer to C60 in 1992,100 the use of fullerenes as electron acceptors in OPVs emerged

as a promising route to higher efficiency devices.101 Further development was greatly

accelerated by the bulk heterojunction architecture after its introduction in 1995.13 With

an improvement in device structures and development of higher performing materials

PCEs gradually climbed to the current values of ~7%.102-104

Device Operating Principles

The general operating processes in an OPV cell are shown schematically in Figure

1-7. The first process as shown in Figure 1-7a is the absorption of a photon by either

the electron donor (D) or electron acceptor (A) to create an exciton. The exciton then

diffuses to a D-A heterojunction interface where, due to the energetic offset of the type 2

heterojunction, the electron is transferred to A and the hole remains on D as shown in

Figure 1-7c. The charge transfer exciton is separated resulting in free charges that are

then transported within their respective phases to the electron and hole collecting

electrodes. Driving the hole and electron transport to their respective electrodes is the

presence of a built in electric field originating from the work function difference between

the two electrode materials. The hole and electron are then transferred to their

respective electrodes resulting in the generation of a photocurrent. It should be noted

that a similar process depicted in Figure 1-7 can also occur when the exciton is initially

created on the A phase.

43

Figure 1-7. Schematic illustration of the processes occurring in an OPV device.

Absorption of a photon to create an exciton (a), exciton diffusion (b), charge transfer (c), charge seperation and transport (d), and charge collection (e).

From the above discussion and Figure 1-7 we can identify six main processes

necessary for the operation of an organic photovoltaic cell. These include absorption of

a photon to generate an exciton (1), exciton diffusion to a D-A interface (2), Charge

transfer (3), charge seperation (4), charge transport (5), and charge collection (6). By

increasing the efficiency of these processes the overall efficiency of the device will be

increased as long as it is not at the expense of another process, which is rarely the

case.

Through the use of materials that absorb a greater portion of the suns light,

exciton generation can be greatly enhanced. Initially, solution processed OPVs relied

mainly on P3HT and MDMO-PPV as the donor materials and PC61BM as the acceptor

material. With these materials only photons between 350-650 nm were being absorbed,

with PC61BM contributing only weakly to the total absorbance. To increase the

absorption in the visible region, PC71BM is now widely being used as an electron

acceptor material. Development of donor materials that absorb more strongly with

44

absorbance extending in the red and near-IR region of the solar spectrum has been a

large focus of synthetic efforts and has resulted in the development of OPVs which

efficiently capture photons out to ~850 nm.105

The exciton diffusion length in most disordered organic materials is ~5-10 nm and

although some work has been done on improving this it has not been a major focus.

Most strategies to increase the exciton diffusion length involve increasing the lifetime of

the exciton. One method of doing so is through the use of a heavy metal containing

polymer where the generated short-lived singlet state may be converted to the long-

lived triplet state.106,107,108 Another method is through the use of highly crystalline

materials with limited defects.109

The process by which charge separation occurs in OPVs is an active area of

research with no well-established mechanism. In contrast to traditional inorganic

materials where exciton binding energies are typically

45

immediately after charge transfer.114,115 However, recent experimental results indicate

that this may not be the correct mechanism as it has been shown that direct absorption

to the charge transfer state, whereby the electron is excited only to the charge transfer

state and not above, results in a similar internal quantum efficiency as direct absorption

by the donor or acceptor.116 Although not the focus of this work, exciton dissociation is

an important process with a significant influence on the maximum PCE attainable in

OPVs.

An alternative theory on how the charge separation process occurs involves a two-

step process by which energy is transferred from D to A via Förster resonance energy

transfer.44,117-119 This process involves the generation of an exciton on D, Förster

resonance energy transfer to generate an exciton on A, followed by electron transfer

from the HOMO of D to the HOMO of A. The second part of this process, i.e. electron

transfer from the HOMO of D to the HOMO of A, is the same as occurs when the

exciton is generated directly on A. This mechanism still involves the generation of a

charge transfer state as depicted in Figure 1-7d; however, its implications may play a

key role on designing new materials and devices architectures to reduce recombination

in OPVs.118

This brings us to charge transport and charge collection. In a typical bilayer

device the charge transport is going to depend primarily on the charge mobility in the

material. A higher mobility will allow for charges to move across the device faster,

thereby reducing recombination and enhancing charge transport. As was previously

discussed, the charge mobility is generally going to be higher when the material can

adopt a more ordered or more crystalline morphology. Furthermore the material must

46

be capable of having a high mobility when the morphology is adequate. This

requirement for high mobility has led to the development of high mobility materials as

well as to the development of processing techniques to enhance ordering and

mobility.120,121 Efficient charge collection requires that the electrode energy offsets be

appropriate for the HOMO and LUMO levels of the materials as depicted in Figure 1-6.

There should also be no barriers to charge collection, such as traps, present at the

electrode interfaces. Through the application of appropriate metals, polymer interlayers,

and metal oxides this charge collection process has been significantly improved.122,123

The fact that a solar cell generates electrical power is a function of two main items,

these being the photovoltage and photocurrent. When a photon is absorbed an electron

is promoted to a higher energy state, meaning that there is now an energy difference

between the hole in the HOMO and the electron in the LUMO. An energy offset,

typically estimated to be ~0.3 eV,44,111 is required to drive electron or hole transfer at the

D-A interface as shown in Figure 1-7c, resulting in a further loss in the energy difference

between the electron and hole. As mentioned above the electron and hole now form a

charge transfer exciton, which requires some amount of energy to separate. Lastly, an

additional energy loss may occur upon charge transfer from the D or A material to the

electrode. Thereby, when the hole and electron reach their respective electrodes the

energy difference between the two is significantly less than the energy difference in the

original exciton, but there remains an energy difference which is in part what determines

the open circuit voltage (VOC).

The second part of the power generated corresponds with the number of electrons

and holes collected by the electrodes, ie. the photocurrent. The power attainable at any

47

point in the J-V curve while the device is under illumination is equal to the product of the

current and voltage. Hence, the maximum power (Pmax) occurs at the point where J

× V is at a maximum as shown in Figure 1-8. The ratio of this Pmax to the power

obtained if Pmax occurred at JSC × VOC (theoretical maximum power, Pth max) is known as

the FF. The name fill factor originates from the concept of the theoretical maximum

power box (JSC × VOC) being filled to some ratio

48

(1-3)

(1-4)

Where Jmax and Vmax are the values of J and V at Pmax, and FF is the fill factor as

defined in equation 1-4.

The Bulk-Heterojunction (BHJ)

A major factor limiting the short circuit current in bilayer OPVs is the short exciton

diffusion lengths of 5-10 nm common to many disordered organic semiconductors.124,125

Excitons created further than 5-10 nm from the D-A interface will recombine either

radiatively or non-radiatively before reaching the D-A interface and will not contribute to

the photovoltaic response. Too ensure that the majority of excitons will reach the D-A

interface in a bilayer device the layers must be

49

the hole collecting electrode as shown schematically in Figure 1-9a.127 The purpose of

the pure D and A phases next to the respective electrodes being to decrease electron-

hole recombination at the electrode/BHJ interface. In reality this morphology is not

obtained in a typical BHJ cell, but instead a more randomly distributed morphology of D

and A phases is obtained as illustrated schematically in Figure 1-9b. In this typical real

morphology there are several problems that may arise including isolated phases where

no path for charge transport to the electrode exists (x), long tortuous paths for charge

transport (y), and large domains (z).

Figure 1-9. Schematic cross-sections of BHJ morphologies showing the ideal (a) and

typical (b) BHJ morphologies where orange and blue represent the D and A phases respectively. In the typical morphology location x represents an isolated domain, y represents a location where the hole would have a long path length to the electrode, and z indicates locations where a generated exciton may not reach a D-A interface.

When D and A phase separation is obtained on an appropriate scale the efficiency

of the BHJ OPV cell is significantly improved. For example, when the size of the D and

A phases are decreased in MDMO-PPV:PC61BM blends from hundreds of nms to tens

of nms the short circuit current density (JSC) improves from 2.33 to 5.25 mA/cm2 and the

power conversion efficiency (PCE) improves from 0.9 to 2.5%.14 In another example,

when the D and A phases are reduced from hundreds of nms to tens of nm in a

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PDPPTPT:PC71BM blend the PCE improves from 2.0 to 5.5%.128 See Figure 1-9 (5

pages forward) for chemical structures.

Morphology Control

The morphology in a BHJ OPV cell is vital to the efficient operation of the cell as

discussed above. To achieve a more desirable BHJ film morphology several different

strategies have been adopted. Generally these devices are processed through spin

coating where the film quickly dries and a metastable morphology is established. One

of the primary methods of morphology control involves the use of an appropriate casting

solvent. This casting solvent needs to result in a phase separated morphology with

domains on the scale of tens of nanometers or a morphology with little to no phase

separation. If the domain sizes immediately after film casting are too large, techniques

such as thermal annealing or solvent annealing will not be able to address this problem,

as these techniques drive further phase separation. The importance of the casting

solvent was first demonstrated by Shaheen, et al. when the use of toluene was

compared with chlorobenzene as casting solvents for MDMO-PPV:PC61BM BHJ OPV

cells.14 Chlorobenzene resulted in smaller phases, a JSC improvement from 2.33 to 5.25

mA/cm2, and a PCE improvement from 0.9 to 2.5% as previously mentioned. The use

of casting solvents was recently explored more thoroughly by the Nguyen group and

again it was demonstrated that solvents which produce more minimally phase

separated morphologies result in higher efficiency devices.129 An extreme example of

the influence of the casting solvent on device performance is work done by the Watkin’s

group on a dibenzochrysene (DBC) derivative (Figure 1-9) blended with PC61BM.130 In

this work an optimized PCE of 0.005% was reached with chlorobenzene as the casting

51

solvent; however, upon switching the casting solvent to chloroform a PCE of 1.51% was

realized.

A commonly used method to improve the morphology and P