Organic Materials for Electronic Devices
by
Lichang Zeng
Submitted in Partial Fulfillment
of the
Requirements for the Degree
Doctor of Philosophy
Supervised by
Professor Shaw H. Chen
and
Professor Ching W. Tang
Department of Chemical Engineering
Arts, Sciences and Engineering
Edmund A. Hajim School of Engineering and Applied Sciences
University of Rochester
Rochester, New York
2010 ii
To my family iii
CURRICULUM VITAE
Lichang Zeng was born in 1978 in Fujian, China. In 2001, he received a
Bachelors of Engineering degree in Polymer Materials from Zhejiang University,
Hangzhou, China. He continued on at Zhejiang University receiving his Master of
Science degree in 2004. He then moved to the University of Rochester to pursue his doctorate in Chemical Engineering under the joint supervision of Professors
Shaw H. Chen and Ching W. Tang, receiving a Master of Science degree in 2009.
His field of research was in organic electronic materials and devices.
Selected Publications in Referee Journals
1. Zeng, L. C.; Tang, C. W.; Chen, S. H. “Effects of Active Layer Thickness and
Thermal Annealing on Polythiophene:Fullerene Bulk Heterojunction
Photovoltaic Cells.” Applied Physics Letters, 2010, 97, 053305.
2. Zeng, L. C.; Blanton, T. N.; Chen, S. H. “Modulation of Phase Separation
between Spherical and Rod-Like Molecules Using Geometric Surfactancy.”
Langmuir 2010, 26, 12877.
3. Zeng, L. C.; Lee, T. Y. S.; Merkel, P. B.; Chen, S. H. “A New Class of
Non-Conjugated Bipolar Hybrid Hosts for Phosphorescent Organic
Light-Emitting Diodes.” Journal of Materials Chemistry 2009, 19, 8772.
4. Zeng, L. C.; Yan, F.; Wei, S. K. H.; Culligan, S. W.; Chen, S. H. “Synthesis
and Processing OF Conjugated Oligomers Into Monodomain Films by iv
Thermal and Solvent-Vapor Annealing.” Advanced Functional Materials
2009, 19, 1978.
5. Chen, A. C. A.; Wallace, J. U.; Wei, S. K. H.; Zeng, L. C.; Chen, S. H.
“Light-Emitting Organic Materials with Variable Charge Injection and
Transport Properties.” Chemistry of Materials 2006, 18, 204 and 6083.
v
ACKNOWLEDGEMENT
I am deeply grateful to my advisors Professor Shaw H. Chen and Professor
Ching W. Tang for their support, guidance, and perseverance that have inspired me and made this thesis possible. The rigorous training I have received from such exemplary scientists will greatly benefit my future career.
I would like to thank Professor Lewis Rothberg and Professor Mitchell
Anthamatten for serving on my advisory committee. I thank them for many valuable discussions and comments over the years. I would also like to express my gratitude for the technical advice and experimental assistance to Professor
Steve Jacobs of the Institute of Optics, Dr. Semyon Papernov, and Mr. Kenneth L.
Marshall of Laboratory for Laser Energetics, Professor Man-Kit Ng, Professor
Hong Yang, Professor Matthew Yates, Professor Yongli Gao, Dr. Paul B. Merkel and Dr. Jane Ou of the University of Rochester, and Dr. Thomas Blanton, Dr.
Andrew J. Hoteling, Dr. Diane Freeman, Dr. David Levy, Mr. Mike Culver, Mr.
Andrea Childs, and Mr. Kevin Klubek, all of Eastman Kodak Company. Mr.
Joseph Madathil of the University of Rochester, and Mr. Joseph Henderson of
Laboratory for Laser Energetics have provided a great deal of assistance in the construction of the device fabrication and measurement systems at the University of Rochester.
My sincere gratitude goes to Dr. Feng Yan, Dr. Sean Culligan, Dr. Andrew
Chien-An Chen, Dr. Zhijian Chen for sharing me with their various expertise, vi
from organic synthesis to device fabrication. Moreover, Dr. Feng Yan synthesized a significant portion of the compounds used in Chapter 2. I also thank Mr. Simon
Ku-HsienWei for his assistance in the preparation of some thin films used in
Chapter 2, and Mr. Thomas Yung-Hsin Lee for his help in the synthesis of some of the compounds and intermediates used in Chapter 3. I would also like to thank
Dr. Jason U. Wallace for his insightful discussions with me throughout the past years.
My gratitude and thanks are due to my fellow graduate students over the years for their camaraderie: Dr. Feng Yan, Dr. Sean Culligan, Dr. Anita
Trajkovska, Dr. Andrew Chien-An Chen, Dr. Zhijian Chen, Dr. Chunki Kim, Dr.
Jason U. Wallace, Simon Ku-Hsien Wei, Thomas Yung-Hsin Lee, Minlu Zhang,
Kevin Klubek, Wei Xia, Hui Wang, Hao Lin, Sunny Hsiang Ning Wu, Qing Du,
Mohan Ahluwalia, Matthew Smith, Sang min Lee, Jonathan Welt, and Eric
Glowacki. My deep appreciation also goes out to my personal friends, Shujing
Wang, Pin Lu, Xueyuan Liu, Chunlin Miao, Zhimin Shi, Lei Sun, Yuping Li,
Ling Cai, Shuang Liu, et al, for bringing the fun to my life in Rochester.
Finally, my family deserves the most credit for this work. I especially thank my sister Suqin, who encouraged and helped me pursue advanced study in the
United States. I thank my brother and sisters for their support and encouragement.
I thank my father Mr. Kebing Zeng and my mother Mrs. Zixiu Wei for always being there whenever I need them. I also thank my in-laws Mr. Guoliang He and vii
Mrs. Guijie Wang, for their support and taking good care of my daughter. My deepest love goes to my wife, Xin, who has sacrificed so much to help me get here, and whose patience and goading keeps me going. My thanks and apologies for all the time away, go to my daughter, Zoey, who has been a constant source of joy and motivation. I dedicate this thesis to my family.
This research was supported by the New York State Energy Research and
Development Authority, the New York State Center for Electronic Imaging
Systems, the Eastman Kodak Company, the Department of Energy, and the
Department of Energy Office of Inertial Confinement Fusion with the Laboratory for Laser Energetics. viii
ABSTRACT
Through light absorption and emission as well as charge carrier generation, transport and recombination, π-conjugated molecules are central to electronic devices including organic field-effect transistors, organic light-emitting diodes, and organic solar cells. This thesis reports on materials development via molecular design, material synthesis and processing, device fabrication and characterization. Major accomplishments are summarized as follows.
A series of oligo(fluorene-co-bithiophene)s, OF2Ts, have been synthesized and characterized for an investigation of the effects of oligomer length and pendant aliphatic structure on thermotropic properties, light absorption and emission, and anisotropic field-effect mobilities. Solvent-vapor annealing at room temperature was shown to be capable of orienting OF2Ts into monodomain glassy-nematic films with an orientational order parameter emulating that achieved with conventional thermal annealing on a rubbed polyimide alignment layer.
Comprising hole- and electron-transporting moieties with flexible linkages, non-conjugated bipolar compounds have been developed for use as hosts for electrophosphorescence. These materials are characterized by an elevated glass transition temperature, morphological stability against crystallization, LUMO and ix
HOMO levels unaffected by chemical bonding, and triplet energy unconstrained by the electrochemical energy gap. Phosphorescent OLEDs containing solution-processed emitting layers were fabricated with TRZ-3Cz(MP)2,
TRZ-1Cz(MP)2 and Cz(MP)2 hosting Ir(mppy)3 for an illustration of how chemical composition and hence charge transport properties affect device performance.
Bulk heterojunction organic solar cells comprising an active layer of
P3HT:PCBM blend at a 1:1 mass ratio with thickness from 130 to 1200 nm have been fabricated and characterized before and after thermal annealing. Before thermal annealing, both short circuit current density and power conversion efficiency decrease with an increasing film thickness, resulting in an inverse spectral response for thick-film devices. Thermal annealing decreases the thin-film device efficiency but substantially increases that of the thick-film devices while eliminating the inverse character of spectral response therefrom.
A conjugated oligomer-C60 Dyad has been synthesized to demonstrate its ability to modulate the extent of phase separation between rod-like OFTB and spherical PCBM. While thermal annealing of the OFTB:PCBM at a 1:1 mass ratio results in a eutectic mixture, OFTB:Dyad:PCBM film at a 9:2:9 mass ratio undergoes phase separation into interspersed 30-nm amorphous domains at approximately equal fractions upon thermal annealing. Geometric surfactancy is x
inferred by analogy to the widely reported formation of microemulsions in traditional oil-surfactant-water systems and ternary polymer blends.
xi
TABLE OF CONTENTS
Curriculum Vitae iii
Acknowledgements v
Abstract viii
List of Charts xiv
List of Reaction Schemes xv
List of Figures xvi
List of Tables xxviii
Forward xxix
Chapter 1 Background and Introduction 1
1.1 Organic Semiconductors 1
1.2 Organic Field-Effect Transistors 4
1.3 Organic Light-Emitting Diodes 8
1.4 Organic Photovoltaic Cells 13
1.5 Formal Statement of Research Objectives 19
References 22
Chapter 2 Synthesis and Processing of Monodisperse Oligo (fluorene-co-bithiophene)s into Oriented Films by 42 Thermal and Solvent Annealing. 2.1 Introduction 42 xii
2.2 Experimental 45
2.3 Results and discussion 59
2.4 Summary 71
References 74
Chapter 3 Development of Non-Conjugated Bipolar Host Materials
for Phosphorescent Organic Light-Emitting Diodes 80
3.1 Introduction 80
3.2 Experimental 83
3.3 Results and discussion 95
3.4 Summary 111
References 113
Chapter 4 Effects of Active Layer Thickness and Thermal Annealing
on the P3HT-PCBM Photovoltaic Cells 119
4.1 Introduction 119
4.2 Experimental 120
4.3 Results and discussion 122
4.4 Summary 132
References 134
xiii
Chapter 5 Modulation of Phase Separation Between Spherical and
Rod-Like Molecules Using Geometric Surfactancy 136
5.1 Introduction 136
5.2 Experimental 137
5.3 Results and discussion 144
5.4 Summary 154
References 156
Chapter 6 Conclusions and Future Studies 158
6.1 Conclusions 158
6.2 Future studies 161
Appendix 1 1H NMR and MALD/I-TOF Mass Spectra for Chapter 2 164
Appendix 2 1H NMR and MALDI/TOF Mass Spectra for Chapter 3 181
Appendix 3 1H NMR and MALD/I-TOF Mass Spectra for Chapter 5 188
Appendix 4 POM Images for Chapter 5 193
xiv
LIST OF CHARTS
2.1 Molecular structures of OF2T-1 through -8 and PF2T 45
3.1 Representative non-conjugated bipolar compounds as well as independent electron- and hole-transport moieties with their thermal transition temperatures determined by DSC heating scans. Symbols: G, glassy; K, crystalline; I, isotropic. 83
5.1 Molecular structures of OFTB, PCBM, and Dyad used in this study. Symbols: G, glassy; K, crystalline; I, isotropic. 138
xv
LIST OF REACTION SCHEMES
2.1 Synthesis of OF2T-1 through -8 46
3.1 Synthesis scheme of non-conjugated bipolar hybrids, TRZ-1Cz(MP)2, TRZ-3Cz(MP)2, OXD-2Cz(MP)2, and hole-transporting Cz(MP)2. 85
5.1 Synthesis schemes for OFTB and Dyad. 138
xvi
LIST OF FIGURES
1.1 Schematic diagram of orgnic field-effect transistors in a top-gate configuration. 41 1.2 Typical J-V characteristics under dark and illuminated conditions accompanied by device performance parameters. 42 − 2.1 DSC thermograms at ±20 oC min 1 for samples of OF2Ts and
PF2T preheated to above Tc or Tm (whichever is higher) and then − quenched to −30oC at −100 oC min 1 before recording the reported second heating and cooling scans. Vertical arrows on the thermograms of OF2T-3 locate transition temperatures identified by hot-stage polarizing optical microscopy. Symbols: G, glassy; K, crystalline; N, nematic; I, isotropic. 58 2.2 (a) UV-vis absorption spectra in molecular extinction coefficient, ε; and (b) fluorescence (with excitation at 430 nm) spectra of − − OF2T-1, OF2T-4, OF2T-7 and OF2T-8 in toluene at 10 7 to 10 6 M. 61 2.3 Cyclic voltammetric scans of (a) OF2T-4, (b) OF2T-5, (c)
−4 OF2T-7 and (d) OF2T-8 in anhydrous CH2Cl2 at 2.5 ×10 M with 0.1 M tetraethylammonium tetrafluoroborate as the supporting electrolyte. 62 2.4 Polarizing optical micrographs of OF2T-8 films spin-cast from chloroform on a rubbed polyimide alignment layer observed at 45 degrees with respect to rubbing direction: (a) pristine, and (b) after thermal annealing at 140oC for 5 min. 63 2.5 (a) Schematic diagram of the organic field-effect transistors constructed for this study, where S and D denote source and drain, respectively; output (b) and transfer (c) curves for the OFETs xvii
comprising OF2T-8 with chain alignment parallel to current flow. 64 2.6 Polarized absorption spectra of films spin-cast from 0.8 wt% solutions at 3000 rpm for 65 s on rubbed polyimide alignment layers: (a) OF2T-8 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (b) OF2T-8 film from chloroform or chlorobenzene, thermally annealed at 140oC for 5 min, and then cooled to room temperature; (c) OF2T-8 film from chloroform and then vacuum-dried at room temperature for 12 h; and (d) PF2T film from chlorobenzene, and then vacuum-dried at room temperature for up to 48 h. The
reported Sab values are accompanied by an experimental error of
±0.02. Symbols A|| and A⊥ represent absorbance parallel and perpendicular to rubbing direction, respectively. 67 2.7 Polarized absorption spectra of films spin-cast from 0.8 wt% solutions at 3000 rpm for 65 s on rubbed polyimide alignment layers: (a) OF2T-7 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (b) OF2T-4 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (c) OF2T-7 film from chlorobenzene, followed by exposure to saturated chlorobenzene vapor for 30 s, and then vacuum-dried for up to 48 h, all at room temperature; (d) OF2T-4 film from chlorobenzene, followed by exposure to saturated chlorobenzene vapor for 30 s, and then vacuum-dried for
up to 48 h, all at room temperature. The reported Sab values are
accompanied by an experimental error of ±0.02. Symbols A|| and
A⊥ are as defined in Figure 2.6. 69 3.1 TGA thermograms of hybrid compounds recorded at a heating rate of 10 oC/min under nitrogen atmosphere. The decomposition xviii
temperatures at a weight loss of 5% are 399, 403 and 407 oC for TRZ-1Cz(MP)2, TRZ-3Cz(MP)2 and OXD-2Cz(MP)2, respectively. 96 3.2 DSC heating and cooling scans at ±20 oC/min of samples comprising (a) hole- and electron-transport moieties, (b) mixtures thereof, and (c) non-conjugated bipolar compounds that have been preheated to beyond their melting points followed by quenching to –30 oC. Symbols: G, glassy; K, crystalline; I, isotropic. 97 3.3 Polarizing optical micrographs of films from spin-cast chlorobenzene of OXD:2Cz(MP)2 mixture (a) before and (b) after thermal annealing at 32 oC for 3 days; (c) that of an OXD-2Cz(MP)2 film before and after thermal annealing at 100 oC for 3 days; and (d) that of an OXD:PVK mixture at 30:70 mass ratio after thermal annealing at 100 oC for 3 days. 98 3.4 Cyclic voltammetric scans of compounds in acetonitrile/toluene − (1:1 by volume) at 10 3 M with 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte. 100 3.5 Fluorescence spectra with excitation at 360 nm of approximately 45-nm-thick, spin-cast films of Cz(MP)2, TRZ-1Cz(MP)2, and TRZ:1Cz(MP)2; thermal annealing was performed at 20 oC
above Tg under argon for ½ h. 101 3.6 (a) UV-vis absorption spectra in molecular extinction coefficients, ε, of OXD-2Cz(MP)2, Cz(MP)2 and OXD. Phosphorescence spectra of (b) Cz(MP)2, (c) OXD, and (d) OXD-2Cz(MP)2 at 77
o −4 K in ethyl acetate at 10 M, for which the ET values were determined by the 0-0 transitions as indicated by arrows. 105 3.7 (a) Molecular structure and energy diagram of a conjugated bipolar compound CzOXD, and (b) Molecular structure and xix
energy diagram of a non-conjugated bipolar compound TRZ-1Cz(MP)2 accompanied by those of Cz(MP)2 and TRZ. 107 3.8 (a) Current density as a function of driving voltage for phosphorescent OLEDs with emitting layers comprising Cz(MP)2, TRZ-3Cz(MP)2, and TRZ-1Cz(MP)2 doped with
Ir(mppy)3 at a 10:1 mass ratio. Inset: electroluminescence (EL) spectrum with TRZ-3Cz(MP)2 as the host. (b) Luminance and current efficiency as functions of current density for the same phosphorescent OLEDs as described in (a). 109 4.1 Schematic diagram o the BHJ-OPV device architecture and chemical structures of P3HT and PCBM 123 4.2 J-V characteristics under 100 mW/cm2 light illumination before and after thermal annealing at 110oC for 20 min of 130-nm BHJ-OPV devices without the LiF layer to improve electron collection-efficiency over Al cathode alone. 123 4.3 J-V characteristics under 100 mW/cm2 light illumination of BHJ-OPV devices with varying active layer thicknesses before thermal annealing. 124 4.4 (a) UV-vis absorption spectra of P3HT:PCBM blend films with varying active layer thicknesses before thermal annealing, and (b) Spectral responses from BHJ-OPV devices with varying active layer thicknesses before thermal annealing; the dotted curve represents the spectral response from a 1200-nm device under illumination through the semitransparent cathode. 125 4.5 J-V characteristics under 100 mW/cm2 white light illumination of a 1200-nm BHJ-OPV device thermally annealed at 110oC up to 120 min. 128 4.6 (a) UV-vis absorption spectra of a 1200-nm film before and after xx
thermal annealing at 110 oC for 20 min, and (b) Spectral responses of a 1200-nm BHJ-OPV device thermally annealed at 110oC up to 120 min. 129 4.7 J-V characteristics in dark of BHJ-OPV devices with a 130- and 1200-nm active layer thermally annealed at 110oC for 20 min. The dotted curve represents the J-V characteristics of a 130-nm device without a LiF layer. 131 4.8 Electric-field dependence of spectral responses under reverse bias of a 1200-nm BHJ-OPV device after thermal anneaming at 110oC for 20 min. 132 5.1 DSC heating scans of OFTB:Dyad:PCBM at three compositions
as indicated. Dashed curves: samples preheated to 310oC and quenched to –30oC before heating at 20oC/min. Solid curves: samples preheated to 310oC and quenched to –30oC before
o annealing at 10 C above respective Tgs, as determined with dashed
curves, for 12 h and then cooled to room temperature before
heating at 20oC/min. Complete transition to isotropic liquid for the annealed 10:0:10 blend occurred at 242oC as indicated by an arrow. Symbols: G, glassy; K, crystalline; I, isotropic. 145 5.2 Samples of OFTB:Dyad:PCBM at 10:0:10 mass ratio, pure
PCBM and OFTB were preheated to 310oC and then quenched to room temperature before annealing at 103oC for 48 h followed by cooling to room temperature for powder XRD analysis. Weaker crystalline diffraction peaks resulted from a shorter annealing time, e.g. 12 h. The annealing temperature was placed at 10 and
o o 22 C above Tg of the blend and OFTB, respectively, and 189 C
below the Tm of PCBM. A quenched but unannealed 10:0:10 xxi
blend was also characterized for identification of broad amorphous peaks. 146 5.3 The OFTB:PCBM phase diagram constructed with the DSC
thermograms at 20oC/mim and hot-stage POM for phase identification. With the exception of PCBM, all samples were
preheated to 310oC, quenched to –30oC, and then annealed at 10oC
above their respective Tgs for 12 h to maximize crystallization. The samples were then cooled at –20oC/min to room temperature for collecting heating scans at 20oC/min. Preheating PCBM to 310oC followed by quenching to –30oC did not result in glass transition, thereby obviating thermal annealing at 10oC above its
Tg to further induce crystallization. The liquid lines were constructed with the end melting (open circles) and end dissolution (solid circles) temperatures for neat and excess components, respectively. Open triangles represent crystallization temperatures for the specified compositions. Symbols S and L represent solid and liquid regimes, respectively. 147 5.4 XRD patterns (collected at 0.02º/step), 10 s per step and POM micrographs as the insets for 100-nm-thick spin-cast films of OFTB:Dyad:PCBM blends at (a) 10:0:10 mass ratio and (b)
9:2:9 mass ratio after thermal annealing at 10oC above their
respective Tgs for 12 h followed by cooling to room temperature. Essentially the same XRD and POM results were observed for the 7:6:7 film as reported in (b) for the 9:2:9 film. Preheating to
o 310 C as conducted for powders was avoided to preserve film
integrity. No diffraction peaks are visible at 2θ between 10 and 30o as shown in Figure S.5. 149 xxii
5.5 AFM phase images of 100-nm-thick spin-cast films of OFTB:Dyad:PCBM at three compositions before (a, b, c) and
after (d, e, f) thermal annealing for 12 h at 10oC above their
respective Tgs followed by cooling to room temperature. Preheating to 310oC as conducted for powders was avoided to preserve film integrity. Featureless phase images of (a), (b), (c), and (f) represent the absence of phase separation down to about 1 nm. 151 5.6 Section analysis of the phase image of a 100-nm-thick spin-cast film comprising OFTB:Dyad:PCBM at a mass ratio of 9:2:9
o after thermal annealing at 10 C above Tg for 12 h followed by
cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity. The domain sizes are typically between 30 and 40 nm. 152 5.7 A schematic diagram of Dyad acting as a geometric surfactant to modulate phase separation between OFTB and PCBM. 153 5.8 AFM topographic images of 100-nm-thick spin-cast films of OFTB:Dyad:PCBM at three compositions before (a, b, c) and
o after (d, e, f) thermal annealing at 10 C above their respective Tgs for 12 h followed by cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity. The root-mean square protrusions are 0.33, 0.25, and 0.21 nm for a, b, and c, respectively, and 5.6, 3.1, and 0.23 nm for d, e, and f, respectively. 154
1 A1.1 H NMR (400 MHz) spectrum of OF2T-1 in CDCl3 at 298 K. 165
1 A1.2 H NMR (400 MHz) spectrum of OF2T-2 in CDCl3 at 298 K. 166
1 A1.3 H NMR (400 MHz) spectrum of OF2T-3 in CDCl3 at 298 K. 167 xxiii
1 A1.4 H NMR (400 MHz) spectrum of OF2T-4 in CDCl3 at 298 K. 168
1 A1.5 H NMR (400 MHz) spectrum of OF2T-5 in CDCl3 at 298 K. 169
1 A1.6 H NMR (400 MHz) spectrum of OF2T-6 in CDCl3 at 298 K. 170
1 A1.7 H NMR (400 MHz) spectrum of OF2T-7 in CDCl3 at 298 K. 171
1 A1.8 H NMR (400 MHz) spectrum of OF2T-8 in CDCl3 at 298 K. 172 A1.9 MALD/I TOF MS spectrum of OF2T-1 using DCTB as the matrix. 173 A1.10 MALD/I TOF MS spectrum of OF2T-2 using DCTB as the matrix. 174 A1.11 MALD/I TOF MS spectrum of OF2T-3 using DCTB as the matrix. 175 A1.12 MALD/I TOF MS spectrum of OF2T-4 using DCTB as the matrix. 176 A1.13 MALD/I TOF MS spectrum of OF2T-5 using DCTB as the matrix. 177 A1.14 MALD/I TOF MS spectrum of OF2T-6 using DCTB as the matrix. 178 A1.15 MALD/I TOF MS spectrum of OF2T-7 using DCTB as the matrix. 179 A1.16 MALD/I TOF MS spectrum of OF2T-8 using DCTB as the matrix. 180
1 A2.1 H NMR (400 MHz) spectrum of TRZ-2Cz(MP)2 in CDCl3 at 298K 182
1 A2.2 H NMR (400 MHz) spectrum of TRZ-3Cz(MP)2 in CDCl3 at 298 K. 183
1 A2.3 H NMR (400 MHz) spectrum of OXD-2Cz(MP)2 in CDCl3 at 298 K. 184 A2.4 MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as xxiv
the matrix. 185 A2.5 MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as the matrix. 186 A2.6 MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as the matrix. 187
1 A3.1 H NMR (400 MHz) spectrum of OFTB in CDCl3 at 298 K. 189
1 A3.2 H NMR (400 MHz) spectrum of Dyad in CDCl3 at 298 K. 190 A3.3 MALD/I TOF MS spectrum of OFTB using DCTB as the matrix 191 A3.4 MALD/I TOF MS spectrum of Dyad using DCTB as the matrix 192 A4.1 Polarizing optical micrographs of 100-nm-thick films comprising OFTB:Dyad:PCBM at three compositions before (a, b, c) and
o after (d, e, f) thermal annealing at 10 C above their respective Tgs for 12 h followed by cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity. 194
xxv
LIST OF TABLES
2.1 Highest occupied molecular orbital, HOMO, energy levels,
orientational order parameters, Sab, and anisotropic field-effect hole
mobilities, μ|| and μ⊥, for OF2Ts and PF2T 69
2.2 Orientational order parameters, Sab, of monodomain films comprising OF2T-4, -7 and -8 achieved through thermal, quasi-solvent and solvent-vapor annealing 71 3.1 Electrochemical properties of compounds determined by the oxidation and reduction scans presented in Figure 3.4. 102 4.1 Performance parameters of BHJ-OPV devices with varying active layer thicknesses before and after thermal annealing at 110 oC for 20 min. 126 4.2 Performance parameters of the 1200-nm BHJ-OPV device after thermal annealing at 110 oC up to 120 min. 130
xxvi
FORWORD
This thesis summarizes my PhD research in collaboration with my advisers, my fellow students, and scientists within and outside the University of Rochester.
My participation and contributions to the research are as what follows.
Chapter 1 of my thesis is to introduce the background information pertinent to the researches discussed in the following chapters. I did the literature study and wrote this chapter.
I’m the leading author of Chapter 2 and have collaborated with Professor
Shaw H. Chen, Feng Yan, Sean W. Culligan, and Simon K. H. Wei. Feng Yan synthesized compound OF2T-1, -2, -3, -4, -5, and -7. Sean W. Culligan measured the hole mobilities for OF2T-4 and -5. This chapter has been published in
Advanced Functional Materials 2009, 19, 1978
Chapter 3 of my thesis was co-authored with Professor Shaw H. Chen,
Thomas Y. H. Lee, and Paul B. Merkel. The low-temperature phosphorescence spectra were collected by Paul B. Merkel. I’m the leading author for this chapter published in the Journal of Materials Chemistry 2009, 19, 8772
Chapter 4 of my thesis originates in the joint efforts with Professor Ching W.
Tang and Professor Shaw H. Chen. I carried out all the experiments reported in this chapter, and it has been published in Applied Physics Letters 2010, 97,
053305.
Chapter 5 of my thesis was co-authored with Professor Shaw H. Chen, and xxvii
Thomas N. Blanton of the Eastman Kodak Company. All the X-ray diffraction data were collected by Blanton, and I’m responsible for all other data. This chapter has been published in Langmuir 2010, 26, 12877. 1
CHAPTER 1
BACKGROUND AND INTRODUCTION
1.1 ORGANIC SEMICONDUCTORS
Electronic devices with organic materials as active components, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs), have transitioned from scientific curiosities to potentially important technologies for consumer electronics and renewable energy converted from sun light. This process has been facilitated by global collaborative efforts between chemists, material scientists, and device physicists during the past couple of decades.[1-9] Organic materials offer the advantages of thin-film formation, light weight, large area, and mechanical flexibility, all of which are desirable for electronic devices. Moreover, organic materials are compatible with roll-to-roll processing that could significantly reduce device fabrication costs.
The central components for organic electronic devices are organic semiconductors responsible for light absorption and emission as well as charge generation, transport and recombination. Organic semiconductors are typically
π-conjugated molecules comprised primarily of carbon, hydrogen, and other elements such as nitrogen, sulfur, silicon, boron, phosphorus, selenium, etc. 2
Aromatic hydrocarbon and alkenes are typical building blocks for π-conjugated molecules, but heterocylics such as pyrrole, furane, thiophene, and pyridine are also important constituents. Depending on the molecular size, these organic semiconductors can be categorized into small molecules, oligomers, and polymers.
The electrical and optical properties of the π-conjugated molecules are determined by the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).[10] The HOMO and LUMO energy levels represent the ionization potential and electron affinity, respectively, characterizing the materials abilities to donate and accept electrons. In other words, materials with high HOMO levels usually function as electron-donating and hole-transport media, while those with low LUMO levels function as electron-accepting and electron-transport media. Some materials that have both high HOMO levels and low LUMO levels exhibit bipolar transport characteristics, i.e., they are capable of transporting both holes and electrons. Increasing HOMO levels of hole-transport materials facilitates hole injection from anodes to lower the operation voltage for electronic devices. However, materials with high-lying HOMO levels are prone to oxidation by ambient air and hence are less stable. Lowering the LUMO levels of electron-transport materials allows for the fabrication of electronic devices using stable high-work-function cathode metals. Furthermore, the LUMO-HOMO energy gap, EG, dictates the positions of the absorption and emission bands of conjugated molecules.[11] 3
Material properties are greatly affected by molecular packing behaviors and the broadly defined solid morphology.[12-14] Organic semiconductors with crystalline, amorphous and liquid crystalline morphologies have found potential applications in various electronic devices. In organic molecular crystals, atoms and molecules are arranged with positional and orientational order. Pentacene and rubrene single crystals have demonstrated excellent charge transport capabilities,[15] yielding hole mobilities as high as 10.7 cm2 V-1s-1 [16] and 20 cm2 V-1s-1,[17] respectively. However, the application of these single crystals in electronic devices has met with limited success,[18] because organic single crystals are usually small, fragile, and difficult to handle. Polycrystalline thin films fabricated from either solution or vapor phase have been actively explored for OFET and OPV devices.
Charge transport in these polycrystalline films is impeded by domain boundaries, and the mobility values are affected by crystal grain size and molecular orientation.[19, 20] Amorphous materials are usually optically homogenous, isotropic and transparent, and have been widely applied in OLEDs.[21] Liquid crystals are fluids with orientational and/or positional order, and can be aligned under mechanical, electrical or magnetic field. Arresting these ordered structures in solid state without encountering crystallization, glassy liquid crystalline conjugated molecules [22] have been demonstrated for polarized OLEDs [23] and OFETs with anisotropic charge-transport characteristics.[24] 4
1.2 ORGANIC FIELD-EFFECT TRANSISTORS
Organic field-effect transistors based on polymers [25] and small molecules [26] emerged in the late 1980s and have been developed into an attractive cost-effective technology for the construction of logic circuits. Some of the potential applications of OFETs include active-matrix driving schemes for flexible and large-area flat-panel displays,[27] radio-frequency identification tags,[28] and sensors.[29]
VG
Gate Insulator Organic Semiconductor
Source Drain VD Substrate Figure 1.1: Schematic diagram of organic field-effect transistors in a top-gate configuration
OFET devices are generally constructed following the principle of thin film transistors [30, 31] consisting of three parts: a dielecric layer, a thin layer of organic semiconductors, and three electrodes (Figure 1.1). Two of the electrodes, the source and the drain, are in direct contact with organic semiconducting layer. As the third electrode, the gate is isolated from the semiconducting layer by the dielectric layer.
For device operation, the conductivity of channels between the source and drain is modulated by a voltage applied at the gate. The source electrode is always grounded.
Applying a negative voltage (VG) greater than a threshold voltage, VT, at the gate 5
(with a p-type semiconductor) induces hole injection from the source to the semiconductor and hole accumulation at the interface between the semiconductor and the dielectric layer. A conducting channel is formed at this interface, and holes can be driven from source to drain by applying a second, independent negative voltage (VD) at drain, consistuiting the on state. Without the negative voltage at the gate (VG = 0), no conducting channel will be formed, and the device is in the off state. At a low drain voltage, VD < VG, the source-drain current (ISD) rises linearly with VD (Equation 1.1) before approaching the saturation regime (Equation 1.2).
ISD = (W/L)Cμ(VG – VT – VD/2)VD (1.1)
2 ISD = (W/L)Cμ(VG – VT) (1.2)
in which W is the channel width, L is the channel length, C is the capacitance per unit area of the dielectric layer, and μ is the field effect mobility of the semiconductor.
Equations 1.1 and 1.2 show that dielectric layers with a high capacitance and the semiconductor layer with a high charge carrier mobility are favorable to raise
ISD at constant VG and VD. High capacitance can be achieved by using very thin dielectric layer,[32] or by employing insulating materials with high dielectric
[33] [34] [35] constant (k), such as Al2O3 (k = 7.6), TiO2 (k = 41), polyvinyl alcohol (k = 6
7.8) [36] and cyanoethylpullulan (k = 18.5).[37] However, polar functional groups present on the surfaces of many high-k dielectric materials serves as charge traps at the semiconductor-dielectric layer interface, thus lowering the field-effect mobility.[38] On the other hand, high charge carrier mobilities were found in OFETs with a trap-free low-k dielectric polymer [38a, 39] or self-assembled monolayer.[40]
Small molecules such as acences,[15, 41] heterocenes [41] and oligothiophenes [42] tend to crystallize and have yielded impressive field-effect hole mobilities greater than 1 cm2 V-1.s-1. Nevertheless, most of these molecules have poor solubility in common organic solvents, preventing the implementation of cost-effective solution processing. Solubilization of these small molecules was accomplished by introducing bulky aliphatic groups or employing branched molecular structures,[31c] enabling solution-processed OFETs with hole mobilities up to 1.7 cm2 V-1.s-1.[43]
Benefiting from high viscosity and excellent film-forming ability, soluble conjugated polymers have been one of the most investigated classes of materials for
OFETs. Hole mobilities in the range from 0.1 to 1.4 cm2V-1s-1 have been achieved with thiophene-containing polymers.[44] Furthermore, n-channel OFETs with electron mobility of 0.45-0.85 cm2V-1.s-1 have also been reported with a copolymer comprising naphthalene tetracarboxylic diimide and bithiophene.[45]
The charge carrier mobilities are highly dependent on the film morphology of organic semiconductors. Grain boundaries existing in polycrystalline films break the continuity of charge-transport channels.[20] The effect of grain size on the device 7
performance is still under debate.[31, 46] Nevertheless, large crystalline grain size could be induced by heating the substrates during vacuum deposition,[42b, 47] and by post-deposition thermal and/or solvent-vapor annealing.[20, 48] Furthermore, in-plane molecular organization of organic semiconductors favors charge transport through hopping along the π−π stacking direction. Therefore, macroscopically aligned organic molecules potentially have high mobilities with anisotropic charge carrier transport characteristics.[49] Alignment of organic semiconductors has been demonstrated by mechanical stretching of polythiophene films,[50] and by epitaxial growth of small-molecules on single-crystalline inorganic substrates.[51] Some conjugated molecules, such as hexabenzocoronenes,[52] oligothiophenes,[53] as well as fluorene oligomers [24, 54] and co-oligomers [55] together with their polymer analogues [56], exhibit liquid crystalline phases at elevated temperatures, offering alternative routes for molecular self-assembly. These semiconducting liquid crystals have been aligned by friction transfer,[57] nanoconfinement,[58] and thermal annealing on a uniaxially buffed alignment layer [56b] or on a photoalignment layer.[59] However, these alignment techniques involve thermal treatment in the high-temperature liquid crystal regime, which is incompatible with commonly used
o flexible plastic substrates such as polyethylene terephthalate (Tg = 80 C) and
o [60] polyethylene naphthalate (Tg = 150 C) that cannot withstand high temperatures.
Development of liquid crystalline conjugated molecules and alignment techniques 8
executable at room temperature is highly desirable and will be sought after in this thesis.
1.3 ORGANIC LIGHT-EMITTING DIODES
Since the seminal papers by Tang et al. using small molecules in 1987,[61] and
Friend et al. using conjugated polymers in 1990, [62] OLEDs have received tremendous attention in academic communities and industry. With continuing improvement in both efficiency and lifetime, OLEDs have recently been commercialized for small mobile electronic devices and TV sets. Compared to liquid crystal displays, OLED displays are thin and light weight, capable of sharp contrast and a wide viewing angle. Furthermore, OLEDs are also potentially useful for energy-efficient and inexpensive solid-state lighting.
The simplest OLED consists of a transparent indium tin oxide anode, an emitting layer comprising organic materials, and a reflective metal cathode.
Charge-injection, -transport and -blocking layers are usually inserted between the electrode and the emitting layer.[63] Holes and electrons are injected from anode and cathode into the organic layers, move in opposite directions, and recombine in the emitting layer to generate excitons that emit light to be out-coupled through the transparent anode. The external quantum efficiency, EQE, defined as the ratio of collected photons to injected electrons, is determined by the probability of electron-hole recombination (γ), the fraction of emissive excitation due to spin statistics (χ), the efficiency of radiative decay (Φ) and the portion of photons 9
out-coupled from the device (ξ): EQE = γ⋅χ⋅ Φ⋅ξ. The adoption of the multilayer structure helps to both balance charge-carrier fluxes for efficient recombination and confine excitons inside the emitting layer to produce highly efficient devices.[63]
It has been widely accepted that singlet and triplet excitons in OLEDs are formed by charge recombination at a ratio of 1:3 according to a random spin distribution of electrons and holes.[64] It was assumed that fluorescent emitters can only utilize singlet excitons for emission, setting the maximum EQE at 5%.[65]
However, theoretical calculation has shown that the fraction of singlet excitons can exceed the 25% limit for certain conjugated polymers.[66] It has also been suggested that triplet-triplet annihilation can contribute to fluorescence emission.[67] Recently, highly efficient fluorescence OLEDs have been reported with an EQE exceeding
10%.[68]
A great deal of attention has focused on phosphorescent OLEDs (PhOLEDs) based on triplet emitters introduced by Forrest et al. in 1998,[69] and an EQE above
20% have been achieved.[63] Triplet emitters are organometallic complexes comprising heavy transition metal ions and chelating chromophores.[70] The strong spin-orbit coupling induced by the heavy metal ions promotes rapid intersystem crossing, and consequently results in efficient emissive decay from the lowest excited triplet state to the singlet ground state.[71] Therefore, an internal quantum efficiency of 100% is accessible with PhOLEDs. 10
Because their triplet excitons have long excited-state lifetimes, triplet emitters are usually doped into host materials to avoid concentration quenching of emission.[72] The triplet emitters should be completely miscible with the host materials to prevent phase separation for the formation of glassy films stable against thermally activated crystallization. The host materials should also have their triplet energies (ETs) greater than those of triplet emitters to confine excitons on the latter, and HOMO/LUMO levels matching those of adjacent layers to facilitate charge injection. Preferably, host materials should have bipolar charge transport capability to achieve balanced electron and hole fluxes through the emitting layer for the sake of efficiency and lifetime. Much effort has been invested in developing triplet host materials for efficient and stable PhOLEDs as to be elaborated in what follows.
Typically, host materials should have a minimum ET of 2.7, 2.5, and 2.0 eV to accommodate a blue-, green-, and red-emitting guest, respectively. Carbazole has been widely used as a building block for the construction of amorphous triplet
[73] hosts because of its high ET of 3.0 eV. Both computational and experimental results suggest that derivatization with π-conjugated groups at the 3-, 6- and
9-positions of carbazole has less adverse effect on the triplet energies than at the 2-
[73b, c] [74] and 7-positions. Other building blocks with high ETs, such as arylsilanes, arylphosphate oxides,[75] siloles,[76] triazines,[77] and fluorenes,[78] have also been incorporated into PhOLED host materials. Previous efforts based on these building blocks have created a number of high-ET host materials, and additional inert bulky 11
substitution groups have been introduced to arrive at materials with stable morphology. These efforts have culminated in highly efficient PhOLEDs with full-color emission by vacuum deposition.
Generally, a large ET implies a wide LUMO-HOMO bandgap. The
LUMO-HOMO energy difference of organic compound is equal to the sum of the triplet energy, the singlet-triplet splitting, and the exciton binding energy.[73c, d]
Therefore, triplet host materials with large ETs usually have a high LUMO level and/or a deep HOMO level,[73a, 73j, 74, 77] which presents large barriers to electron and/or hole injection while limiting their bipolar charge-transport capability. A large barrier for charge injection results in high driving voltage, and unipolar charge-transport capability leads to unbalanced charge transport and consequently low recombination efficiency. In PhOLEDs using unipolar hosts, charge carriers and excitons tend to accumulate close to the interface with the emitting layer,[79] resulting in efficiency roll-off at high current densities[80] and device degradation.[64c, 81] These problems have been alleviated to some extent by using mixed hosts comprising electron- and hole-transport molecules. The injection of electrons and holes is facilitated by components in the mixed hosts with energy levels better matched with the adjacent layers.[82] Furthermore, fluxes of electrons and holes can be balanced by varying the compositional ratio without affecting energy levels.[82d] Decreased driving voltage, improved device efficiency and suppressed efficiency roll-off have been demonstrated by using mixed host.[82] 12
However, achieving the desired bipolar transport capability requires a high concentration of charge-transport additives at the risk of phase separation over time to adversely affect device stability.
Bipolar host materials capable of transporting both electrons and holes represent a viable approach to circumvent the potential phase-separation problem as encountered in mixed hosts. For example, 4,4’-bis(9-carbazolyl)-2,2’-biphenyl is
[83] among the widely used bipolar host materials. However, the relatively low ET of 4,4’-bis(9-carbazolyl)-2,2’-bipheneyl (2.6 eV) precludes its application to blue-emitting PhOLEDs. Recently, new bipolar host materials have been developed, in which electron- and hole-transport moieties are chemically bonded.[84] Modulation of π-conjugation is accomplished by controlling dihedral angle or through meta-linkage between the two moieties to arrive at high ETs. An
EQE of 24 % has been reported by using the bipolar host consisting of pyridine
[84a] and carbazole moieties, in which the meta-linkage managed to retain a high ET.
Nevertheless, the ETs of these bipolar hosts are still limited by the LUMO-HOMO energy gap. One aspect of this thesis will seek to develop bipolar high-ET host materials where the ET is unconstrained by the LUMO/HOMO levels, and the electron- and hole-transport properties can be incorporated without affecting energy levels.
13
1.4 ORGANIC PHOTOVOLTAIC SOLAR CELLS
Solar cells convert sunlight directly into electrical energy. The first solar cell was invented in 1954 using semiconducting crystalline silicon as the active medium, which is widely used today in the solar cell market.[85] In recent years organic semiconductors have emerged as an attractive alternative for the construction of solar cells. Modulation of light absorption, energy levels, and phase separation behavior can, in principle, be accomplished with organic materials through molecular design and synthesis. Moreover, organic materials enable solution processing at a much lower cost than their inorganic counterparts. Earlier work on organic photovoltaic (OPV) cells involved exciton dissociation at the interface of organic material and a metal electrode with a poor power conversion efficiency.[86]
Tang reported the first efficient OPV cell based on a donor-acceptor bilayer heterojunction comprising small molecules.[87] The active layer thickness in these planar devices, however, cannot exceed the exciton diffusion length typically less than 20 nm,[88] thus limiting the ability to harvest sunlight. This problem was partly overcome by using bulk heterojunctions [89] prepared by blending a donor (D) and an acceptor (A) with nanoscale phase separation into bicontinuous pathways for electrons and holes generated at the D-A interface to conduct efficiently to the two electrodes. Mechanistically, light is converted into photocurrent in four steps:[90] exciton generation following light absorption; exciton diffusion to the D-A 14
interface; exciton dissociation (or charge transfer between D and A) into free charge carriers; and charge transport to and collection at electrodes.
J
⋅ ⋅ Dark η = Pout = JSC VOC FF Pin Pin V
0 Vmax VOC J maxVmax FF = J J V max SC OC Illuminated
J SC
Figure 1.2. Typical J-V characteristics under dark and illuminated conditions accompanied by device performance parameters.
The solar power conversion efficiency, η = JSCVOCFF/Pin, where JSC, VOC, FF, and Pin are the short circuit current density, the open circuit voltage, the fill factor, and incident solar power density, respectively. Moreover, FF = JmaxVmax/JSCVOC, where Jmax and Vmax are the current density and voltage yielding the maximum electrical power, respectively. These important performance parameters together with representative illuminated and dark current density-voltage, J-V, characteristics are depicted in Figure 1.2. Recent advances in the elucidation of factors affecting the power conversion efficiency and lifetime of bulk heterojunction OPVs are summarized in what follows. 15
For the construction of bulk-heterojunction OPVs, small molecules can be vacuum sublimed,[91] while polymers are normally deposited by solution processing, both into thin films. The discovery of fast photo-induced electron transfer from
[92] poly(p-phenylenevinylene)s to C60 has prompted intense interest in polymer-fullerene bulk heterojunctions. [6,6]-Phenyl C61-phenyl C61-butyric acid methyl ester (PCBM) and its C70 analogue have been the most promising electron acceptors to date.[93] Poly(p-phenylenevinylene)s,[94] polythiophenes,[95] and polyfluorenes[96] are among the widely used electron-donating polymers.
Intensive efforts have been devoted to developing low-bandgap polymers for effectively capturing the solar spectrum especially in the near infrared region. A powerful molecular design approach exploits the push-pull strategy, where electron-rich and -deficient moieties are placed in conjugation to lower the LUMO level and to elevate the HOMO level at the same time.[97] Many low-bandgap polymers have been successfully synthesized [98] with light absorption extended well beyond 1 μm.[99] Some of the typical electron-donating groups are thiophene,[100] cyclopentadithiophene,[101] dithienosilole,[102] and carbazole,[103] while benzothiodiazole,[104] thienopyrazine,[105] quinoxaline,[106] and diketo-pyrrole-pyrrole[107] are commonly used electron-accepting groups. The alignment of energy levels between donors and acceptors must be taken into account when designing new materials. It was suggested that a minimum difference of 0.3 eV between the donor’s and the acceptor’s LUMO levels is required to 16
overcome the exciton binding energy for efficient exciton dissociation at the donor-acceptor interface.[108] Furthermore, in the bulk heterojunction OPVs, the maximum achievable open circuit voltage is closely related to the difference between the donor’s HOMO level and the acceptor’s LUMO level.[109] The intensive efforts in materials development have led to a steady improvement in device performance, achieving an η of 7.6 % recently.[110] However, organic materials tend to have band-like absorption spectra with absorption coefficients varying with wavelength.[11] At the same time, the typical active layer thickness of current polymer OPV devices is about 100 to 200 nm, losing significant amount of photons at weakly absorbing wavelength regions.[89c] One aspect of this thesis will investigate factors governing performance of devices with thick active layers.
The performance of polymer-fullerene bulk heterojunction OPVs depends critically on nanoscale film morphology.[112] The film morphology is significantly affected by molecular structure, chemical composition, solution processing condition, and post treatment. While the in-plane π−π stacking of poly(3-hexylthiophene) (P3HT) is favorable to charge transport in OFET, poly(thieno[3,4-b]thiophene-co-benzodithiophene)s were found to stack in a face-down fashion that are particularly desirable for charge transport in OPVs parallel to substrate surface.[111] Though P3HT/PCBM blend at a mass ratio close to
1:1 was found to produce optimal morphology for efficient devices,[113] other polymers including poly(2-methoxy, 5-(3’,7’-dimethyl-octyloxy)-p-phenylene 17
vinylene) (MDMO-PPV) [94] and most of the low-bandgap polymers [96-107] require a much higher fraction of fullerenes to generate bicontinuous channels for efficient devices. Fullerene derivatives are more soluble in chlorinated solvents than in non-halogenated ones, and consequently, tend to form finer domains in polymer:fullerene blend films spin coated from chlorinated solvents.[112b, 114]
Incorporating small amounts of additives, such as high-boiling-point alkanedithioles,[115] chloronaphthalene,[116] nitrobenzene,[117] and certain non-volatile transition metal complexes,[118] into the polymer:fullerene solutions for spin coating has been shown to enhance device performance that is attributable to improved morphologies. Both solvent-vapor annealing [119] and thermal annealing [120] are able to promote P3HT crystallization in P3HT/PCBM blends, thereby enhancing hole mobility and device efficiency.
It is not surprising that morphological instability was observed in polymer:fullerene blends. The morphological instability in the polymer:fullerene blends originated in interfacial tension, which tends to increase the domain size by minimizing the interfacial area and hence energy.[121] Rapidly growing PCBM domains were observed in MDMO-PPV/PCBM films annealed at temperatures between 60 and 120 oC.[122] This morphological evolution was found to be responsible for the degradation in device performance.[123] Similarly, prolonged annealing of the P3HT:PCBM films resulted in an overgrowth of domain size and hence deteriorating device performance.[124] One strategy to stabilize the desired 18
nanoscale morphology is to covalently link the donor and the acceptor together to form D-A dyads [125] or diblock copolymers.[126] Unfortunately, no phase separation was observed in the D-A dyads, suggesting molecular-level mixing between the donor and acceptor moieties.[127] In diblock copolymers, the interplay between interfacial tension and covalent bonding results in thermodynamically stable periodic nanostructures such as lamellae, gyroid(bicontinuous) phase, hexagonal array of cylinders, and cubic array of spheres.[128] The lamellar organization has
[126a] been demonstrated in a PPV-b-C60 diblock copolymer and also in a copolymer consisting of triphenylamine and perylenediimide diblocks,[126b] with the lamella layers parallel to the film plane, which is unfavorable to charge transport.
As an alternative strategy to improve morphological stability, an amphiphilic diblock copolymer can be added to a composite of two immiscible homopolymers.
The idea has been demonstrated for nonconjuated polymers by having an amphiphilic diblock copolymer locate itself at the polymer-polymer interface,[129] thereby diminishing the interfacial tension[130] while retaining the bicontinuous structure.[131] This strategy has been extended to the polymer donor:acceptor composite whose morphology is stabilized against thermally induced growth in domain size through doping with a diblock copolymer comprising donor and acceptor blocks.[132] As a result, the solar cell’s power conversion efficiency was shown to be stable with thermal annealing in the presence of a donor-acceptor diblock copolymer.[132a] Considering the fact that conjugated polymers are rod-like, 19
while fullerenes are spherical, this thesis seeks to utilize this geometric dissimilarity to modulate the phase-separated morphology using geometric amphiphiles.
1.5 FORMAL STATEMENT OF RESEARCH OBJECTIVES
Tremendous efforts have been invested in organic electronics, producing impressive achievements in recent years. OFETs are being actively explored as flexible display backplane, and chemical and biological sensor. Widespread use of
OFETs in large-area devices will rely on the fabrication of uniform films by cost-effective solution-processing techniques. Liquid crystalline conjugated oligomers and polymers represent one class of solution-processable materials, where high charge carrier mobility is realized by uniaxial alignment of molecules.
However, current alignment techniques require thermal treatment at high temperatures, which is incompatible with flexible substrates that are critical to roll-to-roll processing. It is highly desirable to develop a room-temperature alignment technique for promising conjugated molecules to circumvent high-temperature processing.
Consumer electronic products incorporating both small- and large-area high-quality OLED display panels are now commercially available. Meanwhile,
OLEDs are being actively pursued for efficient and affordable solid-state lighting.
One approach to realize low-cost OLEDs is through solution processing with a minimum number of layers, but it should not sacrifice high efficiency sustainable at 20
high current density essential to lighting applications. High-ET bipolar host materials with low LUMO levels and high HOMO levels are desirable to satisfy these requirements. Such host materials are rarely reported in literature, and a systematic design strategy toward these host materials would be highly appreciated.
The improvement of OPV’s power conversion efficiency is being vigorously attempted to address the global issue of energy shortage. To fully exploit the light-absorption capability of organic materials for high-efficiency devices, a thick active layer is advantageous in principle. However, the thickness of the active layer reported in current literature has been limited to less than 400 nm, and the understanding of thick-layer devices is lacking. The morphology of donor:acceptor blends is critical to device performance, while the evolution of morphology is one of the sources of device instability. A strategy to arrive at thermodynamically stable bicontinuous nanoscale phase-separated morphology is highly desirable.
Therefore, this thesis was motivated by the following objectives through the development of novel materials and processing techniques for implementation in organic electronic devices with a goal of furnishing fundamental understanding:
(1) To design and synthesize monodispersed glassy nematic conjugated oligomers that can be aligned into monodomain oriented films through thermal and solvent-vapor annealing suitable for use in OFETs with anisotropic charge transport characteristics. 21
(2) To develop morphologically stable high-ET non-conjugated bipolar host materials, where the ET is unconstrained by the LUMO/HOMO levels and the electron- and hole-transport properties are incorporated without affecting energy levels.
(3) To fabricate and characterize the P3HT:PCBM bulk heterojunction photovoltaic devices with active layer thickness up to 1 micron with an emphasis on understanding J-V characteristics and spectral response in relation to active layer thickness and thermal annealing.
(4) To explore geometric surfctancy as a new concept for the modulation and stabilization of phase separation between spherical and rod-like molecules with a potential application including efficient and stable bulk heterojunction OPVs.
22
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CHAPTER 2 SYNTHESIS AND PROCESSING OF CONJUGATED OLIGOMERS INTO MONODOMAIN FILMS BY THERMAL AND SOLVENT-VAPOR ANNEALING
2.1. INTRODUCTION
Organic materials with an extended π-conjugation have found numerous potential applications to electronics and photonics,[1-5] most notably in light-emitting diodes, field-effect transistors, and solar cells. Conjugated polymers can be prepared into thin films through solution processing across a large area, thus offering a cost advantage over small molecules that are typically deposited by vacuum sublimation. As an integral part of flexible electronics, polymer-based organic field-effect transistors have been actively pursued over the past decade, generating a wealth of information on charge-carrier mobility as a function of polymer morphology, gate insulator, and device architecture.[6-11] Uniaxial alignment of conjugated backbones offers additional desirable features, such as polarized light emission to obviate polarizers in liquid crystal displays and anisotropic charge transport to suppress cross-talk in logic circuit and pixel switching elements. Numerous backbone alignment strategies have been demonstrated for liquid crystalline poly(9,9’-dioctylfluorene),[11-20] PF, and poly(9,9’-dioctylfluorene-co-bithiophene),[21-25] PF2T, including rubbed polymer 43
films,[12-14, 19, 21] photoalignment induced by polarized irradiation,[15, 25] and friction transfer.[16, 19, 22] In addition to liquid crystallinity, both PF and PF2T are capable of preserving polymer backbone alignment in the solid state through glass transition. The hole mobility along the uniaxially aligned PF2T film has been reported to be 10 times that of the perpendicular component and about 3 times that observed in an isotropic film.[21] For a facile hole injection from gold with a
Fermi level at –5.2 eV, PF2T is better suited for field-effect transistors than PF in terms of the highest occupied molecular orbital (HOMO) energy level, –5.5 versus –5.8 eV.[14, 26] Regioregular poly(3-hexylthiophene) is superior to PF and
PF2T in terms of hole mobility on the order of 0.1 cm2/V·s[27, 28] and ease of hole injection (with a HOMO level at –4.9 eV) from gold, but is inferior in oxidative stability as with polythiophenes in general.[29] With somewhat improved oxidative stability because of the higher HOMO level of –5.1 eV, thermotropic liquid crystalline thieno[3, 2-b]thiophene polymers have bee reported with a hole mobility of 0.2 to 0.6 cm2/V·s in crystalline domains. [30]
Normally synthesized by step-growth polymerization, π-conjugated polymers are characterized by distributed chain lengths. The divergent-convergent approach to the synthesis of conjugated oligomers[31-33] offers monodisperse molecular systems readily soluble in benign organic solvents to facilitate purification by recrystallization, column chromatography, or vacuum sublimation.
Compared to polydisperse conjugated polymers with a high molecular weight, 44
monodisperse conjugated oligomers are more amenable to structural optimization for thermotropic properties, such as the type of mesomorphism as well as glass-transition and clearing temperatures. In the absence of chain entanglements and defects, relatively short and uniform chains are also conducive to the formation of monodomain films by monodisperse conjugated oligomers. To take advantage of these favorable material traits, a number of monodisperse oligo(fluorene)s,[31-40] OFs, and oligo(fluorene-co-bithiophene)s,[41-43] OF2Ts, have been reported, but challenges remain in the understanding of how molecular structures affect material properties that are critical to the performance of polarized light-emitting diodes and anisotropic field-effect transistors. Of particular interest is the ability of conjugated oligomers to form glassy liquid crystalline films with elevated phase transition temperatures and superior stability against thermally activated crystallization.
This study was motivated to unravel the roles played by the backbone length and aliphatic pendants to fluorene units’ C-9 positions in thermotropic properties and morphological stability through the synthesis and characterization of a series
OF2Ts. Top-gate field-effect transistors have been constructed for the characterization of hole mobility using morphologically stable glassy-nematic materials. Particularly relevant to flexible electronics, solvent annealing at room temperature has also been conducted in an attempt to emulate thermal annealing for producing monodomain glassy-nematic films. 45
Chart 2.1. Molecular Structures of OF2T-1 through -8 and PF2T
2.2 EXPERIMENTAL
Materials Synthesis
The fluorene-co-bithophene conjugated oligomers OF2T-1 though -8 depicted in Chart 2.1 were synthesized and purified according to Scheme 2.1 following the procedures described below. All chemicals, reagents, and solvents were used as received from commercial sources without further purification except for tetrahydrofuran (THF) and toluene, which had been distilled over sodium/benzophenone before use. All reactions were carried out under argon atmosphere and anhydrous conditions unless noted otherwise. Intermediates 2-1,
2-5a-e, and 2-12 were synthesized according to the procedures reported 46
previously.[31-33] PF2T was acquired from American Dye Source (Quebec,
− Canada) with a number-average molecular weight of 36,200 g mol 1 with a
1 polydispersity index of 3.1. H NMR spectra were acquired in CDCl3 with an
Avance-400 spectrometer (400 MHz). Elemental analysis was carried out by
Quantitative Technologies, Inc. Molecular weights were measured with a
TofSpec2E MALD/I TOF mass spectrometer (Micromass, Inc., Manchester,
U.K.) with 2-[(2E)-3-(4-tert-butylphenyl)-2- methylpropenylidene]malanoitrile
(DCTB) as the matrix.
o (i) Pd(dppf)Cl2, THF, room temperature. (ii) BuLi, THF, −78 C and then o o SnBu3Cl, −78 C to room temperature; (iii) Pd(PPh3)4, toluene, 90 C. Scheme 2.1. Synthesis of OF2T-1 through -8 as depicted in Chart 2.1 47
5-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophene, 2-3.
A Grignard reagent, 2-2, prepared by reacting 5-bromo-2,2’-bithiophene (4 g, 16.3 mmol) with Mg (0.6 g, 25 mmol) in THF (80 ml), was added dropwise into a solution of 2-bromo-[9,9-bis(n-propyl)]-fluorene (2-1, 4.12 g, 12.5 mmol) in THF
(30 ml) containing Pd(dppf)Cl2 (90 mg, 1.3 mmol) as a catalyst. The reaction mixture was stirred at room temperature overnight, quenched with 0.2 M HCl, and extracted with methylene chloride. The organic extracts were combined, washed with brine and water, and finally dried over MgSO4. Upon evaporating off the solvent, the crude product was purified by column chromatography on silica gel with hexane as the eluent to yield 2-3 (0.75 g, 86%) as yellow crystals. 1H NMR
(400 MHz, CDCl3, 298 K): δ (ppm) 7.70-7.68 (d, J=4.4 Hz, 2H), 7.59-7.55 (m,
2H), 7.36-7.29 (m, 4H), 7.23-7.22 (m, 2H), 7.18-7.17 (d, J=3.6 Hz, 1H),
7.06--7.03 (m, 1H), 2.00-1.96 (m, 4H), 0.73-0.67 (m, 10H).
5-[9,9-bis(n-propyl)fluoren-2-yl]-5’-tributylstanyl-2,2’-bithiophene, 2-4.
Into a stirred solution of 2-3 (1.24 g, 3 mmol) in THF (30 ml) was added n-BuLi
(2.5 M in hexane, 1.6 ml, 4.0 mmol) dropwise at –78oC, where the reaction mixture was stirred for 3 h before adding tributylstannyl chloride (1.3 g, 4 mmol) in one portion. The reaction mixture was allowed to warm up to room temperature overnight before pouring into water for extraction with diethylether. The organic 48
extracts were combined, washed with brine and water, and dried over MgSO4.
Upon evaporating off the solvent, 2-4 was obtained in a quantitative yield. 1H
NMR (400 MHz, CDCl3, 298 K): δ (ppm) 7.69-7.67 (m, 2H), 7.59-7.55 (m, 2H),
7.34-7.28 (m, 6H), 7.18-7.16 (d, J=4.0 Hz, 1H), 7.10-7.08 (d, J=3.2 Hz, 1H),
1.98-1.95 (m, 4H), 1.65-0.93 (m, 27H), 0.77-0.69 (m, 10H).
5-[9,9-bis(n-propyl)fluoren-2-yl]-5’-[7-bromo-9,9-bis(2-ethylhethyl)fluoren-2-yl]-
2,2’-bithiophene, 2-6a.
Toluene (50 mL) was added to a mixture of 2-4 (3.52 g, 5 mmol),
2,7-dibromo-9,9-bis(2-ethylhethyl)-fluorene (2-5a) (6.85 g, 12.5 mmol) and
o Pd(PPh3)4 (60 mg, 0.05 mmol). The reaction mixture was stirred at 90 C overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride at
20:1 to 9:1 (v/v) as the eluent to yield 2-6a (2.3 g, 52%) as a yellow solid. 1H
NMR (400 MHz, CDCl3, 298 K): δ (ppm) 7.71-7.46 (m, 10H), 7.35-7.30 (m, 5H),
7.23-7.22 (d, J=3.6 Hz, 2H), 2.13-2.00 (m, 8H), 0.98-0.72 (m, 32H), 0.63-0.58 (m,
8H).
5-[9,9-bis(n-propyl)fluoren-2-yl]-5’-[7-bromo-9,9-bis(2-methylbutyl)fluoren-2-yl]
-2,2’-bithiophene, 2-6b. 49
The procedure for the synthesis of 2-6a was followed to prepare 2-6b from 2-4 and 2,7-dibromo-9,9-bis (2-methylbutyl)fluorene (2-5b) as a yellow solid in a
1 57% yield. H NMR (400 MHz, CDCl3, 298K): δ (ppm) 7.71-7.46 (m, 10H),
7.35-7.30 (m, 5H), 7.23-7.22 (d, J=3.6 Hz, 2H), 2.25-1.86 (m, 8H), 1.05-0.62 (m,
22H), 0.45-0.30(m, 6H).
2-[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-7-(2,2’-bithiophen-5-y l)-9,9-bis(2-methylbutyl)fluorene, 2-7.
The procedure for the synthesis of 2-3 was followed to prepare 2-7 from 2-6b and
1 2-2 as a yellow solid in a 75% yield. H NMR (400 MHz, CDCl3, 298 K): δ (ppm)
7.71-7.57 (m, 10H), 7.37-7.27 (m, 6H), 7.24-7.22 (m, 4H), 7.19-7.18 (d, J=4.0 Hz,
1H), 7.06-7.03 (m, 1H), 2.20-1.86 (m, 8H), 1.05-0.62 (m, 22H), 0.40-0.30(m, 6H)
2-[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-7-(5’-tributylstanyl-2,
2’-bithiophen-5-yl)-9,9-bis(2-methylbutyl)fluorene, 2-8.
The procedure for the synthesis of 2-4 was followed to prepare 2-8 from 2-7 in a
1 quantitative yield. H NMR (400 MHz, CDCl3, 298K): δ (ppm) 7.71-7.57 (m,
10H), 7.37-7.27 (m, 7H), 7.23-7.22 (m, 2H), 7.18-7.17 (d, J=3.6 Hz, 1H),
7.10-7.09 (d, J=3.2 Hz, 1H), 2.25-1.86 (m, 8H), 1.65-0.60 (m, 49H), 0.45-0.30(m,
6H)
50
2-[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-7-(5’-[7-bromo-9,9-bi s(2-ethyl-hexyl)fluoren-2-yl]-2,2’-bithiophen-5-yl)-9,9-bis(2-methylbutyl)fluorene,
2-9.
The procedure for the synthesis of 2-6a was followed to prepare 2-9 from 2-8 and
1 2-5a as a yellow solid in a 51% yield. H NMR (400 MHz, CDCl3, 298 K): δ
(ppm) 7.74-7.44 (m, 16H), 7.40-7.30 (m, 7H), 7.23-7.21 (m, 4H), 2.25-1.86 (m,
12H), 1.05-0.62 (m, 54H), 0.45-0.30(m, 6H)
5,5’-Bis[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophene, OF2T-1.
A Grignard reagent, 2-10, prepared by the reaction of
2-bromo-[9,9-bis(n-propyl)]-fluorene (0.99 g, 3 mmol) with Mg (0.08 g, 3.3 mmol) in THF (10 mL), was added dropwise into a stirred solution of
5,5’-dibromo-2,2’-bithiophene (2-11, 0.40 g, 1.2 mmol) in THF (20 ml) containing Pd(dppf)Cl2 (20 mg, 0.03 mmol) as a catalyst. The reaction mixture was stirred at room temperature overnight, quenched with 0.2 M HCl, and extracted with methylene chloride. The organic extracts were combined, washed with brine and water, and dried over MgSO4. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride at 10:0 to 9:1 (v/v) as the eluent to yield OF2T-1
1 (0.75 g, 86%) as a yellow powder. H NMR (400 MHz, CDCl3, 298 K): δ (ppm)
7.71-7.69 (m, 4H), 7.61-7.57 (m, 4H), 7.37-7.30 (m, 8H), 7.23-7.22 (d, J=3.6 Hz, 51
2H), 2.01-1.98 (m, 8H), 0.73-0.69 (m, 20H). MALD/I TOF MS (DCTB) m/z
+ ([M] ): 662.3. Calcd. for C46H46S2: C, 83.34; H, 6.99. Found: C, 83.12; H, 6.92.
2,7-Bis[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-9,9-bis(n-propyl) fluorene, OF2T-2.
Toluene (20 mL) was added into a mixture of 2-4 (2.47 g, 3.5 mmol),
2,7-dibromo-9,9-bis(n-propyl)-fluorene (2-5c) (0.49 g, 1.2 mmol) and Pd(PPh3)4
(90 mg, 0.075 mmol). The reaction mixture was stirred at 90oC overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride at 4:1 to 2:1 (v/v) as the eluent to give OF2T-2 (0.76 g, 58%) as a yellow powder. 1H NMR (400 MHz,
CDCl3, 298 K): δ (ppm) 7.71-7.69 (m, 6H), 7.63-7.57 (m, 8H), 7.36-7.31 (m,
10H), 7.24-7.23 (d, J=3.6 Hz, 4H), 2.04-1.98 (m, 12H), 0.73-0.69 (m, 30H).
+ MALD/I TOF MS (DCTB) m/z ([M] ): 1074.5. Calcd for C73H70S4: C, 81.52; H,
6.56. Found: C, 81.24; H, 6.41.
2,7-Bis[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-9,9-bis(n-octyl)fl uorene, OF2T-3.
The procedure for the synthesis of OF2T-2 was followed to prepare OF2T-3 from
2-4 and 2,7-dibromo-9,9-bis(n-octyl)fluorene (2-5d) as a yellow powder in a 69%
1 yield. H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 7.71-7.69 (m, 6H), 7.63-7.57 52
(m, 8H), 7.36-7.31 (m, 10H), 7.24-7.23 (d, J=3.6 Hz, 4H), 2.02-1.98 (m, 12H),
1.18-1.09 (m, 20H), 0.85-0.78 (m, 30H). MALD/I TOF MS (dithranol) m/z
+ ([M] ): 1214.4. Calcd for C83H90S4: C, 81.99; H, 7.46. Found: C: 81.65; H, 7.26
2,7-Bis[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-9,9-bis(2-ethyl-h exyl)fluorene, OF2T-4.
The procedure for the synthesis of OF2T-2 was followed to prepare OF2T-4 from
1 2-4 and 2-5a as a yellow powder in a 72% yield. H NMR (400 MHz, CDCl3, 298
K): δ (ppm) 7.71-7.69 (m, 6H), 7.62-7.58 (m, 8H), 7.36-7.29 (m, 10H), 7.23-7.22
(d, J=4.0 Hz, 4H), 2.08-1.98 (m, 12H), 0.93-0.55 (m, 50H). MALD/I TOF MS
+ (DCTB) m/z ([M] ): 1214.7. Calcd for C83H90S4: C, 81.99; H, 7.46. Found: C,
82.08; H,7.28
2,7-Bis[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-9,9-bis(2-methyl- butyl)fluorene, OF2T-5.
The procedure for the synthesis of OF2T-2 was followed to prepare OF2T-5 from
1 2-4 and 2-5b as a yellow powder in a 59% yield. H NMR (400 MHz, CDCl3, 298
K): δ (ppm) 7.72-7.69 (m, 6H), 7.63-7.57 (m, 8H), 7.36-7.31 (m, 10H), 7.24-7.23
(d, J=3.6 Hz, 4H), 2.21-2.18 (m, 2H), 2.02-1.98 (m, 10H), 0.93-0.80 (m, 4H),
0.78-0.61 (m, 28H), 0.35-0.30 (m, 6H). MALD/I TOF MS (DCTB) m/z ([M]+):
1130.4. Calcd for C77H78S4: C, 81.72; H, 6.95. Found: C, 81.30; H, 6.78. 53
2,7-Bis[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl]-9,9-bis(2-methyl- propyl)fluorene, OF2T-6.
The procedure for the synthesis of OF2T-2 was followed to prepare OF2T-6 from
2-4 and 2,7-dibromo-9,9-bis(2-methylpropyl)-fluorene (2-5e) as a yellow powder
1 in a 80% yield. H NMR (400 MHz, CDCl3, 298 K): δ (ppm) 7.73-7.69 (m, 6H),
7.64-7.57 (m, 8H), 7.36-7.31 (m, 10H), 7.24-7.23 (d, J=4.0 Hz, 4H), 2.07-1.98 (m,
12H), 1.03-0.95 (m, 2H), 0.73-0.69 (m, 20H), 0.46-0.45 (d, J=6.8 Hz, 12H).
+ MALD/I TOF MS (DCTB) m/z ([M] ): 1102.4. Calcd for C75H74S4: C 81.62 H
6.76. Found: C, 81.49; H, 6.56
5,5’-Bis[7-[5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-2-yl]-9’,9’-bis(2-et hyl-hexyl)fluoren-2-yl]-2,2’-bithiophene, OF2T-7.
Toluene (20 mL) was added into a mixture of 2-6a (1.94 g, 2.2 mmol),
5,5’-tributylstanyl-2,2’-bithiophene (2-12) (0.56 g, 0.8 mmol) and Pd(PPh3)4 (70 mg, 0.060 mmol). The reaction mixture was stirred at 90oC overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride at 4:1 to 2:1 (v/v) as the eluent to yield OF2T-7 (0.95 g, 67%) as a yellow powder. 1H NMR (400
MHz, CDCl3, 298 K): δ (ppm) 7.71-7.69 (m, 8H), 7.62-7.58 (m, 12H), 7.37-7.30
(m, 12H), 7.23-7.22 (d, J=3.6 Hz, 6H), 2.08-2.00 (m, 16H), 0.94-0.53 (m, 80H). 54
+ MALD/I TOF MS (DCTB) m/z ([M] ): 1767.9. Calcd for C120H134S6: C, 81.49; H
7.64. Found: C,81.21; H, 7.64.
5,5-Bis[7-(5’-[7-(5’-[9,9-bis(n-propyl)fluoren-2-yl]-2,2’-bithiophen-5-yl)-9,9-bis(
2-methy-lbutyl)fluoren-2-yl]-2,2’-bithiophen-5-yl)-9,9-bis(2-ethylhexyl)fluoren-2- yl]-2,2’-bithiophene, OF2T-8.
The procedure for the synthesis of OF2T-7 was followed to synthesize OF2T-8 from 2-9 and 2-12 as a yellow powder in a 63% yield. 1H NMR (400 MHz,
CDCl3, 298K): δ (ppm) 7.72-7.58 (m, 32H), 7.38-7.29 (m, 16H), 7.24-7.23 (m,
10H), 2.30-1.96 (m, 24H), 1.05-0.57 (m, 104H), 0.45-0.35 (m, 12H). MALD/I
+ TOF MS (DCTB) m/z ([M] ): 2703.3. Calcd for C182H198S10: C, 80.78; H 7.37.
Found: C, 80.46; H, 7.27
Phase Transition Temperatures
Thermal transition temperatures were determined by differential scanning
−1 calorimetry (Perkin-Elmer DSC-7) with a continuous N2 purge at 20mL min .
Samples were preheated to above Tc or Tm (whichever is higher) and then cooled
− down −30oC at −100 oC min 1 before the reported second heating and cooling
− scans were recorded at 20 oC min 1. The nature of phase transition was characterized by hot-stage polarizing optical microscopy (DMLM, Leica, FP90 central processor and FP82 hotstage, Mettler Toledo). 55
Absorption and Fluorescence Spectra in Dilute Solution
Dilute solutions of OF2Ts in toluene were prepared at a concentration of
− − 10 7 to 10 6 M. Absorption spectra were acquired with an HP 8453E UV-vis-NIR diode array spectrophotometer. Fluorescence spectra were collected with a spectrofluorimeter (Quanta Master C60SE, Photon Technology International) at an excitation wavelength of 430 nm. The fluorescence quantum yield of OF2Ts in diluted solution was measured following the procedure reported previously.[33]
Electrochemical Characterization
Cyclic voltammetry was conducted on an EC-Epsilon potentiostat
− (Bioanalytical Systems Inc.) at a concentration of 10 4 M in anhydrous dichloromethane containing 0.1 M tetraethylammonium tetrafluoroborate as the supporting electrolyte. A silver/silver chloride wire (2-mm diameter), a platinum wire (0.5-mm diameter), and a platinum disk (1.6-mm diameter) were used as the reference, counter, and working electrodes, respectively, to complete a standard
3-electrode cell. Prior to use, the supporting electrolyte was dissolved in a minimum amount of ethanol for treatment with activated charcoal followed by filtration through Celite powder and addition of water (twice the volume of the ethanol) for recrystallization at 0oC. Recrystallization was repeated until the oxidation and reduction scans attributable solely to the electrolyte were observed. 56
The difference between the oxidation potential of OF2Ts and that of ferrocene, both against Ag/AgCl, were used as the oxidation potential of OF2Ts over Fc/Fc+
(Fc=ferrocene).[44] HOMO levels of the OF2Ts were calculated via HOMO = −
(1.4 ± 0.1) × qVCV − (4.6 ± 0.08) eV, where q is the electron charge, VCV is the oxidation potential of an OF2T over Fc/Fc+.[45]
Preparation and Characterization of Neat Films
Optically flat fused silica substrates (25.4-mm diameter x 3-mm thickness, transparent down to 200 nm, Esco Products) were coated with a thin film of polyimide (Nissan SUNEVER grade 610 polyimide varnish) following by soft-baking at 80oC for 10 min and hard-curing at 250oC for 1 h, and uniaxial rubbing for use as an alignment layer. Films of OF2Ts were prepared by spin coating from a dilute solution onto fused silica substrates containing alignment coatings at 3000 rpm for 65 s followed by drying under vacuum overnight. The solution concentration was adjusted to control the film thickness to within 50 to
70 nm. Thermal annealing was performed under argon atmosphere at a temperature 10oC above the glass transition temperature for 5 min. Quasi-solvent annealing was performed by spin coating from a 0.8 wt% solution in chlorobenzene at 3000 rpm for 65 s followed by drying under vacuum for up to
48 h. Solvent-vapor annealing was executed by exposing spin-cast films from chlorobenzene to saturated chlorobenzene vapor in a closed jar for a varying 57
period of time before drying under vacuum for up to 48 h, all conducted at room temperature. Absorption dichroism was characterized by a UV-Vis-NIR spectrophotometer (Perkin-Elmer Lambda 900) equipped with linear polarizers
(HNP’B, Polaroid) for the calculation of orientational order parameter. Film thickness was measured by optical profilometry (Zygo NewView 5000).
Fabrication and Characterization of Field-Effect Transistors
The 40-nm-thick gold source/drain electrodes were deposited by evaporation through a shadow mask onto a rubbed polyimide layer as a part of the top-gate field-effect transistor with channel lengths ranging from 75 to 120 μm. The OF2T films were spin cast from chloroform, vacuum dried, and thermally annealed as described above. A 500 to 800 nm thick, poly(chloro-p-xylylene) film was prepared on top of the OF2T film by chemical vapor deposition to serve as the gate insulator.[46] Device fabrication was completed by evaporation of a 30- to
40-nm-thick gold gate electrode through a shadow mask, resulting in a channel width of 5 mm. The thickness of the evaporated gate insulator was measured with a profilometer (Sloan Dektak 3). The current-voltage characteristics of the resultant transistors were characterized with an Agilent 4156C precision semiconductor parameter analyzer in ambient atmosphere. Hole mobilities were calculated in the saturation regime at a drain voltage of −100 V using the standard
MOSFET equations.[47] 58
K I G + K
OF2T-1 G + K K I G N I K K I 1 2 OF2T-2 G N I G N I K I OF2T-3 G I N N I G OF2T-4 G N I G N I OF2T-5 G N I G N I K I OF2T-6 G N I G N I OF2T-7 G N I I G N OF2T-8 G Endothermic N I
N I K G+K PF2T
G+K I K N
0 100 200 300 400 Temperature, oC
− Figure 2.1. DSC thermograms at ±20 oC min 1 for samples of OF2Ts and PF2T
o preheated to above Tc or Tm (whichever is higher) and then quenched to −30 C at − −100 oC min 1 before recording the reported second heating and cooling scans. Vertical arrows on the thermograms of OF2T-3 locate transition temperatures identified by hot-stage polarizing optical microscopy. Symbols: G, glassy; K, crystalline; N, nematic; I, isotropic.
59
2.3 RESULTS AND DISCUSSION
Depicted in Chart 2.1 are the molecular structures of oligo(fluorene-co-bithiophene)s, identified as OF2T-1 through -8, synthesized for the present study. For a comparison with OF2Ts, commercially available PF2T is included in Chart 2.1 accompanied by its number- and weight-average molecular weights, M n and M w. As compiled in Figure 2.1, the differential scanning calorimetric (DSC) thermograms of samples preheated to above whichever is higher – the crystalline melting point, Tm, or nematic-to-isotropic transition temperature, Tc – were used in conjunction with hot-stage polarizing optical microscopy for the determination of their thermotropic properties. A compound is classified as glassy-nematic if it exhibits a glass transition temperature, Tg, followed by a Tc on heating, and a Tc followed by a Tg on cooling without experiencing crystalline melting or crystallization.
Two structural parameters, the oligomer length and aliphatic pendants, affect the ability of an OF2T to form a morphologically stable glassy-nematic film.
With four n-propyl pendants at the C-9 positions of the two fluorene units,
OF2T-1 is an amorphous solid but prone to crystallization. Oligomers OF2T-2 through -6 incorporates an additional fluorene-bithiophene unit to OF2T-1. As revealed by the DSC thermograms, replacing linear aliphatic pendants to the C-9 position of the central fluorene unit with branched counterparts managed to 60
improve morphological stability against crystallization, as glassy-nematic
OF2T-4 and -5 are compared to OF2T-2 and -3. Note the lower Tg of OF2T-4
o o than that of OF2T-5, 97 versus 123 C, with about the same Tc at 164 C thanks to the bulkier 2-ethylhexyl than the 2-methylbutyl pendants. Relatively compact
2-methylpropyl pendants, however, were ineffective in suppressing crystallization as evidenced by the DSC heating scan of OF2T-6, viz. thermally induced crystallization from isotropic liquid at 198oC followed by crystalline melting at
245oC. With an addition of one and two fluorene-bithiophene units to OF2T-5,
o OF2T-7 and -8 are both glassy-nematic systems with Tg at 102 and 133 C and Tc at 240 and 324oC, giving rise to a wide nematic fluid temperature range of 138 and 191oC, respectively. The long-term morphological stability of glassy-nematic films of OF2T-7, and -8 were further ascertained by the absence of crystallization in spin-cast films left at room temperature for over a year. In sharp contrast to
OF2T-7 and -8, PF2T exhibited complex thermal transition behaviors as shown in Figure 2.1, including crystallization on heating and cooling under the same thermal treatment.
The UV-Vis absorption and fluorescence spectra were gathered at 10–7 to
10–6 M in toluene. The results presented in Figure 2.2 show a red shift in both sets of spectra at an increasing backbone length. Whereas the molar extinction coefficient, ε , increases by about 105 M–1cm–1 with each increment of fluorene-bithiophene unit, a slight increase in fluorescence quantum yield with 61
oligomer length is observed.[48] The oxidation potentials were measured for
OF2T-4, -5, -7, and -8 in anhydrous methylene chloride using the Ag/AgCl reference electrode and adjusted to ferrocene, which has an oxidation potential of
0.46 V relative to Ag/AgCl, for the calculation of HOMO levels using Eq.(4) in
Ref.[45], resulting in –5.3 ±0.2 eV for all four oligomers. This procedure has been demonstrated to yield HOMO levels equivalent to direct measurement by ultraviolet photoemission spectroscopy.[49] The data presented in Table 2.1 indicate the independence of HOMO level on the number of repeat units up to 52 in PF2T. Having a HOMO level at –5.3±0.2 eV, OF2T-4 through -8 and PF2T represent an optimization between PF and poly(3-hexylthiophene) with respect to oxidative stability and ease of hole injection from gold.
4 Φ (a) (b) PL
-1 3 OF2T-1 OF2T-1 0.36
cm OF2T-4 OF2T-4 0.47 -1 OF2T-7 M 2 OF2T-7 0.48 5 OF2T-8 OF2T-8 0.53 , 1010 , ε
1 PL Normalized
0 300 400 500 600 400 450 500 550 600 650 700 λ, nm λ, nm
Figure 2.2. (a) UV-vis absorption spectra in molecular extinction coefficient, ε; and (b) fluorescence (with excitation at 430 nm) spectra of OF2T-1, OF2T-4, − − OF2T-7 and OF2T-8 in toluene at 10 7 to 10 6 M.
62
4 4 (a) 2 2 (b) A A A 0 μ μ 0 μ −2 −2 −4
Current, Current, OF2T-4 Current, Current, OFTh-5 −4 −6 −6 −8 00.40.81.200.40.81.2 Potential, V vs Ag/AgCl Potential, V vs Ag/AgCl 4 4 (c) (d) 2 2 A A μ 0 μ 0
−2 −2 Current, Current, OF2T-7 Current, OF2T-8 −4 −4
−6 −6 00.40.81.200.40.81.2 Potential, V vs Ag/AgCl Potential, V vs Ag/AgCl Figure 2.3. Cyclic voltammetric scans of (a) OF2T-4, (b) OF2T-5, (c) OF2T-7
−4 and (d) OF2T-8 in anhydrous CH2Cl2 at 2.5 ×10 M with 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte.
Thin films of OF2Ts were spin-cast from chloroform onto fused silica substrates containing rubbed polyimide alignment layers and then thermally
o o annealed at 10 C above their Tgs, viz. from 107 to 143 C, for 5 min followed by cooling to room temperature. The resultant glassy-nematic films were monodomain across 2-cm diameter, i.e. devoid of the Schlieren textures characteristic of nematic mesomorphism, as Figure 2.4b is compared to 2.4a. 63
(a) (b)
100 μm 100 μm
Figure 2.4. Polarizing optical micrographs of OF2T-8 films spin-cast from chloroform on a rubbed polyimide alignment layer observed at 45 degrees with respect to rubbing direction: (a) pristine, and (b) after thermal annealing at 140oC for 5 min.
Linear absorption dichroism was characterized to evaluate the orientational order parameter, Sab = (R–1)/(R+2), in which R denotes the absorbance parallel over that perpendicular to the nematic director defined by the rubbing direction. As reported in Table 2.1, the Sab value increases from 0.76 to 0.84 with an increasing oligomer length. In contrast, the PF2T film exhibited an Sab value of 0.70 and
0.60 after thermal annealing at 275-285 oC for 3-15 min on a rubbed polyimide layer[21] and at 280oC for 5 min on a photoalignment layer,[25] respectively. For the characterization of anisotropic hole mobilities, top-gate field-effect transistors as depicted in the Figure 2.5a were fabricated, in which monodomain glassy-nematic
OF2T films were prepared by thermal annealing as described above with poly(chloro-p-xylylene) serving as the dielectric layer.
64
(a) Gold (30-40 nm) Dielectric Layer (500-800 nm) Organic Semiconductor S (50-70 nm) D Rubbed Polyimide (15 nm) Fused Silica (3 mm)
−2.5 A 6 −2.0 −100 − (b) V Voltage, Gate − 1.5 −80 −1.0 −60 − 0.5 −40 Drain Current, 10 Current, Drain − −20 0.00 −20 −40 −60 −80 −100 Drain Voltage, V
−5 −10 0.002 Drain Current, ( −10−6 (c)
−10−7
−8 −
−10 A) Drain Current, A Current, Drain 1/2
−9 −10 0 0 −20 −40 −60 −80 −100 Gate Voltage, V Figure 2.5. (a) Schematic diagram of the organic field-effect transistors constructed for this study, where S and D denote source and drain, respectively; output (b) and transfer (c) curves for the OFETs comprising OF2T-8 with chain alignment parallel to current flow.
Based on the source-drain current versus gate voltage relationships included in
Figure 2.5b and c, respectively, hole mobilities were calculated in the saturation regime at a drain voltage of –100 V. As noted in Table 2.1, hole mobilities 65
parallel (μ||) and perpendicular (μ⊥) to the nematic director in the OF2T series generally increase with oligomer length but fall short of those of PF2T on a rubbed polyimide alignment layer,[21] albeit comparable to PF2T on a photoalignment layer.[25] Nonetheless, one should be cautious about comparing field-effect mobility data, as the results are known to be affected by device structure,[6, 7, 47] alignment method,[21, 22, 25] and the material used as the dielectric layer.[8, 9]
Table 2.1. Highest cccupied molecular orbital, HOMO, energy levels, orientational order parameters, Sab, and anisotropic field-effect hole mobilities, μ|| and μ⊥, for OF2Ts and PF2T
HOMO [a] 103 μ [b] 103 μ [b] Compound S [b] || ⊥ [eV] ab [cm2 V−1 s−1] [cm2 V−1 s−1] OF2T-4 -5.4 0.78 0.56 ±0.21 0.089 ±0.034 OF2T-5 -5.4 0.76 0.43 ±0.05 0.12 ±0.02 OF2T-7 -5.3 0.82 1.8 ±0.8 0.32 ±0.14 OF2T-8 -5.3 0.84 2.0 ±0.9 0.75 ±0.43 PF2T -5.3 0.70 [c]; 0.78 [d] 15 ± 5[c]; 2 [d] 1.5 ± 0.5 [c]; 0.2 [d]
[a] The reported HOMO levels are accompanied by an uncertainty of ±0.2 eV.
[b] Orientational order parameter, Sab, measured to ±0.02 by polarized UV-vis absorption, and anisotropic hole mobilities, μ|| and μ⊥, measured for field-effect transistors, all comprising monodomain glassy-nematic films of OF2Ts spin-cast from chloroform on rubbed polyimide alignment coating and thermally annealed at 10oC above their respective Tgs for 5 min followed by cooling to room temperature. [c] Data taken from Ref.[21]. The PF2T film was thermally annealed at 275-285oC for 3-15 min on rubbed polyimide film before cooling to room temperature, and poly(vinylphenol) served as the dielectric layer in a top-gate device. [d] Data taken from Ref.[25]. The PF2T film was thermally annealed at 280oC for 5 min on a photoalignment layer before cooling to room temperature, and the underlying SiO2 served as the dielectric layer in a top-contact-bottom-gate device. 66
To take advantage of the relative ease of molecular alignment, monodisperse conjugated oligomers have been actively explored as organic semiconductors capable of anisotropic charge transport. With singly defined molecular structures, monodisperse conjugated oligomers are invaluable to elucidating structure-property relationships. From a practical perspective, these compounds are readily soluble to facilitate purification and solution processing. In addition to thermal annealing, quasi-solvent annealing was tested in this study via spin coating from a high-boiling solvent on a rubbed polyimide alignment layer followed by vacuum drying, both performed at room temperature. The feasibility was demonstrated with OF2T-8 spin-cast from chlorobenzene, resulting in Sab =
0.81, a value very close to that from thermal annealing of a film spin-cast from chloroform or cholobenzene, Sab = 0.84; see Figures 2.6a and b. The success in quasi-solvent annealing of OF2T-8 at room temperature originated in the oligomer’s ability to undergo uniaxial orientation in a relatively wet environment thanks to the slow evaporation of chlorobenzene during spin coating. The attempt at quasi-solvent annealing of OF2T-8 using low-boiling chloroform yielded an inferior Sab at 0.33 as expected (see Figure 2.6c). Presumably because of chain entanglements or defects as a result of its relatively high molecular weight, PF2T was irresponsive to quasi-solvent annealing using chlorobenzene as shown in
Figure 2.6d. 67
0.8 1.5 OF2T-8 OF2T-8 (a) A (b) 42 nm 68 nm || 0.6 A Sab = 0.81 || 1.0 Sab = 0.84 0.4 0.5 Absorbance Absorbance 0.2 A⊥ A⊥ 0.0 0.0 300 400 500 600 300 400 500 600 λ, nm λ, nm
1.5 0.8 OF2T-8 PF2T (c) (d) 68 nm 0.6 55 nm S = 0 A|| 1.0 Sab = 0.33 ab A|| 0.4 A⊥ Absorbance Absorbance 0.5 A ⊥ 0.2
0.0 0.0 300 400 500 600 300 400 500 600 λ, nm λ, nm Figure 2.6. Polarized absorption spectra of films spin-cast from 0.8 wt% solutions at 3000 rpm for 65 s on rubbed polyimide alignment layers: (a) OF2T-8 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (b) OF2T-8 film from chloroform or chlorobenzene, thermally annealed at 140oC for 5 min, and then cooled to room temperature; (c) OF2T-8 film from chloroform and then vacuum-dried at room temperature for 12 h; and (d) PF2T film from chlorobenzene, and then vacuum-dried at room temperature for up to 48 h. The reported Sab values are accompanied by an experimental error of ±0.02.
Symbols A|| and A⊥ represent absorbance parallel and perpendicular to rubbing direction, respectively.
Quasi-solvent annealing via spin coating of OF2Ts in chlorobenzene at 0.8 wt% and 3000 rpm yielded a decreasing Sab value with a decreasing solute length:
0.81, 0.77, and 0.52 for OF2T-8, -7, and -4, respectively, cf. Figures 2.6a, 2.7a 68
and b. Note the increasing departure of Sab values at a decreasing oligomer length from those achieved with thermal annealing. Subsequent thermal annealing at
o 10 C above Tgs for 5 min improved the Sab values to 0.85, 0.80, and 0.73 for
OF2T-8, -7 and -4, respectively, equal to or slightly inferior to the results from thermal annealing. At the same mass concentration in chlorobenzene, the solution viscosity is expected to increase with an increasing oligomer length. Moreover, the depression in solvent vapor pressure decreases with the molar concentration of an involatile solute, namely, a high solvent evaporation rate is expected of a long solute at the same mass concentration. The difference in evaporation rate, however, will diminish as spin coating proceeds. In this process, the difference in solution viscosity continues to grow beyond the initial value in favor of the long solute. Therefore, the longer the solute, the thicker the liquid film with a higher solute mass concentration, and hence the thicker the dry oligomer film, as borne out by the experimental data: 55, 42, 31, and 27 nm for PF2T, OF2T-8, -7, and -4, respectively. The importance of solvent evaporation to dry film thickness is also evident by comparing chloroform with chlorobenzene as the solvents for spin coating. At the same OF2T-8 mass concentration and spin rate as stated above, the thicker dry film from chloroform than chlorobenzene, 68 versus 42 nm, can be attributed to the higher solution viscosity during spin coating as a result of the higher evaporation rate of chloroform. In fact, the solvent evaporation rate seems 69
to have predominated over the fact that cholrobenzene has a higher viscosity than chloroform, i.e. 0.80 over 0.54 cp at 20oC.[50]
0.8 0.8 OF2T-7 OF2T-4 (a) (b) 31 nm 27 nm 0.6 0.6 Sab = 0.77 Sab = 0.52 A || A 0.4 0.4 || Absorbance Absorbance 0.2 0.2 A⊥ A⊥ 0.0 0.0 300 400 500 600 300 400 500 600 λ, nm λ, nm 0.8 0.8 OF2T-7 (c) OF2T-4 (d) 0.6 31 nm 0.6 27 nm Sab = 0.82 Sab = 0.77 A|| A|| 0.4 0.4 Absorbance Absorbance 0.2 0.2 A⊥ A⊥ 0.0 0.0 300 400 500 600 300 400 500 600 λ, nm λ, nm Figure 2.7. Polarized absorption spectra of films spin-cast from 0.8 wt% solutions at 3000 rpm for 65 s on rubbed polyimide alignment layers: (a) OF2T-7 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (b) OF2T-4 film from chlorobenzene and then vacuum-dried at room temperature for up to 48 h; (c) OF2T-7 film from chlorobenzene, followed by exposure to saturated chlorobenzene vapor for 30 s, and then vacuum-dried for up to 48 h, all at room temperature; (d) OF2T-4 film from chlorobenzene, followed by exposure to saturated chlorobenzene vapor for 30 s, and then vacuum-dried for up to 48 h, all at room temperature. The reported Sab values are accompanied by an experimental error of ±0.02. Symbols A|| and A⊥ are as defined in Figure 2.6.
70
In addition to quasi-solvent annealing, solvent-vapor annealing was conducted by exposing films spin-cast from chlorobenzene to saturated chlorobenzene vapor at room temperature for 30 s followed by vacuum drying also at room temperature up to 48 h. The dichroic absorption spectra for resultant
OF2T-7 and -4 films were depicted in Figure 2.7c and d, respectively. As shown in Table 2.2, solvent vapor annealing achieves the Sab values identical with those
o attained with thermal annealing at 10 C above their Tgs for 5 min. In comparison to solvent-vapor or thermal annealing, quasi-solvent annealing amounted to kinetically trapping the orientational order of OF2Ts in view of the generally lower Sab values. The solvent-vapor annealing process can be described as permeation of an organic vapor into a spin-cast OF2T film to plasticize liquid crystal molecules for spontaneous self-organization motivated by the rubbed polyimide layer. As encountered in quasi-solvent annealing, a spin-cast PF2T film failed to be oriented by solvent-vapor annealing using chlorobenzene or chloroform from 30 s to 8 h. It should also be noted that both quasi-solvent annealing and solvent-vapor annealing using chlorobenzene followed by drying in vacuo, all at room temperature, produced monodomain glassy-nematic films of
OF2Ts across 2-cm diameter observed under polarizing optical microscopy with the same micrographs as that from thermal annealing shown in Figure 2.4b.
Solvent annealing has been documented for performance enhancement of organic solar cells through modulation of nanoscale crystalline morphology in 71
phase-separated poly(thiophene)s and fullerene derivatives.[51-56] Furthermore, solvent-vapor annealing has been shown to induce crystalline modification for the improvement of field-effect mobility of a solution-processable organic semiconductor.[57] The work reported here is the first demonstration of solvent annealing for the realization of monodomain glassy-nematic films of conjugated oligomers. Work is in progress to incorporate photoalignment on plastic substrates at room temperature prior to solvent annealing of conjugated oligomer films without resorting to high temperature baking of polyimide alignment layers.
Table 2.2. Orientational order parameters, Sab, of monodomain films comprising OF2T-4, -7 and -8 achieved through thermal, quasi-solvent and solvent-vapor annealing
Sab Compound Thermal Quasi-solvent Solvent-vapor OF2T-4 0.78 0.52 0.77 OF2T-7 0.82 0.78 0.82 OF2T-8 0.84 0.81 0.83
2.4 SUMMARY
A series of OF2Ts were synthesized and characterized to evaluate the effects of oligomer length and pendant aliphatic structures on thermotropic properties.
The UV-vis absorption and fluorescence spectra were gathered in toluene, and the
HOMO energy levels were calculated using the oxidation potentials measured by 72
cyclic voltammetry in methylene chloride. Top-gate field-effect transistors comprising uniaxially aligned OF2T films were constructed for the characterization of anisotropic hole mobilities. In addition to the widely practiced thermal annealing to attain monodomain glassy-nematic films, two approaches to solvent annealing were explored at room temperature. Key observations are recapitulated as follows:
(1) For a given oligomer length, branched aliphatic pendants are more effective
than linear pendants in suppressing crystallization. For the relatively short
oligomer consisting of two repeat units end-capped with fluorene, the
2-methylbutyl group has emerged as the optimum branched pendant
compared to 2-ethylhexyl and 2-methylpropyl groups in terms of
glass-transition temperature and resistance to crystallization. With a Tg at
o o 102 and 133 C and a Tc at 240 and 324 C, respectively, oligomers
comprising three and five repeat units also end-capped with fluorene are
capable of forming glassy-nematic films without encountering crystallization
at room temperature for over a year.
(2) At an increasing oligomer length, the light absorption and fluorescence
spectra in toluene undergo a bathochromic shift accompanied by a
substantial enhancement in molar extinction coefficient but a modest
increase in fluorescence quantum yield. The HOMO energy levels are
constant at –5.3±0.2 eV for all OF2Ts and PF2T with 52 number-average 73
repeat units. Field-effect hole mobilities increase with oligomer length but
fall short of those of PF2T on a rubbed polyimide alignment layer, albeit
comparable to PF2T on a photoalignment layer.
(3) Quasi-solvent annealing of OF2Ts and PF2T films via spin coating from
chlorobenzene at the same mass concentration and spin rate produced
kinetically trapped Sab values in dry films with thicknesses that decrease with
a decreasing oligomer length. Solvent-vapor annealing was performed by
exposing spin-cast films to saturated chlorobenzene vapor at room
temperature, yielding Sab values identical to those from thermal annealing.
While amenable to thermal annealing at an elevated temperature, PF2T was
not responsive at all to either approach of solvent annealing. 74
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CHAPTER 3
DEVELOPMENT OF NON-CONJUGATED BIPOLAR
HYBRID HOSTS FOR PHOSPHORESCENT ORGANIC
LIGHT-EMITTING DIODES
3.1 INTRODUCTION
Since the invention of relatively efficient fluorescent organic light- emitting diodes (OLEDs) in 1987, [1] a new generation of flat-panel displays has emerged with a potential for capturing a substantial market share of consumable electronics, such as television sets and computer monitors. While full-color OLED displays require the emission of blue, green and red light, white OLEDs are potentially useful for efficient and inexpensive solid-state lighting and as backlights for liquid crystal displays. [2-4] Compared to molecular materials that can be vacuum- deposited into thin films, solution-processable materials, such as π-conjugated polymers and monodisperse oligomers, offer cost advantage and ease of scale-up to large-area thin films. Fluorescence or phosphorescence is responsible for light emission from organic luminophores. Electrophosphorescence is superior to electrofluorescence in terms of readily accessible internal quantum yield, 100 versus
25 %. Despite the intensive efforts worldwide over the past decade, device efficiency and lifetime have remained critical issues. For the fabrication of an efficient phosphorescent OLED, a triplet emitter is typically doped in a host material with 81
[5-8] sufficiently high triplet energy, ET, to realize blue, green or red emission. A higher ET of the host than the guest ensures exciton transfer from the former to the
latter where light emission occurs. In cases where the triplet emitters serve as charge
traps, exciton formation is expected at the emitter without back-transfer to the host
because of the higher ET of the latter. Compared to exciton transfer from the host,
charge trapping on the emitter as the source of phosphorescence is advantageous in
terms of the higher internal quantum yield, [6, 9, 10] less concentration quenching
because of the lower doping level, [9, 11] and the emission spectrum solely from the
emitter, [11, 12] albeit at the higher driving voltage.[12]
Most of the existing triplet host materials are capable of preferentially
transporting holes or electrons.[7, 13, 14] Charge injection and transport layers are added
between electrodes and the emitting layer as needed to improve efficiency. [5-8, 13, 15]
Nevertheless, charge recombination tends to occur close to the interface with the charge-transport layer for lack of bipolar transport capability in general of the emitting layer. [16, 17] Under a high current density pertaining to practical application,
confinement of excitons to the interfacial region could expedite triplet-triplet
annihilation, resulting in efficiency roll-off. [18-21] Furthermore, a narrow
recombination zone is detrimental to operational stability because only a fraction of
molecules contribute to charge transport, exciton formation, and light emission. [22-24]
To substantially improve device efficiency and lifetime, it is imperative that excitons be evenly distributed through the emitting layer and that the accumulation of charges and excitons at interfaces be prevented. To this effect, it has been demonstrated that 82
mixed hosts can effectively decrease driving voltage while improving device
efficiency sustainable at high current densities. [11, 12, 25-30] A typical phosphorescent
layer is comprised of a host mixed with a charge-transport component at 25 to 50
wt%, to which 1 to 10 wt% of a triplet emitter is doped. The desired bipolar transport
capability entails a high concentration of the charge-transport additive, at which
doping level phase separation is destined to take place over time unless miscibility
has been taken into account in the design of both components, thus adversely
affecting long-term operational stability of OLEDs. Bipolar charge-transport host materials via chemical modification represent a viable approach to circumventing the potential phase-separation problem. A bipolar compound can be constructed by chemically bonding an electron- and a hole-transport moiety with and without a finite
extent of π-conjugation between the two moieties, resulting in conjugated and non-
conjugated bipolar compounds, respectively. For all the conjugated bipolar host materials that have been developed for full-color emission, [31-40] few non-conjugated
bipolar compounds have been reported, [41-42] of which none carry a flexible linkage
consisting of σ-bonds between the two moieties. Conjugated and non-conjugated bipolar hybrid molecules without a flexible linkage tend to be rigid and bulky, thus limiting solubility and the ability to form morphologically stable glassy films. This study aims at a new class of non-conjugated bipolar hybrids comprising aliphatic
linkages between the two charge-transport moieties. Representative compounds have
been synthesized and characterized to assess their potential for use as hosts for triplet
emitters. Furthermore, the ability of bipolar hybrid hosts to modulate charge injection 83
into and transport through the emitting layer has been unraveled through the
fabrication and characterization of PhOLEDs.
N
N
N O N N
O N N N
TRZ-3Cz(MP)2: G 144oC I OXD-2Cz(MP)2: G 138oC I
N N
o o o o TRZ-1Cz(MP)2: G 98 C I TRZ: K 242 C I OXD: K 246 C I Cz(MP)2: G 59 C I
Chart 3.1: Representative non-conjugated bipolar compounds as well as independent electron- and hole-transport moieties with their thermal transition temperatures determined by DSC heating scans shown in Figure 3.2 below. Symbols: G, glassy; K, crystalline; I, isotropic.
3.2 EXPERIMENTAL
Material Synthesis
All chemicals, reagents, solvents, and poly(N-vinylcarbazole) with a weight-
average molecular weight of 1.1 × 106 g/mole were used as received from
commercial sources without further purification except toluene and tetrahydrofuran 84
(THF) that had been distilled over sodium and benzophenone, respectively. All
reactions were carried out under argon and anhydrous conditions unless noted
1 otherwise. H NMR spectra were acquired in CDCl3 with an Avance-400
spectrometer (400 MHz) at 298 K using trimethylsilane (TMS) as an internal
standard. Elemental analysis was carried out by Quantitative Technologies, Inc.
Molecular weights were measured with a TofSpec2E MALD/I TOF mass
spectrometer (Micromass, Inc., Manchester, U.K.) with 2-[(2E)-3-(4-tert-
butylphenyl)-2-methylpropenylidene]malanoitrile (DCTB) as the matrix. The target
compounds depicted in Chart 3.1 were synthesized and purified according to reaction
Scheme 3.1 following the procedures described below.
3,6-Dibromo-9-(2-methylpropyl)carbazole, 3-2.
Into a suspension of 3-1 (10.0 g, 44.8 mmol) and silica gel (230-400 mesh, 100 g) in
methylene chloride (400 ml) was added NBS powder (15.9 g, 89.6 mmol) at room
temperature. The reaction mixture was stirred in the dark for 3 h before silica gel was
removed by filtration. The filtrate was washed with water and dried over MgSO4.
Upon evaporating off the solvent, the crude product was purified by column chromatography with hexane as the eluent to yield 3-2 (16.9 g, 99 %) as white
1 crystals. H NMR (400 MHz, CDCl3): δ (ppm) 8.14-8.13 (d, J=4.0 Hz 2H), 7.55-7.53
(m, 2H), 7.27-7.25 (m, 2H), 4.04-4.02 (d, J=7.6 Hz 2H), 2.32-2.30 (m, 1H), 0.97-0.94
(m, 6H).
85
N NBS, CH2Cl2 Silica gel N Br 3-2 Br N N
3-1 N Br Br Br NBS, CH2Cl2 1) BuLi, THF N 3-2 Silica gel 2) O Pd(PPh ) ,NaCO , O 3 4 2 3 Br O B toluene/H2O N 3-3 O B 3-5 3-4 O
N N
Br p-dibromobenzene 9-BBN Allytributyltin Pd(PPh3)4,K2CO3 THF Pd(PPh3)4,LiCl, THF/H N THF 3-7 N 20 3-6
Cl
N N
N N 1) BuLi, THF O B 3-9 O 2) O Pd(PPh3)4,Na2CO3, toluene/H2O O B O N 3-8 N N N FeCl3,CHCl3 N 3-1 N N N Cz(MP)2 TRZ-1Cz(MP)2 N
Br N
N N N
N N Br 10 Br NN 3-6 9-BBN THF Pd(PPh3)4,Na2CO3, N toluene/H2O N
N TRZ-3Cz(MP)2
N
N N N N N O O N O N N 3-11 3-6 9-BBN Br Br O THF Pd(PPh3)4,Na2CO3, toluene/H2O N N N OXD-2Cz(MP)2
Scheme 3.1: Synthesis scheme of non-conjugated bipolar hybrids, TRZ-1Cz(MP)2,
TRZ-3Cz(MP)2, OXD-2Cz(MP)2, and hole-transporting Cz(MP)2.
86
3-Bromo-9-(2-methylpropyl)carbazole, 3-3.
The procedure for the synthesis of 3-2 was followed with 1 eq. NBS to prepare 3-3
1 from 3-1 as white crystals in an 84% yield. H NMR (400 MHz, CDCl3): δ (ppm)
8.20-8.19 (d, J=1.6 Hz 1H), 8.05-8.03 (d, J=8.0 Hz 1H), 7.53-7.38 (m, 3H), 7.28-7.21
(m, 2H), 4.04-4.02 (m, 2H), 2.32-2.30 (m, 1H), 0.97-0.94 (m, 6H).
2-(9-(2-Methylpropyl)carbazol-3-yl)-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, 3-4.
BuLi (2.5 M in hexane, 6.95 ml, 17.4 mmol) was added dropwise into a solution of 3-
3 (4.2 g, 13.9 mmol) in THF (80 ml) at −78 oC, where the mixture was stirred for 3 h before adding 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane (4.05 g, 21.75 mmol) in one portion. The reaction mixture was allowed to warm up to room temperature over a period of 12 h, quenched with water, and then extracted with ether. The organic extracts were combined, washed with brine and water, and dried over MgSO4. Upon evaporating off the solvent, the crude product was purified by
column chromatography on silica gel with hexane/methylene chloride 3:1 (v/v) as the
1 eluent to yield 3-4 (3.95 g, 81 %) as a white powder. H NMR (400 MHz, CDCl3): δ
(ppm) 8.60 (s, 1H), 8.15-8.10 (d, J=2.0 Hz 1H), 7.90-7.85 (d, J=2.0 Hz 1H), 7.45-
7.35 (m, 3H), 7.25-7.15 (m, 1H), 4.10-4.09 (d, J=4.0 Hz 2H), 2.42-2.30 (m, 1H),
1.39(s, 12H), 0.97-0.95 (d, J=8.0 z 6H).
3-Bromo-6-(9-(2-methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazole, 3-5. 87
Toluene (30 ml) and H2O (10 ml) were added into a mixture of 3-4 (1.2 g, 3.43
mmol), 3-2 (3.27 g, 8.59 mmol), Pd(PPh3)4 (0.22 g, 0.17 mmol), and Na2CO3 (3.43 g,
34 mmol). The reaction mixture was stirred at 90 oC for 12 h, cooled to room
temperature, and extracted with methylene chloride. The organic extracts were
combined, washed with water, and dried over MgSO4. Upon evaporating off the
solvent, the crude product was purified by gradient column chromatography on silica
gel with hexane/methylene chloride 9:1 to 3:1 (v/v) to yield 3-5 (0.62 g, 34%) as a
1 white powder. H NMR (400 MHz, CDCl3): δ (ppm) 8.38-8.34 (m, 2H), 8.29-8.28 (d,
J=1.6 Hz 1H), 8.19-8.17 (d, J=7.6 Hz 1H), 7.85-7.78 (m, 2H), 7.55-7.42 (m, 5H),
7.30-7.23 (m, 2H), 4.15-4.10 (m, 4H), 2.45-2.38 (m, 2H), 1.13-1.10 (m, 12H).
3-Allyl-6-(9-(2-methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazole, 3-6
THF (20 ml) was added into a mixture of 3-5 (0.62 g, 1.18 mmol), allyltributyltin
(0.78 g, 2.36 mmol), Pd(PPh3)4 (0.068 g, 0.06 mmol), and LiCl (0.092 g, 3.54 mmol).
The reaction mixture was stirred at 90 oC for 24 h. After evaporating off the solvent,
the crude product was purified by gradient column chromatography on silica gel with
hexane/methylene chloride 9:1 to 4:1 (v/v) as the eluent to yield 3-6 (0.35g, 61 %) as
1 a white powder. H NMR (400 MHz, CDCl3): δ (ppm) 8.40-8.38 (m, 2H), 8.19-8.17
(d, J=7.6 Hz 1H), 8.00 (s, 1H), 7.93-7.79 (m, 2H), 7.50-7.41( m, 4H), 7.34-7.22 (m,
3H), 6.18-6.08 (m, 1H), 5.20-5.10 (m, 2H), 4.15-4.11 (m, 4H), 3.61-3.59 (d, J=6.4 Hz
2H), 2.50-2.39 (m, 2H), 1.03-1.01 (m, 12H).
88
1-Bromo-4-(3-(6-(9-(2-methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazol-3-
yl)propyl)-benzene, 3-7.
9-BBN (0.5 M in THF, 6 ml, 3.0 mmol) was added dropwise into a solution of 3-6
(0.46 g, 0.95 mmol) in THF (3 ml) at 0 oC. The mixture was stirred at room
temperature for 15 min and then at 35 oC for 18 h before transferring into a mixture of
p-dibromobenzene (1.12 g, 4.7 mmol), Pd(PPh3)4 (0.055 g, 0.047 mmol), K2CO3
(0.98 g, 7.1 mmol), H2O (3 ml) and THF (5 ml). The reaction mixture was stirred at
85 oC for 40 h, cooled to room temperature, and extracted with chloroform. The
organic extracts were combined, washed with water, and dried over MgSO4. Upon
evaporating off the solvent, the crude product was purified by column
chromatography on silica gel with hexane/chloroform 4:1 (v/v) as the eluent to yield
1 3-7 (0.36 g, 59 %) as a white powder. H NMR (400 MHz, CDCl3): δ (ppm) 8.40-
8.38 (m, 2H), 8.19-8.17 (d, J=7.6 Hz, 1H), 7.98-7.97 (d, J=0.8 Hz, 1H), 7.81-7.79 (m,
2H), 7.50-7.39 (m, 6), 7.33-7.23 (m, 3H), 7.10-7.08 (d, J=8.4 Hz, 2H), 4.15-4.11 (m,
4H), 2.86-2.83 (t, J=7.6 Hz, 2H), 2.69-2.65 (t, J=7.6 Hz, 2H), 2.44-2.42 (m, 2H),
2.09-2.03 (m, 2H), 1.04-1.01 (m, 12H).
2-(4-(3-(6-(9-(2-Methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazol-3- yl)propyl) phenyl)-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, 3-8.
The procedure for the synthesis of 3-4 was followed to prepare 3-8 from 3-7 as a
1 white powder in an 88 % yield. H NMR (400 MHz, CDCl3): δ (ppm) 8.40-8.38 (m,
2H), 8.19-8.17 (d, J=7.6 Hz, 1H), 7.99 (s, 1H), 7.84-7.78 (m, 2H), 7.75-7.74 (d, J=8.8 89
Hz, 2), 7.50-7.43 (m, 4H), 7.32-7.23 (m, 3H), 4.15-4.10 (m, 4H), 2.86-2.83 (t, J=7.6
Hz, 2H), 2.69-2.65 (t, J=7.6 Hz, 2H), 2.44-2.42 (m, 2H), 2.09-2.03 (m, 2H), 1.34 (s,
12H), 1.04-1.01 (m, 12H).
3-(9-(2-Methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazole, Cz(MP)2.
A solution of 3-1 (0.3 g, 1.34 mmol) in chloroform (16 ml) was slowly added into a
mixture of Iron(III) chloride (0.92 g, 5.37 mmol) and chloroform(30 ml) under argon.
The reaction mixture was stirred at room temperature for 4 h, quenched with 10 %
sodium hydroxide aqueous solution (100 ml), and extracted with chloroform. The
organic extracts were combined, washed with water and dried over MgSO4. Upon evaporating off the solvent, the crude product was purified by column chromatography on silica gel with hexane/chloroform 1:4 (v/v) as the eluent to yield
Cz(MP)2 (0.23 g, 76 %) as a white powder (Found: C, 86.08; H, 7.22; N, 6.24. Calc.
1 for C32H32N2: C, 86.45; H, 7.25; N, 6.30%.); H NMR (400 MHz, CDCl3): δ (ppm)
8.40 (s, 2H), 8.19-8.18 (d, J=3.8Hz, 2H), 7.83-7.81 (m, 2H), 7.50-7.41 (m, 6H), 7.26-
7.23 (m, 2H), 4.15 (s, 4H), 2.50-2.39 (m, 2H), 1.03-1.02 (m, 12H).
2-(4-(3-(6-(9-(2-Methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazol-3-
yl)propyl) phenyl)-4,6-diphenyl-1,3,5-triazine, TRZ-1Cz(MP)2.
Toluene (10 ml) and H2O (2 ml) were added into a mixture of 3-8 (0.34 g, 0.49
mmol), 3-9 (0.16 g, 0.62 mmol), Pd(PPh3)4 (0.056 g, 0.049 mmol), and Na2CO3 (0.39 g, 3.68 mmol). The reaction mixture was stirred at 95 oC for 40 h, cooled to room 90
temperature, and then extracted with methylene chloride. The organic extracts were
combined, washed with water, and dried over MgSO4. Upon evaporating off the
solvent, the crude product was purified by column chromatography on silica gel with
hexane/methylene chloride 4:1 (v/v) to yield TRZ-1Cz(MP)2 (0.28 g, 69 %) as a
yellow powder (Found: C, 84.52; H, 6.39; N,8.74. Calc. for C56H51N5: C, 84.71; H,
1 6.47; N, 8.82%); H NMR (400 MHz, CDCl3): δ (ppm) 8.79-8.76 (m, 4H), 8.71-8.69
(d, J=8.4 Hz, 2H), 8.41-8.39 (m, 2H), 8.19-8.17 (d, J=7.6 Hz, 1H), 8.02 (s, 1H), 7.81-
7.79 (m, 2H), 7.60-7.55 (m, 6),7.49-7.42 (m, 6H), 7.35-7.33 (m, 2H), 7.25-7.20 (m,
1H), 4.14-4.11 (m, 4H), 2.93-2.82 (m, 4H), 2.44-2.40 (m, 2H), 2.20-2.10 (m, 2H),
1.03-1.01 (m, 12H). MALD/I TOF MS (DCTB) m/z ([M]+): 793.4.
2,4,6-Tris(4-(3-(6-(9-(2-methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazol-3-
yl) propyl)-phenyl)-1,3,5-triazine, TRZ-3Cz(MP)2.
9-BBN (0.5 M in THF, 3.30 ml, 1.65 mmol) was added dropwise into a solution of 3-
6 (0.21 g, 0.42 mmol) in THF (2 ml) at 0 oC. The mixture was stirred at room
temperature for 15 min and then at 35 oC for 18 h before transferring into a mixture of
3-10 (0.07 g, 0.13 mmol), Pd(PPh3)4 (0.030 g, 0.026 mmol), Na2CO3 (3.82 g, 36.0
o mmol), H2O (18 ml) and toluene (30 ml). The reaction mixture was stirred at 85 C for 40 h, cooled to room temperature, and then extracted with chloroform. The organic extracts were combined, washed with water, and dried over MgSO4. Upon
evaporating off the solvent, the crude product was purified by column
chromatography on silica gel with hexane/methylene chloride 3:2 (v/v) as the eluent 91
to yield TRZ-3Cz(MP)2 (0.147 g, 65 %) as a pale yellow powder (Found: C, 85.43;
1 H, 7.01; N, 7.11. Calc. for C126H123N9: C, 85.82; H, 7.03; N, 7.15%). H NMR (400
MHz, CDCl3): δ (ppm) 8.69-8.67 (d, 6H), 8.40-8.39 (t, J=2.2Hz, 6H), 8.18-8.16 (d,
J=3.8Hz, 3H), 8.01 (s, 3H), 7.82-7.78 (m, 6H), 7.47-7.30 (m, 27H), 4.12-4.09 (m,
12H), 2.92-2.82 (m, 12H), 2.48-2.35 (m, 6H), 2.19-2.11 (m, 6H), 1.03-0.99 (m, 36H).
MALD/I TOF MS (DCTB) m/z ([M]+): 1762.1.
1,3-Bis(5-(4-(3-(6-(9-(2-methylpropyl)carbazol-3-yl)-9-(2-methylpropyl)carbazol-3-
yl)propyl)-phenyl)-1,3,4-oxadiazol-2-yl)benzene, OXD-2Cz(MP)2.
9-BBN (0.5 M in THF, 3.0 ml, 1.50 mmol) was added dropwise into a solution of 3-6
(0.20 g, 0.41 mmol) in THF (3 ml) at 0 oC. The mixture was stirred at room
temperature for 15 min and then at 35 oC for 18 h before transferring into a mixture of
3-11 (0.11 g, 0.20 mmol), Pd(PPh3)4 (0.033 g, 0.029 mmol), Na2CO3 (0.60 g, 4.35
o mmol), H2O (2 ml) and toluene (5 ml). The reaction mixture was stirred at 85 C for
40 h, cooled to room temperature, and then extracted with chloroform. The organic extracts were combined, washed with water, and dried over MgSO4. Upon
evaporating off the solvent, the crude product was purified by column
chromatography on silica gel with chloroform/ethyl acetate 50:1 (v/v) as the eluent to
yield OXD-2Cz(MP)2 (0.12 g, 44 %) as a pale yellow powder (Found: C, 82.38; H,
1 6.42; N, 8.42. Calc. for C92H86N8O2: C, 82.73; H, 6.49; N, 8.39%) H NMR (400
MHz, CDCl3): δ (ppm) 8.84 (s, 1H), 8.40-8.39 (m, 4H), 8.32-8.30 (m, 2H), 8.18-8.16
(d, J=7.6 Hz, 2H), 8.11-8.08 (d, J=8.4 Hz, 4H), 8.00 (s, 2H), 7.82-7.78 (m, 4H), 7.66- 92
7.63 (t, J=8.4 Hz, 1H), 7.49-7.32 (m, 16H), 7.25-7.21 (m, 2H), 4.13-4.11 (m, 8H),
2.90-2.88 (t, J=7.6 Hz, 4H), 2.82-2.80 (t, J=7.6 Hz, 2H), 2.46-2.40 (m, 4H), 2.20-2.10
(m, 4H), 1.03-1.01 (m, 24H). MALD/I TOF MS (DCTB) m/z ([M]+): 1334.8.
Morphology, Thermal Stability, and Phase Transition Temperatures
Thermogravimetric analysis was performed in a TGA/DSC system (SDT Q600,
TA Instruments) at a ramping rate of 10 oC/min under a nitrogen flow of 50 ml/min.
Thermal transition temperatures were determined by differential scanning calorimetry
(Perkin-Elmer DSC-7) with a continuous N2 purge at 20 ml/min. Samples were
o o preheated to above Tm and then cooled down to −30 C at −100 C/min before the reported second heating and cooling scans were recorded at 20 oC/min. The nature of
phase transition was characterized by hot-stage polarizing optical microscopy
(DMLM, Leica, FP90 central processor and FP82 hotstage, Mettler Toledo).
− − Absorption spectra of dilute solutions in chloroform at a concentration of 10 7 to 10 6
M were acquired on a UV-vis-NIR spectrophotometer (Lambda-900, Perkin-Elmer).
Electrochemical Characterization
Cyclic voltammetry was conducted on an EC-Epsilon potentiostat (Bioanalytical
− Systems Inc.) at a concentration of 10 3 M in acetonitrile/toluene (1:1 by volume)
containing 0.1 M tetraethylammonium tetrafluoroborate as the supporting electrolyte. 93
A silver/silver chloride (Ag/AgCl) wire, a platinum wire, and a glassy carbon disk (3-
mm diameter) were used as the reference, counter, and working electrodes,
respectively, to complete a standard 3-electrode cell. The supporting electrolyte was
purified as described in Chapter 2, and the solvents acetonitrile and toluene were
distilled over calcium hydride and sodium/benzophenone, respectively. The dilute
sample solutions in acetonitrile:toluene (1:1 by volume) exhibit reversible reduction
and oxidation waves against the Ag/AgCl reference electrode. The reduction and
oxidation potentials were adjusted to ferrocene serving as an internal standard with an
oxidation potential of 0.51 ± 0.02 V over Ag/AgCl. The resultant reduction and
+ oxidation potentials, E1/2(red) and E1/2(oxd), relative to (Fc/Fc ) were used to
calculate the LUMO and HOMO levels as −4.8eV−qE1/2(red) and −4.8eV−qE1/2(oxd), respectively, where q is the electron charge. [43, 44]
Triplet Energy Measurement
Phosphorescence spectra were gathered using a Fluorolog-3 spectrofluorometer
(Jobin Yvon, Horiba) and were corrected for the efficiency of the monochromator and
− the spectral response of the photomultiplier tube. Samples (10 4 M) were dissolved in
ethyl acetate in NMR tubes and inserted into a small liquid nitrogen Dewar to
measure the phosphorescence spectra at 77 K. As has been customary, [45, 46] the maximum of highest-energy 0-0 vibronic band in the phosphorescence spectrum was assigned as energy of the lowest triplet state. Phosphorescence measurements were 94
also carried out with 10% butyl iodide added to the ethyl acetate to enhance the 0-0
vibronic transition [47] and to help differentiate between phosphorescence and
fluorescence.
Thin Film Preparation and Characterization
Films of hybrid compounds and mixtures were prepared by spin coating from 2
wt% chlorobenzene solutions at 2500 rpm on microscope glass slides followed by
drying under vacuum overnight. The thicknesses of the resultant films were
determined by optical interferometry (Zygo New Views 5000). Thermal annealing
was performed under argon at elevated temperatures. The film morphology, including
melting points where applicable, was characterized by hot-stage polarizing optical
microscopy. Photoluminescence was characterized using a spectrofluorimeter
(Quanta Master C-60SE, Photon Technology International) with a liquid light guide
directing excitation at 360 nm onto the sample film at normal incidence.
Phosphorescent OLED Device Fabrication and Characterization
Glass substrates coated with patterned ITO were thoroughly cleaned and treated
with oxygen plasma prior to deposition of a 10-nm thick MoO3 layer by thermal
evaporation at 0.1 nm/s. The emitting layers comprising host:Ir(mppy)3 at a mass
ratio of 10:1 were then prepared by spin-coating from 2.0 wt% toluene solutions at 95
4000 rpm for 2 min in a nitrogen-filled glove box with oxygen level less than 1 ppm.
The resultant film thickness is 40~50 nm determined by optical interferometry (Zygo
NewView 5000). Layers of TPBI (30 nm) and CsF (1 nm) were then consecutively deposited at rates of 0.1 nm/s and 0.02 nm/s, respectively. The devices were completed by thermal evaporation of Al (100 nm) at 1 nm/s through a shadow mask to define an active area of 0.1 cm2. All evaporation processes were carried out at a
− base pressure less than 4 × 10 6 Torr. All devices were encapsulated with cover glass and glue for characterization with a source-measure unit (Keithley 2400) and a spectroradiometer (PhotoResearch PR650). Only the front-view performance data were collected.
3.3 RESULTS AND DISCUSSION
In addition to precluding phase separation, the flexible linkages connecting the two charge-transport moieties in the non-conjugated bipolar hybrid molecules serve to increase entropy because of the more abundant conformations, which is conducive to solubility in benign solvents to facilitate materials purification and solution processing. Furthermore, the increased entropy with flexible linkages presents a higher free energy barrier to crystallization from glassy state, thereby improving morphological stability against crystallization over relatively rigid conjugated and non-conjugated bipolar hybrid molecules without flexible linkages. Depicted in Chart
3.1 are three representative non-conjugated bipolar hybrid compounds with propylene 96
linkages, TRZ-1Cz(MP)2, TRZ-3Cz(MP)2, and OXD-2Cz(MP)2 that were
synthesized for an investigation of their thermal, morphological, electrochemical,
fluorescence, and phosphorescence properties. These hybrid compounds consist of a
hole-transport Cz(MP)2 [48-50] and an electron-transport TRZ [51-54] or OXD. [27, 30, 53,
54]
0
TRZ-1Cz(MP)2 TRZ-3Cz(MP)2 OXD-2Cz(MP)2 50 Weight Losses, %
100 0200400600 Temperature, oC
Figure 3.1: TGA thermograms of hybrid compounds recorded at a heating rate of 10 oC/min under nitrogen atmosphere. The decomposition temperatures at a weight loss of 5% are 399, 403 and 407 oC for TRZ-1Cz(MP)2, TRZ-3Cz(MP)2 and OXD- 2Cz(MP)2, respectively.
The results from thermogravimetric analysis compiled in Figure 3.1 reveal their
thermal stability to 400 oC at about 5 wt% weight loss. Solid morphologies of the
three independent building blocks, three bipolar compounds, and their corresponding
mixtures, TRZ:1Cz(MP)2, TRZ:3Cz(MP)2, and OXD:2Cz(MP)2 were
characterized by differential scanning calorimetry and hot-stage polarizing optical
microscopy. 97
(a) I G Cz(MP)2 G I K I TRZ K I K I OXD K I Endothermic
0 100 200 300 400 Temperature, oC
(b) G K+I I TRZ:1Cz(MP)2 G K+I I + K+I K G K I TRZ:3Cz(MP)2 G+K K+I I G I OXD:2Cz(MP)2 G I Endothermic
0 100 200 300 400 Temperature, oC
(c) G I TRZ-3Cz(MP)2 G I G I TRZ-1Cz(MP)2 G I G I OXD-2Cz(MP)2 G I Endothermic
0 100 200 300 400 Temperature, oC
Figure 3.2: DSC heating and cooling scans at ±20 oC/min of samples comprising (a) hole- and electron-transport moieties, (b) mixtures thereof, and (c) non-conjugated bipolar compounds that have been preheated to beyond their melting points followed by quenching to –30 oC. Symbols: G, glassy; K, crystalline; I, isotropic.
98
The DSC thermograms compiled in Figure 3.2 indicate that the building blocks and their mixtures are crystalline, semicrystalline or amorphous with a glass transition
o temperature, Tg, below 65 C, while all the hybrid compounds are amorphous with a
o Tg near or above 100 C.
(a) OXD:2Cz(MP)2 (b) OXD:2Cz(MP)2
200 μm 200 μm
(c) OXD-2Cz(MP)2 (d) OXD:PVK
200 μm 200 μm
Figure 3.3: Polarizing optical micrographs of films from spin-cast chlorobenzene of OXD:2Cz(MP)2 mixture (a) before and (b) after thermal annealing at 32 oC for 3 days; (c) that of an OXD-2Cz(MP)2 film before and after thermal annealing at 100 oC for 3 days; and (d) that of an OXD:PVK mixture at 30:70 mass ratio after thermal annealing at 100 oC for 3 days.
99
The amorphous OXD:2Cz(MP)2 mixture observed under differential scanning calorimetry and polarizing optical microscopy was further evaluated for phase
o separation via thermal annealing at 32 C, viz. a reduced temperature, T/Tg = 0.91, for three days. While the pristine, spin-cast film of OXD:2Cz(MP)2 was amorphous under polarizing optical microscopy (Figure 3.3a), phase separation and/or crystallization occurred upon thermal annealing at 32 oC for 3 days (Figure 3.3b). The
thermally annealed film was further characterized by polarizing optical microscopy to
yield melting points at 150 and 185 oC, which fall between the melting points of
Cz(MP)2 (144 oC) and OXD (240 oC) also determined by polarizing optical microscopy for spin-cast films left at room temperature for 2 to 3 days. These results suggest a complex phase behavior of the OXD:2Cz(MP)2 mixture. As shown in
Figure 3.3c, the amorphous character persisted in the OXD-2Cz(MP)2 film upon
thermal annealing at 100 oC, i.e. the same reduced temperature and annealing time as
for OXD:2Cz(MP)2, indicating that the hybrid compound is not vulnerable to phase
separation and that it is resistant to thermally activated crystallization. Poly(N-
vinylcarbazole), PVK, mixed with OXD at a 70:30 mass ratio was employed as the host for iridium(III) bis[2-(4,6-difluorophenyl)-pyridinato-N, C2’]picolinate (FIrpic).
[27] The pristine, spin-cast film of this bipolar host material was found to be
amorphous with the same optical micrograph as shown in Figure 3.3a or c, but phase
separation and/or crystallization emerged in Figure 3.3d upon thermal annealing at
100 oC for 3 days. Moreover, the crystallites observed under polarizing optical microscopy were found to melt at 230 oC, close to the melting point of OXD. 100
60 60 60
30 (a)30 (b) 30 (c) A A A A A μ μ μ μ μ 0 0 0
Current, Current, −30 TRZ Current, −30 OXD Current, −30 Cz(MP)2
−60 −60 −60 −2 −1 0 1 −2 −1 0 1 −2 −1 0 1 Potential, V vs Ag/AgCl Potential (V vs Ag/AgCl) Potential, V vs Ag/AgCl 60 60 60 (f) 30 (d) 30 (e) 30 A A A A μ μ μ μ 0 0 0 −30 Current, Current, Current, Current, −30 TRZ-1Cz(MP)2 −60 TRZ-3Cz(MP)2 Current, −30 OXD-2Cz(MP)2 −90 −60 −60 −2 −1 0 1 −2 −1 0 1 −2 −1 0 1 Potential, V vs Ag/AgCl Potential, V vs Ag/AgCl Potential, V vs Ag/AgCl
Figure 3.4: Cyclic voltammetric scans of compounds in acetonitrile/toluene (1:1 by − volume) at 10 3 M with 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte.
The electrochemical properties of the non-conjugated bipolar compounds and
their building blocks are characterized by cyclic voltammetry. The results presented
in Figure 3.4 illustrate that the oxidation and reduction scans of the hybrids are
essentially the composites of those of their constituent hole- and electron-transport
moieties. The key data summarized in Table 3.1 indicates that the HOMO levels of
TRZ-1Cz(MP)2, TRZ-3Cz(MP)2 and OXD-2Cz(MP)2 are placed at −5.2 eV, a
value identical to that of the hole-transporting Cz(MP)2. The LUMO levels of −2.6
eV for TRZ-1Cz(MP)2 and TRZ-3Cz(MP)2, and −2.5 eV for OXD-2Cz(MP)2, are
also equal within error to those of the electron-transporting TRZ and OXD,
respectively. With a HOMO level at −5.2 eV, which is close to the work function of 101
PEDOT:PSS at 5.1 eV, and a LUMO level at −2.5 to −2.6 eV, which is relatively close to the work function of LiF/Al at 2.9 eV, these non-conjugated bipolar compounds are expected to facilitate hole and electron injection in phosphorescent
OLED devices. With an interruption of π-conjugation between the two moieties by a propylene linkage, the HOMO and LUMO levels of non-conjugated bipolar hybrid compounds are essentially imported from those of the hole- and electron-transport moieties as independent chemical entities. The LUMO and HOMO levels in conjugated bipolar hybrid compounds, however, are generally affected by the finite extent of π-conjugation between the two moieties.
Cz(MP)2, pristine
TRZ:1Cz(MP)2, pristine
TRZ-1Cz(MP)2, pristine and annealed
TRZ:1Cz(MP)2, annealed PL Intensity / Film Thickness,FilmPL a.u. Intensity /
400 450 500 550 600 Wavelength, nm
Figure 3.5: Fluorescence spectra with excitation at 360 nm of approximately 45-nm- thick, spin-cast films of Cz(MP)2, TRZ-1Cz(MP)2, and TRZ:1Cz(MP)2; thermal o annealing was performed at 20 C above Tg under argon for ½ h.
102
Table 3.1: Electrochemical properties of compounds determined by the oxidation and reduction scans presented in Figure 3.4.
E (red)[a,b] E (red)[c] E (oxd)[a,b] E (oxd)[c] 1/2 1/2 1/2 1/2 HOMO[d] LUMO[e] Compound vs Ag/AgCl vs Fc/Fc+ vs Ag/AgCl vs Fc/Fc+ (eV) (eV) (V) (V) (V) (V) TRZ TRZ −1.67 −2.18 −2.6
OXD OXD −1.86 −2.37 −2.4
Cz(MP)2 Cz(MP)2 0.92 0.41 −5.2
TRZ −1.69 −2.20 −2.6 TRZ-1Cz(MP)2 Cz(MP)2 0.92 0.41 −5.2
TRZ −1.72 −2.23 −2.6 TRZ-3Cz(MP)2 Cz(MP)2 0.89 0.38 −5.2
OXD −1.83 −2.34 −2.5 OXD-2Cz(MP)2 Cz(MP)2 0.88 0.37 −5.2
[a] Half-wave potentials, E1/2, determined as the average of forward and reverse reduction or oxidation peaks. [b] Reduction and oxidation scans of 10−3 M solutions in acetonitrile/toluene (1:1 by volume) with 0.1 M tetrabutylammonium tetrafluoroborate as the supporting electrolyte. [c] Relative to ferrrocene/ferrocenium (Fc/Fc+) with an oxidation potential at 0.51±0.02 V vs Ag/AgCl − − [d] Calculated using the equation HOMO= 4.8 eV qE1/2(oxd) eV, where q is the electron charge and + E1/2(oxd) is the oxidation potential over Fc/Fc . − − [e] Calculated using the equation LUMO= 4.8 eV qE1/2(red) eV, where q is the electron charge and E (red) is the reduction potential over Fc/Fc+. 1/2
In addition to phase separation detected by microscopy (see Figure 3.3), fluorescence spectroscopy was employed to uncover molecular aggregation in non- conjugated bipolar compounds and their equivalent mixtures. To facilitate data interpretation, TRZ-1Cz(MP)2 was used along with TRZ:1Cz(MP)2 based on the facts that TRZ was found to be essentially nonemissive [55] and that OXD showed fluorescence in the 400 to 450 nm region largely overlapping with that of Cz(MP)2. 103
Approximately 45-nm-thick amorphous films of TRZ-1Cz(MP)2, TRZ:1Cz(MP)2, and Cz(MP)2 prepared by spin-coating from chlorobenzene were photoexcited at 360
nm, and their fluorescence spectra normalized with film thickness are presented in
Figure 3.5. The pristine TRZ-1Cz(MP)2 and TRZ:1Cz(MP)2 films exhibited similar
fluorescence in the same spectral range from 450 to 600 nm, representing a red-shift
from that of Cz(MP)2 by about 90 nm. These broad and red-shifted fluorescence
peaks at 2.4 eV originated from exciplex formation between TRZ and Cz(MP)2
moieties in the hybrid compound and their equimolar mixture, as expected of the offset between the HOMO level of Cz(MP)2 and the LUMO level of TRZ in addition to the Coulomb attraction energy. [56] The lower fluorescence intensity from
the pristine film of TRZ-1Cz(MP)2 than that of TRZ:1Cz(MP)2 could have arisen
from the less extent of inter- and/or intra-molecular exciplex formation in the former
caused by steric hindrance in the presence of a propylene spacer. Thermal annealing
o of the TRZ-1Cz(MP)2 film at 20 C above its Tg for ½ h did not result in any change
in the amorphous character and the fluorescence spectrum. Upon thermal annealing
under the same condition, polycrystalline domains emerged from the pristine
TRZ:1Cz(MP)2 film with a melting point at 178 oC, which is distinct from those of
TRZ and Cz(MP)2 at 242 and 144 oC, respectively as noted above. As shown in
Figure 3.5, phase separation accompanied by crystallization led to much reduced,
blue-shifted exciplex emission as well as weak emission between 400 and 450 nm
attributable to Cz(MP)2. Thus, the hybrid compound is preferable over its equivalent
mixture from the standpoint of morphological stability, but the issue of exciplex 104
formation in both material systems must be addressed. Whereas exciplex emission
involving bipolar mixed hosts is frequently observed in photoluminescence, it is
absent in electroluminescence where triplet emitters serve as charge traps with
improved device efficiencies at lower driving voltages compared to the constituent
unipolar hosts. [11, 12, 29] To ensure effective charge trapping on the emitter, its LUMO
level must be lower than that of the electron-transport component, while its HOMO
level higher than that of the hole-transport component, an idea to be incorporated in
the design of non-conjugated bipolar hybrid compounds.
The absorption spectra of Cz(MP)2, OXD, and OXD-2Cz(MP)2 at 10–7 to 10–6
M in chloroform are presented in Figure 3.6a, serving to identify selective photoexcitation wavelengths for the determination of their lowest triplet energies, ET, to within ±0.02 eV through phosphorescence measurements. Phosphorescence spectra were obtained for Cz(MP)2, OXD, and OXD-2Cz(MP)2 at 10–4 M in ethyl acetate at
–196 oC (or 77 K). As illustrated in Figure 3.6b, Cz(MP)2 shows a 0,0-
phosphorescence band corresponding to an ET at 2.77 eV upon excitation at 360 nm.
The OXD moiety shows only very weak phosphorescence due largely to inefficient intersystem crossing. Thus, butyl iodide was added at 10 wt% to the OXD and OXD-
2Cz(MP)2 solutions to enhance phosphorescence, particularly the 0,0-vibronic transition, [47] and to help confirm the identity of the emitting state.
105
(a) (b) 2.0 Cz(MP)2 Cz(MP)2 E = 2.77 eV 1 T λ
− = 360 nm OXD ex
cm OXD-2Cz(MP)2 1 − M 5 1.0 , 10 a.u. Intensity, ε
0.0 250 300 350 400 400 450 500 550 600 Wavelength, nm Wavelength, nm
(c) (d) OXD OXD-2Cz(MP)2 E = 2.71 eV λ = 332 nm E = 2.71 eV λ = 360 nm T ex T ex Intensity, a.u. Intensity, a.u. a.u.
400 450 500 550 600 400 450 500 550 600 Wavelength, nm Wavelength, nm
Figure 3.6: (a) UV-vis absorption spectra in molecular extinction coefficients, ε, of OXD-2Cz(MP)2, Cz(MP)2 and OXD. Phosphorescence spectra of (b) Cz(MP)2, (c) o −4 OXD, and (d) OXD-2Cz(MP)2 at 77 K in ethyl acetate at 10 M, for which the ET values were determined by the 0-0 transitions as indicated by arrows.
With photoexcitation at 332 nm, the relatively sharp highest-energy 0,0-vibronic band shown in Figure 3.6c establishes an ET of 2.71 eV for OXD. When the hybrid compound OXD-2Cz(MP)2 is excited at 360 nm (Figure 3.6d), its phosphorescence spectrum is identical to that of OXD (Figure 3.6c) even though OXD is not excitable at 360 nm. These results indicate highly efficient triplet energy transfer from
Cz(MP)2 (ET=2.77 eV) to OXD (ET=2.71 eV) in OXD-2Cz(MP)2 and further 106
suggest that the two moieties retain their energy levels as independent entities and
that the effective ET of the hybrid equals that of the lower-ET moiety due to efficient intramolecular triplet energy transfer. Phosphorescence spectra were also collected for TRZ and TRZ-3Cz(MP)2 to arrive at ET=3.03 and 2.75 eV, respectively, an
observation also consistent with highly efficient triplet energy transfer from TRZ to
the lower energy Cz(MP)2 moiety. Furthermore, the ET value of a non-conjugated
bipolar hybrid compound is determined by the lower value of the two independent
moieties. The ET value of a conjugated bipolar compound, however, is consistently
less than those of the two independent moieties because of the finite π-conjugation
between them.[29, 31, 45]
Let us proceed to assess yet another merit of non-conjugated bipolar compounds
compared to their conjugated counterparts. In a conjugated bipolar compound, the ET
[31-40] value is consistently less than EG, the LUMO–HOMO energy gap, by the sum of
[57] [58] singlet-triplet splitting, ΔST, and exciton binding energy, EB. For example,
CzOXD has its HOMO/LUMO levels at –5.6/–2.4 eV based on the reported half-
[38] wave potentials with an ET at 2.5 eV, which is 0.7 eV less than EG (see Figure
3.7a). On the other hand, the ET value of a non-conjugated bipolar compound, such as
TRZ-1Cz(MP)2, is not limited by its EG. The oxidation potentials of TRZ and OXD
and the reduction potential of Cz(MP)2 are beyond our CV measurement range. 107
(a) LUMO = −2.4eV Δ EB + ST = 3.2eV = 2.5eV G CzOXD T E E HOMO = −5.6eV CzOXD (b)
LUMO = −1.8eV Δ EB + ST LUMO = −2.6eV LUMO = −2.6eV Δ EB + ST = 3.4eV = 2.8eV = 2.6eV = 2.8eV G T T TRZ-1Cz(MP)2 G E E E E
HOMO = −5.2eV HOMO = −5.2eV = 4.1eV = 3.0eV G Cz(MP)2 T E E TRZ-1Cz(MP)2
Cz(MP)2 TRZ HOMO = −6.7eV TRZ
Figure 3.7: (a) Molecular structure and energy diagram of a conjugated bipolar compound CzOXD, and (b) Molecular structure and energy diagram of a non- conjugated bipolar compound TRZ-1Cz(MP)2 accompanied by those of Cz(MP)2 and TRZ.
Their optical bandgaps were estimated at the onset of their absorption spectra in dilute solutions to yield HOMO levels at −6.7 and −6.2 eV for TRZ and OXD, respectively, and LUMO level at −1.8 eV for Cz(MP)2. As shown in Figure 3.7b, the
ETs of independent electron- and hole-transport moieties are less than their respective
EGs as in a typical conjugated system. The Cz(MP)2 moiety is characterized by an ET and HOMO/LUMO levels at 2.8 eV, and −5.2/−1.8 eV, respectively, while the TRZ moiety carries an ET and HOMO/LUMO levels at 3.0 eV, and −6.7/−2.6 eV,
respectively (see Figure 3.7b). The hybrid TRZ-1Cz(MP)2 has an ET at 2.8 eV, as 108
verified by low temperature phosphorescence spectroscopy, and HOMO/LUMO
levels at −5.2/−2.6 eV by cyclic voltammetry. In contrast to CzOXD, TRZ-
1Cz(MP)2 has an ET greater than its EG and its HOMO/LUMO levels corresponding
to the hole- and electron-transport moieties without modification in the absence of
inter-moiety conjugation. The caveat here is that the ETs of both the electron- and
hole-transport moieties must be greater than the hybrid’s EG to arrive at a non- conjugated bipolar compound with an ET greater than its EG, as is also applicable to
TRZ-3Cz(MP)2 and OXD-2Cz(MP)2.
Bipolar hybrid TRZ-3Cz(MP)2 and TRZ-1Cz(MP)2 and hole-transporting
Cz(MP)2 were employed as hosts for the fabrication of bilayer phosphorescent
OLEDs in the device architecture, ITO/MoO3(10 nm)/emitting layer(40~50
nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI)(30 nm)/CsF(1 nm)/ Al
(100 nm). Prepared by spin casting, the emitting layer consisted of a host doped with green-emitting tris(2-(p-methylphenyl)pyridine)iridium, Ir(mppy)3, at a 10:1 mass
ratio. While MoO3 and CsF were applied as the hole- and electron-injection layers,
respectively, TPBI was applied as the electron-transporting and hole-blocking layer.
A typical electroluminescence spectrum is shown as the inset in Figure 3.8a,
[12] consistent with the triplet emission of Ir(mppy)3, suggesting effective confinement
of triplet excitons on the emitter. This is expected of the higher ETs of all three hosts
[59] than that of Ir(mppy)3, 2.8 eV for Cz(MP)2 over 2.4 eV for Ir(mppy)3.
Furthermore, Ir(mppy)3 has a HOMO level at −5.0 eV, 0.2 eV higher than those of 109
the hosts, thus serving as a hole trap to preclude exciplex formation between the TRZ
and Cz(MP)2 moieties.
150 2 (a) Cz(MP)2 125 TRZ-3Cz(MP)2 TRZ-1Cz(MP)2 100 75
50 a.u. EL, 400 500 600 700 25 Wavelength, nm
Current Density, Density, Density, Current mA/cm mA/cm 0 05101520 Driving Voltage, V 40 105 (b) Luminance, cd/m Luminance, 30 104
20 103
10 Cz(MP)2 102 TRZ-3Cz(MP)2 2 TRZ-1Cz(MP)2 Current Efficiency, Current cd/A 0 101 0.1 1 10 100 Current Density, mA/cm2
Figure 3.8: (a) Current density as a function of driving voltage for phosphorescent OLEDs with emitting layers comprising Cz(MP)2, TRZ-3Cz(MP)2, and TRZ-
1Cz(MP)2 doped with Ir(mppy)3 at a 10:1 mass ratio. Inset: electroluminescence (EL) spectrum with TRZ-3Cz(MP)2 as the host. (b) Luminance and current efficiency as functions of current density for the same phosphorescent OLEDs as described in (a).
110
It is also shown in Figure 3.8a that driving voltage decreases with an increasing
TRZ content in the hybrid hosts, suggesting improved electron injection from the
adjacent TPBI into the emitting layer. Current efficiency and luminance as functions
of current density are shown in Figure 3.8b. Compared to Cz(MP)2, the higher
efficiency with TRZ-3Cz(MP)2 as the host is attributable to the more balanced
electron and hole fluxes through the emitting layer, which leads to the more efficient
electron-hole recombination. Furthermore, the presence of electron-transporting TRZ
in TRZ-3Cz(MP)2 generates a broader charge recombination zone to alleviate
efficiency roll-off at high current densities. As shown in Figure 3.8b, at the current
density of 0.5 mA/cm2, the device with Cz(MP)2 as the host has a luminance of 105
cd/m2, corresponding to current efficiency of 21 cd/A and external quantum
efficiency of 5.9 %, which diminish to 5.4 cd/A and 1.5 % at 100 mA/cm2, respectively, representing a 74 % loss in efficiencies. In contrast, at the current density of 0.5 mA/cm2, the device with TRZ-3Cz(MP)2 as the host has a luminance
of 160 cd/m2, corresponding to current efficiency of 32 cd/A and external quantum
efficiency of 9.2 %, which roll off to 16.8 cd/A and 4.9 % at 100 mA/cm2, respectively, representing a 47 % loss in efficiencies. The efficiencies achieved with
TRZ-3Cz(MP)2 as the host are among the best of solution-processed phosphorescent
OLEDs using bipolar hosts. [32, 33, 38, 41] Implemented in more sophisticated device
architectures, optimum non-conjugated bipolar hybrids can be expected to yield much
higher efficiencies than the bilayer device architecture as presently reported. With a
further increase in the TRZ content, however, TRZ-1Cz(MP)2 resulted in current 111
efficiencies comparable to those with Cz(MP)2 as the host, presumably because of
exciton quenching by MoO3 as the recombination zone is shifted toward the anode at
an increased electron transport. [60] The results obtained to date have demonstrated the
potential of non-conjugated bipolar hybrid hosts with flexible linkages for
substantially improving PhOLED device performance through optimization of change
injection into and transport through the emitting layer.
3.4 SUMMARY
Potentially useful as the host materials for the fabrication of efficient and stable,
single-layer phosphorescent OLEDs for information display and solid-state lighting, a
new class of non-conjugated bipolar compounds have been synthesized and characterized for their thermal, morphological, electrochemical, fluorescence, and phosphorescence properties. Comprising hole- and electron-transport moieties chemically bonded by an aliphatic spacer, the potential of these materials has been demonstrated for solution processing and for the formation of uniformly amorphous thin films with superior stability against thermally activated crystallization. Because of the absence of π-conjugation between the two charge-carrier moieties, the
LUMO/HOMO levels and the triplet energies of the two moieties as independent entities are retained in the resultant non-conjugated bipolar compounds. Furthermore, the flexibility in molecular design is enhanced by the fact that the triplet energy of a non-conjugated bipolar compound is not constrained by its electrochemical energy gap. Exciplex formation is inevitable in neat films between the hole- and electron- 112
transport moieties, but its adverse effects on spectral purity and device efficiency can be prevented by having triplet emitters serve as charge traps. All these material traits are conducive to the optimization of properties for intended device applications. The current efficiencies of PhOLEDs consisting of Ir(mppy)3 doped in Cz(MP)2, TRZ-
3Cz(MP)2, and TRZ-1Cz(MP)2 at an increasing TRZ content reach the maximum at 32 cd/A with TRZ-3Cz(MP)2, which is among the best of solution processed devices using bipolar hosts. The driving voltage, however, decreases monotonically with an increasing TRZ content, suggesting improved electron injection from the adjacent TPBI layer into the emitting layer. 113
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CHAPTER 4
EFFECTS OF ACTIVE LAYER THICKNESS AND
THERMAL ANNEALING ON P3HT-PCBM
PHOTOVOLTAIC CELLS
4.1 INTRODUCTION
With a steady improvement in energy conversion efficiency during the past decades, organic photovoltaics (OPV) has evolved into a promising technology for renewable energy made possible by the first report of planar donor-acceptor heterojunction [1] where efficient exciton dissociation takes place. The device efficiency got another boost with the introduction of bulk heterojunction OPV (BHJ-
OPV) [2] comprising blends of donor and acceptor materials that phase-separate into nanoscale interpenetrating networks to dramatically increase the donor-acceptor interface per unit volume. The most promising BHJ-OPV devices to date consist of conjugated polymers, such as poly(3-hexylthiophene) (P3HT), blended with soluble fullerene derivatives, such as [6,6]-phenyl C61 butyric acid methyl ester (PCBM).
Both thermal annealing [3, 4] and solvent-vapor annealing [5, 6] have been performed to optimize phase separation between P3HT and PCBM and to induce crystallization in the P3HT domain with an objective of improving device efficiency. In addition, 120
thermal annealing of completed devices has been reported to improve charge collection efficiency by promoting active layer/electrode interaction [4]. The active layer thickness has been limited largely to 100-200 nm, although a thicker active layer should absorb more incident light to be more efficient in power conversion.
Thick-film polymer BHJ-OPV devices, however, have been found to be less efficient than thin-film devices because of inadequate mobility for supporting charge transport to electrodes for collection.[7, 8] There have been no systematic studies of how chemical purity of conjugated polymers affects device efficiency. Using a copper phthalocyanine:C60 active layer, Hiramoto et al. have reported the effects of chemical purity via repeated sublimations and of active layer thickness up to microns on BHJ-
OPV device efficiency.[9]
In this study we have examined the thickness-dependent device characteristics of BHJ-OPV with a P3HT:PCBM layer thickness from 130 to 1200 nm. In particular, it will be demonstrated that thick-film devices can be made as efficient as their thin- film counterparts through thermal annealing. Furthermore, the observed OPV device characteristics can be accounted for by vertical phase separation in the active layer and Li+ diffusion from the Al/LiF cathode into the neighboring active layer.
4.2 EXPERIMENTAL
Device Fabrication 121
Pre-patterned indium tin oxide (ITO) substrates (1.5 × 1.5 inch) with surface resistance of 15 Ω / square were cleaned and treated with oxygen plasma prior to the deposition of a thin MoO3 layer by thermal sublimation at 0.03 nm/s. Regioregular
P3HT (Rieke Metal) and PCBM (Nano-C) at a 1:1 mass ratio were dissolved in o- dichlorobenzene by stirring at 50 oC under nitrogen atmosphere overnight. The active layers were spin-coated from 0.2 g of the solution onto the MoO3-coated ITO substrates at 300 rpm for 10 s plus 600 rpm for 60 s, and kept in a covered petri dish for solvent vapor annealing for 3 h to create bulk heterojunctions [5]. The solution concentration was varied to adjust the film thickness measured with an optical interferometer (Zygo NewView 5000). The devices were completed by successive deposition of LiF and Al at 0.01 and 1 nm/s, respectively, through a shadow mask to
2 define an active area of 0.1 cm . The finished device has the structure: ITO/MoO3(3 nm)/P3HT:PCBM(x nm)/LiF(0.8 nm)/Al(100 nm), where x is varied from 130 to
1200 nm. Incremental thermal annealing was conducted by placing the tested device on a hotplate set at 110oC for a pre-determined period of time before quenching to room temperature on an aluminum block for the next round of measurement. Spin coating and thermal annealing were performed in a nitrogen-filled glove box with a oxygen level less than 1ppm, and the thermal evaporation was performed at a base
− pressure less than 4 × 10 6 torr.
Device Characterization 122
Devices were characterized in ambient atmosphere without encapsulation. The current density (J) – voltage (V) characteristics were recorded with a Keithley 2400 under the illumination of white light at 100 mW/cm2. The white light was generated using an Oriel xenon lamp solar simulator equipped with an AM 1.5 G color filter, and the intensity is calibrated with a silicon diode traceable to National Renewable
Energy Laboratory. Defined as the ratio of maximum attainable electrical power to the incident light power (Pin), the power conversion efficiency (η) was calculated as
JSCVOCFF/Pin, where JSC, VOC and FF are short circuit current density, open circuit voltage, and fill factor, respectively. The reported η values are accompanied by an error of ±5%. For the measurement of spectral response, a beam of white light is passed through a monochromator before being focused onto the devices with photocurrent recorded by the Keithley 2400. The monochromatic photocurrent was recorded at a step size of 10 nm. The external quantum efficiency (ηEQE) is calculated as the ratio of collected electrons to incident photon with a typical error of ±5%. For the measurement of spectral response under applied field, the dark current under the same applied field was subtracted from the recorded photocurrent.
4.3 RESULTS AND DISCUSSION
Depicted in Figure 4.1 is the schematic diagram of device architecture together with the chemical structures of P3HT and PCBM. Recently, there has been a
[10] growing interest in adding a metal oxide layer on the ITO anode, such as MoO3, to improve hole-collection efficiency. The bulk heterojunction between P3HT and 123
PCBM were created by solvent vapor annealing. [5] To facilitate electron collection, a thin LiF layer was inserted between the active layer and the Al cathode. [11]
e− Al (100 nm) LiF (0.8 nm)
P3HT:PCBM (x nm)
+ MoO3 (3 nm) h ITO Glass P3HT PCBM
Figure 4.1. Schematic diagram of the BHJ-OPV device architecture and chemical structures of P3HT and PCBM.
15 J, mA/cm2 Unannealed 10 Annealed 5 −1.0 −0.5 0 0.5 1.0
V, V −5
−10
Figure 4.2. J-V characteristics under 100 mW/cm2 light illumination before and after thermal annealing at 110oC for 20 min of 130-nm BHJ-OPV devices without the LiF layer to improve electron collection-efficiency over Al cathode alone.
124
As the baseline experiment, a BHJ-OPV device with a 130-nm active layer but without a LiF layer was fabricated and characterized to yield the J-V relationship as displayed in Figure 4.2. It was found that η = 1.3 and 0.9% before and after thermal annealing at 110oC for 20 min, respectively, both inferior to η = 2.9 and 1.1% from devices with a LiF layer processed under the same conditions as to be shown below.
Therefore, a LiF layer was consistently adopted in all the BHJ-OPV devices in the present study.
20 J, mA/cm2 130 nm 220 nm 15 370 nm 540 nm 10 830 nm 1200 nm 5 −1.0 −0.5 0 0.5 1.0
−5 V, V
−10
Figure 4.3: J-V characteristics under 100 mW/cm2 light illumination of BHJ-OPV devices with varying active layer thicknesses before thermal annealing.
A series of OPV devices with an active layer thickness varied from 130 to 1200 nm were fabricated and characterized. Their J-V curves before thermal annealing are plotted in Figure 4.3, and the relevant performance parameters are compiled in Table
4.1. Note that all the devices exhibit a weak dependence of photocurrent on the 125
applied field with virtually the same FF values around 0.62. As the film thickness
2 increases from 130 to 1200 nm, JSC decreases sharply from 8.4 to 1.3 mA/cm accompanied by a VOC decreasing from 0.58 to 0.50 V. The weak dependence of photocurrent on applied field for thick-film device precludes recombination as a
[12] possible cause for the low JSC.
5 0.8
4 (a) 130 nm (b) 130 nm 220 nm 0.6 220 nm 370 nm 370 nm 3 540 nm 540 nm
EQE 0.4 830 nm 830 nm η 2 1200 nm 1200 nm 1200 nm Absorbance 0.2 1
0 0 400 500 600 700 800 400 500 600 700 800 Wavelength, nm Wavelength, nm
Figure 4.4: (a) UV-vis absorption spectra of P3HT:PCBM blend films with varying active layer thicknesses before thermal annealing, and (b) Spectral responses from BHJ-OPV devices with varying active layer thicknesses before thermal annealing; the dotted curve represents the spectral response from a 1200-nm device under illumination through the semitransparent cathode.
The absorption spectra of P3HT:PCBM films with varied thicknesses are presented in Figure 4.4a. All the films show a main peak at 515 nm with two shoulders at 560 and 610 nm, which originates from the highly crystalline P3HT domains with improved stacking. [4, 5] As the film thickness reaches 830 nm, essentially all incident light between 350 and 610 nm is absorbed in one optical path. The spectral responses 126
of the devices under short circuit conditions are presented in Figure 4.4b. The ηEQE spectrum of the 130-nm device tracks the P3HT:PCBM absorption spectrum quite well as expected.
Table 4.1: Performance parameters of BHJ-OPV devices with varying active layer thicknesses before and after thermal annealing at 110oC for 20 min.
Unannealed Annealed Thickness J V FF η J V FF η (nm) SC OC SC OC (mA/cm2) (V) (%) (mA/cm2) (V) (%) 130 8.2 0.58 0.61 2.9 4.4 0.57 0.42 1.1 220 7.6 0.58 0.61 2.7 5.3 0.60 0.54 1.7 370 6.4 0.58 0.62 2.3 6.7 0.61 0.57 2.4 540 3.9 0.55 0.61 1.3 8.7 0.61 0.53 2.8 830 1.6 0.52 0.65 0.5 11.3 0.61 0.45 3.1 1200 1.3 0.50 0.64 0.4 9.2 0.61 0.40 2.3
As the film thickness increases from 130 nm, the relative ηEQE around absorption peak decreases and eventually dwindles to a valley for film thickness beyond 540 nm with the maximum ηEQE shifted to the absorption edge. This inverse spectral response indicates that photons that are more strongly absorbed by P3HT are less likely to be converted into collected charge carriers, as observed in planar heterojunction devices with a layer thickness much greater than exciton diffusion length. [13] The apparent filtering effect arises from excitons created in the neighborhood of ITO electrode have limited donor-acceptor interface within their diffusion length to dissociate efficiently. We propose that vertical phase separation in 127
a thick film yield a separated P3HT layer on ITO as inspired by numerous recent reports. [14-16] This P3HT layer absorbs most of incident photons to produce excitons that fail to diffuse sufficiently deep into the region rich in P3HT:PCBM interfaces for dissociation into charge carriers, resulting in a low photocurrent density. On the other hand, photons with wavelength in the weakly absorbing region can penetrate deeper into the active layer where more abundant P3HT:PCBM interfaces are available for efficient exciton dissociation. Meanwhile, the resultant holes and electrons are transported in spatially separated P3HT and PCBM domains, respectively, without encountering significant recombination, thereby achieving FF values in thick-film devices comparable to those in thin-film devices. Furthermore, the reduced exciton dissociation efficiency in vertically separated films is also
[17] responsible for the inferior VOC in thick-film devices. Overall, η decreases steadily from 2.9% for the 130-nm device to 0.4% for the 1200-nm device before thermal annealing. To confirm the presence of a separated P3HT layer on ITO as an explanation for the inverse spectral response in thick-film devices, Al(100 nm) was replaced with a semitransparent Al(2 nm)/Ag(10 nm) having a transmittance of about
70% in the 350 to 650 nm spectral region. As shown in Figure 4.4b, illumination through the semitransparent metal electrode of a 1200-nm device yielded a spectral response consistent with its absorption spectrum with a much improved JSC = 3.8 mA/cm2.
The finished devices with a 1200-nm active layer were further subjected to thermal annealing at 110oC over incremental durations. The J-V characteristics as a 128
function of annealing time are plotted in Figure 4.5, and their performance parameters are summarized in Table 4.2. The JSC increases steadily upon thermal annealing, reaching the maximum value of 10.1 mA/cm2 after annealing for 40 min, accompanied by VOC rising from 0.50 to 0.62 V and FF decreasing from 0.64 to 0.40, corresponding to an improvement of η from 0.4 to 2.5%. The maximum η of 2.6% was achieved with thermal annealing for 90 min, beyond which the η value decreases slightly to 2.2% because of the deteriorated JSC.
15 J, mA/cm2 0 min 40 min 10 4 min 60 min 10 min 90 min 20 min 120 min 5 −1.0 −0.5 0 0.5 1.0
V, V −5
−10
Figure 4.5: J-V characteristics under 100 mW/cm2 white light illumination of a 1200- nm BHJ-OPV device thermally annealed at 110oC up to 120 min.
Similar to what has been reported previously, [5] thermal annealing of solvent-vapor- annealed films did not significantly alter the absorption spectra, as shown in Figure
4.6a. However, the ηEQE around the absorption maximum is substantially improved upon thermal annealing, thus reversing the inverse spectral response (Figure 4.6b) through morphological modification of the separated P3HT layer on ITO. It has been reported that thermal annealing of polymer/fullerene bilayer films induces the 129
[13] [18] penetration of C60 and PCBM domains into the underlying electron-donating polymer layers, creating extended donor/acceptor interfaces to improve OPV device efficiency. By analogy the thermally induced penetration of PCBM domains into the separated P3HT layer in thick-film devices is expected to create P3HT/PCBM interfaces in the separated P3HT layer on ITO for efficient exciton dissociation but apparently at the expense of increased charge recombination, thus diminishing FF value with thermal annealing.
5 0.8 (a) 0 min 4 (b) 0.6 4 min 0 min 10 min 3 20 min 20 min 40 min
EQE 0.4 η 60 min 2 90 min
Absorbance 120 min 0.2 1
0 0 400 500 600 700 800 400 500 600 700 800 Wavelength, nm Wavelength, nm
Figure 4.6: (a) UV-vis absorption spectra of a 1200-nm film before and after thermal annealing at 110oC for 20 min, and (b) Spectral responses of a 1200-nm BHJ-OPV device thermally annealed at 110oC up to 120 min.
Devices with active layers of other thicknesses were also subjected to thermal annealing at 110oC for 20 min, and their performance parameters are included in
Table 4.1. Similar to the 1200-nm device, η of 830- and 540-nm devices is improved upon thermal annealing from 0.5 and 1.3% to 3.1 and 2.8%, respectively. In contrast, the same thermal treatment of 130- and 220-nm devices leads to inferior η values, 130
from 2.9 and 2.7% to 1.1 and 1.7%, respectively. The dark J-V curves (Figure 4.7) reveals significant current leakage under reverse bias for thin film devices, which could have been caused by the conducting channels created by diffusion of Li+ into the active organic layer upon thermal annealing. [19] Moreover, Li+ diffusion would also consume active organic materials to decrease JSC. In the absence of a LiF layer,
2 however, JSC was retained at 8.2 mA/cm upon thermal annealing (Figure 4.2), and no insignificant current leakage was detected in the dark J-V curve (Figure 4.7).
Table 4.2: Performance parameters of the 1200-nm BHJ-OPV device after thermal annealing at 110oC up to 120 min.
η Annealing time JSC VOC FF (min) (mA/cm2) (V) (%) 0 1.3 0.50 0.64 0.4 4 3.5 0.58 0.56 1.2 10 6.5 0.60 0.45 1.8 20 9.2 0.61 0.40 2.3 30 9.8 0.62 0.40 2.4 40 10.1 0.62 0.40 2.5 60 9.8 0.63 0.41 2.5 90 9.1 0.63 0.44 2.6 120 7.6 0.62 0.47 2.2
131
105 103 130 nm, Al 130 nm, LiF/Al 1
2 10 1200 nm, LiF/Al − 10 1 − 10 3 , mA/cm
J − 10 5 10−7 − 10 9 −1.0 −0.5 0.0 0.5 1.0 V, V
Figure 4.7: J-V characteristics in dark of BHJ-OPV devices with a 130- and 1200-nm active layer thermally annealed at 110oC for 20 min. The dotted curve represents the J-V characteristics of a 130-nm device without a LiF layer.
Let us proceed to examine the spectral response from a 1200-nm device that has been thermally annealed at 110oC for 20 min, where the extent of Li+ diffusion is overshadowed by the active layer thickness. It is noted that incident photons in the
350-610 nm spectral range are absorbed to practically the full extent, viz. greater than
99% (Figure 4.6a). Nonetheless, ηEQE reaches its maximum value at 610 nm but rolls off toward the short wavelength regime. Light in the long wavelength region is absorbed primarily by crystalline P3HT domains, [4, 5] where more efficient charge generation is expected of the lower ionization potential compared to amorphous domains. [20] In contrast, charge generation is less efficient and hence the inferior
ηEQE in the short wavelength region, where light absorption is contribute mainly by amorphous P3HT domains. The sensitivity of charge generation to P3HT morphology originates in the weak built-in electric field strength in thick-film 132
devices. Therefore, an applied electric field is expected to facilitate charge generation, thus significantly improving ηEQE across the entire light absorption region, as borne out in Figure 4.8.
0.8 0 V 0.6 −5 V −20 V
EQE −40 V η 0.4
0.2 1200 nm
0 400 500 600 700 800 Wavelength, nm
Figure 4.8: Electric-field dependence of spectral responses under reverse bias of a 1200-nm BHJ-OPV device after thermal annealing at 110oC for 20 min.
4.4 SUMMARY
Bulk heterojunction organic photovoltaic cells comprising an active layer of
P3HT:PCBM blend film with thickness from 130 to 1200 nm were fabricated and characterized before and after thermal annealing. Before thermal annealing, both short circuit current density and power conversion efficiency decrease with an increasing film thickness, resulting in an inverse spectral response for thick-film devices. After thermal annealing at 110oC for 20 min, the efficiency of the 130-nm device decreases from 2.9 to 1.1%, while that of the 1200-nm device increases from
0.4 to 2.6% without the inverse spectral response. Contrary to previous reports, we 133
have demonstrated that thick-film OPV devices can outperform thin-film devices with thermal annealing. The variations of the photovoltaic characteristics with active layer thickness and thermal annealing are understandable in terms of vertical phase separation in P3HT:PCBM films and Li+ diffusion from the Al/LiF cathode into the neighboring active layer. 134
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136
CHAPTER 5 MODULATION OF PHASE SEPARATION BETWEEN SPHERICAL AND ROD-LIKE MOLECULES USING GEOMETRIC SURFACTANCY
5.1 INTRODUCTION
Traditional amphiphiles comprising polar and nonpolar moieties are capable of serving not only as co-solvents for hydrophobic solutes in water [1] but also as surfactants for the creation of microstructures involving water and oil, such as bilayers, micelles, droplet and bicontinuous microemulsions.[2] In addition, macromolecular surfactancy originating from the Flory-Huggins interaction is exemplified by diblock copolymers that mediate the formation of similar microstructures between two parent homopolymers.[3-7] A geometric amphiphile consisting of a disk-like moiety linked to a rod-like moiety with an undecylene spacer has been demonstrated for co-solvency.[8]
In analogy to traditional surfactancy, the present study was motivated to demonstrate the feasibility of geometric surfactancy in terms of microstructure formation using a dyad comprising geometrically dissimilar spherical and rod-like moieties. To construct an example geometric surfactant, a propylene linker was inserted between an electron-donating conjugated oligomer and an electron- accepting C60-derivative relevant to organic photovoltaics. The flexible linker is essential to promoting preferential packing of the conjugated oligomer and the
C60-derivative as separate chemical entities. Both geometric packing and electronic interaction should be accounted for geometric surfactancy. As 137
demonstrated for traditional oil-surfactant-water systems and ternary polymer blends, the formation of stable microstructures can be expected under appropriate conditions, considering the enthalpy and entropy changes accompanying molecular self-organization in addition to interfacial energy. Three model compounds were employed to prepare blends for the characterization of powder and film morphologies by differential scanning calorimetry (DSC), hot-stage polarizing optical microscopy (POM), X-ray diffraction (XRD), and atomic force microscopy (AFM) to ascertain the formation of microemulsions.
5.2 EXPERIMENTAL
Materials Synthesis
All chemicals, reagents, and solvents were used as received from commercial sources without further purification except tetrahydrofuran (THF) and toluene which had been distilled over sodium/benzophenone before use. The target compounds depicted in Chart 5.1 were synthesized following reaction Scheme 5.1.
[6,6]-phenyl C61-butyric acid methyl ester, PCBM, at a purity level of 99.5% was
1 purchased from Nano-C (MA). H NMR spectra were acquired in CDCl3 at 298
K with an Avance-400 spectrometer (400 MHz). Elemental analysis was carried out by Quantitative Technologies, Inc. Molecular weights were measured with a
TofSpec2E MALD/I TOF mass spectrometer (Micromass, Inc., Manchester,
U.K.) using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylpropenylidene] malanoitrile
(DCTB) as the matrix.
138
o o o o o OFTB: G 81 C, 165 C K 222 C I PCBM: K1 274 C K2 292 C I
Dyad: G 181oC I
Chart 5. 1. Molecular structures of OFTB, PCBM, and Dyad used in this study. Symbols: G, glassy; K, crystalline; I, isotropic.
S N N S S S N N Bu3Sn SnBu3 S S Br 5-1 OFTB-1 S N N S O S S N N SnBu3 N N S S 1. LiTMP/THF Br S S SnBu 2. SnBu Cl 3 Pd(PPh3)4, toluene 3 5-1 5-2 5-3
S S N N Br Br N N SnBu S S 3 S S Br Pd(PPh ) , toluene Pd(PPh3)4, toluene 3 4 5-4 5-5
S N N Br CHO 9-BBN/THF S S C60,sarcosine CHO Toluene Pd(PPh3)4,Na2CO3 THF/H2O 5-6 S N N S S N
Dyad-1
Scheme 5. 1. Synthesis schemes for OFTB and Dyad. 139
4,7-Bis(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-2,1,3-benzothiadiazole,
OFTB.
Toluene (15 mL) was added to a mixture of 5-1 (1.43 g, 4.0 mmol), 4,7-bis(5- tributylstannylthien -2-yl)-2,1,3-benzothiadiazole (1.75 g, 2.0 mmol), and tetrakis(tri phenylphosphine)palladium (Pd(PPh3)4) (0.023 g, 0.02 mmol). The reaction mixture was stirred under argon at 90oC overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride 20:1 to 8:1 (v/v) as the eluent to
1 yield OFTB (1.1 g, 65 %) as a dark-red powder. H NMR (400 MHz, CDCl3,
298K): δ (ppm) 8.18-8.16 (m, 2H), 7.94 (s, 2H), 7.75-7.67 (m, 8H), 7.49-7.42 (m,
2H), 7.43-7.29 (m, 6H), 2.18-2.15 (m, 4H), 1.97-1.94 (m, 4H), 0.95-0.83 (m, 8H),
0.69-0.57 (m, 16H), 0.33-0.27 (m, 6H). MALD/I TOF MS (DCTB) m/z ([M]+):
908.4. Calcd for C60H64N2S3: C, 79.25; H, 7.09; N, 3.08. Found: C, 79.23; H,
7.03; N, 3.09.
4-(Thien-2-yl)-7-(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-2,1,3- benzothia diazole, 5-2.
Toluene (25 mL) was added to a mixture of 5-1 (1.32 g, 3.4 mmol), 4-thienyl-7-
(5-tributylstannylthien-2-yl)-2,1,3-benzothiadiazole (1.6 g, 3.4 mmol), and tetrakis(triphenyl phosphine)palladium, Pd(PPh3)4, (0.034 g, 0.029 mmol). The reaction mixture was stirred under argon at 90oC overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride 20:1 to 9:1 (v/v) as the eluent to
1 yield 5-2 (1.1 g, 53 %) as an orange solid. H NMR (400 MHz, CDCl3, 298K): δ 140
(ppm) 8.15-8.14 (m, 2H), 7.92-7.90 (m, 2H), 7.72-7.69 (m, 4H), 7.48-7.46 (m,
2H), 7.43-7.28 (m, 3H), 7.23-7.20 (m, 2H), 2.25-1.85 (m, 4H), 1.00-0.55 (m, 12H),
0.35-0.25 (m, 6H).
4-(5-(9,9-Bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-7-(5-(7-bromo-9,9-bis(2- methylbutyl)-fluoren-2-yl))thien-2-yl)-2,1,3-benzothiadiazole, 5-4.
Into a solution of 5-2 (1.1 g, 1.82 mmol) in THF (20 mL), Li- tetramethylpiperidine, LTMP, (2.18mmol) prepared by treating tetramethylpiperidine (2.18 mmol) with n-butyllithium (2.18 mmol) in THF (5 mL) was added dropwise at –78oC. The reaction mixture was stirred under argon for 3 h before adding tributyltin chloride (0.76 g, 2.3 mmol) in one portion. The solution was allowed to warm up to room temperature overnight before adding a large amount of water for extraction with ether. The extract was washed with water and brine before drying over MgSO4. Upon evaporating off the solvent under reduced pressure, the crude product (5-3) was dried in vacuo at 70oC overnight for use without further purification. To a mixture of 5-3 (1.92 g, 2.18 mmol), 2,7-dibromo-9,9-bis(2- methylbutyl)-fluorene (2.53 g, 5.46 mmol) and
Pd(PPh3)4 (0.020 g, 0.017 mmol) was added toluene (20 mL). The reaction mixture was stirred under argon at 90oC overnight. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride 20:1 to 9:1 (v/v) as the eluent to yield 5-
1 4 (1.13 g, 63 %) as a dark-red solid. H NMR (400 MHz, CDCl3, 298K): δ (ppm)
8.23-8.21 (m, 2H), 7.80-7.98 (m, 2H), 7.79-7.68 (m, 7H), 7.66-7.50 (m, 5H), 7.49-
7.32 (m, 3H), 2.30-1.90 (m, 8H), 1.05-0.60 (m, 24H), 0.40-0.30 (m, 12H). 141
4-(5-(9,9-Bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-7-(5-(7-allyl-9,9-bis(2- methylbutyl)-fluoren-2-yl]thien-2-yl)-2,1,3-benzothiadiazole, 5-5.
Toluene (20 mL) was added to a mixture of 5-4 (0.88 g, 0.89 mmol), allytributyltin (0.63 g, 1.89 mmol), and Pd(PPh3)4 (0.043 g, 0.04 mmol). The reaction mixture was stirred under argon at 90oC overnight. Upon evaporating off the, the crude product was purified by gradient column chromatography on silica gel with hexane:methylene chloride 8:1 to 6:1 (v/v) as the eluent to yield 5-5 (0.53
1 g, 46 %) as a dark-red solid. H NMR (400 MHz, CDCl3, 298K): δ (ppm) 8.22-
8.21 (m, 2H), 7.99-7.98 (m, 2H), 7.78-7.68 (m, 8H), 7.53-7.52 (m, 2H), 7.49-7.19
(m, 4H), 6.10-6.00 (m, 1H), 5.14-5.05 (m, 2H), 3.53-3.51 (d, J = 5.6 Hz, 2H),
2.30-1.88 (m, 8H), 1.05-0.60 (m, 24H), 0.40-0.30 (m, 12H).
4-(3-(7-(5-(7-(5-(9,9-Bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-2,1,3-benzothia- diazol-4-yl)-thien-2-yl)-9,9-bis(2-methylbutyl)fluoren-2-yl)propyl)benzaldehyde,
5-6.
Into a solution of allyl compound 5-5 (0.50 g, 0.53 mmol) in THF (4 mL) was added 9-BBN (3.16 mL, 0.5M in THF) at 0oC. The mixture was stirred at 35oC for two days before transferring to a mixture of 4-bromobenzaldehyde (0.487g, 2.64 mmol), Pd(PPh3)4 (0.026 g, 0.023 mmol), potassium carbonate (0.69 g, 5 mmol), water (2.5 mL) and THF (5 mL). The reaction mixture was stirred at 85oC under argon for two another days. After cooling to room temperature, methylene chloride (30 mL) was added for washing with water and brine before drying over
MgSO4. Upon evaporating off the mixed solvents, the crude product was purified 142
by gradient column chromatography on silica gel with hexane:methylene chloride
9:1 to 4:1 (v/v) as the eluent to yield 5-6 (0.32 g, 57 %) as a dark-red solid. 1H
NMR (400 MHz, CDCl3, 298K): δ (ppm) 10.04 (s, 1H), 8.22-8.21 (d, J = 2.0 Hz,
2H), 7.99 (s, 2H), 7.88-7.86(d, J = 7.6 Hz, 2H), 7.80-7.68 (m, 8H), 7.54-7.52 (m,
2H), 7.47-7.20 (m, 7H), 2.83-2.79 (m, 4H), 2.26-1.93 (m, 10H), 1.05-0.60 (m,
24H), 0.40-0.30 (m, 12H).
N-Methyl-2’-(4-(3-(7-(5-(7-(5-(9,9-bis(2-methylbutyl)fluoren-2-yl)thien-2-yl)-
2,1,3-benzo-thiadiazol-4-yl)thien-2-yl)-9,9-bis(2-methylbutyl)fluoren-2-yl)propyl) phen-2-yl)pyrrolidino-[3’, 4’:1,2][60]fullerene, Dyad.
Toluene (200 mL) was added to a mixture of benzaldehyde compound (5-6) (0.32 g, 0.31 mmol), [60]fullerene (0.24 g, 0.33 mmol) and sarcosine (0.30 g, 3.33 mmol). The reaction mixture was stirred in an oil bath at 130oC overnight followed by cooling down to room temperature for washing with water and then drying over MgSO4. Upon evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel with hexane:toluene 2:1 to 1:1 (v/v) as the eluent to yield Dyad (0.20 g, 37 %) as a dark powder. 1H NMR
(400 MHz, CDCl3, 298K): δ (ppm) 8.16 (d, J = 3.2 Hz, 2H), 7.93 (s, 2H), 7.73-
7.66 (m, 8H), 7.60-7.58 (d, J = 7.6 Hz, 10H), 7.48-7.46 (m, 2H), 7.42-7.13(m, 7H),
5.00-4.98 (d, J = 9.6 Hz, 1H), 4.92 (s, 1H), 4.27-4.25 (d, J = 9.6 Hz, 1H), 2.83 (s,
3H), 2.67-2.64 (m, 4H), 2.25-1.93 (m, 10H), 0.94-0.81 (m, 8H), 0.67-0.54 (m,
16H), 0.34-0.24 (m, 12H). MALD/I TOF MS (DCTB) m/z ([M]-):1801.6. Calcd for C132H79N3S3: C, 87.92; H, 4.42; N, 2.33. Found: C, 87.58; H, 4.30; N: 2.33.
143
[6,6]-phenyl C61-butyric acid methyl ester, PCBM.
Acquired from Nano-C (Westwood, MA) at 99.5% purity. 1H NMR (400 MHz,
CDCl3, 298K): δ (ppm) 8.00-7.97 (q, J = 2.0 Hz, 2H), 7.62-7.58 (m, 2H), 7.55-
7.53 (m, 1H), 3.73 (s, 3H), 2.99-2.95 (m, 2H), 2.60-2.56 (t, J = 7.6 Hz, 2H), 2.26-
2.22 (m, 2H).
Bulk-Phase Thermal Transitions and Morphologies.
Thermal transition temperatures were determined by DSC (Perkin-Elmer
DSC-7) with a continuous N2 purge at 20 mL/min. The three pure components
OFTB, PCBM, and Dyad were preheated to 310oC followed by quenching to –
30oC for thermal analysis by DSC at a heating rate of 20oC/min. One set of blends was preheated to 310oC followed by quenching to –30oC before recording their heating scans at 20oC/min. The other set of blends was preheated to 310oC followed by quenching to –30oC, and then annealed at 10oC above their respective glass transition temperatures for 12 h. Heating scans were also collected at
20oC/min after cooling to room temperature. The nature of phase transitions was characterized by hot-stage POM (DMLM, Leica, FP90 central processor and FP82 hot stage, Mettler Toledo). The powder samples for X-ray diffraction analysis were preheated to 310oC followed by quenching to room temperature and then annealing at 103oC for 12 to 48 h with subsequent cooling to room temperature.
X-ray diffraction data for powder samples on (100) silicon wafer were collected in reflection mode geometry using a Rigaku D2000 Bragg-Brentano diffractometer equipped with a copper rotating anode, diffracted beam graphite monochromator 144
tuned to CuKα radiation, and scintillation detector. Diffraction patterns were collected at 2θ from 2 to 30º with a step size of 0.02º.
Film Preparation and Characterization.
Sample mixtures at predetermined mass ratios were spin-coated onto microscope glass slides from 1 wt% chloroform solutions at 1000 rpm followed by drying under vacuum. The thicknesses of the resultant films were determined by optical interferometry (Zygo New Views 5000). Thermal annealing was performed under argon at 10oC above glass transition temperatures for 12 h followed by cooling to room temperature. The phase and topography images of both the pristine and thermally annealed films were recorded on a Nanoscope III
(Digital Instrument, CA) AFM in tapping mode with a NT-MDT NSG01 cantilever at a typical scan rate of 0.5 Hz under ambient condition. X-ray diffraction data for thin films on microscope glass slides were collected using the same instrument under the same conditions as described above.
5.3 RESULTS AND DISCUSSION
Depicted in Chart 5.1 are the molecular structures of the three components employed in this study. The synthesis and purification procedures together with the characterization data for OFTB and Dyad are described in the Experimental
Section, while PCBM was used as received from a commercial source at a purity level of 99.5%. We chose to work with OFTB in light of a recent report on organic photovoltaics using its long-chain polymer analogue as the electron-donor 145
and PCBM as the electron-acceptor.[9] As indicated by the DSC data accompanying molecular structures, OFTB has a glass transition temperature, Tg,
o o at 81 C followed by crystallization at 165 C and a crystalline melting point, Tm, at
o o o 222 C; Dyad a Tg at 181 C; and PCBM a crystalline transformation at 274 C and
o a Tm at 292 C.
G I 10:0:10 G + K K + I I
G I G I 9:2:9
G I I 7:6:7
Endothermic G
0 100 200 300 400 o Temperature, C
Figure 5. 1. DSC heating scans of OFTB:Dyad:PCBM at three compositions as indicated. Dashed curves: samples preheated to 310oC and quenched to –30oC before heating at 20oC/min. Solid curves: samples preheated to 310oC and o o quenched to –30 C before annealing at 10 C above respective Tgs, as determined with dashed curves, for 12 h and then cooled to room temperature before heating at 20oC/min. Complete transition to isotropic liquid for the annealed 10:0:10 blend occurred at 242oC as indicated by an arrow. Symbols: G, glassy; K, crystalline; I, isotropic.
Three blends, OFTB:Dyad:PCBM at 10:0:10, 9:2:9 and 7:6:7 mass ratios, were prepared by co-dissolution in chloroform for an investigation of their morphologies. Thoroughly dried powders were characterized by DSC for their thermal transition temperatures, and the results are shown in Figure 5.1 in which 146
phases were identified by POM micrographs. Presented as dashed curves, preheated and thermally quenched samples of the 10:0:10, 9:2:9 and 7:6:7 blends
o exhibit single Tgs at 93, 105, and 118 C, respectively, which is consistent with compositional uniformity as to be corroborated by AFM of spin-cast films shown in Figure 5.6 below.
quenched 10:0:10
annealed 10:0:10
Intensity, a. u. annealed PCBM annealed OFTB
0102030 2 Theta, degrees
Figure 5. 2. Samples of OFTB:Dyad:PCBM at 10:0:10 mass ratio, pure PCBM and OFTB were preheated to 310oC and then quenched to room temperature before annealing at 103oC for 48 h followed by cooling to room temperature for powder XRD analysis. Weaker crystalline diffraction peaks resulted from a shorter annealing time, e.g. 12 h. The annealing temperature was placed at 10 and o o 22 C above Tg of the blend and OFTB, respectively, and 189 C below the Tm of PCBM. A quenched but unannealed 10:0:10 blend was also characterized for identification of broad amorphous peaks.
147
300 L C
o 250 L + SOFTB L + SPCBM 200
150 Temperature, Temperature, SOFTB + SPCBM
100 0 20406080100 PCBM wt%
Figure 5.3. The OFTB:PCBM phase diagram constructed with the DSC thermograms at 20oC/mim and hot-stage POM for phase identification. With the exception of PCBM, all samples were preheated to 310oC, quenched to –30oC, o and then annealed at 10 C above their respective Tgs for 12 h to maximize crystallization. The samples were then cooled at –20oC/min to room temperature for collecting heating scans at 20oC/min. Preheating PCBM to 310oC followed by quenching to –30oC did not result in glass transition, thereby obviating thermal o annealing at 10 C above its Tg to further induce crystallization. The liquid lines were constructed with the end melting (open circles) and end dissolution (solid circles) temperatures for neat and excess components, respectively. Open triangles represent crystallization temperatures for the specified compositions. Symbols S and L represent solid and liquid regimes, respectively.
The preheated and thermally quenched blends were then annealed at 10oC above their respective Tgs for 12 h followed by cooling to room temperature. The acquired DSC thermograms are shown as solid curves in Figure 5.1. Thermal annealing of the 9:2:9 and 7:6:7 blends resulted in DSC thermograms identical to those of unannealed samples, whereas thermal annealing of the 10:0:10 blend led to partial crystallization with a sharp melting peak at 205oC followed by a 148
complete transition to an isotropic liquid at 242oC. The cooling scans were also gathered at –20oC/mim for all three blends following heating scans. The absence of crystallization during cooling to room temperature suggests that the crystals that melted from 205 to 242oC have resulted exclusively from thermal annealing.
The origin of the sharp crystalline melting at 205oC was further examined by powder XRD as shown in Figure 5.2 to differentiate between co-crystallization and the formation of a eutectic mixture.
Partial crystallization in the annealed 10:0:10 blend is evidenced by the presence of several narrow diffraction peaks attributable to crystalline components on top of two broad amorphous peaks centered at 10.4 and 19.6 degrees inferred from a control experiment with an unannealed blend. Although amorphous phases have no crystalline short or long range order, there still exists a range of most probable distances between neighboring atoms as well as defined bond distances for functional groups, such as fullerene, phenyl ring, and so on. This semi-short- range order is responsible for the broad peaks observed in diffraction patterns of amorphous materials. The powder XRD pattern of the semi-crystalline binary blend represents a superposition of diffraction patterns of OFTB and PCBM, indicating that these two pure components crystallized independently during thermal annealing. Using the Scherrer technique,[10] the average crystallite size was estimated at 26 and 23 nm for OFTB and PCBM, respectively, both greater than the minimum size of 2.5 nm for a crystallite to exhibit its characteristic XRD pattern.11 It is concluded that OFTB and PCBM comprise a eutectic mixture with
o o a Tm at 205 C, below those of the two pure components, 222 C (OFTB) and
292oC (PCBM). The phase diagram was constructed for the OFTB:PCBM binary 149
system as shown in Figure 5.3, a behavior similar to that of the binary blend of poly(3-hexylthiophene) with PCBM.[12]
(a) (b)
100 μm 100 μm Intensity, a. u. Intensity, a. a. u. Intensity, 10:0:10 9:2:9
246810246810 2 Theta, degrees 2 Theta, degrees
Figure 5.4. XRD patterns (collected at 0.02º/step), 10 s per step and POM micrographs as the insets for 100-nm-thick spin-cast films of OFTB:Dyad:PCBM blends at (a) 10:0:10 mass ratio and (b) 9:2:9 mass ratio o after thermal annealing at 10 C above their respective Tgs for 12 h followed by cooling to room temperature. Essentially the same XRD and POM results were observed for the 7:6:7 film as reported in (b) for the 9:2:9 film. Preheating to o 310 C as conducted for powders was avoided to preserve film integrity.
Approximately 100-nm-thick films of the three ternary blends were prepared by spin coating from chloroform followed by drying in vacuo at room temperature.
The pristine films appeared amorphous, according to POM micrographs (Figure
A4.1a-c). The combination of XRD pattern and POM micrograph shown in Figure
5.4a indicates that the 10:0:10 film turned polycrystalline after thermal annealing
o o at 10 C above its Tg for 12 h. The diffraction peak at 2θ=4.37 appearing in the film XRD pattern is also present in the powder XRD patterns of OFTB and the
10:0:10 blend identified by arrows in Figure 5.2. Furthermore, the average 150
crystallite size in the annealed film was estimated at 30 nm, in agreement with 26 nm estimated for OFTB crystallites in the annealed powders of 10:0:10 blend.
Compared to powder XRD, film XRD produced a pattern with fewer diffraction peaks at 2θ less than 10o and no peaks between 10 and 30o. This result is attributed to the likely preferred orientation of crystallites such that one set of lattice planes lies parallel to the sample surface thus in the right geometry to be observed during data collection. The reduced sample volume of the thin film compared to that of the powder and the limited scattering power of organic materials comprised of low atomic-number elements, such as hydrogen and carbon, are additional contributing factors to fewer diffraction peaks in the
10:0:10 thin film than its powder. As revealed by XRD pattern and POM micrograph shown in Figure 5.4b, the 9:2:9 and 7:6:7 films remained amorphous
o after thermal annealing at 10 C above their respective Tgs for 12 h.
Spin-cast films were further characterized by AFM for their compositional and/or morphological characteristics. In principle, phase contrast can be attributed to phase separation arising from lateral variations in composition or morphology regardless of topography.[13] Empirically, the features observed under transmission electron microscopy have been employed to establish AFM phase contrast as a valid tool for elucidating phase separation in block copolymers [14, 15], binary polymer blends [15], and PCBM-polymer blends. [16] Phase images are compiled in Figure 5.5 for films before and after thermal annealing at 10oC above
Tg for 12 h with subsequent cooling to room temperature. Without encountering crystallization, the cooling process served to kinetically trap in solid state the film morphologies prevailing under the annealing conditions. 151
Z-scale (a) (b) (c) 20 o
0 o 5 μm 2 μm 2 μm Z-scale (d) (e) (f) 20 o
0 o
10:0:10 9:2:9 7:6:7
Figure 5.5. AFM phase images of 100-nm-thick spin-cast films of OFTB:Dyad:PCBM at three compositions before (a, b, c) and after (d, e, f) o thermal annealing for 12 h at 10 C above their respective Tgs followed by cooling to room temperature. Preheating to 310oC as conducted for powders was avoided to preserve film integrity. Featureless phase images of (a), (b), (c), and (f) represent the absence of phase separation down to about 1 nm.
Phase angle Phase
0.0 0.5 1.0 1.5 2.0 Distance, μm
Figure 5.6. Section analysis of the phase image of a 100-nm-thick spin-cast film comprising OFTB:Dyad:PCBM at a mass ratio of 9:2:9 after thermal annealing o at 10 C above Tg for 12 h followed by cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity. The domain sizes are about 30 nm. 152
The pristine films are compositionally uniform, according to Figures 5.5a, b, and c, in addition to being amorphous under POM. Upon thermal annealing, the 10:0:10 film was found to crystallize, appearing as phase contrast shown in Figure 5.5d, with a melting range from 208 to 242oC under hot-stage POM, an observation in agreement with the bulk phase diagram within experimental error. The 7:6:7 film remained amorphous after thermal annealing, according to its DSC thermogram,
POM micrograph, and XRD pattern, and compositionally uniform according to
Figure 5.5f versus 5.5c.
Because of the difference both in geometry and molecular-level interaction,
OFTB and PCBM are expected to be partially miscible at best. As indicated by the optical micrograph and XRD pattern, the 9:2:9 film remained amorphous after thermal annealing. Hence, the phase contrast shown in Figure 5.5e is interpreted as compositional separation into two interspersed amorphous domains. Section analysis was performed to yield about 30 nm on average for both amorphous domains at approximately equal fractions (see Figure 5.6). Whereas the observed film morphology remained intact when left at room temperature for up to six months, its long-term morphological stability has yet to be systematically investigated in terms of chemical composition and thermal treatment prior to cooling to room temperature. The AFM phase images shown in Figure 5.5e resemble the transmission-electron and optical micrographs of microemulsions in ternary polymer blends [5, 6] and the cryo-transmission electron micrographs of traditional oil-surfactant-water microemulsions [17] including domain sizes, all observed at a surfactant content of about 10 wt%. At such a low content, it is plausible that facilitated by the flexible spacer, Dyad molecules orient themselves 153
at the interface between the OFTB-rich and the Dyad-rich amorphous domains as visualized in Figure 5.7. The envisioned molecular self-organization is rationalized by geometric packing of Dyad molecules at the interface for the sake of enthalpy at the expense of entropy of mixing with an optimum interfacial energy, thus rendering a negative Gibbs free energy change accompanying the formation of microemulsions.
Figure 5.7. A schematic diagram of Dyad acting as a geometric surfactant to modulate phase separation between OFTB and PCBM.
This behavior is akin to that of a traditional surfactant promoting the formation of microemulsions on the basis of polar-nonpolar amphiphilicity.[2, 18]
Another analogy pertains to microemulsion formation induced by diblock copolymer as a macromolecular surfactant between two parent homopolymers
[3-7] through Flory-Huggins interaction. That a single Tg emerged from the thermally annealed 9:2:9 blend is understandable with the detection limit of 20-50 nm in length scale afforded by the conventional DSC instrument [18-20] employed in the present study. The results reported here represent the first demonstration of phase separation between a conjugated oligomer and a C60-derivative modulated by a dyad with geometric amphiphilicity. It is remarked in passing that the AFM topographic images compiled in Figure 5.8 mirror the phase images in Figure 5.5. 154
In particular, it is noted that nanoscale phase separation shown in Figure 5.5e is responsible for the surface roughness observed in Figure 5.8e.
Z-scale (a) (b) (c) 15 nm
0 nm 5 μm 2 μm 2 μm Z-scale (d) (e) (f) 15 nm
0 nm
10:0:10 9:2:9 7:6:7
Figure 5.8. AFM topographic images of 100-nm-thick spin-cast films of OFTB:Dyad:PCBM at three compositions before (a, b, c) and after (d, e, f) o thermal annealing at 10 C above their respective Tgs for 12 h followed by cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity. The root-mean square protrusions are 0.33, 0.25, and 0.21 nm for a, b, and c, respectively, and 5.6, 3.1, and 0.23 nm for d, e, and f, respectively.
5.4 SUMMARY
Based on polar-nonpolar amphiphilicity, traditional surfactants are capable of solubilizing hydrophobic solutes in water and mediating microstructure formation between oil and water. From an entirely different perspective, diblock copolymers can induce self-organization between the two parent homopolymers into similar 155
microstructures by exploiting Flory-Huggins interaction. Comprising a rod-like and a disk-like moiety, a geometric amphiphile has been demonstrated for co- solvency. In the present study a geometric amphiphile, Dyad, consisting of C60 and π-conjugated oligomer moieties linked by a propylene spacer was tested for its ability to modulate phase separation between OFTB and PCBM using DSC,
POM, XRD and AFM techniques. Key observations are recapitulated as follows.
Powder XRD patterns indicate that OFTB and PCBM crystallized independently
o from the 1:1 blend upon thermal annealing at 10 C above its Tg for 12 to 48 h. A
o eutectic mixture emerged with a Tm at 205 C, which is definitively lower than those of OFTB and PCBM at 222 and 292oC, respectively. Thermal annealing of a spin-cast film of OFTB:Dyad:PCBM at a mass ratio of 9:2:9 at 10oC above its
Tg for 12 h produced an amorphous film according to POM and XRD. The analysis of its AFM phase image concluded phase separation into two interspersed
30-40 nm domains at approximately equal fractions. Geometric surfactancy is therefore inferred from the formation of microemulsions in analogy to widely reported traditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 film under a similar condition resulted in compositional uniformity across an amorphous film as shown by POM, XRD and
AFM. 156
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[10]. Jenkins, R. and Snyder, R.L. Introduction to X-ray Powder Diffractometery;
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New York, 1967: pp17-9.
[12]. Müller, C.; Ferenczi, T. A. M.; Campoy-Qiles, M.; Frost, J. M.; Bradley, D.
D. C.; Smith, P.; Stingelin-Stutzmann, N.; Nelson, J. Adv. Mater. 2008, 20,
3510-3515.
[13]. Garcia, R.; Magerle, R.; Perez, B. “Nanoscale compositional mapping with
gentle forces.” Nat. Mater. 2007, 6, 405-411.
[14]. Zhang, Q. L.; Tsui, O. K. C.; Du, B. Y.; Zhang, F. J.; Tang, T.; He, T. B.
Macromolecules 2000, 33, 9561-9567.
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Chem. Int. Ed. 2006, 45, 3364-3368
[16]. van Duren, J. K. J.; Yang, X. N.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval,
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CHAPTER 6
CONCLUSIONS AND FUTURE STUDIES
6.1 CONCLUSIONS
Under the general theme of organic electronics, this thesis aims at materials development via molecular design, materials synthesis and processing, device fabrication and characterization. Monodisperse glassy-nematic conjugated oligomers were synthesized for processing into monodomain glassy-nematic films through thermal and solvent-vapor annealing. Non-conjugated hybrid compounds with a high triplet energy and bipolar charge transport capability were developed for use as host materials in phosphorescent organic light emitting diodes. Bulk heterojunction organic photovoltaic cells were fabricated using a P3HT:PCBM blend to investigate the effects of active layer thickness and thermal annealing on the device performance. In addition, a conjugated oligomer-fullerene dyad was synthesized to modulate the extent of phase separation between spherical and rod-like molecules on the basis of geometric surfactancy. Major accomplishments emerging from this thesis research are recapitulated as follows.
A series of oligo(fluorene-co-bithiophene)s, OF2Ts, have been synthesized and characterized to investigate the effects of oligomer length and pendant 159
aliphatic structure on glassy-nematic mesomorphism. The OF2Ts comprising more than one repeat unit and their polymer analogue, PF2T, carrying 52 number-average repeat units, have a HOMO level at –5.3±0.2 eV, and the anisotropic field-effect mobilities increase with the oligomer length. Spin coating from high-boiling chlorobenzene with and without subsequent exposure to saturated chlorobenzene vapor constitute solvent-vapor annealing and quasi-solvent annealing, respectively. Solvent-vapor annealing yields monodomain glassy-nematic films in which OF2Ts are aligned as well as with thermal annealing. Quasi-solvent annealing, however, amounts to kinetically trapping a lower orientational order than solvent-vapor or thermal annealing.
While amenable to thermal annealing at elevated temperatures, PF2T shows no alignment at all following either strategy of solvent annealing.
Comprising hole- and electron-transporting moieties with flexible linkages, representative non-conjugated bipolar hybrids have been synthesized and characterized for a demonstration of their potential use as host materials for the fabrication of phosphorescent organic light-emitting diodes. The advantages of this material class include solution processing into amorphous films with elevated glass transition temperatures, stability against phase separation and crystallization, and provision of LUMO and HOMO levels and triplet energies contributed by the two independent moieties without constraint by the electrochemical energy gap. 160
While exciplex formation between the hole- and electron-transporting moieties is inevitable, its adverse effects on spectral purity and device efficiency can be avoided by trapping charges on triplet emitters, as demonstrated for Ir(mppy)3 in
TRZ-3Cz(MP)2, and TRZ-1Cz(MP)2. With these two bipolar hybrids and hole-transporting Cz(MP)2 as the hosts, the maximum current efficiency of the bilayer PhOLED is achieved with TRZ-3Cz(MP)2, but the driving voltage decreases monotonically with an increasing TRZ content.
Bulk heterojunction organic photovoltaic cells comprising an active layer of a
P3HT:PCBM blend film with a thickness varied from 130 to 1200 nm were fabricated for an investigation of the effects of active layer thickness and thermal annealing on device performance. Without thermal annealing, the best efficiency was obtained from thin-film devices. The short circuit current density decreases monotonically with an increasing active layer thickness, and an inverse spectral response is observed in thick-film devices. Thermal annealing at 110oC for 20 min decreases the 130-nm device efficiency from 2.9 to 1.1%, but increases that of the
1200-nm device from 0.4 to 2.6%. Additionally, thermal annealing also eliminates the inverse character of spectral response from the 1200-nm device. The variations in photovoltaic characteristics with film thickness and thermal annealing are interpreted in terms of vertical phase separation in the P3HT:PCBM film and Li+ diffusion from the Al/LiF cathode into the active layer. 161
A Dyad consisting of C60 linked to a conjugated oligomer by a propylene spacer was synthesized to demonstrate its ability to modulate phase separation between OFTB and PCBM using DSC, POM, XRD and AFM techniques. Upon
o thermal annealing at 10 C above its Tg for 12 to 48 h, the 1:1 blend of OFTB and
PCBM resulted in a eutectic mixture. Thermal annealing of a
o OFTB:Dyad:PCBM film with a 9:2:9 mass ratio at 10 C above its Tg for 12 h resulted in an amorphous film. Its AFM phase image indicated phase separation into two interspersed 30-nm amorphous domains at approximately equal fractions.
Geometric surfactancy is inferred from the formation of microemulsions in analogy to widely reported traditional oil-surfactant-water systems and ternary polymer blends. In contrast, thermal annealing of a 7:6:7 film under a similar condition resulted in an amorphous film with compositional uniformity.
6.2 FUTURE STUDIES
Materials development via molecular design, materials synthesis, device fabrication and characterization will continue to be of utmost importance to bringing organic electronics materials to fruition. Some of the potential future studies emerging from this thesis research are suggested below.
With the successful demonstration of uniaxial aligning glassy nematic offluorene-bithiophene cooligomers through solvent-vapor annealing, it is 162
desirable to further explore the potentials of this novel alignment technology in electronic devices. In addition to identifying additional conjugated molecules that are suitable for alignment by solvent-vapor annealing, it is of great interest to assess the opportunity of aligning materials on plastic substrates [1] other than rubbed polyimide on glass substrates.
The development of a wide variety of bipolar hybrid compounds establishes the foundation for pursing superior materials for efficient and stable PhOLED.
Multi-layer device with enhanced efficiency and stability can be fabricated with selected hybrid compounds through vacuum sublimation. Alternatively, attaching crosslinkable functional groups [2] onto these materials will enable the fabrication of solution-processed multi-layer devices. Furthermore , computational chemistry can be exploited to identify hole- and electron-transport moieties and flexible linkages with superior stability against decomposition during device operation.
The realization of efficient bulk heterojunction OPV with active layer thickness up to 1200 nm has dramatically widened the applicable thickness range for OPV. To further confirm the proposed morphology, it is impeative to carry out transmission electronic microscopic study on the film crosssection, and also to study the compositional depth profile through secondary ion mass spectroscopy characterization. The optimal composition of P3HT:PCBM blends could be highly dependent on the film thickness and morpholoy, and the device efficiency 163
could be further improved with compositional optimization. The concept of thick-film devices can also be explored in recently developed low-bandgap conjugated polymers [3] to further improve the device efficiency.
Molecular geometry is an important factor affecting molecular organization.
To further verify the microemulsion morphology identified by AFM, it is essential to carry out neutron scattering studies of ternary blends at selected compositions.
Systems with phase-separated morphology are not only actively pursued for OPV, but also attractive for proton exchange membranes in fuel cells [4], which are to be further explored.
References:
[1]. Choi, M. C.; Kim, Y.; Ha, C. S. Prog. Polym. Sci. 2008, 33, 581
[2]. Rehmann, N.; Ulbricht, C.; Kohnen, A.; Zacharias, P.; Gather, M. C.;
Hertel, D.; Holder, E.; Meerholz, K.; Schubert, U. Adv. Mater. 2008, 20,
129
[3]. Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709
[4]. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E.
Chem. Rev. 2004, 104, 4587 164
APPENDIX 1
1H NMR and MALD/I-TOF Mass Spectra for Chapter 2
165
SS
1 Figure A1.1. H NMR (400 MHz) spectrum of OF2T-1 in CDCl3 at 298 K. 166
SS SS
1 Figure A1.2. H NMR (400 MHz) spectrum of OF2T-2 in CDCl3 at 298 K. 167
SS SS
1 Figure A1.3. H NMR (400 MHz) spectrum of OF2T-3 in CDCl3 at 298 K. 168
SS SS
1 Figure A1.4. H NMR (400 MHz) spectrum of OF2T-4 in CDCl3 at 298 K. 169
SS SS
1 Figure A1.5. H NMR (400 MHz) spectrum of OF2T-5 in CDCl3 at 298 K. 170
SS SS
1 Figure A1.6. H NMR (400 MHz) spectrum of OF2T-6 in CDCl3 at 298 K. 171
SS SS SS
1 Figure A1.7. H NMR (400 MHz) spectrum of OF2T-7 in CDCl3 at 298 K. 172
SS SS SS SS SS
1 Figure A1.8. H NMR (400 MHz) spectrum of OF2T-8 in CDCl3 at 298 K. 173
Figure A1.9. MALD/I TOF MS spectrum of OF2T-1 using DCTB as the matrix. 174
Figure A1.10. MALD/I TOF MS spectrum of OF2T-2 using DCTB as the matrix. 175
Figure A1.11. MALD/I TOF MS spectrum of OF2T-3 using DCTB as the matrix.
176
Figure A1.12. MALD/I TOF MS spectrum of OF2T-4 using DCTB as the matrix.
177
Figure A1.13. MALD/I TOF MS spectrum of OF2T-5 using DCTB as the matrix. 178
Figure A1.14. MALD/I TOF MS spectrum of OF2T-6 using DCTB as the matrix.
179
Figure A1.15. MALD/I TOF MS spectrum of OF2T-7 using DCTB as the matrix.
180
Figure A1.16. MALD/I TOF MS spectrum of OF2T-8 using DCTB as the matrix.
181
APPENDIX 2
1H NMR and MALDI/TOF Mass Spectra for Chapter 3
182
N N N N N
1 Figure A2.1. H NMR (400 MHz) spectrum of TRZ-2Cz(MP)2 in CDCl3 at 298K 183 N N N N N N N N N
1 Figure A2.2. H NMR (400 MHz) spectrum of TRZ-3Cz(MP)2 in CDCl3 at 298 K. 184 N N O N N O N N N N
1 Figure A2.3. H NMR (400 MHz) spectrum of OXD-2Cz(MP)2 in CDCl3 at 298 K. 185
Figure A2.4. MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as the matrix.
186
Figure A2.5. MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as the matrix.
187
N
N N
O N N N O N
N
Figure A2.6. MALD/I TOF MS spectrum of TRZ-1Cz(MP)2 using DCTB as the matrix.
188
APPENDIX 3
1H NMR and MALDI/TOF Mass Spectra for Chapter 5
189 OFTB
1 Figure A3.1 H NMR (400 MHz) spectrum of OFTB in CDCl3 at 298 K.
190 Dyad
1 Figure A3.2. H NMR (400 MHz) spectrum of Dyad in CDCl3 at 298 K.
191
Figure A3.3. MALD/I TOF MS spectrum of OFTB using DCTB as the matrix
192
Figure A3.4. MALD/I TOF MS spectrum of Dyad using DCTB as the matrix
193
APPENDIX 4 POM Images for Chapter 5
194
(a) (b) (c)
100 μm 100 μm 100 μm
(d) (e) (f)
100 μm 100 μm 100 μm
10:0:10 9:2:9 7:6:7
Figure A4.1 Polarizing optical micrographs of 100-nm-thick films comprising
OFTB:Dyad:PCBM at three compositions before (a, b, c) and after (d, e, f) thermal
o annealing at 10 C above their respective Tgs for 12 h followed by cooling to room temperature. Preheating to 300oC as conducted for powders was avoided to preserve film integrity.