STRUCTURE-FUNCTION RELATIONSHIPS IN ORGANIC CHARGE-TRANSFER COMPLEXES BY KATELYN PATRICIA GOETZ A Dissertation Submitted to the Graduate Faculty of WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Physics May 2016 Winston-Salem, North Carolina Approved By: Oana D. Jurchescu, Ph.D., Advisor Laurie E. McNeil, Ph.D., Chair Natalie A. W. Holzwarth, Ph.D. George E. Matthews, Ph.D. Richard T. Williams, Ph.D. Acknowledgments Many people have been invaluable in the completion of my two Wake Forest University degrees. First and foremost, I would like to thank my advisor, Dr. Oana Jurchescu. She has been a great mentor in my academic career, and was always available to talk about science and life. I do not believe I would have had the opportunities I had during my graduate career if I had pursued it elsewhere. Thanks also to the entire Wake Forest Physics Department for inspiring and teaching me, and also for being living examples of what a good research career is and can be. I would especially like to thank Dr. Keith Bonin for (possibly unknowingly) providing the push I needed to major in physics by teaching an enthusiastic and interesting electronics class. Dr. Eric Carlson also deserves a thank you for organizing the society of physics students chapter in our department. I have very much enjoyed participating in science outreach through demo days at Sci Works and work with K-12 classrooms in the area. Eric Chapman has been invaluable throughout my nine years in the department for technical help and good conversations. The numerous people who have directly advanced my dissertation research through academic collaboration also have my gratitude. These include all members of the Ju- rchescu group from 2009-2016, but especially Jack Owen for being a good friend and fellow student. Additionally, my fellow graduate students Dr. Jeremy Ward, Dr. Yaochuan Mei, and Dr. Pete Diemer have set good examples as creative researchers and learners. Zach Lamport and Andrew Zeidell, later group members, have also been inspiring through good conversations, academic or otherwise. The papers I co- authored would not have been possible without the work of UNC researchers Dr. Laurie McNeil and Dr. Derek Vermeulen and the research group of Dr. Veaceslav Coropceanu at Georgia Tech. I would also like to thank the crystal growth group of Christian Kloc at Nanyang Technological University in Singapore for hosting and teaching me through the National Science Foundation (NSF) East Asia and Pacific Summer Institute. Dr. Tatsuo Hasegawa and Dr. Jun'ya Tsutsumi taught me quite a bit about optical spectroscopy and alternative methods of device fabrication at the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba, Japan. Dr. Curt Richter, Dr. David Gundlach, and Dr. Sujitra Pookpanratana at the National Institute of Standards and Technologies have earned my thanks for their collaborative work and conversations. Many thanks to all others who have worked on papers and projects with me. Finally, I would like to thank fellow graduate student Alex Taylor for being an excellent study partner and even better boyfriend. I hope to continue having fun ii with you for years to come. I would also like to thank my entire family, extended and immediate, for being an unending positive presence in my life. This includes Mom, Kathryn Goetz, Dad, Devon Goetz, and siblings, Kirsten, Maggie, Jake, and Jeff. Even our arguments are forces for bettering each other. To my siblings, I am excited to see what we can do as adults. iii Table of Contents Acknowledgments ii List of Figures vii List of Tables viii List of Abbreviations ix Abstract x Chapter 1 Introduction 1 1.1 Introduction . 2 1.2 General Characteristics of Charge-Transfer Complexes . 6 1.2.1 Donors and Acceptors . 6 1.2.2 Crystal Structure . 9 1.3 Electronic Properties of CT Complexes . 12 1.4 The Interplay Between Degree of Charge Transfer and Electrical Prop- erties . 19 1.5 Outlook and Outline of This Thesis . 24 References . 27 Chapter 2 Crystal Growth of Organic Charge-Transfer Complexes 40 2.1 Introduction . 41 2.2 Single Crystal Growth of Charge-Transfer Complexes . 42 2.2.1 Solution Growth . 42 2.2.2 Vapor Growth . 43 2.3 Crystals of Novel Organic Charge-Transfer Complexes . 46 2.4 Thin-Film Growth of Organic Charge-Transfer Complexes . 49 2.5 Summary . 50 References . 53 Chapter 3 Electrical Characterization of Organic Charge-Transfer Com- plexes 57 3.1 Introduction . 58 3.2 Space-Charge-Limited Current . 59 3.2.1 Low-Voltage Regime . 60 iv 3.2.2 Space-charge-limited Current . 61 3.3 Ambipolar Organic Field-Effect Transistors . 63 3.4 Comparison of Mobility Calculation by SCLC and OFET Measure- ments for an Ambipolar CT Complex . 69 3.5 Charge-Transport in Novel Organic Charge-Transfer Complexes . 72 3.5.1 Perylene{TCNQ in Three D:A Stoichiometries . 72 3.5.2 Anthracene and Pyrene with Acceptor PDIF-CN2 . 75 3.6 Summary . 77 References . 79 Chapter 4 The Effect of Librational Motion on Charge Transport in Stilbene{F4TCNQ 81 4.1 Introduction . 82 4.2 Methods . 84 4.2.1 Crystal Growth . 84 4.2.2 Structure Determination . 84 4.2.3 Sample Fabrication and Electrical Characterization . 85 4.2.4 IR and Raman Spectroscopy . 86 4.2.5 Computational Methodology . 87 4.3 Results . 88 4.3.1 Electrical Properties . 88 4.3.2 Crystal Structure and Electronic Structure Calculations . 90 4.3.3 Structure and Thermodynamics . 91 4.4 Discussion . 98 4.5 Conclusions . 101 References . 102 Chapter 5 The Effect of Polymorphism on Charge Transport in the Charge-Transfer Complex DBTTF-TCNQ 107 5.1 Introduction . 108 5.2 Experiment and Results . 111 5.2.1 Structural Characterization of DBTTF{TCNQ . 111 5.2.2 Degree of Charge Transfer . 116 5.2.3 Charge Transport in DBTTF{TCNQ Polymorphs . 120 5.3 Discussion . 123 5.4 Conclusions . 127 References . 128 Chapter 6 Summary 133 Chapter 7 Curriculum Vitae 135 v List of Figures 1.1 DBTTF-TCNQ SC-OFETs with various contact metals . 3 1.2 Examples of donors and acceptors . 7 1.3 Charge-transfer complex energetics . 9 1.4 π-stacking motifs in 1:1 CT complexes . 10 1.5 OFET device geometries . 14 1.6 Organic metal electrodes for organic CT complexes in OFETs . 15 1.7 Organic metal electrodes for organic CT complexes in OFETs . 18 1.8 TCNQ with bonds sensitive to charge-transfer . 19 1.9 Mixed and segregated-stack BEDT-TTF{TCNQ . 21 1.10 Conductivity versus Degree of Charge Transfer . 22 2.1 Color change upon CT complex formation . 43 2.2 Vapor Growth Furnaces . 44 2.3 Crystal growth of Perylene{TCNQ in multiple stoichiometries . 45 2.4 Green Anthracene-PDIF-CN2 Crystals . 47 2.5 Anthracene-PDIF-CN2 preliminary crystal structure . 48 2.6 Pyrene-PDIF-CN2 preliminary crystal structure . 48 3.1 Metal-Insulator-Metal Device Structures . 59 3.2 Current-Voltage characteristics for a metal-insulator-metal structure . 63 3.3 OFET Device Structure . 64 3.4 OFET Channel . 66 3.5 Ambipolar OFET Operation . 68 3.6 Ambipolar DBTTF-TCNQ OFET . 69 3.7 Ambipolar DBTTF-TCNQ SCLC Curve . 70 3.8 Crystals and structures of the Perylene{TCNQ CT complex system . 73 3.9 Current-Voltage characteristics the Perylene{TCNQ crystal system . 74 3.10 Optical reflectance of Anthracene and Pyrene{PDIF-CN2 . 76 3.11 Transfer integrals for Anthracene{PDIF-CN2 . 77 3.12 The Transport Properties of Pyrene{PDIF-CN2 . 78 4.1 Skeletal structures and crystals of stilbene, F4TCNQ, and STB{F4TCNQ 83 4.2 Thermal ellipsoid plots for STB{F4TCNQ . 86 4.3 Current-Voltage Characteristics for two-contact measurements of STB{ F4TCNQ . 88 4.4 Temperature dependent electrical characteristics of STB{F4TCNQ . 89 vi 4.5 Temperature dependence of the STB{F4TCNQ lattice constants and charge carrier effective masses. 90 4.6 Depiction of librationl motion in trans-stilbene . 92 4.7 CT thermal ellipsoids at 143 K, with and without disorder . 93 4.8 Fourier difference maps above and below the transition for STB{F4TCNQ 94 4.9 Libration of the C7=C7a stilbene bond in STB{F4TCNQ . 95 4.10 Degree of charge transfer (q) as a function of temperature. 97 4.11 Transfer integral dependence on the phonon displacements . 99 4.12 Temperature dependent electrical characteristics of related compounds 101 5.1 Crystalline polymorphs of DBTTF{TCNQ . 110 5.2 Selected area electron diffraction images for DBTTF{TCNQ polymorphs112 5.3 XPS for DBTTF{TCNQ polymorphs . 113 5.4 Polarized absorption spectra for DBTTF{TCNQ polymorphs . 114 5.5 Polarized IR spectra for DBTTF{TCNQ polymorphs . 116 5.6 Molecular orientation of DBTTF and TCNQ with respect to crystal surface . 117 5.7 Raman spectra for α and β-DBTTF{TCNQ . 118 5.8 Electrical characteristics of α-DBTTF{TCNQ . 121 5.9 Electrical characteristics of β-DBTTF{TCNQ . 122 5.10 UPS of DBTTF{TCNQ Polymorphs . 123 5.11 DFT calculations for DBTTF{TCNQ Polymorphs . 125 vii List of Tables 1.1 Properties of CT complexes used in OFETs . 16 1.2 CT complexes listed by q and σ ..................... 23 2.1 Solution growth methods of CT complexes . 51 2.2 Physical vapor transport growth methods of CT complexes and Parent Compounds . 52 viii List of Abbreviations Abbreviation Meaning HOMO Highest Occupied Molecular Orbital LUMO Lowest Unoccupied Molecular Orbital XRD X-ray Diffraction SCLC Space-charge-limited current OFET Organic Field-effect Transistor µ Charge-carrier mobility q Degree of charge transfer (unless otherwise listed) CT Charge Transfer D Donor A Acceptor ix Abstract Organic charge-transfer complexes are ordered combinations of charge-donating (D) and charge-accepting (A) compounds.