Excitonics for Organic Electronics by Grayson Ingram A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Materials Science and Engineering University of Toronto © Copyright by Grayson Ingram 2018 Excitonics for Organic Electronics Grayson Ingram Doctor of Philosophy Department of Materials Science and Engineering University of Toronto 2018 Abstract Organic semiconductors have immense potential as replacements for traditional inorganic materials in optoelectronics applications, in particular for organic light-emitting diodes (OLEDs). At the core of the device physics governing the stability and efficiency of OLEDs are tightly bound electron-hole pairs known as excitons. Here, the link between OLED excitonics and operational stability is studied in active OLEDs. First, the exciton distribution is investigated in active OLEDs with doped and undoped emissive layers. In both types of OLEDs, a surprisingly narrow exciton formation zone was measured given the bipolar nature of the materials used. Next, the influence of defects on exciton diffusion is investigated. The effective singlet exciton diffusion length is measured as a function of defect concentration and operational conditions and described in a unified model. Exciton-defect interactions are central to the efficiency and stability of OLEDs; consequently, quantifying these interactions under realistic operating conditions is a major step towards a comprehensive understanding of OLED excitonics. Finally, building on these results, a simple model for singlet exciton driven degradation in OLEDs in presented. This model accounts for the time and current density dependence of the host and defect emission in degrading OLEDs and is thoroughly validated using new experimental data, as well as literature data from leading academic and industrial research ii groups. Results of a first look at applying these conclusions to photovoltaic devices are also included. There exists an abundance of evidence in the literature suggesting singlet exciton driven degradation is a major degradation mechanism. Undoubtedly, providing mathematical tools to quantify this process will be invaluable. iii Table of Contents Table of Contents ........................................................................................................................... iv List of Tables ................................................................................................................................. vi List of Figures ............................................................................................................................... vii Abbreviations ............................................................................................................................... xiii Chapter 1 Introduction .................................................................................................................... 1 Chapter 2 Experimental Methods ................................................................................................. 31 Chapter 3 Exciton Diffusion in CBP ............................................................................................ 38 Chapter 4 Exciton Distribution in a Doped Emissive Layer OLED ............................................. 59 Chapter 5 Impact of Defects on Exciton Diffusion ...................................................................... 66 Chapter 6 Electroluminescence of Degrading CBP Films ............................................................ 82 Chapter 7 Excitonic Degradation of OLEDs ................................................................................ 96 Chapter 8 OPV Degradation ....................................................................................................... 102 Chapter 9 Conclusions and Future Work .................................................................................... 114 References ................................................................................................................................... 121 Appendix A: Boundary Condition Validation ............................................................................ 132 Appendix B: Supplementary Information for Chapter 5 ............................................................ 134 Appendix C: Equation Derivations ............................................................................................. 137 Appendix D: OLED External Degradation ................................................................................. 143 iv Appendix E: Supplementary Information for Chapter 6 ............................................................. 144 Appendix F: Environmental Contaminant Diffusion Simulation ............................................... 146 v List of Tables Table 1-1: Selected physical properties of germanium and anthracene .......................................11 Table 3-1: Parameter values used in the calculation of OLED emission .....................................48 Table 5-1: Extracted relative defect concentrations for three sets of OLEDs .............................77 Table 5-2: Extracted effective diffusion lengths and saturation intensities for three sets of OLEDs ..........................................................................................................................................79 Table 8-1: Bright EL areas as a fraction of total pixel area for OPV’s driven at 25mA/cm2 forward bias, and Jsc as a fraction of its value at the start of outdoor testing, measured 4, 43, and 163 days after the outdoor testing period ended ..........................................................................110 Table E-1: Details on extracted data from Figure 6-3 ...............................................................147 Table E-2: Fit metrics for fits to Equation 7-3 in Figure 7-1 .....................................................148 Table E-3: Fit metrics for fits to Equation 7-3 in Figure 7-2 .....................................................148 vi List of Figures Figure 1-1: Single layer OLED schematic depicting the processes of 1) charge injection 2) charge transport 3) exciton formation and 4) exciton decay accompanied by light emission .........6 Figure 1-2: The bilayer OLED structure reported by Tang and Van Slyke. Reproduced with permission. ......................................................................................................................................7 Figure 1-3: Energy level diagram of a generic multilayer OLED including hole injection layer, electron blocking layer, emissive layer, hole blocking layer and electron injection layer sandwiched between two electrodes. ..............................................................................................8 Figure 1-4: Schematic view of the p-electron cloud above and below the plane of an anthracene molecule. Reproduced with permission, copyright 1966 Scientific American Inc. .....................10 Figure 1-5: Simplified potential energy curves and vibrational wavefunctions and demonstration of Franck-Condon transitions with strength determined by wavefunction overlap. This leads to mirror symmetry of the emission and absorption spectra. Qi here is a configuration coordinate. Reproduced with permission, copyright 2015 John Wiley & Sons, Inc. ......................................16 Figure 1-6: Illustration of Wannier-Mott, Frenkel, and charge transfer excitons. Filled circles represent molecules. ......................................................................................................................18 Figure 1-7: Molecular state viewed a) through electron configuration and b) through exciton energy levels. The view taken in b) greatly simplified the intuitive picture of transitions between molecular states. Reproduced with permission, copyright 2015 John Wiley & Sons, Inc. ..........19 Figure 1-8: Jablonski-diagram, indicating the singlet and triplet manifold with vibrational levels. Also indicated are radiative and nonradiative transitions as arrows between different states. On the left side, a schematic absorption spectrum is indicated. Reproduced with permission, copyright 2015 John Wiley & Sons, Inc. ......................................................................................20 Figure 1-9: Illustration of the electron transitions involved in Förster and Dexter energy transfer. ........................................................................................................................................................23 vii Figure 1-10: Lifetime evolution of fluorescent (a) and phosphorescent (b) blue, green, and red OLEDs. The data points for the white OLEDs were not distinguished between stacked, hybrid, or separate phosphorescent/fluorescent devices. Reprinted with permission from Scholz, S., Kondakov, D., Lüssem, B. & Leo, K. Degradation mechanisms and reactions in organic light- emitting devices. Chem. Rev. 115, 8449–8503 (2015). Copyright 2015 American Chemical Society. ..........................................................................................................................................29 Figure 1-11: Dark spots growing on an OLED. Reprinted with permission from Smith, P. F., Gerroir, P., Xie, S., Hor, A. M. & Popovic, Z. Degradation of Organic Electroluminescent Devices . Evidence for the Occurrence of Spherulitic Crystallization in the Hole Transport Layer. 80, 5946–5950 (1998). Copyright 1998 American Chemical Society. .........................................30