Spectroscopic and Computational Studies of Morphology-Dependent Exciton Dynamics in TIPS-Pentacene Nanoparticles
Date
2022
Authors
Hudson, Rohan Joshua
Editors
Advisors
Kee, Tak W.
Huang, David M.
Huang, David M.
Journal Title
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Type:
Thesis
Citation
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Abstract
Singlet fission (SF) is an exciton multiplication process whereby a singlet exciton converts into two lower-energy triplet excitons, which presents opportunities for bypassing the theoretical effficiency limit of single-junction photovoltaic devices. However, a deeper understanding of the photophysical processes underpinning SF must be developed in order to meaningfully improve photovoltaic efficiencies. 6,13-(triisopropyl silylethynyl)pentacene (TIPS-Pn) has been extensively studied as a model system for intermolecular SF, but the complex relationship between solid-state morphology and exciton dynamics in this material has previously complicated interpretation of results from such models. In particular, nanoparticle (NP) dispersions are a popular medium for spectroscopic study of solid-state excitonic processes, but significant quantitative discrepancies exist in the literature regarding exciton dynamics within TIPS-Pn NPs. This thesis presents a series of spectroscopic and computational investigations aimed at clarifying the relationship between intermolecular morphology, NP structure and exciton dynamics within TIPS-Pn NPs. A novel mode of morphological control in these NP systems is investigated in Chapter 3, using poly(vinyl alcohol) as a chemical additive to convert the TIPS-Pn morphology from amorphous to crystalline. Experimental studies demonstrate that this transformation occurs rapidly and irreversibly through a surface-mediated interaction, and computational modelling suggests that a periodic dispersion interaction between hydroxyl- and TIPS-groups induces reorganization of the TIPS-Pn molecules. Characterization of this effect thus offers a reproducible and reliable strategy for accessing the crystalline phase of TIPS-Pn in NP models. Chapter 4 uses ultrafast spectroscopy and electronic structure calculations to quantify triplet exciton mobility in crystalline TIPS-Pn NPs. Careful study of diffusionlimited triplet–triplet annihilation demonstrates that triplet mobility is highly anisotropic along different crystallographic axes, with diffusion coefficients varying by over seven orders of magnitude. Predicted exciton diffusion lengths range from almost 0.5 μm to less than 1nm depending upon crystal orientation, with significant implications for the alignment of crystalline TIPS-Pn layers in device applications. The effect of particle size on exciton dynamics in TIPS-Pn NPs is explored in Chapters 5 and 6. Chapter 5 studies a series of different-sized amorphous TIPS-Pn NPs, and shows that both the rate and yield of SF vary substantially with NP size. Upon decreasing particle size, SF slows, triplet decay accelerates and an additional, previously undiscovered non-radiative singlet quenching channel becomes significant. All of these effects are ascribed to increased morphological disorder within these NPs at smaller sizes, hence indicating that not all NP sizes are reasonable models of bulk amorphous TIPS-Pn and care must be taken when interpreting results from such systems. Contrasting with this, Chapter 6 observes negligible changes in NP morphology or exciton dynamics with particle size for crystalline TIPS-Pn NPs. Kinetic Monte Carlo modelling shows that while size-dependent triplet decay kinetics are possible due to exciton confinement in very small NPs, this will only occur at conditions that are experimentally unlikely. Small crystalline TIPS-Pn NPs are therefore expected to be excellent model systems for bulk crystalline TIPS-Pn.
School/Discipline
School of Physical Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2022
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