Time-and Polarization-Resolved Photoluminescence Studies of Directional Coupling in Isolated Semiconductor Nanostructures

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Labastide, Joelle A
organic semiconductors; time-resolved fluorescence; P3HT; Tetraazaterrylene; time-resolved polarization; Physical Chemistry
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TIME-AND POLARIZATION-RESOLVED STUDIES OF DIRECTIONAL COUPLING IN ISOLATED SEMICONDUTOR NANOSTRUCTURESMAY 2015JOELLE A. LABASTIDE, B.S. UNIVERSITY OF MASSACHUSESSTS AMHERSTPh.D., UNIVERSITY OF MASSACHUSETTS AMERSTDirected by Professor Michael D. BarnesDevelopment of new materials and assembly strategies for organic semiconductor-based optoelectronic materials is a problem of great interest worldwide, as researchers seek to resolve the questions pertinent to the creation of inexpensive, reliable, efficient, and stable active layer components. Organic semiconductors as the basis for photovoltaic active layers show significant promise for these applications. However, there is still much that needs to be understood about the molecular scale structural features that impact the function-essential processes such as exciton generation, diffusion, and dissociation, and charge transfer and transport, which are all strongly dependent on the ability of photogenerated species to move along and between chromophores within molecular assemblies[1-4], and sensitive functions of molecular scale structure[5-10] . This thesis will address some of the important materials science questions aimed at gaining a thorough understanding, and control, over the properties that promote or inhibit different functionalities. Specifically, how molecular-scale packing affects the nature, and directionality of chromophore coupling in organic semiconducting nanostructures, and how the strength and directionality of chromophore coupling affects movement of photogenerated charges through these materials, along specific directions.In order to address these questions, time-dependent polarization resolved photoluminescence experiments, enabling direct measurement of contrast resolved on picosecond timescales, were developed and explored in the context of two material platforms: single nanofibers of poly 3-Hexythiophene and single crystals of tetraazaterrylene. The combination of fine-timing resolution provided by our new technique and orientation information afforded by isolated nanostructures allowed the directional effects of chromophore coupling on exciton diffusion to be interrogated directly. Using this method, we discovered a fast equilibration processes that populates differently coupled chromophore states on a femtosecond timescale, the effect of small shifts in inter- vs. intra-chain coupling strengths in P3HT nanofibers can reduce the effective dimensionality of a nanostructure, and a direction specific charge transfer interaction in tetraazaterrylene extended crystals.