Harnessing Vibrations for Efficient Exciton Dynamics in Semiconducting Energy Materials

Abstract

This dissertation describes our study of the fundamental role vibrations play in the excited-state dynamics of semiconducting energy materials. We examine these effects in self-assembled organic semiconducting nanostructures and small molecules, focussing on the implications for exciton transport, energy transfer, and light emission. Special use of ultrafast laser spectroscopy techniques such as impulsive vibrational spectroscopy and transient absorption microscopy is made to directly observe vibronic couplings and exciton transport. In self-assembled poly(3-hexylthiophene) nanofibers we observe exceptional exciton transport that cannot be explained with current models of exciton transport, despite low energetic and structural disorder. By directly measuring the excited-state vibrations, we are able to construct non-adiabatic simulations which reveal that zero-point motion enables access to delocalized states which mediate transport. This new transient delocalisation mechanism of transport can enable higher efficiencies and new device architectures. We follow this up by combining polyfluorene nanofibers with inorganic quantum rods for the purpose of energy transfer, and observe high levels of energy funnelling to the rods. Such behaviour has strong prospects for multielectron photocatalysis and upconversion. Finally, we assess the role of vibrations in the emission dynamics of several archetypal thermally-activated delayed-fluorescence emitters. We reveal their excited-state vibrations and track changes over time due to environmental relaxation. This serves to rationalize favourable emission bandwidths, low Stokes shifts, low non-radiative rates, and spin-orbit coupling enhancements. Our results challenge current pictures of exciton dynamics, and assert the varied and profound role vibrations have on properties such as energy transport and light emission. Traditionally, the uniquely strong vibrational couplings of organic semiconductors have been thought of as deleterious, but here they present themselves as an asset. For exciton transport especially, we propose design rules to harness vibrations which may enable the next generation of efficient optoelectronic devices.