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DIRECT REAL-TIME REAL-SPACE IMAGING OF ENERGY TRANSPORT IN ORGANIC SEMI-CONDUCTORS 10.17863/CAM.56305 Chen, Yuk Shek Rao, Akshay Efficient energy transport and charge separation in organic semi-conductors are of vital importance in natural light harvesting for photosynthesis and have huge implications for designing a new generation of organic optoelectronic devices such as photovoltaics, light emitting diodes, sensors and so on. It has been a long-standing goal to understand the nature and mechanisms behind the movement of photo-excited species after the absorption of a photon in organic materials and across interfaces. In this thesis, we explore the energy transport and charge separation dynamics in two systems: a nanotubular J-aggregate formed from the self-assembly of molecular pseudoisocyanine (PIC), and a lateral heterojunction formed between a perylene diimide (PDI) and pentacene. Through femtosecond transient absorption microscopy (with sub-10 fs temporal and sub-10 nm spatial precision), supplemented by various other experiments and modelling, we show that ultrafast energy transport in the PIC systems can be achieved through strong light-matter coupling to form exciton-polaritons which have transport lengths of up to 250 nm at effective velocities of up to 5x10⁶ ms⁻¹. The formation of exciton-polaritons in robust cavity-free organic semiconductors opens up doors to a new generation of light harvesting devices. We also demonstrate a direct visualisation of ultrafast lateral charge separation and movement at the PDI-pentacene interface. We find that excitons proximal to the interface readily dissociate into free electrons and holes, with the latter injected into pentacene and may diffuse efficiently with a diffusion constant D in excess of 200 cms⁻¹, much larger than reported values for excitons in organic and inorganic semiconductors. The ability to visualise ultrafast charge separation at a junction with nanometre resolution will help to develop a more thorough understanding of the physics that underpins most modern optoelectronic devices. University of Cambridge 2020-02-18 Thesis Doctoral Doctor of Philosophy (PhD) eng https://www.repository.cam.ac.uk/handle/1810/309207 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/fa4f2398-9c2f-4a08-a8f5-3708e302940f/download 0abfd5dc4460827a73bff7ff93ef3d39 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/dd930beb-a251-4644-8778-97196425be10/download 353adac0d1ebdfd65ab16480263c3c87 https://www.rioxx.net/licenses/all-rights-reserved/ optoelectronics transient absorption microscopy organic semiconductors energy transport