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Title: Direct real-time real-space imaging of energy transport in organic semi-conductors
Author: Chen, Yuk Shek
ISNI:       0000 0004 9354 1921
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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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.
Supervisor: Rao, Akshay Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: optoelectronics ; transient absorption microscopy ; organic semiconductors ; energy transport