DIRECT REAL-TIME REAL-SPACE IMAGING OF ENERGY TRANSPORT IN ORGANIC SEMI-CONDUCTORS

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.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Chen, Yuk Shek

Advisor dc:contributor.advisor

• Rao, Akshay

• optoelectronics
• transient absorption microscopy
• organic semiconductors
• energy transport

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/dd930beb-a251-4644-8778-97196425be10/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.56305

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/309207