Molecular structure dependent electron-phonon coupling in organic semiconductors
File(s)
Author(s)
Pagano, Katia
Type
Thesis
Abstract
Organic semiconductors (OSCs) provide a promising route to the realisation of cost-effective, lightweight electronic devices. Their solution-processability, flexibility and tuneable optoelectronic properties make them suitable to produce scalable, high-performance devices, with the potential to replace inorganic-based technologies. OSCs are, however, intrinsically limited by their low dielectric constants, which induces strong electron-phonon coupling and large reorganisation energies. This results in strongly bound and highly localised excitons, which makes free charge carrier generation in organic solar cells difficult, and introduces a large energetic barrier to charge transport, which limits the charge carrier mobilities of OSCs. For continued improvement in OSC device performances and for their mass commercialisation to be realised, fundamental understanding of electron-phonon coupling, and the subsequent structure-property relationships are therefore essential. Experimental studies directly tracking electron-phonon coupling are lacking. Here, we employ a variety of steady-state and transient spectroscopy techniques in order to study the impact of molecular structure on electron-phonon coupling in a range of OSCs. We utilise a unique combination of advanced ultrafast spectroscopic techniques, including transient absorption spectroscopy (TAS) and femtosecond stimulated Raman spectroscopy (FSRS), to track ultrafast excited-state formation and the subsequent ultrafast photoinduced structural relaxation dynamics in conjugated polymers with different backbone structures and sidechain content. FSRS probes time-resolved molecular vibrational Raman spectra allowing for the ultrafast temporal tracking (<100 fs) of electron-phonon coupling, enabling the detailed investigation of the ultrafast structural dynamics of the molecular lattice following photoexcitation. Specifically, we study the impact of modifying ground-state molecular structure (backbone structure and sidechain content) and nature (homopolymer versus donor acceptor type polymer) in three different series of conjugated polymers. We observe molecular structure-dependent coupling to the excited state and directly track the phonon modes that couple to their electronic transitions.
Version
Open Access
Date Issued
2024-04-12
Date Awarded
02/11/2024
License URL
Advisor
Kim, Ji-Seon
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)