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Excited states in organic semiconductors: theoretical studies of optical spin injection, polaron formation and singlet fission

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Title: Excited states in organic semiconductors: theoretical studies of optical spin injection, polaron formation and singlet fission
Authors: Szumska, Anna Antonina
Item Type: Thesis or dissertation
Abstract: My research focuses on the modelling of excited states in organic semiconductors applied to three different areas, namely, optical spin injection singlet fission, and optical detection of polarons. One of the challenges in the field of spintronics is spin injection, which has been achieved optically in inorganic crystalline semiconductors, but not yet in organic semiconductors. Here, we apply group theory and computational methods to design molecular materials in which spin can be injected optically via circularly polarized light (CPL). When designing molecules for their optical excitation properties, additional design rules can be defined by considering the relationship between the symmetry of the molecule and its excited state properties using group theory. The theory reveals that molecules with C3h symmetry are good candidates, because they support circularly polarized transitions. Among the molecules with the correct symmetry using TDDFT calculation we are looking for candidates with low lying circularly polarized triplets corresponding to ms = +/-1 . To preserve the spin the triplet must be excited directly from the ground state, thus it is required to have a relatively high oscillator strength. We designed a series of molecules with C3h symmetry which were synthesized, some for the first time, by our collaborator. We present preliminary experimental validation of our approach with spectroscopic studies of the new family of molecules. The research shows how symmetry can be used in molecular design for spintronics applications. My second study concerns understanding the dynamics of singlet fission in organic semiconductors. Here I present a study on dynamics of singlet fission in diluted pentacene films and calculations which can explain the differences in quintet rates of dissociation between molecular pairs. We found out that the rate of the dissociation can be influenced by the degree of delocalization and charge-transfer character of the orbitals and excited states in the two different packing structures considered. My last study concerns understanding the behaviour of conjugated polymer electrodes operating in aqueous solution. We used calculations of absorption of polarons to quantify the depth of charge in polymer electrode. We used calculations of reaction potentials to identify likely side reactions that could limit the efficiency in the presence of oxygen.
Content Version: Open Access
Issue Date: Mar-2020
Date Awarded: Aug-2020
URI: http://hdl.handle.net/10044/1/82292
DOI: https://doi.org/10.25560/82292
Copyright Statement: Creative Commons Attribution NonCommercial No Derivatives Licence
Supervisor: Nelson, Jenny
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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