Investigating optical absorption in organic semiconductors using a coarse-grained approach
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Author(s)
Virbule, Alise
Type
Thesis
Abstract
Organic semiconductors have attracted great interest as candidate materials for solar cells, light-emitting diodes and other photonic applications. The performance of such devices depends upon the spectral range and strength of the optical absorption, as well as other material properties. Optical transition energies and strengths are controlled in turn by the chemical structure of the (macro)molecule and its conformation; however, the large conformational phase space of conjugated polymers means that the chemical structure – optical property relationship is not trivial to determine. In the solid state, the conformations and interactions of the molecules are a function of process conditions and hard to control, and yet they control the optoelectronic properties of the material. Whilst many studies have sought to develop and validate computationally efficient methods for prediction of transition energies, relatively few have addressed prediction of transition strength. Methods such as time dependent density functional theory are widely used to complement experimental studies but are too computationally expensive to access the length scales required to fully explain the observed structure-property relationships, especially for polymers with large repeat units or complex phase behaviour. Coarse-grained models of optical properties such as tight-binding exciton models can be used to examine the structure-property relationships at longer length scales, but these use approximations to the electron-hole interactions that do not accurately reproduce the experimentally observed trends in absorption strength as a function of molecular structure. I therefore implement a coarse-grained model using parameters from ab initio calculations as a tool to further our understanding of the relationship between the conformation of pi-conjugated macromolecules and their optical properties. I apply this approach to study two conjugated polymers and show how effects of conformational changes on their optical absorption spectra can be linked to changes in the parameters that characterise the electronic intermonomer interactions in the model.
Version
Open Access
Date Issued
2021-03
Date Awarded
2023-03
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Nelson, Jenny
Lischner, Johannes
Sponsor
Engineering and Physical Sciences Research Council (EPSRC)
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)