Impacts of microstructure and energetic offset on charge transfer processes in organic photovoltaics
File(s)
Author(s)
Müller, Jolanda Simone
Type
Thesis
Abstract
Organic photovoltaics have rapidly improved in efficiency over the past decades, yet key questions remain about structure–property relationships. We study donor–acceptor systems with varying energetic offsets and microstructure, focusing on materials designed for enhanced morphological stability. First, we examine the impact of energetic offset on charge generation in bulk heterojunctions based on PBDB-T derivatives. While smaller offsets reduce voltage losses, they impair charge separation below 0.45 eV, limited by both reduced exciton dissociation and charge transfer (CT) state dissociation rates. Next, we explore how the impact of offset varies in chemically bonded and non-bonded systems. For this, we compare conjugated block copolymers with all-polymer blends and blends with small molecule acceptors based on the same monomer building blocks. All systems display a trade-off between current generation and voltage loss. Block copolymers show the lowest voltage losses while all-polymer blends display remarkable charge generation even at low offsets. To understand the origin of the reduced voltage losses we perform quantum-chemical simulations on conformational ensembles of representative donor–acceptor dimers. Covalent bonding raises the CT state energy and increases hybridisation with local excitations, consistent with low non-radiative losses and reduced charge separation. Bonded systems need a higher ionisation energy offset between building blocks to achieve the same exciton to CT gap as non-bonded analogues. Increasing the in-built offset may therefore improve charge separation while maintaining low voltage losses. Finally, we examine how degradation influences voltage losses by analysing recombination in bulk heterojunction systems with varied morphological stability, controlled through side-chain engineering. Improved morphological stability preserves bulk recombination pathways and avoids interfacial recombination that occurs in less stable systems. Together, these results highlight that energetic alignment must be tuned to the microstructure. Morphology and electronic coupling jointly shape the excited-state landscape, and co-optimising both is key to advancing performance and lifetime.
Version
Open Access
Date Issued
2025-07-18
Date Awarded
2025-10-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Nelson, Jenny
Heeney, Martin
Sponsor
Engineering and Physical Sciences Research Council
Royal Society (Great Britain)
Grant Number
EP/T028513/1
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)