Photo-physical Studies of Electron Acceptor Function in Organic Photovoltaics
Creators: name: Keiderling, Chaz
Publisher: name: Imperial College London
Year: 2017

description: Substantial progress has been made in achieving increasingly high organic photovoltaics (OPV) power conversion efficiencies. Progress has largely been derived from the development of new donor materials, as until recently a notable scarcity of successful electron acceptor materials remained. [6,6]-phenyl C60 butyric acid methyl ester (PC60BM) or other fullerene derivatives have been the centrepiece of the accepting materials used throughout OPV research, and they have only recently been accompanied by rare examples of novel non-fullerene alternatives. However, the current understanding of the properties of effective electron acceptors for OPV is still relatively limited. In this thesis, attention is paid to the photophysical properties of accepting materials with respect to charge photogeneration and recombination. The work is comprised of three closely linked studies. The first is a detailed exploration of the photophysical properties expressed by PCxBM. This is followed by a consideration of a newly synthesised PDI derivative, tasked with specific design rules to address prior limitations identified by previous perylene diimide (PDI) acceptors. The third study entails a broad comparison of a selection of accepting materials utilising bilayer fabrication to remove morphological variation. Chapter 3 outlines detailed understandings of the photophysical properties of PCxBM excitons. In particular, exciton characteristics are reported as a function of film morphology. Transient absorption measurements show a significant reduction in intersystem crossing and strong shifts in excited state absorption spectra in neat PCxBM films when compared to dispersed molecules within an amorphous polystyrene system. Additionally, neat PCxBM films show an increased formation of free charges when compared to the dispersed systems. An alternative model of excitonic formation from PCxBM systems between aggregated and dispersed systems is proposed. Chapter 4 introduces a newly synthesised amorphous perylene, PDISO, that is designed to remove the typical problem of aggregation in blends. Using a range of spectroscopic and microscopy techniques, the progression of absorption to charge formation is characterised for PCDTBT:PDISO blend films, drawing a direct comparison to the same processes in PCDTBT:PC70BM blend films. In this way, PDISO is demonstrated to overcome key loss mechanisms previously reported to limit efficiencies in PDI blends and determines the optimum blend ratio for charge generation with PCDTBT. Chapters 5 and 6 jointly establish an understanding of solution-processed bilayers, which is followed by a comparison across several acceptors with the two polymers PCDTBT &amp; PBDTTT-CT (chosen for their ability to generate charges). The comparison demonstrates that recombination dynamics fundamentally illustrate dramatic differences between acceptors. This provides evidence that morphologically independent charge dynamics can significantly affect overall device performance post separation. Correlations among the bilayer device performance, acceptor dielectric-mobility product and recombination dynamics are demonstrated. The outcomes in this thesis together draw a complex picture of multiple factors that affect the performance of electron-accepting materials in OPV. This provides a suitable platform for identifying important parameters when designing and testing new accepting materials. It also highlights potentially critical gaps in the current experimental understanding of fundamental charge interaction and recombination dynamics.