Investigation of materials and devices for organic photovoltaics by EPR and EDMR

This research project will study fundamental processes in materials and devices for organic photovoltaic systems through investigation by EPR (Electron Paramagnetic Resonance) and EDMR (Electrically Detected Magnetic Resonance) spectroscopy. It primarily falls within the EPSRC Analytical Science and Materials for Energy Applications research areas within the Physical Sciences theme and has practical applications to the Energy theme as well.
Organic photovoltaic systems involve a blend of donor and acceptor molecules that, after excitation by light, form singlet excitons that can then dissociate into a spin-correlated radical pair by charge transfer at a donor-acceptor interface. This radical pair is characterised by coupled unpaired electron spins that can be detected by EPR spectroscopy. Efficient charge separation over recombination of the radical pair is critical for an effective organic photovoltaic system.
In the past, EPR and EDMR spectroscopy have been used to investigate a range of different spin centres in silicon solar cells and in fullerene-based organic solar cells. Currently, research in organic photovoltaics is focused on non-fullerene acceptors that have led to increased energy conversion efficiencies. Photovoltaic cells based on non-fullerene acceptors have been showing tremendous growth in efficiency in the last decade at a much faster rate than similar fullerene-based acceptors. Since EPR and EDMR can follow the evolution from a charge-transfer state to separated charges, the aim of the project is to use these methods to gain a better insight into this process for donor- acceptor blends including state-of-the-art non-fullerene acceptors. Different continuous-wave and pulse EPR and EDMR experiments will be used to characterise the nature and dynamics of the spin centres in materials for organic photovoltaics. In particular, the interactions of the spin centres with their molecular environment, including hyperfine interactions to nearby magnetic nuclei and dipolar and exchange interactions between spin centres, will be investigated using advanced pulse EPR and EDMR techniques. The aim is to gain molecular-level insight into the charge separation, recombination, and transport processes relevant for solar cell operation and to identify molecular requirements for efficient energy conversion. Recent developments in digital electronics have enabled the integration of shaped pulses into pulse EPR sequences. Compared to traditional rectangular pulses, shaped pulses allow increased spin control and the design of new experiments with increased sensitivity and with increased accuracy in the characterisation of magnetic interactions. Shaped pulses have been demonstrated to achieve increased sensitivity for structural characterisation of biological systems by EPR but have so far not been used to aid the investigation of spin centres in materials for organic electronics. This project will involve development of EPR experiments tailored to exploit the advantages of shaped pulses in the characterisation of spins in these materials. Within EDMR, the use of shaped pulses is currently still largely unexplored, therefore, this project will also aim to develop the pulse EDMR method further through the inclusion of shaped pulses and the design of novel pulse sequences that leverage the increased spin control they provide. This project, at the interface of physics, chemistry, and materials science, is aligned with the EPSRC's focus on interdisciplinarity in research. The insights gained from EPR and EDMR spectroscopy on the fundamental processes involved in the conversion of solar energy to electricity in organic photovoltaics will have great significance for the field of solar energy technology. The developed methods and approaches are additionally relevant to many other research areas within the Physical Sciences theme, including optoelectronics, spintronics and quantum computing.