Insights into the photovoltaic mechanism of organic solar cells from Electron Spin Resonance spectroscopy
Jack Palmer a, Claudia Tait a
a Department of Chemistry, University of Oxford, Oxford, UK
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
London, United Kingdom, 2023 June 12th - 14th
Organizers: Tracey Clarke, James Durrant and Trystan Watson
Poster, Jack Palmer, 281
Publication date: 30th March 2023

The remarkable success of non-fullerene acceptors in improving power conversion efficiencies of organic photovoltaics compared to earlier fullerene-based acceptors [1] has renewed interest in the details of the photovoltaic mechanism, and differences between the two types of acceptors. Since charge-transfer states, separated charges and triplet states involved in the photovoltaic mechanism are all paramagnetic, they can be characterised by electron spin resonance (ESR) spectroscopy.

By using the spins as probes, ESR spectroscopy can provide complementary information to more widely used optical characterisation techniques. In particular, the formation mechanism of photoinduced paramagnetic states is encoded in the observed spin polarisation of ESR spectra, a result of non-equilibrium population distributions. Based on the spin polarisation, ESR can distinguish between singlet- and triplet-born charge-transfer states, as well as between intersystem crossing and geminate recombination triplets.

We are using continuous-wave and transient EPR to investigate both fullerene- and non-fullerene-based blends by detecting and characterising paramagnetic states generated following photoexcitation and to examine the evolution of the spin states with time and identify differences in spin dynamics between the two types of blends.

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