Charge and spin delocalisation in photoinduced polarons of polymer donors and non-fullerene acceptors for organic photovoltaics – a multi-frequency pulse EPR study

Abstract

Organic photovoltaics rely on efficient charge separation and transport, processes facilitated by charge delocalisation across the π-conjugated backbone of donor and acceptor molecules. By probing the interactions between the unpaired electron spin associated with photoinduced charged states and magnetic nuclei in their molecular environment, electron paramagnetic resonance (EPR) spectroscopy enables precise experimental quantification of spin and charge delocalisation. We first characterise the EPR spectral signatures of the positive and negative polarons on polymer donors and non-fullerene acceptors in the PBDB-T:ITIC and PM6:Y6 blends, as well as the corresponding fullerene-based blends PBDB-T:PC₆₁BM and PM6:PC₆₁BM, by multi-frequency EPR spectroscopy. Reliable separation of overlapping donor and acceptor signatures is enabled by EDNMR-induced EPR spectroscopy exploiting unique nuclear hyperfine couplings in the non-fullerene acceptors ITIC and Y6. Then, by combining the measurement of electron–nuclear hyperfine couplings by ENDOR with DFT modelling and a regularised least-squares fitting approach, we quantify the extent of spin and charge delocalisation. The experimental results reveal intramolecular delocalisation of the positive polarons on the PBDB-T and PM6 donors across approximately 6 nm. Delocalisation of the negative polarons, on the other hand, depends on the nature of the acceptor: for ITIC, the electron spin is found to be localised on a single molecule, whereas for Y6, contributions from spins localised on a single molecule, as well as spins delocalised over two adjacent molecules in different π–π stacked configurations, are required to explain the experimental ENDOR and HYSCORE data. Validation of computational predictions by experimental results is shown to be crucial for the accurate estimation of charge delocalisation, and therefore conclusions on its relevance in determining device efficiency in organic photovoltaics.