Publication date: 11th March 2026
Since the achievement of high power conversion efficiencies (PCEs) in PM6:Y6-based organic solar cells (OSCs), increased attention has been devoted to understanding the fundamental photophysical processes governing device performance, with the aim of identifying and further minimizing remaining losses. Charge recombination remains a major loss pathway limiting PCE, and multiple recombination mechanisms have been identified. The most prevalent is bimolecular recombination between charge carriers residing on donor and acceptor molecules. In addition to this pathway, triplet-charge annihilation, involving the interaction between a triplet exciton and a charge carrier, has also been reported to contribute to efficiency loss.[1]
Both recombination processes are governed by spin-selection rules due to the spins associated with each charge carrier. Electrically Detected Magnetic Resonance (EDMR) can selectively probe these spins, and the processes in which the corresponding charge carriers are involved. Controlled manipulation of spins in EDMR experiments can therefore provide detailed insights into the local environment and mutual interactions of charged states during recombination processes in fully operational, miniaturized devices.[2]
In this work, we investigated OSCs based on a range of donor:acceptor blends, including two with non-fullerene acceptors (PM6:Y6 and PBDB-T:ITIC) and two with fullerene acceptors (PM6:PCBM and PBDB-T:PCBM). The spectral signatures of the individual donor and acceptor molecules exhibit a notable degree of overlap, in particular for the non-fullerene acceptor blends. Therefore donor-only and acceptor-only devices were additionally investigated to unambiguously disentangle the different contributions. Then, we turned our focus on the nature and dynamics of the recombination processes. By selectively manipulating the spin of acceptor charge carriers while monitoring the response of donor spins, a weak spin-spin coupling was identified, consistent with bimolecular recombination. The simultaneous observation of triplet exciton signatures provided clear evidence for the additional presence of a triplet-charge annihilation process. By exploring the role of different recombination processes, this study aims to provide new insights into the main performance-limiting loss mechanisms in a range of donor:acceptor blends.
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