Publication date: 17th February 2025
Understanding and controlling the dynamics and interactions of photoinduced charged states between photoexcitation and charge extraction is key to improving Organic Solar Cell (OSC) efficiency. The two main processes following charge separation are charge transport and recombination; both involve unpaired electrons and are spin-dependent: to hop to the next localized state or to recombine with an opposite charge, the unpaired electrons’ spins must have a singlet (antiparallel) configuration. If they are in a triplet (parallel) configuration, transport or recombination is blocked.[1] Spin manipulation with microwaves allows controlled switching between these configurations, directly affecting device current, and is at the basis of Electrically Detected Magnetic Resonance (EDMR), an operando technique enabling valuable molecular-level insights into spin-dependent charge transport and recombination. [2]
EDMR measurements are performed by detecting changes in current flowing through a fully assembled miniature solar cell in an external magnetic field, following excitation with microwave pulses. By exploiting differences in resonance conditions for spins in different molecular environments, charged states on donor and acceptor molecules can be selectively probed and characterised, and their role in charge transport and recombination processes can be investigated.
In this work, we use EDMR to investigate organic solar cells based on two non-fullerene acceptor blends (PBDB-T:ITIC and PM6:Y6) and their corresponding fullerene analogues (PBDB-T:PCBM and PM6:PCBM). Through Rabi nutation experiments [3], in which spins are coherently manipulated by microwave pulses of increasing length, we detect clear spin-locking signatures characteristic of coupled pairs of interacting spins on donor and acceptor molecules, encountering to recombine and leading to transient current quenching. We find that this bipolar recombination, rather than unipolar charge transport, is the dominant process leading to the observed EDMR signal.
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