Thermally Activated Reverse Electron Transfer Limits Carrier Generation Efficiency in PM6:Y6 Non‐Fullerene Organic Solar Cells

Transient absorption and time‐resolved fluorescence measurements in a wide temperature range are used to investigate the mechanism of charge carrier generation in efficient organic solar cells based on a PM6:Y6 donor–acceptor blend. The generation mechanisms differ significantly under excitation of a donor or acceptor. The investigations reveal a temperature‐dependent interplay between the formation of interfacial charge transfer (CT) states and intra‐moiety CT states of the acceptor, their separation into free charge carriers and carrier recombination. The efficient charge carrier generation is ensured by the carrier separation over a small energy barrier, which is easily surmountable at room temperature. However, the overall yield of charge carrier generation at room temperature is reduced by the recombination of charge carriers due to the thermally activated back transfer of electrons from the acceptor to the donor via the highest occupied molecular orbit (HOMO) levels, which is enabled by the small energy offset between HOMO levels of the donor and the acceptor. Combined transient absorption and time‐resolved photoluminescence studies reveal several parallel charge carrier generation pathways, which determine the performance efficiency of a non‐fullerene organic solar cell. The small offset between donor and acceptor highest occupied molecular orbit levels enables high open‐circuit voltage, yet opens a thermally activated recombination loss channel by reverse electron transfer from the acceptor to the donor.