Ultrafast energy transfer from polymer donors facilitating spectral uniform photocurrent generation and low energy loss in high-efficiency nonfullerene organic solar cells

The mysterious photoinduced charge generation process and its implications for device performance are key issues for the state-of-the-art nonfullerene acceptor (NFA) based Organic Solar Cells (OSCs). Compared to the well explored hole transfer (HT) from photoexcited NFAs, its counterpart, the photoinduced process after donor light absorption and its implications for device performance remain unclear. Herein, by combining ultrafast spectroscopy and photovoltaic characterization, we unambiguously show the dominant ultrafast (& SIM;80 fs) and lossless Forster resonance energy transfer (FRET) from photoexcited polymer donors to Y6. In striking contrast to fullerene OSCs, the FRET process with an efficiency of over 65% is followed by reverse hole transfer (HT), bypassing direct electron transfer, becoming a more efficient charge generation channel in high-efficiency nonfullerene OSCs, especially for Y-series NFA based OSC systems. This synergistic two-step process facilitates spectral uniform photocurrent generation, also lowering the non-radiative recombination energy loss by suppressing the formation of intermedia non-luminous CT exciton. This study emphasizes the great potential of FRET and the central role of HT and also provides new guidelines to engineer FRET for high-efficiency OSCs.