Efficient Nonfullerene Organic Solar Cells with Small Driving Forces for Both Hole and Electron Transfer

State-of-the-art organic solar cells (OSCs) typically suffer from large voltage loss (V_loss ) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the V_loss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T-TT and a small-molecular acceptor (O-IDTBR), which have identical bandgaps and close energy levels. The Fourier-transform photocurrent spectroscopy external quantum efficiency (FTPS-EQE) spectrum of the blend overlaps with those of neat PffBT2T-TT and O-IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQE_EL ) of ≈1 × 10^-4 , which leads to a significantly minimized nonradiative V_loss of 0.24 V. Despite the small driving forces and a low V_loss , a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.