Fig. 1. Understanding charge separation processes in highly efficient non-fullerene organic solar cell (OSC) with small photovoltage loss.

a, b Schematic energy diagram illustrating the time evolution of charges in a conventional fullerene and b non-fullerene OSC systems. Conventional fullerene systems are designed with large donor–acceptor (D/A) offsets and generally have high degree of electronic disorder at the interface (Urbach energy ≥ 40 meV), whereas non-fullerene systems are designed with small offsets and generally have low disorder (Urbach energy ~25–30 meV). Photoexcited states evolve through the following three stages: (1) dissociation of singlet excitons into CTEs or directly into free charges, (2) separation of CTEs into free charges, and (3) charge recombination. Additional losses through triplet exciton formation is often found for fullerene systems (process 4). c Absorption spectra of pristine P3TEA and SF-PDI₂, and P3TEA:SF-PDI₂ blend on quartz substrate. We preferentially create photoexcitations in P3TEA (pump at 670 ± 40 nm) for all sub-nanosecond transient absorption measurements presented in this work. For pump–push–probe measurements, a push pulse (time-delayed to the pump) arrives to interact with excited states and create a characteristic optical response that is subsequently monitored by a broadband probe pulse. The push pulse has an energy below the optical gap (push energy = 800 ± 10 nm) and does not generate more photoexcitations directly from the ground state (see Supplementary Information). d Chemical structure of donor polymers (P3TEA, P3TAE, and PffBT2T-TT) and acceptor molecules (SF-PDI₂, O-IDTBR, FTTB-PDI₄, and PCBM) involved in this study. Additional material and device characterizations are found in the Supplementary Information.