Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells
Ture F. Hinrichsen, Christopher C. S. Chan, Chao Ma, David Paleček, Alexander J. Gillett, Shangshang Chen, Xinhui Zou, Guichuan Zhang, Hin‐Lap Yip, Kam Sing Wong, Richard H. Friend, He Yan, Akshay Rao, Philip C. Y. Chow
Abstract
Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor-acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination.