Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

Authors：Hinrichsen, Ture F.; Chan, Christopher C. S.; Ma, Chao; Palecek, David; Gillett, Alexander; Chen, Shangshang; Zou, Xinhui; Zhang, Guichuan; Yip, Hin-Lap; Wong, Kam Sing; Friend, Richard H.^*; Yan, He^*; Rao, Akshay^*; Chow, Philip C. Y.^*

Source：Nature Communications, 2020, 11(1): 5617.

DOI：10.1038/s41467-020-19332-5

• ENERGY-LOSSES
• POLYMER
• RECOMBINATION

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.