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 Gillett; Shangshang Chen; Xinhui Zou; Guichuan Zhang; Hin-Lap Yip; Kam Sing Wong; Richard H Friend; He Yan; Akshay Rao; Philip C Y Chow

doi:10.1038/s41467-020-19332-5

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

Nat Commun. 2020 Nov 5;11(1):5617. doi: 10.1038/s41467-020-19332-5.

Authors

Affiliations

¹ Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK.
² Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay,, Hong Kong, China.
³ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,, Hong Kong, China.
⁴ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, China.
⁵ Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK. rhf10@cam.ac.uk.
⁶ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,, Hong Kong, China. hyan@ust.hk.
⁷ Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK. ar525@cam.ac.uk.
⁸ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,, Hong Kong, China. pcyc@hku.hk.
⁹ Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China. pcyc@hku.hk.

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.