With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.
KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: H.K.H.L., W.C.T., and J.D. thank the Welsh Assembly Government of the Ser Cymru Solar Program, and Z.L. and S.D. thank the Welsh Assembly Government Ser Cymru II fellowship scheme for financial support. E.M.S. and W.C.T. thank the National Research Network in Advanced Engineering and Materials and EPSRC funded Project EP/M025020/1. This work is partially funded by the European regional Development Fund through the Welsh Government. A.J.C. and W.C.T. acknowledge funding from the European Social Fund via the Welsh Government and EPSRC Project EP/L015099/1. J.L. and J.S.K. acknowledge the UK EPSRC for the Plastic Electronics Centre for Doctoral Training (EP/L016702/1) funding and CSEM Brasil for studentship.