The role of charge recombination to triplet excitons in organic solar cells

Gillett, A.J. and Privitera, A. and Dilmurat, R. and Karki, A. and Qian, D. and Pershin, Anton and Londi, G. and Myers, W.K. and Lee, J. and Yuan, J. and Ko, S.-J. and Riede, M.K. and Gao, F. and Bazan, G.C. and Rao, A. and Nguyen, T.-Q. and Beljonne, D. and Friend, R.H. (2021) The role of charge recombination to triplet excitons in organic solar cells. NATURE, 597 (7878). pp. 666-671. ISSN 0028-0836

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Abstract

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) using nonfullerene acceptors (NFAs) have now reached 18%1 . However, this is still lower than inorganic solar cells, for which PCEs >20% are commonplace2 . A key reason is that OSCs still show low open-circuit voltages (VOC) relative to their optical band gaps 3 , attributed to non-radiative recombination4 . For OSCs to compete with inorganics in efficiency, all non-radiative loss pathways must be identified and where possible, removed. Here, we show that in most NFA OSCs, the majority of charge recombination at open-circuit proceeds via formation of non-emissive NFA triplet excitons (T1); in the benchmark PM6:Y6 blend5 , this fraction reaches 90%, contributing 60 mV to the reduction of VOC. We develop a new design to prevent recombination via this non-radiative channel through the engineering of significant hybridisation between the NFA T1 and the spintriplet charge transfer exciton ( 3CTE). We model that the rate of the back charge transfer from 3CTE to T1 can be reduced by an order of magnitude, allowing re-dissociation of the 3CTE. We then demonstrate NFA systems where T1 formation is suppressed. This work therefore provides a clear design pathway for improved OSC performance to 20% PCE and beyond.

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