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

Abstract

The use of non-fullerene acceptors (NFAs) in organic solar cells has led to power conversion efficiencies as high as 18%1. However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%2. A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps3, owing to non-radiative recombination4. For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend5, this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to molecular triplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more.

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Fig. 1: Triplet formation pathways and organic solar cell materials.
Fig. 2: Spectroscopic investigations of triplet formation in model NFA blends.
Fig. 3: The role of hybridization in organic solar cell blends.

Data availability

The data that support the plots within this paper and other findings of this study are available at the University of Cambridge Repository (https://doi.org/10.17863/CAM.75316).

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Acknowledgements

A.J.G. and R.H.F. acknowledge support from the Simons Foundation (grant number 601946) and the EPSRC (EP/M01083X/1 and EP/M005143/1). This project has received funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 670405). A.K. and T.-Q.N. were supported by the Department of the Navy, Office of Naval Research award number N00014-21-1-2181. A.K. acknowledges funding by the Schlumberger foundation. A.Privitera, R.D., A.Pershin, G.L., M.K.R. and D.B. were supported by the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska Curie grant agreement number 722651 (SEPOMO project). Computational resources in Mons were provided by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the Fonds de la Recherche Scientifiques de Belgique (FRS-FNRS) under grant number 2.5020.11, as well as the Tier-1 supercomputer of the Fedération Wallonie-Bruxelles, infrastructure funded by the Walloon Region under grant agreement number 1117545. D.B. is a FNRS Research Director. F.G. acknowledges the Stiftelsen för Strategisk Forskning through a Future Research Leader programme (FFL18-0322). Transient electron paramagnetic resonance measurements were performed in the Centre for Advanced ESR (CAESR) in the Department of Chemistry at the University of Oxford, and this work was supported by the EPSRC (EP/L011972/1). We thank T. Biskup and A. Sperlich for their assistance with simulation and interpretation of the transient electron paramagnetic resonance data.

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A.J.G., T.-Q.N. and R.H.F. conceived the work. A.J.G. performed the transient absorption measurements. A.Privitera and W.K.M. conducted the transient electron paramagnetic resonance measurements. R.D., A.Pershin and G.L. carried out the quantum chemical calculations. A.K., D.Q., J.Y. and S.-J.K. fabricated and tested the organic solar cell devices. A.J.G. and J.Y. performed the photoluminescence quantum efficiency measurements. J.L. synthesized SiOTIC-4F and the IEICO derivatives. M.K.R., F.G., G.C.B., T.-Q.N, D.B. and R.H.F. supervised their group members involved in the project. A.J.G., A.R. and R.H.F. wrote the manuscript, with input from all authors.

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Correspondence to Alexander J. Gillett, Thuc-Quyen Nguyen, David Beljonne or Richard H. Friend.

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Peer review information Nature thanks Till Biskup, Yingping Zou and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Gillett, A.J., Privitera, A., Dilmurat, R. et al. The role of charge recombination to triplet excitons in organic solar cells. Nature 597, 666–671 (2021). https://doi.org/10.1038/s41586-021-03840-5

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