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A unified description of non-radiative voltage losses in organic solar cells

Abstract

Recent advances in organic solar cells based on non-fullerene acceptors (NFAs) come with reduced non-radiative voltage losses (ΔVnr). Here we show that, in contrast to the energy-gap-law dependence observed in conventional donor:fullerene blends, the ΔVnr values in state-of-the-art donor:NFA organic solar cells show no correlation with the energies of charge-transfer electronic states at donor:acceptor interfaces. By combining temperature-dependent electroluminescence experiments and dynamic vibronic simulations, we provide a unified description of ΔVnr for both fullerene- and NFA-based devices. We highlight the critical role that the thermal population of local exciton states plays in low-ΔVnr systems. An important finding is that the photoluminescence yield of the pristine materials defines the lower limit of ΔVnr. We also demonstrate that the reduction in ΔVnr (for example, <0.2 V) can be obtained without sacrificing charge generation efficiency. Our work suggests designing donor and acceptor materials with high luminescence efficiency and complementary optical absorption bands extending into the near-infrared region.

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Fig. 1: Emission spectral lineshapes of D:A blends as a function of tLE−CT and ΔELECT.
Fig. 2: Non-radiative voltage losses as a function of ΔELE−CT and tLE−CT.
Fig. 3: ΔVnr versus interfacial ECT.
Fig. 4: Charge generation efficiencies versus device ΔVnr.

Data availability

The authors declare that all relevant data are included in the paper and its Supplementary Information.

Code availability

The codes used in this paper are deposited on GitHub (https://github.com/chenxiankai/three-state-abs-and-emission).

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Acknowledgements

We thank K. Vandewal (Hasselt University) for insightful discussions and H. Wu (South China University of Technology) for providing the EQEEL value in a Y11 neat film. The research in Linköping was supported by the Swedish Strategic Research Foundation through a Future Research Leader program to F.G. (FFL 18-0322), Swedish Research Council VR (grant nos. 2016-06146, 2018-06048 and 2019-00677), and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (faculty grant no. SFO-Mat-LiU #2009-00971); the work at Arizona was funded by the Department of the Navy, Office of Naval Research, under award no. N00014-20-1-2110 and the University of Arizona. F.G. is a Wallenberg Academy Fellow and O.I. is a Wallenberg Academy Scholar.

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X.-K.C., D.Q., V.C., J.L.B. and F.G. conceived the project; X.-K.C. carried out all of the theoretical simulations. D.Q. developed new blends. D.Q. made the devices and conducted the spectroscopy measurements together with Y.W. T. K. and M.H. contributed to the measurements of the electroluminescence spectra. T.K., W.T., O.I., V.C., J.L.B. and F.G. contributed to the result analysis. D.Q. and J. Y. conducted the cyclic voltammetry measurements. H.Y., J.Y., M.Z., Y.Z., Y.L. and J.H. developed the donor and acceptor materials. Y.S. developed two of PBDT-TS1-based blends. X.-K.C., D.Q., V.C., J.L.B. and F.G. wrote the manuscript. F.G. supervised the project. All authors discussed the results and commented on the final manuscript.

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Correspondence to Deping Qian, Veaceslav Coropceanu, Jean-Luc Bredas or Feng Gao.

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Supplementary Figs. 1–17, Notes 1–8 and Tables 1–8.

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Chen, XK., Qian, D., Wang, Y. et al. A unified description of non-radiative voltage losses in organic solar cells. Nat Energy 6, 799–806 (2021). https://doi.org/10.1038/s41560-021-00843-4

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