The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets

Abstract

Organic solar cells utilize an energy-level offset to generate free charge carriers. Although a very small energy-level offset increases the open-circuit voltage, it remains unclear how exactly charge generation is affected. Here we investigate organic solar cell blends with highest occupied molecular orbital energy-level offsets (∆EHOMO) between the donor and acceptor that range from 0 to 300 meV. We demonstrate that exciton quenching at a negligible ∆EHOMO takes place on timescales that approach the exciton lifetime of the pristine materials, which drastically limits the external quantum efficiency. We quantitatively describe this finding via the Boltzmann stationary-state equilibrium between charge-transfer states and excitons and further reveal a long exciton lifetime to be decisive in maintaining an efficient charge generation at a negligible ∆EHOMO. Moreover, the Boltzmann equilibrium quantitatively describes the major reduction in non-radiative voltage losses at a very small ∆EHOMO. Ultimately, highly luminescent near-infrared emitters with very long exciton lifetimes are suggested to enable highly efficient organic solar cells.

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Fig. 1: Donor and acceptor materials and their energy-level alignment.
Fig. 2: Exciton dynamics of the donor and acceptor materials and their blends.
Fig. 3: Exciton splitting efficiency and its correlation with external and internal quantum efficiencies.
Fig. 4: J–V characteristics, FTPS and EL of all the solar cells.
Fig. 5: Voltage loss analysis as a function of Eoffset.
Fig. 6: Experimental and analytic analysis of ΔVOC,non-rad.

Data availability

All data generated or analysed during this study are included in the published article and its Supplementary Information data files. Source data are provided with this paper.

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Acknowledgements

A.C. and C.J.B. gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project no. 182849149-SFB 953. C.J.B. gratefully acknowledges financial support through the ‘Aufbruch Bayern’ initiative of the state of Bavaria (EnCN and SFF) and the Bavarian Initiative ‘Solar Technologies go Hybrid’ (SolTech) and funding from DFG project DFG INST 90/917. C.C.L. thanks the European Union for the financial support. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements no. 761112 (PRESTIGE) and no. 820789 (OLEDSOLAR).

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T.H. and C.J.B. conceived the idea for this study and supervised the work. A.C. fabricated all the measured devices and samples, performed JV characterizations, EQE measurements, FTPS spectroscopy, CV measurements and absorbance measurements. C.L.C. and V.G.G. synthesized the polymers used in this work and performed the DFT calculations. A.O. and A.C. performed the TRPL measurements. J.W. and A.C. performed the steady-state PL and EL measurements. K.F. performed the optical transfer-matrix calculation of IQE. L.L. developed the effective two-states model and utilized this theoretical framework to fit the experimental data. T.H., I.M., C.J.B. and A.C. evaluated and interpreted the experimental findings. The manuscript was written and commented on by all the authors.

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Correspondence to Larry Lüer or Thomas Heumüller or Christoph J. Brabec.

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Supplementary Information

Supplementary Figs. 1–38, Tables 1–5, Notes 1–4 and refs. 1–9.

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Classen, A., Chochos, C.L., Lüer, L. et al. The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets. Nat Energy 5, 711–719 (2020). https://doi.org/10.1038/s41560-020-00684-7

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