Charge-transfer electronic states in organic solar cells

Abstract

In organic solar cells, the charge-transfer (CT) electronic states that form at the interface between the electron-donor (D) and electron-acceptor (A) materials have a crucial role in exciton-dissociation, charge-separation and charge-recombination processes. Since the introduction of active layers consisting of D–A bulk heterojunctions, CT states have been the focus of extensive experimental and theoretical studies. In this Review, we assess the current understanding of CT states and describe how factors such as the geometry of the D–A interface, electronic polarization and the extent of electron delocalization affect their nature and influence the radiative and non-radiative decay processes. We focus on the description and application of fundamental concepts, which provides the framework to discuss the path to organic solar cells with efficiencies comparable to those in inorganic photovoltaic technologies.

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Fig. 1: Charge-carrier formation in organic solar cells.
Fig. 2: Diabatic and adiabatic states.
Fig. 3: Quantum-mechanical description of CT states.
Fig. 4: Effects of interface morphology on CT states.
Fig. 5: Energetic distribution of CT states.
Fig. 6: Relationship between CT states and energy losses.
Fig. 7: Blends with small local-exciton–charge-transfer energy offsets.
Fig. 8: Three-state model.

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Acknowledgements

The authors thank their collaborators for shaping their understanding of organic photovoltaic materials and devices, in particular H. Ade, A. Bakulin, G. Bazan, D. Beljonne, J. Cornil, R. Friend, F. Gao, O. Inganas, B. Kippelen, L. Kronik, K. Leo, S. Marder, M. McGehee, Q. Nguyen, J. Reynolds, Z. Shuai, K. Vandewal, H. Yan and Y. Yi, as well as their research groups. The authors are also grateful to P. Armistead for supporting their research programme in this field and acknowledge funding of this work by the U.S. Department of the Navy, Office of Naval Research, under the MURI ‘Center for Advanced Organic Photovoltaics’ (award nos. N00014-14-1-0580 and N00014-16-1-2520) and under award no. N00014-17-1-2208.

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Coropceanu, V., Chen, X., Wang, T. et al. Charge-transfer electronic states in organic solar cells. Nat Rev Mater 4, 689–707 (2019). https://doi.org/10.1038/s41578-019-0137-9

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