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On the origin of the open-circuit voltage of polymer–fullerene solar cells

Nature Materials volume 8, pages 904–909 (2009)Cite this article

Abstract

The increasing amount of research on solution-processable, organic donor–acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor–acceptor devices.

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Figure 1: The chemical structures of the donor polymers used.
Figure 2: The E Q EPV spectra of polymer–fullerene devices.
Figure 3: Electroluminescence emission and corresponding injected current versus voltage curves of polymer–fullerene devices.
Figure 4: Comparison of the measured E Q EEL with the product of the E Q EPV spectrum with the black-body spectrum at room temperature.
Figure 5: The Voc obtained by means of the detailed balance approach versus the measured Voc.

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Acknowledgements

K.V., A.G. and J.V.M. acknowledge the institute for the promotion of science and technology in Flanders (IWT-Vlaanderen) the IWT-project polyspec, the FWO project nano-fibres and the European project solar-n-type. K.T. and O.I. thank the Swedish Energy Agency for funding through the programme Tandem. All authors acknowledge M.R. Andersson at Chalmers University for supplying APFO3 and LBPP5 and Markus Scharber for supplying PCPDTBT. D. Vanderzande, W. D. Oosterbaan, P. Adriaensens and S. Chambon are thanked for valuable discussions.

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Authors and Affiliations

  1. IMEC-IMOMEC, vzw, Institute for Materials Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium

    Koen Vandewal, Abay Gadisa & Jean V. Manca

  2. Department of Physics, Biomolecular and Organic Electronics, Center of Organic Electronics (COE), Chemistry and Biology, Linköping University, 58183 Linköping, Sweden

    Kristofer Tvingstedt & Olle Inganäs

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  1. Koen Vandewal
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Contributions

K.V. and A.G. prepared the devices and carried out FTPS measurements in Hasselt. K.T. and K.V. prepared devices and carried out the electroluminescence measurements in Linköping. K.V. wrote the paper. All authors provided comments on the manuscript. J.V.M. and O.I. directed the research.

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Correspondence to Koen Vandewal.

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Vandewal, K., Tvingstedt, K., Gadisa, A. et al. On the origin of the open-circuit voltage of polymer–fullerene solar cells. Nature Mater 8, 904–909 (2009). https://doi.org/10.1038/nmat2548

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