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Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance

Authors

  • Wolfgang Tress,

    Corresponding author
    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
    Current affiliation:
    1. Biomolecular and Organic Electronics, IFM, Linköping University, 58183 Linköping, Sweden
    • Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany.
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  • André Merten,

    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
    Current affiliation:
    1. Fraunhofer-Institut für Photonische Mikrosysteme IPMS, Maria-Reiche-Str. 2, 01109 Dresden, Germany
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  • Mauro Furno,

    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
    Current affiliation:
    1. Novaled AG, Tatzberg 49, 01307 Dresden, Germany
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  • Moritz Hein,

    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
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  • Karl Leo,

    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
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  • Moritz Riede

    1. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany
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Abstract

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.

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