Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Authors

  • Daniel Rauh,

    1. Bavarian Center for Applied Energy, Research e.V. (ZAE Bayern), Am Hubland, 97074 Würzburg, Germany
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  • Carsten Deibel,

    Corresponding author
    1. Experimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilian-University of Würzburg, Am Hubland, 97074 Würzburg, Germany
    • Experimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilian-University of Würzburg, Am Hubland, 97074 Würzburg, Germany
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  • Vladimir Dyakonov

    Corresponding author
    1. Bavarian Center for Applied Energy, Research e.V. (ZAE Bayern), Am Hubland, 97074 Würzburg, Germany
    2. Experimental Physics VI, Faculty of Physics and Astronomy, Julius-Maximilian-University of Würzburg, Am Hubland, 97074 Würzburg, Germany
    • Bavarian Center for Applied Energy, Research e.V. (ZAE Bayern), Am Hubland, 97074 Würzburg, Germany.
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Abstract

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donor–acceptor phase separation until re-emission from these deep states. In order to clarify which mechanism is dominant temperature- and illumination-dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) and PTB7:PC71BM (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC61BM at temperatures below 300 K and PTB7:PC71BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.

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