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Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole-Based Solution-Processed Small Molecule Solar Cells

Authors

  • Christopher M. Proctor,

    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA
    2. Department of Materials, University of California, Santa Barbara, CA 93106, USA
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  • Chunki Kim,

    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA
    2. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA
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  • Dieter Neher,

    1. University of Potsdam, Institute of Physics and Astronomy, 14476 Potsdam, Germany
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  • Thuc-Quyen Nguyen

    Corresponding author
    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA
    2. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA
    • Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106, USA.
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Abstract

Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fill factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono-DPP system (2 × 10−5 cm2 V−1 s−1 versus 34 × 10−5 cm2 V−1 s−1). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.

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