Charge Formation, Recombination, and Sweep-Out Dynamics in Organic Solar Cells

Authors

  • Sarah R. Cowan,

    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
    2. National Renewable Energy Lab, 1617 Cole Blvd., Golden, CO 80401, USA
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  • Natalie Banerji,

    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
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  • Wei Lin Leong,

    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
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  • Alan J. Heeger

    Corresponding author
    1. Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
    • Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, CA 93106, USA.
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  • This manuscript is dedicated to the memory of Professor Alan G. MacDiarmid

Abstract

This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep-out to the electrodes. Exciton formation and relaxation in poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) and poly-3(hexylthiophene) (P3HT) are discussed based on a fluorescence up-conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re-examined in light of these femtosecond time-resolved measurements. Transient photoconductivity, time-delayed collection field, and time-delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias- and charge-density-dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

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