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Investigation of Driving Forces for Charge Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing S-Shaped Current–Voltage Characteristics

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 SE-581 83 Linköping, Sweden
    • Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str, 1, 01069 Dresden, Germany.
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  • Steef Corvers,

    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

The role of drift and diffusion as driving forces for charge carrier extraction in flat heterojunction organic solar cells is examined at the example of devices showing intentional S-shaped current–voltage (J-V) characteristics. Since these kinks are related to energy barriers causing a redistribution of the electric field and charge carrier density gradients, they are suitable for studying the limits of charge extraction. The dynamics of this redistribution process are experimentally monitored via transient photocurrents, where the current response on square pulses of light is measured in the μs to ms regime. In combination with drift-diffusion simulation data, we demonstrate a pile-up of charge carriers at extraction barriers and a high contribution of diffusion to photocurrent in the case of injection barriers. Both types of barrier lead to S-kinks in the J-V curve and can be distinguished from each other and from other reasons for S-kinks (e.g. imbalanced mobilities) by applying the presented approach. Furthermore, it is also helpful to investigate the driving forces for charge extraction in devices without S-shaped J-V curve close to open circuit to evaluate whether their electrodes are optimized.

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