Recombination Losses Above and Below the Transport Percolation Threshold in Bulk Heterojunction Organic Solar Cells |
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| Authors: | Aren Yazmaciyan Martin Stolterfoht Paul L. Burn Qianqian Lin Paul Meredith Ardalan Armin |
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| Affiliation: | 1. School of Mathematics and Physics, The University of Queensland, Brisbane, Australia;2. Institute of Physics and Astronomy, University of Potsdam, Potsdam‐Golm, Germany;3. Centre for Organic Photonics and Electronics (COPE), School of Mathematics and Physics and School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Australia;4. School of Physics and Technology, Wuhan University, Wuhan, Hubei Province, P. R. China;5. Department of Physics, Swansea University, Singleton Park, Swansea, Wales, UK;6. Centre for Engineered Quantum Systems (EQuS), School of Mathematics and Physics, The University of Queensland, Brisbane, Australia |
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| Abstract: | Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second‐order recombination losses dominate the shape of the current density–voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric‐field dependence of first‐order losses, which includes electric‐field‐dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first‐ and second‐order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold. |
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| Keywords: | bulk heterojunctions charge transport organic solar cells percolation threshold recombination losses |
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