The efficiency of polymer solar cells critically depends on the intimacy of mixing of the donor and acceptor semiconductors used in these devices to create charges and on the presence of unhindered percolation pathways in the individual components to transport holes and electrons. The visualization of these bulk heterojunction morphologies in three dimensions has been challenging and has hampered progress in this area. Here, we spatially resolve the morphology of 2%-efficient hybrid solar cells consisting of poly(3-hexylthiophene) as the donor and ZnO as the acceptor in the nanometre range by electron tomography. The morphology is statistically analysed for spherical contact distance and percolation pathways. Together with solving the three-dimensional exciton-diffusion equation, a consistent and quantitative correlation between solar-cell performance, photophysical data and the three-dimensional morphology has been obtained for devices with different layer thicknesses that enables differentiating between generation and transport as limiting factors to performance.
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The authors gratefully acknowledge D. Veldman for assistance with the PIA measurements and N. Lousberg for preparing a cross-section of a complete device by FIB processing. The research was supported by a TOP grant of the Chemical Sciences (CW) division of the Netherlands Organization for Scientific Research (NWO) and is part of the Joint Solar Programme (JSP). The JSP is co-financed by the Foundation for Fundamental Research on Matter (FOM), Chemical Sciences of NWO and the Foundation Shell Research. This work was further supported by the Deutsche Forschungsgemeinschaft under Priority Programme 1355 ‘Elementary Processes of Organic Photovoltaics’, Senter/Novem in the EOS project Zomer (EOSLT03026) and is part of the research program of the Dutch Polymer Institute (DPI, project 524).
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Oosterhout, S., Wienk, M., van Bavel, S. et al. The effect of three-dimensional morphology on the efficiency of hybrid polymer solar cells. Nature Mater 8, 818–824 (2009). https://doi.org/10.1038/nmat2533
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