The power conversion efficiency of small-molecular-weight and polymer organic photovoltaic cells has increased steadily over the past decade. This progress is chiefly attributable to the introduction of the donor–acceptor heterojunction1,2 that functions as a dissociation site for the strongly bound photogenerated excitons. Further progress was realized in polymer devices through use of blends of the donor and acceptor materials3,4,5: phase separation during spin-coating leads to a bulk heterojunction that removes the exciton diffusion bottleneck by creating an interpenetrating network of the donor and acceptor materials. The realization of bulk heterojunctions using mixtures of vacuum-deposited small-molecular-weight materials has, on the other hand, posed elusive: phase separation induced by elevating the substrate temperature inevitably leads to a significant roughening of the film surface and to short-circuited devices. Here, we demonstrate that the use of a metal cap to confine the organic materials during annealing prevents the formation of a rough surface morphology while allowing for the formation of an interpenetrating donor–acceptor network. This method results in a power conversion efficiency 50 per cent higher than the best values reported for comparable bilayer devices, suggesting that this strained annealing process could allow for the formation of low-cost and high-efficiency thin film organic solar cells based on vacuum-deposited small-molecular-weight organic materials.
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We thank N. Stroustrup for his assistance, and gratefully acknowledge the financial support of the Air Force Office of Scientific Research, the National Renewable Energy Laboratory, and the Global Photonic Energy Corporation. S.U. is currently on leave from Nippon Oil Corporation, Central Technical Research Laboratory, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan.
The authors declare that they have no competing financial interests.
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Peumans, P., Uchida, S. & Forrest, S. Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films. Nature 425, 158–162 (2003). https://doi.org/10.1038/nature01949
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