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Emerging from the field of dye-sensitized solar cells, organometal halide perovskite-based solar cells have recently attracted considerable attention. In these devices, the perovskite light absorbers can also be used as charge transporting materials, changing the requirements for efficient device architectures. The perovskite deposition can vary from merely sensitizing the TiO2 electron transporting scaffold as an endowment of small nanoparticles, to completely filling the pores where it acts as both light absorber and hole transporting material in one. By decreasing the TiO2 scaffold layer thickness, we change the solar cell architecture from perovskite-sensitized to completely perovskite-filled. We find that the latter case leads to improvements in device performance because higher electron densities can be sustained in the TiO2, improving electron transport rates and photovoltage. Importantly, the primary recombination pathway between the TiO2 and the hole transporting material is blocked by the perovskite itself. This understanding helps to rationalize the high voltages attainable on mesoporous TiO2-based perovskite solar cells.
Leijtens et al. (Mon,) studied this question.