Semiconducting lead triiodide perovskites (APbI3 ) have shown remarkable performance in applications including photovoltaics and electroluminescence. Despite many theoretical possibilities for A+ in APbI3 , the current experimental knowledge is largely limited to two of these materials: methylammonium (MA+ ) and formamidinium (FA+ ) lead triiodides, neither of which adopts the ideal, cubic perovskite structure at room temperature. Here, a volume-based criterion is proposed for cubic APbI3 to be stable, and two perovskite materials MA1-x EAx PbI3 (MEPI, EA+ = ethylammonium) and MA1-y DMAy PbI3 (MDPI, DMA+ = dimethylammonium) are introduced. Powder and single-crystal X-ray diffraction (XRD) results reveal that MEPI and MDPI are solid solutions possessing the cubic perovskite structure, and the EA+ and DMA+ cations play similar roles in the symmetrization of the crystal lattice of MAPbI3 . Single crystals of MEPI and MDPI are grown and made into plates of a range of thicknesses, and then into metal-perovskite photodiodes. These devices exhibit tripled diffusion lengths and about tenfold enhancement in stability against moisture, both relative to the current benchmark MAPbI3 . In this study, the systematic approach to materials design and device fabrication greatly expands the candidate pool of perovskite semiconductors, and paves the way for high-performance, single-crystal perovskite devices including solar cells and light emitters.
Keywords: crystal structures; mixed cations; perovskites; photodiodes; stabilities.
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