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One of the organic components in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to the long-term operation of organic–inorganic hybrid perovskite-based solar cells. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA + ) is gradually replaced by cesium (Cs + ), and iodide (I − ) is substituted by bromide (Br − ), i.e. , Cs y FA 1− y Pb(Br x I 1− x ) 3 . Higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic structures. We find that while some correlation exists between the tolerance factor and structure, the tolerance factor does not provide a holistic understanding of whether or not a perovskite structure will fully form. By screening 26 solar cells with different compositions, our results show that Cs 1/6 FA 5/6 PbI 3 delivers the highest efficiency and long-term stability among the I-rich compositions. This work sheds light on the fundamental structure–property relationships in the Cs y FA 1− y Pb(Br x I 1− x ) 3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information for this compositional space.