Phase Stability and Electronic Properties of Hybrid Organic–Inorganic Perovskite Solid Solution (CH(NH(2))(2))(x)(CH(3)NH(3))(1–x)Pb(Br(y)I(1–y))(3) as a Function of Composition

[Image: see text] Compositional mixing provides the means to maintain the structural stability of a hybrid organic–inorganic perovskite for efficient and robust photovoltaic applications. Here we present a theoretical, first-principles study of the electronic and energetic properties of the solid solution (CH(NH(2))(2))(x)(CH(3)NH(3))(1–x)PbBr(y)I(1–y), the mixing of two organic molecules with various orientations, formamidinium and methylammonium, and two halides, bromide and iodide. Our results show the variation in the band gap as a function of composition (x and y) provides several candidates that exceed the 27.5% Schockley–Queisser efficiency. The variation in the composition of hybrid perovskite shows specific regions where either the hexagonal or cubic phase dominates. We discuss the balance between the band gap and phase stability and indicate regions where the phase transition temperature between cubic and hexagonal phases is far from room temperature, indicating that these compositions are more robust at room temperature against phase transitions.