Thermodynamic Origin of Photoinstability in the CH(3)NH(3)Pb(I(1–x)Br(x))(3) Hybrid Halide Perovskite Alloy

[Image: see text] The formation of solid-solutions of iodide, bromide, and chloride provides the means to control the structure, band gap, and stability of hybrid halide perovskite semiconductors for photovoltaic applications. We report a computational investigation of the CH(3)NH(3)PbI(3)/CH(3)NH(3)PbBr(3) alloy from density functional theory with a thermodynamic analysis performed within the generalized quasi-chemical approximation. We construct the phase diagram and identify a large miscibility gap, with a critical temperature of 343 K. The observed photoinstability in some mixed-halide solar cells is explained by the thermodynamics of alloy formation, where an initially homogeneous solution is subject to spinodal decomposition with I and Br-rich phases, which is further complicated by a wide metastable region defined by the binodal line.