Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH₃NH₃PbI₃ ^∗

The organic-inorganic hybrid perovskite CH₃NH₃PbI₃ has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH₃NH₃PbI₃ solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH₃NH₃PbI₃ is thermodynamically unstable with respect to the phase separation into CH₃NH₃I+PbI₂, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH₃NH₃PbI₃ is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH₃NH₃PbI₃ and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.