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The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significantinterest for its high performance in converting solar light into electricalpower with an efficiency exceeding 20%. Unfortunately, chemical stability isone major challenge in the development of the CH3NH3PbI3 solar cells. It wascommonly assumed that moisture or oxygen in the environment causes the poorstability of hybrid halide perovskites, however, here we show from thefirst-principles calculations that the room-temperature tetragonal phase ofCH3NH3PbI3 is thermodynamically unstable with respect to the phase separationinto CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent ofthe humidity or oxygen in the atmosphere. When the structure is distorted tothe low-temperature orthorhombic phase, the energetic cost of separationincreases, but remains small. Contributions from vibrational andconfigurational entropy at room temperature have been considered, but theinstability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb bySn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become morestable and do not phase-separate spontaneously. Our study highlights that thepoor chemical stability is intrinsic to CH3NH3PbI3 and suggests thatelement-substitution may solve the chemical stability problem in hybrid halideperovskite solar cells.

Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3