Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3

TL;DR

This study reveals that the hybrid halide perovskite CH3NH3PbI3 is intrinsically thermodynamically unstable at room temperature, leading to phase separation, which challenges prior assumptions that environmental factors cause its instability.

Contribution

First-principles calculations demonstrate the inherent thermodynamic instability of CH3NH3PbI3, independent of environmental conditions, and suggest element substitution as a solution.

Findings

CH3NH3PbI3 is thermodynamically unstable at room temperature.

Phase separation into CH3NH3I + PbI2 is exothermic and intrinsic.

Substituting elements enhances stability and prevents spontaneous phase separation.

Abstract

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 the 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.