1Key Laboratory for Computational Physical Sciences (MOE), State Key Laboratory of Surface Physics, and Department of Physics, Fudan University, Shanghai 200433 2Key Laboratory of Polar Materials and Devices (MOE), East China Normal University, Shanghai 200241 3Center for Sustainable Chemical Technologies and Department of Chemistry, University of Bath, Bath BA2 7AY, UK 4Beijing Computational Science Research Center, Beijing 100094
Abstract：The organic-inorganic hybrid perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ 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 CH$_{3}$NH$_{3}$PbI$_{3}$ 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 CH$_{3}$NH$_{3}$PbI$_{3}$ is thermodynamically unstable with respect to the phase separation into CH$_{3}$NH$_{3}$I + PbI$_{2}$, 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 CH$_{3}$NH$_{3}$PbI$_{3}$ is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH$_{3}$NH$_{3}$ 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 CH$_{3}$NH$_{3}$PbI$_{3}$ and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.
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2018, Vol. 35 
