Thermodynamic Origin of the Photostability of the Two-Dimensional Perovskite PEA 2 Pb(I 1- x Br x ) 4 .

The two-dimensional (2D) mixed halide perovskite PEA 2 Pb(I 1- x Br x ) 4 exhibits high phase stability under illumination as compared to the three-dimensional (3D) counterpart MAPb(I 1- x Br x ) 3 . We explain this difference using a thermodynamic theory that considers the sum of a compositional and a photocarrier free energy. Ab initio calculations show that the improved compositional phase stability of the 2D perovskite is caused by a preferred I-Br distribution, leading to a much lower critical temperature for halide segregation in the dark than for the 3D perovskite. Moreover, a smaller increase of the band gap with Br concentration x and a markedly shorter photocarrier lifetime in the 2D perovskite reduce the driving force for phase segregation under illumination, enhancing the photostability.