All-inorganic halide perovskites have attracted a great interest as a promising light harvester of perovskite solar cells due to their enhanced chemical stability. In this work we investigate the material properties of solid solutions CsPb(I 1−x Br x ) 3 in cubic phase by applying the virtual crystal approximation approach within a density functional theory framework. First we check the validity of constructed pseudopotentials of the virtual atoms (X = I 1−x Br x ) by verifying that the lattice constants follow the linear function of mixing ratio. We then suggest an idea of using the hybrid HSE functional with linear increasing value of exact exchange term as increasing the Br content x, which produces the band gaps of CsPbX 3 in good agreement with the available experimental data. The calculated light absorption coefficients and reflectivity show the systematic varying tendency to the Br content. We calculate the phonon dispersions of CsPbX 3 , CsX and PbX 2 as slightly changing their volumes, revealing the phase instability of CsPbX 3 and calculating the thermodynamic potential function differences. By projecting Gibbs free energy differences onto the plane of ∆G = 0, we determine the P − T diagram for CsPbX 3 to be stable against the chemical decomposition, highlighting that the area of being stable extends gradually as the Br content increases.

Perovskite solar cells have continued to fascinate over the past decade due to fast increasing power conversion efficiency and very low fabrication cost but still suffered from poor stability. Interface engineering is evolved to be one of the most promising solutions to the instability problem. In this work, we perform a first-principles study on the MAPbI 3 /CsPbI 3 interface system, aiming at clarifying the underlying mechanism of interfacial enhancement of solar cell performance. We devise the atomistic modeling of superlattices as increasing the number of included unit cells and carry out structural optimizations, revealing that the binding strength between the perovskite layers becomes stronger while the band gap decreases as the supercell size increases. Using enough large supercells of the interface system, we further estimate the formation energies of the interfacial vacancy defects and activation barriers for vacancy-mediated I atom migrations. Our calculations show the shallow transition states for most of the defects and the higher activation barriers for I atom migrations across the interface, providing an evidence of performance enhancement by the interface formation. By giving an insightful understanding of the MAPbI 3 /CsPbI 3 heterojunction, this work definitely contributes to the design of interface systems for high-performance solar cells.

We investigate the variation of structural, elastic, electronic, and optical properties of all-inorganic bromide and chloride perovskite solid solutions of CsPb(Br1−xClx)3 using first-principles calculations within the virtual crystal approximation.