Theoretical Investigation of the Role of Mixed A⁺ Cations in the Structure, Stability, and Electronic Properties of Perovskite Alloys

Experimental studies have demonstrated the importance of the combination of different chemical species at the A-, B-, or X-sites in metal-halide ABX 3 perovskites to improve the performance of perovskite solar cells (PSCs). However, from our understanding, further efforts at the atomistic scale are required to unveil the role of alloying in PSCs. Here, we performed a density functional theory investigation on perovskite alloy materials, namely, Cs x MA 1– x PbI 3, MA x FA 1– x Sn 0.50 Pb 0.50 I 3, and MA x FA 1– x PbBr 2.50 I 0.50 ( x = 0.00, 0.25, 0.50, 0.75, 1.00). Equilibrium orthorhombic supercell structures were obtained for all systems with distorted octahedral environments, in which the magnitude depends on the chemical species. Besides, energetically stable crystals, in comparison with the parent structures, were found only for Cs x MA 1– x PbI 3, even though the remaining alloys presented stronger bonds. Furthermore, we addressed the role of the spin–orbit coupling effects to the electronic structure, which was critical to estimate the power conversion efficiency (PCE) with radiative recombinations, e.g., a PCE exceeding 23% was obtained. From our analyses, alloys with Cs content stood out as the best photovoltaic material.