Mixed A-Cation Perovskites for Solar Cells: Atomic-Scale Insights into Structural Distortion, Hydrogen Bonding, and Electronic Properties

Hybrid lead halide perovskites containing a mixture of A-site cations such as the formamidinium (CH(NH₂)2⁺, FA) and the smaller cesium (Cs⁺) cations have attracted considerable interest due to their improved stability and solar cell performance. However, the structural changes at the atomic scale and modifications to the optoelectronic properties of these mixed cation perovskites are not fully understood. Here, we investigate the FA_1-xCs_xPbI₃ (x ≤ 0.25) system using a combination of static and dynamic ab initio computational methods. We find that the incorporation of Cs⁺ cations into the parent FAPbI₃ structure induces a chemical pressure or lattice strain effect through Cs/FA ion size mismatch resulting in structural distortion and stronger FA-iodide (N-H···I) hydrogen bonding interactions. The dynamic tilting of PbI₆ octahedra and the rotational motion of FA cations are also suppressed, which leads to symmetry-breaking of the lattice. Such symmetry-breaking distortions of the Pb/I lattice give rise to a Rashba-type effect, which spin-splits the frontier electronic bands making the band gap indirect. Our results suggest that the direct-indirect band gap transition may be a factor in the reduced charge-carrier recombination rate in these mixed cation perovskites.