How cation nature controls the bandgap and bulk Rashba splitting of halide perovskites

Because of instability issues presented by metal halide perovskites based on methylammonium (MA), its replacement to $$ has emerged as an alternative to improve the materials' durability. However, the impact of this replacement on electronic properties, especially gap energy and bulk Rashba splitting remains unclear since electrostatic interactions from organic cations can play a crucial role. Through first-principles calculations, we investigated how organic/inorganic cations impact the electronic properties of $$ and $$ perovskites. Although at high temperatures the organic cation can assume spherical-like configurations due to its rotation into the cages, our results provide a complete electronic mechanism to show, from a chemical perspective based on ab initio calculations at $$ , how the $$ dipoles suppression can reduce the $$ gap energy by promoting a degeneracy breaking in the electronic states from the $$ framework, while the dipole moment reinforcement is crucial to align theory $$ experiment, increasing the bulk Rashba splitting through higher $$ off-centering motifs. The lack of permanent dipole moment in $$ results in $$ polymorphs with a pronounced $$ on-centering-like feature, which causes suppression in their respective bulk Rashba effect.