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 Cs $$ \mathrm{Cs} $$ 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 MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ and CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ 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 0 K $$ 0\ \mathrm{K} $$ , how the MA $$ \mathrm{MA} $$ dipoles suppression can reduce the MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ gap energy by promoting a degeneracy breaking in the electronic states from the PbI 3 $$ {\mathrm{PbI}}_3 $$ framework, while the dipole moment reinforcement is crucial to align theory ↔ $$ \leftrightarrow $$ experiment, increasing the bulk Rashba splitting through higher Pb $$ \mathrm{Pb} $$ off-centering motifs. The lack of permanent dipole moment in Cs $$ \mathrm{Cs} $$ results in CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ polymorphs with a pronounced Pb $$ \mathrm{Pb} $$ on-centering-like feature, which causes suppression in their respective bulk Rashba effect.

Bulk Rashba Effect Splitting and Suppression in Polymorphs of Metal Iodine Perovskites

We quantified the bulk Rashba splitting and suppression in polymorphs of MA(Pb, Sn, Ge, or Si)I₃ perovskites. The low-computational-cost DFT-1/2 quasiparticle correction was performed for all structures, combined with the inclusion of spin-orbit coupling (SOC) effects. The presence of SOC and symmetry breaking from the metal off-centering octahedral distortion are indispensable and essential conditions for Rashba splitting, whose magnitude emerges from the Pb → Si sequence. Additionally, the quasiparticle correction provides energy bandgaps for MAPbI₃ (cubic, tetragonal, and orthorhombic), MASnI₃ (cubic and tetragonal), and MAGeI₃ (cubic) that are in outstanding agreement with experimental results. However, while gap energies are yielded collaboratively from the metal off-centering and relative octahedral tiltings, the bulk Rashba suppression is reached for metal on-centering (octahedral platonic-like) configurations that are thermodynamically stable even when the charge polarization is kept invariant among metal-I bonds in the polymorphs.