Tuning Rashba Splitting for Bright Ground-State Excitons in 2D CsPbBr₃ Perovskites through Structural Distortions

Tuning the exciton fine structure of lead halide perovskites to brighten the dark excitonic ground state is crucial for enhancing their optoelectronic performance. While Rashba splitting is linked to dark-to-light exciton flipping, the specific nature of this phenomenon remains unclear. Here, we systematically studied 18 Cs₂PbBr₄ structures, representing 2D systems of CsPbBr₃ with varying degrees of distortion, using density functional theory (DFT) and the Model-Bethe-Salpeter Equation (m-BSE). We demonstrate that spin-orbit coupling (SOC) combined with inversion symmetry breaking induces spin splitting in both the valence band (VB) and conduction band (CB), typically leading to a dark ground exciton due to band misalignment, even in the presence of Rashba splitting. However, controlled inversion symmetry breaking in states with weaker SOC─such as the VB states in perovskites─enables tunable Rashba splitting of the VBM. It was found that structures with VBM exhibiting linear Rashba and linear Dresselhaus splitting, where the Rashba coefficient exceeds the Dresselhaus coefficient, create an elliptical spin texture that aligns the VB maximum (VBM) and CB minimum (CBM), potentially brightening the excitonic ground state. This behavior is driven by significant bond angle distortions from the ideal cubic perovskite geometry that enhances Rashba splitting of the VBM through orbital noncentrosymmetry and facilitates a tetragonal-to-orthorhombic phase transition that further splits the excitonic states and flattens the VBM. These findings establish a structure-property relationship linking structural distortions and Rashba splitting that elucidates their role in brightening ground excitons and their implications for bright ground states in perovskites used in advanced optoelectronic applications.