Tuning Rashba-Splitting to Brighten Ground State Exciton Via Structural Distortion in 2D Cs₂PbBr₄

Tuning the exciton fine structure in lead halide perovskites has emerged as a pivotal objective for researchers aiming to push the boundaries of optoelectronic performance and unlock new frontiers of efficiency and functionality. Although Rashba splitting has been associated with brightening the ground exciton state, the precise nature of the Rashba splitting responsible for this phenomenon remains unclear. In this study, we employed density functional theory (DFT) and the Model-Bethe-Salpeter Equation (m-BSE) to systematically investigate eighteen 2D Cs₂PbBr₄ systems with varying degrees of structural distortion. Our findings reveal that spin-orbit coupling (SOC) combined with inversion symmetry breaking induces spin splitting in both the valence band (VB) and conduction band (CB), leading to a misalignment between the valence band maximum (VBM) and the conduction band minimum (CBM). This misalignment typically results in a dark ground state exciton, even in the presence of Rashba splitting. However, by controlling inversion symmetry breaking in states with lower SOC, such as the VB in perovskites, the nature of Rashba splitting in the VBM can be modulated. To summarize the study outcomes, a comparative analysis of two systems from the eighteen distorted 2D Cs₂PbBr₄highlights two distinct excitonic behaviors. The first system, <i>2-S-180,</i> has two 180⁰and two slightly distorted consecutive tetrahedral angles. While, the second system, <i>2-L-P, </i>has four highly-distorted tetrahedral angles, where each two opposite angles are equal and consecutive angles are unequal. In 2-S-180, the excitonic state is localized around the Γ point, with the CBM displaying a four-fold spin texture influenced by a combination of linear and cubic Dresselhaus splitting. This arrangement shifts the CBM away from the linear Rashba splitting peak in the VBM, resulting in the first excitonic state being a forbidden dark state with low charge carriers’ effective masses. Conversely, in 2-L-P, the pronounced elliptical spin texture caused by a combination of linear Dresselhaus and linear Rashba splitting, with a dominant Rashba coefficient, leads to a flattened VB. This flattened VB allows the delocalization of excitons characterized by low exciton binding energy and high charge carriers’ effective masses, ultimately producing a bright ground exciton. Tuning Rashba splitting in the VB can be achieved by engineering orbital non-centrosymmetric wavefunction distributions through deviations from ideal tetrahedral geometry. The large deviations can trigger a tetragonal-to-orthorhombic phase transition and flatten the VB structure, potentially aligning the VBM with the CBM and resulting in bright ground excitons. In conclusion, this study establishes a structure-property-performance relationship, linking structural distortions to Rashba-splitting nature, elucidating their effect on the bright ground state exciton formation. This fundamental understanding paves the way for future research, including high-throughput and machine-learning approaches, to accelerate the discovery and development of materials with bright ground states, driving innovation in cutting-edge optoelectronic applications.