Electronic structure of 2D hybrid perovskites: Rashba spin-orbit coupling and impact of interlayer spacing

Two-dimensional (2D) lead perovskite materials are of interest and under investigation in the solar cell and light-emitting device research community stemming from their high stability and intriguing anisotropic properties. Here we report electronic properties with and without spin-orbit coupling (SOC) together with the influence of van der Waals interaction. Particular attention is given to Rashba SOC, anisotropic band structure effects, and the impact of the electronic structure as a function of interlayer spacing with successively longer organic cations. The results show that larger cations, with a series from butyl-, hexyl-, octyl-, and decyl-diammonium, decrease the electrostatic interaction between the PbI4 planes in the 2D layered perovskites. SOC splitting of the conduction band states lowers the bandgap from 2.21 eV to 1.43 eV in the butyl-diammonium layered perovskite and results in a bandgap of about 1.5 eV in the analogs with longer cation chains. The k-dependent SOC effects (Rashba and Dresselhaus SOC) in the 2D and 3D structures are smaller than the k-independent SOC and are compared to Rashba SOC in III-V semiconductors, SrTiO3, and other 2D hybrid perovskites with respect to symmetry and I-Pb-I angles. The symmetry of the p-orbitals and the bandgap shifts were utilized to perform an analysis of the SOC coupling parameter in the structures in comparison with relativistic effects of isolated Pb. We also report that spacing directly affects the curvature of the bands and the charge carrier mobility perpendicular to the inorganic planes and thus affects the directional charge transport in the 2D perovskite. A distance of 6 nm is the maximum length between the 2D layers to retain a similar effective mass of holes (3m(0)) in-plane as out-of-plane to allow effective hole charge carrier transport perpendicular to the inorganic layer.