Structural dynamics in hybrid halide perovskites: Bulk Rashba splitting, spin texture, and carrier localization

TL;DR

This study investigates how thermal structural fluctuations influence Rashba splitting, spin texture, and carrier localization in hybrid halide perovskites, challenging the idea that quasi-indirect band gaps explain their long carrier lifetime.

Contribution

The paper provides a detailed analysis of the dynamic behavior of Rashba splitting and carrier localization in (CH3NH3)PbI3, highlighting the impact of lattice fluctuations on electronic properties.

Findings

Rashba splitting energy is affected by Pb-atom displacement and thermal fluctuations.

Dynamic lattice effects reduce Rashba splitting and cause hole localization.

Long carrier lifetime may be due to hole localization rather than quasi-indirect band gap.

Abstract

The extended carrier lifetime in hybrid halide perovskites was attributed to a quasi-indirect band gap that arises due to Rashba splitting in both conduction and valence band edges. In this paper, we present results for an effective relativistic band structure of (CH3NH3)PbI3 with the focus on the dispersion of electronic states near the band edges of (CH3NH3)PbI3 affected by thermal structural fluctuations. We establish a relation between the magnitude of Rashba splitting and a deviation of Pb-atom from its centrosymmetric site position in the PbI6 octahedron. For the splitting energy to reach the thermal energy kT~26 meV (room temperature), the displacement should be of the order 0.3 Ang, which is far above the static displacements of Pb-atoms in the tetragonal phase of (CH3NH3)PbI3. The significant dynamic enhancement of the Rashba splitting observed at earlier simulation times (less than 2 ps) later weakens and becomes less than the thermal energy despite the average displacement of Pb-atoms remaining large (0.37 Ang). It is randomization of Pb-displacement vectors and associated cancelation of the net effective magnetic field acting on electrons at the conduction band edge is responsible for reduction of the Rashba splitting. The lattice dynamics also leads to deterioration of Bloch character for states in the valence band leading to subsequent localization of holes, which affects bipolar mobility of charge carriers in (CH3NH3)PbI3. These results call into question the quasi-indirect band gap as a reason for the long carrier lifetime observed in (CH3NH3)PbI3 at room temperature. An alternative mechanism involves dynamic localization of holes and their reduced overlap with electrons in reciprocal space.