Correction to Rashba Spin-Orbit Coupling Enhanced Carrier Lifetime in CH3NH3PbI3.

Organometal halide perovskites are promising solar-cell materials for next-generation photovoltaic applications. The long carrier lifetime and diffusion length of these materials make them very attractive for use in light absorbers and carrier transporters. While these aspects of organometal halide perovskites have attracted the most attention, the consequences of the Rashba effect, driven by strong spin-orbit coupling, on the photovoltaic properties of these materials are largely unexplored. In this work, taking the electronic structure of methylammonium lead iodide as an example, we propose an intrinsic mechanism for enhanced carrier lifetime in 3D Rashba materials. Based on first-principles calculations and a Rashba spin-orbit model, we demonstrate that the recombination rate is reduced due to the spin-forbidden transition. These results are important for understanding the fundamental physics of organometal halide perovskites and for optimizing and designing the materials with better performance. The proposed mechanism including spin degrees of freedom offers a new paradigm of using 3D Rashba materials for photovoltaic applications. The organometal halide perovskites (OMHPs) have attracted significant attention due to the rapid increase in their photovoltaic power conversion efficiency. In the past 2 years, the reported efficiency of OMHP-based solar cells has almost doubled from 9.7%1 to over 20%,2–4 making OHMPs very promising for low-cost and high-efficiency photovoltaics. Methylammonium lead iodide, CH3NH3PbI3 (MAPbI3), and other closely-related hybrid perovskites such as Cl-doped and Br-doped MAPbI3 (MAPbI3−xClx and MAPbI3−xBrx), (NH2)2CHPbI3 (formamidinium lead iodide, FAPbI3), and Sn-doped MAPbI3 (MAPbxSn1−xI3), all display band gaps (1.1 to 2.1 eV) in the visible light region, favorable for photovoltaic applications.5–13 The class of materials also possesses strong light absorption, fast charge generation and high carrier mobility.14,15 In particular, exceptionally long carrier lifetime and diffusion length have been observed in MAPbI3 and MAPbI3−xClx, making them better solar cell candidates than other semiconductors with similar band gaps and absorption coefficients.16–18 Intense research has been directed toward understanding and further enhancing the long carrier lifetime and diffusion length in OMHPs. Previous studies reported a relatively low defect 2 concentration in MAPbI3, which reduces the scattering centers for nonradiative charge carrier recombination. Recently, it has been suggested that the spatial carrier segregation caused by disorder-induced localization24 or domains acting as internal p-n junctions25–27 may reduce the recombination rate. Though many of the OMHPs are 3D Rashba materials driven by strong spin-orbit coupling (SOC) and bulk ferroelectricity,28–31 the effects of spin and orbital degrees of freedom on photovoltaic applications are largely unexplored beyond band gap engineering.29 In this work, we focus on an intrinsic mechanism for the enhancement of long carrier lifetime due to the Rashba splitting. Using first-principles calculations and effective models, we find that the Rashba splitting arising from SOC under inversion symmetry breaking can result in spin-allowed and spin-forbidden recombination channels. The spin-forbidden recombination path has a significantly slower transition rate due to the mismatch of spin and momentum. The spin-allowed recombination path, though kinetically favorable, can be suppressed under appropriate spin texture due to the low population of free carriers. Taking the electronic structures of MAPbI3 under various distortions as examples, we show that the proposed mechanism is possible under room temperature, and is potentially responsible for the long carrier lifetime in OMHPs. This spindependent recombination mechanism highlights the possibility of using 3D Rashba materials for efficient photovoltaic applications. Fig. 1 illustrates the mechanism for enhancing the carrier lifetime in a generic 3D Rashba material. The strong spin-orbit coupling effect from heavy elements (e.g., Pb, Sn, I and Br) and the polar distortion (e.g., aligned molecular dipoles in OMHPs) give rise to the Rashba effect, which lifts the two-fold degeneracy of bands near the band gap. Near the band gap, the spin degeneracies of the conduction and valence bands are lifted, giving rise to “inner" and “outer" bands with opposite spin textures, characterizing spin rotation direction as “clockwise" (χ = −1) and “counterclockwise" (χ = +1) (Fig. 1). The photo excitation process creates free electrons and holes, which can quickly relax to band extrema in the presence of inelastic phonon scattering. When the spin textures of conduction band minimum (CBM) and valence band maximum (VBM) are opposite, the radiative recombination of Cχ=−1 → Vχ=+1 is a spin-forbidden transition due