Small Electron Polarons in CsPbBr3: Competition between Electron Localization and Delocalization

Nicklas Österbacka,Paul Erhart,Stefano Falletta,Alfredo Pasquarello,Julia Wiktor

doi:10.1021/acs.chemmater.0c02345

Abstract

We study the nature of excess electrons in CsPbBr3 and identify several single and double polaronic states. We emphasize the importance of proper inclusion of the self-interaction corrections for the stability of small electron polarons in this material. We demonstrate that spin–orbit coupling (SOC) has a significant impact on the energetics of the polaronic states. In particular, we find that SOC disfavors electron localization and leads to different polaronic geometries. Additionally, by carrying out thermodynamic integration, we show that small electron polarons are thermally stabilized in CsPbBr3. The small energy differences between the localized and delocalized electronic states could possibly reconcile the apparently conflicting properties of high charge-carrier mobilities and low recombinations rates.

Highlights

Metal halide perovskites have gained remarkable interest as promising materials for optoelectronic applications, for instance, in efficient solar cells.[1−3] The outstanding performance of solar devices based on halide perovskites stems from long lifetimes and diffusion paths of photogenerated electrons and holes,[4−6] among other things
We showed that electron polarons in CsPbBr3 can form in several geometries, in the case of both single and double charge localization
We showed that the effect of spin−orbit coupling (SOC) corresponds to a complex interplay between the shifts in the CBM and the polaronic level

Summary

■ INTRODUCTION

Metal halide perovskites have gained remarkable interest as promising materials for optoelectronic applications, for instance, in efficient solar cells.[1−3] The outstanding performance of solar devices based on halide perovskites stems from long lifetimes and diffusion paths of photogenerated electrons and holes,[4−6] among other things. The conduction band edge of halide perovskites, especially of the Pb-based ones, has been shown to be strongly affected by relativistic effects.[42−44] To assess the influence of SOC on polaronic states in CsPbBr3, we perform additional calculations using the Vienna ab initio package (VASP)[28,29] on top of the geometries relaxed with. We determine the minimum energy path between the configuration of the delocalized electron (pristine supercell) and that of the most stable single polaron state (P1 geometry) This is done by carrying out nudged elastic band (NEB)[47] calculations using CP2K without SOC. Due to the differences in the local geometries, the thermal stabilization calculated for the single polaron cannot be trivially applied to these configurations

■ DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES