Halogen vacancy migration at surfaces of CsPbBr3 perovskites: insights from density functional theory

R-I Biega,L Leppert

doi:10.1088/2515-7655/ac10fe

Abstract

Migration of halogen vacancies is one of the primary sources of phase segregation and material degradation in lead-halide perovskites. Here we use first principles density functional theory to compare migration energy barriers and paths of bromine vacancies in the bulk and at a (001) surface of cubic CsPbBr3. Our calculations indicate that surfaces might facilitate bromine vacancy migration in these perovskites, due to their soft structure that allows for bond lengths variations larger than in the bulk. We calculate the migration energy for axial-to-axial bromine vacancy migration at the surface to be only half of the value in the bulk. Furthermore, we study the effect of modifying the surface with four different alkali halide monolayers, finding an increase of the migration barrier to almost the bulk value for the NaCl-passivated system. Migration energies are found to be correlated to the lattice mismatch between the CsPbBr3 surface and the alkali halide monolayer. Our calculations suggest that surfaces might play a significant role in mediating vacancy migration in halide perovskites, a result with relevance for perovskite nanocrystals with large surface-to-volume ratios. Moreover, we propose viable ways for suppressing this undesirable process through passivation with alkali halide salts.

Highlights

Halide perovskites are exciting materials with exceptional optoelectronic properties, wide tunability, and a broad range of applications spanning solar cells [1,2,3], light-emitting diodes (LEDs) [4, 5], photo-detectors [6,7,8] and x-ray scintillators [9]
The average relative variation of Pb–Br bonds per layer as compared to the bulk Pb–Br bond length of 2.93 Å is shown in figure 2(a), where we have averaged over all axial and equatorial Pb–Br bonds, respectively
We performed a first principles density functional theory (DFT) study of Br vacancy migration in CsPbBr3 and showed that the migration barrier within the close-packed bulk structure of cubic CsPbBr3 is about twice as large as that at either of the distinctly terminated (001) surfaces of the system

Summary

Introduction

Halide perovskites are exciting materials with exceptional optoelectronic properties, wide tunability, and a broad range of applications spanning solar cells [1,2,3], light-emitting diodes (LEDs) [4, 5], photo-detectors [6,7,8] and x-ray scintillators [9]. All-inorganic lead-halide perovskites CsPbX3 have seen their own surge of interest, in particular because colloidal CsPbX3 nanocrystals can exhibit very high photoluminescence quantum yields, with band gap energies and emission spectra tunable over the entire visible spectral region [14]. Even all-inorganic halide perovskites can exhibit poor stability under electric fields [15]. Material degradation and phase separation in both organic-inorganic and all-inorganic halide perovskites have been attributed to the migration of mobile ionic species [16]

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Conclusion