Probing local coordination and halide miscibility in single-, double-, and triple-halide perovskites using EXAFS

TL;DR

This study uses cryogenic X-ray absorption spectroscopy to analyze local halide coordination in mixed-halide perovskites, revealing insights into halide mixing and stability crucial for optimizing perovskite solar cell materials.

Contribution

It provides detailed local structural information on halide mixing in triple-halide perovskites, advancing understanding beyond bulk measurements.

Findings

Triple-halide perovskites form a single phase with bromide-mediated miscibility.

Signatures of halide mixing identified via Pb L3-edge EXAFS and wavelet transforms.

All three halides coordinate at short range, confirmed by wavelet analysis of Br K-edge EXAFS.

Abstract

Lead-halide perovskites are a promising material platform as semiconductors in next-generation solar cells because of their solution processability, defect tolerance, and tunable optoelectronic properties. While iodide-bromide perovskite compositions have shown promise as wide bandgap absorbers, they also suffer from significant instabilities under operating conditions. Triple-halide perovskites, where chloride is additionally incorporated, have demonstrated improved stability and performance over their double-halide counterparts; however, relatively little is understood about halide miscibility and incorporation in these novel materials. While bulk metrics such as lattice spacing and optical bandgap can be consistent with incorporation of chloride into a single phase, these results are not sufficient to fully describe the material as having homogeneous mixing on the X site. This uncertainty motivates the use of a more local probe to study short-range halide coordination and illuminate the role of chloride in triple-halide perovskites. We use cryogenic X-ray absorption spectroscopy (XAS) to characterize lead-halide bonds in a range of single-, double-, and triple-halide perovskite compositions. We show formation of a single-phase triple-halide perovskite whose miscibility is mediated by bromide content. We identify signatures of halide mixing from the Pb L3-edge EXAFS of mixed double- and triple-halide perovskites using both quantitative fits and Cauchy wavelet transforms. Finally, using wavelet transforms of the Br K-edge EXAFS, we demonstrate via forward scattering amplified 3rd shell halide-halide interactions that all three halides coordinate at short range in a fully mixed perovskite phase. These results are a step forward in the understanding of local structure that is required to fully describe and optimize halide incorporation for novel perovskite compositions.