Tuning the Optical Absorption Edge of Vacancy-Ordered Double Perovskites through Metal Precursor and Solvent Selection.

Abstract

Vacancy-ordered double perovskites with the formula A 2 MX 6 (where A is a +1 cation, M is a +4 metal, and X is a halide ion) offer improved ambient stability over other main-group halide AMX 3 perovskites and potentially reduced toxicity compared to those containing lead. These compounds are readily formed through a number of synthetic routes; however, the manner in which the synthetic route affects the resulting structure or optoelectronic properties has not been examined. Here, we investigate the role of distinct precursors and solvents in the formation of the indirect band gap vacancy-ordered double perovskite Cs2TeBr6. While Cs2TeBr6 can be synthesized from TeBr4 or TeO2, we find that synthesis from TeBr4 is more sensitive to solvent selection, requiring a polar solvent to enable the conversion of TeBr4. Synthesis from TeO2 proceeds in all of the organic solvents tested, provided that HBr is added to solubilize TeO2 and enable the formation of [TeBr6]2-. Furthermore, the choice of metal precursor and solvent impacts the product color and optical absorption edge, which we find arises from particle size effects. The emission energy remains unaffected, consistent with the idea that emission in these zero-dimensional structures arises from the isolated [TeBr6]2- octahedra, which undergo dynamic Jahn-Teller distortion rather than band-edge recombination. Our work highlights how even minor changes in synthetic procedures can lead to variability in metrics such as the absorption edge and emission lifetime and sheds light on how the optical properties of these semiconductors can be controlled for light-emitting applications.