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

Vacancy-ordered double perovskites with the formula <i>A</i> ₂ <i>MX</i> ₆ (where <i>A</i> is a +1 cation, <i>M</i> is a +4 metal, and <i>X</i> is a halide ion) offer improved ambient stability over other main-group halide <i>AMX</i> 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.