Resolving Optical Bandgap Discrepancies in Vacancy‐Ordered Double Perovskites via Iodine Vacancy Control

Vacancy‐ordered double halide perovskites (A ₂ BX ₆ ) are emerging as promising lead‐free alternatives for optoelectronic applications owing to their high chemical stability and low toxicity. However, substantial discrepancies in reported optical bandgap values have hindered the rational design and optimization of device performance. Herein, we propose a bandgap‐engineering strategy that modulates iodine content during mechanochemical synthesis using additional iodine sources, including HI and I ₂ . This approach enables tunable bandgaps, strongly correlated with variations in nominal iodine‐site deficiency, with iodine deficiency inducing pronounced bandgap widening. Density functional theory (DFT) calculations suggest that iodine vacancies weaken Te–I orbital overlap, leading to an increase in the bandgap, whereas iodine enrichment suppresses vacancy formation and consequently narrows the bandgap. As a representative case, cesium tellurium iodide (Cs ₂ TeI ₆ ), which exhibits one of the largest reported bandgap discrepancies, was investigated in detail. Systematically increasing the HI content tuned the optical bandgap of Cs ₂ TeI ₆ from 1.78 to 1.58 eV, indicating that bandgap modulation is strongly correlated with nominal iodine‐site deficiency. Similar trends in other A ₂ BX ₆ compounds synthesized under iodine‐rich conditions confirm the generality of this behavior. These findings highlight halide‐vacancy engineering as an effective strategy for tuning the optical bandgap of A ₂ BX ₆ materials.