Reversible Luminescence in Te 4+ ‐Doped Cs 2 InCl 5 (H 2 O) by Solvent‐Induced Switchable Phase Transformations for Dynamic Information Writing and Erasing

Abstract Lead‐free halide perovskites demonstrate potential in fluorescent anti‐counterfeiting and information encryption due to their dynamic responsiveness to external stimuli and exceptional luminescent properties. However, current materials face problems such as complex synthesis schemes, irreversible solvent responses, and high optical visibility of inks. This work adopts a hydrochloric acid‐assisted synthesis strategy, achieving batch preparation of Te 4+ ‐doped Cs 2 InCl 5 (H 2 O) perovskites at room temperature within only 5 min, with a high yield of ≈95%. Notably, solvent‐induced reversible structural phase transitions between Cs 2 InCl 5 (H 2 O) and Cs 3 InCl 6 enable dynamic fluorescence switching between orange emission and a colorless state. Systematic investigations reveal that the removal/recovery of crystallization water during phase evolution, which disrupts/reconstructs the Te 4+ coordination field, serves as the key governing fluorescence “ON‐OFF” switching. Building upon this foundation, a colorless and transparent Cs 2 InCl 5 (H 2 O): Te 4+ @PVA ink has been developed, enabling reversible writing/erasing of encrypted information patterns. These patterns retain excellent readability even after 10 reversible cycles. Furthermore, two additional ink formulations are prepared through solvent ratio modulation and two‐component polydimethylsiloxane (PDMS) encapsulation. A multi‐level anti‐counterfeiting strategy is designed by synergistically combining multiple inks. The printed patterns and encrypted information exhibit enhanced resistance to forgery and reverse‐engineering attempts, achieving better protection levels for confidential data.