Thermochromic reversible luminescence in Bi3+-doped Cs2ZrCl6 perovskites for multi-level optical anti-counterfeiting

Conventional materials with single emission centers face limitations in emission tunability and anti-counterfeiting security. Thermochromic-induced reversible luminescence modulation offers an effective approach for multi-level optical encryption. In this work, we demonstrate an information‐encryption strategy based on a vacancy-ordered perovskite that undergoes reversible, delayed luminescence shifts. Blue-emitting Bi 3+ -doped Cs 2 ZrCl 6 crystals were synthesized via a facile precipitation method and exhibit dual emission centers under different excitations. Under 365 nm excitation, the Bi 3+ emission at 465 nm undergoes a thermochromic‐induced red shift to 500 nm upon annealing at 110–180 °C; this visibly distinct green state persists for seven days at room temperature and fully reverts to blue over ≈40 days. In contrast, the intrinsic Cs 2 ZrCl 6 self-trapped-exciton emission at 254 nm excitation remains constant, providing an internal reference for enhanced anti-counterfeiting security. Comprehensive photoluminescence and UV–Vis diffuse-reflectance analyses reveal that annealing modulates the band gap via Bi 3+ -induced intermediate states, thereby driving the reversible emission shift without altering crystal structure or oxidation states. By incorporating the Bi 3+ -doped Cs 2 ZrCl 6 powder into conventional screen-printing ink (5:1 mass ratio), we fabricate nearly invisible security patterns that switch from blue to green under 365 nm UV after heating and automatically recover, enabling multi-cycle thermal encryption. This study not only elucidates the band-gap–mediated mechanism of thermochromic luminescence in lead-free perovskites but also establishes a versatile platform for next-generation, reversible optical security and anti-counterfeiting applications.