Ultra-High-Efficiency and Reversible Hydrochromism in a CsCaCl3:Cd2+/Mn2+ Perovskite for Advanced Information Encryption

Lead-free metal halide perovskites have attracted considerable interest due to their excellent optical properties and environmental friendliness. However, their practical application is often hindered by a low photoluminescence quantum yield (PLQY). Herein, we report a novel Cd2+/Mn2+ codoped CsCaCl3 perovskite crystal that achieves a record-breaking PLQY of 93%, which is realized through a novel energy transfer pathway. By varying the Mn2+ doping concentration in CsCaCl3:4%Cd2+, the crystal’s emission color can be tuned from cyan to yellow across the 400–700 nm wavelength range. Interestingly, the material also exhibits a distinct hydrochromic behavior. Upon humid air exposure, it undergoes a structural transformation from CsCaCl3:Cd2+/Mn2+ to Cs2CaCl4·2H2O:Cd2+/Mn2+, accompanied by a dramatic emission color shift from yellow to orange. This process exhibits excellent reversibility over multiple wetting–drying cycles. The combination of ultrahigh PLQY, tunable luminescence, and stimuli-responsive hydrochromism makes this material highly promising for advanced applications in dynamic information encryption and anticounterfeiting. Moreover, the CsCaCl3:4%Cd2+/3%Mn2+ crystal exhibits tunable energy-transfer emission under different temperatures, achieving a high relative thermal sensitivity of 5.48%·K–1, demonstrating its potential for optical thermometry.