Hydrochromic Perovskite System with Reversible Blue‐Green Color for Advanced Anti‐Counterfeiting

Abstract The intrinsic instability of halide perovskites toward to external stimulus, has created a competitive advantage for designing stimuli‐responsive materials. However, the external environment tuning reversibly fluorescence emission of perovskite system is still limited. In this work, humidity is verified to act as a new option to modulate the emission properties of mixed‐halide perovskite. The perovskite nanocrystals (PNCs) photoirradiated in dichloromethane are easily and stably redispersed in water, and emit bright fluorescence which is quite different from the original. Moreover, the perovskites confined on glass slide can reversibly switch their fluorescence between blue and green colors under moisture. It is demonstrated that the factors of different solubilities of CsCl and CsBr in water, the structural transformation of perovskites and the confine of glass matrix play key roles in the reversible transformation. Finally, the combination of hydrochromic CsPb(Br x Cl y ) 3 and water‐resistant CsPb(Br x Cl y ) 3 ‐polymethyl methacrylate have been applied in advanced anti‐counterfeiting, which greatly improves the information security. This work not only give an insight into the effects of humidity on fluorescence and structures of PNCs, but also offer a new class of hydrochromic PNCs materials based on reversible emission transformation for potential application in sensors, anti‐counterfeiting and information encryption.