Sn-Doped Cs₃Cu₂I₅ Microcrystals with High Photoluminescence and Hydrochromic Stability for Anti-Counterfeiting and Encryption Applications

Cu-based perovskites have excellent optical properties, environmental friendliness, and broad optical applications. However, their development is hindered by complex preparation methods and poor hydrochromic stability. In this work, a solvent-evaporation-based crystallization strategy combined with Sn doping for the preparation of Cs3Cu2I5 microcrystals (MCs) is reported. This strategy enhances the photoluminescence quantum yield (PLQY) and hydrochromic reliability. In addition, first-principles calculations reveal the effect of Sn doping on the lattice structure and exciton-phonon coupling of the Cs3Cu2I5 MCs. The results show that Sn doping reduces the lattice spacing as a result of the substitution radius difference between Cu+ and Sn2+ ions, which causes stronger exciton-phonon coupling and enhances the PLQY. Moreover, the Sn-doped Cs3Cu2I5 MCs show excellent reliability on heating and under hydrochromic cycles and long-term luminescence conditions. Based on their hydrochromic and temperature-responsive properties, the Sn-doped Cs3Cu2I5 MCs are applied as encryptable printing materials for information encryption and decryption and as a fluorescence-based temperature sensor, combined with Machine-Learning to guide the manufacturing of fluorescent thermometers. The findings provide insights into the commercial value of copper-based perovskites and demonstrate their potential anti-counterfeiting, fluorescence temperature sensing applications.