Ion-doped lead-free double perovskite Cs2NaBiCl6 with multiple excitation and tunable emission towards light emitting and anti-counterfeiting applications

Lead-free double perovskite is emerging as an intriguing optical material particularly due to its non-toxicity, higher instability, and tunable multicolor luminescence. Herein, Cs2NaBiCl6 (CNBC) double perovskite was employed as a versatile host to accommodate ionic dopants for multicolor emission properties. By Br- ion doping, the excitation peak can be modulated continuously from 357 to 374 nm, such as the CNBC:30%Br- sample, which just covers the emitting light of commercial ultraviolet (UV) similar to 365 nm chip for possible lighting applications. Moreover, doping of Mn2+ into CNBC:30%Br- will retain the UV similar to 365 nm excitation characteristics but with much strengthened intensity. The CNBC:30%Br-, Mn2+ yields robust broadband emission at 585 nm from the 4T1 (4G) -> 6A1 (6S) transition of Mn2+, which after optimizing the Mn2+ concentration (about 15%) is about 72 times stronger than the emission of CNBC upon 365 nm excitation. Energy transfer from the self-trapping exciton (STE) state of halogen double perovskite to the Mn2+ activator, as well as the concentration quenching of Mn2+, were rationally proposed on the basis of experiments and theories. Using our prepared CNBC:30%Br-, 15%Mn2+ phosphor, a white light-emitting diode (WLED) with CIE chromaticity coordinate (0.35, 0.33) was fabricated by mixing commercial BaMgAl10O17:Eu phosphors on a UV similar to 365 nm chip. Most interestingly, the CNBC:30%Br-, 15%Mn2+ can further host Er3+ and Yb3+ ions for a novel material that can be excited by 365 nm light for orange luminescence and by 980 nm for unique green up-conversion, respectively. With the help of screen printing, information encryption/decryption and optical anti-counterfeiting were practically carried out under irradiation of 365 and 980 nm, which can be further developed into a promising multi-level fluorescent anti-counterfeiting technology using these lead-free double perovskite systems.