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) ∼ 365 nm chip for possible lighting applications. Moreover, doping of Mn²⁺ into CNBC:30%Br⁻ will retain the UV ∼ 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 ∼ 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.