High‐Efficiency Dual‐Mode Polychromatic Emission of Rare‐Earth‐Based Double Perovskite with Low Phonon Energy toward High‐Level Anti‐Counterfeiting and Information Encryption

The development of rare‐earth‐doped luminescent materials with single component, multicolor, and multimode still remains a challenge. Here, rare‐earth‐based double perovskite single crystals (Cs 2 NaYbCl 6 DPSCs) are fabricated with low phonon energy of 134 cm −1 , which offers great potential to serve as highly efficient luminescent host materials. Subsequently, Er 3+ , Tm 3+ , and Ho 3+ are successfully incorporated into Cs 2 NaYbCl 6 host to form Cs 2 NaLn x Yb 1− x Cl 6 (Ln = Er, Tm, and Ho) DPSCs, realizing efficient dual‐mode polychromatic emission. Under 980 nm laser excitation, Cs 2 NaLn x Yb 1− x Cl 6 DPSCs show high upconversion (UC) efficiencies of 12.9%, 7.2%, and 3.1% for Cs 2 NaEr 0.3 Yb 0.7 Cl 6 , Cs 2 NaTm 0.02 Yb 0.98 Cl 6 , and Cs 2 NaHo 0.3 Yb 0.7 Cl 6 , respectively. Meanwhile, Cs 2 NaEr x Yb 1− x Cl 6 DPSCs exhibit the adjustable size (0.5–10 mm) and tunable UC or down‐shifting emission colors from yellow, green to red upon 980, 1540, or 380 nm excitation. Moreover, the DPSCs demonstrate weak concentration and thermal quenching effect, and excellent resistance against oxygen, heat and humidity. Finally, the anti‐counterfeiting and information encryption applications based on multicolor emitting DPSCs are designed and realized.