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 (Cs2NaYbCl6 DPSCs) are fabricated with low phonon energy of 134 cm−1, which offers great potential to serve as highly efficient luminescent host materials. Subsequently, Er3+, Tm3+, and Ho3+ are successfully incorporated into Cs2NaYbCl6 host to form Cs2NaLnxYb1−xCl6 (Ln = Er, Tm, and Ho) DPSCs, realizing efficient dual‐mode polychromatic emission. Under 980 nm laser excitation, Cs2NaLnxYb1−xCl6 DPSCs show high upconversion (UC) efficiencies of 12.9%, 7.2%, and 3.1% for Cs2NaEr0.3Yb0.7Cl6, Cs2NaTm0.02Yb0.98Cl6, and Cs2NaHo0.3Yb0.7Cl6, respectively. Meanwhile, Cs2NaErxYb1−xCl6 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. All‐inorganic rare‐earth‐based Cs2NaYbCl6:Er3+ double perovskite single crystals with lower phonon energy of 134 cm−1 are successfully synthesized by modified hydrothermal synthesis method, and exhibit significantly efficient dual‐mode fluorescence behavior. Under 380, 980, or 1540 nm laser excitation, they exhibit efficient red, yellow, or green emission, respectively. Based on this, the high‐level information encryption is carried out.