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

Abstract 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 Cs₂NaLn_xYb_1−xCl₆ (Ln = Er, Tm, and Ho) DPSCs, realizing efficient dual‐mode polychromatic emission. Under 980 nm laser excitation, Cs₂NaLn_xYb_1−xCl₆ DPSCs show high upconversion (UC) efficiencies of 12.9%, 7.2%, and 3.1% for Cs₂NaEr_0.3Yb_0.7Cl₆, Cs₂NaTm_0.02Yb_0.98Cl₆, and Cs₂NaHo_0.3Yb_0.7Cl₆, respectively. Meanwhile, Cs₂NaEr_xYb_1−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.