Mixed‐Cation Engineering for Enhanced Efficiency and Stability of Mo ⁴⁺ ‐Doped Lead‐Free Perovskite for Near‐Infrared Phosphors

ABSTRACT Broadband near‐infrared phosphors are essential for advancing energy‐efficient phosphor‐converted light‐emitting diodes in applications such as night vision and bioimaging. While Mo 4+ doped 0D metal halides offer a promising route to near‐infrared emission, achieving both high efficiency and stability remains challenging. Herein, we report a mixed‐cation strategy to synthesize Mo 4+ ‐doped (Rb 1− x Cs x ) 2 HfCl 6 vacancy‐ordered double perovskites, which exhibit broadband near‐infrared emission centered at 925 nm with a full width at half‐maximum over 180 nm. The optimized composition delivers a high photoluminescence quantum yield of 82.89% under 298 nm excitation and 77.64% under 763 nm excitation. Density functional theory and experiments confirm the formation of a thermodynamically stable Rb + /Cs + alloy, enabling the material to retain 80.5% of its room‐temperature photoluminescence intensity at 150°C. A prototype NIR pc‐LED is fabricated, demonstrating its potential for sensing and imaging. This work opens new horizons for optimizing the luminescence efficiency and stability of broadband near‐infrared luminescent metal halides.