B‐Site Co‐Occupation of Sb/In Achieves Record‐High Quantum Yield in Lead‐Free Hybrid Halide Phosphors for Lighting and Anticounterfeiting Applications

Lead‐free halide phosphors with high photoluminescence efficiency and structural robustness offer a promising path for sustainable optoelectronic applications. However, most systems suffer from low quantum yields and poor stability due to uncontrolled environments and inefficient lattice regulation. In this work, we present a 0D heterometallic hybrid halide, (C 3 H 12 N 2 ) 2 [(Sb 0.5 In 0.5 Cl 6 )(H 5 O 2 )2Cl] (denoted as DAP‐SbInCl), where the equal co‐occupation of Sb 3+ and In 3+ ions at the same site modulates the lattice structure and electronic configuration, yielding an ultrahigh photoluminescence quantum yield (PLQY ≈ 98%). In contrast, the Sb‐only and In‐only analogues exhibit much lower PLQYs of ≈18% and ≈6%, respectively. The co‐occupation of Sb 3+ and In 3+ ions induce symmetry distortions around the Sb 0.5 In 0.5 Cl 6 octahedra, leading to band‐gap narrowing and enhanced radiative recombination. Moreover, DAP‐SbInCl demonstrates excellent thermal stability and superior solution processability, and high color rendering index of 96, making it suitable for use as a yellow phosphor for white light‐emitting diodes (WLED). The resulted high luminous efficacy (178.7 lm W −1 ) and high thermal stability (the intensity of luminescence reduces only ≈12% when the temperature is elevated to 420 K) are better than many commercially available high‐output WLEDs. This work demonstrates an effective B‐site engineering strategy for developing highly efficient Sb‐based hybrid halide phosphors.