Ag Doping Triggered Electronic Structural Engineering in Halide Quadruple Perovskite Toward Near‐Unity Luminescence Quantum Yield

Abstract Ion doping is an effective strategy to regulate the optical performance in metal halide perovskites for desirable light emission manipulations. In this work, a Mn 2+ and Ag + co‐doping strategy is utilized to improve the luminescence in Cs 4 CdBi 2 Cl 12 quadruple perovskite. Mn 2+ and Ag + ions are proven to be incorporated into Cs 4 CdBi 2 Cl 12 lattice. Mn 2+ ions occupy Cd 2+ sites and Ag + ions occupy Cd 2+ sites or vacancies. The alloyed Mn 2+ ions bring efficient orange‐red luminescence attributed to the energy transfer process from the host to Mn 2+ ions. A small amount of Ag + doping can optimize the band structure and increase the exciton binding energy, thus promoting efficient radiative recombination from d–d transition of Mn 2+ ions and boosting the PLQY up to 94.6%. Furthermore, Cs 4 Cd 0.75 Mn 0.25 Bi 2 Cl 12 :1.5%Ag + show good phase and emission stability in ambient environment and is employed in white light‐emitting diodes with commercial blue and green phosphors on a near‐ultraviolet LED chip. This work provides insights for understanding the composition‐structure‐property relationship in vacancy‐ordered quadruple perovskites and designing highly efficient luminescent lead‐free metal halides for solid‐state light emitting applications.