Unraveling the Near-Unity Narrow-Band Green Emission in Zero-Dimensional Mn²⁺-Based Metal Halides: A Case Study of (C₁₀H₁₆N)₂Zn_1–<i>x</i>Mn_<i>x</i>Br₄ Solid Solutions

Zero-dimensional (0D) Mn²⁺-based metal halides are potential candidates as narrow-band green emitters, and thus it is critical to provide a structural understanding of the photophysical process. Herein, we propose that a sufficiently long Mn-Mn distance in 0D metal halides enables all Mn²⁺ centers to emit spontaneously, thereby leading to near-unity photoluminescence quantum yield. Taking lead-free (C₁₀H₁₆N)₂Zn_1-<i>x</i>Mn_<i>x</i>Br₄ (<i>x</i> = 0-1) solid solution as an example, the Zn/Mn alloying inhibits the concentration quenching that is caused by the energy transfer of Mn²⁺. (C₁₀H₁₆N)₂MnBr₄ exhibits highly thermal stable luminescence even up to 150 °C with a narrow-band green emission at 518 nm and a full width at half maximum of 46 nm. The fabricated white light-emitting diode device shows a high luminous efficacy of 120 lm/W and a wide color gamut of 104% National Television System Committee standard, suggesting its potential for liquid crystal displays backlighting. These results provide a guidance for designing new narrow-band green emitters in Mn²⁺-based metal halides.