Blue-excitable lead-free metal halide microcrystals (MCs) with high efficiency and narrow emission are highly desired for next-generation wide-gamut displays. Herein, an ultrafast self-assembly strategy is developed to prepare hybrid manganese bromide MCs with tunable alkyl-chain lengths. With increasing alkyl-chain length, the emission intensity of the obtained MCs first improves significantly and then deteriorates, accompanied by a gradual narrowing of the full width at half maximum (FWHM) from 46.37 to 41.97 nm. The optimized [(C₂H₅)₄N]₂MnBr₄ MCs exhibit strong narrow-band green emission at 518 nm with a FWHM of 43.30 nm and a high photoluminescence quantum yield of 98.70% under 455 nm excitation. This superior emission performance is ascribed to cooperative weak interactions, an appropriate Mn-Mn distance, and highly localized electronic transitions within isolated [MnBr₄]²⁻ tetrahedra. Moreover, the [(C₂H₅)₄N]₂MnBr₄ MCs show moderate thermal stability, retaining 52.03% of their initial emission intensity at 413 K. By integrating [(C₂H₅)₄N]₂MnBr₄ with K₂SiF₆: Mn⁴⁺ red phosphors on a blue chip, a white light-emitting diode with a high luminous efficacy of 134.60 lm/W and a wide gamut of 112.6% NTSC is achieved, demonstrating the great promise of ultrafast-assembled hybrid manganese bromides for efficient and wide gamut display backlighting.

Hybrid manganese bromides have emerged as promising lead-free narrow-band green emitters for wide-gamut displays, but their practical application is still limited by insufficient operational stability. Herein, Zn²⁺ substitution is employed to enhance the stability of tetraethylammonium manganese bromide (TEA₂MnBr₄) microcrystals (MCs) synthesized via an ultrafast self-assembly method. With increasing Zn²⁺ contents, the photoluminescence intensity of TEA₂Mn_1−xZn_xBr₄ MCs gradually decreases, while the x = 0.10 MCs still exhibit efficient narrow-band green emission at 518 nm and a high photoluminescence efficiency of about 85% under blue excitation. More importantly, Zn²⁺ incorporation substantially enhances the storage and thermal stability, as well as operational stability of the corresponding green LED devices (retaining above 99% of initial luminous efficacy after 160 h continuous operation at 20 mA). Finally, a white LED fabricated using the x = 0.10 MCs achieves a high luminous efficiency of about 136 lm/W and covers about 112% of the National Television System Committee 1931 color gamut. These results demonstrate that Zn²⁺ substitution is an effective strategy for stabilizing hybrid manganese bromide emitters and highlight their potential for lead-free wide-gamut display applications.