Centimeter-Scale Ethyltriphenylphosphonium Manganese(II) Halide Single Crystals as Thermally Robust Green Phosphors for White LEDs

Lead-free manganese(II) halide hybrids are promising phosphors for solid-state lighting, but the effect of halide substitution within a fixed organic-cation framework remains insufficiently resolved. Here, centimeter-scale ETP2MnCl4 and ETP2MnBr4 single crystals were grown at room temperature by antisolvent diffusion using ethyltriphenylphosphonium (ETP) as the organic cation. Single-crystal X-ray diffraction and powder X-ray diffraction identified zero-dimensional structures composed of isolated [MnX4]2– tetrahedra separated by ETP cations. Both compounds exhibited Mn2+-centered green emission under blue-compatible excitation. ETP2MnCl4 and ETP2MnBr4 emitted at 518 nm with a fwhm of 43 and 520 nm with a fwhm of 44 nm. ETP2MnBr4 reached a photoluminescence quantum yield (PLQY) of 64.0%, compared with 28.1% for ETP2MnCl4, and retained 73.9% of its room-temperature PL intensity at 390 K. In phosphor-converted WLEDs, the chloride formulation delivered a luminous efficacy of 102.54 lm·W–1, whereas the bromide formulation achieved a CRI of 84.4. The matched-cation comparison shows that chloride-to-bromide substitution preserves the basic emission mechanism but changes local distortion, Mn2+-Mn2+ separation, optical-gap response, decay kinetics, electron–phonon coupling descriptors, and thermal performance. These results provide a composition-guided basis for selecting lead-free Mn2+ halide phosphors according to efficiency, color temperature, color rendering, and operating-temperature requirements.