Why Fe doping kills photoluminescence in CsPbCl₃ but not in CsPbBr₃: Role of midgap Fe 3d states and electron-phonon coupling

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Understanding the impact of transition-metal doping on the optoelectronic properties of halide perovskite nanocrystals is essential for their rational design in photonic applications. We establish the microscopic origin of photoluminescence (PL) quenching in Fe-doped CsPbCl₃ using spin-polarized density functional theory calculations. The emergence of Fe 3d midgap states creates efficient electron-trapping centres that drive nonradiative recombination, accounting for the reduced PL intensity. Extending this analysis to Fe-doped CsPbX₃ (X = Cl, Br), we show experimentally that although PL intensity is suppressed in both systems relative to their pristine counterparts, their high-doping behaviour diverges: CsPbCl₃ becomes completely non-emissive, whereas CsPbBr₃ retains a finite, saturated PL intensity. Despite this contrast, electronic structure calculations reveal nearly identical midgap states in both materials, indicating that electronic effects alone cannot explain the distinct PL responses. Phonon calculations likewise fail to capture this difference. In contrast, electron-phonon coupling calculations based on the deformation potential approach reveal significantly stronger coupling in Fe-doped CsPbCl₃, enabling efficient dissipation of electronic excitation energy into lattice vibrations and leading to complete PL quenching. These results identify electron-phonon coupling as the key factor governing halide-dependent PL quenching and provide a unified microscopic framework for dopant-induced nonradiative processes in halide perovskites.