Blue emission band governed by chlorine vacancy defect states in the photoluminescence of Cs2NaInCl6 microcrystal

The double perovskite Cs2NaInCl6 microcrystals (MCs) have drawn growing interest, yet the origin of their luminescence remains debated. Herein, systematic comparison of excited-state photophysical dynamic and electronic structures before and after Pb2+ doping assigns the emission to specific excited-state configurations of Cs2NaInCl6 MCs. By combining steady-state, temperature-dependent, time-resolved, and power-dependent photoluminescence spectroscopy with density functional theory calculations, the 445 nm emission band in Cs2NaInCl6 MCs is identified as Cl vacancy related defect states. Such defect states, corroborated by electron localization function mapping, exhibit strong electron localization that enhances the electron density around adjacent In3+, which consequently accounts for the nearly temperature-independent peak position and linewidth of the blue emission band. Furthermore, the weak 515 nm broadband emission is attributed to self-trapped exciton (STE), and trace Pb2+ doping breaks the parity-forbidden transition, enhancing the STE emission and boosting the photoluminescence quantum yield to 67.55%. Moreover, the defect emission intensity is progressively enhanced with increasing Pb2+ doping concentration, which is attributed to the reduced defect activation energy upon Pb2+ incorporation. Under band edge excitation (290–300 nm), a minor subset of photoexcited carriers relaxes to defect states and recombines radiatively. Therefore, the STE emission band has a marginal contribution from defect emission with nanosecond-scale lifetime. In contrast, emission under sub-bandgap excitation (310–350 nm) comes purely from defect states. The anomalous excitation wavelength-dependent emission behavior is thereby clarified. These findings resolve the debate on the luminescence origin in Cs2NaInCl6 and provide a basis for understanding and engineering the optical properties of double perovskites.