${\mathrm{Bi}}^{3+}$-doped ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$ (${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6:\mathrm{Bi}$) nanocrystals (NCs) have emerged as promising lead-free perovskites. However, the photophysical pathways governing their emission remain to be elucidated. Combining spectroscopic measurements with density functional theory, this work elucidates the role of ${\mathrm{Bi}}^{3+}$ doping in enabling singlet-to-triplet transitions and the origin of the asymmetric photoluminescence (PL) line shape. ${\mathrm{Bi}}^{3+}$ doping in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$ NCs enables triplet self-trapped exciton (STE) emission, evidenced by two distinct microsecond-scale lifetimes, a redshift in the delayed PL spectrum matching the singlet-triplet energy gap, temperature-dependent PL (TDPL) deconvolution resolving two peaks, and integrated intensity analysis yielding two binding energies split by the singlet-triplet gap, as well as transient absorption measurements revealing the intersystem crossing timescale. Theoretical calculations reveal that ${\mathrm{Bi}}^{3+}$ doping introduces strong spin-orbit coupling and breaks local inversion symmetry, which inverts the singlet and triplet energy levels and thereby facilitates the singlet-to-triplet transition. The PL spectrum broadening arises from multiple recombination processes, including free excitons and singlet/triplet self-trapped excitons, as well as ionized impurity and phonon scattering. The spectral asymmetry is mainly attributed to the cumulative emission from long-lived triplet STEs. Additionally, ${\mathrm{Bi}}^{3+}$ doping in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$ NCs induces pronounced phonon-assisted absorption, indicating the presence of strong electron-phonon coupling. The correlation between TDPL broadening and temperature-dependent Raman spectroscopy establishes longitudinal optical phonons as the dominant contributors to electron-phonon coupling. This work unravels the microscopic mechanism underlying triplet emission in ${\mathrm{Cs}}_2{\mathrm{AgInCl}}_6$:Bi NCs, offering a predictive basis for the rational design of luminescent lead-free perovskites.

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• Luminescence
• Nanocrystals
• Optical & microwave phenomena
• Nanoparticles
• Density functional theory
• Photoluminescence
• Time-resolved photoluminescence
• Transient absorption spectroscopy