Lead-free halide double perovskite (DP) colloidal nanocrystals (NCs) have attracted considerable attention as less toxic and more environmentally stable alternatives to lead-halide perovskites. [1] The broad self-trapped excitonic (STE) photoluminescence (PL), strongly Stokes-shifted from the absorption onset, makes them promising for transparent luminescent concentrators [2] and white-light LEDs. [3] A record PL quantum yield of 70% was recently achieved in Cs-Ag-Na-Bi-In-Cl (CANBIC) DP NCs. [4] However, further development is limited by incomplete understanding of their photophysics, particularly the lack of experimental validation of the Bi–Ag STE state predicted by density functional theory. [4] To address this research gap, we performed ultrafast transient X-ray absorption spectroscopy (trXAS) on CANBIC NCs at the SPring-8 Angstrom Compact Free Electron Laser (SACLA) in Japan. Using a 335 nm ultraviolet pump and 13.4 keV probe, we tracked the Bi L₃-edge X-ray absorption near-edge structure (XANES). In the absence of pumping, the Bi L₃-edge at 13,415 eV arises from Bi 2p_3/2 → 6d transitions. [5] At 1 ps delay, we observed a redshift of the edge and bleaching of the white line, attributed to combined effects of optical excitation (Bi 6s₂ → 6sp) and Bi³⁺ → Bi^(3–δ)+ reduction via hole localization on non-Bi sites. [6] The transient spectra revealed three features: (A) a peak at 13,411 eV associated to the Bi 2p_3/2 → 6s transition, enabled by optical excitation, (B) a peak at 13,418 eV due to the redshifted edge, and (C) a white-line bleach at 13,425 eV. Peak A decays within tens of ps, whereas B and C persist for hundreds of ps. This behavior suggests electron refilling of the Bi 6s state via hole trapping on Ag. Our results demonstrate that in CANBIC NCs: (i) Bi 6p orbitals host the optically excited electron, (ii) Bi is not the site of hole localization, and (iii) hole trapping occurs on a ~10 ps timescale. These findings provide direct experimental evidence of the Bi–Ag STE PL mechanism, advancing the photophysical understanding of lead-free DP NCs.

References
[1] I. López-Fernández et al. Advanced Functional Materials, 34 (6), 2307896 (2024)
[2] L. Zdrazil et al. ACS Applied Energy Materials, 4 (7), 6445 (2021)
[3] J. Luo et al. Nature, 563 (7732), 541 (2018)
[4] Z. Liu et al. Nano Letters, 22 (21), 8567 (2022)
[5] S. Salem-Sugui et.al. Physical Review B 43, 5511 (1991)
[6] Y. Obara et.al. Structural Dynamics 4, 044033 (2017)