Thermally Activated Delayed Fluorescence from Perovskite‐Derivative CsAgCl₂ Nanocrystals for High‐Resolution X‐Ray Imaging

Abstract Thermally activated delayed fluorescence (TADF) materials hold great promise as next‐generation efficient emitters due to their theoretical 100% internal quantum efficiency. However, such an intriguing photophysical mechanism is extremely rare in perovskites or their derivatives. Herein, the colloidal synthesis of two‐dimensional CsAgCl₂ nanocrystals (NCs) with typical TADF feature is reported first. The TADF mechanism is investigated in detail by combining first‐principles calculations with experimental results, including temperature‐dependent spectral analysis and fs‐transient absorption spectra. A small singlet−triplet state splitting energy (0.082 eV) and a large spatial separation of charge density |Ψ|² of valence band maximum and conduction band minimum are observed, which are prerequisites for TADF. Moreover, these CsAgCl₂ NCs deliver an ultralow X‐ray detection limit of 170.5 nGy_air s⁻¹ and a high imaging resolution of 11.5 lp mm⁻¹, much superior to the commercial scintillators.