Photoluminescence Mechanism and Excited-State Fine Structure of Cs ₂ (Na,Ag,K)(In,Bi)Cl ₆ Double Perovskite Nanocrystals

Lead-free halide double perovskite nanocrystals are promising candidates for optoelectronic applications due to their chemical stability and compositional tunability. Among them, Cs₂(Na,Ag,K)(In,Bi)Cl₆ nanocrystals stand out for high photoluminescence quantum yields, yet the nature of their emissive state remains debated. Here, we study temperature- and magnetic field-dependent photoluminescence dynamics to reveal a fine structure composed of a long-lived dark triplet state and a short-lived bright singlet state, separated by ∼190 meV. This remarkably large dark-bright splitting arises from strong electron-hole exchange between tightly confined carriers. By varying Bi and Na content, we demonstrate that the fine structure and radiative dynamics can be tuned through compositional alloying. Our results reveal that the emissive state is a tightly bound, molecular-like charge-transfer exciton and offer design principles for tailoring the emission properties of lead-free perovskite nanomaterials.