Colloidal Synthesis of Blue-Emitting Cs₃TmCl₆ Nanocrystals via Localized Excitonic Recombination for Down-Conversion White Light-Emitting Diodes

Lead-halide perovskite nanocrystals (NCs) have gained significant attention for their promising applications in lighting and display technologies. However, blue-emitting NCs have struggled to match the high efficiency of their red and green counterparts. Moreover, many reported blue-emitting perovskite NCs contain heavy metal lead (Pb), which poses risks to human health and the environment. In this study, we synthesized rare-earth-based Cs₃TmCl₆ NCs via the hot injection method, which exhibit a broadband blue emission at 440 nm. Combined experimental and theoretical studies indicate that the broadband emission in Cs₃TmCl₆ arises from self-trapped excitons due to the excited-state structural distortion of the [TmCl₆]^3- cluster. Furthermore, the ultrafast dynamics of charge carriers were analyzed using time-resolved photoluminescence and transient absorption measurements. Encouraged by the remarkable thermal, light, and water stabilities of Cs₃TmCl₆ NCs, as evidenced by experimental and theoretical results, a white light-emitting diode was further designed and fabricated using the Cs₃TmCl₆ NCs as the color converter. The device exhibits outstanding performance, achieving a long half-lifetime of 336 h and a large color-rendering index of 87.0. Combining eco-friendly features and a facile synthesis method, the rare-earth-based Cs₃TmCl₆ NCs mark a significant breakthrough as a reliable blue emitter, showcasing their future potential in lighting and display applications.