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Metal halide double perovskites are highly attractive for optoelectronic applications because of their superior optical properties. Here, a novel halide double perovskite, Cs2NaLuCl6, exhibiting bright blue emission originating from the host self‐trapped exciton (STE) is reported. Through doping with Sb3+, the dopant‐induced STE is formed in the crystals with a photoluminescence quantum yield up to 94%. The energy transfer from the dopant STE to Er3+ is superior to the direct excitation of Er3+, which is more efficient than the energy transfer from the host STE to Er3+. Furthermore, the energy transfer efficiency from STE to Er3+ is boosted from 8% to 33% through Sb3+ doping in Cs2NaLu0.95Er0.05Cl6. Due to the strong electron‐phonon coupling and efficient energy transfer, the as‐prepared crystals exhibit intense dual‐band emissions and remarkable thermal stability. Importantly, the STE emission variation after the Er3+‐substitution is explored based on the electron‐phonon coupling effect. These findings provide new insights into the excited state dynamics of halide double perovskites and extend their luminescence applications such as white light‐emitting diodes and night vision. [ABSTRACT FROM AUTHOR]

Carbon dots (CDs) possess intrinsically broad emission spectra, making them suitable for modulating high‐quality white light‐emitting diodes (WLEDs). However, the chief technical challenge in applying CDs to WLEDs is the aggregation‐caused quenching (ACQ) effect. A common solution is polymer post‐modification on the surface of CDs, which is a low‐yield, hard‐to‐purified, and complicated process. Herein, polymerized CDs with broad emission (full width at half maxima, FWHM = 187 nm) are synthesized via a one‐step solvothermal method. The obtained polymerized CDs display excellent solution‐processed and film‐forming capabilities, with absolute photoluminescence quantum yield as high as 4.7% in thin film state, creating a firm foundation for assembling solution‐processed WLEDs. Moreover, the polymerization steps of CDs are analyzed and described in detail. The polymer chains prevent ACQ and create new energy levels, resulting in the broad emission of CDs in an aggregated state. Subsequently, WLEDs are fabricated using polymerized CDs as the single active emitting material. The optimal device achieves the maximum brightness of 337.50 cd m−2 and FWHM of 242 nm, covering nearly the entire visible spectral window. The results present a new approach to obtaining polymerized CDs and further fabricating WLED with a straightforward, cost‐effective method. [ABSTRACT FROM AUTHOR]