<p>Lead-free halide double perovskite-based white light-emitting diodes (WLEDs) have recently emerged as promising candidates for advanced lighting technologies. However, their inherent characteristics, such as indirect bandgaps and forbidden electronic transitions, significantly limit photoluminescence (PL) efficiency, posing critical challenges to their widespread application. Herein, we selected highly efficient blue-emitting Cs₂NaInCl₆:Bi³⁺ nanocrystals (NCs) as a matrix, into which lanthanide ions (Ln³⁺) were incorporated by ion doping, yielding a series of Ln³⁺-doped perovskite NCs, Cs₂NaInCl₆:Bi³⁺,Ln³⁺(Ln³⁺ = Eu³⁺, Tb³⁺ and Dy³⁺), with an impressive photoluminescence quantum yield (PLQY) of 85.1%. Doping with Ln³⁺ effectively regulates the optical bandgap and PL efficiency of these materials. Femtosecond transient absorption (fs-TA) spectroscopy reveals changes in self-trapped exciton (STE) dynamics and reduced deep trap-related states upon Ln³⁺ doping, indicating effective modulation of self-trapping and defect passivation. The lead-free halide double perovskite-based WLED devices were demonstrated by coating the highly fluorescent green-emissive Cs₂NaInCl₆:Bi³⁺,Tb³⁺ NCs with red-emitting (Sr, Ca)AlSiN₃:Eu phosphor on a commercial ultraviolet (UV) chip, achieving a color-rendering index (CRI) of 87.5 and a correlated color temperature (CCT) of 5723&#xa0;K. This study is laying a theoretical foundation for the regulation of the optical properties of lead-free halide double-perovskite NCs and defining an ideal model for the development of efficient WLEDs.</p> Graphical Abstract <p>We developed a family of Ln³⁺-Bi³⁺ co-doped NCs, Cs₂NaInCl₆:Bi³⁺,Ln³⁺ (Ln³⁺  = Eu³⁺, Tb³⁺, Dy³⁺), with multicolor luminescence and excellent stability. The luminescence mechanism revealed energy transfer from Bi³⁺ to Ln³⁺ and achieved efficient self-trapping excitation emission of these materials. WLED device was fabricated using green emissive Cs₂NaInCl₆:Bi³⁺,Tb³⁺ NCs (PLQY = 85.1%) with excellent optical stability.</p> <p></p>