Improving Lead‐Free Double Perovskite Cs₂NaBiCl₆ Nanocrystal Optical Properties via Ion Doping

Abstract The exploration of lead‐free halide perovskite nanocrystals (NCs) with intriguing optical properties is highly desirable owing to the toxicity and instability of lead halide perovskite NCs. Here, a new kind of uniform lead‐free double perovskite Cs₂NaBiCl₆ NCs are reported as versatile hosts to accommodate ionic dopants for improving optical properties especially the photoluminescence (PL). In contrast to the low deep‐blue PL with a quantum yield of only 1.7% of the as‐synthesized pristine Cs₂NaBiCl₆ NCs, the PL of the Cs₂NaBiCl₆ NCs can be impressively regulated and enhanced via doping Ag⁺, Mn²⁺, or Eu³⁺ ions in the double perovskite lattices. The femtosecond time‐resolved transient absorption spectroscopy is adopted to unravel the PL enhancement mechanism of the ion doping in the Cs₂NaBiCl₆ NCs. For the Ag⁺‐doping, the excitonic absorption energy of the Cs₂NaBiCl₆ NCs can be tuned from 3.82 to 3.48 eV with the significant improvement of the PL quantum yield (PLQY) from 1.7% to 20%. The Mn²⁺‐doped Cs₂NaBiCl₆ NCs show broad orange–red emission peak centered at 585 nm with a PLQY of 3%, owing to the ⁴T₁→⁶A₁ transition of octahedrally coordinated Mn²⁺. Eu³⁺‐doped Cs₂NaBiCl₆ NCs are endowed with strong Eu³⁺⁵D₀→⁷F_J (J = 1, 2) orange–red emission at 591 and 615 nm.