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

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 T-4(1)->(6)A(1) transition of octahedrally coordinated Mn2+. Eu3+-doped Cs2NaBiCl6 NCs are endowed with strong Eu3+5D0 -> F-7(J) (J = 1, 2) orange-red emission at 591 and 615 nm.