Co-Doping of Cerium and Bismuth into Lead-Free Double Perovskite Cs₂AgInCl₆ Nanocrystals Results in Improved Photoluminescence Efficiency

Lead-free double cation metal halide perovskites have recently attracted considerable attention, with continuing research efforts focusing on the improvement of their stability and photoluminescence quantum yield (PL QY). In this study, Ce³⁺ has been co-doped together with Bi³⁺ into lead-free double perovskite Cs₂AgInCl₆ nanocrystals (NCs) in order to improve their crystallinity and PL QY. Both uncoordinated chloride ions and silver vacancies could be eliminated using this co-doping strategy, and the resulting Ce³⁺,Bi³⁺-co-doped Cs₂AgInCl₆NCs showed adjustable PL emission peaks in the range of 589 to 577 nm by varying the doping amount of Ce³⁺ with a fixed feeding ratio of bismuth precursor set at 1%. Cs₂AgInCl₆NCs doped with 1% Bi alone reached a PL QY of 10% for the PL peak centered at 591 nm, while those co-doped with 1% Bi and 2% Ce together achieved the highest PL QY of 26% for the PL peak centered at 580 nm. The use of Ce³⁺ as a dopant promoted the localization of self-trapped excitons to prevent PL quenching, although the ion's 5d excited state may potentially provide an energetically favorable indirect route for the radiative relaxation process. This also resulted in a blue shift of the PL maximum and increased the exciton binding energy, thus promoting the radiative recombination of self-trapped excitons.