Enhancing Optical Properties of Lead-Free Cs₂NaBiCl₆ Nanocrystals via Indium Alloying

This study presents the synthesis and characterization of Cs₂NaBiCl₆ nanocrystals (NCs) doped with varying concentrations of In³⁺ to improve their luminescent properties. Utilizing a colloidal solution method, we systematically varied the In³⁺ concentration to identify the optimal alloying level for enhancing the photoluminescence (PL) properties of the Cs₂NaBiCl₆ NCs. Structural analysis confirmed that the In-alloyed NCs maintained high crystallinity and a uniform cubic shape. The optical properties were significantly improved with the In³⁺ alloying, reaching a peak photoluminescence quantum yield (PLQY) at an In/(Bi + In) ratio of 0.7. This optimal alloying concentration led to a nearly 2-fold increase in the average exciton lifetime to 6.98 ns, indicating an enhanced self-trapped exciton generation and improved radiative recombination efficiency. Temperature-dependent PL spectra revealed that the Cs₂NaBi_0.30In_0.70Cl₆ NCs exhibited a higher exciton binding energy and longitudinal optical phonon energy compared to the undoped NCs, suggesting superior thermal stability and reduced nonradiative recombination pathways. Density functional theory (DFT) calculations were employed to elucidate the charge density distribution, highlighting significant charge localization at the In-Cl and Bi-Cl bonds, which is attributed to the enhanced charge mobility in the alloyed NCs. The findings of this research not only address the limitations of current lead-free perovskites but also establish In-alloyed Cs₂NaBiCl₆ NCs as a promising candidate for blue light-emitting devices. This work advances the development of environmentally friendly optoelectronic technologies.