Na⁺/Bi³⁺ Co-Alloying Enhances the Optical Properties of Monodisperse Lead-Free Silver–indium Double Perovskite Nanocrystals

Lead-free double perovskite nanocrystals (NCs) garnered significant attention for their success in addressing the toxicity and stability concerns associated with lead-based perovskite NCs. However, they suffer from a lower photoluminescence quantum yield (PLQY). Here, we propose a coalloying strategy to enhance the PLQY of Bi:Cs2AgInCl6 NCs, employing a colloidal synthesis strategy to prepare monodisperse, homogeneous cubic-shaped Bi:Cs2Ag1–xNaxInCl6 NCs (0 ≤ x ≤ 1). Benefiting from the Na+-induced breakdown of lattice symmetry in Bi:Cs2AgInCl6 NCs leads to significant improvement in their photoluminescence. By adjusting the alloying concentration of Na+, the band gap increases from 3.25 to 3.66 eV, and the exciton lifetime extends from 24.89 to 49.82 ns. Notably, after coalloying with 20% Na+, the PLQY of Bi:Cs2AgInCl6 NCs is significantly increased from 30.2% to 56.5%. Temperature-dependent photoluminescence spectra and density functional theory calculations confirm that Na+ alloying increases the exciton binding energy, enhances the electron–phonon coupling, and changes the local lattice symmetry. Furthermore, Bi:Cs2Ag0.8Na0.2InCl6 NCs demonstrated excellent long-term stability under environmental conditions, making them an ideal material choice for photovoltaic device manufacturing. Therefore, this coalloying strategy not only provides an approach for enhancing the optical properties of lead-free perovskite nanomaterials but also holds significant potential for practical applications.