Na<sup>+</sup>/Bi<sup>3+</sup> Co-Alloying Enhances\nthe Optical Properties of Monodisperse Lead-Free Silver–indium\nDouble Perovskite Nanocrystals

Lead-free double perovskite nanocrystals (NCs) garnered\nsignificant\nattention for their success in addressing the toxicity and stability\nconcerns associated with lead-based perovskite NCs. However, they\nsuffer from a lower photoluminescence quantum yield (PLQY). Here,\nwe propose a coalloying strategy to enhance the PLQY of Bi:Cs<sub>2</sub>AgInCl<sub>6</sub> NCs, employing a colloidal synthesis strategy\nto prepare monodisperse, homogeneous cubic-shaped Bi:Cs<sub>2</sub>Ag<sub>1–<i>x</i></sub>Na<sub><i>x</i></sub>InCl<sub>6</sub> NCs (0 ≤ <i>x</i> ≤\n1). Benefiting from the Na<sup>+</sup>-induced breakdown of lattice\nsymmetry in Bi:Cs<sub>2</sub>AgInCl<sub>6</sub> NCs leads to significant\nimprovement in their photoluminescence. By adjusting the alloying\nconcentration of Na<sup>+</sup>, the band gap increases from 3.25\nto 3.66 eV, and the exciton lifetime extends from 24.89 to 49.82 ns.\nNotably, after coalloying with 20% Na<sup>+</sup>, the PLQY of Bi:Cs<sub>2</sub>AgInCl<sub>6</sub> NCs is significantly increased from 30.2%\nto 56.5%. Temperature-dependent photoluminescence spectra and density\nfunctional theory calculations confirm that Na<sup>+</sup> alloying\nincreases the exciton binding energy, enhances the electron–phonon\ncoupling, and changes the local lattice symmetry. Furthermore, Bi:Cs<sub>2</sub>Ag<sub>0.8</sub>Na<sub>0.2</sub>InCl<sub>6</sub> NCs demonstrated\nexcellent long-term stability under environmental conditions, making\nthem an ideal material choice for photovoltaic device manufacturing.\nTherefore, this coalloying strategy not only provides an approach\nfor enhancing the optical properties of lead-free perovskite nanomaterials\nbut also holds significant potential for practical applications.