Understanding the cation-selective ligand passivation for AgBiS₂ nanocrystal photovoltaics

Ternary AgBiS₂ nanocrystals (NCs) have emerged as a promising candidate for efficient environment-friendly photovoltaic (PV) applications. However, the different binding affinity of the molecular ligand towards two different metal cations with distinctive chemical natures results in a cation-selective ligand binding scenario. Such disparity interrupts the comprehensive surface passivation of NCs by using a single functional group, thereby limiting the electronic coupling and bespoke energy level tuning that are directed to PV performances. Herein, we explore the detailed binding nature of the thiol- and carboxylic acid-based molecular ligands onto the AgBiS₂ NC surface. We observe that the binding preferences between the metal cations and functional groups qualitatively follow the hard-soft acid-base theory, whereas 3-mercaptopropionic acid (MPA) having both functional groups enables comprehensive surface binding onto both Ag and Bi. In addition, the AgBiS₂ NC film fabricated via ligand exchange using a MPA ligand renders a robust p-type character. Integration of the MPA-treated NCs with iodine-passivated NCs as a bilayer architecture, with an effective cascade energy level structure, leads to improved PV performances. Our work provides fundamental insights into the design of effective molecular ligands for high performance of ternary NC-based optoelectronics.