Surface Chemistry of Ternary Nanocrystals: Engineering the Deposition of Conductive NaBiS2 Films

The ability to engineer the surface chemistry of complex ternary nanocrystals is critical to their successful application in photovoltaic, thermoelectric, and other energy conversion devices. For many years, several studies have shed light on the surface chemistry of unary and binary semiconductor nanocrystals as well as their surface modification with monodentate and multidentate ligands in a variety of applications. In contrast, our understanding of the surface chemistry and ligand modification of ternary and other complex multinary nanocrystals remains relatively limited. Recently, our group reported the synthesis of colloidal NaBiS2 semiconductor nanocrystals with sizes tunable between 2 and 60 nm and a light absorption edge of ca. 1.4 eV. Here, we use a combination of infrared and nuclear magnetic resonance spectroscopies to show that the as-made NaBiS2 nanocrystals are capped by oleylamine and neodecanoate ligands. We investigate biphasic liquid–liquid exchange as a means to replace these native ligands with either carboxylate-terminated lipoic acid or with small iodide ligands, leading in both cases to solubility in polar solvents, such as methanol, water, and dimethylformamide. We also investigate a layer-by-layer, biphasic solid–liquid exchange approach to prepare films of NaBiS2 nanocrystals capped with halide ligands—iodide, bromide, and chloride. Upon exchange and removal of the native ligands, we show that the resistance of NaBiS2 nanocrystal films greatly decreases, with their measured conductivity being comparable to that of films made of isostructural PbS nanocrystals, which have been used in solar cells. Lastly, we report the first solar cell device made of NaBiS2 nanocrystal films with a limited power conversion efficiency (PCE) of 0.07. Further nanostructuring and ligand optimization may enable the preparation of much more efficient energy conversion devices based on NaBiS2 as well as other nontoxic and Earth-abundant, biocompatible multinary semiconductors.