Direct Synthesis of Sulfide-Capped Nanoparticles for Carbon-Free Solution-Processed Photovoltaics

Colloidal nanoparticles have demonstrated significant promise in the fabrication of solution-processed Cu(In,Ga)(S,Se)2 photovoltaics. However, carbonaceous impurities from long-chain native ligands retained in the films during and after heat treatments have necessitated the exploration of postsynthetic ligand-exchange procedures, which increases process complexity and solvent usage and could potentially reduce the cost advantages that solution processing aims to deliver over conventional vacuum processing. In this report, a new method to directly synthesize anion impurity-free CuInS2 (CIS) nanoparticles with both inorganic and thermally degradable thioacetamide (TAA) ligands is developed to bypass the need for ligand exchange completely. Metal thiolate complexes were isolated from solutions of Cu2S or Cu and In precursors in amine–thiol mixtures. The isolated metal thiolate complexes were solubilized in the readily available and low-toxicity solvent sulfolane and thermally decomposed to CIS nanoparticles in the presence of TAA. H2S formed during thiolate decomposition was used in conjunction with TAA as ligands for nanoparticles. High-mass-concentration ink in the low-toxicity polar solvent dimethyl sulfoxide was used to easily deposit thin films using scalable blade coating. A CuInSe2 photovoltaic device with a total area power conversion efficiency of 7.1% was prepared from a carbon-free CIS nanoparticle film. Further, synthetic methods were successfully adapted to the more complex quaternary material Cu2ZnSnS4, where phase-pure material synthesis was observed. The developed methods represent a paradigm shift in the synthesis of metal sulfide nanomaterials and the subsequent solution processing of photovoltaics without the need for ligand exchange.