Carbon Impurity Minimization of Solution-Processed, Thin-Film Photovoltaics via Ligand Engineering of CuInS₂ Nanoparticles

Colloidal semiconductor nanoparticles (NPs) have long been used as a reliable method for depositing thin films of semiconductor materials for applications, such as photovoltaics via solution-processed means. Traditional methods for synthesizing colloidal NPs often utilize heavy, long-chain organic species to serve as surface ligands, which, during the fabrication of selenized chalcogenide films, leaves behind an undesirable carbonaceous residue in the film. In an effort to minimize these residues, this work looks at using N-methyl-2-pyrrolidone (NMP) as an alternative to the traditional species used as surface ligands. In addition to serving as a primary ligand, NMP also serves as the reaction medium and coating solvent for fabricating CuInS2 (CIS) NPs and thin-film solar cells. Through the use of the NMP-based synthesis, a substantial reduction in the number of carbonaceous residues was observed in selenized films. Additionally, the resulting fine-grain layer at the bottom of the film was observed to exhibit a larger average grain size and increased chalcopyrite character over those of traditionally prepared films, presumably as a result of the reduced carbon content. As a result, a gallium-free CuIn(S,Se)2 device was shown to achieve power-conversion efficiencies of over 11% as well as possessing exceptional carrier generation capabilities with a short-circuit current density (JSC) of 41.6 mA/cm2, which is among the highest for the CIGSSe family of devices fabricated from solution-processed methods