Hybrid Ligand Exchange of Cu(In,Ga)S2 Nanoparticles for Carbon Impurity Removal in Solution-Processed Photovoltaics

The solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics from colloidal nanoparticles has long suffered from deleterious carbonaceous residues originating from long chain native ligands. This impurity carbon has been observed to hinder grain formation during selenization and leave a discrete residue layer between the absorber layer and the back contact. In this work, organic and inorganic ligand exchanges were investigated to remove tightly bound native oleylamine ligands from Cu­(In,Ga)­S2 nanoparticles, thereby removing the source of carbon contamination. However, incomplete ligand removal, poor colloidal stability, and/or selective metal etching were observed for these methods. As such, an exhaustive hybrid organic/inorganic ligand exchange was developed to bypass the limitations of individual methods. A combination of microwave-assisted solvothermal pyridine ligand stripping followed by inorganic capping with diammonium sulfide was developed and yielded greater than 98% removal of native ligands via a rapid process. Despite the aggressive ligand removal, the nanoparticle stoichiometry remained largely unaffected when making use of the hybrid ligand exchange. Furthermore, highly stable colloidal ink formulations using nontoxic dimethyl sulfoxide were developed, supporting stable nanoparticle mass concentrations exceeding 200 mg/mL. Scalable blade coating of the ligand-exchanged nanoparticle inks yielded remarkably smooth and microcrack free films with an RMS roughness less than 7 nm. Selenization of ligand-exchanged nanoparticle films afforded substantially improved grain growth as compared to conventional nonligand-exchanged methods, yielding an absolute improvement in device efficiency of 2.8%. Hybrid ligand exchange nanoparticle-based devices reached total area power conversion efficiencies of 12.0%, demonstrating the feasibility and promise of ligand-exchanged colloidal nanoparticles for the solution processing of Cu­(In,Ga)­(S,Se)2 photovoltaics.