Hydrazine-free surface modification of CZTSe nanocrystals with all-inorganic ligand

The optoelectronic properties of semiconductor nanoparticles (NPs) depend sensitively on their surface ligands. However, introducing certain organic ligands to the solution-synthesized CZTSe NPs unfavorably suppresses the interaction among those NPs. These organic ligands prevent the NPs from dissolving in water and create an insulating barrier for charge transportation, which is the key property for semiconductor devices. In our study, by adopting Na₂S to displace the associated organic ligands on Cu₂ZnSnSe₄ (CZTSe), we obtained high solubility NPs in an environmentally friendly polar solvent as well as excellent charge transport properties. Toxicity of CZTSe: Na₂S NPs was determined to be around 10 mg/L. Because of the inorganic ligand S^2- around CZTSe NPs, thin films can be easily fabricated by solution processing out of benign solvents like water and ethanol. After annealing, a homogeneous CZTSSe absorbing layer without carbon point defects was obtained. As the S^2- effectively facilitates the electronic coupling in nanocrystal thin films, carrier mobility of the surface-engineered CZTSe enhances from 4.8 to 8.9 cm²/(Vs). This raises the possibility for engineering chalcogenide materials by controlling the surface properties during the fabrication process.