Exchange of Thiol Ligands on CuInS₂ Quantum Dots in High Boiling Solvents

As-prepared quantum dots are covered with long-chain ligands to prevent aggregation. When quantum dots are used in optoelectronic devices such as solar cells and QD-LED, ligand exchange is necessary to replace long-chain ligands with short-chain ones to increase the efficiency of charge transfer from the quantum dots to the electrode. In this study, we successfully exchanged 1-dodecanethiol (DDT) ligands on CuInS₂ quantum dots with mercaptopropionic acid (MPA) ligands by using a two-phase system of high-boiling hydrophilic and hydrophobic solvents. The ligand exchange to MPA was achieved by using diethylene glycol (DEG) or ethylene glycol (EG) as the hydrophilic phase and tetradecane as the hydrophobic phase. The ligand exchange rate increased with increasing ligand exchange temperature. When quantum dot sensitized solar cells (QDSSCs) were fabricated using the ligand-exchanged quantum dots, a positive correlation was observed between the progress of ligand exchange and short-circuit current density. This is because charge transfer efficiency from the quantum dots to the TiO₂ electrode was improved by the ligand exchange. This work has shown that QDs synthesized using DDT can be applied to optoelectronic devices.