Ligand and Solvent Effects on Hole Transport in Colloidal Quantum Dot Assemblies for Electronic Devices

Semiconductor quantum dots (QDs) in colloidal form have attracted growing interest for their potential applications in solution-processable electronic devices. Controlled electronic doping in QD assemblies is one of the challenges required for advancing the development of novel solar cells, photodetectors, and transistors based on this system. While several n-type QD films with excellent conductivity have been successfully demonstrated, in general, p-type QD films have shown poor conductivity. Ligand and solvent engineering were found to permit significant enhancements of hole transport in lead sulfide (PbS) QD films. Capping with a carboxylate ligand generally produces p-type doping of PbS QD films; furthermore, among various carboxylate ligands, thiophene-2,5-dicarboxylic acid provides PbS QD films with exceptionally high hole mobility values, and solvents with a high solvency power for the ligand are important for enhancing carrier mobility. With an appropriate combination of ligand molecule and solvent, QDs can be packed more closely into films, resulting in orders-of-magnitude enhancement in the field-effect hole mobility, reaching values of 0.20 ± 0.06 cm2 V–1 s–1. The new guideline presented in this study will be vital for constructing high-performance QD-based p–n junction-type devices, especially photovoltaics