Ligand Influence on CuInS2 Quantum Dot Photoconductive Films

In this work, we investigate the effect of ligand chemistry on the optical and electrical properties of copper indium disulphide (CuInS₂) quantum dots (QDs) and evaluate their suitability for photodetection with simple device structures. CuInS₂ QDs capped with dodecanethiol (DDT) ligands were synthesized, followed by processes to exchange the DDT with thioglycolic acid (TGA), mercaptopropionic acid (MPA), or thioacetamide (TAA) ligands. Photoluminescence (PL) and UV-Visible absorption studies revealed that while TGA- and MPA-capped QDs retained strong emission, TAA-capped QDs exhibited significant quenching, indicating surface defect formation due to poor ligand binding. Metal-semiconductor-metal (MSM) test structures were fabricated using the QD films as the active layer to study their electrical properties under dark and UV-illuminated conditions. Devices based on MPA- and TGA-capped QD films demonstrated currents that were 7-9 times higher than those of devices with native DDT ligands, with significantly enhanced photocurrent-to-dark current ratios of 2.6 and 1.7, respectively, highlighting the effective charge transport pathways enabled by the shorter ligands. The device with TGA-capped QD film also responded well to 20 kHz pulsed UV excitation, underscoring the strong potential of this simple MSM structure for photodetection and optical switching applications.