Enhanced Photoresponse of Quantum Dot-Coated ITZO Phototransistors by Atomic Layer Deposition of ZnO

Quantum dots (QDs), which exhibit strong absorption in the visible light spectrum, can be used as photoactive materials to enhance visible light detection by phototransistors. In this study, CdSe/ZnS QDs were spin-coated onto indium–tin–zinc oxide (ITZO) thin-film transistors to form QD-coated phototransistors with enhanced photoresponsivity. To ensure compatibility with the photolithography process for the QD films, a ZnO layer was applied by atomic layer deposition (ALD) as a passivation layer to provide solvent orthogonality. Notably, beyond this protective function, the ALD ZnO treatment enhanced the interfacial charge transport. This is attributed to the partial replacement of the insulating oleic acid ligands on the QD surfaces via vapor-phase ligand exchange and reduction in the conduction band barrier between the QD shell and the ITZO channel. Under low-light conditions (0.02 mW/cm2), the devices achieved a responsivity of 266.7 A/W at VG = 10 V and detectivity of 9.3 × 1010 jones at VG = 10 V, both higher than those of the untreated device (200 A/W, 5.6 × 1010 jones). Furthermore, scanning photocurrent microscopy mapping analysis revealed that the ALD ZnO-treated device exhibited a maximum photocurrent of 286 μA, while the untreated device showed only 192 nA under 532 nm illumination at 1.58 mW/cm2. This represents an approximately 1000-fold enhancement in the photocurrent, thereby indicating the synergistic contributions of photoconductive and photogating effects.