Methods processed in aqueous solutions offer numerous advantages, including simplicity, affordability, eco-friendliness and scalability. This paper presents a facile chemical precipitation approach to synthesize water-soluble CuInS2 (CIS) nanocrystals (NCs) in an oil bath, offering a promising Pb- and Cd-free alternative. In this method, different concentrations of thioglycolic acid have been used as a capping agent compared to other articles. The oil bath temperature was altered in a wide range of 25–150 °C and was optimized to find the best bandgap energy/band edge positions for higher energy conversion efficiencies. The characterization of the synthesized NCs was achieved using UV–visible spectroscopy, tauc plot, energy dispersive X-ray (EDX), X-ray diffraction (XRD) and scanning electron microscope (SEM). The EDX analysis revealed the presence of Cu, In, and S in a 1:8.5:14.5 ratio or in more accurate form of Cu0.11In0.89S1.53. These CIS NCs hold significant potential for application in quantum dot-sensitized solar cells (QDSCs) as the light absorbing layer. By deposition only one CIS QDs on nanoparticles (NPs) and hollow spheres (HSs) of TiO2, we observed high performance in QDSCs. The photovoltaic parameters were examined by various photovoltaic analyses. It was found that, with the oil bath temperature increasing, the short-circuit photocurrent density (Jsc) and open-circuit voltage (Voc) of CIS QDSCs gradually increased, leading to enhanced cell performance. The pioneer cell with TiO2 NPs/TiO2 HSs/CIS/ZnS photoanode, including the CIS QDs synthesized at a temperature of 120 °C, revealed a Jsc = 24.37 mA/cm2, Voc = 576 mV, FF = 0.37 and a power conversion efficiency (PCE) of about 5.4%.
