<p>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&#xa0;°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 Cu_0.11In_0.89S_1.53. 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&#xa0;°C, revealed a Jsc = 24.37&#xa0;mA/cm², Voc = 576 mV, FF = 0.37 and a power conversion efficiency (PCE) of about 5.4%. </p> Graphical abstract <p>Schematic of the CuInS₂ QDs sensitized solar cell (a) and corresponding flat band energy diagram (b) and J-V characteristics of devices (c)</p> <p></p>