In this present work, CuNiCoS4 thiospinel nanocrystals were synthesized by hot injection and characterized by X-ray diffractometry (XRD), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). The XRD, EDS, and HR-TEM analyses confirmed the successful synthesis of CuNiCoS4. The obtained CuNiCoS4 thiospinel nanocrystals were tested for photodiode and capacitance applications as interfacial layer between Au and p-type Si by measuring I–V and C–V characteristics. The fabricated Au/CuNiCoS4/p-Si device exhibited good rectifying properties, high photoresponse activity, low series resistance, and high shunt resistance. The C–V characteristics revealed that capacitance and conductance of the photodiode are voltage-and frequency-dependent. The fabricated device with CuNiCoS4 thiospinel nanocrystals can be employed in high-efficiency optoelectronic applications