Metal oxide nanoparticles were synthesized by a hydrothermal method inside a self-designed stainless steel autoclave, sealed, and heated at different temperatures. X-ray diffraction (XRD) analysis revealed the single-phase structure of CuO nanoparticles with an average grain size of 52.64 nm at temperatures of 100 °C, respectively. The EDX analysis determined that the sample was of high purity and did not contain impurities. Visible and ultraviolet spectroscopy identified a broad absorption band for CuO nanoparticles at a wavelength of 570 nm. Using the Tauc diagram, the bandgap was calculated, as its value was equal to 1.7 eV. The photovoltaic performance values of a solar cell are sensitized to the natural red dye extracted from beetroot. The solar cell produced a conversion efficiency of 2.08. The synthesized CuO nanoparticles could be potential candidates for use in electronic devices.