Copper oxide-titanium dioxide (TiO₂) p–n junctions are promising materials for photovoltaic devices and may reduce production costs due to their low cost and inexpensive production methods compared with silicon solar cells. The present review compares solar cells made with copper oxides combined with TiO₂–TiO₂/Cu₂O and TiO₂/CuO heterojunctions, and “cascade heterojunction systems.” First, we describe the main properties of titanium (iv) dioxide (TiO₂), cuprous oxide (Cu₂O), and cupric oxide (CuO), and their potential applications. Next, we explain the concept of copper oxide and TiO₂ heterojunctions. We summarize and present the photovoltaic characteristics (efficiency, fill factor, circuit current density, and open circuit voltage), thickness, preparation method, and electrode type for solar cells comprising copper oxide and TiO₂. The efficiency of the solar cells ranged from 0.0005% to 1.62%. The thickness of the TiO₂ and cupric oxide layers ranged from 0.06 to 16 µm, and from 0.18 to 1.5 µm, respectively, depending on the fabrication method. Additionally, we review and discuss the available combinations of copper oxide with other materials (Cu₂O with ZnO, CuO with ZnO, and CuO with Si), as well as the effect of the thickness of the copper (i) oxide and copper (ii) oxide on the solar cell performance. Finally, we present aspects to improve the conversion efficiency of heterojunction solar cells with copper oxides combined with TiO₂. This review will be useful for the construction and further development of thin-film solar cells.

Several Cu₂O and TiO₂ thin films and four additional TiO₂/Cu₂O structures were fabricated by direct current (DC) magnetron sputtering. The process parameters were selected on the basis of earlier studies and numerical simulations. We examined the morphology of a cross-section of the PV structures, roughness and topography, and the transmission spectra of the thin films. Additionally, the properties of the samples were determined by X-ray diffraction. Next, the morphology cross-sectional and layer compositions of the solar cells was evaluated by scanning electron microscopy. Only one of the TiO₂/Cu₂O structures appeared smooth and homogeneous with columnar-type growth. For the as-grown films, diffraction peaks were observed and identified as brookite, rutile, CuO, and Cu₂O and the average roughness of the samples was 0.5, 1.2, 5.4, and 4.0 nm, respectively. Finally, the transmission spectra of the thin films were recorded. Transmission and reflection spectra of ultraviolet-visible spectroscopy were analyzed, and the optical band gap and absorption coefficient of the oxidized layers were calculated. In the region of 400 to 1000 nm, transmittance varied from 5% to 70% in the TiO₂ samples, and from 15% to 40% in the Cu₂O samples, and reflectance of the TiO₂ and Cu₂O samples ranged from 20% to 90%. In the region of 1.5 eV to 3.5 eV, the mean absorption coefficient varied from ∼10⁵ 1/cm to ∼3 · 10⁵ 1/cm for TiO₂ thin film, and from ∼2 · 10⁵ to ∼6 · 10⁵ 1/cm for Cu₂O thin film. The optical band gap values of the samples shifted slightly toward bulk anatase-3.5 eV, bulk rutile-3.1 eV, and copper(I) oxide. Finally, silver contacts were used for the electrodes. One of the fabricated TiO₂/Cu₂O PV structures was found to be sensitive to electromagnetic radiance during the experiment.

In this study, titanium dioxide/copper oxide thin-film solar cells were prepared using the reactive direct-current magnetron sputtering technique. The influence of the deposition time of the top Cu contact layer on the structural and electrical properties of photovoltaic devices was analyzed. The structural and morphological characterization of the TiO₂/CuO/Cu₂O solar cells was fully studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and current–voltage (I-V) characteristics. Additionally, using van der Pauw sample geometries, the electrical properties of the titanium dioxide and copper oxide layers were investigated. From the XRD study, solar cells were observed in cubic (Cu₂O), monoclinic (CuO), and Ti₃O₅ phases. In addition, the crystallite size and dislocation density for copper oxide layers were calculated. Basic morphological parameters (thickness, mechanism of growth, and composition of elements) were analyzed via scanning electron microscopy. The thicknesses of the titanium dioxide and copper oxide layers were in the range of 43–55 nm and 806–1223 nm, respectively. Furthermore, the mechanism of growth and the basic composition of the elements of layers were analyzed. The I-V characteristic curve confirms the photovoltaic behavior of two titanium dioxide/copper oxide thin-film structures. The values of short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells were: 4.0 ± 0.8 µA/cm², 16.0 ± 4.8 mV and 0.43 ± 0.61 µA/cm², 0.54 ± 0.31 mV, respectively. In addition, the authors presented the values of I_sc, P_max, FF, and R_sh. Finally, the resistivity, carrier concentration, and mobility are reported for selected layers with values reflecting the current literature.