Copper‐oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar‐cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin‐film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin‐film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar‐cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p‐Cu2O/n‐AlGaN is much more promising for the application as efficient solar cells than that of p‐Cu2O/n‐ZnO heterojunction devices that have been favored up to now.

Using photoelectron spectroscopy, we investigate the band alignments of the Cu2O/ZnO heterointerface and compare the findings with the corresponding values for Cu2O/GaN. While for Cu2O/ZnO, we find a valence band offset (VBO) of 2.17 eV and a conduction band offset (CBO) of 0.97 eV, both values are considerably reduced for Cu2O/GaN where the numbers are 1.47 eV (VBO) and 0.24 eV (CBO), respectively. The large CBO between ZnO and Cu2O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu2O/ZnO solar cells.

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Series of CuxO thin-films in the entire range of compositions 1≤x≤2 were obtained by varying the oxygen flux in an rf-sputter deposition process. Growth windows for three crystalline phases, i.e., the thermodynamically stable cuprous oxide Cu2O and cupric oxide CuO as well as the metastable paramelaconite Cu4O3, were observed. The crystalline phases persist non-stoichiometrically over a wide range of compositions. These flux-range windows are separated by ranges where highly disordered, almost amorphous material is obtained. All samples were analysed with respect to their thermoelectric properties, i.e., Seebeck coefficient, electrical, and thermal conductivity. Clear trends of these transport parameters were found and used to determine the thermoelectric figure of merit ZT. The ZT-values at room temperature are highest for the two thermodynamically stable crystalline phases CuO and Cu2O.