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.

Sodium−oxygen cells with sodium superoxide (NaO 2 ) as discharge product show charge and discharge characteristics with very low overvoltage, different from lithium/oxygen cells. Here, it is shown that the discharge of a nonaqueous sodium/oxygen cell proceeds via the electrochemical formation of superoxide (O 2 − ), its dissolution in the liquid electrolyte, and subsequent precipitation together with sodium ions as solid sodium superoxide. Charge proceeds in the counter-direction by consumption of dissolved superoxide anions and dissolution of NaO 2 . Indirect evidence for the solution-precipitation route is provided by theoretical results on the electronic structure of NaO 2 and the conclusion that the electronic conductivity of NaO 2 is too low to allow direct electrochemical growth and dissolution. Direct evidence for the solution-precipitation route is provided by results from charge/ discharge studies of a three-electrode cell in which preformed NaO 2 is being decomposed without direct electronic contact to the charging circuit. An analytical model for the overvoltage as a function of electrode coverage with electrically insulating discharge product complements the theoretical and experimental results and supports the mechanistic findings.

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