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.

A series of ZnO1−xSx films with 0⩽x⩽1.0 was deposited by radio-frequency reactive sputtering on different substrates. The structural characterization by x-ray diffraction measurements revealed that the films have wurtzite symmetry and correlated investigations of the layer composition by photoelectron spectroscopy showed that the lattice constant varies linearly with x. The composition dependence of the band gap energy in the ternary system was determined by optical transmission and the optical bowing parameter was found to be about 3eV.

No abstract

Charge storage based on conversion reactions is a promising concept to store electrical energy. Many studies have been devoted to conversion reactions with lithium; however, still many scientific questions remain due to the complexity of the reaction mechanism combined with surface film formation. Replacing lithium by sodium is an attractive approach to widen the scope of conversion reactions and to study whether the increase in ion size changes the reaction mechanisms and whether the cell performance benefits or worsens. In this study, we use thin film electrodes as a additive-free model system to study the conversion reaction of CuO with sodium (CuO/Na) by means of electrochemical methods, microscopy, and X-ray photoelectron spectroscopy. The reaction mechanism and film formation are being discussed. Some important differences to the analogue lithium-based system (CuO/Li) are found. Whereas CuO has been reported as charge product in CuO/Li cells, charging is incomplete in the case of CuO/Na and only Cu2O is formed. As an important finding, oxygen appears to be redox active and Na2O2 forms during charging from Na2O. Moreover, surface film formation due to electrolyte decomposition is much more severe as compared to CuO/Li. Depth profiling is used to probe the inner composition of the surface film, revealing a much thicker surface film with more inorganic components as compared to the lithium system. It is also found that the surface film disappears to a large extent during charging.

Smart windows possess the potential to contribute significantly to reducing the world-wide energy consumption in the future. The properties of the thermochromic metal oxide VO2 are closest to the material requirements set by economic considerations for the use as an active layer in thermochromic glazings. We show that the required parameters can be achieved by modifying VO2 on the atomic level, i.e., by simultaneous co-doping with Sr and W on the cation site. In particular, the transition temperature ϑc can be adjusted in the range between 15 and 68 °C by varying W contents, whereas the incorporation of Sr mainly increases the band gap yielding a greyish color of the films. Interestingly, the simultaneous presence of W and Sr enhances both effects. The co-doping leads to values of the luminous transmittance Tlum and the solar transmittance Tsol fulfilling the requirements set by application. The variation of the solar transmittance ΔTsol of the plain thin films on a glass substrate already is larger than 5% for all samples promising that the required value of ΔTsol=10% is achievable by using such thin films as part of an optically engineered multilayer system.