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.

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.

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.

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Research on sodium-ion batteries (NIBs) is well motivated by the large abundance of sodium and strategies for commercializing such systems are mainly based on principles known from lithium-ion technology. From a scientific perspective it is an intriguing questions of how lithium and sodium compare in their redox chemistry with identical electrodes. Graphite, for example, was long thought to be inactive as electrode in NIBs but has been shown to be active under the right conditions1 . In metal-batteries, Li2O2 forms in the case of lithium whereas NaO2 can form in the case of sodium2, 3. The differences in physic-chemical properties might also provide an opportunity for improved performance in next-generation electrodes that are so far not performing well enough for LIBs. For example, conversion reactions are being studied for many years with the hope to achieve higher capacity electrodes but many challenges remain. In this presentation, will focus on lithium and sodium ion storage in graphite and copper oxide (CuO). We will first discuss the peculiar properties of sodium-ion intercalation into graphite. Inactive when carbonates are used as electrolyte solvents, a highly reversible storage mechanisms (> 1000 cycles without significant capacity decay) with low irreversible capacity and fast kinetics is found when ether solvent molecules are used. This is because ether molecules can co-intercalate into the graphite structure without leading to exfoliation. This redox process is more reversible as compared to lithium. In the second example, we will discuss CuO as model system for conversion reactions. We prepared CuO thin films by sputtering and studied the surface film formation by XPS (surface analysis and depth profiling) and SEM. As important finding, the reaction mechanism changes when replacing lithium by sodium. In the latter case, CuO conversion is only partially reversible but oxygen becomes redox active as well and Na2O2 forms as additional intermediate. On the contrary, oxygen is inactive in Li-CuO cells. Further, significant differences occur in the surface film formation on CuO electrodes in Li and Na cells.4 The results clearly demonstrate that significant differences in the redox chemistry in LIBs and NIBs exist for identical electrode materials and these differences need to be understood and used accordingly to achieve high performance electrodes. 1.            Jache, B.; Adelhelm, P., Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angewandte Chemie-International Edition 2014, 53, (38), 10169-10173. 2.            Hartmann, P.; Bender, C. L.; Vracar, M.; Duerr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P., A rechargeable room-temperature sodium superoxide (NaO2) battery. Nature Materials 2013, 12, (3), 228-232. 3.            Adelhelm, P.; Hartmann, P.; Bender, C. L.; Busche, M.; Eufinger, C.; Janek, J., From lithium to sodium: cell chemistry of room temperature sodium-air and sodium-sulfur batteries. Beilstein Journal of Nanotechnology 2015, 6, 1016-1055. 4.            Klein, F.; Pinedo, R.; Hering, P.; Polity, A.; Janek, J.; Adelhelm, P., Reaction Mechanism and Surface Film Formation of Conversion Materials for Lithium- and Sodium-Ion Batteries: An XPS Case Study on Sputtered Copper Oxide (CuO) Thin Film Model                   Electrodes. The Journal of Physical Chemistry C 2016. - advance online - DOI: 10.1021/acs.jpcc.5b10642