The band alignment of p-Cu2O/n-AlxGa1–xN heterojunction with x up to 0.15 was studied by X-ray photoelectron spectroscopy. The conduction band offset between binary Cu2O and ternary AlxGa1–xN is found to decrease with increasing x. The data suggest that a flatband situation in the conduction band of p-Cu2O/n-AlxGa1–xN heterojunctions can be achieved for x about 0.4, which is an Al-content where n-type doping is still feasible. Thus, n-AlxGa1–xN with x between 0.4 and 0.6 may be a suitable window material for heterojunction solar cells with a p-Cu2O absorber layer. The current-voltage characteristics of the p-Cu2O/n-AlxGa1–xN heterojunctions under illumination confirm the anticipated improvement of the photovoltaic properties with increasing x.

Polycrystalline Cu2O thin films were prepared on c-sapphire substrates by reactive radio-frequency sputtering at various temperatures between 500 and 925 K employing a metallic target and utilizing an argon/hydrogen/oxygen gas mixture. It is demonstrated that the use of hydrogen in the sputter deposition process beneficially affects the transport properties of the Cu2O films obtained. Correlating the amount of hydrogen incorporated into the thin films, the film morphology and the transport and luminescence properties demonstrate that in this approach hydrogen is predominantly accumulated at the grain boundaries of the polycrystalline films, leading to a lower film resistivity due to the reduction of grain boundary scattering. It is demonstrated that a suitable employment of hydrogen in the growth process of Cu2O material for solar cell applications improves the material properties significantly.

Several growth methods were employed to investigate the photovoltaic behavior of GaN/Cu2O heterojunctions by depositing cuprous oxide thin films on top of gallium nitride templates. The templates consist of a thin layer of GaN:Si grown on a sapphire substrate by metal organic vapor deposition. The deposition procedure was followed up by photolithographic structuring and thermal evaporation of metal contacts. For device characterization, J-V characteristics and external quantum efficiency were measured, pointing to a possible energy barrier in the conduction band. To gain further insight X-ray photoelectron spectroscopy was applied.

The p-type conducting Copper-oxide compound semiconductors (Cu 2 O, CuO) provide a unique possibility to tune the band gap energies from 2.1 eV to the infrared at 1.40 eV into the middle of the efficiency maximum for solar cell applications. By a pronounced non-stoichiometry the electronic properties may vary from insulating to metallic conduction. They appear to be an attractive alternative absorber material in terms of abundance, sustainability, non-toxicity of the elements, and numerous methods for thin film deposition that facilitate low cost production. The synthesis and characterization of Cu 2 O thin films used as p-type absorbers in heterojunction solar cells will be reported. We discuss properties of the undoped non-stoichiometric Cu 2 O, controlled p-type doping by nitrogen, analysis of band offsets by X-ray photoelectron spectroscopy (XPS). In addition we show proof of concept for an increase in photovoltaic conversion efficiency in AlGaN/Cu 2 O heterostructures due to a more favorable band alignment.