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.

Tin dioxide (SnO2) thin films were deposited on quartz glass substrates by chemical vapor deposition using SnI2 and O-2 as reactants. The growth experiments were carried out in the substrate temperature range of 300-900 degrees C. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, spectrophotometry and Raman spectroscopy were used to characterize the films. The films were polycrystalline with their crystallites having a preferred orientation, which was dependent on the film thickness. The average grain size increased with increasing thickness of the films. The binding energies of Sn 3d(5/2) and O 1s for all samples showed the Sn4+ and O-Sn4+ bonding state from SnO2. The absolute average transmittance of SnO2 films exceeded 90% in the visible and infrared range. The obtained SnO2 films had optical band gaps between 3.78 and 3.92 eV. (C) 2015 Elsevier Ltd. All rights reserved.

Tin dioxide (SnO2) thin films were grown on c-plane sapphire substrates by chemical vapor deposition using SnI2 and O2 as reactants. The growth experiments were carried out at a fixed substrate temperature of 510°C and different O2 flow rates. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV–Vis–IR spectrophotometry and Hall-effect measurement were used to characterize the films. All films consisted of pure-phase SnO2 with a rutile structure and showed an epitaxial relationship with the substrate of SnO2(100)||Al2O3(0001) and SnO2[010]||Al2O3<11–20>. The crystalline quality and properties of the films were found to be sensitively dependent on the O2 flow rate during the film growth. The absolute average transmittance of the SnO2 films exceeded 85% in the visible and infrared spectral region. The films had optical band-gaps (3.72–3.89eV) that are in line with the band gap of single-crystal SnO2. The carrier concentration and Hall mobility of the films decreased from 3.3×1019 to 9×1017cm−3 and from 19 to 2cm2V−1s−1, respectively, while the resistivity increased from 0.01 to 3Ωcm with increasing of the O2 flow rate from 5 to 60sccm.

Thin films of doped VO2 were deposited, analyzed, and optimized with regard to their solar energy transmittance (Tsol) and visible/luminous light transmittance (Tlum) which are important parameters in the context of smart window applications in buildings. The doping with alkaline earth metals (AEM) like Mg, Ca, Sr, or Ba increased both Tsol and Tlum due to a bandgap widening and an associated absorption edge blue-shift. Thereby, the brown-yellowish color impression of pure VO2 thin films, which is one major hindrance limiting the usage of VO2 as thermochromic window coating, was overcome. Transparent thin films with excellent switching behavior were prepared by sputtering. Highly doped V1−xMexO2 (Me = Ca, Sr, Ba) kept its excellent thermochromic switching behavior up to x(Me) = Me/(Me + V) = 10 at. % doping level, while the optical bandgap energy was increased from 1.64 eV for undoped VO2 to 2.38 eV for x(Mg) = 7.7 at. %, 1.85 eV for x(Ca) = 7.4 at. %, 1.84 eV for x(Sr) = 6.4 at. % and 1.70 eV for x(Ba) = 6.8 at. %, as well as the absorption edge is blue shifted by increasing AEM contents. Also, the critical temperature ϑc, at which the semiconductor-to-metal transition (SMT) occurs, was decreased by AEM doping, which amounted to about −0.5 K/at. % for all AEM on average. The critical temperature was determined by transmittance-temperature hysteresis measurements. Furthermore, Tsol and Tlum were calculated and were found to be significantly enhanced by AEM doping. Tlum increased from 32.0% in undoped VO2 to 43.4% in VO2 doped with 6.4 at. % Sr. Similar improvements were found for other AEM. The modulation of the solar energy transmittance ΔTsol, which is the difference of the Tsol values in the low and high temperature phase, was almost constant or even slightly increased when the doping level was increased up to about 10 at. % Ca, Sr, or Ba.

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.

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. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3685719]