Properties of CuCdTeO films: from solid solutions to composites

The use of different technologies and methods aimed to the development of novel materials is one of the topics in current materials research that has been triggered by the on-going, increasingly demanding, technological needs which include environmentally friendly energy sources. This area may be considered among the front runners in the current world research agenda. One of the photovoltaic technologies currently under development is based on the II-VI semiconductor cadmium telluride (CdTe). The search for novel materials for photovoltaics led us to grow and study CdTe-based films modified by incorporating Cu and O using both reactive co-sputtering and deposition from single composite targets made from mixtures of CdTe and CuO powders with different relative concentrations. The single-target method is simple, robust, with good reproducibility and overcomes some issues of the reactive co-sputtering technique. In general, the electrical resistivity and the optical transmittance (i.e. the electronic properties) can be readily controlled by the concentration of CuO in the target. For high CuO concentrations, the films are mainly composites of CdTe:Cu:O and Cu2-xTe. The thermal stability of these phases was tested by in-situ X-ray diffraction performed from room temperature up to 400 oC. Theoretical work carried out by other groups and by us involving copper and/or oxygen in a CdTe host will also be presented and compared to the experimental findings.