High Efficiency CdTe/CdS Thin Film Solar Cells with a Novel Back Contact

The back contact is probably the most critical step in the fabrication of high efficiency CdTe/CdS thin film solar cells. Usually, it is done first by etching the CdTe surface in order to form a Te rich surface and second by depositing a small amount of Cu, on the order of 2nm, on top of etched CdTe surface. In this way a CuxTe layer forms that seems to behave as a non rectifying contact for CdTe. If more copper is used, there is the risk that Cu in excess diffuses into the CdTe/CdS structure causing shunt paths by segregating into the grain boundaries or lowering the cell efficiency by doping CdS thus increasing its resistivity. In this paper, we propose a novel approach to avoid the Cu diffusion. First of all we do not etch the CdTe surface, second we deposit on top of a not etched surface 100-200nm of a buffer layer, that is As2Te3, followed by the deposition of 10-20nm of Cu. If the deposition of Cu is done at 150-200°C substrate temperature, a reaction between Cu and As2Te3 happens that forms a CuxTe layer by a substitution reaction. This type of contact resulted to be stable and non rectifying.