ZnO/ZnO:Al window and contact layer for thin film solar cells: high rate deposition by simultaneous RF and DC magnetron sputtering

RF-magnetron sputtering of zinc oxide films (ZnO:Al and ZnO) is nearly exclusively used for the deposition of the window and contact layer for heterojunction thin film solar cells based on absorber materials like CuInSe₂, CdTe or CulnS₂. A drawback of this sputtering technique is the small deposition rate. This is caused by the low DC-voltage that develops at the target, since the sputtering rate in the energy range below 1 keV depends linearly on the acceleration voltage in the cathode fall. In order to increase the target voltage a simultaneous excitation by RF (13.56 MHz) and DC has been used. In this way, the advantages of both excitation modes have been combined for sputtering from oxidic targets (ZnO and ZnO:Al). Excitation by DC increases the deposition rate up to a factor of 1.5. On the other hand RF-excitation decreases the specific resistance significantly. This new deposition method has been used for the preparation of ZnO:Al/ZnO/CdS/CuInS₂-solar cells