Microtribological rating of transparent conducting oxide for thin film solar cells
In this study, the mechanical properties of transparent conducting oxide (TCO) deposited on glass substrates, namely, aluminium doped zinc oxide (ZnO:Al) and fluorine doped tin oxide (SnO2:F) for the application of amorphous or amorphous/microcrystalline silicon solar cells, were investigated. The ZnO : Al thin films were deposited by either inline reactive ac magnetron sputtering or radio frequency magnetron sputtering utilising ceramic targets. The SnO2:F thin films were prepared using either inline or offline chemical vapour deposition (CVD). The micromechanical properties such as elastic and permanent deformation, microtribological properties and scratch resistance were measured to assess the film properties. The micro-structure-analysis method (MISTAN) was used to characterise those properties in situ with high local resolution. The inline reactive ac magnetron sputtered ZnO:Al films show relatively good scratch resistance. The surface structure of ZnO:Al films can be roughened by an etching process in order to enhance the scattered light into the cells. At the same time, the microfriction of the ZnO:Al films under the same load is higher after the etching process. Also the elastic and permanent deformation of inline reactive ac magnetron sputtered ZnO:Al films are increased after etching. The etching process has a significant impact on the radio frequency magnetron with ceramic targets deposited ZnO:Al films, since the scratch resistance of these films can be significantly improved by the etching process. The results show that the SnO2:F films possess poor mechanical resistance and planar surface structure.