Recent development on surface-textured ZnO:Al films prepared by sputtering for thin-film solar cell application

This study addresses the optimization of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) thin films as front contact in silicon thin-film solar cells. The front contact has to be highly conductive and highly transparent for the visible as well as for the infrared spectrum. Furthermore, it has to scatter the incident light efficiently leading to an effective light trapping inside the silicon layers. To materialize the scattering phenomenon, the surface of the magnetron-sputtered ZnO:Al thin films is textured by wet-chemical etching. In this contribution we focus on an optimized balance between electrical and optical needs maintaining a surface topography well suited for light trapping. In a first step we study the influence of vacuum annealing on ZnO:Al films on glass substrates. An increase in transmission is observed while the carrier concentration is gradually decreased. Application of vacuum-annealed ZnO:Al films in silicon thin-film solar cells allows the determination of the relationship between the front-contact carrier concentration and the short-circuit current density. Also, an optimized carrier concentration for the solar module application has been estimated. In a second step we apply this knowledge for direct fabrication of ZnO:Al layers with optimized carrier concentration by varying the target doping concentration (TDC). Nevertheless, changing the TDC alters the ZnO:Al properties and especially the texturing behavior of the thin films as well. Thus, we present a parameter study of TDC and substrate temperature during sputtering to prepare front contacts with surface topography enabling efficient light trapping and ideally balancing optical and electrical properties for solar module applications.