Dependence of ZnO:Al properties on the substrate to target position in RF sputtering

Abstract

Al doped ZnO (ZnO:Al) is commonly used as front contact in Cu(In,Ga)Se 2 based thin film solar cells, and for this application is often deposited by RF magnetron sputtering from a ceramic target. In the present study, we use a configuration in which the substrates are immobile under the sputter target and we find the ZnO:Al film properties to depend strongly on the substrate position relative to the target erosion area (or “racetrack”). The zone immediately under the racetrack has higher resistivity correlated to a lesser crystallinity. The differences between these zones (inside the racetrack, under the racetrack, outside the racetrack) are studied as a function of the substrate: bare soda-lime glass (SLG), (CBD)CdS coated SLG, undoped ZnO coated SLG. It is shown that the evolution of the ZnO:Al films non homogeneity is substrate dependent. The impact on complete CIGSe/CdS/ZnO/ZnO:Al devices is measured and show that the influence of the substrates has to be taking in account; the device prediction made from models using the ZnO:Al properties measured on bare glass are inaccurate.