Preparation and characterization of RF sputtered ZnO layers for application in thin films solar cells

Transparent conductive electrodes (TCO) serve as window and contact layers and they should combine high optical transmittance with low electrical resistivity. Recently ZnO has been considered to be the most promising TCO material in view of cost and properties. Chalcopyrite based solar cells commonly consist of Cu(In,Ga)Se2 (CIGS) absorber layer and a sputter sequence of undoped and doped ZnO layers in a structure: glass/Mo/CIGS/CdS/ZnO/ZnO:Al. In some cases, ZnO plays the role of the buffer layer itself as an alternative of the mostly used toxic CdS. An advanced application of this material is considered to be involving in Organic Solar Cells (OSCs) as Electron Transporting Material (ETM). The work presents fabrication of uniform resistive thin (50-100 nm) undoped ZnO films serving as buffer layers and conductive Al-doped ZnO films (Al:ZnO) with thickness in the range of 100-500 nm. In the latter case the film thickness was optimized to combine high transmittance and low resistivity. Both types of films are with good adhesion and possess a transmittance over 90% in the visible region. Highly transparent bi-layer films (ZnO/ZnO:Al) were finally embedded in the solar cell structure: glass/Mo/CIGS/ZnO/ZnO:Al/Al.