Application of Stacked ZnO Films as a Window Layer to Cu(InGa)Se2-Based Thin-Film Modules

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Abstract
To overcome the disadvantages of DC sputtering while keeping its advantages, a stacked structure of ZnO films as a window layer has been developed for application to Cu(InGa)Se2 (CIGS)-based thin-film modules. Application of multilayered, 2 wt% Al2O3-doped ZnO (AZO) films prepared from a 2 wt% Al2O3-doped ZnO ceramic target and use of a combination of RF and DC sputtering techniques (i.e., sputtering conditions of RF 300 W/DC 1 A/DC 1 A) can lead to better crystallinity and electrical properties and, as a result, an improved device performance of CIGS-based thin-film modules. To improve the humidity resistance of the ZnO window layer, which is the top layer of CIGS-based thin-film modules, AZO is replaced with 5.7 wt% Ga2O3-doped ZnO (GZO) prepared from a 5.7 wt% Ga2O3-doped ZnO ceramic target. By adjusting the stacked structure and the layer thickness by DC sputtering, an improvement in device performance, especially the fill factor (FF), is achieved with the stacked structure of AZO (base)/AZO (middle)/0.6 GZO (top) as the window layer, and a total thickness of about 600 nm is realized, which means a thinner (about 60% of AZO top layer thickness) top layer.