Replacement of the CBD–CdS buffer and the sputtered i-ZnO layer by an ILGAR-ZnO WEL: optimization of the WEL deposition

As shown earlier the window extension layer (WEL) concept for thin film solar cells based on chalcopyrites results in device performances exceeding those of corresponding chemical bath deposited cadmium sulfide (CBD–CdS) buffered reference cells. The WEL concept is extended and it will be demonstrated, that now a single WEL successfully replaces both, the conventional buffer and the intrinsic part of the window bi-layer usually deposited by sputtering. Thus, one part of the window is deposited directly onto the absorber by a soft process called ion layer gas reaction (ILGAR). The optimization of ILGAR-ZnO WELs on Cu(In,Ga)(S,Se)2 absorbers with respect to the efficiencies of the completed solar cells is presented. This effort results in ‘total area’ efficiencies of 14.5% (best cell) which are comparable to those of devices with CBD–CdS buffer (14.7%—best cell) without any antireflecting coating.