OPTIMIZED REFLECTION OF CdS/ZnO WINDOW LAYERS IN Cu(In,Ga)Se_2 THIN FILM SOLAR CELLS

摘要

ZnO/CdS/Cu(In,Ga)Se_2 thin film heterojunction solar cells with a wide range of thicknesses for the CdS layer (up to 85 nm) and the ZnO layer (up to 400 nm) are characterized by current/voltage, quantum efficiency, and optical reflection measurements. We investigate the correlation between the short circuit current density and the ZnO/CdS layer thicknesses, focussing on the counteracting effects of light absorption and reduced optical reflection induced by each layer. In case of CdS both effects almost compensate each other for CdS thicknesses between 0 and 40 nm. Thus, the short circuit current density does barely change within this range of CdS thicknesses. The optimal ZnO thickness is found to be 50 nm. We present a solar cell with optimized reflection properties and 17.8 % total area efficiency. The layer system MgF_2 / CdS / ZnO proves to serve as an excellent 3-layer antireflection coating.