In the Cu(In,Ga)Se-2 system the bandgap can be varied between 1.0 and 1.7 eV, which offers the possibility of tandem devices. For a development in this direction, superstrate solar cells have been fabricated with CuGaxSey absorber layers. The p-n junction is formed by vacuum evaporation of p-type CuGaxSey directly on the n-type ZnO grown by RF-magnetron sputtering. The grain size and lattice parameters of CuGaxSey depend on the Cu/Ga ratio. Optical transmission measurements have been used to determine the bandgap of CuGaxSey layers. The Ga-rich phase CuGa3Se5 has a bandgap of 1.9 eV compared to 1.7 eV of the CuGaSe2 phase. The Ga-rich layers are p-type conducting and they form a photovoltaic diode on n-ZnO. Photovoltaic properties of CuGaxSey/ZnO superstrate solar cells are correlated with the growth conditions and structural properties of the absorber layer. The formation of an interfacial layer of about 10 nm thickness and precipitates on top of the ZnO grains, have been observed with transmission electron microscopy.