[Show abstract][Hide abstract] ABSTRACT: 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 CuGa x Se y absorber layers. The p±n junction is formed by vacuum evaporation of p-type CuGa x Se y directly on the n-type ZnO grown by RF-magnetron sputtering. The grain size and lattice parameters of CuGa x Se y depend on the Cu/Ga ratio. Optical transmission measurements have been used to determine the bandgap of CuGa x Se y layers. The Ga-rich phase CuGa 3 Se 5 has a bandgap of 1.9 eV compared to 1.7 eV of the CuGaSe 2 phase. The Ga-rich layers are p-type conducting and they form a photovoltaic diode on n-ZnO. Photovoltaic properties of CuGa x Se y /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. q 2000 Elsevier Science S.A. All rights reserved.