Cu(In, Ga)Se-2 absorber layers were deposited on glass coated with a ZnO/ZnO:Al double layer for the fabrication of superstrate solar cells. Their photovoltaic performance and material properties were compared with cells in the substrate configuration which were grown on Mo-coated glass. Measurements of the capacitance-voltage characteristics were used to study the carrier density of the absorber material. Compared to substrate cells. superstrate cells revealed a low acceptor concentration. Profiling with secondary mass spectroscopy revealed a low concentration of Na, which is an effective acceptor for Cu(In, Ga)Se-2. In contrast to the Mo layer in substrate cells, the ZnO/ZnO:Al bilayer acts as barrier against the diffusion of Na from the glass substrate. Furthermore, the carrier density analysis revealed a high concentration of trap states close to the interface in the absorber layer of superstrate solar cells. Light soaking saturates such trap states and increases the net carrier density, which improves the open circuit voltage and the efficiency of the solar cells.