The electronic properties of the interface region in the ZnO/CdS/Cu(In,Ga)Se-2 devices have been investigated in the various metastable states induced by voltage bias and illumination. Capacitance spectroscopy has been used to gain information about the level spectrum in the interface region of absorber and space-charge distribution within the structures. The results of capacitance spectroscopy are analyzed in conjunction with the current-voltage characteristics. We have differentiated between the metastable effect due to the changes of the space-charge distribution in the absorber and a process involving the persistent changes of the Fermi-level position at the interface. We attribute the first one to the electronic processes due to relaxing donor-type V-Se centers. The second one in our opinion involves the process of forming a quasi-equilibrium between the positive and negative charges in the immediate vicinity of the interface. In the admittance and DLTS spectra of interface levels a signal belonging to bulk donors (most probably to In-Cu defects) has been identified. (C) 2003 Elsevier Science B.V. All rights reserved.