The reversible persistent changes of the fill factor (FF) induced by the illumination and voltage bias along with changes in the electronic properties of the ZnO/CdS/Cu(In,Ga)Se-2 photovoltaic devices have been studied. Admittance spectroscopy and capacitance-voltage characterization reveal a correlation between the FF and the space charge distribution within the absorber. Our experiments provide evidence that a major source of FF loss in efficient devices is caused by excess negative charge close to the interface. We explain the persistent changes in the net acceptor concentration in the interface region by the relaxation effects due to compensating donors-the same mechanism, which leads to metastable changes of the doping level in the bulk of the absorber. (C) 2003 Elsevier B.V. All rights reserved.