Current-voltage characteristics of ZnO/CdS/CuGaSe2 single-crystal solar cells with solar conversion efficiency values of eta = 3.5%, 6.0%, 6.7% and 9.7% were analyzed using the single diode equation. The effect of each of the achieved parameters on the fill factor was calculated. The calculations revealed that the fill factor reduction due to the series resistance remained below Deltaff = 4.4% under illumination, while this effect would have been much higher if the illumination had not reduced the series resistance markedly. The calculation furthermore revealed that the fill factor reduction due to the shunt resistance remained below Deltaff = 3.6% under illumination. This effect would have been negligible if the illumination had not also reduced the shunt resistance in all studied cells. The increase of the saturation current density under illumination has brought about considerably high fill factor losses (at least Deltaff = 8.3%) in all studied cells. Already the dark saturation current density and the diode ideality factor in such cells have been found to be much higher than the ones in the cells based on CuInSe2. This seems to be the most substantial restriction to the fill factor, and thus to the performance, of solar cells based on CuGaSe2. An explanation for this different behavior seems to lie in the different band structures of these cells. (C) 2003 Elsevier B.V. All rights reserved.