The electrical properties of CdTe and Cu(In-A,Ga1-x)Se-2 (CIGS) solar cells change depending on their illumination conditions. The dark and light current-voltage (I-V) curves of both solar cells exhibit a distinct cross over. The reason for the cross over of the CIGS solar cells is located in the CdS buffer layer and its interface to the absorber. Light which is absorbed in the CdS buffer layer results in a steeper I-V curve in contrast to the dark curve. The buffer layer of the CdTe solar cell has a similar influence on the I-V curve. In this case, however, the effect described above overlaps with a second feature. Many CdTe solar cells have a Schottky contact as their back contact. This is expressed in the I-V curve as a current limitation at positive voltage bias (roll over). However, under light bias even this diode can be influenced. By means of quantum efficiency (QE) measurements under voltage bias it is possible to analyze both effects. All experimental observations can be explained by traps in CdS and/or CdTe assuming strongly different capture cross sections for holes and electrons. A model describing the influence of the illumination on the I-V and QE curves is presented.