The current-voltage curves of many ZnO/CdS/CuInSe2 solar cells display significant distortion when only red light illumination is employed. This distortion generally disappears or partially disappears for a period of time following illumination with blue light. Similarly, the dark diode curve shortly after illumination containing blue light is shifted significantly from the equilibrium dark curve. This effect is more common than generally realized and indicates a mechanism that is potentially detrimental to photovoltaic efficiency. A model is presented that is based on a low free-electron concentration and a high concentration of deep levels in the CdS window layer. This model is consistent with observed variations in current-voltage, capacitance, and laser scan data with illumination wavelength and history.