A simple approach, which can estimate the diode ideality factor of a high efficiency pn junction solar cell under illumination by using its current-voltage data, is explained. We have proposed that an analytical method based on Lambert W-function is sufficient for the extraction of the diode ideality factor of a solar cell modeled by double junction behavior with considerable compliance. Various illumination intensities are also considered in order to specify the reliable limit of the method. The dependence of the ideality factor and the reverse saturation current with light intensity has also been investigated in order to provide insight into the alteration of electrical conduction at junction interface at room temperature. (C) 2011 Elsevier Ltd. All rights reserved.