This paper presents that in analytical method based on Lambert W-function can be applied to estimate the value of the diode ideality factor n, of a ZnO/CdS/Cu(In,Ga)Se-2 (CIGS) solar cell by using its dark current-voltage characteristics. The method is tested at different temperatures in the dark and found that the resulting n(T) values are in good agreement with those estimated experimentally from the slopes of the straight-line regions of Log I-V plots. The suggested values of n(T) under illumination are also determined using the exact explicit analytic solutions for the current-voltage relation expressed in terms of Lambert W-functions and experimentally estimated parasitic series and shunt resistances (R-s, R-sh), diode saturation current (I-o), open circuit voltage (V-oc) and short circuit current (I-sc) values at various temperatures. Temperature dependence of the diode ideality factor revealed that after illumination still tunnelling enhanced interface recombination mechanism dominates the Current transport with relatively low tunnelling energy as compared to the dark case. (c) 2008 Elsevier Ltd. All rights reserved.