The necessity to pre-estimate the mathematical model is discussed and the parameters extracted from the density current-voltage (J-V) curves are evaluated based on a double-exponential model. Two devices, epitaxial CIS and polycrystalline CIGS, are fabricated and compared for J-V characteristics in the dark and under illumination. After pre-estimation, the epitaxial CIS device under illumination was over-estimated, while crystalline CIGS device as well as epitaxial CIS device in the dark showed well-behaved property. Parameters (ideality factor, saturation current density, series and shunt resistance) were extracted by double-exponential model from these well-behaved J-V characteristics. Moreover, the meanings of these parameters are discussed and compared to describe physical mechanism of devices. (C) 2010 Elsevier Ltd. All rights reserved.