Band gap grading is able to improve the performance of CIGS solar cells. In order to get a better understanding of the parameters influencing the effect of a grading, a closed form analytical expression of the current density is derived. The advantages and drawbacks of this analytical expression with respect to numerical simulation results are assessed. The analytical analysis has limited validity due to restrictions on the absorption characteristic and on the recombination in the space charge layer. Nevertheless it leads to good results, it has the advantage of being faster than numerical simulations and it clarifies the interrelation of the meaningful parameters. (C) 2010 Elsevier B.V. All rights reserved.