Band-gap grading in a CIGS absorber and a conduction band offset at n/p hetero-interface are two important parameters of band-gap engineering aiming at high efficient CIGS solar cells. To obtain optimal CIGS absorber's band-gap grading profile an automatic optimization loop based on Nelder-Mead simplex optimization algorithm has been implemented. The optimization problem is described with an objective function, which-by varying the input parameters-is minimized or maximized. In our study two types of objective functions are used; optical and electrical. As the most optimal profile a parabolic, double graded band-gap profile with a positive or nearly zero conduction band offset at n/p hetero-interface is calculated. Structures with different CIGS absorber thicknesses and bulk and/or hetero-interface recombination lifetimes are examined and their optimized parameters are discussed in the light of experimental achievements. (c) 2005 Elsevier B.V All rights reserved.