Band-gap engineering in CIGS solar cells using Nelder–Mead simplex optimization algorithm

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.