Improving the absorption spectrum and performance of CIGS solar cells by optimizing the stepped band gap profile of the multilayer absorber

Abstract

In this paper, we investigate a way to improve the performance of thin films CIGS-based solar cells by optimizing their spectral responses. Band gap profile grading, aroused this last decade as a very promising strategy to achieve higher efficiency. This work was performed on a CIGS-based thin-film solar cell model, using cadmium sulphide CdS as a buffer layer, with zinc oxide ZnO as window layer. Focusing on electrical properties such as band gap and electron susceptibility, we modified gallium content of the absorber layer of a reference cell. Using aforementioned band gap grading strategy, we stacked three CIGS layers with different gallium contents, and we modelled a stepped band gap profile using SCAPS-1D. The comparative analysis of the results obtained through numerical simulation, with those of six different band gap profiles of the absorber layer, supports the statement that, stepped band gap profile is a promising approach for engineering improvement of high-efficiency CIGS-based solar cells. In fact, the absorption spectrum of the optimized cell has improved compared to the other band gap profiles simulated in this work; with a peak external quantum efficiency (EQE) of 94%. Efficiencies losses dues to recombination were reduced and the generated photocurrent is higher than that of classical profiles. Record efficiency of 28.03% was achieved with a fill factor of 86.42%. We believe absorber layer stacked in a stepped band gap profile, as a complementary approach to improve performances, is definitely a must-have technology for all future generations high efficiency thin films solar cells.