Improvement of CIGS solar cell efficiency with graded bandgap absorber layer

There are several ways to increase the efficiency of a solar cell. In addition to increasing efficiency, the important subject is to build the cell at a lower cost and try to reduce the heat losses of the cell. In this article, research was conducted on CIGS solar cell in which some methods were proposed that in addition to reducing the losses of a CIGS solar cell, we could achieve maximum efficiency with the minimum thickness of the absorber layer. To simulate and perform mathematical calculations, the Atlas software of Silvaco was used in this research. It should be noted that in all proposed structures, the thickness of the layer is minimized, and also the absorber layer is p-type. In the first stage, the structure of a ZnO:Al/Zn0.83Mg0.17O/CdS/Graded CIGS/MO was simulated in which the thickness of the graded CIGS (absorber layer) is 1 µm. Graded CIGS is a structure in which the bandgap of material CuIn1−xGaxSe2 changes linearly from x1 to x2. In this study, the x variation is from 0.7 to 0.1, so that the decrease in x corresponds to the decrease in band gap linearly from 1.45 to 1.07 eV. In this stage, the cell efficiency was 17.1%. In the second stage, the buffer layer was upgraded to increase the cell efficiency, that is, instead of CdS layer with the bandgap of 2.4 eV, ZnO0.5S0.5 with the bandgap of 2.8 eV was used that made the efficiency to become 19.0%. In the third stage, an electron reflector layer was added to the structure of the first stage; indeed at this stage, the effect of the electron reflector layer on the solar cell of the first stage was displayed. In the fourth stage (last stage), CGS (CGS is CuGaSe2) layer was used as the electron reflector layer, which caused the cell efficiency to reach 28.3%.