Effects of Ga content on Cu(In,Ga)Se2 solar cells studied by numerical modeling

The band-gap energy of the CuIn1-xGaxSe2 (CIGS) films is varied in the range from 1.04 to 1.68eV with the corresponding Ga content in the CIGS films from x = 0 to 1. With the addition of Ga in the CuInSe2 films for the CIGS-based solar cells, the high open-circuit voltage is achieved, and hence the performance of the CIGS solar cells is improved. However, the performance of the CIGS solar cells with the high Ga content falls short of the expectations. The simulation results suggest that the defect density in the surface layers and/or the bulk defect density of the CIGS films are the critical factors responsible for the limitation of the performance for the CIGS solar cells with the high Ga content. Due to the surface band-gap widening, the conduction band offset between the surface region and bulk region of the CIGS films is formed. The simulation results indicate that the open-circuit voltage of the CIGS solar cells increases with the increase of the conduction band offset. The band-gap grading in the space charge region and near the back surface region with the spatial gradient distribution of the Ga content significantly improves the performance of the CIGS solar cells. (C) 2007 Elsevier Ltd. All rights reserved.