Abstract
We investigated the characteristics of Cu(In, Ga)Se2 solar cells with bandgap (Eg) grading. Two precursor types were employed: Mo/Cu0.75Ga0.25/In/Ga2Se3 (CIGSe-1) and Mo/Cu/In/Ga2Se3 (CIGSe-2). In CIGSe-1, the range of depths with a high Ga content is wider than that in CIGSe-2; thus, the region in which the main electron-trapping clusters and high-population deep donor defects can form is larger, and the defect density is higher. In the defect energy level range, various other defects and defect clusters exist with a defect density of 2.83 × 1015 cm−3 within the CIGS-1 absorber layer and 2.37 × 1015 cm−3 within the CIGS-2 absorber layer. The average efficiency values are 5.71% for CIGSe-1 and 6.82% for CIGSe-2. Additionally, the average VOC deficit (Eg/q − VOC) values are 0.758 V for CIGSe-1 and 0.731 V for CIGSe-2. As a result, in the 7 CIGSe-2 samples, the open-circuit voltage and efficiencies are improved. Thus, it is demonstrated that appropriate Eg grading in a CIGSe layer with a wider Eg on the back surface can result in improved performance.
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