The characteristics of Cu(In, Ga)Se 2 thin-film solar cells by bandgap grading

Abstract

We investigated the characteristics of Cu(In, Ga)Se 2 solar cells with bandgap (E g ) grading. Two precursor types were employed: Mo/Cu 0.75 Ga 0.25 /In/Ga 2 Se 3 (CIGSe-1) and Mo/Cu/In/Ga 2 Se 3 (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 × 10 15 cm −3 within the CIGS-1 absorber layer and 2.37 × 10 15 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 V OC deficit (E g /q − V OC ) 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 E g grading in a CIGSe layer with a wider E g on the back surface can result in improved performance. © 2019 The Korean Society of Industrial and Engineering Chemistry