Effect of Se Composition in CdSe\u3cem\u3e1-X\u3c/em\u3eT\u3cem\u3eeX\u3c/em\u3e/CdTe Solar Cells

Cadmium Telluride solar cell is a leading thin film photovoltaic in the market today. Constant enhancement of efficiency in the last decade has made its position firmer in research interest as well. The record cell efficiency for CdTe solar cells is 22.1%. The theoretical limit suggests that there is still room for efficiency improvements. Reducing the band gap of CdTe solar cells can improve the short circuit current and overall cell performance. This work focuses on bandgap engineering of the CdTe layer by incorporating Selenium (Se) in to form the compound CdSexTe1-x (CST) which can have a lower bandgap than CdTe, and it will therefore enhance the short circuit current density (JSC) and cell efficiency. In this study three compositions of Se 7%, 15%, and 19% have been studied to understand the effects of Se on cell performance. The cell structure was glass/ITO/CdS/CST/CdTe/Back contact. The total thickness of CdTe and CST were maintained at 4 µm. All samples were exposed to the CdCl2 heat treatment at temperatures of 390, 410, 420 and 430°C. The thickness of the CST layer was also varied; three thicknesses 0.5,1, 1.5 and 2µm were investigated. Cells with CST layers with 19% Se composition and thickness of µm exhibited the best performance among three compositions. Among the four CdCl2 annealing temperature 420°C proved to produce the best overall cell performance. As the composition and thickness of the CST alloy increased a shift in the absorption edge from ~ 860 to 900 nm was observed in the QE indicating that the inclusion of CST resulted in improved JSC due to the smaller bandgap