Optical modeling of periodic nanostructures in ultra-thin CdTe solar cells with an electron reflector layer

CdTe solar cells have become a competitive player in the commercial photovoltaic market competing with CIGS and Si wafers. Considerable research attention has been devoted to increasing the overall conversion efficiency of CdTe which reached 22%. Among the challenges facing this technology is the scarcity of Te. In this paper, we investigated the use of an ultra-thin CdTe layer, which would result in a substantial decrease in Te consumption by almost 90%. However, this substantial decrease in the absorber thickness must be accompanied by some embedded nano-light-trapping structures. The purpose of these nanostructures is to increase the light absorption of the UV-VIS and NIR wavelengths which leads to an increase in the generated photo-current. Also, an electron reflector layer must be added between the metallic contact and the thin absorber layer to mitigate the high recombination rate. Additionally, three different metallic contacts were explored: Au, Ag, and Al. The absorption in the CdTe layer which is responsible for the photo-generated current and the parasitic absorption in the metallic back contact, as well as the short circuit current densities, were determined and analyzed. The numerical Wave optics finite element method and the analytical transmission line theory were utilized. It was noted that these nanostructures showed a substantial increase in the absorption in the CdTe absorber layer in the visible and NIR ranges. A net increase in the short circuit current density of 36% has been realized with the top nano-structured CdTe layer comparing to the flat reference cell of the same absorber volume. Furthermore, the Ag metallic back-contact outperforms both Al and Au and achieved the highest short circuit current density and the lowest parasitic absorption. These findings would open a new route to integrate optimized light trapping nano-structures into the commercial CdTe solar cells to further enhance their material utilization and efficiency.