Design of the TCO (ZnO:Al) thickness for glass/TCO/CdS/CIGS/Mo solar cells

Modelling solar cells has, in recent years, become a fundamental tool for their optimum design. Here, we present an optical study of CuInGaSe2 (CIGS) solar cells in order to improve them by selecting the appropriate transparent conductive oxide (TCO) layer thickness. The system of interest in our case was glass/TCO/CdS/CIGS/Mo, considering radiation incident perpendicular to the cell from the glass and taking into account the absorbent nature of the TCO, CdS and CIGS layers. The reflectance of this layered system was calculated using the optical matrix method considering the three absorbent layers and the reflectance at the Mo substrate. In addition, the transmittance of the TCO layer was also calculated. Then, having the constant thickness of the CIGS (1 mu m) and CdS (0.1 mu m) layers, the weighted average of the cell reflectance and the TCO transmittance in the AM1.5 solar spectrum were calculated for different TCO thickness in the range from 0.1 to 1 mu m. Finally, we defined a figure of merit that involves the total cell reflectance, the transmittance and the sheet resistivity of the TCO as a function of the TCO layer thickness. Our results indicate that the ZnO film thickness can be increased to 1 mu m with a possible improvement of the cell performance compared to cells with typical TCO thickness around 500 nm.