ZnO:Al/Al back reflector with good adhesion on a flexible polyimide substrate for thin film silicon solar cells

For thin film solar cells, a back reflector is required to enhance conversion efficiency by reflecting the unabsorbed light back into the absorber layers. The most widely used back reflector in thin film solar cells is high reflectance transparent conducting oxide (TCO)/Ag thin films deposited on substrates such as glass, steel and polyimide. Compared with the other two types of substrates, polyimide substrate has a lot of advantages, including low price, light weight, and high power-to-weight-ratio. For these reasons, thin film solar cells on polyimide substrates are of great importance for both terrestrial and space applications. However, the main limitation of the polyimide substrate is the bad adhesion of the coating layer on it. In this study, a cheap back reflector comprising of Al doped ZnO (AZO) and Al thin films was deposited by magnetron sputtering on a flexible opaque polyimide (PI) substrate at a substrate temperature of 300°C. A thin barrier layer-quartz (SiO₂) was deposited before the Al layer to enhance the adhesion. The films were characterized by UV-VIS spectrophotometer, scan electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The adhesion between the coating layers and the PI substrate was also tested. Our results show that the adhesion of cheap back reflector on the PI substrate has been greatly enhanced by the SiO₂ barrier layer. Thus, AZO/Al back reflector would be a promising alternative to TCO/Ag back reflector for thin film solar cells.