Optimization of ZnO:Al based back reflectors for applications in thin film flexible solar cells

Back reflectors based on Aluminum and Molybdenum as the reflecting materials and ZnO:Al as the transparent conductive oxide, were deposited at low temperature on polyethylene naphthalate by magnetron sputtering. The optimization of the layer thickness of the constituent materials and the final texture of the structure was performed in order to achieve structures with appropriate properties to be applied to amorphous silicon solar cells. In this sense, average total reflectance values of 71% in the wavelength range of 400-800 nm were obtained for the layer stack of 80 nm-thick ZnO:Al layer on 235 nm-thick Aluminum deposited directly on the flexible substrate. However, this structure showed a poor adherence onto the substrate. This issue was improved using a thin Molybdenum layer between the metal and the substrate, maintaining similar optical properties. On the other hand, textured back reflectors were obtained etching the transparent conductive oxide with NH4Cl aqueous solution. After the etching process, the highest enhancement was achieved for the sample that used an Mo/Al bilayer as reflective metal. In this case, a roughness of around 29 nm and a haze factor of 25% at 600 nm were achieved.