Instabilities in reactive sputtering of ZnO:Al and reliable texture-etching solution for light trapping in silicon thin film solar cells

Texture etched zinc oxide is often used as transparent front contact for silicon thin film solar cells. Reactive sputtering is a potentially low-cost process. However, process stability, film uniformity, and reproducibility are challenges to be solved. Oscillations of the control signal and subsequent reaction of the plasma emission control with moving substrates from rotatable metallic targets cause fluctuations of aluminum doped zinc oxide (ZnO:Al) properties. Solutions to overcome such variations during the reactive sputtering process are discussed. However, effects on film properties, especially on etching behavior, cannot be totally removed. To achieve good light scattering properties for solar cell application ZnO:Al films are usually etched in dilute hydrochloric acid. An etch process based on hydrofluoric acid has been developed to tune the surface texture for a given ZnO:Al material. One feature of this process is the relaxed requirement on ZnO:Al film properties as the reactively sputtered ZnO:Al films do not necessarily possess optimized film structure for the HCl etch. Solar cells with optimized ZnO:Al front contacts achieved conversion efficiency well above 11 %.