Novel etching method on high rate ZnO:Al thin films reactively sputtered from dual tube metallic targets for silicon-based solar cells

Highly transparent and conductive aluminum-doped zinc oxide thin films (ZnO:Al) were reactively sputtered from metallic targets at high rate of up to 90 nm m/min. For the application as transparent light scattering front contact in silicon thin film solar cells, a texture etching process is applied. Typically, it is difficult to achieve appropriate etch features in hydrochloric acid as the deposition process must be tuned and the interrelation is not well understood. We thus introduce a novel two-step etching method based on hydrofluoric acid. By tuning the etch parameters we varied the surface morphology and achieved a regular distribution of large craters with the feature size of 1-2 μm in diameter and about 250 nm in depth. Microcrystalline silicon single junction solar cells (μc-Si:H) and amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with high efficiency of up to 8.2% and 11.4%, respectively, were achieved with optimized ZnO:Al films as light scattering transparent front contact.