Chemical etching is widely applied to texture the surface of sputter-deposited zinc oxide for light scattering in thin-film silicon solar cells. Based on experimental findings from the literature and our own results we propose a model that explains the etching behavior of ZnO depending on the structural material properties and etching agent. All grain boundaries are prone to be etched to a certain threshold, that is defined by the deposition conditions and etching solution. Additionally, several approaches to modify the etching behavior through special preparation and etching steps are provided.

Magnetron sputtered aluminum-doped zinc oxide (ZnO:Al) is used as a window layer in silicon-based thin-film solar cells due to its high transparency, high conductivity, and ability to provide effective light trapping after etching in a dilute HCl solution. A challenge with this method is the strong influence of sputtering conditions on the density and shape of the ZnO:Al etch features. Here we present a novel chemical etch process based on HF that enables the modification of surface features through the etch process itself, thus allowing the separate optimization of ZnO:Al deposition and texturization. The different etch characteristics of HCl and HF are studied on single crystal ZnO. Using the different etch characteristics of HF we effectively texturize polycrystalline ZnO:Al films which previously exhibited only poor light trapping in silicon thin-film solar cells. The light trapping improvement is seen by an 18% increase in short circuit current density when used in microcrystalline silicon solar cells. Additionally, using mixtures of HCl and HF we are able to tune the feature size and shape on a given ZnO:Al sample.Scanning electron microscope images of high-rate deposited ZnO:Al etched in HCl and HF.

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