The catalytic effect of iron(III) on the etching of ZnO:Al front contacts for thin-film silicon solar cells

Sputter-deposited ZnO:Al thin films, used as front contact in thin-film Si solar cells, were etched in diluted HCl containing catalytic amounts of Fe(III) salts. The Fe(III) effectively catalyzes the HCl-based etch process, leading to a crater-like morphology that is qualitatively similar to the one generated by the uncatalyzed etching process at the same HCl concentrations. Utilizing this catalyzed process, an increase of the etching rate by a factor of approximately 1.8–2.6 was observed. This allows for a well-controllable tuning of the etch duration without changing HCl concentration or temperature. The process has been evaluated with a selection of Fe(III) salts at different concentrations of the acid and of the catalyst. Optical, electrical and scanning force microscopic characterization of such catalytically etched ZnO:Al films has shown that the catalytical process leads to slightly smaller morphological features on the film compared to the uncatalyzed etching, accompanied by a shift of light scattering intensity to higher angles. This etching behavior may be used beneficially for light trapping in thin-film Si solar cells. In addition to these application-oriented aspects, this approach provides a deeper insight into the mechanistical details of the ZnO thin-film dissolution.