References

1.
J. Hüpkes, J. Müller, B. Rech
Texture Etched ZnO:Al for Silicon Thin Film Solar Cells. In: Ellmer KK, A.;  Rech, B. (ed) Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells
2.
S.O. Klemm, S. Pust, J. Hüpkes, A.W. Hassel, K.J.J. Mayrhofer

Electrochemical Texturing of Al-doped ZnO

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Electrochemical texturing of Al-doped ZnO

Surface morphology as a function of the solution pH Zoom Image
Surface morphology as a function of the solution pH

Transparent conducting oxides (TCO) play an important technical role, for instance as an electrical contact in thin film solar cells. ZnO was found to be a promising material for photovoltaics due to its optical end electrical characteristics, with the material optimization process presently ongoing. In case of thin film solar cells, a simple transmission of light is not desired, but rather a scattering which leads to a prolongation of the light path through the absorber layer (e.g. silicon) [1]. This tuning of the optical properties by surface texturing is usually achieved by a chemical etching step. The image beside shows the surface morphology as a function of the solution pH in acetate buffer (0.1 M) for an etch time of 500 s. As evident from the scanning electron images, a significant roughening of the surface occurs, with the etch pits most probably originating from the grain boundaries.

As shown by downstream analysis of zinc ions using a SFC setup [2] (see figure below), a linear relation between etch rate and proton concentration was observed. The reaction mechanism was furthermore shown to be diffusion limited by proton transport, both sensible to the solution pH and buffer concentration.

Downstream analysis of zinc ions Zoom Image
Downstream analysis of zinc ions

A fundamentally different dissolution mechanism was observed by using galvanostatic techniques to trigger anodic dissolution. As evident from the SEM images, a selective removal of the grain boundaries was observed reaching deep into the substrate. The original surface level remains unaffected as indicated by surface profiling with a stylus being larger than the etch pit dimensions. This novel etching procedure now allows tuning of the surface texture by both chemical and electrochemical procedures subsequently or in parallel. For a detailed investigation of the dissolution mechanism see reference 2.

Change of grainboundaries under different current densities in the SEM Zoom Image
Change of grainboundaries under different current densities in the SEM
 
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