Electrochemical Texturing and Deposition of Transparent Conductive Oxide Layers for the Application in Silicon Thin-Film Solar Cells

Doped zinc oxide layers are widely used in thin-film solar cells for several purposes, for instance as transparent contacts, as a source of light scattering and as part of the back reflector. Magnetron sputtered, aluminum-doped zinc oxide thin films provide very high transparency and conductivity, and are usually flat in the as-deposited state. To introduce light scattering, a surface texture is conventionally introduced by post-deposition etching in diluted hydrochloric acid. However, the ability to obtain suitable surface morphologies by chemical dissolution is strongly dependent on the deposition process. Thus, optimization of zinc oxide thin films requires a careful trade-off between optical, electric, and morphological properties. This markedly limits the process window and excludes layers with excellent optical and electric properties due to a lack of suitable texturing processes. Electrochemical methods can help to overcome these limitations by making novel zinc oxide structures accessible. Both deposition and dissolution can be achieved using electrochemical methods. In this context, an advanced understanding of the stability of polycrystalline zinc oxide thin films in aqueous solutions is crucial. This work investigates the zinc oxide/electrolyte interface under various conditions in order to further the understanding of the interfacial reactions and the zinc oxide itself. Cathodic electrochemical deposition was used for the growth of zinc oxide films and nano-structures from aqueous solutions. This method utilizes specific manipulation of the interfacial pH at the substrate surface by reduction of a suitable precursor such as nitrate or molecular oxygen. The dependence of the zinc oxide precipitation and crystallization on several parameters such as the deposition potential, the bath temperature, the substrate, and the composition of the electrolyte were investigated. Temperatures above 50 $^{\circ}$C were found to be necessary for the crystallization of well defined hexagonal crystals. The comparison of electrochemical deposition on indium tin oxide and zinc oxide substrates revealed the fundamental influence of the substrate on the nucleation. While the growth on zinc oxide seed layers seemed to proceed epitaxially, conserving the preferential c-axis orientation and crystallite size of the substrate, the nucleation on indium tin oxide substrates depended largely on the applied potential. With increasing cathodic potential the density of nucleation sites increased. The crystallite size decreased simultaneously