By Jorj I. Owen, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Jürgen Hüpkes, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Hongbing Zhu, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH; Eerke Bunte, IEK5-Photovoltaik, Forschungszentrum Jülich GmbH
Magnetron-sputtered ZnO:Al is often used as a front contact in thin film silicon solar cells due to its transparent conductive oxide (TCO) properties that allow texturization by chemical etch processes to introduce light trapping. The transparency, conductivity, and surface texture after etching depend strongly on the sputtering conditions. Consequently, the typical preparation method is to find the right balance in TCO properties and light scattering, leading to a very narrow sputtering parameter window. It is preferable to separate the electro optical optimization from that of texturization to allow for a larger process window and improve ZnO:Al film properties further. This paper presents some methods of controlling the surface features using various mixtures of two step etching processes in aqueous solutions of HF and HCl. Results include methods for controlling the density of craters, texturizing compact ZnO:Al films, and fabricating novel modulated surfaces with more than one characteristic feature size. The two step etch process enables the creation of good surface textures even on high rate material that, via state of the art HCl etching, tend to lead to poor solar cell performance.