Textured transparent conductive oxide thin films with uniform properties on large scale

The transparent conductive oxide (TCO) thin films are wide band-gap degenerate semiconductor materials with low resistivity and high transmittance over a large spectral range. The interest about such kind of materials has been increased in the last years due to the extreme commercial interest for employing them in most devices like solar cells, flat panel display, optical waveguides, light emitting diodes, photodiodes, gas sensors, surface acoustic devices, piezoelectric transducers and varistors. In all application fields TCO electrodes have to exhibit both high transparency and high electrical conductivity. Furthermore, in solar cells related field they can fulfill additional functions, i.e. light scattering for improved cell efficiency [1, 2]. The light scattering ability of a TCO layer in a thin film solar cell is determined by the feature sizes and shapes of its surface. Nowadays indium-tin-oxide (ITO) is the leading material for TCO film applications and the Al-doped ZnO (ZnO:Al) is the most interesting alternative candidate [3]. Radiofrequency RF magnetron sputtering is considered one of the best deposition techniques to prepare TCO films. In fact, ITO and ZnO:Al thin films with very low resistivity can be produced by RF magnetron sputtering, but the problem of a spatial distribution of thickness and resistivity related to the inhomogeneous erosion pattern of the target [3, 4] tends to remain. Since in all the above mentioned applications the uniformity of thickness and resistivity is a very important factor, further studies especially addressed at the up-scaling of the research results to industrially relevant substrate sizes are needed. In this work we demonstrate: a) the possibility to prepare high quality, large area ITO and ZnO:Al thin films with good spatial resistivity and thickness distribution via RF magnetron sputtering, b) the effectiveness of a chemical post etching to produce tunable surface nanotextures, since a simple sputtering process permit to obtain only smooth films. These textures permit a lateral diffusion into the thin film absorbers of incoming solar light and the traveling length becomes some order of magnitude longer than that of the directly transmitted light. Since the resistivity is only slightly increased by the chemical etching, this post-deposition step allows separating the optimization of electrooptical properties from light scattering behavior.