J. Herrero

Leveling effect of sol–gel SiO2 coatings onto metallic foil substrates

Abstract The leveling capability of sol–gel SiO 2 layers onto titanium and Kovar (Fe54 Ni29 Co17) foil substrates has been analyzed by atomic force microscopy (AFM) and profilometer roughness measurements as a function of the sol–gel preparation parameters. SiO 2 coatings have been prepared by immersion of metallic foils in a solution where the [alkoxide]/[EtOH] ratio was between 0.05 to 0.7, and subsequent withdrawal of the samples at a constant rate between 8 and 49 cm/min. By increasing the [alkoxide]/[EtOH] ratio and/or the withdrawal velocity, the SiO 2 layer thickness and leveling capability increase but its mechanical integrity decreases. By increasing SiO 2 film thickness, better coverage of large-scale heterogeneities but poorer coverage of short-scale features have been observed. A compact (cracks and striations free) coverage which minimizes the roughness sample surface at short and large scales has been obtained by applying successive SiO 2 layers with various thicknesses.

Arrangement of flexible foil substrates for CuInSe2-based solar cells

Abstract Metallic foils of aluminium, titanium, stainless steel and Ni-alloys (Kovar™) have been arranged as flexible substrates for thin film solar cells. For this purpose, they have been coated with SiO 2 by a sol–gel process. Following this, Mo has been deposited by DC-sputtering onto the SiO 2 . The morphology, structure and optical properties of these samples have been analysed as grown and after annealing in a selenium atmosphere at temperatures ranging from 400 to 600°C. No selenium is incorporated in the samples but oxygen appears at all annealing temperatures as crystalline MoO 2 , decreasing the total optical reflectance. Only on Ti and Kovar™ foils does the molybdenum remain crack-free after annealing.

Performance of sol-gel SiO2 coatings onto glass/SnO2 superstrates

Abstract From commercial glass/SnO 2 superstrates, by dipping in an appropriate sol–gel solution, new SiO 2 /glass/SnO 2 /SiO 2 superstrates have been obtained, with improved transmittance controlled by SiO 2 deposition parameters such as the alcohol/alkoxide ratio in the sol–gel solution and the sample withdrawal velocity. In this way, close to 15% visible transmittance enhancement has been achieved, without infrared optical losses. The electrical characteristics of the sol–gel-coated samples have been evaluated by applying electro-deposited coatings on them. Electrodeposition of a CuInSe 2 absorber layer onto SiO 2 -coated conducting superstrates has been found to be heterogeneous at low cathodic potentials, but can be made homogeneous by increasing the work electrode polarisation, in order to achieve the disruptive voltage for the SiO 2 electrical insulation.

SiO2 sol–gel-coated conducting substrates for CuInSe2 electrodeposition

Abstract Glass–SnO 2 samples, which are typical superstrates for thin-film solar cells, have been dipped in an appropriate sol–gel solution, and thus improved SiO 2 –glass–SnO 2 –SiO 2 superstrates have been obtained with a higher transmission and a smoother surface, owing to the antireflective and levelling properties of these SiO 2 films. The electrodeposition of the photovoltaic absorber CuInSe 2 has been found homogeneous on SnO 2 and heterogeneous on SiO 2 -coated SnO 2 . CuInSe 2 grows only onto specific SiO 2 points, and the residual substrate area remains CuInSe 2 free. Optical characterization of the samples has been performed in the visible and near-infrared spectral range. Morphological analysis have been carried out using atomic force microscopy.