Ángel Morales

Preparation and characterization of sol-gel TiO2 antireflective coatings for silicon

Sol–gel polymeric TiO2 films have been obtained by dip coating. These films showed antireflective properties on silicon substrates as well as suitable adherent properties. The film thickness varied from 55 to 122 nm, and the refractive index presented values between 1.95 and 2.10. Both these parameters can be easily tailored by varying the withdrawal rate and the [TbuTi]/[Ethanol] ratio in the solution. The structural properties of the films showed that amorphous films are obtained by sintering at 450°C, meanwhile, the nanocrystalline titania anatase phase is formed after heating the films at 850°C for 5 s. By using these films, the hemispherical reflectance of polished silicon wafers is reduced in all the wavelength range and particularly, it is decreased from 37 to 1.5% at approximately 600 nm. At the same time, the solar averaged reflectance of the uncoated silicon is reduced from 0.38 to 0.12 after the sol–gel TiO2 coating.

Sol–gel TiO2 antireflective films for textured monocrystalline silicon solar cells

Abstract The aim of this study is validate the sol–gel method as a procedure to prepare antireflective films (AR) for textured monocrystalline solar cells. In this way, sol–gel TiO 2 films were deposited on both microscope slides and textured monocrystalline silicon solar cells by dip coating. The solutions were prepared by mixing tetrabutyl ortotitanate, water and ethanol using a basic compound as catalytic agent. The sintering of the samples was performed in the 400–800 °C range, by using a heating rate of 30°/min. It was observed that the sintering atmosphere influenced the morphological and antireflective properties of the films. The film thickness was tailored by varying the withdrawal rate. GAXRD analysis showed that nanocrystalline titania anatase and rutile were formed and the average crystallite size increased with the sintering temperature. The solar averaged reflectance of the textured monocrystalline solar cells was reduced from 0.09 to 0.02 after the sol–gel TiO 2 deposition.

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.

Effect of Additives on the Durability and Properties of Antireflective Films for Solar Glass Covers

Generally, single layer antireflective (AR) coatings are based on SiO 2 for which the low refractive index is achieved by increasing the porosity of the films. On the other hand, the increase of porosity on any material results in a decrease of its mechanical properties. In addition to increasing the solar transmittance, it is equally important that the AR film is long-term stable for its application in solar collectors. In this sense, the objective of our work has been to develop a film that exhibits good AR properties and physical durability as well. In the present study, sol-gel silica films have been deposited on borosilicate glass. The effect of adding some organic compounds to the precursor solution in the properties of the films has been studied. Moreover, accelerated weathering tests have been performed in order to investigate the outdoor durability. A value of solar transmittance as high as 0.974 has been obtained by using a porosity promoter additive. However, the durability tests have shown the necessity of incorporating a hydrophobic additive to the precursor solutions in order to avoid the degradation of the optical properties of the films. 1.5% decrease in solar transmittance is observed after 1900 h in the weathering chamber for films prepared with methyltriethoxysilane.

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.

SiO2/TiO2 Antireflective Coatings With Photocatalytic Properties Prepared by Sol–Gel for Solar Glass Covers

The glass covers of solar systems are usually coated with antireflective (AR) coatings on both sides that allow to increase the efficiency of the whole system. At the same time, the accumulation of dust and dirt particles on the surface of the AR coated glass decreases the transmittance of the covers, even to values lower than the uncoated glass. This decrease in the cover transmittance reduces the solar radiation that reaches the absorber and, in this way, a decrease in the solar plant efficiency would be obtained. So, the use of films that combine high transmittance and self-cleaning capacity seems to be a promising development. This paper reports the preparation of sol–gel porous TiO2 and SiO2 bilayers on borosilicate glass. The porosity and thickness of both layers have been optimized in order to obtain the optimal photocatalytic and optical properties. Solar transmittance values of 0.964 were obtained for the TiO2/SiO2 coated glass. The highest value of transmittance reached is 0.993 and it is placed at 600 nm. The self-cleaning properties were evaluated using methylene blue and trichloroethylene (TCE) as model organic molecules using UV-A light. The results showed that the SiO2/TiO2 samples degraded the colorant faster than the SiO2 sample. Moreover, the degradation of TCE in air in a continuous fix bed photoreactor for samples with different TiO2 film thicknesses was also performed. The TCE conversion was found dependent on the TiO2 thickness. Nevertheless, the increase in the thickness of this layer reduced transmittance properties. The study demonstrates that it is possible to combine antireflective and self-cleaning properties in the same material. [DOI: 10.1115/1.4007298]

Towards standardized testing methodologies for optical properties of components in concentrating solar thermal power plants

Precise knowledge of the optical properties of the components used in the solar field of concentrating solar thermal power plants is primordial to ensure their optimum power production. Those properties are measured and evaluated by different techniques and equipment, in laboratory conditions and/or in the field. Standards for such measurements and international consensus for the appropriate techniques are in preparation. The reference materials used as a standard for the calibration of the equipment are under discussion. This paper summarizes current testing methodologies and guidelines for the characterization of optical properties of solar mirrors and absorbers.

4 – A new generation of absorber tubes for concentrating solar thermal (CST) systems

Heat collector elements or solar receiver tubes are a key element in the development and performance of parabolic trough collector technology. They are responsible to collect solar energy and to transfer the heat collected to the heat thermal fluid. Main challenges to be met in the future are to increase durability and to reduce cost and maintenance. In order to increase durability, vacuum maintenance is the major challenge because thermal losses and selective absorber stability are directly dependent on vacuum level. Selective absorber thermal stability, both in vacuum and air, has to be improved to raise operation temperature in solar thermal plants and in this way, to increase efficiency and to minimize damage if vacuum is lost.