Accelerated aging tests on ENEA-ASE solar coating for receiver tube suitable to operate up to 550 °c

A patented solar coating for evacuated receiver, based on innovative graded WN-AlN cermet layer, has been optically designed and optimized to operate at high temperature with high performance and high thermal stability. This solar coating, being designed to operate in solar field with molten salt as heat transfer fluid, has to be thermally stable up to the maximum temperature of 550 °C. With the aim of determining degradation behaviour and lifetime prediction of the solar coating, we chose to monitor the variation of the solar absorptance αs after each thermal annealing cycle carried out at accelerated temperatures under vacuum. This prediction method was coupled with a preliminary Differential Thermal Analysis (DTA) in order to give evidence for any chemical-physical coating modification in the temperature range of interest before performing accelerated aging tests. In the accelerated aging tests we assumed that the temperature dependence of the degradation processes could be described by Arrhenius behaviour and we hypothesized that a linear correlation occurs between optical parameter variation rate (specifically, Δαs/Δt) and degradation process rate. Starting from Δαs/Δt values evaluated at 650 and 690 °C, Arrhenius plot gave an activation energy of 325 kJ mol-1 for the degradation phenomenon, where the prediction on the coating degradation gave a solar absorptance decrease of only 1.65 % after 25 years at 550 °C. This very low αs decrease gave evidence for an excellent stability of our solar coating, also when employed at the maximum temperature (550 °C) of a solar field operating with molten salt as heat transfer fluid. © 2016 Author(s)

Determining the optical properties of Transparent and Conductive Oxides for thin film solar cells

This paper presents a method for determining the complex refractive index of a Transparent and Conductive Oxide that is embedded in a multilayer structure. The procedure assumes that the optical properties of others materials forming the structure are known and only the real and the imaginary part of the refractive index are unknowns. The refractive index is determined by exploiting the matrix model of light propagation along with experimental measurements of the light transmitted through the complete multilayer device. A simple multilayer structure, formed by a ZnO thin film deposited on a thick glass substrate, is analyzed to evidence the reliability of the method. Results show that the by adopting for the ZnO the refractive index extracted with the proposed procedure, a reliable description of light propagation through multiple layers can be obtained. © 2017 IEEE

Sol-gel synthesis of ZnO transparent conductive films: The role of pH

The sol-gel synthesis of undoped and B- or Al-doped ZnO thin films were critically examined with particular reference to the influence of the pH of the reaction medium on some of their specific characteristics, such as thickness, morphology, doping level and optical properties, in view of their application in the photovoltaic field. Using triethanolamine (TEA) as chelating agent, a range of basic pH from 7.66 to 8.76 was explored starting from a very concentrated zinc acetate dehydrate (ZAD) solution in ethanol, [Zn2+] = 1.0 M, and keeping the ZAD/TEA = 1. A more basic environment gives more porous films whose thickness and crystallinity are higher than those achieved at lower pH. It was found that the morphology, as well as the sheet resistance (Rs) of films, depends on both pH and doping. Increasing the pH the Rs decreases for both undoped and doped films. At a certain pH undoped films exhibit a granular microstructure and lower Rs than B- or Al-doped films which exhibit a finer texture, characterized by a lower porosity. Optical properties strongly depend on the pH as well. Increasing the pH, a noticeable blue shift effect was observed, that was attributed mainly to structural changes and to a lesser extent to the Burnstein-Moss effect. © 2014 Elsevier B.V

Effect of the RF sputtering power on microstructural, optical and electrical properties of Al doped ZnO thin films

ZnO:Al (AZO) thin films have been deposited by radio frequency (RF) magnetron sputtering and RF power applied to the target has been varied in the range 600-1200 W. RF power effect on structural, electrical and optical properties was investigated and the relationship existing between these properties and the film lattice defect distribution was discussed. At the increasing of the RF power it was found that AZO films, having a preferential growth orientation along (002) direction, showed a decrease of the lattice distance indicating a less defected structure. Furthermore, at the increase of the RF power a higher optical absorption by free carriers, coupled with an increase of the band gap value, was observed. Resistivity varied from 1.1 × 10- 3 cm at 600 W down to a minimum value of 5.6 × 10- 4 cm at 1200 W, whereas the carrier density increased up to 1 × 1021 cm- 3. Lattice defect variation of AZO films was analyzed by photoluminescence (PL) measurements. Presence and amount of different lattice defects were evaluated for AZO films deposited at different RF powers. At 600 W the film structure was dominated by zinc vacancies (VZn), whereas for higher RF power the PL band associated with VZn decreased and interstitial oxygen (Oi) band remarkably increased. PL analysis revealed that extrinsic Al doping is the dominant effect on the conductivity enhancement. It was hypothesized that at higher RF power a more effective diffusion phenomenon can give more effective Al doping and less amount of zinc vacancies. As a consequence, Al atoms are more effectively trapped into the structure. © 2015 Elsevier B.V

Advanced light-scattering materials: Double-textured ZnO:B films grown by LP-MOCVD

Double-textured ZnO:B layers with enhanced optical scattering in both short and long wavelength regions have been successfully fabricated using MOCVD technique through a three step process. Growth of double-textured structures has been induced by wet etching on polycrystalline ZnO surface.Our double-layer structure consists of a first ZnO:B layer wet etched and subsequently used as substrate for a second ZnO:B layer deposition. Polycrystalline ZnO:B layers were etched by utilizing diluted solutions of fluoridic acid (HF), chloridric acid (HCl) and phosphoric acid (H3PO4) and their effect on surface morphology modification was systematically investigated. The morphology of the second deposited ZnO layer strongly depended on the surface properties of the etched ZnO first layer. Growth of cauliflower-like texture was induced by protrusions presence on the HCl etched surface. Optimized double-layer structure shows a cauliflower-like double texture with higher RMS roughness and increased spectral haze values in both short and long wavelength regions, compared to conventional pyramidal-like single texture. Furthermore, this highly scattering structure preserves excellent optical and electrical properties. © 2013 Elsevier B.V. All rights reserved

Plasma etched c-Si wafer with proper pyramid-like nanostructures for photovoltaic applications

Dry and maskless texturing process of mono-crystalline silicon wafers using CF4/O2 plasma in a reactive ion etching (RIE) system has been developed with the aim of obtaining optimized surface texture characterized by low optical reflection loss for photovoltaic applications. In this study c-Si has been successfully subjected to plasma etching at unusual process conditions, specifically high value of substrate temperature (15 °C) coupled with high value of process pressure (40 Pa). Different c-Si surface textures are produced at different etch durations. Specifically, pyramid-like morphology appears at very short etch time and crater-like nanostructures form at longer etch time, whereas the former gives the lowest average reflectance (<5%). Furthermore, we clearly observe a slowing down of the etching rate starting from 5 min and in correspondence with the appearance of the sponge-like structure coupled with pores formation. Geometric parameters of morphological features, such as RMS and average inclination angle of the surface features, have been correlated with some specific texture functions such as angular distribution of scattered light Ps(θ) obtained by Angular Resolved Scattering (ARS) technique. Shape and slope of the texture features strongly determine the amount of forward scattered light and, as a consequence, the amount of reflectance decrease. Surface texture obtained after only 3 min of etch and characterized both by well distributed small normal pyramids and by high average inclination angle, proves to be more effective in giving reflection reduction respect to other surface textures characterized by inhomogeneously distributed large cavities. Finally, we can well suppose that this small pyramid-like texture on Si-substrates can be used for the preparation of highly efficient heterojunction-based solar cells (Si-HJT). © 2018 Elsevier B.V