Optical and structural analysis of solar selective absorbing coatings based on AlSiOx:W cermets

It is reported in this work the development and study of the optical and structural properties of a solar selective absorber cermet based on AlSiOx:W. A four-layer composite film structure, W/AlSiOx:W(HA)/AlSiOx:W(LA)/AlSiOx, was deposited on stainless steel substrates using the magnetron sputtering deposition method. Numerical calculations were performed to simulate the spectral properties of multilayer stacks with varying metal volume fraction cermets and film thickness. The chemical analysis was performed using X-ray photoelectron spectroscopy and the results show that in the high metal volume fraction cermet layer, AlSiOx:W(HA), about one third of W atoms are in the W-O oxidation state, another third in the Wx+ oxidation state and the last third in the W4+, W5+ and W6+ oxidation states. The X-ray diffractograms of AlSiOx:W layers show a broad peak indicating that both, W and AlSiOx, are amorphous. These results indicate that this film structure has a good spectral selective property that is suitable for solar thermal applications, with the coatings exhibiting a solar absorptance of 94-95.5% and emissivities of 8-9% (at 100 degrees C) and 10-14% (at 400 degrees C). The samples were subjected to a thermal annealing at 450 degrees C, in air, and 580 degrees C, in vacuum and showed very good oxidation resistance and thermal stability. Morphological characterizations were carried out using scanning electron microscopy and atomic force microscopy. Rutherford Backscattering experiments were also performed to analyze the tungsten depth profile.The authors acknowledge the support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013. The authors are also grateful to the financial support of FCT, POCI and PORL operational programs through the project POCI-01-0145-FEDER-016907 (PTDC/CTM-ENE/2882/2014), co-financed by European community fund FEDER. The authors also acknowledge GIST Japan for using the XPS-Kratos.info:eu-repo/semantics/publishedVersio

Rf sputtered aluminium doped zinc oxide films enhanced damp heat stability by means of plasma etching treatment

ZnO Al films, obtained by RF sputtering and textured by wet etching, have been subjected to argon plasma etching treatment in order to stabilize their electrical properties over time, being well known the electrical degradation of these transparent conductive films when exposed to air. Specifically focused on photovoltaic application as front electrode, the effect of argon plasma treatment on long term electrical stability of ZnO Al thin film was monitored by performing damp heat degradation cycles in environmental test chamber. There was evidence for strong slowdown of electrical degradation when ZnO surface had been subjected to Ar plasma treatment. Different etching times have been used and it has been observed that surface modification induced by plasma treatment gave its more pronounced beneficial effect after the first 10 min of the process. In particular, for no plasma treated samples the carrier mobility worsening is about three times larger than that related to Ar treated samples after long lasting aging test. The stabilization mechanism of the electrical properties was investigated by studying the main modifications of the plasma etched materials in terms of optical, chemical, morphological and structural properties. Structural analysis revealed that crystalline rearrangement occurs down to 30 nm depth below the plasma treated film surface, where a thin nanocrystalline layer forms. PL analysis revealed a different distribution of deep defect centres for plasma treated film surface compared to untreated one. In particular, donor like lattice defects Vo and Zni decreased whereas Vzn defects became predominant. Crystalline rearrangement and change of type and or amount of deep defect centres inside the plasma modified top layer have been hypothesized as responsible for the enhancement of the electrical stabilit