TEM and XPS Investigations of Multi-Layer Solar Selective Absorber: Thermal Stability

The solar energy resource is still relatively under-exploited although it could cover most of our energy needs. Beyond the development of photovoltaics, there is a worldwide challenge to deploy large-scale concentrated solar thermal power plants. To address this issue, materials with high absorption in the visible range, low emissivity and good temperature resistance are required. To this end, multi-layered optical thin films produced by plasma processes are developed in the NANOPLAST project (nanoplast-project.cnrs.fr, ANR-19-CE08-0019).<br/><br/>This study is focussed on the investigation of the absorbing active layer with or without air annealing at 500°C.<br/>First, composite monolayers W-SiC:H are obtained by reactive magnetron sputtering from a W target combined with tetramethylsilane (TMS-Si(CH₃)₄) diluted in Ar plasma.<br/>TEM and XPS characterisations were carried out to get insight into the nature of the absorber and the anti-reflective top films. Depending on the TMS fraction in the discharge, the quantity of Si introduced in the W-SiC:H material can be tuned.<br/><br/>In dedicated conditions (5 and 8% TMS), W nano-crystallites could be identified by coupling STEM-HAADF (Scanning Transmission Electron Microscopy - High-Angle Annular Dark Field) images, EDS mapping and High Resolution imaging. In other conditions (20 and 28% TMS), amorphous layers are observed. In XPS, we were able to analyse the different surface chemical environments of the samples. Under low TMS content, we observed a significant occurrence of W-C bonding whereas no Si is incorporated in the layer. By decomposing the obtained survey spectra and identifying the related components, we were able to achieve quantification of the species as a function of the TMS concentration.<br/><br/>Second, multi-layers of SiC:H/metallic W/SiC:H were also prepared. Upon annealing at 500°C in air for duration up to 48h, XPS profiles indicate that excepting a diffusion of oxygen from the surface in the upper SiC:H layer, the W interlayer acts as a blocking layer for oxidation. However after annealing for 96h, the triple layer is severely modified with segregation of the different species involved.<br/>TEM images confirmed these major modifications.