Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.