New method to optimize a solar absorber graded film profile

Normalized dc conductivity was used to analyze the inhomogeneously graded depth profile structure of deposits from dc magnetron reactive sputtering in a roll coater. The graded coating was obtained by moving the substrate through the deposition zone with a non-uniform oxidation along the zone mainly due to the asymmetrical position of the oxygen inlet to the right of the target. With this arrangement, the composition of the deposited film was gradually changed from metal to metal oxide as the substrate moved from left to right through the zone. The profile control therefore relied on the non-uniform oxidation along the sputtering zone. The study shows that the normalized dc conductivity of stationary samples in this roll coater offers a simple and effective method to optimize the graded composition in spectrally selective solar absorber coatings. A solar absorptance of 0.91 with thermal emittance of 0.05 at 100 °C was achieved for a single graded film without antireflection treatment.