Spectrally selective Al/AlN/Al/AlN tandem solar absorber by inline reactive ac magnetron sputtering

Spectrally selective Al/AlN/Al/AlN tandem solar absorbers were deposited onto soda lime glass substrates using inline ac magnetron sputtering in a reactive atmosphere containing argon and nitrogen. To achieve a reproducible and homogenous deposition process, the deposition of multilayer Al/AlN/Al/AlN films under different process conditions was investigated. Two main variables, ac power and the speed of substrate movement in the chamber, were varied in the ranges of 1-10 kW and 10.47-31.4 mm s^-1 respectively to obtain films with high absorption and low emittance. The effects of film thickness of different layers and deposition conditions on the optical performance of selective coatings were also studied. Structural features and surface morphology of the films were investigated by X-ray diffraction and field emission scanning electron microscopy analyses respectively. In the present study, tandem solar absorber films deposited onto glass substrate with optimised sputtering parameters can consistently achieve solar absorptance α of 0.864 and thermal emittance ε of 0.03 at 80°C.