Aluminium and tantalum nitride barriers against copper diffusion in solar absorbers

The thermal stability of magnetron sputtered solar absorber coatings was investigated at temperatures of 200–500°C. Diffusion barriers of aluminium (Al) and tantalum nitride (TaN x ) were studied against thermal diffusion of copper substrate atoms. The diffusion barriers studied were experimental DC magnetron sputtered Al and TaN x layers between a copper substrate and an absorber coating. The absorbers were aged by means of heat treatment in air. The influence of diffusion barriers was analysed by optical and microstructural analyses before and after the ageing procedures. Without a barrier coating, copper substrate atoms diffused through the absorber to the surface, which degraded optical selectivity. The diffusion can be prevented or retarded with a dense diffusion barrier layer. The 500-nm-thick Al barrier layer prevented Cu diffusion and retained optical selectivity at 300°C. At higher temperatures, diffusion was observed but the Al barrier retarded the degradation when compared to an absorber without the barrier layer.