Mechanical and microstructural properties of broadband anti-reflective TiO₂/SiO₂ coatings for photovoltaic applications fabricated by magnetron sputtering

Anti-reflective coatings are used in photovoltaic systems to minimize reflectance thus optimizing efficiency. Besides excellent optical properties, these coatings need to provide good mechanical properties due to hostile environmental conditions. Therefore, this study aims at designing anti-reflective coatings based upon multi-layers of TiO₂ and SiO₂ by magnetron sputtering on glass and silicon substrates at room temperature. Afterwards, these coatings were thermally annealed at 400, 500 and 600 °C to study the influence of such treatment on their optical and mechanical properties. Samples prepared at room temperature and annealed at 400 °C showed an optimized reflectance of about 2%. Advanced materials characterization techniques were used to elucidate the microstructural evolution of the multi-layers. The multi-layers annealed at 400 °C demonstrated a dense and smoothly packed microstructure with homogenous grains (50 nm in size), with a phase transformation from amorphous to anatase. In addition, exposure and damage of the glass substrate were detected for elevated temperatures (500 and 600 °C), thus increasing the reflectance. Nanoindentation revealed an improved hardness, elasticity, and resistance against plastic deformation for the sample annealed at 400 °C. Consequently, anti-reflective coatings with post-annealing treatment at 400 °C, unify two complementary aspects such as improved mechanical properties (extended durability) and enhanced collection abilities over a broader wavelength range (increased efficiency).