Mechanical properties and field performance of hydrophobic antireflective sol-gel coatings on the cover glass of photovoltaic modules

The front cover glass required for photovoltaic (PV) module insulation is the first surface in receiving irradiation towards solar cell, and the first surface in limiting the photon flux impinging it due to optical losses, which can be counteracted by means of antireflective (AR) coatings. The soiling adherence inherently disrupts the intended function of the AR coatings, thus reducing the power output of the PV plants. In this work, hydrophobicity of highly antireflective layer stacks was pursued to deal against soiling adherence. Whereas antireflection property has a direct effect on the minimization of cell-to-module losses, antisoiling property reduces maintenance expenditures, both contributing to the reduction of levelized cost of energy. Properties of methyl-silylated silica and polyfluoroalkyl-silica mono- and bi-layer stacks were compared to achieve the most rational AR design based on a proper trade-off between cost-efficiency, processability, optical properties, mechanical properties and reliability during real life operation. Nanoindentation permitted to compare hardness of different coatings; cohesion and adhesion were studied by nanoscratch, both related to cleaning resistance studied by reciprocating wear test. Effect of AR layer stacks on electrical response of monocrystalline silicon cells was assessed during outdoor exposure as well as the effect of environmental relative humidity on the optical properties. Methyl-silylated silica mono-layer and polyfluoroalkyl-silica bi-layer both prepared in two-steps were the more rational designs based on adhesion, mechanical properties and abrasion resistance to cleaning process. They showed an equivalent overgeneration of 2% when applied on cover glass of PV mono-modules compared to uncoated ones along one-year exposure.