Optical breathing of nano-porous antireflective coatings through adsorption and desorption of water

Abstract

We report on the direct consequences of reversible water adsorption on the optical performance of silica-based nanoporous antireflective (AR) coatings as they are applied on glass in photovoltaic and solar thermal energy conversion systems. In situ UV-VIS transmission spectroscopy and path length measurements through high-resolution interferometric microscopy were conducted on model films during exposure to different levels of humidity and temperature. We show that water adsorption in the pores of the film results in a notable increase of the effective refractive index of the coating. As a consequence, the AR effect is strongly reduced. The temperature regime in which the major part of the water can be driven-out rapidly lies in the range of 55°C and 135°C. Such thermal desorption was found to increase the overall transmission of a coated glass by ~ 1%-point. As the activation energy of isothermal desorption, we find a value of about 18 kJ/mol. Within the experimental range of our data, the sorption and desorption process is fully reversible, resulting in optical breathing of the film. Nanoporous AR films with closed pore structure or high hydrophobicity may be of advantage for maintaining AR performance under air exposure.

Quantitative image analysis for evaluating the abrasion resistance of nanoporous silica films on glass

“…The calculated porosity of the potassium silicate-derived thin films is comparably low, i.e., ~0–8%, while that of the SiO 2 nanoparticle-derived films is much higher, i.e., ~20–30%, within the geometrical optimum of a system of close-packed, monodisperse balls (26%). Noteworthy, the absolute porosity value may be influenced by capillary condensation of water which occurs under ambient conditions 29 . Then, the value of n p is strictly not a convolution of silica and air, but of silica and water with n H2O ~ 1.33.…”

“…For the solution-derived films, aqueous potassium silicates (BASF, Düsseldorf, Germany) with a nominal silicate concentration of ~6 wt% were employed (for details, see Refs. 3 , 29 ). Here as well, coatings were deposited by dipping under ambient atmosphere, followed by subsequent washing in demineralized water so as to remove residual alkaline carbonates 3 .…”

“…The sensitivity of this approach is limited only by the quality of the image collection, enabling to resolve even small variations that occur between comparably similar coatings, or between small steps of mild abrasion. For this, we consider two kinds of SiO 2 sol-gel coatings as model systems: films based on colloidal SiO 2 with adjustable colloid size and comparably weak adhesion 28 , and more durable films which are derived from the deposition of aqueous potassium silicate solutions 3 29 with varying film density.…”

Robust operation of mesoporous antireflective coatings under variable ambient conditions

“…Previous studies determined the effect of adsorption and desorption of water by varying temperature. 49 To our knowledge, however, this is the first reported work on the effects of humidity cycling on ARC stability.…”

Large-area wet-chemical deposition of nanoporous tungstic silica coatings

“…Experiments were conducted at 30 1C on microscope slides coated on one side with a tungsten doped coating and a reference coating, both having undergone an acid wash and subsequent thermal treatment prior to measurement. This experimental set-up was previously developed 27 for the study of capillary condensation in similar but undoped nanoporous films.…”