High performance single layer nano-porous antireflection coatings on glass by sol–gel process for solar energy applications D.B. Mahadik, R.V. Lakshmi, Harish C. Barshilia n Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, HAL Airport Road, Kodihalli, Bangalore 560017, India article info Article history: Received 12 November 2014 Received in revised form 25 February 2015 Accepted 19 March 2015 Keywords: Anti-reflection coating Glass Sol–gel Dip coating Nano-porous abstract Antireflection coatings have received a great importance due to their ability to enhance the efficiency of the solar cells and solar selective coatings by minimizing the reflections of the incident light from the front surface. In this study, a silica (SiO 2 ) sol, prepared using sol–gel process, was deposited on cleaned glass substrates by dip coating method and subjected to heat treatment at 400 °C. The thickness and porosity of the coating were optimized to achieve high transmittance. The thickness was optimized by varying the lifting speed of the substrate from the sol. The porosity was induced in the coating by using polymeric additives and through heat treatments. The optimized single layer SiO 2 coating on cleaned glass substrate exhibited a maximum transmittance of 97.5% at λ ¼500 nm wavelength. The hybrid sol was found to give reproducible coatings up to a period of 30 days when stored at 16 °C. The present process provides a simple and cost effective method for the preparation of antireflection coatings, which have huge potential to enhance the efficiency of solar cells, receiver tubes and other solar devices. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Antireflection (AR) coatings have been widely used to increase the solar radiation absorption in solar collectors to reduce front surface reflection of selective absorbers [1] as well as collector glass covers of solar selective coatings [2]. Silicon solar cells, with the band gap of 1.1 eV, convert light into electrical energy with a spectral response range of 380–900 nm. At present, the visible light trans- mittance of low iron glass, commonly used for the photovoltaic cells, is only  92% [3,4]. The market prospects of antireflection coatings that can decrease light reflections from the surface of solar devices (both photovoltaic and photothermal) to increase efficiency of solar power generation are promising. Antireflection coatings reduce the reflection considerably by improving the quality of optical lens systems [5]. Many applications require an increase in the transmit- tance, or a reduction in the front surface reflection of transparent materials. In solar thermal systems, AR coatings have been used in parabolic trough collectors, either in selective absorbers or glass envelopes that are positioned around the receiver tube. The refractive index mismatch between a material and the air produces reflectance losses that can be reduced by applying an antireflection coating with an intermediate refractive index between air and the substrate material. An ideal homogeneous single-layer AR coating having an optical thickness of one-quarter of a wave- length will have its refractive index n c ¼ (n a n s ) 1/2 (where n a and n s are the refractive indices of the air and the substrate, respectively) [6,7]. The refractive index of soda lime glass is roughly 1.54, which implies an ideal refractive index of 1.24 for an AR coating. The antireflection coatings can either be materials with an intermediate refractive index between the substrate and air [8,9] or porous coatings [10] or refractive index-graded materials [6,11,12]. Most widely used materials for AR coatings are dielectric materials such as SiO 2 , alumina and titania with refractive indices of 1.45, 1.65 and 2.30 [1,8]. SiO 2 is the most ideal material due to its low refractive index, good durability and environmental resistance. Also, since glass is used for most of the applications, SiO 2 based AR coatings have strong adherence and hence is an added advantage. AR coatings are developed by various approaches such as multilayers, porous nanostructures, coatings with nanoparticles, etc. The preparation of AR coatings can be achieved either by dry deposition techniques (CVD, PVD, etc.) or by wet techniques (spray coating, brush casting, spin coating, dip coating, etc.) and through sol–gel process. Wet deposition techniques are advantageous for the preparation of multi-phase materials since any non-volatile compound that is dispersed or dissolved into the solution will be homogeneously distributed in the coatings deposited. Dip coating is an ideal method to prepare thin layers from chemical solutions since it offers a good control on the coating thickness. The most important advantage of sol–gel process is the ability to tailor the microstructure of the deposited film, thereby inducing porosity Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2015.03.023 0927-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 80 2508 6494; fax þ91 80 2521 0113. E-mail address: harish@nal.res.in (H.C. Barshilia). Solar Energy Materials & Solar Cells 140 (2015) 61–68