Functional nano-textured titania-coatings with self-cleaning and antireflective properties for photovoltaic surfaces

Highlights

• Relation between surface nano-texture and functional properties of sol–gel multilayer TiO₂ coatings are presented.

• TiO₂ thin films show good self-cleaning properties and transmittance higher than 95%.

• Good crystalline films at 400 °C, also reported.

Abstract

Photoactive TiO₂-only transparent coatings having self-cleaning and antireflection (AR) properties were prepared by forming first a nanosol through controlled hydrolysis of tetraisopropyl orthotitanate (TIPT), followed by deposition of this nanosol on glass substrates by dip-coating with a final calcination step to form the surface nano-textured thin film. The samples were characterized in terms of nano-structure and -texture by X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy and atomic force microscopy, while AR properties were investigated by transmittance measurements. Self-cleaning properties were analyzed by measuring the changes of water contact angle, and by photocatalytic degradation of a dye. The aim was to analyze how to prepare these materials and the relation of the properties of titania with the surface nano-texture, particularly in relation to obtain the properties required to their use as functional coatings for PV cells. Films with good optical characteristics and high transmittance (<1% loss in transmittance) can be obtained at low speed of dip-coating (6 mm/s) and high nitric acid concentration (0.5 M). Under optimized conditions, calcination at low temperature (400 °C) may already be sufficient to produce coatings with good functional properties, making the procedure compatible with the use of some flexible substrates. A preliminary mechanism of formation of the surface nano-texturing is also proposed.

Introduction

The use of photovoltaic (PV) devices to transform directly the solar energy into electrical energy is fast wide spreading to move to a decentralized and more sustainable production of energy. However, this development is posing new problems, because the decentralized use, often on the roof of building in cities, creates the issues of (i) a quick fouling of the PV devices and (ii) the need to decrease reflective properties to improve light adsorption and especially reduce the visual impact of light reflection.

The top layer of silicon solar cells, the PV cell with wider commercial use, is a cover glass having different functions (Deubener et al., 2009): (i) reduce the high reflection coefficient of silicon to improve cell efficiency, (ii) act as a radiation barrier and optical-coupling element, and (iii) protect against debris and aggressive agents present in air. Surface fouling, particularly in cities and when limited periodic cleaning is possible, still remains an issue and an improvement of antireflection (AR) properties is also generally necessary.

AR properties could be improved by (i) creating appropriate surface profiles (texturing) or (ii) depositing an AR coating, while fouling issues can be minimized by exploiting the photo-catalytic and self-cleaning properties of a cover film (Dobrzański and Szindler, 2012, Goetzberger and Hoffmann, 2005, Dobrzański and Drygala, 2008a, Dobrzański and Drygala, 2008b, Dobrzański et al., 2008). TiO₂-based thin films are often used as cover layer for PV cells, due to titania properties of (i) high photo-reactivity (Schiavello, 1988, Pelizzetti and Sepone, 1989, Fujishima et al., 1999) and -stability, (ii) super-hydrophilic behavior during irradiation, (iii) good mechanical, chemical and thermal resistance, (iv) low toxicity and cost as well (Gronet and Truman, 2007, Gensler et al., 2013, Fujishima et al., 2002). The photocatalytic properties of TiO₂ and the hydrophilic effect caused by UV irradiation are well known aspects of the surface chemistry and reactivity of titania (Yates, 2009, Long et al., 2010, Taga, 2009, Zhao et al., 2008, Fujishima and Zhang, 2006). These two aspects are related. Yates (2009) suggested that photoinduced hydrophilicity in titania is due to the photo-oxidation of a non-wetting monolayer of adsorbed hydrocarbon molecules. This monolayer of adsorbed hydrocarbon molecules is initially present over the entire surface, being in equilibrium with the gas phase hydrocarbon. When photooxidation is initiated, the adsorbed hydrocarbon will slowly decrease in the region external to the droplet edge, permitting the water droplet to wet the external surface. The hydrocarbon layer under the droplet is not photooxidized because the water bulk shields this surface from extensive exposure to O₂. Therefore, the contact angle decreases, with a change from hydrophobic-like to hydrophilic-like characteristics of TiO₂ film during UV exposure.

However, much less investigated is how to obtain multifunctional photoactive self-cleaning and AR films, which should combine various additional properties to photoreactivity and photoinduced hydrophilicity: high optical properties of transmittance, AR properties, good mechanical resistance to scratches, and good adherence to glass substrate. In addition, the method of preparation has to be low cost and easily scalable for industrial production.

The specific functional characteristics of TiO₂ are closely related to its crystal structure and morphology, which depend on many factors: manufacturing method, process conditions and final heat treatment. (Centi and Perathoner, 2009a, Centi and Perathoner, 2012) However, the relation of these parameters with the nano-texture of the surface is typical not analyzed (Centi and Perathoner, 2009b, Ampelli et al., 2008), although it is known that an efficient self-cleaning requires a specific surface roughness to minimize the contact angle of water drops (lotus effect) and allow the efficient removal of the deposited dust particles during raining. These characteristics should be combined to an efficient photocatalytic activity, related to semiconductor properties of titania, in order to reduce (by photo-oxidation) the accumulation of grease, hydrocarbons and other contaminants on the surface. These contaminants will not only cause a lowering of the transmittance, but also reduce the effectiveness of dust removal, with thus a synergetic effect.

AR properties are depending on the surface nano-texture aspects as well (Passalacqua et al., 2014, Blanco et al., 2015), because light scattering depends on surface roughness (Harada et al., 2013) and nano-texture (Attia et al., 2002). Optical properties of the films, including transparency, are indirectly dependent on the surface roughness. The latter is related to the way the thin film is prepared (nucleation rate, film thickness, etc.) (Harada et al., 2013). These parameters influence also the optical properties of the coating (Blanco et al., 2015). The properties and behavior of titania thin films to be used with optimal performances in photovoltaic glass surfaces may thus depend in a complex way from the preparation, which in turn influences various properties (including surface nano-texture) and related functional properties (self-cleaning and AR behavior, transparency, etc.). However, there are no specific studies concerning the relation of the above properties with the surface nano-texture of titania, particularly in relation to obtaining the multifunctional properties required to their use as functional coatings for PV cells.

Many recent papers have been published on the self-cleaning and/or AR properties of titania-based thin films coatings, for example prepared by thermal evaporation and cathodic arc plasma deposition (Bedikyan et al., 2013), spray pyrolysis deposition of WO₃–TiO₂ nanoparticle (Noh and Myong, 2014), self-assembly of a block-copolymer in combination with silica-based sol–gel chemistry and preformed TiO₂ nanocrystals (Guldin et al., 2013), chemical vapor deposition of SiO₂–TiO₂ thin films (Klobukowski et al., 2013), deposition of a polyimide–titania hybrid film prepared using nano-crystalline titania (Yen et al., 2013), RF magnetron sputtering of titania (Abdullah et al., 2013), anodization of a Ti layer deposited by sputtering technique (Manea et al., 2013), dip-coating in silica-titania colloid solutions (Zhang et al., 2013), layer-by-layer assembly of silica-titania core–shell nanoparticles and silica nanoparticles as building blocks (Li et al., 2013), dip-coating in sol solutions to prepare multilayer SiO₂, TiO₂ and SiO₂–TiO₂ hybrid thin film (Ye et al., 2013), cluster beam deposition method (Mao et al., 2012) and soft lithography modification of sol–gel glasses (Zhang et al., 2012a, Zhang et al., 2012b).

In general, TiO₂ films have been often prepared by physical deposition methods as pulsed laser deposition (Yamamoto et al., 2001), reactive evaporation (Mergel et al., 2000, Zeman and Takabayashi, 2002) and chemical vapor decomposition (Nakamura et al., 2001, Watanabe et al., 2002, Kaliwoh et al., 2002). In these procedures, although applicable on industrial scale, there is a low efficiency of material (Ti-source) utilization, with a consequent impact on costs and sustainability of the preparation process. Solution chemistry methods, such as dip-coating and related techniques (spin-coating, Doctor Blade coating, etc.) have advantages from this perspective (Brinker et al., 1992). As reviewed very recently by Carretero-Genevrier and coworkers (2014), good functional properties (AR, self-cleaning, optical, etc.) in metal oxide thin films, prepared by deposition of sol–gel derived solutions, could be obtained only in hybrid multilayer films. Accordingly, various recent papers on TiO₂-based thin film coatings focused on these aspects, for example the preparation of multilayer films formed by first a mesostructured SiO₂ film followed by a mesoporous TiO₂ film (Yao and He, 2014) or similarly SiO₂–TiO₂ bilayer films (Miao et al., 2013, San Vicente et al., 2012, Chen et al., 2011, Prado et al., 2010), triple layer materials formed by SiO₂, TiO₂ and SiO₂–TiO₂ hybrid thin films (Ye et al., 2013), bilayer films formed by a first Me-functionalized nanoporous SiO₂ film on which a ultrathin crystalline TiO₂ nano-perforated layer is deposited (Faustini et al., 2010, Innocenzi and Malfatti, 2013). These are typically indicated as low-cost methodologies and advantageous over the other type of methodologies cited before. However, in practical applications, e.g. under the presence of cycles of heating/cooling during day and night as well variable degree of humidity, the different expansion characteristics of the layers may induce cracks and film rupture. It is thus preferable to look at the possibility to realize films by sol–gel derived methods composed only of titania. Mu et al. (2012) and Jaguiro et al. (2010) showed that an assembly of TiO₂ nanorods on glass substrate allows obtaining notable AR and self-cleaning properties. However, it is interesting to analyze whether similar properties could be obtained by controlling the preparation parameters in a simple method (easily scale-up) such as the dip-coating procedure with nanosol solutions. Avoiding the need of coating with different materials simplifies the process and reduces the costs, besides to avoid the drawbacks indicated above.

Dip-coating of cover glass with nanosol solutions is one of the preferable methods for creating a thin surface coating by a chemical method. The advantages are the (i) good homogeneity, (ii) easy composition control, (iii) low processing temperature, (iv) large area coating possibility, (v) scalability and low equipment cost. However, the surface nano-texture characteristics of the films prepared by this method have been limitedly investigated, particularly in relation to the preparation of coating films for cover glasses in PV devices (Manea et al., 2013, Chen et al., 2013, Singh et al., 2012, Shimizu et al., 2012, Fleury et al., 2012, Lai et al., 2012, Seo et al., 2010, Kim et al., 2010).

Aim of this work is to investigate the role of the preparation parameters in obtaining, by dip-coating of nanosol solutions, TiO₂-only thin films having the specific surface nano-texture and the other characteristics to make them suitable for their use as transparent functional surface coatings for PV cells. The surface morphology, optical properties and roughness of TiO₂ thin films are analyzed by using different methods: scanning electron microscopy, atomic force microscopy and UV/Vis spectroscopy, as well as measurements of water droplet contact angle and photocatalytic properties. Specific aspects investigated in the preparation are the calcination temperatures, the number of multiple steps during the dip-coating process, the rate of extraction of the glass during the dip-coating process and the nitric acid (used as catalyst) concentration during the preparation of the nanosol. These are the main critical parameters to control from an industrial preparation perspective.