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Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
CHAPTER 12
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency Swarnalata Sahoo, Sukanya Pradhan, Sonalee Das SARP-Laboratory for Advanced Research in Polymeric Materials CIPET, Bhubaneswar, India
1. Introduction Reflect, reuse, and refocus on superhydrophobic antireflective (AR) polymer coatings with improved solar cell efficiency provide many valuable properties in different surroundings. The most copious renewable energy source in the world is solar energy that converts solar energy or light energy to another form of usable energy [1–3]. The crucial factor of the solar cell is it does not emit any greenhouse gases in the process of energy generation production. However, as compared to other fuel sources, the cost of solar is high. Usually, two types of technologies such as solar photovoltaics (PV); where photo means light and voltaic means electricity and solar thermal that controls the solar energy. A PV cell, or solar cell, is an electrical tool that converts sun’s heat into direct current electricity such as current, voltage, or resistance, etc. by using semiconductor such as silicon. Further, when the light rays strike cell, the semiconductor material absorbs a portion of light energy that transferred to the semiconductor thereby inducing loose electrons to flow freely in a certain direction. In this way, the flow of electron produces electrical energy from light energy. Whereas, solar thermal is an electrical device that utilizes light energy directly from the sun for electricity (electrical energy) production. The electrical efficiency is a physical property which represents the amount of electrical energy produced by a cell for certain insolation. Further, the maximum efficiency can be given by the ratio between output powers of a cell to the incident solar power. Therefore, the conversion efficiency or electrical efficiency is a crucial factor in the PV system. However, it is the most expensive. Hence, the selection of the material is driven by choosing cost efficiency and other properties. In addition, the higher power conversion efficiency (PCE) is a crucial research area that leads to the development of PV solar devices. Moreover, higher power efficiency indicates the solar devices more cost competitive as compared with other traditional sources of energy. Several works of literature reported regarding the uses of conjugated polymer coatings in electronics and PV solar cell to enhance the efficiency of solar cell [3–8]. Polymer offers many prospective applications in solar cell technologies that can help to achieve the total cost efficiency by Superhydrophobic Polymer Coatings https://doi.org/10.1016/B978-0-12-816671-0.00013-8
© 2019 Elsevier Inc. All rights reserved.
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optimizing three major parameters such as lower cost, durability, and greater design flexibility as compared with other materials in current use. Several reports have also been disused about the utilization of carbon nanotubes (CNTs), graphene, and semiconducting polymer which can be easily deposited on Si wafers in order to increase the percentage of solar efficiency. Hence, nowadays the polymer coating-based solar cell have gained maximum power efficiency conversion of 5% [8, 9]. The organic polymer coating deposition by several techniques such as screen printing, spray deposition, and inkjet printing are utilized to reduce the polymer coating based PV solar cell which can compete for the need of current grid electricity. In addition, all the technique permits devices to be successfully fabricated on plastic-based substrates for the development of flexible surfaces at lower temperature. Hence, qualities such as flexibility and lightweight are primarily the reason for the reduction of cost solar cell (PV). Moreover, various works of literature have also been developed the more efficient solar cells which produce the efficiency of more than 10%. The design of solar cell (Fig. 1) is complex which consists of mainly three layers of many different materials such as topmost layer, middle layer, and bottom layer. The topmost layer of solar cell made up of glass material with an AR coat and metallic strips in which the glass protects the materials, while the AR coat helps to transmit more sunlight to reach the semiconductors. AR coating [5] on the top of glass cover in the topmost layer produces better transmission, lower reflection, and more efficiency that can be achieved by using silicon nitride or titanium dioxide coatings of nanometer scales. The middle layer of solar cell plays a vital role which created solar energy through PV effect. Primarily, the middle layer consists of two semiconductor layers in which the first layer is made up of n-type and the second layer is made up of p-type materials. The n-type layer is usually made
Fig. 1 Design of solar cell.
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
by silicon mixed with less amount of phosphorous which makes the silicon material negatively charged; whereas p-type layer made by silicon mixed with small amounts of boron which makes the silicon positive charged. Similarly, the bottom layer consists of a rear metallic electrode with the metallic grid present lower to the p-type semiconductor creates an electric current. Besides these three above layer, the final layer known as the reflective layer is present to reduce the loss of light energy in the solar cell system. The percentage of efficiency is usually decreased by the reflection process and by dust. Hence, current interest in superhydrophobic characteristics of ARCs has been diverted owing to its potential utility in solving key technological issues. Superhydrophobic AR coating is the properties of the material which repels water, solid particles, and viscous liquids. Primarily, it acts as an antidust coating [6] and makes the surface highly water repellent (superhydrophobic) in which water contact angle (WCA) is greater than 150 degrees. In this case, if the water droplets fall on the AR-coated surface, it starts rolling down carrying dust particles. The superhydrophobic antireflective polymer (SHAP) coatings of the surface can be made using two pathways such as by making a rough surface with a low surface energy material and the chemical surface modification with a material having low surface energy. Similarly, the major parameter, that is, refractive index (RI) of the medium determines the amount of percentage of light transmission of the medium. Moreover, high transparency plays a vital role in improving the performance of solar devices and optical equipment such as solar panels, lenses, windows, etc. Generally, the SHAP coating is a type of coating which is used to reduce the reflection of light rays and to enhance the transmittance of incoming light as well as scattering [10]. Duparre et al. [11] investigated that can be a trade-off between both the features such as scatter loss and hydrophobicity. The SHAP coating by chemical vapor deposition method exhibits good trade between scatter loss and hydrophobicity [12]. In this chapter, recent developments in SHAP coatings for solar cell applications are described. Many approaches for SHAP have been discussed to achieve the ideal AR efficiency by paying particular focus to devise design and prospective improvements. The approaches such as layer-by-layer assembly, dip coating, sol-gel, vapor deposition, spin coating, spray coating, etching, lithography, nanoimprinting, vacuum sintering, and micro-replication have been discussed [7]. In addition, the fundamentals of antireflection and superhydrophobicity properties are also described in detail.
2. Theoretical aspect of antireflective coatings To determine the superhydrophobicity and wettability properties of a solid surface, contact angle determination is mainly used. If the solid surface is smooth and homogeneous the contact angle of a liquid to the solid surface is determined by Young’s equation (1) [13]: γ γ (1) cosθ ¼ SV SL γ LV
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where γ SV, γ LV, and γ SL are the interfacial tension values of solid-vapor, liquid-vapor, and solid-liquid interfaces, respectively. However, the theoretical model assumed by the scientist Wenzel and Cassie-Baxter was for chemical heterogeneous and rough solid surface. Generally, the model developed by Wenzel reported that the liquid penetrates completely into the indentation in a solid surface, which is given as follows [14]: cos θW ¼ rcosθ
(2)
where θW indicates the contact angle of the rough surface, r is the roughness factor of the surface, and θ is the Young’s contact angle on a symmetrical smooth surface. From Eq. (2) it has been concluded that if θ is less than 90 degrees, the roughness of the surface increases with enhanced wetting properties of the surface. If θ is greater than 90 degrees, the roughness factor decreases with decreasing roughness factor. Further, another model, Cassie-Baxter proposed that, when θ is greater than 90 degrees, the air bubbles are penetrated due to the roughness properties of the surface. In this case, the liquid creates two interfaces such as liquid-solid interface and liquid-vapor interface. Hence, the apparent contact angle is expressed as the following equation [15]: cosθC ¼ f1 cosθ1 + f2 cosθ2
(3)
where θC is the apparent contact angle to the solid surface and f1, f2 are the apparent contact angle of liquid-solid and liquid-vapor interface, respectively. At the liquid-vapor phase, the interface is (1 f ), if the fraction of solid (f ) phase is wetted by the liquid phase. For vapor, if θ ¼ 180 degrees, then the apparent contact angle can be calculated as the following equation: cos θC ¼ fcosθ + ð1 f Þ cos180° ¼ fcosθ + ð1 f Þ
(4)
In this model, Cassie-Baxter assumed that the contact angle of the interface is assumed to be static or constant. Depending on the chemical heterogeneity, surface reorganization and surface roughness the contact values can vary with the probe liquid over the contact area (Fig. 2) [16].
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Fig. 2 Liquid droplet on solid surfaces. (A) Young’s model, (B) Wenzel’s model, (C) Cassie’s model.
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
3. Types of antireflective coatings 3.1 Introduction The concept of antireflective coatings (ARCs) had been first noticed in the 19th century by Lord Rayleigh when he observed the tarnishing on a glass increasing its transmittance instead of reducing it. This has led to the development of achieving anti reflection property by varying the RI. But, in 1817, the actual ARC was developed by Fraunhofer during an experiment which concluded the reduction in reflection as a result of etching a surface in sulfur and nitric acid vapor atmosphere. The prime requirement of optical and optoelectronic equipment is maximum efficiency in the light collection. In this regard, the topmost covering of the solar panels has served well for the purpose through its better transmission and glare reduction properties which are achieved by the coatings made up of silicon nitride or titanium dioxide coatings of nanometer range. It has been studied earlier that a normal solar panel absorbs only one-fourth of the incident solar radiation, thereby reflecting the remaining radiation which could contribute to the net efficiency. The emergence of antireflective coatings that has been commercially manufactured had solved these issues through various modifications.
3.2 Types of antireflective coatings ARCs have been classified into following types on the basis of uniformity, layer composition, and surface topography: 3.2.1 Type I This type of ARCs is divided into subtypes on the basis of its homogeneity represented in Fig. 3. They are: Homogenous antireflective coatings
This type of coating consists of a single homogenous layer of RI “n” which is contributed to the restriction on the RI and the thickness of the coating. The RI (n) must be equal to
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Fig. 3 (A) Homogenous (B) inhomogenous antireflective coating.
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√ nair √ nsubstrate and thickness equal to one-fourth of the wavelength (λ). However, multiple layers in this type of coating help in achieving null reflectance at certain wavelength though the above rule is not obeyed [5]. Inhomogeneous antireflective coatings
The inhomogeneous type of ARC achieves its properties by following the RI gradient approach [17]. The reflectance from the abrupt interfaces is gradually reduced with the depth due to the changes in RI value of air to the substrate. This type of coating can be assumed to be consisting of a large number of sublayers if the RI between the adjacent sublayers is negligible [18]. 3.2.2 Type II This type of coating has been classified into the following types on the basis of its layer composition represented in Fig. 4. Unit layer antireflection coatings
The selection criteria of the material for this type of coating is a tedious task as the RI of BK7 glass is 1.5151 at 633 nm and the calculation says that the RI must be approximately 1.22. These type of coatings are generally used for moderate suppression of reflectance to around 2.5% over a wide range of spectral range from 450 to 1100 nm at normal incidence. The unit layer antireflection coating from magnesium fluoride is the common coating of this type as it RI is around 1.38. Although the performance characteristics
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Fig. 4 Plot between reflectivity and wavelength showing the comparison of single-layer MgF2, V, and broadband coatings.
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
of magnesium fluoride-based unit layer ARC are not unique, still its broad range of wavelength zone is its major advantage. Double-layer antireflection coating
The double-layer antireflection coating has been employed in order to further reduce the reflectance. However, the prime and necessary condition for this type of coating with equal optical thickness n1d1 ¼ n2d2 ¼ λ/4) is to give zero reflectance is n1 =n2 ¼ √n0 =ns This type of coatings is known as V-coatings due to its shape profile diagram and also as quarter-quarter coatings owing to their thickness relationship [19]. These double-layer coatings are specially used in laser applications where resistance to higher intensity laser radiation is extremely important [20]. Multilayer antireflection coatings
In this type of coating, multiple layers of alternating layers of low RI materials and high index materials are used to obtain reflectance as low as possible at a particular wavelength. According to the mathematical model for multilayer coatings, it has been concluded that by considering the RI and the thickness of the film, the net sum of all the reflected vectors must be minimized. In a study [21] on multilayers of Si substrate, it has been reported that the incorporation of additional Si layer in between the greater index Ge and the lower index SiO2 has resulted in maximum blue transitivity at 480 nm. Similarly, in many biological species such as Coleoptera, alternate high and low RI layers facilitate optical interference. Gradient refractive index antireflection coatings
This type of coatings have different RI profile with different curves, viz., as linear, parabolic, cubic, quintic, exponential, exponential-sinusoid, etc. and comply with Rayleigh effect. Sheldon and Haggerty [22] have demonstrated the gradient RI profiles under transverse electric waves corresponding to linear, concave, and convex-parabolic and cubic curves. In a different study, Xi et al. [23] have also proposed the gradient RI profile of the structure derived from 450 SiO2 nanorods which has resulted a net RI of 1.0526 which corresponds to a quintic profile. However, the disadvantages of this type of coating could be attributed to two major factors (a) the difference in RI between the upper surface of GRIN coating and ambient air (b) the existence of mismatch due to the complex and real RI obtained for the absorbing substrate and the transparent ARC. This shortcoming violates the Rayleigh effect and suggests that a multilayer ARC facilitates destructive interference.
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3.2.3 Type III (based on the surface topography) The requirement of the omnidirectional antireflection property has driven to the emergence of this type of coating based on surface topography. Porous antireflection coating
These types of coatings are generally made up of porous silicon and find its major application in the field of solar energy harvesting [24, 25]. It consists of nanometer-sized voids with a large hydrogenated surface. But yet there is no perfect study [26] reported which states the relationship between RI and porosity. Studies on porous silicon have demonstrated the reflective properties on the outer region of p/n+ junction with regards to solar cells by etching the outermost region in the presence of HF/HNO3. Biomimetic photonic nanostructures (“moth’s eye”)
The optical systems have been fabricated by nature so well that it justifies scientific sense to reproduce them. For example, scallop eyes inspired chromatic aberration minimization, horsefly cornea inspired extreme UV-optics and human deformable eye lenses inspired liquid lenses, etc. The camouflaging strategy of some insects and the exceptional photon collection capability of nocturnal creatures have motivated scientists to analyze the eyes of such creatures, especially moths and butterflies and transparent wings of hawk moths. Studies demonstrated that these nanostructures have a gradient RI relationship between chitin and ambient of which makes it clear that antireflection property is chiefly due to exceptional transmission. Boden et al. [27] studied the antireflectivity which showed that moth’s eye arrays of 250 nm exhibited similar performance as the doublelayer antireflection which has been already discussed in the earlier section. Textured surface antireflection coating
Surface texturing also provides the information about the substrate AR as explained by many scientific communities and the reason for antireflection is due to light trapping and multiple internal reflections phenomena. A new texturing geometry was explained to produce maximum efficacy solar cells with a null antireflection property that has three perpendicular planes that provide a platform for multiple internal reflections [28]. Surface texturing imparts a reduction of reflectance to the tune of approximately 10% in monocrystal Si and reflection losses are reportedly minimized to nearly 1% in case of amorphous Si using surface texture. Antireflection grating
The prime requirement for broadband antireflection over a large region gets fulfilled by the surface relief gratings. The grating structures are based on the similar principle of creating a continuous gradient of RI and their efficacy has been proved in the solar, microwave, and THz wavelength ranges. However, antireflection have been observed to be
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
less effective for solar cells because gratings that help to propagate the zeroth diffraction orders and do not provide to higher diffraction orders which apparently contribute to the total energy collected in the solar cell.
4. Recent progress toward the development of superhydrophobic antireflective polymer coatings with improved solar cell efficiency Wang et al. [28] studied the synthesis and characterization of superhydrophobic ARC with high transmittance that can be used for solar cell applications. The authors prepared superhydrophobic sol-gels by hydrolyzing tetraethoxysilane (TES) and thereafter, reacting it with hexamethyldisilazane (HMDS). The prepared superhydrophobic sol-gels were aged for 48, 72, 96, or 168 h, respectively, to render it superhydrophobic. The antireflective glass coatings were fabricated through layer-by-layer (LBL) deposition technique using poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) polyelectrolyte indicating a transmittance of 96%. Heat-treated ARCs up to 220°C rendered PAH/PAA multilayer to be hydrophobic with uniform distribution of pore size. This observation corroborated with the SEM micrographs confirming the presence of nanopores distributed uniformly throughout the surface. The authors observed that aging of the superhydrophobic sol-gels led to an increase in superhydrophobicity with an average WCA of 162.6, 164.4, and 163.5 degrees, respectively due to the NH3 produced during the hydrolysis reaction of TEOS with HDMS. After spin coating, the superhydrophobic AR polymer coatings displayed high transmittance of 96.4% 0.2% with a WCA of 158.4 degrees and contact angle hysteresis (CAH) 1.8 degrees as compared to neat AR coatings. This was due to the large RI of superhydrophobic sol-gel. After calcination of superhydrophobic AR PAH/PAA Coatings with nanosilica, the WCA increased to 161 degrees with CAH of 5 degrees. The morphological studies of the superhydrophobic polymer AR Polymer Coatings using AFM (Fig. 8 reproduced from Ref. [28]) indicated submicrometer-sized structures which were composed of many nanometer-sized structures. This resulted in a structure similar to the lotus leaf with dual scale roughness property. Impact test of the coatings indicated high transmittance of 96% with an increase in CAH of 4.6 0.2 degrees. Advancing WCA of the coatings increased 7.2 5.0 degrees. The observed increase of CAH was due to the high speed of the plastic bullet during the impact test which led to the overall increase in the RMS roughness of the coatings. Thus, the authors concluded that the synthesized superhydrophobic AR coating based on (PAH/PAA) polyelectrolyte calcined with nanosilica obtained through LBL technique indicated high transmittance with improved WCA and good mechanical properties (Fig. 5). Prado et al. [2] developed multifunctional sol-gel ARCs with improved superhydrophobic self-cleaning capacity for solar cell application. The AR coating was prepared based on tetraethyl orthosilicate (TEOS), Pluronic F127 using ethanol and distilled water. The
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(A)
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Fig. 5 AFM morphology of the AR polymer coating (A) before heat treatment, (B) after heat treatment, (C) after immersion in water for 1 week.
superhydrophobic films were synthesized using titanium n-butoxide (n-BuTi) and absolute ethanol. AR-coated glass was developed using mesoporous TiO2/SiO2 layers. The TiO2 AR-coated glass presented a transmittance of 95.9% as compared to a neat glass with a transmittance of 91.2%. It was observed from methylene blue degradation tests that all the TiO2 AR-coated glass is photoactive. The unique morphological arrangement of nanostructured inner SiO2 and outer TiO2 layers and the increase in the surface area has resulted in improved self-cleaning ability with high photocatalytic activity. Choi and Huh [29] described the synthesis and characterization of one-step biomimetic antireflective, self-cleaning coating for light harvesting application in organic solar cells. The authors prepared the template by combining UV-assisted molding with subsequent hydrophobization of nanoparticles through one-step replication method. The prepared template was then subjected to further ultraviolet-ozone (UVO) treatment that resulted in the formation of dual nanometer-scale roughness with microstructured surface. The one-step replication method provides large surface area conducive for different applications. Polyurethane acrylate (PUA) functionalized prepolymer with a
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
perfluoropolyether (PFPE) backbone was considered to be the UV-curable precursor anchored with Al2O3 nanoparticles. The solar cell was fabricated using ITO-coated glass. The presence of fluorine groups led to both self-cleaning and development of AR properties due to its low surface energy and relatively low RI. The developed ITO-coated glass indicated superhydrophobicity with contact angle >156 degrees due to multiscale hierarchical surface morphology and low surface energy of fluorine. The AR properties of the ITO-coated glass were analyzed through UV-Vis spectroscopy. The ITOdeposited glass used as a substrate for solar cell devices indicated >8% reflectance over a visible region due to the high reflective power of ITO. The reflectance of ITO-coated glass was found to be higher by twofolds as compared to the optical bare glass. The UV-cured flat PFPE layer exhibited transmittance greater than the uncoated ITO glass. On the other hand, the cured dual rough PFPE layer on ITO glass indicated slightly lower reflection in comparison to uncoated ITO glass. This was due to the low RI (i.e., 1.37) of the transparent cured PFPE layer which gave rise to an increased transmission owing to the insertion of an intermediate RI layer in between air (i.e., 1.00) and the glass substrate (i.e., 1.52) which suppresses the interfacial Fresnel’s reflection. The durability and stability of the coatings were investigated using QUV Accelerated Weathering Tester with UVA-340 lamp irradiated at 60°C. It was observed that the transmittance, AR and superhydrophobic properties of the coatings remained unaltered even after 200 h of UV exposure due to the higher bonding energy of CdF bonds, that is, 488 kJ mol1. This indicates that the coatings can be used as solar cells with higher withstanding efficiency toward long exposure to sunlight. Thus, the authors concluded that the methodology adopted for the fabrication of organic solar cells using one-step UV-assisted replica molding with acrylate-functionalized PFPE precursor on ITO glass can have practical applications in the field of solar cells due to its large surface area, dual scale roughness, low surface energy, and low RI. Gan et al. [30] fabricated AR transparent polymer-coated graphene/silicon (G/Si) electrodes for application in Schottky junction PV solar cells. In comparison to the conventional monocrystalline Si solar cells, the assembly of G/Si only requires the transfer of graphene films onto silicon substrates at room temperature thereby avoiding hightemperature furnaces and vacuum systems. This assembly has also led to achieving the PCE of nearly 2% [31]. Apart from this method chemical doping, creation of micronanostructure on Si substrate and light trapping with antireflection coatings (ARC) can be other alternative strategies to increase antireflection properties and PCEs (10%). [32, 33]. Polymers are also novel candidates which can be applied in the field of ARC coatings as compared to inorganic materials owing to their lightweight, flexibility, stretching, and bending capabilities. The authors demonstrated the applicability of polymethylmethacrylate (PMMA) ARC coatings on G/Si substrates as transparent conductive electrodes with excellent light absorption for solar cell applications. The optical property of the PMMA ARC coatings on G/Si substrates was investigated using
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transmission electron microscopy (TEM). It was observed that the transmittance of PMMA/G-Si ARC coatings (i.e., 97.4%) was slightly lower than the neat graphene coatings (i.e., 96%) in the region of 400–800 nm. However, the PV performance of the PMMA/G-Si ARC coatings was superior as compared to the uncoated ones with PCE of 6.55%. The observed phenomenon was due to the increase in short circuit current density (Jsc), which reduces the resistance of Schottky junctions thereby enhancing the light absorption ability. For further improvement in the PCE of the PMMA/G-Si ARC coatings, it was chemically doped via HNO3 vapor treatment, which indicated an increase in PCE to about 13.34%. Thus the above methodology can be a cost-effective technique for the fabrication of solar cells with excellent light transmission ability (Fig. 9 reproduced from Ref. [30]) (Fig. 6). Leem et al. [34] fabricated highlytransparent antireflective moth-eye nanopatterned UV cured Norland Optical Adhesive (NOA) 63 coatings (ARC) on glass for application in solar PV systems. Glass has been a candid material for application in the field of optoelectronics due to its high optical transparency, low cost, and good thermal stability. They are basically used to safeguard the PV systems against UV radiation, external shock, and corrosive acidic environment. However, the optical reflection of glass degrades with time due to its high RI (nearly 1.5) which can reduce the PCE of solar cells. Hence, in order to overcome this disadvantage, we require ARCs that can reduce the optical losses caused by Fresnel surface reflection (FSR). The FSR phenomenon of glass can be decreased via the formation of biomimetic moth-eye structures on the surface of the glass, composed of tapered conical gratings having wavelengths smaller than the incident light [35–39]. In recent decades, soft imprint lithographic method has gained momentous attention in
Graphene
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Fig. 6 Fabrication of polymer-coated G/Si solar cells.
n-Si
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
comparison to other nanolithography techniques for fabricating micro- or nano-patterns onto UV-curable poly-dimethylsiloxane (PDMS) polymers. PDMS offers various advantages which include low free surface energy, flexibility, transparency, and hardness owing to its simplicity, low-cost, and tenability. In addition, the NOA63 polymers are quite beneficial as ARC owing to their similar RI of 1.56 with almost no absorption in the wavelength region between 350 and 1800 nm. The author observed that less work has been carried out regarding the performance improvement of III–V semiconductor-based multijunction PV module systems with conical nanogratings (NGs) patterned polymer films. Hence, the author investigated the effect of UV, thermal, and wetting behavior on the efficiency behavior of PDMS/NOA63 NG ARC coatings. The NOA63 NGs coatings exhibited a hydrophobic surface with a contact angle of 112 degrees and solar-weighted transmittance (Tsw) of 93.2%. Superhydrophobic polymeric coating surface with CNT covering improves the solar cell by producing a large contact angle. Zhao et al. [40] has been successfully synthesized the silica-covered CNT by using sol-gel method and they described that the surface energy of the polymeric material was significantly reduced with static WCA of 156 degrees, thereby making the surface superhydrophobic. This also shows good acid, alkali, and aging resistance, which might be used as a good practical application in the future. Similarly, Jinguang and Qi [41] have been described the recent development in the fabrication and ARC surfaces based on nanostructure arrangement of silicon and nonsilicon materials. Moreover, Prado et al. [2] described the antireflective hydrophobic coatings based on porous silica and found the increase in transmission of light for wavelength range of 350–850 nm; which would be helpful for depositing a hydrophobic layer of thickness less than 3.5 nm without destroying any properties of ARC. Hiralal et al. demonstrated the ZnO nanowire-based ARCs for organic PV solar cell and found that the coating produces the additional benefits to the solar cell such as it lowers the rate of degradation and also lowers the absorption of UV radiation. The presence of ZnO on AR coating is a photocatalytic semiconductor in nature which catalyzes redox reactions in the presence of water and oxygen thereby permitting the breakdown of organic molecules on its coating surface. This can be used as a self-cleaning agent for organic materials for reduction issues from soiling [42].
5. Fabrication of superhydrophobic coatings Wetting behavior is generally classified into four different categories based on the WCA (θ), that is, 0 degrees < θ < 10 degrees (superhydrophilic), 10 degrees < θ < 90 degrees (hydrophilic), 90 degrees < θ < 150 degrees (hydrophobic), and 150 degrees < θ < 180 degrees (superhydrophobic). Recent decades have witnessed momentous attention toward the development of superhydrophobic surface. Polymers reinforced with nanofillers with distinctive features like low cost, good processability, flexibility, good scratch,
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and abrasion resistance have been used as candid materials for the fabrication of superhydrophobic coatings [43]. The two important factors which are responsible for the fabrication of superhydrophobic coatings include roughening of the surface and chemical modification of coatings with low surface energy materials [44]. Different models such as Wenzel’s and Cassie’s are used for correlating and understanding the relation between superhydrophobicity phenomenon, surface roughness, and chemical modification [15, 45, 46].
6. Future prospect and challenges From the above literature findings, it is imperative that the AR coating materials have shown rapid application with potential prospects in the field of lenses, lasers, solar cells, diodes, optical and optoelectronic devices, screens, sensors, antiglare glasses for automotive applications, and even military equipment. It is necessary to focus some parameters of investments such as efficiency, stability, energy yield, and span of life instead of focusing solely on initial capital investment because the capital cost of installation of the solar panel is too high. Hence, the future scope can be divided into two parts; wherein, the first part aims to the improvement of the materials performance by continuous optimizing process and the second part aims to build a request on manufacturing technology. The performance improvement includes many multifunctional activities such as selfcleaning, light-trapping, durability, and so on [12]. Moreover, ARCs should have significant properties such as thermal, mechanical, long term durability, etc. This is due to the high mechanical strength of the SHAP coating makes the material suitable for selfcleaning and AR. Hence, high power conversion and cost reduction are obtained. Hybrid ARCs with self-cleaning ability have found extensive application in low emission applications. The conventional single and bilayer ARCs are still used for laser applications owing to minimum reflectance at smaller wavelengths. However, in the present era, the single and bilayer coatings are replaced by advanced ARCs that is, moth-eye ARCs due to its economic feasibility. Bionic nanostructures based on moth eye have resolved the issues related with omnidirectional ARCs to be used for different optical equipment and solar cells with improved transmittance. The fabrication process is also cost-effective and feasible for stacking multilayer coatings. In recent decades, ARCs have found wide application in the field of PV cells; however, the coatings lack mechanical resilience and strong adhesion strength. In addition, ARCs used for outdoor application gets peeled off from the substrate due to poor adhesion strength. For instance, ARCs based on ceramic material show brittle fracture due to microcracks and voids. Thus, it is imperative that the mechanical strength, adhesion strength, and thermal stability of the ARCs need to be improved for finding applications in harsh
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
environmental conditions. The other factors which need prime attention are the compressive and tensile failure of ARCs. The prime focus should be toward the development of polymer ARCs reinforced with nanofillers, which exhibits ductile failure with improved mechanical stability. Another area which needs attention is the development of cost-effective single ARCs and hybrid multifunctional ARCs with improved mechanical strength, thermal stability, superamphiphobic property, scratch resistivity, and antiglare property for finding application in the field of solar cells. Hence, the developments of innovative materials for SHAP coating techniques are the most essential part to expand the range of applications which can lead to the large-scale production in the industry. In addition, it is also required to carry out accurate measurements and different models for better understanding of the fabrication of SHAP coatings and to overcome the above challenges. The following are some of the main parts in which the research could be directed to resolve issues: • Although ARCs have exemplified their application in the field of solar PV still it poses some challenges owing to their performance over time period. • The biological aspect of antireflection remains unsought which could be explored by considering photonic nanostructures in butterflies, squids, etc. • Novel developments in optical devices provide a platform containing the immense potential for customization of ARCs to meet the requirements of the cutting edge technology and product improvisation.
7. Conclusions In the present chapter, the recent progress in SHAP coatings has been described in detail. But, there are still some issues for the fabrication process in the manufacture of SHAP coatings. Based on the findings polymeric ARCs can be used to increase the efficiency, antireflection or transmission, and self-cleaning performance which encourage the technology or industry to increase the mechanical strength of the developed products. ARCs have been used in a wide range of applications starting from optical and optoelectronic devices to automotive and aerospace applications. Further, in other fields such as electrochromism and green architectural strategies have also led to the integration of AR property and electrochromism into unity. The performance characteristics of the solar cell also get impaired by the accumulation of dust, microbes, and moisture. The development of hydrophobic, hydrophilic, and antimicrobial coatings is also achieving greater demand due to its hybrid characteristics. In conclusion, a simple, cost-effective with high-performance solar cell can be developed which might provide new ideas and new concept for the development of SHAP coating surfaces.
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Superhydrophobic antireflective polymer coatings with improved solar cell efficiency Swarnalata Sahoo, Sukanya Pradhan, Sonalee Das SARP-Laboratory for Advanced Research in Polymeric Materials CIPET, Bhubaneswar, India
1. Introduction Reflect, reuse, and refocus on superhydrophobic antireflective (AR) polymer coatings with improved solar cell efficiency provide many valuable properties in different surroundings. The most copious renewable energy source in the world is solar energy that converts solar energy or light energy to another form of usable energy [1–3]. The crucial factor of the solar cell is it does not emit any greenhouse gases in the process of energy generation production. However, as compared to other fuel sources, the cost of solar is high. Usually, two types of technologies such as solar photovoltaics (PV); where photo means light and voltaic means electricity and solar thermal that controls the solar energy. A PV cell, or solar cell, is an electrical tool that converts sun’s heat into direct current electricity such as current, voltage, or resistance, etc. by using semiconductor such as silicon. Further, when the light rays strike cell, the semiconductor material absorbs a portion of light energy that transferred to the semiconductor thereby inducing loose electrons to flow freely in a certain direction. In this way, the flow of electron produces electrical energy from light energy. Whereas, solar thermal is an electrical device that utilizes light energy directly from the sun for electricity (electrical energy) production. The electrical efficiency is a physical property which represents the amount of electrical energy produced by a cell for certain insolation. Further, the maximum efficiency can be given by the ratio between output powers of a cell to the incident solar power. Therefore, the conversion efficiency or electrical efficiency is a crucial factor in the PV system. However, it is the most expensive. Hence, the selection of the material is driven by choosing cost efficiency and other properties. In addition, the higher power conversion efficiency (PCE) is a crucial research area that leads to the development of PV solar devices. Moreover, higher power efficiency indicates the solar devices more cost competitive as compared with other traditional sources of energy. Several works of literature reported regarding the uses of conjugated polymer coatings in electronics and PV solar cell to enhance the efficiency of solar cell [3–8]. Polymer offers many prospective applications in solar cell technologies that can help to achieve the total cost efficiency by Superhydrophobic Polymer Coatings https://doi.org/10.1016/B978-0-12-816671-0.00013-8
© 2019 Elsevier Inc. All rights reserved.
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optimizing three major parameters such as lower cost, durability, and greater design flexibility as compared with other materials in current use. Several reports have also been disused about the utilization of carbon nanotubes (CNTs), graphene, and semiconducting polymer which can be easily deposited on Si wafers in order to increase the percentage of solar efficiency. Hence, nowadays the polymer coating-based solar cell have gained maximum power efficiency conversion of 5% [8, 9]. The organic polymer coating deposition by several techniques such as screen printing, spray deposition, and inkjet printing are utilized to reduce the polymer coating based PV solar cell which can compete for the need of current grid electricity. In addition, all the technique permits devices to be successfully fabricated on plastic-based substrates for the development of flexible surfaces at lower temperature. Hence, qualities such as flexibility and lightweight are primarily the reason for the reduction of cost solar cell (PV). Moreover, various works of literature have also been developed the more efficient solar cells which produce the efficiency of more than 10%. The design of solar cell (Fig. 1) is complex which consists of mainly three layers of many different materials such as topmost layer, middle layer, and bottom layer. The topmost layer of solar cell made up of glass material with an AR coat and metallic strips in which the glass protects the materials, while the AR coat helps to transmit more sunlight to reach the semiconductors. AR coating [5] on the top of glass cover in the topmost layer produces better transmission, lower reflection, and more efficiency that can be achieved by using silicon nitride or titanium dioxide coatings of nanometer scales. The middle layer of solar cell plays a vital role which created solar energy through PV effect. Primarily, the middle layer consists of two semiconductor layers in which the first layer is made up of n-type and the second layer is made up of p-type materials. The n-type layer is usually made
Fig. 1 Design of solar cell.
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
by silicon mixed with less amount of phosphorous which makes the silicon material negatively charged; whereas p-type layer made by silicon mixed with small amounts of boron which makes the silicon positive charged. Similarly, the bottom layer consists of a rear metallic electrode with the metallic grid present lower to the p-type semiconductor creates an electric current. Besides these three above layer, the final layer known as the reflective layer is present to reduce the loss of light energy in the solar cell system. The percentage of efficiency is usually decreased by the reflection process and by dust. Hence, current interest in superhydrophobic characteristics of ARCs has been diverted owing to its potential utility in solving key technological issues. Superhydrophobic AR coating is the properties of the material which repels water, solid particles, and viscous liquids. Primarily, it acts as an antidust coating [6] and makes the surface highly water repellent (superhydrophobic) in which water contact angle (WCA) is greater than 150 degrees. In this case, if the water droplets fall on the AR-coated surface, it starts rolling down carrying dust particles. The superhydrophobic antireflective polymer (SHAP) coatings of the surface can be made using two pathways such as by making a rough surface with a low surface energy material and the chemical surface modification with a material having low surface energy. Similarly, the major parameter, that is, refractive index (RI) of the medium determines the amount of percentage of light transmission of the medium. Moreover, high transparency plays a vital role in improving the performance of solar devices and optical equipment such as solar panels, lenses, windows, etc. Generally, the SHAP coating is a type of coating which is used to reduce the reflection of light rays and to enhance the transmittance of incoming light as well as scattering [10]. Duparre et al. [11] investigated that can be a trade-off between both the features such as scatter loss and hydrophobicity. The SHAP coating by chemical vapor deposition method exhibits good trade between scatter loss and hydrophobicity [12]. In this chapter, recent developments in SHAP coatings for solar cell applications are described. Many approaches for SHAP have been discussed to achieve the ideal AR efficiency by paying particular focus to devise design and prospective improvements. The approaches such as layer-by-layer assembly, dip coating, sol-gel, vapor deposition, spin coating, spray coating, etching, lithography, nanoimprinting, vacuum sintering, and micro-replication have been discussed [7]. In addition, the fundamentals of antireflection and superhydrophobicity properties are also described in detail.
2. Theoretical aspect of antireflective coatings To determine the superhydrophobicity and wettability properties of a solid surface, contact angle determination is mainly used. If the solid surface is smooth and homogeneous the contact angle of a liquid to the solid surface is determined by Young’s equation (1) [13]: γ γ (1) cosθ ¼ SV SL γ LV
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where γ SV, γ LV, and γ SL are the interfacial tension values of solid-vapor, liquid-vapor, and solid-liquid interfaces, respectively. However, the theoretical model assumed by the scientist Wenzel and Cassie-Baxter was for chemical heterogeneous and rough solid surface. Generally, the model developed by Wenzel reported that the liquid penetrates completely into the indentation in a solid surface, which is given as follows [14]: cos θW ¼ rcosθ
(2)
where θW indicates the contact angle of the rough surface, r is the roughness factor of the surface, and θ is the Young’s contact angle on a symmetrical smooth surface. From Eq. (2) it has been concluded that if θ is less than 90 degrees, the roughness of the surface increases with enhanced wetting properties of the surface. If θ is greater than 90 degrees, the roughness factor decreases with decreasing roughness factor. Further, another model, Cassie-Baxter proposed that, when θ is greater than 90 degrees, the air bubbles are penetrated due to the roughness properties of the surface. In this case, the liquid creates two interfaces such as liquid-solid interface and liquid-vapor interface. Hence, the apparent contact angle is expressed as the following equation [15]: cosθC ¼ f1 cosθ1 + f2 cosθ2
(3)
where θC is the apparent contact angle to the solid surface and f1, f2 are the apparent contact angle of liquid-solid and liquid-vapor interface, respectively. At the liquid-vapor phase, the interface is (1 f ), if the fraction of solid (f ) phase is wetted by the liquid phase. For vapor, if θ ¼ 180 degrees, then the apparent contact angle can be calculated as the following equation: cos θC ¼ fcosθ + ð1 f Þ cos180° ¼ fcosθ + ð1 f Þ
(4)
In this model, Cassie-Baxter assumed that the contact angle of the interface is assumed to be static or constant. Depending on the chemical heterogeneity, surface reorganization and surface roughness the contact values can vary with the probe liquid over the contact area (Fig. 2) [16].
gLV q gSV
(A)
qw
gSL
(B)
qc
(C)
Fig. 2 Liquid droplet on solid surfaces. (A) Young’s model, (B) Wenzel’s model, (C) Cassie’s model.
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
3. Types of antireflective coatings 3.1 Introduction The concept of antireflective coatings (ARCs) had been first noticed in the 19th century by Lord Rayleigh when he observed the tarnishing on a glass increasing its transmittance instead of reducing it. This has led to the development of achieving anti reflection property by varying the RI. But, in 1817, the actual ARC was developed by Fraunhofer during an experiment which concluded the reduction in reflection as a result of etching a surface in sulfur and nitric acid vapor atmosphere. The prime requirement of optical and optoelectronic equipment is maximum efficiency in the light collection. In this regard, the topmost covering of the solar panels has served well for the purpose through its better transmission and glare reduction properties which are achieved by the coatings made up of silicon nitride or titanium dioxide coatings of nanometer range. It has been studied earlier that a normal solar panel absorbs only one-fourth of the incident solar radiation, thereby reflecting the remaining radiation which could contribute to the net efficiency. The emergence of antireflective coatings that has been commercially manufactured had solved these issues through various modifications.
3.2 Types of antireflective coatings ARCs have been classified into following types on the basis of uniformity, layer composition, and surface topography: 3.2.1 Type I This type of ARCs is divided into subtypes on the basis of its homogeneity represented in Fig. 3. They are: Homogenous antireflective coatings
This type of coating consists of a single homogenous layer of RI “n” which is contributed to the restriction on the RI and the thickness of the coating. The RI (n) must be equal to
d
(A)
n
2
1 0
d
(B)
n
2
1 0
Fig. 3 (A) Homogenous (B) inhomogenous antireflective coating.
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√ nair √ nsubstrate and thickness equal to one-fourth of the wavelength (λ). However, multiple layers in this type of coating help in achieving null reflectance at certain wavelength though the above rule is not obeyed [5]. Inhomogeneous antireflective coatings
The inhomogeneous type of ARC achieves its properties by following the RI gradient approach [17]. The reflectance from the abrupt interfaces is gradually reduced with the depth due to the changes in RI value of air to the substrate. This type of coating can be assumed to be consisting of a large number of sublayers if the RI between the adjacent sublayers is negligible [18]. 3.2.2 Type II This type of coating has been classified into the following types on the basis of its layer composition represented in Fig. 4. Unit layer antireflection coatings
The selection criteria of the material for this type of coating is a tedious task as the RI of BK7 glass is 1.5151 at 633 nm and the calculation says that the RI must be approximately 1.22. These type of coatings are generally used for moderate suppression of reflectance to around 2.5% over a wide range of spectral range from 450 to 1100 nm at normal incidence. The unit layer antireflection coating from magnesium fluoride is the common coating of this type as it RI is around 1.38. Although the performance characteristics
7.0 6.0 Reflectivity (%)
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5.0 V-type 4.0 Single layer MgF2 3.0 Broadband 2.0 1.0 0.0 400
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Wavelength (nm)
Fig. 4 Plot between reflectivity and wavelength showing the comparison of single-layer MgF2, V, and broadband coatings.
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of magnesium fluoride-based unit layer ARC are not unique, still its broad range of wavelength zone is its major advantage. Double-layer antireflection coating
The double-layer antireflection coating has been employed in order to further reduce the reflectance. However, the prime and necessary condition for this type of coating with equal optical thickness n1d1 ¼ n2d2 ¼ λ/4) is to give zero reflectance is n1 =n2 ¼ √n0 =ns This type of coatings is known as V-coatings due to its shape profile diagram and also as quarter-quarter coatings owing to their thickness relationship [19]. These double-layer coatings are specially used in laser applications where resistance to higher intensity laser radiation is extremely important [20]. Multilayer antireflection coatings
In this type of coating, multiple layers of alternating layers of low RI materials and high index materials are used to obtain reflectance as low as possible at a particular wavelength. According to the mathematical model for multilayer coatings, it has been concluded that by considering the RI and the thickness of the film, the net sum of all the reflected vectors must be minimized. In a study [21] on multilayers of Si substrate, it has been reported that the incorporation of additional Si layer in between the greater index Ge and the lower index SiO2 has resulted in maximum blue transitivity at 480 nm. Similarly, in many biological species such as Coleoptera, alternate high and low RI layers facilitate optical interference. Gradient refractive index antireflection coatings
This type of coatings have different RI profile with different curves, viz., as linear, parabolic, cubic, quintic, exponential, exponential-sinusoid, etc. and comply with Rayleigh effect. Sheldon and Haggerty [22] have demonstrated the gradient RI profiles under transverse electric waves corresponding to linear, concave, and convex-parabolic and cubic curves. In a different study, Xi et al. [23] have also proposed the gradient RI profile of the structure derived from 450 SiO2 nanorods which has resulted a net RI of 1.0526 which corresponds to a quintic profile. However, the disadvantages of this type of coating could be attributed to two major factors (a) the difference in RI between the upper surface of GRIN coating and ambient air (b) the existence of mismatch due to the complex and real RI obtained for the absorbing substrate and the transparent ARC. This shortcoming violates the Rayleigh effect and suggests that a multilayer ARC facilitates destructive interference.
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3.2.3 Type III (based on the surface topography) The requirement of the omnidirectional antireflection property has driven to the emergence of this type of coating based on surface topography. Porous antireflection coating
These types of coatings are generally made up of porous silicon and find its major application in the field of solar energy harvesting [24, 25]. It consists of nanometer-sized voids with a large hydrogenated surface. But yet there is no perfect study [26] reported which states the relationship between RI and porosity. Studies on porous silicon have demonstrated the reflective properties on the outer region of p/n+ junction with regards to solar cells by etching the outermost region in the presence of HF/HNO3. Biomimetic photonic nanostructures (“moth’s eye”)
The optical systems have been fabricated by nature so well that it justifies scientific sense to reproduce them. For example, scallop eyes inspired chromatic aberration minimization, horsefly cornea inspired extreme UV-optics and human deformable eye lenses inspired liquid lenses, etc. The camouflaging strategy of some insects and the exceptional photon collection capability of nocturnal creatures have motivated scientists to analyze the eyes of such creatures, especially moths and butterflies and transparent wings of hawk moths. Studies demonstrated that these nanostructures have a gradient RI relationship between chitin and ambient of which makes it clear that antireflection property is chiefly due to exceptional transmission. Boden et al. [27] studied the antireflectivity which showed that moth’s eye arrays of 250 nm exhibited similar performance as the doublelayer antireflection which has been already discussed in the earlier section. Textured surface antireflection coating
Surface texturing also provides the information about the substrate AR as explained by many scientific communities and the reason for antireflection is due to light trapping and multiple internal reflections phenomena. A new texturing geometry was explained to produce maximum efficacy solar cells with a null antireflection property that has three perpendicular planes that provide a platform for multiple internal reflections [28]. Surface texturing imparts a reduction of reflectance to the tune of approximately 10% in monocrystal Si and reflection losses are reportedly minimized to nearly 1% in case of amorphous Si using surface texture. Antireflection grating
The prime requirement for broadband antireflection over a large region gets fulfilled by the surface relief gratings. The grating structures are based on the similar principle of creating a continuous gradient of RI and their efficacy has been proved in the solar, microwave, and THz wavelength ranges. However, antireflection have been observed to be
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
less effective for solar cells because gratings that help to propagate the zeroth diffraction orders and do not provide to higher diffraction orders which apparently contribute to the total energy collected in the solar cell.
4. Recent progress toward the development of superhydrophobic antireflective polymer coatings with improved solar cell efficiency Wang et al. [28] studied the synthesis and characterization of superhydrophobic ARC with high transmittance that can be used for solar cell applications. The authors prepared superhydrophobic sol-gels by hydrolyzing tetraethoxysilane (TES) and thereafter, reacting it with hexamethyldisilazane (HMDS). The prepared superhydrophobic sol-gels were aged for 48, 72, 96, or 168 h, respectively, to render it superhydrophobic. The antireflective glass coatings were fabricated through layer-by-layer (LBL) deposition technique using poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) polyelectrolyte indicating a transmittance of 96%. Heat-treated ARCs up to 220°C rendered PAH/PAA multilayer to be hydrophobic with uniform distribution of pore size. This observation corroborated with the SEM micrographs confirming the presence of nanopores distributed uniformly throughout the surface. The authors observed that aging of the superhydrophobic sol-gels led to an increase in superhydrophobicity with an average WCA of 162.6, 164.4, and 163.5 degrees, respectively due to the NH3 produced during the hydrolysis reaction of TEOS with HDMS. After spin coating, the superhydrophobic AR polymer coatings displayed high transmittance of 96.4% 0.2% with a WCA of 158.4 degrees and contact angle hysteresis (CAH) 1.8 degrees as compared to neat AR coatings. This was due to the large RI of superhydrophobic sol-gel. After calcination of superhydrophobic AR PAH/PAA Coatings with nanosilica, the WCA increased to 161 degrees with CAH of 5 degrees. The morphological studies of the superhydrophobic polymer AR Polymer Coatings using AFM (Fig. 8 reproduced from Ref. [28]) indicated submicrometer-sized structures which were composed of many nanometer-sized structures. This resulted in a structure similar to the lotus leaf with dual scale roughness property. Impact test of the coatings indicated high transmittance of 96% with an increase in CAH of 4.6 0.2 degrees. Advancing WCA of the coatings increased 7.2 5.0 degrees. The observed increase of CAH was due to the high speed of the plastic bullet during the impact test which led to the overall increase in the RMS roughness of the coatings. Thus, the authors concluded that the synthesized superhydrophobic AR coating based on (PAH/PAA) polyelectrolyte calcined with nanosilica obtained through LBL technique indicated high transmittance with improved WCA and good mechanical properties (Fig. 5). Prado et al. [2] developed multifunctional sol-gel ARCs with improved superhydrophobic self-cleaning capacity for solar cell application. The AR coating was prepared based on tetraethyl orthosilicate (TEOS), Pluronic F127 using ethanol and distilled water. The
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superhydrophobic films were synthesized using titanium n-butoxide (n-BuTi) and absolute ethanol. AR-coated glass was developed using mesoporous TiO2/SiO2 layers. The TiO2 AR-coated glass presented a transmittance of 95.9% as compared to a neat glass with a transmittance of 91.2%. It was observed from methylene blue degradation tests that all the TiO2 AR-coated glass is photoactive. The unique morphological arrangement of nanostructured inner SiO2 and outer TiO2 layers and the increase in the surface area has resulted in improved self-cleaning ability with high photocatalytic activity. Choi and Huh [29] described the synthesis and characterization of one-step biomimetic antireflective, self-cleaning coating for light harvesting application in organic solar cells. The authors prepared the template by combining UV-assisted molding with subsequent hydrophobization of nanoparticles through one-step replication method. The prepared template was then subjected to further ultraviolet-ozone (UVO) treatment that resulted in the formation of dual nanometer-scale roughness with microstructured surface. The one-step replication method provides large surface area conducive for different applications. Polyurethane acrylate (PUA) functionalized prepolymer with a
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
perfluoropolyether (PFPE) backbone was considered to be the UV-curable precursor anchored with Al2O3 nanoparticles. The solar cell was fabricated using ITO-coated glass. The presence of fluorine groups led to both self-cleaning and development of AR properties due to its low surface energy and relatively low RI. The developed ITO-coated glass indicated superhydrophobicity with contact angle >156 degrees due to multiscale hierarchical surface morphology and low surface energy of fluorine. The AR properties of the ITO-coated glass were analyzed through UV-Vis spectroscopy. The ITOdeposited glass used as a substrate for solar cell devices indicated >8% reflectance over a visible region due to the high reflective power of ITO. The reflectance of ITO-coated glass was found to be higher by twofolds as compared to the optical bare glass. The UV-cured flat PFPE layer exhibited transmittance greater than the uncoated ITO glass. On the other hand, the cured dual rough PFPE layer on ITO glass indicated slightly lower reflection in comparison to uncoated ITO glass. This was due to the low RI (i.e., 1.37) of the transparent cured PFPE layer which gave rise to an increased transmission owing to the insertion of an intermediate RI layer in between air (i.e., 1.00) and the glass substrate (i.e., 1.52) which suppresses the interfacial Fresnel’s reflection. The durability and stability of the coatings were investigated using QUV Accelerated Weathering Tester with UVA-340 lamp irradiated at 60°C. It was observed that the transmittance, AR and superhydrophobic properties of the coatings remained unaltered even after 200 h of UV exposure due to the higher bonding energy of CdF bonds, that is, 488 kJ mol1. This indicates that the coatings can be used as solar cells with higher withstanding efficiency toward long exposure to sunlight. Thus, the authors concluded that the methodology adopted for the fabrication of organic solar cells using one-step UV-assisted replica molding with acrylate-functionalized PFPE precursor on ITO glass can have practical applications in the field of solar cells due to its large surface area, dual scale roughness, low surface energy, and low RI. Gan et al. [30] fabricated AR transparent polymer-coated graphene/silicon (G/Si) electrodes for application in Schottky junction PV solar cells. In comparison to the conventional monocrystalline Si solar cells, the assembly of G/Si only requires the transfer of graphene films onto silicon substrates at room temperature thereby avoiding hightemperature furnaces and vacuum systems. This assembly has also led to achieving the PCE of nearly 2% [31]. Apart from this method chemical doping, creation of micronanostructure on Si substrate and light trapping with antireflection coatings (ARC) can be other alternative strategies to increase antireflection properties and PCEs (10%). [32, 33]. Polymers are also novel candidates which can be applied in the field of ARC coatings as compared to inorganic materials owing to their lightweight, flexibility, stretching, and bending capabilities. The authors demonstrated the applicability of polymethylmethacrylate (PMMA) ARC coatings on G/Si substrates as transparent conductive electrodes with excellent light absorption for solar cell applications. The optical property of the PMMA ARC coatings on G/Si substrates was investigated using
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transmission electron microscopy (TEM). It was observed that the transmittance of PMMA/G-Si ARC coatings (i.e., 97.4%) was slightly lower than the neat graphene coatings (i.e., 96%) in the region of 400–800 nm. However, the PV performance of the PMMA/G-Si ARC coatings was superior as compared to the uncoated ones with PCE of 6.55%. The observed phenomenon was due to the increase in short circuit current density (Jsc), which reduces the resistance of Schottky junctions thereby enhancing the light absorption ability. For further improvement in the PCE of the PMMA/G-Si ARC coatings, it was chemically doped via HNO3 vapor treatment, which indicated an increase in PCE to about 13.34%. Thus the above methodology can be a cost-effective technique for the fabrication of solar cells with excellent light transmission ability (Fig. 9 reproduced from Ref. [30]) (Fig. 6). Leem et al. [34] fabricated highlytransparent antireflective moth-eye nanopatterned UV cured Norland Optical Adhesive (NOA) 63 coatings (ARC) on glass for application in solar PV systems. Glass has been a candid material for application in the field of optoelectronics due to its high optical transparency, low cost, and good thermal stability. They are basically used to safeguard the PV systems against UV radiation, external shock, and corrosive acidic environment. However, the optical reflection of glass degrades with time due to its high RI (nearly 1.5) which can reduce the PCE of solar cells. Hence, in order to overcome this disadvantage, we require ARCs that can reduce the optical losses caused by Fresnel surface reflection (FSR). The FSR phenomenon of glass can be decreased via the formation of biomimetic moth-eye structures on the surface of the glass, composed of tapered conical gratings having wavelengths smaller than the incident light [35–39]. In recent decades, soft imprint lithographic method has gained momentous attention in
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Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
comparison to other nanolithography techniques for fabricating micro- or nano-patterns onto UV-curable poly-dimethylsiloxane (PDMS) polymers. PDMS offers various advantages which include low free surface energy, flexibility, transparency, and hardness owing to its simplicity, low-cost, and tenability. In addition, the NOA63 polymers are quite beneficial as ARC owing to their similar RI of 1.56 with almost no absorption in the wavelength region between 350 and 1800 nm. The author observed that less work has been carried out regarding the performance improvement of III–V semiconductor-based multijunction PV module systems with conical nanogratings (NGs) patterned polymer films. Hence, the author investigated the effect of UV, thermal, and wetting behavior on the efficiency behavior of PDMS/NOA63 NG ARC coatings. The NOA63 NGs coatings exhibited a hydrophobic surface with a contact angle of 112 degrees and solar-weighted transmittance (Tsw) of 93.2%. Superhydrophobic polymeric coating surface with CNT covering improves the solar cell by producing a large contact angle. Zhao et al. [40] has been successfully synthesized the silica-covered CNT by using sol-gel method and they described that the surface energy of the polymeric material was significantly reduced with static WCA of 156 degrees, thereby making the surface superhydrophobic. This also shows good acid, alkali, and aging resistance, which might be used as a good practical application in the future. Similarly, Jinguang and Qi [41] have been described the recent development in the fabrication and ARC surfaces based on nanostructure arrangement of silicon and nonsilicon materials. Moreover, Prado et al. [2] described the antireflective hydrophobic coatings based on porous silica and found the increase in transmission of light for wavelength range of 350–850 nm; which would be helpful for depositing a hydrophobic layer of thickness less than 3.5 nm without destroying any properties of ARC. Hiralal et al. demonstrated the ZnO nanowire-based ARCs for organic PV solar cell and found that the coating produces the additional benefits to the solar cell such as it lowers the rate of degradation and also lowers the absorption of UV radiation. The presence of ZnO on AR coating is a photocatalytic semiconductor in nature which catalyzes redox reactions in the presence of water and oxygen thereby permitting the breakdown of organic molecules on its coating surface. This can be used as a self-cleaning agent for organic materials for reduction issues from soiling [42].
5. Fabrication of superhydrophobic coatings Wetting behavior is generally classified into four different categories based on the WCA (θ), that is, 0 degrees < θ < 10 degrees (superhydrophilic), 10 degrees < θ < 90 degrees (hydrophilic), 90 degrees < θ < 150 degrees (hydrophobic), and 150 degrees < θ < 180 degrees (superhydrophobic). Recent decades have witnessed momentous attention toward the development of superhydrophobic surface. Polymers reinforced with nanofillers with distinctive features like low cost, good processability, flexibility, good scratch,
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and abrasion resistance have been used as candid materials for the fabrication of superhydrophobic coatings [43]. The two important factors which are responsible for the fabrication of superhydrophobic coatings include roughening of the surface and chemical modification of coatings with low surface energy materials [44]. Different models such as Wenzel’s and Cassie’s are used for correlating and understanding the relation between superhydrophobicity phenomenon, surface roughness, and chemical modification [15, 45, 46].
6. Future prospect and challenges From the above literature findings, it is imperative that the AR coating materials have shown rapid application with potential prospects in the field of lenses, lasers, solar cells, diodes, optical and optoelectronic devices, screens, sensors, antiglare glasses for automotive applications, and even military equipment. It is necessary to focus some parameters of investments such as efficiency, stability, energy yield, and span of life instead of focusing solely on initial capital investment because the capital cost of installation of the solar panel is too high. Hence, the future scope can be divided into two parts; wherein, the first part aims to the improvement of the materials performance by continuous optimizing process and the second part aims to build a request on manufacturing technology. The performance improvement includes many multifunctional activities such as selfcleaning, light-trapping, durability, and so on [12]. Moreover, ARCs should have significant properties such as thermal, mechanical, long term durability, etc. This is due to the high mechanical strength of the SHAP coating makes the material suitable for selfcleaning and AR. Hence, high power conversion and cost reduction are obtained. Hybrid ARCs with self-cleaning ability have found extensive application in low emission applications. The conventional single and bilayer ARCs are still used for laser applications owing to minimum reflectance at smaller wavelengths. However, in the present era, the single and bilayer coatings are replaced by advanced ARCs that is, moth-eye ARCs due to its economic feasibility. Bionic nanostructures based on moth eye have resolved the issues related with omnidirectional ARCs to be used for different optical equipment and solar cells with improved transmittance. The fabrication process is also cost-effective and feasible for stacking multilayer coatings. In recent decades, ARCs have found wide application in the field of PV cells; however, the coatings lack mechanical resilience and strong adhesion strength. In addition, ARCs used for outdoor application gets peeled off from the substrate due to poor adhesion strength. For instance, ARCs based on ceramic material show brittle fracture due to microcracks and voids. Thus, it is imperative that the mechanical strength, adhesion strength, and thermal stability of the ARCs need to be improved for finding applications in harsh
Superhydrophobic antireflective polymer coatings with improved solar cell efficiency
environmental conditions. The other factors which need prime attention are the compressive and tensile failure of ARCs. The prime focus should be toward the development of polymer ARCs reinforced with nanofillers, which exhibits ductile failure with improved mechanical stability. Another area which needs attention is the development of cost-effective single ARCs and hybrid multifunctional ARCs with improved mechanical strength, thermal stability, superamphiphobic property, scratch resistivity, and antiglare property for finding application in the field of solar cells. Hence, the developments of innovative materials for SHAP coating techniques are the most essential part to expand the range of applications which can lead to the large-scale production in the industry. In addition, it is also required to carry out accurate measurements and different models for better understanding of the fabrication of SHAP coatings and to overcome the above challenges. The following are some of the main parts in which the research could be directed to resolve issues: • Although ARCs have exemplified their application in the field of solar PV still it poses some challenges owing to their performance over time period. • The biological aspect of antireflection remains unsought which could be explored by considering photonic nanostructures in butterflies, squids, etc. • Novel developments in optical devices provide a platform containing the immense potential for customization of ARCs to meet the requirements of the cutting edge technology and product improvisation.
7. Conclusions In the present chapter, the recent progress in SHAP coatings has been described in detail. But, there are still some issues for the fabrication process in the manufacture of SHAP coatings. Based on the findings polymeric ARCs can be used to increase the efficiency, antireflection or transmission, and self-cleaning performance which encourage the technology or industry to increase the mechanical strength of the developed products. ARCs have been used in a wide range of applications starting from optical and optoelectronic devices to automotive and aerospace applications. Further, in other fields such as electrochromism and green architectural strategies have also led to the integration of AR property and electrochromism into unity. The performance characteristics of the solar cell also get impaired by the accumulation of dust, microbes, and moisture. The development of hydrophobic, hydrophilic, and antimicrobial coatings is also achieving greater demand due to its hybrid characteristics. In conclusion, a simple, cost-effective with high-performance solar cell can be developed which might provide new ideas and new concept for the development of SHAP coating surfaces.
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