Self-Cleaning Microcavity Array for Photovoltaic Modules - ACS

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2929−2936

Self-Cleaning Microcavity Array for Photovoltaic Modules Felix Vüllers,† Benjamin Fritz,‡ Aiman Roslizar,† Andreas Striegel,† Markus Guttmann,† Bryce S. Richards,†,‡ Hendrik Hölscher,† Guillaume Gomard,†,‡ Efthymios Klampaftis,*,† and Maryna N. Kavalenka*,† †

Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Light Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), Engesserstrasse 13, 76131 Karlsruhe, Germany Downloaded via UNIV OF VIRGINIA on November 15, 2018 at 01:47:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Development of self-cleaning coatings is of great interest for the photovoltaic (PV) industry, as soiling of the modules can significantly reduce their electrical output and increase operational costs. We fabricated flexible polymeric films with novel disordered microcavity array (MCA) topography from fluorinated ethylene propylene (FEP) by hot embossing. Because of their superhydrophobicity with water contact angles above 150° and roll-off angles below 5°, the films possess selfcleaning properties over a wide range of tilt angles, starting at 10°, and contaminant sizes (30−900 μm). Droplets that impact the FEP MCA surface with velocities of the same order of magnitude as that of rain bounce off the surface without impairing its wetting properties. Additionally, the disordered MCA topography of the films enhances the performance of PV devices by improving light incoupling. Optical coupling of the FEP MCA films to a glass-encapsulated multicrystalline silicon solar cell results in 4.6% enhancement of the electrical output compared to that of an uncoated device. KEYWORDS: solar cells, photovoltaics, superhydrophobic, self-cleaning, antisoiling, microstructured

1. INTRODUCTION Soiling of photovoltaic (PV) modules is a term that describes the process of particulate matter or dust (particles with sizes of 500 μm and below) accumulation on their front surfaces.1 Dust in the atmosphere, which is often referred to as aeolian dust, includes soil elements such as sand, clay, or pollen lifted by the wind, pollution caused by power plants and vehicles, volcanic activity, textile fibers, animal cells, and other types of contaminants.1−3 Soiling has a negative impact on the energy yield of PV devices due to increased reflection and absorption of incoming photons by deposited particles. Quantification of this impact is not trivial because both the soiling rate and composition as well as natural cleaning due to wind and rainfall are strongly dependent on local weather conditions.1−7 The resulting energy yield reduction can be up to 50%, especially in arid regions where solar insolation is high and rainfall is low, thus making PV installations particularly attractive.1−6,8 Manual cleaning of PV modules is both labor- and resourceintensive and can significantly increase their operational costs. Researchers have proposed a number of alternative solutions, including stowing PV arrays during the night, mechanical cleaning systems, cleaning processes based on air flow and vibrations, electrodynamic screens, and incorporation of superhydrophobic or superhydrophilic coatings.1−5,9 Superhydrophobic surfaces and their self-cleaning properties have © 2017 American Chemical Society

been identified as one of the best solutions to prevent soiling, and satisfy most requirements for commercial implementation, as they are a passive long-term solution and drastically reduce maintenance costs compared to those of active cleaning procedures.1 A frontside coating has to be mechanically robust with minimal corrugation depth, so that impacting particles do not damage it or get trapped in the surface roughness, and it has to be made from a hydrophobic and water-insoluble material to reduce adhesion of particles, organic compounds, and salts, which lead to cementation and soiling.10,11 The selfcleaning ability of superhydrophobic surfaces is well known from the studies of surfaces of aquatic plant leaves and insects.12−15 Water droplets on superhydrophobic surfaces form a Cassie−Baxter wetting state and easily roll over the surface and carry away contaminants, resulting in selfcleaning.16,17 Artificial nano- or microstructured coatings are widely investigated in the context of PV.18−23 PV modules are often structured with antireflective coatings such as the motheye structure, thus reducing Fresnel reflection losses and improving light harvesting and power conversion efficiency of PV devices.18,19,21,24 Recently, nano- and microstructures on Received: October 13, 2017 Accepted: December 29, 2017 Published: December 29, 2017 2929

DOI: 10.1021/acsami.7b15579 ACS Appl. Mater. Interfaces 2018, 10, 2929−2936

Research Article

ACS Applied Materials & Interfaces

Finally, the Ni-shim was removed from the substrate through wet chemical dissolution of silicon wafer and cleaned by oxygen plasma for 150 min at 22 °C, 1200 W, 450 mTorr (STP2020, R3T, Germany). FEP microcavity array (MCA) films were fabricated from 130 μm fluorinated ethylene propylene (FEP) (FEP500c, DuPont Corp) sheets (refractive index n = 1.34) using hot embossing. FEP was placed on a poly(tetrafluoroethylene) (PTFE) sheet sitting on a polished steel plate, and the Ni-shim was embossed into FEP at 275 °C with 10 kN.33−35 The microstructured FEP film is peeled off the shim without further treatment. 2.2. Characterization Techniques. Fabricated films were imaged by scanning electron microscopy (SEM) (Supra 55P, Zeiss, Germany) at 3 kV. Cross-sectional SEM images were used to determine the aspect ratio (AR) of μ-cavities on nanofur using a self-written MATLAB code (MathWorks Inc.). Static contact angles of 3 μL water droplets on FEP were measured with a contact angle goniometer (OCA 15Pro, Dataphysics, Germany). A goniometer equipped with an electronic tilting base unit (TBU90E, Dataphysics, Germany) was used to determine roll-off angles by depositing 12 μL droplets and tilting the surface until the droplet rolls off. Self-cleaning properties were investigated by attaching tested surfaces to glass slides tilted at 10, 25, 35, and 45° before contaminating them with graphite powder (size: 30−900 μm) (Merck, Germany) or sea sand (size: 100−315 μm) (Merck, Germany) and manually dispensing water droplets on the surface. Bouncing droplet experiments were recorded using a highspeed camera (Fastec IL3, Fastec Imaging Corporation). MCA films were optically coupled to a glass-encapsulated laser-cut screen-printed multicrystalline silicon solar cell (E-Ton Solar, Taiwan) by dispensing droplets of a refractive index-matching liquid (n = 1.50; Cargille Laboratories) on the backside of FEP MCA. Films were then placed on the solar cell, and sufficient manual force was applied to eliminate the air gap between FEP MCA and the solar cell. A xenon lamp and a monochromator-based spectral response system (Oerlikon Solar, Switzerland, monochromatic beam area 0.05 cm2) were used to determine the external quantum efficiency (EQE) under one sun bias light (biased area 2.48 cm2). The EQE was measured at three different spots for unstructured FEP and bare PV device and at three spots each on three different samples for the FEP MCA under normal incidence and subsequently averaged. I−V curves were obtained using a solar simulator classified to class AAA (WXS-90S-L2, WACOM, Japan) and a sourcemeter (Keithley 2400, Tektronix) with an aperture area of 2.48 cm2. Three spots were measured for each architecture, and the results were averaged. 2.3. Ray Tracing Simulations. All simulations were performed with commercial ray tracing software LightTools (Synopsys GmbH). The MCA was modeled by subtracting spheroids with radius r and depth h from a cuboid base. Spheroids are placed in a hexagonal array with lattice constant a = 3 r. Microhairs are modeled as 100 μm high upright cones and positioned at μ-cavities corners. A rectangular light source with arbitrary dimensions is positioned over the cavity array and for oblique light moved to the side to illuminate the surface area regardless of angles of incidence (AOI). A detector is positioned at the bottom of the model, collecting the transmitted light angular distribution. The model sidewalls are perfectly reflective. The reflection coefficient is calculated from a far-field detector. A total of 106 rays per unit cell were simulated, as convergence tests show this amount to give a reliable simulation result. Fluctuating cavity aspect ratios were investigated following Fritz et al.36 The mean aspect ratio (AR) and standard deviation (σAR) were determined from crosssectional SEM images, and individual cavities in the simulation were assigned aspect ratios from a normal distribution based on AR and σAR, with a fixed cavity radius.

PV module surfaces were used to achieve superhydrophobicity to reduce dust accumulation and, thus, mitigate losses in electrical output.1,22 However, not much research has been done on combining superhydrophobicity and reduced Fresnel reflection in one coating.25 Inspired by the surface topography of trichome-covered water plants, we recently developed highly transmissive superhydrophobic polycarbonate (PC) nanofur films using a combination of hot embossing and hot pulling techniques.26 Their self-cleaning properties originate from a dense network of randomly distributed nano- and microhairs decorating the edges of microcavities.27,28 In this work, we develop superhydrophobic self-cleaning coatings that simultaneously enhance the electrical output of PV devices due to improved light harvesting. It is achieved by employing polymers that are already in use in the PV industry and realized using a simplified patterning process involving hot embossing. First, the bioinspired nanofur coating is deconstructed into a disordered microcavity array (MCA) and a layer of nano- and microhairs, and the influence of these structural components on optical properties is studied. Next, we develop a scalable fabrication process to transfer the disordered MCA topography into fluorinated polymers. Fluorinated polymers are promising candidates for obtaining targeted coating functionalities due to their low surface energy and refractive index, which are advantageous toward achieving superhydrophobicity and low Fresnel reflection at the PV module frontside, respectively. Such polymers are used by the PV industry to produce flexible and light-weight PV modules.29 In this work, we use fluorinated ethylene propylene (FEP), which is a highly transmissive, UV-resistant, chemically inert, and mechanically stable polymer developed as a top cover for solar cells among other applications.30 The presented fabrication method is based on hot embossing of the disordered MCA into FEP films. We demonstrate the self-cleaning properties of the flexible FEP MCA films over a wide range of tilt angles and contaminant sizes and analyze the stability of their wetting properties against impacting droplets. Finally, the optical properties of the fabricated FEP MCA films and their impact on photocurrent generation when integrated atop glass-encapsulated multicrystalline silicon PV devices are investigated.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Microcavity Array Films. Thin nanofur films were fabricated from optical-grade polycarbonate (PC) (Makrolon LED2045, Bayer, Germany) (refractive index n = 1.58) using a combination of hot embossing and hot pulling, as described in previous studies.26−28 Nanofur with exposed μ-cavities was produced by oxygen plasma treatment of nanofur for 480 s at 100 W and 100 mTorr using a reactive ion etching system (Sentech GmbH, Germany). The nickel shim (Ni-shim) was fabricated by gluing nanofur films to a silicon wafer before full metallization with 8 nm chromium (adhesive layer) and 150 nm gold (conductive plating base) using evaporation techniques. After electrically connecting the polymer film to wafer with a copper tape, the substrate was masked with a green tape, creating a 1 mm thick and 22.5 mm × 22.5 mm large plating window. Then, the masked wafer was fixed to a custom-built plating holder and immersed in a standard nickel electroplating system with boric acid containing nickel sulfamate electrolyte (T = 52 °C, pH ranging from 3.4 to 3.6), which was specifically developed for nickel electroforming of microand nanostructures.31,32 To achieve a defect-free filling of nanostructured areas, the current density was subsequently increased from 0.1 A·dm−2 at the start of the plating process to 1.0 A·dm−2. This way, a 600 μm nickel layer was electroformed, resulting in a stiff homogeneous shim, able to withstand forces during replication.

3. RESULTS AND DISCUSSION 3.1. Experimental and Numerical Analyses of the Optical Properties of Microcavity Arrays (MCAs). Plant surfaces covered in nano- and microstructures often combine self-cleaning properties with high optical transmission. Inspired by such surfaces, we previously fabricated superhydrophobic 2930


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Figure 1. Experimental and numerical analyses of optical properties of bioinspired nanofur. (a) SEM images of polycarbonate (PC) nanofur, consisting of a dense hair layer supported by a disordered MCA (left). Oxygen plasma treatment removes most micro- and nanohairs and exposes underlying μ-cavities (right). (b) Experimental reflection spectra of nanofur (as-prepared) and of oxygen plasma-treated nanofur (exposed μ-cavities) under normal incidence. Partial removal of micro- and nanohairs reduces reflection by 2%. (c) Three-dimensional model of PC MCA with incorporated microhairs, which are simulated as cones and positioned at cavity corners. (d) Reflection of simulated PC MCA with uniform and disordered AR, PC MCA with incorporated hairs, and experimental measurements of nanofur at λ = 600 nm as a function of angle of incidence.

bioinspired nanofur films from PC using hot pulling (Figure 1a).26−28,37 Its topography consists of a layer of dense high aspect ratio micro- and nanohairs and a disordered MCA supporting the hair layer, as shown in Figure 1a. Such topography is a result of separating a heated sandblasted steel plate from the softened polymer during hot pulling. To distinguish between the impact of micro- and nanohairs and the impact of microcavities on the optical properties, nanofur was exposed to oxygen plasma in a dry etching step. Thus, the majority of the micro- and nanohairs is removed and the underlying microcavities are exposed (Figure 1a). Reflection spectra of nanofur (original) and nanofur with exposed microcavities (μ-cavities) under normal incidence are shown in Figure 1b. Partial removal of the hairs lowers the reflection losses by approximately 2%. To evaluate the influence of the disordered μ-cavity topography and verify the impact of the microhairs on the optical properties of the films under varying angles of incidences (AOIs) of incoming light, a simplified model of the MCA with the option to include microhairs was developed and numerical simulations were carried out. The MCA model with incorporated microhairs is displayed in Figure 1c. As dimensions of the μ-cavities are significantly larger than the considered wavelengths of 300−1200 nm, optical properties are solely governed by ray optics and, thus, ray tracing simulations were conducted to obtain reflection spectra. The modeled cavities are positioned in a hexagonal array with lattice constant a = 3 r, with r being the cavity radius, thus resulting in a complete tiling of the base. The nanofur μ-cavities have an average aspect ratio AR = 0.45, which was experimentally derived from cross-sectional scanning electron microscope (SEM) images. Moreover, numerical simulations of MCA with varying ARs, described in detail in the Experimental Section, have shown minimal reflection values for AR = 0.5. Simulated reflection values of PC MCA with and without AR disorder as a

function of AOI and simulated reflection of PC MCA with microhairs are compared with those from experimental reflection measurement of as-prepared nanofur in Figure 1d. Because reflection does not significantly vary over the spectral range, the reflection values are taken at λ = 600 nm, as to be close to the solar irradiance peak. With increasing AOI, reflection of nanofur increases exponentially up to 25%. The exponential increase in reflection is also observed for simulated PC MCA with incorporated microhairs. The increased reflection of nanofur and MCA with hairs can be explained by the change in the cross-sectional area of hairs with increased AOI. For low AOI, microhairs do not cover a significant area due to their small diameter, thus not influencing the reflection. However, for a higher AOI, the cross-sectional area of the hairs becomes the effective area responsible for reflection and, being more similar to an unstructured polymer, similarly increases reflection. Additionally, light hitting the surface at high AOI has to transmit through several microhairs, thus increasing the number of refractive interfaces the light has to transmit through and increasing reflection as a result. For MCA simulated without microhairs, reflection values are below 7% for all AOIs. To investigate the effect of μ-cavities with varying ARs on the film reflection, the diameter of modeled μ-cavities was fixed while varying their height, thus introducing disorder into the AR of the simulated MCA. The AR of nanofur μ-cavities follows a normal distribution with average AR = 0.45 and standard deviation σAR = 0.13. On the basis of this distribution, modeled μ-cavities were assigned an AR. The introduced disorder increases reflection by less than 0.2% (Figure 1d), as was previously shown by Fritz et al. for microstructured surfaces composed of microcones.36 Thus, the prevalent structural disorder does not need to be adjusted for replication of the MCA introduced in the next section. The increase in reflection of nanofur with increasing AOI compared to that of MCA can, therefore, be attributed to the presence of microhairs 2931


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Figure 2. Fabrication of FEP MCA. (a) Schematic of shim fabrication and the hot embossing process. PC nanofur is galvanically copied by electroforming to fabricate a Ni-shim. The heated shim is pressed into FEP foil resting on a PTFE plate, thus embossing the MCA topography into FEP. The Ni-shim is removed after cooling, resulting in a microstructured FEP film. (b) SEM image of the electroplated Ni-shim. (c) SEM image of the embossed FEP MCA. (d) Cross-sectional SEM of FEP MCA reveals inverted hemispherical indentations in FEP film.

being transferred from the steel to the backside of the FEP film. An SEM image of the embossed FEP MCA surface and a crosssectional SEM image of the film are shown in Figure 2c,d. The embossed structure consists of inverted hemispheres previously observed on PC nanofur. The electroplated Ni-shim was used in more than 70 consecutive fabrication cycles without any signs of wear. Additionally, because of low complexity of shim fabrication, the fabrication process can be scaled up to a continuous roll-to-roll embossing manufacturing.33 3.3. Wetting and Self-Cleaning Properties of MCA Films. To characterize wetting properties of the FEP MCA films, we measured their water contact and roll-off angles. Increasing the surface roughness of polymer films was previously shown to enhance their wetting properties.39−41 Flat FEP is hydrophobic with high water contact angle, θW = 109.63 ± 0.65° (Figure 3a). However, water droplets stick to flat FEP up to approximately 41° tilt, after which they start to creep along the surface. The disordered MCA structure increases the surface roughness of FEP film and results in a superhydrophobic surface with a water contact angle θW = 157.50 ± 5.60° and low roll-off angle θWS = 4.81 ± 3.02°, as shown in Figure 3a−c.

and, as a result, makes a bare MCA more advantageous for applications where high transmittance is necessary, such as PV. 3.2. Fabrication of a Fluorinated Polymer MCA. To achieve superhydrophobic and self-cleaning properties on materials with high surface energy, such as PC, high AR nano- and microstructures with minimal solid water contact area are required. For low AR MCA structures, polymers with low surface energy, such as fluorinated polymers, have to be used to achieve these properties. FEP has high chemical and mechanical resistance against impact and tearing, making it a viable replacement of the commonly used glass top cover of PV modules.30,38 The fabrication process, shown in Figure 2a, was developed to transfer the disordered MCA topography to FEP. The PC nanofur film was galvanically copied by nickel electroforming to create a Ni-shim consisting of densely packed microhemispheres (Figure 2a,b).27,31,37 The Ni-shim was then glued to a steel plate, and a 130 μm thick 2 cm × 2 cm FEP foil was placed between a poly(tetrafluoroethylene) (PTFE) sheet and the shim. In a hot embossing step, the shim is heated above the melting temperature of FEP (Tm = 270 °C) before pressing it into the FEP foil and cooling it down to room temperature.33,35 Using a PTFE sheet between the FEP foil and the steel counterplate prevents roughness from 2932


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Lotus-Effect and self-cleaning properties of FEP MCA were tested at tilt angles ranging from 10 to 45°, thus including optimal tilt angles for most of the world.45 Both MCA and unstructured FEP were contaminated with graphite powder before consecutive water droplets were dispensed on both surfaces. Graphite powder has a large size distribution (30−900 μm) and, therefore, tests the self-cleaning properties for a broad range of contaminant sizes. Figure 3d and Movie M1 (Supporting Information) demonstrate the self-cleaning effect observed on FEP MCA compared to that on unstructured FEP at 10° tilt angle. Independent of the tilt angle, dispensed water droplets roll down the MCA surface, pick up the contaminants, and clean the surface in the process (Figure S1, Supporting Information). The droplets dispensed on unstructured FEP stick to the surface, resulting in no self-cleaning effect (Figure 3d). Optical microscope images of MCA and flat FEP after cleaning (insets in Figure 3d) show flat FEP contaminated with graphite particles, whereas the MCA surface is free of contamination. As soiling of PV modules is of particular importance in arid regions, where dust and sand are two of the most common contaminants, the self-cleaning properties of MCA were additionally tested with sea sand at tilt angles ranging from 10 to 35° (Figure S1). In all tests, the contaminants have successfully been cleaned from the MCA surface, whereas the unstructured FEP displayed no selfcleaning behavior. Rain is a natural water source necessary for self-cleaning of PV devices. When water droplets impact a superhydrophobic surface, a Cassie−Baxter to Wenzel wetting state transition can occur, resulting in droplets sticking to the surface and, thus, loss of the self-cleaning properties. Moreover, droplets from dust rain can deposit particulate matter upon drying, further contaminating PV devices.47,48 Therefore, the surface wetting behavior has to be highly stable against the impact of water droplets at various speeds. To test the stability of the surface and its wetting properties against impacting water, we studied the behavior of water droplets (13 μL) dispensed from heights of 5 and 10 cm, which correspond to 1.08 ± 0.05 and 1.38 ± 0.04 m·s−1, respectively, onto FEP MCA and unstructured FEP tilted at 0, 10, 20, and 30°. The droplet velocities are of the same order of magnitude as the velocities of rain droplets, which range from 2 to 9 m·s−1.49 Droplet movements were recorded with a high-speed camera, and time-lapse images of droplets falling on unstructured FEP and FEP MCA are shown in Figure 4. On unstructured FEP, the droplet hits the surface, deforms, and recovers its hemispherical shape, while constantly being in contact with the substrate. Because of tilt and inherent hydrophobicity of FEP, the droplet slides a short distance down the slope before coming to rest and firmly sticking to the surface. The droplet impacting the MCA also deforms, but because of the surface superhydrophobicity, it does not wet the surface and reforms before being propelled away from the surface. Upon further impact on the MCA, the droplet again deforms and bounces off, until enough kinetic energy is transferred, and the droplet simply rolls off the surface. A higher drop velocity allows droplets impacting the smooth FEP to slide down longer distances, but they still come to a complete stop, whereas droplets impacting the MCA bounce off the substrate independent of tilt angles and drop velocity, as shown in Movies M2 and M3 (Supporting Information). The FEP MCA films possess stable wetting and self-cleaning properties under contamination and impacting droplets and, as a result, have superior antisoiling properties compared to

Figure 3. Wetting and self-cleaning properties of superhydrophobic FEP MCA. (a) Colored water droplets dispensed on flat FEP and FEP MCA form a hemisphere and a sphere, respectively, demonstrating superhydrophobicity of FEP MCA. (b) Photograph of a 3 μL droplet on FEP MCA with a water contact angle θW > 155°. (c) Series of photographs of a 12 μL water droplet rolling off the MCA at a sliding angle θWS < 5°. The contour of the moving droplet is highlighted to emphasize its motion. (d) Self-cleaning properties of MCA (right) compared to those of unstructured FEP (left). Graphite particles are dispersed on both surfaces tilted at 10°. Dispensed water droplets adhere to the unstructured FEP without cleaning the surface. On MCA, water droplets roll down the substrate, picking up the contaminants and effectively cleaning the surface. Magnifications show optical microscope images of contaminated smooth FEP and cleaned MCA after the self-cleaning test.

Most micro- and nanostructured superhydrophobic surfaces exhibit self-cleaning properties also known as the “LotusEffect”, which are of particular interest for PV devices as a way to reduce soiling.2,5,8,9,17,42,43 These self-cleaning properties are a result of minimal solid-contaminant contact area, which reduces the adhesion of contaminants to the surface. Thus, contaminants adhere stronger to water droplets than to the structured surface, enabling droplets to pick them up and carry them off the surface. For a more detailed explanation of this effect, the reader is referred to the studies of Fürstner et al. from 2005 and Barthlott and Neinhuis from 1997.17,43,44 To maximize their efficiency, PV modules are installed at different tilt angles depending on the latitude of installation, the fraction of diffuse versus direct sunlight, and local shading effects resulting from hills, buildings, or trees.45,46 Therefore, the 2933


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Figure 4. Water droplets impacting flat and MCA FEP films at 1.08 ± 0.05 m·s−1. Image series of a 13 μL water droplet sticking to a 30° tilted unstructured FEP (a) and bouncing off a 30° tilted FEP MCA (b) upon impact, demonstrating the antiwetting stability of FEP MCA against droplet impact.

Figure 5. Electrical characterization FEP MCA-coated PV devices. (a) I−V curves and (b) external quantum efficiency (EQE) curves of a glassencapsulated silicon solar cell and a glass-encapsulated silicon solar cell with optically coupled flat FEP and FEP MCA films under simulated sunlight (air mass 1.5 global spectrum) and normal incidence.51 Attachment of FEP increases the electrical gain by 3.7%, and FEP MCA increases it by an additional 0.9%. The electrical enhancement is most pronounced in the visible spectral region from λ = 400 to 700 nm, as seen in the relative gain in EQE shown in the inset.

efficiency (EQE) measurements of glass-encapsulated solar cells coated with unstructured FEP film, coated with FEP MCA film, and uncoated. A broadband improvement over the entire spectral region relevant to electrical conversion of a solar cell can be seen. The improvement is smallest in the infrared regime, whereas it is highest in the visible regime between 400 and 700 nm, where the solar photon flux peaks, resulting in significant improvement in electrical gain (inset Figure 5b). In agreement with the I−V-curves, the introduction of FEP has a bigger impact on the EQE than on the microstructure of the film. However, the reported electrical gain of the FEP MCA might be hindered by unstructured ridges between the μcavities. Further adjustment of the fabrication parameters can eliminate the unstructured ridges and, thus, reduce reflection losses and maximize the electrical output. The impact of the FEP MCA on the electrical output of PV devices under different AOIs is an on-going investigation in our laboratory, and the results will be reported in future work.

those of unstructured FEP. Moreover, superhydrophobicity of FEP MCA allows the top cover of PV modules to stay dry, thus reducing surface conductivity. This additional effect can help mitigate potential-induced degradation known to reduce the electrical output and even cause failure of PV modules in field conditions.50 3.4. Application of FEP MCA Films to PV Modules. To evaluate the effect of self-cleaning FEP MCA on the electrical output of PV devices, the MCA film was optically coupled to a glass-encapsulated multicrystalline silicon solar cell. Additionally, an unstructured FEP film was characterized to resolve the contributions of reduced Fresnel reflection due to lower refractive index of FEP (n = 1.34) and the embossed MCA microstructure. The current−voltage (I−V) curves for a bare PV device compared with a PV device with flat FEP and with FEP MCA attached are displayed in Figure 5a. Coupling of unstructured FEP increases the short-circuit current (ISC) by 3.7% compared to that of the glass-encapsulated device, whereas coupling of FEP MCA results in a 4.6% gain. Thus, the 0.9% difference in gain can be solely attributed to the disordered MCA microstructure, whereas the rest of the improvement is due to the intermediate refractive index of the introduced FEP. Furthermore, Figure 5b displays the impact of a FEP MCA and unstructured FEP through external quantum

4. CONCLUSIONS In conclusion, we developed a polymeric film with disordered microcavity array (MCA) topography, which can serve as a selfcleaning top layer on PV devices that additionally enhances the electrical performance of PV by improved light incoupling. 2934


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investigations, results, literature, and mitigation approaches. Renewable Sustainable Energy Rev. 2013, 22, 698−733. (2) Mani, M.; Pillai, R. Impact of dust on solar photovoltaic (PV) performance: research status, challenges and recommendations. Renewable Sustainable Energy Rev. 2010, 14, 3124−3131. (3) Darwish, Z. A.; Kazem, H. A.; Sopian, K.; Al-Goul, M.; Alawadhi, H. Effect of dust pollutant type on photovoltaic performance. Renewable Sustainable Energy Rev. 2015, 41, 735−744. (4) Costa, S. C.; Diniz, A. S. A.; Kazmerski, L. L. Dust and soiling issues and impacts relating to solar energy systems: literature review update for 2012-2015. Renewable Sustainable Energy Rev. 2016, 63, 33− 61. (5) Jamil, W. J.; Rahman, H. A.; Shaari, S.; Salam, Z. Performance degradation of photovoltaic power system: review on mitigation methods. Renewable Sustainable Energy Rev. 2017, 67, 876−891. (6) Micheli, L.; Muller, M. An investigation of the key parameters for predicting PV soiling losses. Prog. Photovoltaics Res. Appl. 2017, 25, 291−307. (7) Maghami, M. R.; Hizam, H.; Gomes, C.; Radzi, M. A.; Rezadad, M. I.; Hajighorbani, S. Power loss due to soiling on solar panel: a review. Renewable Sustainable Energy Rev. 2016, 59, 1307−1316. (8) Sayyah, A.; Horenstein, M. N.; Mazumder, M. K. Energy yield loss caused by dust deposition on photovoltaic panels. Sol. Energy 2014, 107, 576−604. (9) Ragesh, P.; Anand Ganesh, V.; Nair, S. V.; Nair, A. S. A review on ‘self-cleaning and multifunctional materials’. J. Mater. Chem. A 2014, 2, 14773−14797. (10) Cuddihy, E. F. Theoretical considerations of soil retention. Sol. Energy Mater. 1980, 3, 21−33. (11) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H. A. Mechanically durable superhydrophobic surfaces. Adv. Mater. 2011, 23, 673−678. (12) Koch, K.; Barthlott, W. Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials. Philos. Trans. R. Soc., A 2009, 367, 1487−1509. (13) Darmanin, T.; Guittard, F. Superhydrophobic and superoleophobic properties in nature. Mater. Today 2015, 18, 273−285. (14) Webb, H. K.; Crawford, R. J.; Ivanova, E. P. Wettability of natural superhydrophobic surfaces. Adv. Colloid Interface Sci. 2014, 210, 58−64. (15) Wang, G.; Guo, Z.; Liu, W. Interfacial effects of superhydrophobic plant surfaces: a review. J. Bionic Eng. 2014, 11, 325−345. (16) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (17) Fürstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Wetting and self-cleaning properties of artificial superhydrophobic surfaces. Langmuir 2005, 21, 956−961. (18) Tsui, K.-H.; Lin, Q.; Chou, H.; Zhang, Q.; Fu, H.; Qi, P.; Fan, Z. Low-cost, flexible, and self-cleaning 3D nanocone anti-reflection films for high-efficiency photovoltaics. Adv. Mater. 2014, 26, 2805−2811. (19) Chen, J.-D.; Zhou, L.; Ou, Q.-D.; Li, Y.-Q.; Shen, S.; Lee, S.-T.; Tang, J.-X. Enhanced light harvesting in organic solar cells featuring a biomimetic active layer and a self-cleaning antireflective coating. Adv. Energy Mater. 2014, 4, No. 1301777. (20) Heo, S. Y.; Koh, J. K.; Kang, G.; Ahn, S. H.; Chi, W. S.; Kim, K.; Kim, J. H. Bifunctional moth-eye nanopatterned dye-sensitized solar cells: light-harvesting and self-cleaning effects. Adv. Energy Mater. 2014, 4, No. 1300632. (21) Leem, J. W.; Kim, S.; Lee, S. H.; Rogers, J. A.; Kim, E.; Yu, J. S. Efficiency enhancement of organic solar cells using hydrophobic antireflective inverted moth-eye nanopatterned PDMS films. Adv. Energy Mater. 2014, 4, No. 1301315. (22) Park, Y.-B.; Im, H.; Im, M.; Choi, Y.-K. Self-cleaning effect of highly water-repellent microshell structures for solar cell applications. J. Mater. Chem. 2011, 21, 633−636. (23) Said, S. A.; Al-Aqeeli, N.; Walwil, H. M. The potential of using textured and anti-reflective coated glasses in minimizing dust fouling. Sol. Energy 2015, 113, 295−302.

Simulations have shown that MCA topography significantly decreases the reflection over all AOIs compared with nano- and microhair-covered microcavities. Moreover, we developed a fabrication technique to transfer the MCA to FEP, which can be used as top cover for PV modules. A Ni-shim was fabricated by electroforming of previously reported bioinspired nanofur. The shim was subsequently hot embossed into FEP to create FEP MCA. We demonstrate the self-cleaning properties of the fabricated FEP MCA for a wide range of tilt angles and contaminant sizes and show that water droplets easily bounce off the microstructured surface. Additionally, attaching FEP MCA to a glass-encapsulated multicrystalline silicon solar cell resulted in a 4.6% relative gain in the short-circuit current compared with the uncoated device. These properties are highly beneficial for outdoor applications and combined with high mechanical and UV-resistance will result in a long device lifetime with minimal maintenance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15579. Microscope images of self-cleaning properties of FEP MCA (PDF) Demonstration of self-cleaning properties of FEP MCA (Movie M1); water droplet impacting and bouncing off FEP MCA (Movie M2); water droplet impacting and sticking to unstructured FEP (Movie M3) (MPG) (MPG) (MPG)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.K.). *E-mail: [email protected] (M.N.K.). ORCID

Felix Vüllers: 0000-0002-9730-8540 Hendrik Hölscher: 0000-0002-1033-1669 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Frederik Kotz, Nico Keller, and Richard Thelen for support with characterization of wetting properties and Senta Schauer for fruitful discussions. F.V. gratefully acknowledges a scholarship from the Landesgraduiertenförderung Baden-Württemberg. G.G. acknowledges the support of the Helmholtz Postdoctoral Program. B.F. acknowledges support by the Karlsruhe School of Optics & Photonics (KSOP). A.R. acknowledges a scholarship from the Ministry of Higher Education of Malaysia and the Technical University of Malaysia Malacca. E.K. and B.S.R. acknowledge funding from the Helmholtz Association, including the Recruitment Initiative and Science and Technology of Nanosystems research programme. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/ knmf), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu.)

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REFERENCES

(1) Sarver, T.; Al-Qaraghuli, A.; Kazmerski, L. L. A comprehensive review of the impact of dust on the use of solar energy: history, 2935


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voltaic modules in Europe. Prog. Photovoltaics Res. Appl. 2008, 16, 47− 59. (46) Šúri, M.; Huld, T. A.; Dunlop, E. D.; Ossenbrink, H. A. Potential of solar electricity generation in the European Union member states and candidate countries. Sol. Energy 2007, 81, 1295− 1305. (47) Pye, K. Aeolian Dust and Dust Deposits; Elsevier, 2015. (48) Itani, G. N.; Smith, C. A. Dust rains deliver diverse assemblages of microorganisms to the Eastern Mediterranean. Sci. Rep. 2016, 6, No. 22657. (49) Byers, H. R. General Meteorology; McGraw-Hill: New York, 1959. (50) Luo, W.; Khoo, Y. S.; Hacke, P.; Naumann, V.; Lausch, D.; Harvey, S. P.; Singh, J. P.; Chai, J.; Wang, Y.; Aberle, A. G.; Ramakrishna, S. Potential-induced degradation in photovoltaic modules: a critical review. Energy Environ. Sci. 2017, 10, 43−68. (51) ASTM Standard G173-03, Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface. http://dx.doi.org/10.1520/G0173-03R12. DOI: 10.1520/g017303r12.

(24) Huh, D.; Shin, J.-H.; Byun, M.; Son, S.; Jung, P.-H.; Choi, H.-J.; Kim, Y.-D.; Lee, H. Analysis of long-term monitoring data of PV module with SiOx-based anti-reflective patterned protective glass. Sol. Energy Mater. Sol. Cells 2017, 170, 33−38. (25) Ren, T.; He, J. Substrate-versatile approach to robust antireflective and superhydrophobic coatings with excellent selfcleaning property in varied environments. ACS Appl. Mater. Interfaces 2017, 9, 34367−34376. (26) Vüllers, F.; Gomard, G.; Preinfalk, J. B.; Klampaftis, E.; Worgull, M.; Richards, B.; Hö lscher, H.; Kavalenka, M. N. Bioinspired superhydrophobic highly transmissive films for optical applications. Small 2016, 12, 6144−6152. (27) Röhrig, M.; Mail, M.; Schneider, M.; Louvin, H.; Hopf, A.; Schimmel, T.; Worgull, M.; Hölscher, H. Nanofur for biomimetic applications. Adv. Mater. Interfaces 2014, 1, No. 1300083. (28) Kavalenka, M. N.; Vüllers, F.; Lischker, S.; Zeiger, C.; Hopf, A.; Röhrig, M.; Rapp, B. E.; Worgull, M.; Hölscher, H. Bioinspired airretaining nanofur for drug reduction. ACS Appl. Mater. Interfaces 2015, 7, 10651−10655. (29) Flisom Flexible Solar Modules. www.flisom.com/wp-content/ uploads/2017/08/Datasheet_eFlex_3.1m.pdf. (30) The Chemours Company, Teflon FEP Fluoroplastic Film. www. chemours.com, 2016. (31) Leach, R.; Giusca, C.; Guttmann, M.; Jakobs, P.-J.; Rubert, P. Development of low-cost material measures for calibration of the metrological characteristics of areal surface texture instruments. CIRP Ann. 2015, 64, 545−548. (32) Schanz, G.; Bade, K. Microelectroforming of Metals. In Microengineering of Metals and Ceramics Part II: Special Replication Techniques, Automation and Properties; Wiley-VCH, 2005; pp 395− 420. (33) Worgull, M. Hot Embossing: Theory and Technology of Microreplication; William Andrew, 2009. (34) Greer, A. I. M.; Vasiev, I.; Della-Rosa, B.; Gadegaard, N. Fluorinated ethylene-propylene: a complementary alternative to PDMS for nanoimprint stamps. Nanotechnology 2016, 27, No. 155301. (35) Yeong, Y. H.; Gupta, M. C. Hot embossed micro-textured thin superhydrophobic Teflon {FEP} sheets for low ice adhesion. Surf. Coat. Technol. 2017, 313, 17−23. (36) Fritz, B.; Hünig, R.; Schmager, R.; Hetterich, M.; Lemmer, U.; Gomard, G. Assessing the influence of structural disorder on the plant epidermal cells optical properties: a numerical analysis. Bioinspiration Biomimetics 2017, 12, No. 036011. (37) Kavalenka, M. N.; Vüllers, F.; Kumberg, J.; Zeiger, C.; Trouillet, V.; Stein, S.; Ava, T. T.; Li, C.; Worgull, M.; Hölscher, H. Adaptable bioinspired special wetting surface for multifunctional oil/water separation. Sci. Rep. 2017, 7, No. 39970. (38) Martínez, A. L.; Menéndez, A.; Sánchez, P.; Andrés, L.; Menéndez, M.; Izard, J.; Sánchez, B.; Gómez, D. Solar photovoltaic technology on rough low carbon steel substrates for building integrated photovoltaics: a complete fabrication sequence. Sol. Energy 2016, 124, 216−226. (39) Vogler, E. A. Structure and reactivity of water at biomaterial surfaces. Adv. Colloid Interface Sci. 1998, 74, 69−117. (40) Quéré, D. Wetting and roughness. Annu. Rev. Mater. Res. 2008, 38, 71−99. (41) Marmur, A. Wetting on hydrophobic rough surfaces: to be heterogeneous or not to be? Langmuir 2003, 19, 8343−8348. (42) Barthlott, W.; Mail, M.; Neinhuis, C. Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications. Philos. Trans. R. Soc., A 2016, 374, No. 20160191. (43) Barthlott, W.; Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 1997, 202, 1−8. (44) Bhushan, B. Biomimetics - Bioinspired Hierarchical-Structured Surfaces for Green Science and Technology, 2nd ed.; Springer International Publishing, 2016. (45) Huld, T.; Šúri, M.; Dunlop, E. D. Comparison of potential solar electricity output from fixed-inclined and two-axis tracking photo2936


Self-Cleaning Microcavity Array for Photovoltaic Modules - ACS

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