Self-Cleaning Microcavity Array for Photovoltaic Modules

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Self-Cleaning Microcavity Array for Photovoltaic Modules Felix Vuellers, Benjamin Fritz, Aiman Roslizar, Andreas Striegel, Markus Guttmann, Bryce Sydney Richards, Hendrik Hoelscher, Guillaume Gomard, Efthymios Klampaftis, and Maryna N Kavalenka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15579 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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ACS Applied Materials & Interfaces

Self-Cleaning Microcavity Array for Photovoltaic Modules Felix Vüllers,

Guttmann,

†

†

Benjamin Fritz,

‡

Aiman Roslizar,

†,‡

Bryce S. Richards,

†Institute

†

Andreas Striegel,

†

Hendrik Hölscher,

∗,†

Efthymios Klampaftis,

†

Markus

Guillaume Gomard,

†,‡

∗,†

and Maryna N. Kavalenka

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

E-mail: [email protected]; [email protected]

Abstract Development of self-cleaning coatings is of great interest for photovoltaic (PV) industry, as soiling of the modules can signicantly reduce their electrical output and increase operational costs. We fabricated exible polymeric lms with novel disordered microcavity array (MCA) topography from uorinated ethylene propylene (FEP) by hot embossing. Due to their superhydrophobicity with water contact angles above 150◦ and roll-o angles below 5◦ , the lms posses self-cleaning properties over a wide range of tilt angles, starting at 10◦ , and contaminant sizes (30 µm - 900 µm). Droplets which impact the FEP MCA surface with velocities of the same order of magnitude as rain, bounce o the surface without impairing its wetting properties. Additionally, the disordered MCA topography of the lms enhances the performance of PV devices by improving light incoupling. Optical coupling of the FEP MCA lms to a glass-encapsulated 1

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multicrystalline silicon solar cell results in 4.6% enhancement of the electrical output compared to an uncoated device.

Keywords solar cells, photovoltaics, superhydrophobic, self-cleaning, anti-soiling, 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 bers, animal cells and other types of contaminants. 13 Soiling has a negative impact on the energy yield of PV devices due to increased reection and absorption of incoming photons by deposited particles. Quantication of this impact is not trivial, since both soiling rate and composition as well as natural cleaning due to wind and rainfall are strongly dependent on local weather conditions. 17 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. 16,8 Manual cleaning of PV modules is both labor- and resource-intensive and can signicantly 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 ow and vibrations, electrodynamic screens and incorporation of superhydrophobic or superhydrophilic coatings. 15,9 Superhydrophobic surfaces and their self-cleaning properties have been identied as one of the best solutions to prevent soiling, and which satisfy most requirements for commercial implementation, as they are a passive long term solution and drastically reduce maintenance costs compared to active cleaning 2


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procedures. 1 A front-side coating has to be mechanically robust with minimal corrugation depth, so impacting particles do not damage it or get trapped in the surface roughness, and it has to be made from hydrophobic and water-insoluble material in order to reduce adhesion of particles, organic compounds and salts, which lead to cementation and soiling. 10,11 The self-cleaning ability of superhydrophobic surfaces is well known from the studies of surfaces of aquatic plant leaves and insects. 1215 Water droplets on superhydrophobic surfaces form a Cassie-Baxter wetting state and easily roll over the surface and carry away contaminants, resulting in self-cleaning. 16,17 Articial nano- or microstructured coatings are widely investigated in the context of PV. 1823 PV modules are often structured with antireective coatings such as the moth-eye structure, thus, reducing Fresnel reection losses and improving light harvesting and power conversion eciency of PV devices. 18,19,21,24 Recently nanoand microstructures on PV module surfaces were used to achieve superhydrophobicity in order 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 reection in one coating. 25 Inspired by the surface topography of trichome-covered water plants, we recently developed highly transmissive superhydrophobic polycarbonate nanofur lms 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 simplied 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 inuence of these structural components on optical properties is studied. Next, we develop a scalable fabrication process to transfer the disordered MCA topography into uorinated polymers. Fluorinated polymers are promising candidates for obtaining targeted 3



coating functionalities due to their low surface energy and refractive index, which are advantageous towards achieving superhydrophobicity and low Fresnel reection at the PV module frontside, respectively. Such polymers are used by the PV industry to produce exible and light-weight PV modules. 29 In this work, we use uorinated 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 lms. We demonstrate the self-cleaning properties of the exible FEP MCA lms 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 lms and their impact on photocurrent generation when integrated atop glass-encapsulated multicrystalline silicon PV devices are investigated.

2

Experimental

Fabrication of Microcavity Array Films Thin nanofur lms

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. 2628 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 lms 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 lm to wafer with a copper tape, the substrate was masked with green tape creating a 1 mm thick and 22.5 mm x 22.5 mm large plating window. Then, the masked wafer was xed to a custombuilt plating holder and immersed in a standard nickel electroplating system with boric acid

4


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containing nickel sulphamate electrolyte (T = 52 ◦ C, pH ranging from 3.4 to 3.6), which was specically developed for nickel electroforming of micro- and nanostructures. 31,32 To achieve a defect-free lling 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 sti homogeneous shim able to withstand forces during replication. Lastly, Ni-shim was removed from the substrate through wet chemical dissolving of silicon wafer and cleaned by oxygen plasma for 150 min at 22 ◦ C, 1200 W, 450 mTorr (STP2020, R3T, Germany).

FEP microcavity array lms (MCA) were fabricated from 130 µm uorinated ethylene propylene (FEP) (FEP500c, DuPont Corp, USA) sheets (refractive index n = 1.34) using hot embossing. FEP was placed on a polytetrauorethylene (PTFE) sheet sitting on a polished steel plate, and the Ni-shim was embossed into the FEP at 275 ◦ C with 10 kN. 3335 Microstructured FEP lm is peeled o the shim without further treatment.

Characterization Techniques Fabricated lms 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 of µ-cavities on nanofur using self-written MATLAB code (MathWorks Inc., USA). 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-o angles by depositing

12 µl droplets and tilting the surface until the droplet rolls o. 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 µm - 900 µm) (Merck, Germany) or sea sand (size: 100 µm - 315 µm) (Merck, Germany) and manually dispensing water droplets on the surface. Bouncing droplet experiments were recorded using a high-speed camera (Fastec IL3, Fastec Imaging Corporation, USA). MCA lms were optically coupled to a glass-encapsulated laser-cut screen-printed mul5



ticrystalline silicon solar cell (E-Ton Solar, Taiwan) by dispensing droplets of refractive index matching liquid n = 1.50 (Cargille Laboratories, USA) on the backside of FEP MCA. Films were then placed on the solar cell and sucient 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 eciency (EQE) under one-sun bias light (biased area 2.48 cm2 ). The EQE was measured at 3 dierent spots for unstructured FEP and bare PV device, and at 3 spots each on 3 dierent samples for the FEP MCA under normal incidence and subsequently averaged. I-V curves were obtained using a solar simulator classied to Class AAA (WXS-90S-L2, WACOM, Japan) and a sourcemeter (Keithley 2400, Tektronix, USA) with an aperture area of 2.48 cm2 . Three spots were measured for each architecture and the results were averaged.

Ray Tracing Simulations All simulations were performed with the commercial ray tracing software LightTools (Synopsys GmbH, USA). The MCA was modelled by subtracting spheroids with radius r and depth h from a cuboid base. Spheroids are placed in a hexagonal array with lattice constant √ a = 3r. Microhairs are modelled 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 AOI. A detector is positioned at the bottom of the model, collecting the transmitted lights angular distribution. The models sidewalls are perfectly reective. The reection coecient is calculated from a fareld detector. 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 Mean aspect ratio (AR) and standard deviation (σAR ) were determined from cross-sectional SEM images and individual cavities in the simulation were assigned aspect ratios from a normal distribution based on AR and σAR , with a xed cavity radius. 6


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3

Results and discussion

3.1 Experimental and numerical analysis of the optical properties of microcavity arrays (MCA) Plant surfaces covered in nano- and microstructures often combine self-cleaning properties with high optical transmission. Inspired by such surfaces, we previously fabricated superhydrophobic bioinspired nanofur lms from polycarbonate (PC) using hot pulling (Figure 1 a). 2628,37 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 1 a). Such topography is a result of separating a heated sandblasted steel plate from the softened polymer during hot pulling. In order 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 1 a). Reection spectra of nanofur (original) and nanofur with exposed microcavities (µ-cavities) under normal incidence are shown in Figure 1 b). Partial removal of the hairs lowers the reection losses by approximately 2%. To evaluate the inuence of the disordered µ-cavity topography and verify the impact of the microhairs on the optical properties of the lms under varying angles of incidence (AOI) of incoming light, a simplied 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 1 c). As dimensions of the µ-cavities are signicantly larger than the considered wavelengths of 300 nm - 1200 nm, optical properties are solely governed by ray optics, and, thus, ray tracing simulations were conducted to obtain reection spectra. √ The modelled cavities are positioned in a hexagonal array with a lattice constant a = 3r, 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 simula7



Figure 1: Experimental and numerical analysis 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 reection spectra of nanofur (asprepared) and of oxygen plasma treated nanofur (exposed µ-cavities) under normal incidence. Partial removal of micro- and nanohairs reduces reection by 2%. c) Three-dimensional model of PC MCA with incorporated microhairs, which are simulated as cones and positioned at cavity corners. d) Reection 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. tions of MCA with varying AR, described in detail in the Experimental Section, have shown minimal reection values for AR=0.5. Simulated reection values of PC MCA with and without AR disorder as a function of AOI, and simulated reection of PC MCA with microhairs are compared to experimental reection measurement of as-prepared nanofur in Figure 1 d). Because reection does not signicantly vary over the spectral range, the reection values are taken at λ=600 nm, as to be close to the solar irradiance peak. With increasing AOI reection of nanofur increases exponentially up to 25%. The exponential increase in reection is also observed for simulated PC MCA with incorporated microhairs. Increased reection 8


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of nanofur and MCA with hairs can be explained by the change in cross-sectional area of hairs with increased AOI. For low AOI microhairs do not cover a signicant area due to their small diameter, thus not inuencing the reection. However, for higher AOI the crosssection area of the hairs becomes the eective area responsible for reection, and, being more similar to an unstructured polymer, similarly increases reection. 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 reection as a result. For MCA simulated without microhairs, reection values are below 7% for all AOI. In order to investigate the eect of µ-cavities with varying AR on the lms reection, the diameter of modelled µ-cavities was xed while varying their height, and, 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. Based on this distribution, modelled µ-cavities were assigned an AR. Introduced disorder increases reection by less than 0.2% (Figure 1 d) 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 reection of nanofur with increasing AOI compared to MCA can, therefore, be attributed to the presence of microhairs, 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 uorinated polymer MCA In order 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 uorinated 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 9



shown in Figure 2 a), was developed to transfer the disordered MCA topography to FEP. PC

Figure 2: Fabrication of FEP MCA. a) Schematic of shim fabrication and 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. Ni-shim is removed after cooling, resulting in a microstructured FEP lm. 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 lm. nanofur lm was galvanically copied by nickel electroforming in order to create a Ni-shim consisting of densely packed microhemispheres (Figure 2 a,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 polytetrauorethylene (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 being transferred from the steel to the backside of the FEP lm. An SEM image of the embossed FEP MCA surface and a cross-sectional 10


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SEM image of the lm are shown in Figure 2 c),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, due to low complexity of the 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 lms To characterize wetting properties of the FEP MCA lms, we measured their water contact and roll-o angles. Increasing the surface roughness of polymer lms was previously shown to enhance their wetting properties. 3941 Flat FEP is hydrophobic with high water contact angle θW = 109.63 ± 0.65◦ (Figure 3 a). However, water droplets stick to at 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 lm and results in a superhydrophobic surface with a water contact angle θW = 157.50 ± 5.60◦ and low roll-o angle θW S = 4.81 ±

3.02◦ , as shown in Figure 3 a),b),c). Most micro- and nanostructured superhydrophobic surfaces exhibit self-cleaning properties also known as the "Lotus-Eect", 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 solidcontaminant contact area, which reduces 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 o the surface. For a more detailed explanation of this eect the reader is referred to the studies of Fuerstner et al. from 2005 and Barthlott and Neinhuis from 1997. 17,43,44 In order to maximize eciency, PV modules are installed at dierent tilt angles depending on the the latitude of installation, the fraction of diuse versus direct sunlight, and local shading eects resulting from hills, buildings, or trees. 45,46 Therefore, "Lotus-Eect" 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 11



Figure 3: Wetting and self-cleaning properties of superhydrophobic FEP MCA. a) Colored water droplets dispensed on at 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 o the MCA at a sliding angle θW S < 5◦ . The contour of the moving droplet is highlighted in order to emphasize its motion. d) Self-cleaning properties of MCA (right) compared to 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 eectively cleaning the surface. Magnications show optical microscope images of contaminated smooth FEP and cleaned MCA after the self-cleaning test.

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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 µm - 900 µm), and, therefore, tests the self-cleaning properties for a broad range of contaminant sizes. Figure 3 d) and Movie M1 (Supporting Information) demonstrate the self-cleaning eect observed on FEP MCA compared to unstructured FEP at 10◦ tilt angle. Independently of the tilt angle dispensed water droplets roll down the MCA surface, pick up the contaminants and clean the surface in the process (Supporting Information Figure S1. The droplets dispensed on unstructured FEP stick to the surface, resulting in no self-cleaning eect (Figure 3 d). Optical microscope images of MCA and at FEP after cleaning (insets in Figure 3 d) show at FEP contaminated with graphite particles, while 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, while the unstructured FEP displayed no self-cleaning 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 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 cm and 10 cm, which correspond to 1.08 ± 0.05 m·s−1 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 m·s−1 to 9 m·s−1 . 49 Droplet movements were recorded with a high-speed camera and time-lapse images of droplets falling on unstruc13



Figure 4: Water droplets impacting at and MCA FEP lms 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 o a 30◦ tilted FEP MCA (b) upon impact, demonstrating the anti-wetting stability of FEP MCA against droplet impact. tured 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. Due to tilt and inherent hydrophobicity of FEP, the droplet slides a short distance down the slope before coming to rest and rmly 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 o, until enough kinetic energy is transferred and the droplet simply rolls o the surface. Higher drop velocity allows droplets impacting the smooth FEP to slide down longer distances but they still come to a complete stop, while droplets impacting the MCA bounce o the substrate independent of tilt angles and drop velocity, as shown in Movies M2 and M3 (Supplementary Information). The FEP MCA lms possess stable wetting and self-cleaning properties under contamination and impacting droplets, and, as a result, have superior anti-soiling properties compared to unstructured FEP. Moreover, superhydrophobicity of FEP MCA allows the top cover of PV modules to stay dry, thus, reducing surface conductivity. This additional eect can help mitigate potential-induced degradation known to reduce electrical output and even cause failure of PV modules in eld conditions. 50 14


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

3.4 Application of FEP MCA lms to PV modules In order to evaluate the eect of self-cleaning FEP MCA on the electrical output of PV devices, the MCA lm was optically coupled to a glass-encapsulated multicrystalline silicon solar cell. Additionally, an unstructured FEP lm was characterized in order to resolve the contributions of reduced Fresnel reection 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 to a PV device with at FEP and with FEP MCA attached are displayed in Figure 5 a). Coupling unstructured FEP increases the short-circuit current (ISC ) by 3.7% compared to the glass encapsulated device, while coupling of FEP MCA results in a 4.6% gain. Thus, the 0.9% dierence in gain can be solely attributed to the disordered MCA microstructure, while the rest of the improvement is due to the intermediate refractive index of the introduced FEP. Furthermore, Figure 5 b) displays the impact of a FEP MCA and unstructured FEP through external quantum eciency (EQE) measurements of glassencapsulated solar cells coated with unstructured FEP lm, coated with FEP MCA lm and

15



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, while it is highest in the visible regime between 400 nm and 700 nm, where the solar photon ux peaks resulting in signicant improvement in electrical gain (inset Figure 5 b). In agreement with the I-V-curves, the introduction of FEP has a bigger impact on the EQE than the microstructure of the lm. However, the reported electrical gain of the FEP MCA might be hindered by unstructured ridges between the µ-cavities. Further adjustment of the fabrication parameters can eliminate the unstructured ridges and, thus, reduce reection losses and maximize the electrical output. The impact of the FEP MCA on the electrical output of PV devices under dierent AOI is an on-going investigation in our laboratory and the results will be reported in future work.

4

Conclusion

In conclusion, we developed a polymeric lm with disordered microcavity array (MCA) topography, which can serve as a self-cleaning top layer on PV devices that additionally enhances the electrical performance of PV by improved light incoupling. Simulations have shown that MCA topography signicantly decreases the reection over all AOI compared to 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 electroformin 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 o 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 to the uncoated device. These properties are highly benecial for outdoor applications, and combined with high mechanical

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and UV-resistance will result in a long device lifetime with minimal maintenance.

Acknowledgement 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).

Supporting Information Available 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 o FEP MCA (Movie M2) Water droplet impacting and sticking to unstructured FEP (Movie M3) This material is available free of charge via the Internet at http://pubs.acs.org/.

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Self-Cleaning Microcavity Array for Photovoltaic Modules

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