Self-Templated Fabrication of Robust Moth-Eye-Like Nanostructures

We split the coating into 60 slices (named as hi, i = 0, 1, 2, ..., 60; the .... (47) The prominent reduction in reflection from blank glass to moth-e...
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Article pubs.acs.org/journal/apchd5

Self-Templated Fabrication of Robust Moth-Eye-Like Nanostructures with Broadband and Quasi-Omnidirectional Antireflection Properties Binbin Jin†,‡ and Junhui He*,† †

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Antireflection coatings (ARCs) have been realized by “bottom-up” depositing self-templated moth-eye patterns on glass substrates. The substrates were dip-coated by a layer of silica nanoparticles and further textured into patterns of nanodomes by precursor-derived one-step assembly (POA) of a mesoporous silica layer. Typically, the compact stacked nanodomes were about 260 nm in width and about 90 nm in height, under which was a layer comprising the lower halves of silica nanoparticles and mesoporous silica layer. The protuberant nanodomes with quarter-wavelength thickness and smoothly varying refraction index make significant contributions to the realization of broadband and quasi-omnidirectional antireflection. The mesoporous silica layer grown on the surface of silica nanoparticles and substrate through chemical bonding endows the coating with excellent structural integrity, thus, guaranteeing the high mechanical durability of the coating. The optimized coating reduces the double-sided reflection of glass from 8.75% to 1.88% over the wavelength range of 400−1200 nm. A considerable AR effect was also observed over an angular range as wide as ±40°. Silicon solar cells covered with this nanostructured glass showed enhancement in conversion efficiency by 4.91% at normal incidence as compared to under blank glass, which went up to 31.90% at an incidence angle of 60°. Tape peeling test, sponge washing test, and pencil hardness test showed favorable robustness and functional durability of the coating, which promises great potential for applications in sunlight harvest, solar energy conversion, and optical devices. KEYWORDS: self-templated, moth-eye, mesoporous, antireflective coating, silica

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refractive index is available in nature. To gain low refractive index, porous structures were designed and introduced in coatings.7,8 For example, interparticle voids from aggregated particles were constructed for a single-layer solid silica nanoparticle coating.9 Additionally, hollow and mesoporous silica nanoparticles and nanoporous silica coatings are more straightforward and have been more popular for low-n materials because of their controllable intrinsic porosity.10−14 Recently, our group had successfully fabricated antireflective, mechanically robust and self-healing nanoporous silica coatings through novel precursor-derived one-step assembly (POA) approach.15 Porous polymer coatings have also been proposed as ARCs.16 Nevertheless, such porous ARCs offer AR performance only covering a certain range of wavelength and incidence angle, which are yet to be significantly enhanced toward applications such as solar energy utilization, imaging, and photodetector. Alternatively, the fabrication of ARCs featuring a gradient refractive index (GRIN) is considered to be the most effective approach in suppressing reflection over a wider range of

eflection often exists at the interfaces of transparent substrates; in most cases, such reflection loss is undesirable.1 For example, reflection loss at the air−glass interface lowers the efficiency of solar energy modules, and the reflected sunlight off the screen results in poor contrast in display devices. Antireflection coatings (ARCs), with precisely controlled refractive index (n) and thickness (d), can suppress this unwanted reflection loss and pose considerable application potential in solar cells, camera lens, photodetectors, and so on.2−6 In general, the total reflection loss is nearly 8% at the air− glass interface when light hits on glass substrate at the normal incidence angle, which will be even severer at higher incidence angles. To address the issue, a homogeneous single-layer ARC is commonly used to eliminate the reflection, which should satisfy the following criteria: (i) the thickness of the coating should be λ/4nc, where λ is the incidence wavelength and nc is the refractive index of the coating; (ii) nc = (nans)1/2, where na and ns are the refractive index of air and the substrate. According to these criteria, a single-layer quarter-wavelength coating with a refractive index of 1.22 is required on glass substrates. Unfortunately, no single material with such a low © XXXX American Chemical Society

Received: November 9, 2016

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wavelength and incidence angle.17,18 There are two ways to achieve GRIN, including deposition of multilayers with successively increasing refractive index from air to substrate or fabrication of biomimetic moth-eye nanostructure arrays.19−23 The former approach attains low reflectance by destructive interference of reflected light at adjacent layers. However, it is difficult to precisely control the thickness of each layer. What’s more, thermal mismatch and poor adhesion between layers usually raise problems in their outdoor applications.24 The fabrication of bioinspired moth-eye nanostructures array is another approach to achieve GRIN. Various arrays of nanostructures such as nanorods,25,26 nanocones,27,28 nanograss,29 and nanopillars30,31 have so far been fabricated by ion or electron beam etching,32,33 selfassembly,34 phase separation,35 and nanoimprint lithography.36,37 Although moth-eye nanostructured ARCs have better broadband and omni-directional antireflection properties, their fabrication generally need complicated procedure, and their high aspect ratio geometry also raises problems such as easy destroy or deformation in outdoor applications.14,38 The fabrication of moth-eye nanostructured ARCs that are robust enough for outdoor applications remains a significant challenge. Herein, we report the fabrication of robust silica-based antireflection coatings textured to moth-eye-like nanostructure without using heterogeneous templates,38 wet/dry etching,39 colloidal lithography,40,41 or vacuum technique.42 The current approach is simply based on dip-coating of a layer of solid silica nanoparticles followed by POA of mesoporous silica layer. The mesoporous silica layer grown on the surface of silica nanoparticles delivered the roughness of nanoparticles to the surface of the coating, resulting in the formation of densely packed nanodomes. These protuberant nanodomes with subwavelength spacing were modeled by cutting them into horizontal slices with a certain effective RI, neff, calculated using effective medium equation.43 The obtained coatings exhibited broadband antireflection with an average reflectance of 1.88% in the wavelength range from 400 to 1200 nm. Moreover, 99.3% of transmittance was maintained even at high incidence angles out of 40° as compared to normal incidence. It was found that the power conversion efficiency (PCE) of a Si solar cell covered with the moth-eye-like nanostructured glass could be enhanced by 4.91% at normal incidence as compared to the device under blank glass, and the enhancement went up to 31.90% at an incidence angle of 60°. Furthermore, tape peeling test, sponge washing test and pencil hardness test showed high mechanical robustness of the coatings, thus demonstrating their potential in outdoor applications.

Figure 1. Schematics of the fabrication of moth-eye nanostructures and the cross sections of corresponding specimens.

Figure 2. TEM images of (a) solid silica nanoparticles and (b) mesoporous silica layer; (c) cross-sectional and (d) tilted view SEM images of glass coated with solid silica nanoparticles and mesoporous silica layer fabricated by 10 h precursor-derived self-assembly (S+M10ARC). The insets in (c) and (d) are the corresponding magnified images of coating, and arrows in (d) point to interspaces.

size roughly estimated to be 3.3 and 2.0 nm, respectively. A cross-section SEM image (Figure 2c) of S+M10-ARC shows that subwavelength moth-eye-like nanostructures were surprisingly achieved, where the upper nanodomes were about 260 ± 10 nm in width and 90 ± 4 nm in height, and the lower layer (including lower halves of silica nanoparticles and mesoporous layer) were about 90 ± 2 nm thick. According to the above height information, we could figure out that the heights of mesoporous layer grown on the surface of silica nanoparticles and glass substrate are 60 and 90 nm, respectively. This may be because mesoporous layer was grown on a glass substrate from both substrate surface and nearby lower surfaces of silica nanoparticles, while it was grown on the upper part of silica nanoparticles only from the nanoparticles’ upper part surface as the growth site. Moreover, the differences of surface chemical composition and curvature between glass substrate and silica nanoparticles may also effect the growth of mesoporous layer, thus contributing the difference in thickness. From the top-view SEM image (Figure 2d) and AFM images (Figure 3a,b) of S +M10-ARC, it could be seen again that the glass substrate was homogeneously covered by subwavelength moth-eye-like

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RESULTS AND DISCUSSION Surface Structure of Coatings. A schematic of the steps involved in the fabrication of S+Mx-ARCs is illustrated in Figure 1. First, a monolayer of closely packed silica nanoparticles was dip-coated on glass substrate. Then a thin mesporous silica layer was grown on the surface of silica nanoparticles and substrate (i.e., those parts of substrate through the voids between silica nanoparticles). As shown in Figure 2a, the mean diameter of silica nanoparticles was measured to be 120 ± 5 nm. The mesoporous silica layer was grown through electrostatic interactions between surfactant cations (CTA+) and negatively charged silicate species.44 TEM image (Figure 2b) reveals closely packed mesopores with the center-to-center distance between adjacent mesopores and pore B

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Figure 3. (a) Two- and (b) three-dimensional AFM images and (c) height profile along the white line in (a) of S+M10-ARC.

Figure 4. Structure models of the cross sections of (a) S+M10-ARC glass, (c) S-ARC glass, and (e) M-ARC glass, (b, d, f) are the refractive index profiles of corresponding specimens. In (a), dome-like cylinders and cylinders marked by black boxes constitute the whole coating, anbn is the arbitrary diameter at a height between 90 to 180 nm. h0, h20, h30, and h60 represent different heights (h) at 0, 60, 90, and 180 nm, respectively.

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⎛ anbn ⎞2 Ssolid silica ⎟ × π /( 1202 − 302 )2 × π =⎜ ⎝ 2 ⎠ S

nanostructures with some interspaces between them. The surface roughness (RMS) of S+M10-ARC was estimated to be about 27.2 nm. Figure 3c is the height profile of the white line in Figure 3a. It is noted that the height of the nanodome is about 90 nm from the coated substrate surface but the heights are smaller from the top of nanodome to the joint point of mesoporous layer coated nanoparticles, which are in accord with the SEM observations (Figure 2c,d). These protuberances would have significant impacts on the optical performance of coatings. To better explore the relationship between the surface morphology and optical properties, we also fabricated S+M8ARC and S+M12-ARC, SEM and AFM images of which are shown in Figure S1. The surface morphology of these two coatings are analogous to that of S+M10-ARC, while some silica fragments appear on the surface of S+M12-ARC with an extended growth time of 12 h. Their optical performance will be demonstrated in the later discussion. Calculations of Refractive Index of Coatings. Since the glass substrate is homogeneously covered by subwavelength moth-eye-like nanostructures, it is available for us to reckon the neff of the coating as a function of height by using effective medium theory involving two-dimensional Bruggeman equation for two material mixtures, as shown in eq 1: ⎛ n − n1 ⎞ ⎛ n − n0 ⎞ 0 = F ⎜ eff ⎟ ⎟ + (1 − F )⎜ eff ⎝ neff + n1 ⎠ ⎝ neff + n0 ⎠

(2)

We could get the F value and the full neff profile eventually, as shown in Figure 4b (the details of calculations are provided in the Supporting Information). The plot clearly shows that neff gradually transforms from 0 (0 nm) to h30 (90 nm) and appears as a quasi-step from 90 to 180 nm due to the dense silica layer of the coating. In the calculations, we made some approximations, and the obtained profile would lead us to further construct perfect antireflective nanostructures. The gradual transition in refractive index of nanodomes from 0 to 90 nm together with a quasi-step from 90 to 180 nm significantly eliminates optical interface between air and substrate as compared to S-ARC (Figure 4d) and M-ARC (Figure 4f) and contributes to suppressing reflectance in the visible and near-IR region. Moreover, the quarter-wavelength nanodomes (center thickness: 90 nm) suppress the reflectance at a specific near-ultraviolet (NUV, 300−500 nm) wavelength.45 Optical Performance. S+Mx-ARCs were supposed to demonstrate superior broadband optical properties. Figure 5 shows the reflectance and transmittance (Ttotal) spectra of S +M10-ARC with respect to coatings prepared solely by silica nanoparticles (S-ARC) and mesoporous silica layer (M-ARC), respectively. Clearly, S+M10-ARC exhibits both broadband antireflection property and high transmittance, the reflectance being constantly below 3% and the transmittance being

(1)

where F is the area ratio of silica material in a slice and n1 and n0 are the RI of silica material and air, respectively. For nanodomes, where F increases gradually through the structure from top to bottom, incident light will be reflected at each slice with a phase determined by the distance traveled through the structure. The more phases, the more possibilities of undergoing destructive interference and yielding low reflection for broadband and wide-angle light. According to the analysis of SEM and AFM images of S +M10-ARC, we built up the structural model of the coating. As shown in Figure 4a, we divided the coating into three layers: (i) the first layer, from 0 to 60 nm, totally consists of mesoporous silica layer; (ii) the second layer, from 60 to 90 nm, contains a mesoporous silica layer and partial silica nanoparticles; the area ratio of solid silica to mesoporous silica in the layer increases gradually. The mesoporous layers in these two layers were grown merely on the deposited silica nanoparticles; (iii) the third layer, from 90 to 180 nm, also includes a mesoporous silica layer and silica nanoparticles. The percentage of silica material attains to the maximum at the height of 120 nm. The mesoporous silica layer in the third layer was grown on both silica nanoparticles and glass substrate. Figure 3a shows the two-dimensional AFM image of coating, where we could extract the area ratio, F, of the silica material. We could then substitute F into eq 1 to calculate neff. We split the coating into 60 slices (named as hi, i = 0, 1, 2, ..., 60; the thickness of each slice is 3 nm) and figured out the neff(hi) (the effective RI at the height of i × 3 nm) of the first two layers from 0 to 90 nm. However, the F value from 90 to 180 nm could not be gained directly from the AFM results. To tackle the problem, we divided the coating into two parts: dome-like cylinders and cylinders, as shown in Figure 4a in black boxes. anbn is the arbitrary diameter at hi from 90 to 180 nm. The diameter of the dome-like cylinder is

Figure 5. UV−vis-near IR (a) reflectance and (b) transmittance of SARC glass, M-ARC glass, and S+M10-ARC glass vs blank glass. S-ARC glass was fabricated by dip-coating solid silica nanoparticles and MARC glass was prepared by precursor-derived self-assembly at 60 °C for 30 h.

2 × 1202 − 302 nm, so the percentage of solid silica in the dome-like cylinder could be calculated from eq 2: D

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constantly 5% higher than that of blank glass over the wavelength range of 400 to 1200 nm. The transmittance profile of S+M10-ARC is nearly the same as that of blank glass but overall shifts up by 5−6%, which indicates that the AR effect of S+M10-ARC is nearly wavelength independent. For wavelengths (700−1200 nm) much larger than the size of nanodomes, the M-ARC glass shows higher transmittance than the S+M10-ARC glass due to the fact that it achieves an optimum reflection reduction at the specific wavelength of about 750 nm (determined by the thickness and refractive index of the coating). However, with an increase in the wavelength, the transmittance of M-ARC glass decays faster than that of S+M10-ARC. For near-ultraviolet wavelengths (300−500 nm), the S+M10-ARC glass shows the best AR performance when compared to the S-ARC and M-ARC glasses. Figure S3 shows the haze (defined as the ratio of diffused transmittance to Ttotal) spectra of S+M10-ARC, SARC, M-ARC, and non-ARC glasses. It is noted that S+M10ARC shows the maximum haze, and S-ARC shows slightly lower haze. With an increase in the wavelength, the features of these two surfaces are less resolved, which leads to a reduced scattering effect and, thus, a reduced haze. However, the MARC glass and blank glass show almost zero haze. The surface with enhanced haze can increase optical absorption via scattering effect. Therefore, it is rational that the haze should be maximized without sacrificing Ttotal for the most effective solar cells.46 The effects of Ttotal and haze on the solar cell performance will be discussed later. The broadband AR performance of S+M10-ARC glass can be attributed to the following facts: (i) For long wavelengths over 700 nm, the reduced reflectance can be explained by effective medium theory. The nanodomes (0−90 nm) render a continuous gradient refractive index (Figure 4b). The suppression of reflection occurs through destructive interference of reflected light at different depths of nanodomes, where the light waves of different phases partially or wholly cancel one another. Additionally, the lower layer from 90 to 180 nm works as a single-layer ARC (similar to M-ARC), which can also suppress reflectance for long wavelengths.14 (ii) For short wavelengths, the nanodomes with a center thickness of 90 nm act as a λ/4-wavelength ARC and significantly suppress the reflection in the range from 300 to 500 nm.45 (iii) Moreover, strong light scattering takes place when light impinges on the bottom of nanodomes, leading to increased optical path length and consequently more penetration of light through the coated substrate.46 The combined effects of effective medium, λ/4wavelength antireflection and light scattering result in the broadband AR performance of S+M10-ARC glass. Usually, display devices focus on AR performance in the visible range (400−800 nm), while AR in the visible-infrared region (400−1200 nm) is preferable for solar cells. Therefore, we summarize the average reflectance and transmittance of three different coatings. As shown in Table 1, moth-eye-like nanostructured ARC shows the best optical performance when compared to S-ARC and M-ARC particularly in the visible region. It is widely acknowledged that incident angle dependent transmittance is an important issue for practical applications such as solar cells. Thus, we measured the transmittance of S +M10-ARC glass with respect to blank glass over incidence angles ranging from 0 to 60°. As shown in Figure 6, S+M10ARC (Figure 6b) demonstrates considerably better transmittance than blank glass (Figure 6a). The transmittance varies

Table 1. Average Reflectance and Transmittance of Blank Glass and Glasses Coated with S-ARC, M-ARC, and S+M10ARC in the Wavelength Ranges of 400−800 and 400−1200 nm, Respectively

Rave (400−800 nm) Rave (400−1200 nm) Tave (400−800 nm) Tave (400−1200 nm)

blank glass

S-ARC glass

M-ARC glass

S+M10-ARC glass

9.01% 8.75%

2.76% 3.13%

3.02% 2.24%

2.01% 2.04%

90.73% 89.58%

96.22% 94.71%

96.21% 95.74%

96.43% 95.41%

from 96 to 92% within 0 to 40°. Moreover, the transmittance of S+M10-ARC glass is 5.5% higher than that of blank glass at normal incidence angle, while the increment reaches 7.9% at 60°. The much better wide-angle performance of S+M10-ARC is due to the smooth transition in refractive index, and wideangle light is bent closer to normal incidence while approaching the substrate, according to Snell’s law of refraction.47 The prominent reduction in reflection from blank glass to moth-eye nanostructure coated glass is also shown in Figure 6c,d. As a typical application, we examined the effects of S-ARC, M-ARC and S+M10-ARC on the performance of photovoltaic cell. We measured the current density (J)−voltage (V) characteristics of a standard single-crystal Si solar cell covered with the different glasses under air-mass (AM) 1.5 G illumination at normal incidence. Figure 7a presents the obtained J−V curves, from which the detailed performances were derived and are summarized in Table 2. It is clear that the device covered with the S+M10-ARC glass delivers the highest PCE, which is about 4.91% higher than that obtained under blank glass. The improved PCE mainly comes from the enhanced short-circuit current density (Jsc), which is due to the superior Ttotal and haze discussed in Figures 5 and S2. Furthermore, the angle-of-incidence (AOI)-dependent PCE was also measured. Figure 7b shows the PCE of Si solar cell covered with the S+M10-ARC glass and blank glass with the AOI ranging from −60° to 60°. Clearly, S+M10-ARC presents the higher PEC at all angles, and the enhancement is particularly prominent at high angles (e.g., from 4.91% at 0° to 35.9% at 60°), which is in accord with the AOI-dependent transmittance (Figure 6). Mechanical Properties. To evaluate the mechanical durability of the S+M10-ARC, three complementary tests were performed. First, S-ARC and S+M10-ARC were subjected to 3 M Scotch tape stripping. As shown in Figure 8a, a clear boundary distinguishing between treated and untreated areas is visible for S-ARC under SEM observation. Because the silica nanoparticles were attached to each other and to the substrate only by van der Waals interactions, they could be readily peeled off by the tape, leaving the substrate surface nanoparticle free. On the contrary, S+M10-ARC did not change its coverage (Figure 8b) after peeling the tape off the surface, since the mesoporous layer grown in the interparticle voids actually played a role as a binder (Figure 8c), thus, significantly strengthening the adhesion of nanoparticles to each other and to the substrate. TEM observation (Figure 8d) shows that the mesoporous layer is chemically bonded to silica nanoparticles and links them into a network structure when we mixed 5 mL of silica sol with the precursor solution for 15 h growth. Abrasion resistance of ARCs is essential in preserving consistent surface structure and optical performance. The E

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Figure 6. Contour plots of transmittance as a function of both wavelength and angle of incident light for (a) blank glass and (b) S+M10-ARC glass, respectively. The reflected light can be clearly seen from the surface of (c) blank glass, which is dramatically reduced on the surface of (d) S+M10ARC glass.

structured glass show a PCE enhancement of 4.91% at normal incidence, and the enhancement goes up to 39.10% at the incidence angle of 60°. We proposed the structural model and calculated the RI profile using effective medium theory, which well accounts for the prominent optical performance and would guide us to design new nanostructures. Furthermore, the mesoporous silica layer, which is chemically bonded to silica nanoparticles and substrate, endows the coating with structural integrity and mechanical durability. The coating retains optical performance after adhesive tape peeling test, sponge washing test, and 6H pencil hardness test. The integration of superior optical performance and high mechanical robustness in coatings through the easy fabrication procedure promises great potentials for applications such as sunlight harvest, solar energy conversion, and optical devices.

abrasion resistance of S+M10-ARC was evaluated by sponge washing (ASTMD 4828−92) for up to 100 cycles in 2 min and pencil hardness test. We measured the transmittance, reflectance, and surface morphology of the thin film before and after sponge washing test. As shown in Figure 9a, only a tiny deviation took place in the reflectance and transmittance spectra, which might be due to a minor change in the surface structure and film thickness, as revealed in Figure 9b,c. We also measured the pencil hardness of S+M10-ARC using 4H to 6H pencils. It was observed that the coating was hardly scratched off but leaving graphite scraps in interparticle voids by pressing pencil of hardness up to 6H (Figure S4). Thus, the S+M10ARC exhibits superior abrasion resistance compared to many silica-based ARCs fabricated from hierarchical nanoporous silica (4H-5H),14 mesoporous silica (5H),10 and so on. Therefore, S +M10-ARC shows excellent mechanical strength (passed tape peeling test, sponge washing test, and 6H pencil hardness test). The mesoporous silica layer that is chemically bonded to silica nanoparticles and the substrate accounts for the mechanical durability of the coating.

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EXPERIMENTAL SECTION Materials. Aqueous ammonia (25%), absolute ethanol (99.5%), and cetyltrimethylammonium bromide (CTAB) were purchased from Beihua Fine Chemicals. Tetraethyl orthosilicate (TEOS, 98+%) was obtained from Alfa Aesar. All chemicals were of analytic grade and used without further purification. Glass substrates were purchased from Jiangsu Swift Boat Glass and Plastic Company. Ultrapure water with a resistivity higher than 18.2 MΩ·cm was used in all experiments and was obtained from a three-stage Millipore Milli-Q Plus 185 purification system (Academic). Preparation of Silica Sol and Precursor Solution. SiO2 nanosphere sol was synthesized using the Stöber method. Typically, 5 mL of aqueous ammonia and 5 mL of ultrapure water were mixed with 95 mL of absolute ethanol followed by the injection of 3 mL of TEOS under stirring at 40 °C. After 10 h, 120 nm SiO2 nanosphere sol was obtained. Precursor solution was prepared by dissolving 0.08 g CTAB in a mixture

CONCLUSION

In summary, a novel self-templated approach was applied to fabricate moth-eye-like patterns on glass substrate by dipcoating a layer of silica nanoparticles and a layer of dense mesoporous silica layer. The protuberant nanodomes play a dominant role in the AR enhancement of the near-ultraviolet region and the smoothly varying refraction index significantly reduces reflectance in the visible and NIR region. The optimized coating reduces the double-sided reflection of glass from 8.75% to 1.88% over the wavelength range of 400−1200 nm. Moreover, the high transparency of the coating is maintained out to an incidence angle of 40°. Such characteristics are highly desired for sunlight harvesting and solar energy conversion. Si solar cells covered with this surface nanoF

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Figure 8. SEM images of (a) S-ARC and (b) S+M10-ARC that were partially exposed to 3 M tape (white boxes indicate the exposed areas); (c) schematic illustration of the surface structure change from S-ARC to S+M10-ARC; (d) TEM image of silica nanoparticles subjected to 15 h growth in the precursor solution.

Figure 7. (a) J−V curves of a Si solar cell covered with S+M10-ARC glass, S-ARC glass, M-ARC glass, and blank glass, respectively. (b) AOI-dependent PCE of the Si solar cell covered with S+M10-ARC glass and blank glass, respectively, where the PCE enhancement is also shown.

Table 2. Photovoltaic Performance of a Si Solar Cell Covered with Blank Glass, S-ARC Glass, M-ARC Glass, and S+M10-ARC Glass, Respectively, at AOI = 0° parameters

blank

S-ARC

M-ARC

S+M10-ARC

Voc (mV) Jsc (mA/cm2) FF (%) PCE (%)

574 30.44 63.3 10.99

581 31.16 61.5 11.13

573 31.29 62.7 11.19

578 31.80 62.5 11.53

solvent of 35 mL of water and 15 mL of ethanol, followed by the addition of 5 μL of aqueous ammonia and 40 μL of TEOS. Fabrication of Moth-Eye-Like Coatings. Moth-eye-like coatings were fabricated on glass substrates through a two-step method, including dip-coating and precursor-derived selfassembly. First, a glass substrate was sonicated in deionized water for 15 min, followed by oxygen plasma (84 W, 5 min) treatment. The cleaned glass substrate was immersed in the silica sol for 30 s and withdrawn from the sol at 150 mm min−1. Each glass substrate was subjected to dip-coating twice. Second, the precoated glass substrate was immersed in the precursor solution, and a mesoporous silica layer was growing under quiescent conditions in a Teflon container at 60 °C for 8−12 h. Then the coating was dried in an oven at 100 °C for 10 h, after which the CTAB template was removed by calcinations at 550 °C for 3 h. The obtained coatings were named as S+Mx-ARCs

Figure 9. (a) Reflectance and transmittance spectra of S+M10-ARC glass before and after sponge washing test; (b) SEM image of S+M10ARC after sponge washing test; (c) enlarged view of white box in (b).

(x: growth time of mesoporous silica layer) by fabrication steps involved, for example, S+M10-ARC represents a coating prepared by dip-coating a layer of solid silica nanoparticles and growing a mesoporous silica layer for 10 h by POA. Characterization. The morphology and cross-section of coatings were observed by Hitachi S4800 field-emission scanning electron microscopy (FE-SEM) at 5 kV. Transmission G

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(6) Liu, L. Q.; Wang, X. L.; Jing, M.; Zhang, S. G.; Zhang, G. Y.; Dou, S. X.; Wang, G. Broadband and omnidirectional, nearly zero reflective photovoltaic glass. Adv. Mater. 2012, 24, 6318−6322. (7) Gao, L.; He, J. Surface hydrophobic co-modification of hollow silica nanoparticles toward large-area transparent superhydrophobic coatings. J. Colloid Interface Sci. 2013, 396, 152−159. (8) Liu, X.; He, J. Hierarchically structured superhydrophilic coatings fabricated by self-assembling raspberry-like silica nanospheres. J. Colloid Interface Sci. 2007, 314, 341−345. (9) Rouse, J. H.; Ferguson, G. S. Preparation of thin silica films with controlled thickness and tunable refractive index. J. Am. Chem. Soc. 2003, 125, 15529−15536. (10) Moghal, J.; Kobler, J.; Sauer, J.; Best, J.; Gardener, M.; Watt, A. A.; Wakefield, G. High-performance, single-layer antireflective optical coatings comprising mesoporous silica nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 854−859. (11) Zhang, X.; Lan, P.; Lu, Y.; Li, J.; Xu, H.; Zhang, J.; Lee, Y.; Rhee, J. Y.; Choy, K. L.; Song, W. Multifunctional antireflection coatings based on novel hollow silica-silica nanocomposites. ACS Appl. Mater. Interfaces 2014, 6, 1415−1423. (12) Xu, L.; He, J. Fabrication of highly transparent superhydrophobic coatings from hollow silica nanoparticles. Langmuir 2012, 28, 7512−7518. (13) Hoshikawa, Y.; Yabe, H.; Nomura, A.; Yamaki, T.; Shimojima, A.; Okubo, T. Mesoporous Silica Nanoparticles with Remarkable Stability and Dispersibility for Antireflective Coatings. Chem. Mater. 2010, 22, 12−14. (14) Mizoshita, N.; Ishii, M.; Kato, N.; Tanaka, H. Hierarchical Nanoporous Silica Films for Wear Resistant Antireflection Coatings. ACS Appl. Mater. Interfaces 2015, 7, 19424−19430. (15) Xu, L. G.; He, J. H. A novel precursor-derived one-step growth approach to fabrication of highly antireflective, mechanically robust and self-healing nanoporous silica thin films. J. Mater. Chem. C 2013, 1, 4655−4662. (16) Kuo, C. Y.; Chen, Y. Y.; Lu, S. Y. A facile route to create surface porous polymer films via phase separation for antireflection applications. ACS Appl. Mater. Interfaces 2009, 1, 72−75. (17) Fukada, K.; Shiratori, S. Gradient Functional Characteristic of Polymer/Nanoparticle Stacks on a Polyethylene Naphthalate Film. Ind. Eng. Chem. Res. 2015, 54, 979−986. (18) Raut, H. K.; Nair, A. S.; Dinachali, S. S.; Ganesh, V. A.; Walsh, T. M.; Ramakrishna, S. Porous SiO2 anti-reflective coatings on largearea substrates by electrospinning and their application to solar modules. Sol. Energy Mater. Sol. Cells 2013, 111, 9−15. (19) Lu, Y.; Choi, Y. J.; Lim, H. S.; Kwak, D.; Shim, C.; Lee, S. G.; Cho, K. pH-Induced antireflection coatings derived from hydrogenbonding-directed multilayer films. Langmuir 2010, 26, 17749−17755. (20) Yan, X.; Poxson, D. J.; Cho, J.; Welser, R. E.; Sood, A. K.; Kim, J. K.; Schubert, E. F. Enhanced Omnidirectional Photovoltaic Performance of Solar Cells Using Multiple-Discrete-Layer Tailored- and LowRefractive Index Anti-Reflection Coatings. Adv. Funct. Mater. 2013, 23, 583−590. (21) Zhang, X. X.; Cai, S.; You, D.; Yan, L. H.; Lv, H. B.; Yuan, X. D.; Jiang, B. Template-Free Sol-Gel Preparation of Superhydrophobic ORMOSIL Films for Double-Wavelength Broadband Antireflective Coatings. Adv. Funct. Mater. 2013, 23, 4361−4365. (22) Clapham, P. B.; Hutley, M. C. Nature 1973, 244, 281. (23) Ji, S.; Song, K.; Nguyen, T. B.; Kim, N.; Lim, H. Optimal moth eye nanostructure array on transparent glass towards broadband antireflection. ACS Appl. Mater. Interfaces 2013, 5, 10731−10737. (24) Shimomura, H.; Gemici, Z.; Cohen, R. E.; Rubner, M. F. Layerby-layer-assembled high-performance broadband antireflection coatings. ACS Appl. Mater. Interfaces 2010, 2, 813−820. (25) Cai, J. G.; Ye, J. F.; Chen, S. Y.; Zhao, X. W.; Zhang, D. Y.; Chen, S.; Ma, Y. R.; Jin, S.; Qi, L. M. Self-cleaning, broadband and quasi-omnidirectional antireflective structures based on mesocrystalline rutile TiO2 nanorod arrays. Energy Environ. Sci. 2012, 5, 7575− 7581.

electron microscopy (TEM) images were taken on JEOL JEM2100F at 200 kV. Small pieces were scratched off of the mesoporous layer, dispersed in ethanol under ultrasonication, and casted on holey carbon grids for TEM observation. The roughness and surface topography of the coatings were characterized by atomic force microscopy (AFM) on an MM8-SYS scanning probe microscope (Bruker AXR). The transmittance and reflection spectra in the wavelength of 300− 1200 nm were recorded on a Varian Cary 5000 UV−vis-NIR spectrophotometer, with an integrating sphere attached. The transmittance spectra at incidence angles from 0 to 60° were measured on a Lambda 950 UV/vis spectrometer using a rotating sample stage.

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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/acsphotonics.6b00888.

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Detailed calculations and supplementary figures (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 10 82543535. ORCID

Junhui He: 0000-0001-7199-3351 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very grateful to Mr. Wenbin Wang for help in the photovoltaic performance measurements and related discussion. Financial supports are greatly appreciated from the National Natural Science Foundation of China (Grant Nos. 21571182, 21271177), a Chinese Academy of Sciences Grant (CXJJ-14-M38), the Science and Technology Commission of Beijing Municipality (Z151100003315018), the National High Technology Research and Development Program (“863” Program) of China (Grant No. 2011AA050525), the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS; Grant Nos. KGCX2-YW-370, KGCX2-EW-304-2), and Key Laboratory of Space Energy Conversion Technology, TIPC, CAS.

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REFERENCES

(1) Raut, H. K.; Ganesh, V. A.; Nair, A. S.; Ramakrishna, S. Antireflective coatings: A critical, in-depth review. Energy Environ. Sci. 2011, 4, 3779−3804. (2) Aiken, D. J. High performance anti-reflection coatings for broadband multi-junction solar cells. Sol. Energy Mater. Sol. Cells 2000, 64, 393−404. (3) Ghymn, Y. H.; Jung, K.; Shin, M.; Ko, H. A luminescent downshifting and moth-eyed anti-reflective film for highly efficient photovoltaic devices. Nanoscale 2015, 7, 18642−18650. (4) Leem, J. W.; Song, Y. M.; Yu, J. S. Biomimetic artificial Si compound eye surface structures with broadband and wide-angle antireflection properties for Si-based optoelectronic applications. Nanoscale 2013, 5, 10455−10460. (5) Liang, D.; Kang, Y.; Huo, Y.; Chen, Y.; Cui, Y.; Harris, J. S. Highefficiency nanostructured window GaAs solar cells. Nano Lett. 2013, 13, 4850−4856. H

DOI: 10.1021/acsphotonics.6b00888 ACS Photonics XXXX, XXX, XXX−XXX

ACS Photonics

Article

(26) Diedenhofen, S. L.; Vecchi, G.; Algra, R. E.; Hartsuiker, A.; Muskens, O. L.; Immink, G.; Bakkers, E. P. A. M.; Vos, W. L.; Rivas, J. G. Broad-band and Omnidirectional Antireflection Coatings Based on Semiconductor Nanorods. Adv. Mater. 2009, 21, 973−978. (27) 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. (28) Wang, K. X.; Yu, Z.; Liu, V.; Cui, Y.; Fan, S. Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings. Nano Lett. 2012, 12, 1616−1619. (29) Ravipati, S.; Shieh, J.; Ko, F.-H.; Yu, C.-C.; Chen, H.-L.; Wu, C.T.; Chen, S.-H. Broadband and wide angle antireflection of sub-20 nm GaAs nanograss. Energy Environ. Sci. 2012, 5, 7601−7605. (30) Ji, S.; Park, J.; Lim, H. Improved antireflection properties of moth eye mimicking nanopillars on transparent glass: flat antireflection and color tuning. Nanoscale 2012, 4, 4603−4610. (31) Lin, T.; Ramadurgam, S.; Liao, C. S.; Zi, Y.; Yang, C. Fabrication of Sub-25 nm Diameter GaSb Nanopillar Arrays by Nanoscale SelfMask Effect. Nano Lett. 2015, 15, 4993−5000. (32) Hao, J. Y.; Lu, N.; Xu, H. B.; Wang, W. T.; Gao, L. G.; Chi, L. F. Langmuir-Blodgett Monolayer Masked Chemical Etching: An Approach to Broadband Antireflective Surfaces. Chem. Mater. 2009, 21, 1802−1805. (33) Huang, Y. F.; Chattopadhyay, S.; Jen, Y. J.; Peng, C. Y.; Liu, T. A.; Hsu, Y. K.; Pan, C. L.; Lo, H. C.; Hsu, C. H.; Chang, Y. H.; Lee, C. S.; Chen, K. H.; Chen, L. C. Improved broadband and quasiomnidirectional anti-reflection properties with biomimetic silicon nanostructures. Nat. Nanotechnol. 2007, 2, 770−774. (34) Galeotti, F.; Trespidi, F.; Timo, G.; Pasini, M. Broadband and crack-free antireflection coatings by self-assembled moth eye patterns. ACS Appl. Mater. Interfaces 2014, 6, 5827−5834. (35) Li, X.; Gao, J. P.; Xue, L. J.; Han, Y. C. Porous Polymer Films with Gradient-Refractive-Index Structure for Broadband and Omnidirectional Antireflection Coatings. Adv. Funct. Mater. 2010, 20, 259− 265. (36) Raut, H. K.; Dinachali, S. S.; He, A. Y.; Ganesh, V. A.; Saifullah, M. S. M.; Law, J.; Ramakrishna, S. Robust and durable polyhedral oligomeric silsesquioxane-based anti-reflective nanostructures with broadband quasi-omnidirectional properties. Energy Environ. Sci. 2013, 6, 1929−1937. (37) Raut, H. K.; Dinachali, S. S.; Loke, Y. C.; Ganesan, R.; AnsahAntwi, K. K.; Gora, A.; Khoo, E. H.; Ganesh, V. A.; Saifullah, M. S.; Ramakrishna, S. Multiscale ommatidial arrays with broadband and omnidirectional antireflection and antifogging properties by sacrificial layer mediated nanoimprinting. ACS Nano 2015, 9, 1305−1314. (38) van de Groep, J.; Spinelli, P.; Polman, A. Single-Step SoftImprinted Large-Area Nanopatterned Antireflection Coating. Nano Lett. 2015, 15, 4223−4228. (39) Kitamura, S.; Kanno, Y.; Watanabe, M.; Takahashi, M.; Kuroda, K.; Miyata, H. Films with Tunable Graded Refractive Index Consisting of Spontaneously Formed Mesoporous Silica Nanopinnacles. ACS Photonics 2014, 1, 47−52. (40) Li, Y.; Zhang, J.; Zhu, S.; Dong, H.; Jia, F.; Wang, Z.; Tang, Y.; Zhang, L.; Zhang, S.; Yang, B. Bioinspired silica surfaces with nearinfrared improved transmittance and superhydrophobicity by colloidal lithography. Langmuir 2010, 26, 9842−9847. (41) Zhang, X.; Zhang, J.; Ren, Z.; Li, X.; Zhang, X.; Zhu, D.; Wang, T.; Tian, T.; Yang, B. Morphology and wettability control of silicon cone arrays using colloidal lithography. Langmuir 2009, 25, 7375− 7382. (42) Park, K. C.; Choi, H. J.; Chang, C. H.; Cohen, R. E.; McKinley, G. H.; Barbastathis, G. Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity. ACS Nano 2012, 6, 3789−3799. (43) Stavenga, D. G.; Foletti, S.; Palasantzas, G.; Arikawa, K. Light on the moth-eye corneal nipple array of butterflies. Proc. R. Soc. London, Ser. B 2006, 273, 661−667.

(44) Teng, Z.; Zheng, G.; Dou, Y.; Li, W.; Mou, C. Y.; Zhang, X.; Asiri, A. M.; Zhao, D. Highly ordered mesoporous silica films with perpendicular mesochannels by a simple Stober-solution growth approach. Angew. Chem., Int. Ed. 2012, 51, 2173−2177. (45) Park, H.; Shin, D.; Kang, G.; Baek, S.; Kim, K.; Padilla, W. J. Broadband optical antireflection enhancement by integrating antireflective nanoislands with silicon nanoconical-frustum arrays. Adv. Mater. 2011, 23, 5796−5800. (46) Lin, C. A.; Tsai, M. L.; Wei, W. R.; Lai, K. Y.; He, J. H. The Packaging Glass with a Hierarchically Nanostructured Surface: A Universal Method to Achieve Self-Cleaning Omnidirectional Solar Cells. ACS Nano 2016, 10, 549−555. (47) Perl, E. E.; McMahon, W. E.; Farrell, R. M.; DenBaars, S. P.; Speck, J. S.; Bowers, J. E. Surface structured optical coatings with nearperfect broadband and wide-angle antireflective properties. Nano Lett. 2014, 14, 5960−5964.

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DOI: 10.1021/acsphotonics.6b00888 ACS Photonics XXXX, XXX, XXX−XXX