pubs.acs.org/Langmuir © 2010 American Chemical Society
Fabrication of Antireflection and Antifogging Polymer Sheet by Partial Photopolymerization and Dry Etching Dongha Tahk,† Tae-il Kim,‡ Hyunsik Yoon,† Moonkee Choi,† Kyusoon Shin,† and Kahp Y. Suh*,‡,§ †
School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea, ‡School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Korea, and §World Class University Program on Multiscale Mechanical Design, Seoul National University, Seoul 15-1742, Korea Received December 18, 2009. Revised Manuscript Received January 8, 2010
We present a simple method to fabricate a polymer optical sheet with antireflection and antifogging properties. The method consists of two consecutive steps: photocross-linking of UV-curable polyurethane acrylate (PUA) resin and reactive ion etching (RIE). During photopolymerization, the cured PUA film is divided into two domains of randomly distributed macromers and oligomers due to a relatively short exposure time of 20 s at ambient conditions. Using the macromer domain as an etch-mask, dry etching was subsequently carried out to remove the oligomer domain, leaving behind a nanoturf surface with tunable roughness. UV-vis spectroscopy measurements demonstrate that transmittance of a nanoturf surface is enhanced up to 92.5% as compared to a flat PUA surface (89.5%). In addition, measurements of contact angle (CA) reveal that the etched surface shows superhydrophilicity with a CA as small as 5°. To seek potential applications, I-V characteristics of a thin film organic solar cell were measured under various testing conditions. It is shown that the efficiency can be increased to 2.9% when a nanoturf film with the surface roughness of 34.73 nm is attached to indium tin oxide (ITO) glass. More importantly, the performance is maintained even in the presence of water owing to superhydrophilic nature of the film.
Introduction The performance of Si based optical and optoelectronic devices such as solar cells, display panels, and light sensors is largely limited by the reflection of incident light. To tackle this challenge, thin film coatings with intermediate or gradient refractive indices have been used to suppress undesired reflection from the surface (e.g, Vikuiti from 3M company).1 However, some problems are yet to be solved such as poor adhesion and thermal mismatch.2 Since the discovery of the antireflection function in the moth eyes, an array of nanostructures, in either ordered or disordered format, has been considered as an alternative to thin film coatings. Such a structured surface can benefit from the monolithic construction, allowing for a more stable and robust antireflection coating. In order to prevent blurring that is induced by Mie scattering from surface roughness, structures with dimensions less than 400 nm need to be formed.3 Accordingly, there have been a number of approaches to realize antireflective nanostructures *To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: þ82-2-880-9103. Fax: þ82-2-883-0179.
(1) Gemici, Z.; Schwachulla, P. I.; Williamson, E. H.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2009, 9, 1064. (2) Smith, E. G.; Webber, G. B.; Sakai, K.; Biggs, S.; Armes, S. P.; Wanless, E. J. J. Phys. Chem. B 2007, 111, 5536. (3) Mie, G. Ann. Phys. 1908, 25, 377. (4) Lin, G. R.; Chang, Y. C.; Liu, E. S.; Kuo, H. C.; Lin, H. S. Appl. Phys. Lett. 2007, 90, 181923. (5) Wu, C. T.; Ko, F. H.; Lin, C. H. Appl. Phys. Lett. 2007, 90, 171911. (6) 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. Nat. Nanotechnol. 2007, 2, 770. (7) Kanamori, Y.; Roy, E.; Chen, Y. Microelectron. Eng. 2005, 78-79, 287. (8) Aydin, C.; Zaslavsky, A.; Sonek, G. J.; Goldstein, J. Appl. Phys. Lett. 2002, 80, 2242. (9) Yu, Z. N.; Gao, H.; Wu, W.; Ge, H. X.; Chou, S. Y. J. Vac. Sci. Technol., B 2003, 21, 2874. (10) Sai, H.; Fujii, H.; Arafune, K.; Ohshita, Y.; Yamaguchi, M.; Kanamori, Y.; Yugami, H. Appl. Phys. Lett. 2006, 88, 201116.
2240 DOI: 10.1021/la904768e
Figure 1. Schematic illustration of the fabrication process of a nanoturf surface with tunable roughness. The fabrication involves two serial steps of partial UV curing (∼ 20 s) and subsequent dry etching using Ar plasma.
using nanorods and nanopillars,4-12 gratings,11,13,14 porous structures,14,15 and nanotubes.16 In this work, we report on an alternative way of producing a nanostructured polymer sheet with antireflection and antifogging properties. One notable feature of the current method is that the (11) Sun, C. H.; Min, W. L.; Linn, N. C.; Jiang, P.; Jiang, B. Appl. Phys. Lett. 2007, 91, 231105. (12) Ller, T. L.; Helgert, M.; Sundermann, M.; Brunner, R.; Spatz, J. P. Nano Lett. 2008, 8, 1429. (13) Heine, C.; Morf, R. H. Appl. Opt. 1995, 34, 2476. (14) Nikolajeff, F.; Lofving, B.; Johansson, M.; Bengtsson, J.; Hard, S.; Heine, C. Appl. Opt. 2000, 39, 4842. (15) Striemer, C. C.; Fauchet, P. M. Appl. Phys. Lett. 2002, 81, 2980. (16) Wang, S.; Yu, X. Z.; Fan, H. T. Appl. Phys. Lett. 2007, 91, 061105.
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Figure 2. Planar SEM images showing the morphology of nanoturf surfaces with varying RIE times: (a) 20 s, (b) 30 s, (c) 60 s, (d) 90 s, (e) 120 s, (f) 150 s, (g) 180 s, and (h) 360 s. Each inset shows the corresponding AFM image. As shown, the RMS roughness and the etching depth (in parentheses) increase with time, which are 2.17 (8.00), 11.03 (27.30), 28.09 (46.41), 33.44 (82.17), 34.73 (95.91), 41.61 (106.36), 53.00 (116.17), and 86.08 nm (179.53 nm) for panels a-h, respectively.
Figure 3. (a) UV-vis spectroscopy measurement of transmittance versus wavelength (visible spectrum between 400 and 750 nm) for various exposure times. A flat PET surface is also shown for comparison. (b) Average and standard deviation of transmittance shown in (a).
surface is extremely hydrophilic (superhydrophilic) instead of superhydrophobic,17 so that water completely wets the surface. (17) Kim, T.-I.; Tahk, D.; Lee, H. H. Langmuir 2009, 25, 6576.
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Figure 4. (a) Photographs showing a contrast between flat and superhydrophilic PUA surfaces. No blurring is seen from the superhydrophilic area. Corresponding CAs are also shown on the right panel. (b) UV-vis spectroscopy measurements of transmittance of PET, nanoturf PUA (120 s exposure), flat PUA with a water droplet, and nanoturf PUA (120 s exposure).
This is opposed to what is usually found in the moth eyes as well as in other biomimetic structures such as lotus leaves, cicada wings, and gecko foot hairs.18-22 It is believed that such an antireflection (18) Zhang, J.; Sheng, X.; Jiang, L. Langmuir 2009, 25, 1371.
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Figure 5. I-V characteristics of a thin film organic solar cell under various testing conditions: (a) unmodified bulk heterojunction solar cell (reference), (b) same device as (a) with a nanoturf surface, (c) same device as (b) with a water blanket, and (d) same device as (a) with a flat PUA surface with water droplets. Results are summarized in the table.
and superhydrophilic surface can enhance light transmittance for a long period of time under humid environments at the expense of self-cleaning. The motivation here is that it is sometimes very difficult to create a defect-free superhydrophobic surface over a large area that can resist condensation of water drops.
Experimental Section A schematic procedure for the fabrication of a nanoturf surface is illustrated in Figure 1. First, a silicon wafer was cleaned by nitrogen blowing and ultrasonification for 6 min each in trichloroethylene (TCE) and methanol, followed by rinsing with deionized (DI) water and blow-drying by nitrogen. The cleaned wafer was baked on a hot plate at a temperature between 100 and 120 °C for 10 min to completely remove any residual solvent. Then drops of UV-curable polyurethane acrylate (PUA, 301RM, Minuta Tech., reflective index = 1.53) were dispensed onto the wafer, and a flexible polyethylene terephthalate (PET) film, approximately 50 μm thick, was placed on and roll-pressed against the liquid about 10 times at a pressure of 8-10 bar.23 This process was needed to minimize the thickness of the PUA layer, since a nonuniform thickness distribution could bring about poor optical transmittance. The thickness of the PUA layer was measured to be 10 μm after roll-pressing. The PET film used in this study was surface modified with urethane groups to increase adhesion to the acrylate-containing monomer (Minuta Tech. Korea). Subsequently, the PUA layer was treated with RIE for a variety of etching times from 20 to 360 s. The UV exposure time was fixed at 20 s (λ=250-400 nm, dose=100 mJ/cm2) for partial curing at ambient conditions. After separating the PET back-plane from the wafer, a flat PUA sheet was obtained. Finally, dry etching was performed using Ar plasma at the fixed conditions of 10 sccm, 200 mTorr, and 200 W (PVTronics Etch-100), leaving behind a nanoturf surface with tunable roughness.24 (19) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J.-K. Langmuir 2004, 20, 7665. (20) Sethi, S.; Ge, L.; Ci, L.; Ajayan, P. M.; Dhinojwala, A. Nano Lett. 2008, 8, 822. (21) Gao, X; Yan, X.; Yao, X.; Xu, L.; Zhang, K.; Zhang, J.; Yang, B.; Jiang, L. Adv. Mater. 2007, 19, 2213–2217. (22) Li, Y.; Zhang, J.; Zhu, S.; Dong, H.; Wang, Z.; Sun, Z.; Guo, J.; Yang, B. J. Mater. Chem. 2009, 19, 1806–1810. (23) Choi, S.-J.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem. Soc. 2004, 126, 7744. (24) Johnson, K. S.; Berggren, K. K.; Black, A.; Black, C. T.; Chu, A. P.; Dekker, N. H.; Ralph, D. C.; Thywissen, J. H.; Younkin, R.; Tinkham, M.; Prentiss, M.; Whitesides, G. M. Appl. Phys. Lett. 1996, 69, 2773.
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Results and Discussion Figure 2 shows planar scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of nanoturf surfaces after partial photopolymerization and post dry etching. As seen from the figure, the surface roughness as well as the etching depth increases with time, showing a disordered, randomly distributed network of nanoturf surface. Interestingly, the surface exhibits tunable root mean square (RMS) roughness with etching time, ranging from ∼2 to ∼86 nm. This morphology is largely determined by the formation of two domains of randomly connected macromers and oligomers. It was reported earlier25 that the elastic modulus and the hardness of the cured PUA resin continuously increased over a relatively long period of time from 30 s to 10 h at ambient curing conditions, suggesting that the resin would be partially cross-linked for the short exposure time of 20 s used in this study. According to ref 23, it appears that uncross-linked acrylate monomers or partially cross-linked oligomers are selectively removed during Ar plasma etching, whereas the completely cured domain would resist physical bombardment by Arþ ions. Thus, the macromers can act as an etch-mask against the Ar plasma, leading to disordered, nanostructured surfaces as shown in Figure 2. It should be noted that oxygen plasma did not reproduce the same results due to its low etching selectivity between the two domains. Figure 3 shows UV-vis spectroscopy measurements of tranmitance for various exposure times. As can be seen from the figure, the transmittance was enhanced with a nanoturf surface, except for a highly roughned surface with 360 s etching. In particular, it appears that there exists an optimal etching time of 120 s, for which the maximum transmittance of 92.5% was observed as compared to 89.5% for the flat PUA surface. In the case of other substrates such as quartz and glass, the nanoturf structure also showed a similar trend with increased optical transmittance (Supporting Information Figure S1). Contact angle (CA) measurements of a water droplet demonstrated that the nanoturf surface exihibits superhydrophilicty with a CA less than 5°. This behavior can be explained by the classical Wenzel model, in which hydrophilicity is amplified on a rough (25) Jeong, H. E.; Kwak, R.; Kim, J. K.; Suh, K. Y. Small 2008, 4(11), 1913– 1918.
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surface. According to the Wenzel theory,26 the apparent CA is given by cos θ ¼ r cos θ
ð1Þ
where θ* is the apparent CA on a textured surface, r is the roughness factor, and θ is the equilibrium CA on a smooth surface that is given by Young’s equation, cos θ = (γsv - γsl)/γlv where γ refers to the interfacial tension and the subscripts s, l, and v refer to the solid, liquid, and vapor phases, respectively.27 The CA on the Ar plasma treated, flat PUA surface was 77.5°, indicating that r is about 4.6. Figure 4a shows a contrast between flat and superhydrophilic PUA regions when a droplet is added to the surface. As shown, no blurring or scattering of light was observed from the hydrophilic region. Accordingly, tranmittance was nearly the same even in the present of a water droplet (91.5%), whereas a flat PUA sheet with a droplet showed a sharp descrease in the observed transmittance (Figure 4b). To seek potentional applications of an antireflection and antifogging polymer sheet, a thin film organic solar cell was fabricated and its I-V characteristics were measured under various testing conditions. A detailed procedure for the fabrication of the solar cell can be found elsewhere.28 As can be seen from Figure 5, the conversion efficiency was slighly increased with a nanoturf surface (η = 2.9%) as compared to the reference (η = 2.7%), demonstrating approximately 7.4% improvement under AM 1.5 illumination. The efficiency did not show an appreciable descrease when a water blanket was added, suggesting that superhydrophility plays a significant role in retaining the efficiency. When a flat PUA sheet was used with water droplets, however, the efficiency was reduced to 2.6% because of incresed light scattering from the droplets. In terms of electric power generation, the current results could be beneficial, in particular (26) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (27) Tuteja, A.; Choi, W.; Ma, M.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Science 2007, 318, 1618. (28) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323.
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when operation in a humid environement (e.g., rainy day) is needed.
Summary In this work, we have presented a simple approach to the fabrication of a nanoturf surface as an antireflection and antifogging polymer sheet. The method involves a two-step process of partial photopolymerization and dry etching, which is simple, scalable and can be applied to other polymers. Based on the measurements of transmittance under various etching conditions, the PUA sheet with 120 s etching time (RMS = 34.73 nm and etch depth = 95.91 nm) showed the best antireflection performance (92.5% transmittance). In addition, the nanoturf surface exhibited superhydrophilicity (CA < 5°), allowing for antifogging characteristic in a humid environment. As a consequence, the transmittance was maintained at a similar value even in the presence of a water blanket. Finally, I-V characteristics of a thin film organic solar cell were measured under various testing conditions, which revealed that the current antireflection and antifogging polymer sheet has the potential for improved conversion efficiency of solar cells as well as other optical and optoelectronic devices such as display panels, optical sensors, and projection optics. Acknowledgment. This work was supported by the Korea Research Foundation Grant (MOEHRD) (Grant KRF-J03003) and the World Class University Program (R31-2008-000-100830). This research was also supported in part by a grant from Construction Technology Innovation Program (CTIP) funded by Korea Ministry of Land, Transportation and Maritime Affairs (MLTM)(09CCTI-B050566-02-000000) and Korea Ministry of Environment as “The Eco-technopia 21 project”. Supporting Information Available: UV-vis spectroscopy measurements of the transmittance of a nanoturf structure on various substrates. This material is available free of charge via the Internet at http://pubs.acs.org.
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