pubs.acs.org/Langmuir © 2009 American Chemical Society
Broadband Antireflection Coating Covering from Visible to Near Infrared Wavelengths by Using Multilayered Nanoporous Block Copolymer Films Wonchul Joo, Hye Jeong Kim, and Jin Kon Kim* National Creative Research Initiative Center for Block Copolymer Self-Assembly, Department of Chemical Engineering and Polymer Research Institute, Pohang University of Science and Technology, Kyungbuk 790-784, Korea Received September 22, 2009. Revised Manuscript Received November 11, 2009 Broadband antireflection (AR) covering from visible light to near infrared (NIR) wavelengths (400-2000 nm) was obtained by using three sequential spin-coatings of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymers with different volume fractions of PMMA block (fPMMA) on a glass. PS-b-PMMA having the lowest PMMA volume fraction (fPMMA ∼ 0.3) among three PS-b-PMMAs was first spin-coated on a glass substrate. After spincoating, the film was irradiated by ozone to prevent dissolution during the next spin-coating process. Then PS-b-PMMA with the next larger volume fraction of PMMA block (fPMMA ∼ 0.46) was spin-coated and irradiated again by ozone. Finally, PS-b-PMMA with the largest volume fraction of PMMA block (fPMMA ∼ 0.69) was spin-coated. After three sequential spin-coatings, the entire film was irradiated under UV followed by rinsing with acetic acid, which removed PMMA blocks. This process allowed us to have the triple layers with spongelike nanoporous structures where the refractive index increases from the top to the bottom of the film. The morphology of the triple-layered nanoporous block copolymer films was investigated by scanning electron microscopy. The nanoporous film exhibited excellent broadband AR at wavelengths from 400 to 2000 nm. The measured reflectance curves are in good agreement with the calculation from the characteristic matrix theory. This AR coating would be used for the development of solar cells with high power convergence efficiency.
1. Introduction Nanoporous thin films have been extensively used for templates of inorganic nanostructures,1-5 filters for biomolecules *To whom correspondence should be addressed. Fax: þ82-54-279-8298. E-mail:
[email protected]. (1) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401. (2) Thurn-Albrecht, T.; Schotter, J.; K€asstle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (3) Lee, J. I.; Cho, S. H.; Park, S.; Kim, J. K.; Kim, J. K.; Yu, J.; Kim, Y. C.; Russell, T. P. Nano Lett. 2008, 8, 2315. (4) Park, O.; Cheng, J. Y.; Hart, M. W.; Topuria, T.; Rice, P. M.; Krupp, L. E.; Miller, R. D.; Ito, H.; Kim, H. Adv. Mater. 2008, 20, 738. (5) Gong, Y.; Joo, W.; Kim, J. K. Chem. Mater. 2008, 20, 1203. (6) Kim, J. K.; Lee, J. I.; Lee, D. H. Macromol. Res. 2008, 16, 267. (7) Yang, S. Y.; Ryu, I. C.; Kim, J. K.; Jang, S. K.; Russell, T. P. Adv. Mater. 2006, 18, 709. (8) Yang, S. Y.; Yoon, J. H.; Ree, M. H.; Jang, S. K.; Kim, J. K. Adv. Funct. Mater. 2008, 18, 1371. (9) Peinemann, K.; Abetz, V.; Simon, P. F. W. Nat. Mater. 2007, 6, 992. (10) Ding, S.; Wang, P.; Wan, X.; Zhang, D. W.; Wang, J.; Lee, W. W. Mater. Sci. Eng., B 1999, 83, 130. (11) Wu, Z.; Walish, J.; Nolte, A.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2006, 18, 2699. (12) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (13) Wu, Z.; Lee, D.; Rubner, M. F.; Cohen, R. E. Small 2007, 3, 1445. (14) Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 7293. (15) Joo, W.; Park, M. S.; Kim, J. K. Langmuir 2006, 22, 7960. (16) Min, W.; Jiang, B.; Jiang, P. Adv. Mater. 2008, 20, 3914. (17) Biswas, K.; Gangopadhyay, S.; Kim, H.; Miller, R. D. Thin Solid Films 2006, 514, 350. (18) Xi, J. Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M.; Lin, S.; Liu, W.; Smart, J. A. Nat. Photonics 2008, 2, 76. (19) Diedenhofen, S.; Vecchi, G.; Algra, R. E.; Hartsuiker, A.; Muskens, O. L.; Immink, G.; Bakkers, E. P. A. M.; Vos, W. L.; Rivas, J. G. Adv. Mater. 2009, 21, 973. (20) Huang, Y.; Chattopadhyay, S.; Jen, Y.; Peng, C.; Liu, T.; Hsu, Y.; Pan, C.; Lo, H.; Hsu, C.; Chang, Y.; Lee, C.; Cheng, K.; Chen, L. Nat. Nanotechnol. 2007, 2, 770.
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such as proteins,6-9 dielectric materials for electronic devices,10 and optical materials for photonic crystals or antireflection coatings.11-23 Among many applications, antireflection (AR) coating has gained much attention in the flat panel display industry. It removes light reflection by destructive interference of light at the surface between the air and transparent substrate. The reduction of light reflection, thus the increase in the transmittance of light, allows clearer and brighter viewing of images. To obtain perfect light reflection, one should choose a proper film thickness (t) with an optimum refractive index (n). For a one-layer AR coating, zero reflection is achieved at two requirements:24 (1) nf = (nsn0)1/2, with nf, ns, and n0 being the refractive indices of an AR film, a substrate, and a transmitted medium, respectively; (2) the film thickness should be a quarter of a specific wavelength in the optical medium. For a glass substrate with n = 1.52, n of an AR film must be 1.23 to achieve zero reflectance.24 However, since n of most organic or inorganic materials is higher than 1.23, most AR films are prepared by introducing nanoporous structures into the film. Many nanoporous polymer films have been prepared by phase separation of two homopolymers,25 layer-by-layer assembly,13,14 anodized alumina template,26 colloid particles,27,28 and plasma treatment on polymer surface.29 (21) Yanagishita, T.; Nishio, K.; Masuda, H. Appl. Phys. Express 2008, 1, 067004. (22) Kim, S.; Cho, J.; Char, K. Langmuir 2007, 23, 6737. (23) Park, M. S.; Lee, Y.; Kim, J. K. Chem. Mater. 2005, 17, 3944. (24) Macleod, H. A. Thin-Film Optical Filters; Hilger: Bristol, 1986. (25) Walheim, S.; Sch€affer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (26) Kim, M.; Kim, K.; Lee, N. Y.; Shin, K.; Kim, Y. S. Chem. Commun. 2007, 22, 2237. (27) Koo, H. Y.; Yi, D. K.; Yoo, S. J.; Kim, D. Y. Adv. Mater. 2004, 16, 274. (28) Ha, J.; Park, I. J.; Lee, S. Macromolecules 2008, 41, 8800. (29) Kaless, A.; Schulz, U.; Munzert, P.; Kaiser, N. Surf. Coat. Technol. 2005, 200, 58.
Published on Web 12/03/2009
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Table 1. Characteristics of Three PS-b-PMMAs Employed in This Study notationa
Mn
PDI
fPMMA
remark
SMMA30 91 000 1.08 0.30 anionically synthesized in this lab SMMA46 98 200 1.13 0.46 #P2355-SMMA SMMA69 94 200 1.15 0.69 #P2406-SMMA a The number after SMMA represents the volume fraction of PMMA in PS-b-PMMA.
Previously, we showed that a spongelike nanoporous film prepared with polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) exhibited good AR.15 With control of the volume fraction of PMMA block (fPMMA) in PS-b-PMMA, which was selectively removed by UV irradiation followed by rinsing with acetic acid, almost zero reflectance (less than 0.1%) was achieved at a target wavelength which depended on film thickness. However, a single-layer AR film exhibits the minimum reflection at a specific wavelength, namely a V-shaped reflectance curve is obtained. To obtain AR at broadband wavelengths, two or more layers are needed. Kim and co-workers27 showed that snowmanlike polystyrene colloid particles, which acted as the double layer, exhibited a broadband AR at visible wavelengths. Although excellent AR at visible wavelengths is important in flat panel displays, the broadband wavelengths should be extended to the near IR wavelengths (800-2000 nm) to develop solar cells with high power conversion efficiency. This is because the solar power density at near IR wavelengths (800-2000 nm) is still larger, although at visible wavelengths (400-800 nm) it is strong.30 To achieve this objective, at least three layers are needed, where n should change from lower to higher values with thickness direction from the top surface to the bottom of the film. In this study, we prepared triple-layered nanoporous films by sequential spin-coating of three different PS-b-PMMAs with different fPMMA. However, when the same solvent (here, toluene) is used for spin-coating, the solvent completely dissolves a premade PS-b-PMMA layer during the next spin-coating process. To prevent this problem, we treated the film with ozone after each spin-coating. Ozone treatment causes PS chains to cross-link,31 and thus the dissolution of the premade PS-b-PMMA layer is effectively prevented during the next spin-coating with the same solvent. After three consecutive spin-coatings of PS-b-PMMA with different fPMMA, the PMMA blocks are completely removed by UV irradiation followed by rinsing with acetic acid. We found that the triple-layered film with different porosities (and thus different values of n) in each layer showed an excellent AR covering from visible light to NIR wavelengths. We consider that this AR coating would be used for the development solar cells with high power convergence efficiency.
2. Experimental Section Materials and Fabrication. Table 1 gives molecular characteristics of three PS-b-PMMAs with different volume fractions of PMMA block. SMMA30 was synthesized by using anionic polymerization in the laboratory. The number-average molecular weight (Mn) and polydispersity index (PDI) of SMMA30 were measured by size exclusion chromatography with a multiangle laser light scattering detector. fPMMA was determined by 1H NMR (30) Ireland, P. J.; Wagner, S.; Kazmerski, L. L.; Hulstrom, R. L. Science 1979, 204, 611. (31) Jeong, U.; Ryu, D. Y.; Kim, J. K.; Kim, D. H.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2003, 15, 1247. (32) Brandup, J., Immergut, E. H., Eds. Polymer Handbook; John Wiley & Sons, Inc.: New York, 1989.
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Figure 1. Schematic of the preparation of the triple-layered nanoporous film. spectroscopy (Bruker DPX300) with the mass densities of PS (1.05 g/cm3) and PMMA (1.18 g/cm3).32 SMMA46 and SMMA69 were purchased from Polymer Sources and were used as received. Glass slide, soda lime glass which has a refractive index of 1.52, was purchased from Corning Glass Works (Corning brand, Plan (product #2947)). After being sliced to 1.5 cm 1.5 cm by using a diamond knife, all glass was cleaned by dipping in a mixture of sulfuric acid and hydrogen peroxide for 30 min at 80 °C (piranha treatment). Figure 1 shows a schematic of the preparation of the triplelayered nanoporous film. SMMA30 in toluene (3 wt %) was spincoated on a cleaned glass at a rotating speed of 3000 rpm. The film was exposed to ozone environment (UV/ozone cleaner (GCS1700, AHTECH LTS, Korea) with block UV light) for 80 min at room temperature. We found that only PS blocks were selectively cross-linked by ozone treatment, whereas no chemical degradation of PMMA blocks occurred.31 Then SMMA46 in toluene (3 wt %) was spin-coated at a rotating speed of 2000 rpm on the preformed SMMA30 layer, which was also treated by ozone. We found that the premade SMMA30 layer was maintained without dissolving into toluene after this process. Finally, SMMA69 in toluene (3%) was spin-coated at a rotating speed of 1500 rpm on the preformed SMMA46/SMMA30 double layer. We removed all of the PMMA in the three layers by irradiating with UV (Sankyo Denki, G15T8, maximum intensity at 253.7 nm) for 120 min under vacuum followed by rinsing with acetic acid, which is a selective solvent for PMMA.15,33 Characterization. We used Fourier transform infrared spectroscopy (FTIR; Bruker IFS 66 v/s) with transmission mode to check whether all of PMMA parts were completely removed by UV irradiation followed by rinsing with acetic acid. We found that the absorption peak at 1730 cm-1 corresponding to the carbonyl peak (CdO) in the PMMA block was clearly observed for the as-cast film. However, when the film was irradiated by UV followed by rinsing with acetic acid, the peak at 1730 cm-1 was barely observed. When we compared the ratio of the absorption intensity at 702 cm-1 corresponding to the phenyl ring stretching in the PS block to that at 1730 cm-1, the remaining PMMA after UV treatment was less than 1 vol %. The surface and inner morphology of nanoporous PS-bPMMA films were investigated by atomic force microscopy (AFM; Nanoscope IIIa, Digital Instrument) in tapping mode with a silicon tip (NCH-10, Pointprobe) and by field emission scanning electron microscopy (SEM; Hitachi S-4800) with an accelerated voltage of 8 kV. Osmium tetraoxide (Meiwa Shoji Co., NEOC-ST) was coated on the sample surface and dried in a vacuum for 1 day. The reflectance and transmittance curves of nanoporous films were measured by using a UV-vis spectrophotometer (Varian, Cary 5000) with a specular reflectance module and an incidence angle of 12° at 0.3 s/nm exposure. For the reflectance measurement, (33) Jeong, U.; Kim, H. C.; Rodriquez, R. L.; Tsai, I. Y.; Stafford, C. M.; Kim, J. K.; Hawker, C. J.; Russell, T. P. Adv. Mater. 2002, 14, 274.
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Figure 2. Cross-sectional SEM images (left panels) and height profiles (right panels) obtained from AFM height images of the films before the removal of PMMA: (a, d) single-layered (SMMA30), (b, e) double-layered (SMMA46 on SMMA30), and (c, f) triple-layered (SMMA69 on the SMMA46/SMMA30) block copolymer films. Scale bar is 100 nm. one side of the glass was covered with a black tape to eliminate light reflection at the back side of the glass.15 For the transmittance measurement, the nanoporous films were coated at both sides of the glass. Film thickness and refractive index of each layer were characterized by the characteristic matrix theory (CMT).24 The optical images of glasses with multilayered AR films and a bare glass were taken by using a normal camera (ls753, KODAK) and an IR camera. An IR camera was assembled in the lab as follows. (1) A filter cutting the signals resulting from the NIR wavelengths, which is located between the sensor and lens, was removed from a normal CMOS camera (NX-3000, Microsoft). (2) Another filter (BþW 093, Schneider Technology) that cuts the signals at the visible wavelengths under 800 nm was placed in front of the lens. The samples were placed under a fluorescent lamp inside the room, and the images of all of the samples were taken together.
3. Results and Discussion The left panels in Figure 2 give cross-sectional SEM images of PS-b-PMMA films for the single layer with SMMA30, double layers with SMMA46 on SMMA30, and triple layers (SMMA69 on the SMMA46/SMMA30 layer) before removing the PMMA blocks. Even though the second (and the third) layer was fabricated by spin-coating of PS-b-PMMA in toluene, the preformed layer was maintained without being dissolved by toluene due to the cross-linking of PS block by ozone treatment. The right panels in Figure 2 give the height profiles obtained from AFM height images of the top surface in the three different films. All the films have a very smooth surface with the root-mean-square roughness being less than 1 nm. Thus, we expect that light scattering due to the surface roughness was negligible. From the above results, the double- and triple-layered films are fabricated without damaging the preformed layer. Figure 3 gives SEM images (top surface, and 55° and 90° tilted cross-sectional views) for the single, double, and triple layers after selective removal of the PMMA blocks. The top surface SEM images (Figure 3a, d, g) show that nanopores are successfully 5112 DOI: 10.1021/la9035858
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generated and the porosity in the top layer increases from the single to the triple layers. This is consistent with our expectation, since the top layer in Figure 3a, d, and g is SMMA30, SMMA46, and SMMA69, respectively. As shown in Table 1, fPMMA, which is proportional to the porosity, of SMMA30 is the smallest, whereas it is the largest for SMMA69. Thus, we concluded that the porosity of the triple-layered films decreased from the top to the bottom of the film. Since all the films were not annealed at high temperatures, well-defined nanoporous structures such as cylindrical or lamellar type were not formed. Rather, spongelike nanoporous structures were generated resulting from fast solvent evaporation during spin-coating.15 However, the size of the nanopores was less than ∼50 nm, which can sufficiently prevent the scattering of light. We also found from the cross-sectional SEM images that the top layer in double- and triple-layered film was well attached to the bottom layer without collapsing the nanoporous structure in the premade layer. From 90° tilted SEM images (Figure 3c, f, i), the total film thickness is 111, 245, and 390 nm for SMMA30 single-layered, SMMA46/ SMMA30 double-layered, and SMMA69/SMMA46/SMMA30 triple-layered nanoporous films, respectively. Thus, the thickness (t) of each layer (SMMA69, SMMA46, and SMMA30) in the triple-layered nanoporous films is 145, 134, and 111 nm, respectively. Figure 4a gives the reflectance (R) curves of the triple-layered nanoporous film. The reflectance is lower than 1% in the entire wavelength range from 400 to 2000 nm (covering visible light and NIR wavelengths). It is 0.5-0.8% at visible wavelengths (400-800 nm) with the minimum of 0.5% at 700 nm. Although this value is slightly higher than the reflectance (0.3-0.5%) obtained for the optimum film geometry of the double-layered nanoporous film, AR at NIR wavelengths for the triple layered film is much better than that of the double-layered films (see the Supporting Information). Namely, the reflectance of the triplelayered nanoporous film is 0.9% at 2000 nm, but it is 2.2% for the double-layered nanoporous film. When the CMT is employed to characterize the triple-layered nanoporous film, n and t of the SMMA69 layer, the SMMA46 layer, and the SMMA30 layer are 1.21 and 140 nm, 1.33 and 130 nm, and 1.41 and 110 nm, respectively. The thicknesses of each layer are almost the same as the ones measured by SEM images (Figure 3). Furthermore, the calculated n for each layer is also consistent with the estimated value based on the following equation.15,34 n2 ¼ npolymer 2 ð1 -fpore Þ þ nair 2 fpore
ð1Þ
where npolymer, nair, and fpore are the refractive index of the polymer (in this case, PS) and air, and the pore volume fraction, respectively. Since fpore is the same as fPMMA, the calculated values of n of SMMA69, SMMA46, and SMMA30 are 1.22, 1.35, and 1.44, respectively, when nPS = 1.615 and nair = 1.0. The reflectance of the nanoporous films depends on n and t of each layer. The control of t is simply done by changing the concentration of the solution and the rotating speed during spincoating. Also, n is easily tuned either by synthesizing PS-bPMMA with specific fPMMA or by the addition of PMMA (or PS) homopolymer to PS-b-PMMA. Once the molecular weight of PMMA (or PS) homopolymer is properly chosen to prevent the macrophase transition between the homopolymer and the block copolymer, the fine-tuning of n is easily achieved. (34) Choy, T. C. Effective Medium Theory; Oxford University Press: New York, 1989.
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Figure 3. SEM images of the nanoporous films after selective removal of PMMA with UV irradiation followed by rinsing with acetic acid: (a-c) single-layered, (d-f) double-layered, and (g-i) triple-layered nanoporous films. Images from left to right columns represent top surface SEM image (a, d, g), and 55° (b, e, h) and 90° (c, f, i) tilted cross-sectional SEM images.
Figure 4. Reflectance curves for (a) SMMA69/SMMA46/ SMMA30 triple-layered nanoporous film and (b) another triplelayered nanoporous film with controlled n and t. Symbols (]/4) represent measured reflectances, and solid line represents calculated reflections by the CMT with characteristic parameters given in the inset.
For example, we prepared another triple-layered nanoporous film. Here, n of the top layer was decreased to 1.17 by the addition of 30 wt % PMMA homopolymer (Mn = 100 000 and PDI = 1.07, purchased from Polymer Sources) relative to the PMMA Langmuir 2010, 26(7), 5110–5114
block in SMMA69. The reflectance curve for a new designed film is given in Figure 4b. An excellent broadband AR (less than 0.5%) is seen at visible light wavelengths (400-800 nm) with a minimum value of 0.2% at 520 nm as well as some of NIR regions (up to 1500 nm). However, the reflectance at 2000 nm is 1.2%, which is slightly higher than that (0.9%) for the triple-layered nanoporous film shown in Figure 4a. We consider that the film used in Figure 4b would be very useful for the AR of the solar cell, where the solar power density at wavelengths above 1800 nm is less important.30 Furthermore, this triple-layered nanoporous film showed better reflectance at visible light wavelengths compared with the double-layered nanoporous film. This triple-layered nanoporous film is also characterized by the CMT. n and t of the first layer, the second layer, and the third layer are 1.17 and 110 nm, 1.32 and 105 nm, and 1.42 and 95 nm, respectively. From the results given in Figure 4, an excellent AR covering from visible light to NIR wavelengths is easily achieved for triplelayered nanoporous films. Since the control of t and n of each layer is easily done independently, one can prepare AR films to meet a specific purpose. We consider that since the fabrication of the triple-layered nanoporous film is very simple and robust, it could be used as an effective AR film for solar cells with high power conversion efficiency. Furthermore, if one needs the AR at only visible wavelengths, a double-layered nanoporous film could be used (see the Supporting Information). Although the reflectances (R) given in Figure 4 show an excellent antireflection effect, the transmittance (T) of light is also important, especially for solar cell application. This is because the light transmission can be decreased due to the light scattering or absorption from the nanoporous structures, even when light reflection is observed to be very low. Figure 5a and b gives the transmittance curves of the triple-layered nanoporous films employed in Figure 4a and b, respectively. It is seen that both films showed very high transmittance covering from visible light to the NIR region. Furthermore, the measure values of T in Figure 5 are very similar (less than 1%) to 1-2R, when the values of R in Figure 4 are used. Thus, the light scattering or absorbance which may arise from the nanoporous structures (or roughness) DOI: 10.1021/la9035858
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Figure 6. Optical images for the glasses coated with single-, double-, and triple-layered AR films taken with (a) a normal camera and (b) an IR camera. For reference, the images for the glass without AR film were added.
slightly improved for the single- and double-layered AR films, but there still exists high reflection from the surface. On the other hand, the triple-layered AR films showed much reduced reflection than the double-layered AR films. These results are consistent with reflectance curves in Figures 4 and S1 (Supporting Information). Thus, we conclude that the triple-layered AR film showed excellent broadband AR covering from visible light to NIR wavelengths.
4. Conclusion Figure 5. Transmittance curves of triple layered films employed in Figure 4a and b, respectively.
of the film is negligible in nanoporous films employed in this study. Figure 6 gives the optical images of the glasses coated with single-, double-, and triple-layered AR films taken with both a normal camera and an IR camera. The IR camera completely removed the signals at the visible light wavelength via a filter cutting visible light, whereas the normal camera completely removed the signals at the IR wavelengths. Here, the triple AR film was the same as that used in Figure 4b. It is seen in Figure 6a that the light reflection was significantly reduced for the doubleand triple-layered AR films compared with a bare glass and single-layer AR film. Interestingly, the words of “POSTECH” under a bare glass could not be recognized when the image was with the IR camera (Figure 6b). The readability of the words was
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We fabricated multilayered nanoporous block copolymer films by using ozone treatment after each spin-coating of PS-b-PMMA. Since the nanoporous film has a gradual decrease of refractive index from the top to the bottom of the film, it shows excellent broadband AR covering from visible light to NIR wavelengths. This AR coating could be used to fabricate solar cells with high power conversion efficiency. Acknowledgment. This work was supported by the National Creative Research Initiative Program of the National Research Foundation of Korea (NRF). Supporting Information Available: Reflectances and morphologies of the single- and double-layered nanoporous films prepared by PS-b-PMMA. This material is available free of charge via the Internet at http://pubs.acs.org.
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