Self-cleaning transparent heat mirror with plasma polymer

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Self-cleaning transparent heat mirror with plasma polymer fluorocarbon thin film fabricated by continuous roll-to-roll sputtering process Sung Hyun Kim, Mac Kim, Jae Heung Lee, and Sang-Jin Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00761 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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ACS Applied Materials & Interfaces

Self-cleaning transparent heat mirror with plasma polymer fluorocarbon thin film fabricated by continuous roll-to-roll sputtering process Sung Hyun Kim§,‡, Mac Kim§, Jae Heung Lee§ and Sang-Jin Lee*,§

§

Chemical Materials Solutions Center, Korea Research Institute of Chemical Technology,

Daejeon 34114, Republic of Korea ‡

Department of Nano Fusion Technology, Pusan National University, Busan 46241, Republic

of Korea

Keywords: self-cleaning, transparent heat mirror, plasma polymer fluorocarbon, roll-to-roll sputtering, near-IR reflection

Abstract This paper proposes a novel self-cleaning transparent heat mirror (SC-THM) produced by depositing a plasma polymer fluorocarbon thin film on a silver-and-SiNx multilayer structure fabricated by continuous roll-to-roll sputtering. The optimal structure and thickness of each

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thin film of three-layer and five-layer SC-THM were derived from optical simulation. In the five-layer SC-THM, the visible light transmittance was 60.67% at a wavelength of 406 nm, and the infrared (IR) transmittance was 6.86% at a 1,000-nm wavelength and 2.50% at a 1,500-nm wavelength. The value of the performance parameter Tvis/Tsol was 1.70. The SCTHM exhibited self-cleaning with very good water repellency of more than 111 degrees, achieved by applying a low surface energy-fluorocarbon thin film to the top layer. The study successfully demonstrated the IR blocking properties of an SC-THM through IR reflection and IR irradiation experiments.

Introduction Sunlight consists of ultraviolet (0.2–0.4 µm), visible (0.4–0.7 µm) and infrared (IR) (0.7 µm–2.0 µm) rays in proportions of 5, 40, and 55%, respectively.1 Glass and transparent polymer films that are used in buildings, cars, and greenhouses to ensure visibility and aesthetics allow the passage of most IR light, as well as visible light. The transmission of visible light is essential to brighten interiors and ensure visibility in the daytime, but most infrared rays transmit heat and require enormous energy expenditures for additional cooling or heating. A transparent heat mirror (THM) transmits the visible light in sunlight and reflects IR rays, and it also conserves internal radiant heat. A THM is formed as a thin film on the surface of a glass substrate.2-14 THMs are used to reduce electrical power consumption in energy-efficient buildings, greenhouses, and energy-saving cars because they block external IR rays in hot climates and preserves internal heat in cold climates. Oxide materials such as ITO,2-5,15,16 TiO2,2,4,8 WO3,3,9 HfO2,7 ZnO,17,18 (Al, Ga)-doped ZnO,19-23 and MoO324 which exhibit high visible-light transmittance but relatively low IR blocking properties, have been studied for use in THMs.


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Insulator/metal/insulator THM structures have been studied to assess their ability to enhance IR blocking and ensure the transmittance of visible light.2,3,6-9 The reflective layer is typically composed primarily of a noble metal, such as Ag, Au, or Cu.2,3,7-9,24-27 Of these, Ag has the lowest absorption in the visible region and is well suited for use in THMs because of its refractive index (n = 0.06) and extinction coefficient (k = 3.59).11-13,28,29 The dielectric thin films on both sides of Ag create two interfaces that have opposite phases, resulting in destructive interference and thus producing an antireflection effect.13,14 The insulator of the top layer prevents corrosion and abrasion of the metal, and the insulator of the bottom layer influences the continuous growth of the metal thin film as a nucleation layer.30,31 WO3/Ag/WO3,3

TiO2/Ag/TiO2,2,8

HfO2/Ag/HfO2,7

WO3/Au/WO3,9

AZO/Cu/AZO,23

AZO/Ag/AZO,25,26 and ITO/Ag/ITO27 structures have been examined for use as three-layer structures for THMs and ZrO2/ZrN/ZrO26 and TiO2/TiN/TiO232 structures using metal nitride have also been investigated. Three-layer THMs exhibit high visible light transmittance, but their transmittance in the near-IR region of wavelengths between 700 and 1500 nm, which accounts for a large portion of the IR region of sunlight, has been shown to be more than 20%. Therefore, THMs with better IR blocking characteristics in the near-IR region are required. In this study, a self-cleaning THM (SC-THM) with excellent transmittance of visible light, a self-cleaning surface, and excellent IR blocking in the near-IR region was developed. Ag was used as the reflective metal. SiNx, a high-refractive-index thin film, was used as the bottom-layer thin film. A plasma polymer fluorocarbon (PPFC) thin film was used as the top layer for the first time. This approach to THM development is novel in four ways. First, when the PPFC is deposited on the top layer, the transmittance of the visible light is increased because of the low refractive index, and the reflectance of the near-IR region is remarkably reduced. Second, the PPFC thin film is composed of C–F bonds and has very low surface energy, so its surface exhibits high water repellency. Because THMs are usually formed on ACS Paragon Plus Environment


the exterior surfaces of buildings and cars, a THM with high water repellency will have a self-cleaning effect. As a result, the efficiency of THM can be maintained constantly. Third, all of the layers are fabricated by sputtering in a single chamber. When a carbon nanotube /polytetrafluoroethylene (CNT/PTFE) composite is used as the target, a PPFC thin film can be formed using a midrange-frequency (MF) power source rather than a radio-frequency (RF) power source.33,34 Fourth, the proposed process involves fabrication of SC-THM films continuously on a transparent PET film substrate with a width of 700 mm using large-area roll-to-roll sputtering. When an SC-THM is formed on a film substrate, it can be used in a variety of applications because it can be attached to existing buildings or automobile glass. In this study, three-layer and five-layer SC-THMs with designs derived from optical simulation were fabricated. The transmittance of visible light and IR rays was measured for each SC-THM, and the structure of each multilayer was analyzed. The water repellency and IR shielding properties of the fabricated SC-THM films were also evaluated.

Experimental Procedure We fabricated SC-THM on a 700-mm-wide PET substrate using a roll-to-roll sputtering system (SPW060, ULVAC). Figure 1(a) shows a roll-to-roll sequence diagram for the Ag/SiNx (dyad) and PPFC coating processes. The polymer substrate film comes from the unwinder and is coated through the main roll and finally rewound on the winder roll. A surface treatment was performed to improve the adhesion between the thin film and the substrate, using a thermal heater and Ar/O2 plasma treatment at 300 W during passage of the film through the unwinder module. A SiNx thin film was then deposited by reactive sputtering in an N2 atmosphere with a Si target at an MF cathode while the substrate film was moved


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forward, and the Ag layer was deposited continuously in an Ar atmosphere with a Ag target at a DC cathode. After the dyad (Ag/SiNx) coating was formed, the PPFC thin film was deposited on the MF cathode using a CNT/PTFE composite target in the opposite direction. Due to dielectric properties, polymer materials such as PTFE can be sputtered only through RF (13.56 MHz) power source but CNT/PTFE composite materials could impart low resistance by percolation conduction mechanism of CNTs and could be used for MF and DC power source. In this study, we used the CNT 5 wt%/PTFE 95 wt% composite target with approximately 1 Ω/□ sheet resistance which was stably sputtered at 20 ~ 80 kHz frequency MF power source.33 In the five-layer structure, the dyad was deposited twice in the forward direction, and the PPFC film was deposited in the top layer in the reverse direction. The deposition rates of the SiNx, Ag, and PPFC were very different depending on the process conditions, so the MF and DC power and line speed were controlled during deposition of each target. Base pressure was 1.7×10-4 Pa. The SiNx thin films were deposited at a thickness of 30 nm under MF 3.15 W/cm2 power density condition. Ar (400 sccm) and N2 (100 sccm) were used for sputtering gas and working pressure was 5.5×10-4 Pa. The Ag thin films were deposited at thicknesses of 6, 8, and 10 nm at DC 0.25, 0.33 and 0.41 W/cm2 power density conditions, respectively, and a line speed of 1.0 m/min. The PPFC thin films were deposited at thicknesses of 30, 40 and 50 nm at MF 0.83, 0.91 and 1.04 W/cm2 power density conditions, respectively, and a line speed of 0.2 m/min. Figure 1(b) illustrates the SC-THM film and layer structure deposited on a 700-mm-wide PET film roll. Optical simulation was performed using the Macleod program. The optical transmittances of the SC-THM films were measured in the wavelength range of 300–2400 nm with an optical spectrometer (U-4100, Hitachi). The structure of the SC-THM film was examined with field-effect transmission electron microscopy (FE-TEM, TECNAI G2 F30 S-TWIN, FEI Co.), and an element in each layer was analyzed by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS, ION-



TOF, Münster). Water contact angle measurements were carried out using a contact angle analyzer (PHEONIX 300 Touch, Surface Electro Optics).

Figure 1. (a) Sequential diagram of the deposition procedure for the dyad (Ag / SiNx) and PPFC and (b) schematic illustration of the SC-THM film and layer structure deposited on a 700-mm-wide PET film roll.


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Results and Discussion Prior to fabrication of the SC-THM film, the design of the thin film structure and the thickness of each thin film were developed on the basis of optical simulation. Highrefractive-index SiNx thin film, which is relatively easy to deposit by reactive sputtering, was used as the bottom layer, Ag was used as a metal reflection film, and a low-refractive-index PPFC was used as the top layer to maximize visible light transmission and block IR rays effectively. The transmittance simulation results for various thicknesses of PPFC, Ag and SiNx thin film are shown in Figure S1 to S4. Based on the simulation results, a PPFC thickness of 40 nm and Ag thickness of 10 nm were selected, and the SiNx thickness was varied from 10 nm to 50 nm in increments of 10 nm. The visible light transmittance was high and the IR blocking properties were good when the SiNx thickness was 30 nm. In addition, the visible light transmittance increased with increasing thickness of PPFC, and an appropriate thickness of 50–60 nm was confirmed by the simulation results (see Figure S2). Therefore, we first fabricated a three-layer SC-THM structure (one PPFC layer and a onedyad Ag/SiNx) on a PET substrate by means of a continuous roll-to-roll sputtering process. Figure 2 shows the transmittance measurement results as a function of the PPFC thickness of the top layer of the three-layer THM for fixed Ag and SiNx thicknesses of 10 nm and 30 nm, respectively. The experimental results were very similar to the simulation results. When the thickness of the PPFC was 50 nm, the maximum visible light transmittance was 54.42% at a 400-nm wavelength. The IR transmittance was 8.71% at a 1,000-nm wavelength and 3.56% at a 1,500-nm wavelength. Based on the complex optical admittance, the minimum thickness of the dielectric thin film was found to be λ/8nD, at which point reflectance of multilayer was minimized by destructive interference so that maximized visible light transmittance of the thin film, where nD is the refractive index of the dielectric thin film.3,35,36 The SiNx thin film



had a refractive index of 2.049 at a wavelength of 550-nm, and the minimum thickness of the SiNx thin film was found to be approximately 33 nm. The MF sputtered PPFC thin film had a refractive index of 1.396 at a wavelength of 550-nm, resulting in a calculated minimum thickness of 49 nm. Figure S5 shows refractive index spectra of the PPFC thin films. Refractive index of MF sputtered PPFC thin film by using CNT/PTFE composite target slightly increased, compared to refractive index 1.386 of RF sputtered PPFC thin film by using pure PTFE target. Considering the thickness error associated with the large-area process, the experimental results are very consistent with the theoretical calculation results.

Figure 2. Optical transmittance of three-layer SC-THM (PPFC / Ag / SiNx / PET) as a function of PPFC thickness in the wavelength range 300 to 2400-nm.


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To increase visibility and energy savings in energy-efficient automobiles and buildings, the visible light transmittance should be even higher and the IR blocking properties even better than those achievable with a three-layer structure. Therefore, five-layer (PPFC / Ag / SiNx / Ag / SiNx / PET) films were fabricated to produce a higher-performance SC-THM. A two-dyad structure with a dual Ag / SiNx structure was developed, and a PPFC thin film was formed on the top layer. The results of simulation of the transmittance as a function of the thickness of the top PPFC layer indicated that the transmittance in the visible light region increases as the thickness of the PPFC increases. The optical simulation results showed that the transmittances of the PPFC layer at 50 nm and 60 nm are similar in the visible light region but that the IR blocking is better at a thickness of 50 nm, as shown in Figure S3. Figure 3(a) illustrates the transmittance of the five-layer SC-THM. Based on the simulation results, which indicated that the transmittance of the visible region increases when the Ag thickness decreases, samples with Ag thicknesses of 6 and 8 nm were also fabricated. The results showed that the visible light transmission was decreased and the IR blocking ability deteriorated remarkably at a Ag thickness 6 nm, because Ag did not form a continuous thin film but rather formed islands.18,26,37,38 The transmittance in the visible light range was increased slightly at a Ag thickness 8 nm, but the transmittance in the near-IR region was also high. Therefore, a Ag thickness of 10 nm was judged to be the most suitable for the SC-THM. In the five-layer SC-THM, when the thickness of the PPFC was 50 nm, the visible light transmittance was 60.67% at a wavelength of 406-nm, and the IR transmittance was 6.86% at a 1,000-nm wavelength and 2.50% at a 1,500-nm wavelength. The five-layer SC-THM exhibited very low IR transmittance, less than 1.89 % at a wavelength of 1,000-nm and 0.65% at a wavelength of 1,500-nm, when the thickness of the PPFC was 40 nm, although the visible light transmittance was relatively low at 48.82%. Figure 3(b) shows a graph of the transmittances of the three-layer and five-layer SC-THMs in the visible and IR regions. The



five-layer structure exhibited better performance, with not only high transmittance in the visible light region but also excellent blocking in the IR region. The solar spectrum determined in accordance with ASTM-G173 is included in the graph for comparison. The SC-THMs improved visible light transmittance and near-IR reflection characteristics because of the PPFC applied to the top layer, and they exhibited very good water repellency of more than 111 degrees, as shown in Figure 3(c), because of the low surface energy (12.54×10-3 N/m) of the PPFC thin film. Because SC-THM films are applied primarily to automobile windows and building glass, their self-cleaning capability that results from the high water repellency of their surfaces contributes to their consistent performance. Figure 3(d) shows the results of placement of a large-area SC-THM film on an actual window to assess its effect on visibility. It can be seen that the exterior is clearly visible because of the high visible light transmittance.


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Figure 3. (a) Optical transmittance of five-layer SC-THM (PPFC / Ag / SiNx / Ag / SiNx / PET) as a function of PPFC and Ag thickness, (b) comparison of transmittance of solar spectrum for three-layer and five-layer SC-THMs, (c) water repellency of SC-THM film, and (d) SC-THM film applied to a window to illustrate its effect on visibility.

The performance of the five-layer SC-THM was evaluated using its transmittance and reflection data. Values were calculated for parameters such as the figure of merit (Φ), integrated solar transmittance and reflectance (Tsol, Rsol), integrated visible transmittance (τvis), integrated infrared reflectance (RIR), and integrated visible transmittance and reflectance (Tvis, Rvis), and Tvis/Tsol values were then calculated.10 Solar spectrum data were



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obtained in accordance with ASTM G173 for use as standard parameters for the photovoltaic cells.39,40 For the graphs and formulas used in calculation, refer to the supporting information. Tvis/Tsol is used primarily as a measure of the performance of a transparent heat mirror, and the theoretical maximum is 2. Table 1 shows performance evaluation results of three-layer and five-layer SC-THMs. The Tvis/Tsol of the five-layer SC-THM with a PPFC thin film was 1.70, which suggests excellent performance.

Table 1. Evaluation of performance of SC-THM Structure

λmax (nm)

Tmax

τvis (nm)

RIR (nm)

Φ (nm2)

Tsol

Tvis

Rsol

Rvis

Tvis/Tsol

3-layer

0.400

0.5442

111.73

900.21

5.47×104

0.224

0.354

0.681

0.549

1.58

5-layer

0.404

0.6067

127.06

947.59

7.30×104

0.240

0.407

0.717

0.577

1.70

We analyzed the structure of the five-layer SC-THM by means of FE-TEM and TOFSIMS measurements. Figure 4(a) shows a cross-sectional TEM image of the fabricated SCTHM film, which confirms the successful deposition of the two-dyad Ag / SiNx and PPFC multilayers onto the PET substrate via the continuous roll-to-roll sputtering process. Figure 4(b) illustrates the TOF-SIMS depth profile, showing the changes in the mass intensities of CF-, Ag- and SiN- ions during the sputtering of the PPFC / Ag / SiNx / Ag / SiNx multilayer film. The ion species are in the sequence of CF-, Ag-, SiN-, Ag-, and SiN- over the sputtering, which confirms the correct deposition of the five-layer SC-THM.


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F Figure 4. (a) A cross-sectional TEM image and (b) TOF-SIMS depth profile of the five-layer (PPFC / Ag / SiNx / Ag / SiNx)

SC-THM

When light arrives at the interface of two different materials, transmittance (T), reflection (R), and absorption (A) take place, the sum of which is a constant T + R + A = 1.28 Figure 5(a) shows a graph of the reflectance of the proposed SC-THM. Compared to the transmittance data, the SC-THM exhibits little absorption and reflects most of the IR. An experiment was conducted to verify that the SC-THM reflected heat well. Figure 5(b) shows a photograph of the measurement of the heat reflected from the films by an IR camera, taken while a hand was placed over the PET substrate film and the SC-THM film. The left side of the figure shows the visual images and the right side shows the IR camera images. Unlike the PET substrate, the SC-THM reflected the heat of the hand, as is clearly visible in the IR camera image. Another experiment was conducted to verify the heat mirror effect for the proposed SC-THM. As Fig. 5(c) shows, two transparent boxes made of acrylic were placed under IR lamps, glass and the SC-THM were placed on the boxes, and IR was irradiated for 45 minutes. In the case of the box with glass on top, the temperature in the box increased to



44.6 °C after 45 minutes. In the case of the box with the SC-THM on top, the temperature was just 22.9 °C—a temperature difference of approximately 22 °C. Figure 5(d) shows the temperature change in the box over time during IR irradiation. These experimental results confirm that the proposed SC-THM can transmit visible light sufficiently well and reflect heat in the near-IR region very effectively. In addition, the self-cleaning capability of the surface makes the SC-THM well suited for many applications.

Figure 5. (a) Reflectance of five-layer SC-THM film, (b) heat-reflected hand image on SC-THM film, (c) IR irradiation experiment images of SC-THM film on transparent box, and (d) temperature difference between glass and SC-THM film in IR irradiation experiment


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Conclusion SC-THM films with excellent near-IR blocking properties, sufficient visible transmittance, and self-cleaning surfaces fabricated by means of a continuous roll-to-roll sputtering process were developed in this study. High-refractive-index SiNx thin film was used as the bottom-layer insulator, Ag was used as the reflective metal, and a PPFC layer deposited using a CNT/PTFE composite target was used as the top layer. The five-layer structure, fabricated using two-dyad Ag/SiNx, had higher visible transmittance and better IR blocking properties in the near-IR region than the three-layer structure. It is very important to reflect near-IR rays effectively to achieve energy savings with an IR mirror, because most of the sunlight, except for visible light, occupies the near-IR region, in the 700–1,500 nm wavelength range. The proposed five-layer SC-THM structure exhibited excellent near-IR blocking properties: 1.89–6.86% at a 1,000-nm wavelength and 0.65–2.50% at a 1,500-nm wavelength, depending on the thickness of the PPFC thin film. This structure also exhibited sufficient visible transparency, 49.00–60.67% in the visible range, which makes it suitable for use in energy-efficient buildings and the side windows of energy-saving cars. The performance of the SC-THMs was evaluated, and a high Tvis/Tsol value of 1.7 was calculated. The excellent performance of the SC-THMs was confirmed by means of IR reflecting experiments. In conclusion, the proposed type of SC-THM film is believed to be suitable for use in various applications to improve energy efficiency because it can effectively control sunlight and the self-cleaning nature of the surface ensures that the performance of the SCTHM can be maintained consistently.



ASSOCIATED CONTENT Supporting Information. Supporting Information accompanies this paper at http://pubs.acs.org/journal/aamick Optical transmittance simulation results of multilayers, refractive index of PPFC and deriving of figure of merit factor, integrated solar (SOL) and integrated visible (VIS).

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions S.-J. L designed the study and the experiments. S.-J. L and S. H. K fabricated the fluorocarbon thin films using a test sputter system and a pilot-scale roll-to-roll sputter. S. H. K and M. K. analyzed the properties of the fluorocarbon thin films. J. H. L and S.-J. L wrote the manuscript. All of the authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

This study was supported by the Core Research Project at Korea Research Institute of Chemical Technology (KRICT) (KK-1706-C00) funded by the Ministry of Science and ICT


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and was also supported by the Industrial technology innovation program (No.10079601) funded by the Ministry of Trade, Industry & Energy(MOTIE, Korea)

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Self-cleaning transparent heat mirror with plasma polymer

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