Electrothermally Triggered Broadband Optical Switch Films with

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Electrothermally Triggered Broadband Optical Switch Films with Extremely Low Power Consumption Ryohei Yoshikawa, Mizuki Tenjimbayashi, and Seimei Shiratori ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00251 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Electrothermally Triggered Broadband Optical Switch Films with Extremely Low Power Consumption

Ryohei Yoshikawa, Mizuki Tenjimbayashi, and Seimei Shiratori*

Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

*[email protected]

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ABSTRACT Smart films with transmittance switching capabilities based on thermal stimuli are widely used in many optoelectronic applications. Despite the development of stably switchable materials, transition temperature control and broadband stepwise transmittance switching remain challenging topics. Additionally, reduction of the energy consumption during switching is also required. Here, we introduce an electrothermally driven film with switchable transmittance produced by stacking paraffin-immobilized polydimethylsiloxane gel on a transparent heater based on an aligned Cu/Ni network. The film shows stepwise transmittance switching capability with extremely low power consumption because of the controlled melting point of paraffin and the high-efficiency transparent heater.

KEYWORDS Thermochromic material, paraffin, transparent heater, energy consumption, electrospinning, electroless deposition, aligned network

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Smart windows with transmittance switching abilities based on use of external stimuli are attracting considerable attention for use in applications including infrared reflection, solar modulation, and privacy protection.1–4 Smart windows are designed using controllable molecular arrangements (e.g., liquid crystals) or chromogenic materials with transmittances that can be changed via a phase transition, phase separation, or a redox reaction induced by external stimuli such as applied currents,5,6 heat,7 light,8 or gases.9 Because it is relatively easy to control the functionality of chromogenic materials,7,10 metal oxides such as vanadium (IV) oxide (VO2) and tungsten (VI) oxide (WO3) have previously been used to design smart film structures.11,12 However, most transition metal oxide-based chromogenic materials require high temperatures to switch their optical properties. Additionally, chemical instability and brittleness also restrict practical use.10,13 In contrast, significant interest has been shown in organic thermochromic materials. For example, poly(N-isopropylacrylamide) (PNIPAAm) and paraffin, which show thermal responsiveness near room temperature, are promising for use as stable and cost-effective thermochromic materials.14–16 For further development of smart optoelectronics based on these organic thermochromic materials, stepwise control of the optical transmittance and the tuning transition temperature for practical environments and applications would be useful. Recently, our group succeeded in designing films with stepwise controllable transmittance by controlling the melting temperature of paraffin.17 To use paraffin for smart windows, a structure in which liquid state paraffin is stably immobilized is required because paraffin must shift from the solid state to the liquid state for optical transmittance switching. In addition, to convert these thermochromic materials into electrochromic devices for practical use, development of an electro-thermogenic transparent heater would be a promising 3 ACS Paragon Plus Environment

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step.18,19 Carbon-based materials such as graphene and carbon nanotubes have excellent heating characteristics and high power efficiencies.20,21 Bae et al. reported a graphene film with significant high power efficiency (660 °C cm2/W).22 However, the switching bandwidth is limited because of the low optoelectronic performance of these films. In this context, metal nanonetworks with high transmittance and high conductivity are preferred. Jo et al. reported a highly transparent and conductive heater that used copper nanofibers.23 However, power efficiency improvements are still necessary. The metal nanonetworks in our previous work showed that intersections that involved physical contact caused high thermal losses that led to increased energy consumption.24 This effect is suppressed by fusing these intersections; however, simply reducing the number of intersections is the best approach. Therefore, reducing the number of intersections by forming aligned long metal nanofibers contributes to improvement of power efficiency. Here, we designed an electrothermally driven transmittance switchable film in which the mixed paraffin immobilized polydimethylsiloxane (PDMS) gel (M-PW/PDMS) is stacked on a transparent heater formed using the aligned metal nanofibers. By tuning the solid paraffin to liquid paraffin mixing ratio, the optical transmittance can be changed gradually over a broad switching width. Additionally, the mixed paraffin has long-term stability when packed into the PDMS gels. Metal nanofibers can increase the temperature with low energy consumption. Therefore, our switchable transmittance film is suitable for practical use. In this letter, we first report fabrication of highly aligned metal nanofibers for high power efficient transparent heaters. Second, the optical properties of M-PW/PDMS are characterized. Finally, by depositing optimized M-PW/PDMS on the transparent heater, we constructed a smart window with broadband switching capability that was driven via electrothermogenesis. The fabrication process 4 ACS Paragon Plus Environment

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of the smart window was described in Figure S1. The metal nanofibers were fabricated by selective electroless metallization, in which metals were coated on the polymer nanofiber template by electroless plating.25 The aligned nanofibers were formed on a PET film attached to a drum collector rotating at high speeds by the electrospinning method. A cost-effective Cu/Ni alloy with high oxidation resistance was selected as the coating metal. Figure 1a shows the geometries of the nanofiber networks before and after plating. The fiber diameter increased after plating because the metal was selectively deposited on the catalytic nanofiber’s surface. At low rotation speeds, the fibers formed a random network. In contrast, highly-aligned fiber networks were observed when high rotation speeds were used. The alignment direction corresponds to the direction of drum collector rotation. To quantify the alignment of the fibers, the fiber angles were measured using five binarized scanning electron microscope (SEM) images. Figure 1b shows the degree of alignment of the fibers. Highly uniaxial alignment corresponding to the SEM images (Figure 1a) was confirmed at high rotation speeds. We labeled the fiber networks with low and high rotation speeds as random networks and aligned networks, respectively. To use these networks as transparent heaters, high transmittance ( ) and low sheet resistance ( ) are preferred. Figure 1c shows the opto-electrical performance plots of the random and aligned networks. At the same conductivity, the aligned networks showed much lower sheet resistance than the random networks because they had few dangling edges and fewer undesirable deposits. When we focused on the geometry of the random networks, multiple particle-shaped deposits were observed (Figure 1a). This phenomenon is particularly noticeable at intersections composed of more than three fibers because of locally accelerated metallization.25 These deposits are undesirable because they reduce transparency without contributing to the electron path. The aligned networks, however, show excellent 5 ACS Paragon Plus Environment

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opto-electrical properties. In general, the opto-electrical performance is benchmarked using the percolative figure of merit (Π).26 The Π value is calculated by fitting experimental results using the following equation:

 ⁄ 

  1   Π 

 ,

(1)

where  is the impedance of free space (377 Ω) and  is the percolation exponent. From the fitting curves of the two networks, the aligned networks had Π  111 with   0.598, which is six times higher than the figure of merit for the random networks (Π  17.5 with   2.76). In addition, the electrode shows high mechanical durability and the luminance of a light-emitting diode (LED) was maintained, even during bending and twisting, as shown in Figure 1d.

Figure 1. (a) SEM images of random networks and aligned networks. Angle distributions of (b) random networks and (c) aligned networks. (d) Transmittance and sheet resistance properties of random networks and aligned networks. (e) Photographs of LEDs connected to aligned network electrodes. 6 ACS Paragon Plus Environment

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To assess the heating performance of the aligned networks, a two-terminal side-contact configuration was constructed using silver paste. When a direct current (DC) voltage was applied to the aligned networks, linearity between the applied voltage and the current, which is an ohmic characteristic, was observed as shown in Figure 2a. Therefore, the heating theory of these aligned networks can be explained using Joule heating. In the case of the Joule heating, the saturation temperature ( ) is defined as a function of applied voltage (#) in the following equation.27  

$ %& '

() *+

 , ,

(2)

where R is resistance, C is the film-specific heat, -./ is the thermal loss caused by convection to air, and , is the initial temperature. The saturation temperature was measured by using infrared cameras. As shown in Figure 2b, the saturation temperature is proportional to the square of the applied voltage. Figure 2c shows the time-dependent temperature profiles under various applied voltages. The temperature saturated within 20 s and was maintained until the voltage was turned off. The power efficiency corresponds to the slope of the relationship between saturation temperature and power density. From Figure 2d, aligned networks showed power efficiency of 631.3°C cm2/W, which is much higher than that of previous metal nanonetworks and comparable to that of graphene films.22 This dramatic improvement is attributed to the reduced numbers of intersections, which caused large thermal losses.

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Figure 2. Heating characteristics of aligned network electrode. (a) Current versus voltage plot. The calculated resistance was 56.6 Ω. (b) Saturation surface temperature as a function of applied voltage. The coefficient of determination used for the fitting curve was 0.9982. (c) Time-dependent surface temperature under different applied voltages. Each voltage was turned off at 60 s. (d) Saturation surface temperature versus power density. The slope corresponds to the power efficiency.

Before the switchable transmittance films were constructed, the thermal response of the mixed paraffin was investigated. Figure 3a and b show differential scanning calorimetry (DSC) analysis results for solid paraffin and mixed paraffin, respectively. The solid paraffin showed a single melting peak, whereas the mixed paraffin showed another peak in the lower temperature region due to melting of the liquid paraffin. When solid paraffin was mixed with liquid paraffin, 8 ACS Paragon Plus Environment

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the melting peak of liquid paraffin became dominant and the melting point decreased from 44.72°C to 38.54°C. Additionally, the spread of the melting peak made it possible to change the optical properties gradually over a wide temperature range. The M-PW/PDMS was fabricated by casting a precursor solution on a PET film following by a curing treatment. We confirmed that the mixed paraffin is immobilized uniformly in the PDMS gel networks (Figure S2). We labeled M-PW/PDMS with x% mixed paraffin content as M-PWx/PDMS. Figure 3c shows the temperature-dependent transmittance spectra of M-PW10/PDMS. The transmittance gradually changed with temperature from 33 to 45°C, which corresponds almost exactly with the melting peak width obtained via DSC analysis (Figure 3b). Moreover, the hysteresis was observed in the transmittance spectra due to the thermal history and the different thermal property of paraffin between a solid phase and a liquid phase. As a result, the solid to liquid transition occurs at higher temperature than liquid to solid transition.28,29 Figure 3d shows optical images of M-PW/PDMS with different paraffin contents at room temperature (RT) and 60°C. The films, which are opaque at RT, became transparent with increasing temperature because of the melting paraffin. In the high paraffin content case, the films remained slightly opaque, even at 60°C. The films can also become transparent at higher temperatures; however, improved switchable widths cannot be expected because the opaqueness at RT is almost saturated at x=10%. Figure 3e and 3f show the switching characteristics of parallel transmittance and diffusive transmittance with paraffin content. M-PW10/PDMS showed the widest switching properties in both measurements.

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Figure 3. DSC curves for (a) solid paraffin and (b) mixed paraffin (solid: liquid=1:2). Transmittance switching properties of M-PW/PDMS. (c) Temperature-dependent transmittance spectra, where transmittance ( ) was normalized with respect to the maximum value (,123 ). (d) Optical views of the opaque and transparent states. (e) Parallel transmittance switching properties. (f) Diffusive transmittance switching properties.

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The switchable transmittance film was constructed by depositing M-PW10/PDMS on the transparent heater based on the aligned network, which works by Joule heating. Figure 4a shows optical views with and without applied voltages. When a voltage was applied, the film became transparent to allow characters behind it to be seen clearly. Figure 4b shows the voltage-dependent parallel transmittance and diffusive transmittance characteristics. The transmittance gradually changed over the range from 2 to 6 V. This is because the mixed paraffin has a broad melting peak and liquefaction progressed step-by-step with increasing temperature. The cyclic switching property was demonstrated as shown in Figure 4c. The parallel transmittance varied from 31% to 91% in each cycle without obvious degradation. This high reproducibility is caused by the stable heating performance of the transparent heater and the high immobilization of paraffin by the PDMS gel. We can also confirm that surface morphology is maintained after heating (Figure S3). Figure 4d shows wavelength-dependent transmittance spectra at 0 V and 6 V. The switching width at 550 nm (∆55 + ) was 83.4% at 0.1 W/cm2; broadband switching could thus be realized with lower power consumption than in previous works.20,21 The total energy quantity for transmittance switching can also be saved as shown in Table S1. In conclusion, we constructed an electrothermally driven switchable transmittance film by stacking M-PW/PDMS on transparent heater based on an aligned Cu/Ni network. The transparent heater showed power efficiency of 631.3°C cm2/W with excellent opto-electronic performance because of the highly-aligned current path networks, which resulted in low power consumption and a highly transparent heater. The mixed paraffin had two melting peaks that originated from the solid and liquid paraffin. Melting over a wide temperature range made it possible to tune the optical transmittance using thermal stimuli. This comprehensive strategy can 11 ACS Paragon Plus Environment

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also be applied to other smart films based on thermochromic materials.

Figure 4. Transmittance switching properties of M-PW10/PDMS on a transparent heater. (a) Optical views of the opaque state (0 V) and the transparent state (6 V). (b) Optical values versus applied voltage. Green lines and blue lines represent the parallel transmittance and the diffusive transmittance, respectively. (c) Parallel transmittance switching during cyclic testing. (d) Wavelength-dependent transmittance spectra in the opaque state (0 V) and the transparent state (6 V).

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXX. Figure S1-S3 representing a schematic of fabrication procedure, cross-sectional SEM images, and surface morphology of M-PW/PDMS, respectively. Table S1 shows total energy quantity for transmittance switching. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions R.Y. designed and conducted the experiments, analyzed the results, and wrote the manuscript. M.T. provided scientific advices, supported experiments and wrote the manuscript. S.S. provided scientific advices, commented on the manuscript, and supervised the project. Funding Sources Part of this work was supported by JSPS KAKENHI (grant number JP 16J06070) awarded to M. T. Notes The authors declare no competing financial interest. 13 ACS Paragon Plus Environment

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ACKNOWLEDGMENTS We are grateful to Dr. Yasuhiro Koike and Mr. Hiroaki Nagahama who supported the DSC measurement and Dr. Kenta Homma who supported the optical property measurement. We are also grateful to Dr. Kengo Manabe, Mr. Pecorelli Pietro and Mr. Andrea Testa whose suggestion and comments were valuable for our work. M.T. thanks predoctoral fellowship (PD) from Japan Society of Promotion of Science (JSPS).

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Table of contents

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