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An embedded Ag mesh transparent conductive electrode (TCE) on flexible substrate, which is suitable for polymer dispersed liquid crystal (PDLC) device, is demonstrated. With the combination of soft ultra-violet nanoimprinting lithography and scrape technique, it offers parallel processing with high resolution (10000dpi), as well as remarkable simplicity and fully controllable flexibility to tailor the transmittance and sheet resistance. While being able to achieve maximum transmittance 60% in the on state and the minimum 0.1% in the off state, the PDLC smart window displays low sheet resistance (5.58 Ω/sq.) under low driven voltage (30V) safe for human. The main advantage of adoption of PDLC as an optically scattering element lies in the fact that there needs no mechanical part for in situ tunability. An enhancement factor of 50 of the diffraction intensity is observed experimentally. The embedded Ag mesh TCE for PDLC device has an environmentally-friendly additive manufacturing process inherently. Compared to existing solutions, the fabricated sample shows superior performance in both optoelectronic and mechanic characteristics. We envision that the embedded Ag mesh TCE will enable economically widen application of PDLC devices on flexible substrate.
© 2016 Optical Society of America
1. Introduction
Polymer-dispersed liquid crystals are known to be electro-optically switchable materials and have attracted much attention for a wide range of applications including outdoor display, switchable privacy glass, and energy saving windows [1, 2]. PDLC device has a basis structure of liquid crystal sandwiched by a pair of glass substrates with TCEs. With no applied voltage on the TCEs, liquid crystals are randomly arranged in droplets, resulting in scattering of light as it passes through the liquid crystal cell, consequently the translucent “milky white” appears. Upon a power supply being applied, the electric field between the TCEs causes the liquid crystal’s directors to align along the same direction, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the external voltage. High conductivity, high transmittance, and strong mechanical robustness of the TCEs can lead to excellent performance of the PDLC devices [2].
Typical PDLC-based smart window uses indium thin oxide (ITO) as TCEs, which demonstrates excellent optical and electrical properties. However, some innate drawbacks of the ITO electrode such as brittleness due to its ceramic nature and high cost due to the scarcity of indium restrain its application in nowadays flexible optoelectronic devices [3, 4]. Worse still, it is a challenge to further reducing the sheet resistance of ITO below 10 Ω sq−1, which is a fundamental request for large-area displays and electromagnetic shielding applications. Potential alternatives to ITO have been widely explored including ultra-thin metallic film [5, 6], Ag nanowires [4, 7, 8], conducting polymer [9, 10], carbon nanotubes and graphene [11, 12]. Among these technical schemes, metal-grid based TCE becomes a promising solution to the existing conundrums [3, 13, 14]. Thus far, the metal grid fabrication technique has shown tremendous improvement by employing various methods such as electrospun nanotroughs [15], inject printing [16] and so on. Although those newly emergent candidate methods have intense potential to substitute ITO, they either suffer from complicated procedures or unacceptable optical transparency and electrical conductivity.
Recently, Ag nanowire TCEs are of primary interest and have been successfully integrated into PDLC smart windows [17]. It was demonstrated that the cost of the PDLC smart window with Ag nanowire can be drastically decreased as a comparison to that with ITO. However, its specular transmittance in the on-state is non-uniform over visible regime due to the uneven distribution of the electric current caused by separate stack of the Ag nanowires networks. Another shortcoming is its lack of design elements, which is essential for several applications. Kim et.al constructed all-organic PDLC devices adopting conductive polymer materials as TCEs, which was realized by the vapor-phase deposited polymerization technique. The transmittance deteriorates drastically due to the unmatchable materials [10].
Here, we propose and experimentally demonstrate PDLC devices with the embedded Ag mesh TCEs employing soft ultra-violet nanoimprinting lithography (UV-NIL) and scrape technique. The manufacturing process is compatible with traditional semiconductor process. Soft UV-NIL not only provides parallel processing with high resolution (>10000dpi), but also reduces the cost of master fabrication by using of polymer stamp. Combined with the scrape technique, the procedure is remarkably simple and robust, as well as fast and fully controllable. The fabricated TCE film exhibits not only excellent electro-optical characteristics, but also mechanical robustness. The sheet resistance can be as low as 5.8 Ω sq−1 with high transmittance 90%. It is obvious that lower sheet resistance leads to shorter switching time and less power consumption [17, 18]. By the utility of the embedded Ag mesh based TCE film, PDLC devices can be used as not only smart windows, but also in situ tunable scattering elements. The engineered pattern of the Ag mesh grid offers lots of degree of freedom for design and adjustment mostly through tuning of the applied voltage. An enhanced intensity factor of 50 can be achieved through varying the applied electronic signal, which can find applications in electrically controllable optical diffusers and beam shapers.
2. Fabrication of the Embedded Ag Mesh TCE
Figure 1 shows the fabrication process of the embedded Ag mesh TCE film. Firstly, a layer of photoresist (RZJ-390PG, Ruihong Electronic Chemical Co., LTD) is spin-coated on the glass substrate. To ensure the filling of Ag ink in the micro-groove, microstructure with an aspect ratio of 1~2 is preferred. Conventional optical lithography based on contact exposure technique cannot guarantee the uniformity with high linewidth resolution (~3µm) on large format substrate. Here we use an in-house built digital micro-mirror device (DMD) based maskless lithography system for direct-writing of the micro-groove on photoresist [19] (shown in Figs. 1(a) and 1(b)). Secondly, the micro-groove structure is replicated to a soft mold, as shown in Figs. 1(c) and 1(d). Details can be found in [20, 21]. The exposure time in the second curing is less than 2 sec, thus the process is rather efficient and can be readily scaled up via roll-to-roll NIL. There is no need to adopt any type of electroforming or metallization, thus the process is environmentally friendly and economically. Thirdly, the Ag nanoparticles ink (concentration of 70%, viscosity of 25 cps, and particle diameter in the range of 200 to 300 nm) is filled into the micro-grooves through the scrape technique, as shown in Fig. 1(e). After 20mins sintering at 80°C, the embedded conductive Ag mesh can be formed in the UV resin. A wiping process with nontoxic organic solvents can be employed to clean the surface. It is noteworthy that the solution process can be operated at low temperature, which is free from any complicated vacuum-based process.
Fig. 1 Schematic illustration of the fabrication of PDLC device based on the embedded Ag mesh on flexible substrate. Fabrication of patterned micro-grooves on the photoresist using laser direct-writing lithography. (a) Spin-coating of the photoresist layer and (b) micro-grooves pattern. (c-d) Pattern replication from the photoresist to the soft mold. (e) Dispersing and scratching Ag ink into the microgrooves. (f) PDLC cell assembly.
Finally, the PDLC commercially available from Slichem (The ordinary and extraordinary refractive index is 1.521 and 1.7465, respectively.) is mixed with NOA65 photopolymer (Norland Products, Inc., polymer refractive index is 1.524) with a weight ratio of 1.2:1. After fully mixed, the homogeneous solution is sequentially drop-dispersed on the Ag embedded TCE substrate. Silica microsphere spacers with a diameter of 30.27 ± 0.25 µm are added to maintain uniform thickness. Subsequently, with the assembly of another TCE film, the LC solution is spread over between the two TCE films. It takes 2 minutes for the curing process under radiation of a UV light-emitting diode with an intensity of 1 mW/cm2. It can be observed that the originally transparent mixture solution transitions into translucent solid-state film.
Surface and cross-sectional microstructures of the imprinted UV-resin were observed by field emission scanning electron microscopy (FESEM: JEOL, JSM-5400, USA) and by 3D laser con-focal microscopy (Keyence, VK 9700). The collimated transmittance spectra in the visible range and sheet resistance levels of the fabricated samples were measured using a UV–vis spectrophotometer (UV-2550, SHIMADZU) and a four-point probe (CMT SR2000, A.I.T.), respectively. The bending repetition of the flexible TCE film at a desired bending radius was carried out with a custom-made bending machine. Diffraction efficiencies were calculated by measuring the specular and total transmission from the sample. For specular transmission, the 3-detector module of a Perkin-Elmer Lambda 1050 UV–Vis–NIR Spectrophotometer was used with the sample aligned with respect to the detector window.
3. Results and Discussion
3.1. Embedded Ag-mesh Film as TCEs
In order to evaluate the performance of the embedded Ag mesh TCEs, the hexagonal arrangements mesh grids are exploited firstly. The width and height of the micro-groove is fixed at 2.5µm (~10000dpi), respectively. The transmittance of the bare PET is 92% when incident wavelength is greater than 350nm. Choosing the diagonal length is a tradeoff between the conductivity and optical performance, depending on application. The measured results show that the transmittance of the TCE film is almost constant in the visible region, as shown in Fig. 2(a). With the diagonal length changing from 50 to 250 µm at a step of 50µm, the coverage rate of the Ag mesh decreases from 13.6% to 3.1% and the average transmittance increases from 59% to 91% in the whole spectra. Meanwhile, the haze decreases from 8.4% to 2.7%, as shown in Fig. 2(b). Figure 2(c) shows the relationship between the measured resistance and the diagonal length of the Ag mesh. Sheet resistance as low as 0.7Ω sq−1 can be obtained at the small diagonal length 25µm. Large-format TCE film (55 inches in diagonal) can be fabricated with the proposed scheme.
Fig. 2 Optoelectronic characteristics of the embedded Ag-mesh TCE film. (a) Normalized measured transmission spectra of the bare PET and the Ag-mesh/PET with 50, 100, 150, 200, 250µm diagonal length, respectively. (b) Haze of the Ag-mesh/PET TCE film at the wavelength of 550 nm. (c) The sheet resistance as a function of the diagonal length.
Typically, the optical transmittance and the dc conductivity of the TCE film vary in the opposite direction [22]. We calculate the figure of merit (FoM) for the samples by Eq. (1), which defines as the ratio of the electrical to optical conductivity (). represents FoM and , is the electrical conductivity and optical conductivity, respectively [22–24].
where is the transparency and is the sheet resistance of TCE film. The fabricated embedded Ag-mesh TCE film with different diagonal length has an excellent value ranging from 430 to 892, which is much superior to that of the ITO sample ( of the ITO is in the range of 105 to 162, as mentioned in [24]).Besides the electrical and optical properties, the mechanical stability of TCE film is also vital for its applications. A cyclic bending test and an adhesive tape test are performed on the samples and the results are shown in Figs. 3(a)-3(c). Figure 3(b) presents the variation of the sheet resistance as a function of bending cycles with bending radius ranging from 40 mm to 3 mm. The sheet resistance tends to increase during bending action and can restore to its initial value, and there is no noticeable change () in all cases. and is the measured and initial resistance, respectively. Similarly, the change in the sheet resistance after thousands of consecutive bending cycles at a radius of 3mm is still negligible (as depicted in Fig. 3(a), which exhibits high tolerance to bending stress. The embedded Ag mesh TCE film can maintain its excellent performance even after bending for up to 10000 cycles.
Fig. 3 Bending and mechanical stability test. The sheet resistance of the embedded Ag mesh TCE film and ITO coated PET are measured: (a) after thousands of bending cycles at a bending radius of 3mm. (The inset is the optical microscopy images of the ITO and Ag mesh TCE after bending test. Scale bar, 100 μm). (b) at bending radius in the range of 3-40mm. (c) Changes in the sheet resistance as a function of peeling cycles.
It can be seen from the red plots in Figs. 3(a) and 3(b), the ITO TCE’s resistance increases significantly after 500 bending cycles and it demands relatively large bending radii due to the material brittleness. The inset in Fig. 3(a) indicates that for the ITO based TCE film, there are some obvious cracks along the direction perpendicular to the bending stress, which deteriorates its electrical performance. However, for the Ag mesh based TCE film, it exhibits uniform surface excluding any microscopic cracks due to the embedded Ag nanoparticles in the micro-grooves. The mechanical stability of the embedded Ag mesh TCE film can further be confirmed by exploiting an adhesive tape test. The sheet resistance is measured after every 10 peeling tests using 3M Scotch tape. The ratio of is found to be changed slightly after 100 cycles, as shown in Fig. 3(c). The mechanical robustness of the proposed TCE film can be attributed to its embedded nature, which can help in retaining the Ag nanoparticles firmly attached to the UV resin.
Using the aforementioned scrape technique, the filling height of the Ag nanoparticles in micro-groove mesh can be easily modified without any vacuum or specialized equipment due to its additive manufacturing nature. Figures 4(a)–4(c) show the cross-sections scanning electronic microscopy (SEM) pictures of the micro-grooves after 1 to 3 repetitions of the scraping cycles. Under the action of both the gravity and surface tension, the Ag ink in micro-groove solidifies and forms meniscus shape after sintering. By rule of thumb, after three scraping and sintering cycles, the remaining Ag ink can fully occupied the micro-groove, as shown in Fig. 4(c). The sheet resistance and transmittance are measured as 20 Ω sq−1, 10 Ω sq−1, 7Ω sq−1, and 91%, 90%, 90%, respectively. Consequently, the embedded Ag mesh TCE exhibits excellent process flexibility and superior optoelectronic characteristics, which may benefit its diverse array of applications.
Fig. 4 Cross-section SEM images of the micro-groove after several scraping and sintering cycles. (a) 1 cycle, (b) 2 cycles; (c) 3 cycles. Scale bar, 1µm.
3.2 PDLC Device with Random Ag-mesh TCE as Smart Window
Smart window is the classical application of PDLC which can be electronically switchable between the on and off-state with applied voltage. Classical conductive material, such as the conducting polymer poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), not only suffers high sheet resistance due to its organic nature, but also involves unstable process. The problem can be solved by the combination of the embedded Ag-mesh metal mesh and a thin layer of the spin-coated PEDOT:PSS as hybrid TCE. Figure 5(a) shows the transmittance spectra of the bare PET, PET/Ag-mesh TCE and PET/Ag-mesh/PEDOT:PSS hybrid TCE as a function of wavelength. The conductivity can be much improved at the cost of a little decrease in transmittance in short wavelength regime. Figures 5(b) and 5(c) show SEM picture of the Ag mesh TCE and the hybrid Ag-mesh TCE for a comparison.
Fig. 5 Photos of PET/Ag-mesh and PET/Ag-mesh/PEDOT:PSS hybrid transparent electrodes. (a) Transmittance spectra of bare PET, PET/Ag-mesh and PET/Ag-mesh/PEDOT:PSS as a function of wavelength. SEM pictures of (b) the Ag-mesh/PET electrode (scale bar, 5μm) (The inset shows the detail of the Ag nanoparticle, scale bar, 500 nm) and (c) the PEDOT:PSS hybrid electrode. The inset shows the detail of the Ag nanoparticle on a thin layer of PEDOT:PSS (scale bar, 500 nm).
Figure 6(a) presents the optical microscopy image of PDLC smart window with the embedded random Ag mesh grids. The average distance between any two neighboring nodes is in the range of 150 to 250µm. With the optimum design, there is no evidence of giving rise to generating any moiré phenomenon. Pictures of the fabricated PDLC smart window in the off and on state are shown in Figs. 6(b) and 6(c) respectively. The optical transmittance could reach 60% at 30 V, which is under the human safety voltage. However, in the case of ITO based TCEs, the measured maximum transmittance was only 54% at a higher applied voltage of 60 V, as shown in Fig. 7(a). The relatively lower sheet resistance of the hybrid Ag-mesh TCEs leads to a lower driving voltage than that of the ITO TCE based device. In Fig. 7(b), we measured the haze of the two samples as a function of the applied voltage. The sample based on the hybrid Ag-mesh TCEs shows a 5% haze reduction when compared with the ITO one. There is a maximum 8% decrease in transmittance in short wavelength regime for the hybrid Ag mesh TCE. To our knowledge, the obtained result is the maximum transmittance for the PDLC smart window reported so far.
Fig. 6 Schematic illustration of the embedded Ag-mesh TCE based PDLC smart window. (a) The optical microscopy image of the random grid. The average distance between two neighboring nodes is in the range of 150 to 250µm). In the off- (b) and on- (c) state.
Fig. 7 Characterization of the fabricated PDLC smart windows. Normalized measured transmission (a) and haze (b) as a function of the driving voltage.
3.3 PDLC Device with Engineered Periodic Ag mesh TCE as in situ Tunable Scattering Element
Scientists have made use of a wealth of optical physics to control and manipulate light, for example, in creating mechanically controllable optical diffuser and beam shapers [25, 26]. Attributing to the capability of being electrically controlled, liquid crystals are of technological interest and can be adopted as tunable microlens, beam deflector and lasers etc [27–30]. The main advantage of the adoption of the PDLC as optically tunable element lies in the fact that there needs no mechanical parts for in situ tunability, thus it can enable real-time electronically control of light phase and transparency. The morphology of the engineered periodic Ag mesh can be considered as scattering element, which determines the locations of the diffraction spots. The diffraction intensity can be continuously tuned by the applied voltage.
Figure 8(a) shows the schematic of the experimental setup utilized for scattering pattern analysis. The sample film is located between a light source and a projection screen. Figure 8(b) shows SEM picture of the embedded Ag mesh grating based PDLC scattering element. The grating period and groove width is 5.5µm and 2.5µm, respectively. The locations the diffraction spots can be calculated by the well-known grating equation,
where represents the order of diffraction, and the wavelength, the grating period, the diffraction angle. The measured angle can be determined using the trigonometric relationwhere is the spacing between the diffraction maxima from the center and is the distance between the grating and screen (40cm in the setup). The measured experimental and analytical results show excellent agreement in spots locations. Photographs of the diffraction images obtained as a function of applied voltage with a red He-Ne laser illumination are presented in Fig. 8(c). The scattered light beams with higher diffraction orders (> ± 2nd) are too weak and unobtrusive, thus they are not included in the picture. As the increase of the applied voltage (0V towards 30V), the intensity of all the diffraction spots increases drastically (as listed in Table 1). The physical mechanism behind the phenomenon is that on the application of external voltage, the engineered periodic Ag mesh produces time-averaged electric field with random domains within the PDLC layer. Then, the electrically controllable diffraction can be obtained while maintains light spot location. It can be seen from the measured results that an enhanced factor of 50 (for the ± 1st order) can be achieved, as shown in Table 1. Square and honeycomb arrangements of the Ag mesh TCE are also fabricated and their experimental results are shown in Figs. 8(d)-8(g). Square Ag mesh has a period of 50 μm and width of 3 μm and Honeycomb Ag mesh has a period of 25 μm and width of 3μm depth. Since the Fraunhofer diffraction pattern can be viewed as the Fourier transform of the scattering object, the square arrangement produces discrete diffraction spots in the both horizontal and vertical directions. The honeycomb arrangement produces discrete diffraction spots in six directions at an increasing angle of 60°. Higher order diffraction spots appear as the applied voltage increases. Furthermore, if white light (Xenon lamp of a monochromator, Ocean Optics) is used as source, enhanced ring-shaped diffraction pattern can be clearly visible from the spectral dispersion, shown in Fig. 8(g). The PDLC device based on engineered periodic Ag-mesh can be used as in situ electronically tunable scattering element.Fig. 8 Scattering pattern analysis of the PDLC device with engineered periodic Ag mesh TCE as in situ tunable scattering element. (a) Schematic of the experimental setup utilized for scattering pattern analysis. Scale bar, 10µm. (b) SEM photo of the fabricated Ag mesh grating. (c) Picture of the diffraction spots with the increased external voltage from 0V ~30V. (d) SEM images of the square and honeycomb arrangements of the Ag mesh as TCEs. The diffraction pattern distribution of the PDLC devices with the square arrangement Ag-mesh as TCEs (e), (f) with the increased external voltage. (g) Ring-shaped diffraction pattern realization used the honeycomb arrangement Ag-mesh TCE illuminated by white light.
Table 1. Diffraction efficiency of the ± 2nd, ± 1st, and 0 order increases with the external voltage.
4. Conclusion
Embedded Ag-mesh TCE for PDLC device on flexible substrate is demonstrated as a superior alternative to ITO-based TCEs. With the combination of soft ultra-violet nanoimprinting lithography and scrape technique, it offers parallel processing with high resolution (>10000dpi). Fabrication of the embedded Ag mesh TCE is an environmentally-friendly additive manufacturing process inherently and can be easily scaled up to roll-to-roll process. Compared to existing solutions, the fabricated samples show superior performance in both optoelectronics and mechanics. It is obvious that lower sheet resistance leads to shorter switching time and less power consumption. The PDLC smart window based on random embedded Ag mesh TCE exhibits high transmittance of 60% in the on state and 0.1% in the off state with a low sheet resistance 5.58 Ω/sq. under a low driven voltage (30V) safe for human. What’s more, with periodic Ag mesh TCE, PDLC device can be used as in situ engineered tunable scattering element. An enhanced intensity factor of 50 can be achieved through increasing the applied electronic signal. This study opens an efficient protocol for realizing flexible PDLC device not only having the photon manipulation capability, but also sustaining superior stretchable and bendable resistance potential for future flexible optoelectronic applications.
Funding
This work is supported by the National Natural Science Foundation of China (NSFC) (61405133, 61575133, 51302179, 91323303); the Specialized Research Fund for the Doctoral Program of Higher Education (20133201120027); the Natural Science Foundation of Jiangsu Province (BK20140348); National High Technology Research and Development Program 863 (2015AA042401); a project by the Zhejiang Key Discipline of Instrument Science and technology; a project by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
Acknowledgments
The authors gratefully acknowledge Lidong Liu for providing PDLC materials.
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