Flexible Liquid Crystal Displays Using Liquid Crystal-polymer Composite Film and Colorless Polyimide Substrate

Abstract

Application of liquid crystal (LC) materials to a flexible device is challenging because the bending of LC displays easily causes change in thickness of the LC layer and orientation of LCs, resulting in deterioration in a displayed image quality. In this work, we demonstrate a prototype device combining a flexible polymer substrate and an optically isotropic LC-polymer composite in which the device consists of interdigitated in-plane switching electrodes deposited on a flexible colorless polyimide substrate and the composite consisting of nano-sized LC droplets in a polymer matrix. The device can keep good electro-optic characteristics even when it is in a bending state because the LC orientation is not disturbed in both voltage-off and -on states. The proposed device shows a high potential to be applicable for future flexible LC devices.

I. INTRODUCTION

Flexible displays receive great attention due to their advantages such as their lightweight, thin packaging, impact resistance, and lack of a spatial limit of use compared with conventional glass-based displays [1, 2]. Many different types of flexible displays utilizing liquid crystal displays (LCDs) [3], organic light-emitting diodes(OLEDs) [4, 5], light-emitting diodes (LED) [6] and particle-used electrophoretic displays [7] have already been demonstrated for wearable displays, smart cards, and electronic papers. At present, LCDs with various driving modes, such as twisted nematic (TN), vertical alignment(VA) [8, 9] in-plane switching (IPS) [10], and fringe-field switching (FFS) [11-13], have been commercialized, dominating most of the display markets with high image quality. The present LCDs utilize glass substrates because they can preserve orientation and thickness of LCs and itis advantageous to perform the manufacturing process in the a large-size area. On the other hand, plastic-based OLED is commercialized although the manufacturing process is performed with a glass substrate so that the display can be made with curved form, lightweight, and thinness. To overcome intrinsic demerits of the glass-based LCDs, the plastic substrate-based FFS-LCDs were recent developed by Japan Display Inc. [11-13], whose LCDs are lighter, thinner, and have smaller curvature. However, the LCD has some limitations in bending and mechanical stability because the bending may cause perturbation of LCorientation and cell gap change, and external mechanical shocks may easily break LC orientation, resulting in deterioration of the image quality of LCDs and external mechanical shocks may easily break LC orientation. To address the above concerns plausible approaches were proposed, which build polymer walls between pixels by polymerization induced phase separation. Such relatively rigid parts of polymer walls can help to maintain the cell gap, they are expected to prevent flow of LCs under external pressure [1, 2, 14-17]. However, the issue is not fully solved by using such walls because the deformation of substrates can also induce the distortion against the uniform alignment of LC directors, which causes an optical leakage that gives rise to a bad effect for a dark state of LCDs. Further, it is not a fundamental solution to avoid the flow of LCs in the device. To solve these issues, several reports suggested LC-polymer composites in which nano-size LC droplets are embedded in the polymer matrix so that the composite becomes polarization-independent optically isotropic liquid crystal (OILC) [16-20]. In this way, the flowing property of the LC layer disappears and the OILC film can bend more easily without changing its optical properties

In this paper, we demonstrate a real flexible LCD in which a single substrate utilizing colorless polyimide (CPI)with interdigitated indium-tin-oxide (ITO) electrodes for-plane switching (IPS) is used and LC-polymer composite is coated above the substrate. Since the composite is optically isotropic and CPI has no in-plane retardation, the cell shows a completely dark state under crossed polarizers. The electro-optic characteristics of the proposed flexible LCD in a bending state were studied in detail.

II. EXPERIMENTAL

2.1. Fabrication of Flexible IPS-CPI Electrode Film

The flexible CPI film with patterned IPS electrode was fabricated in 6 steps. i) The CPI varnish was coated on the cleaned Si substrate by spin-coating under 1,000 rpm for 60 s, and it was cured at 60°C, 80°C, 150°C, 230°C, and 300°C for 30 min at each temperature in N2environment, ii) Deposition of an ITO layer on the CPIfilm was carried out by the RF-magnetron sputtering system (SDP-670VT, ULVAC), iii) Photoresist (PR) was coated on the ITO, iv) The photoresist development and bake proceeded to make a patterned ITO electrode, v) The exposed ITO was etched away by a prepared etching solution and residual PR was removed. vi) Finally, the fabricated IPS-CPI electrode film was peeled off from the Si substrate. The detailed process of preparing IPS-CPIwith this photo-lithography technique is schematically shown in Fig. 1. To measure the thickness of the CPI film and ITO electrode, we used surface profiler (P-10, KLA-Tencor Corporation).

FIG. 1. Procedure for fabrication of the flexible in-plane electrode on the colorless polyimide (CPI) film.

2.2. Fabrication and Characterization of Flexible OILCCell

To prepare the OILC mixture, we used high dielectric anisotropic nematic LC mixture, MLC-2053 (Δε = 46.2,ne = 1.7472, no = 1.5122, Δn = 0.235 at 589.3 nm, from Merck Advanced Technology), UV-curable monomer,Norland Optical Adhesive 65 (NOA65, np = 1.524, from Norland Products Inc., USA), and a photo-initiator (Ciba,Irgacure651). The OILC mixture consists of 39.76 wt% ofLC, 59.64 wt% of NOA65, and 0.6 wt% of photo-initiator. The prepared OILC mixture was coated on the CPI-IPSfilms. After coating, the mixture was cured by UV exposure under 150 mW/cm2 for 5 s at room temperature, resulting in a thickness of about 10 µm. The polarizing optical microscope (Nikon, ECLIPSE E600) was used to observe the electro-optic properties of OILC in voltage on and off states. The length and width of patterned ITOelectrode were measured by using a scanning electron microscope (SEM, JSM-5900, JEOL). The voltage-dependent transmittance was measured by lab made set up with crossed polarizers, laser source (He-Ne laser, λ = 632.8 nm),a photo-detector, an oscilloscope (Tektronix, DPO2024B),an amplifier (FLC A400), and a function generator (Agilent,33521A)

III. RESULTS

Generally, ITO is a very weak material against external stresses such as bending, stretching, and vibration. Nevertheless, the thinner substrates with deposited ITO can give better stability, flexibility, and longer lifetime under such stresses [21, 22]. To improve the stability and flexibility of patterned ITO electrode on CPI film, we fabricated a very thin CPI film with a thickness of 13.5 µm and patterned ITO electrode with a thickness of 108 nm in which the interdigitated electrode was patterned with electrode width of 4.24 µm and spacing of 5.60 µm between them to generate an in-plane electric field in the voltage-on state, as shown in Fig. 2.

 

FIG. 2. (a) Thickness of CPI film and patterned ITO electrode layer, (b) SEM and OM image (inset) of patterned ITO electrode, and (c) photograph of IPS-CPI electrode film.

The schematic of the switching principle of the in-Plainfield driven OILC device using the CPI film under crossed polarizers is shown in Fig. 3. When the LC droplet size ina polymer matrix is roughly similar to the wavelength of incident light, it looks quite opaque due to high scattering.

FIG. 3. Schematic of the flexible optically isotropic liquid crystals (a) at voltage-off and (b) voltage-on states.

However, when its size is much smaller than the wavelength of the incident light, less than 250 nm, the scattering is minimized, resulting in a transparent optically isotropic phase [23, 24]. In the proposed OILC film, the LC droplet size is in a range of 100 to 200 nm so that the LC/polymer composite film shows high transparency. Consequently, it appears dark under the crossed polarizers, as shown in Fig. 3(a) and when a voltage is applied, the in-plane electric fields are formed so that the LCs inside droplets orient along the field direction, giving rise to field-induced birefringence known as the Kerr effect [25]. Therefore, the device gives rise to a bright state, when the optic axis of the induced birefringence makes an angle of45° with respect to the crossed polarizer axes, as shown inFig. 3(b). To measure the electro-optic properties, the OILC film is fixed between the crossed polarizers in such a way that the long-side electrode direction is 45° to the polarizers. The He-Ne laser light is made incident on the cell and the transmitted intensity detected and measured by using the photo-diode and the digital oscilloscope while applying 1 kHz frequency square wave voltage to the cell.

The Fig. 4 shows voltage-dependent transmittance of prepared OILC with a flexible IPS substrate. The two inset images show the macroscopic images of black and bright states in the bending state, respectively. No transmitted intensity was observed at 0 V, indicating that there is no light leakage from the film. When applying voltage, the transmittance was increased with the increase of voltage, finally reached to a maximum transmittance at 3.67 V/µm, indicating the induced birefringence is saturated at high enough electric field. The threshold electric field (Eth) and operating electric field (Eop) are defined as the voltage required achieving 10% and 90% transmittance relative to maximum transmittance, respectively. The measured Eopand Eth are 2.37 V/µm and 0.78 V/µm, respectively.

FIG. 4. The dependence of the normalized intensity of the flexible OILC cell with applied E-field. Two insets are photographs of OILC cell in the flat state taken at voltage-off (0 V/µm) and voltage-on (3.67 V/µm) states.​​​​​​​

We also observed the switching behavior of prepared cells through polarizing optical microscopy, as shown in Fig. 5. The aforementioned square wave field is applied to the cell to observe the switching behavior. As expected, the obtained film shows a dark state at the field off state. In addition, there is no indication of the change in color on the rotation of the cell under crossed polarizers, suggesting the obtained film is optically isotropic. Upon applying the voltage, the dark state starts to give rise to brightness, which is an indication of induced birefringence in the OILCfilm. The highest brightness was noticed at 3.67 V/µm. Along bright strip indicates the gap between the electrodes while the dark area indicates the area occupied by the electrodes. One could easily notice few either continuous or discontinuous dark lines, in which the switching does not occur due to a few broken IPS electrodes.

FIG. 5. The POM images of the flexible OILC cell taken at different bias voltages under crossed polarizers.​​​​​​​

Figure 6 shows set up of bending the film and the flexed OILC film with 9 mm of curvature [see Fig. 6(a)]and observed a distinct switching of nano-sized LCdroplets from dark to bright state in top view under the cross polarizers. Under mechanical deformation, the LCorientation, as well as the dark [, see Fig. 6(b)] and white[see Fig. 6(c)] images are not disturbed while keeping an excellent uniformity, confirming an excellent cell gap is kept in the flexible OILC film using CPI film. However,the birefringence in the bright state was not shown over a whole area not because of bending state but due to brokenIPS electrodes because of low-quality photolithography process on a lab scale.

FIG. 6. (a) Photograph and schematic illustration of the flexible OILC cell in the bent state. (b) and (c) top view of OILC cell in OFF and ON state under cross polarizers, respectively.

IV. CONCLUSION

In this work, we demonstrate a fully flexible liquid crystal device with a thickness of less than 30 µm in which colorless polyimides is used as thin substrate and optically isotropic liquid crystal/polymer composite film is formed via photo-induced polymerization induced phase separation. The proposed device is fully flexible because its dark and white states are not disturbed by bending owing to captured LC in a polymer matrix and very lightweight because the device is just polymer film. Althoughpolarizers still remain as a quite big obstacle for the application to a flexible display, there are also good approaches for pursuing the flexibility of polarizers as well, such as coatable polarizers [26, 27]. It is believed that our approach can greatly contribute toward the application of LCs to flexible displays.