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Liquid crystal (LC) is a promising material which is widely used in the area of display devices such as smart phones, to large screen devices because of their anisotropic and electro-optic properties. Several characteristics of LCs such as high birefringence, large electro-optic effect, and low power consumption make them a suitable material to realize various types of display. Polymer dispersed liquid crystal (PDLC) is also material, in which liquid crystal (LC) droplets suspended in a polymer composite, which makes a single-layered PDLC thin film suitable for various application like smart window and display etc [1-4].

Polymer dispersed liquid crystal (PDLC) material, in which liquid crystal (LC) droplets suspended in a polymer composite, attract researcher  and technologists interest as a class of materials for application in next-generation display devices such as transparent displays, projection display, virtual reality display and head-mounted displays [1-12]. Electrically switchable PDLC film between scattering state and transparent state is used as a electrically switchable privacy windows and large-area displays because of its polarization independence, simple and low-cost fabrication.

PDLC technology is based upon a single-layered film that consists of LC droplets suspended in a polymer matrix. Polymer is selected in such a way that the ordinary refractive index of LC matches with the refractive index of polymer matrix and also LC has not to be dissoluble in the polymer for phase separation process [1-5]. Light scattering in PDLC is based on ordering of LC director orientation in the droplets along the applied electric field. LC droplets in polymer matrix are not in preferred macroscopic orientation, results in scattering of light ensuring as a opaque film. Transparent state  can be obtained by reorienting the LC director along the applied electric field as the refractive index of  the polymer matrix matches to ordinary refractive indices of the LC.

PDLC films mainly fabricated by a solvent induced phase separation (SIPS) process [13]; or thermally induced phase separation (TIPS) [14], or polymerization induced phase separation (PIPS) process [2,3,15]. In PIPS method, heat treatment or UV exposure is used to initiate the fabrication process. Advantage of PIPS method that this process involves the phase  separation of a homogeneous mixture of prepolymer and LC, which allow the control over  droplet size and shape [15]. 

In this paper, we propose the fabrication of UV exposure assisted PDLC film on ITO coated glass using monomer and liquid crystal (LC) for application of  electrically switchable windows. The device is fabricated by controlled exposure of UV light during fabrication of PDLC layer for controlling the transmittance of device. We demonstrate transition from two different mode of display fully transparent and fully opaque mode by externally applied voltage. The proposed device has the possible application in electrically controlled smart windows and projected transparent displays.

PDLC films fabricated by PIPS method has LC droplet in polymer matrix. [2,3,15]. These LC droplets between LC domains are randomly oriented, and not in preferred macroscopic orientation. The size of LC droplets are order of few micron therefore the refractive index of the PDLC film is not uniform spatially, results in scattering of light. The proposed device is shown in Figure 1. A PDLC film is sandwiched between  two indium tin oxide (ITO) coated  fused silica substrates. ITO is conducting and transparent material hence top and bottom ITO-coated glass substrates are used as electrodes to apply electric field to the PDLC film. The voltage applied to the electrodes is in the transverse direction to the aligned the domain of LC droplet.  In absence of electric filed PDLC film work as a scattered state as shown in Figure 2 (a). When the applied voltage between the ITO electrode i.e. V = V_th all the LC droplet start to orient along the electric field. This voltage is known as threshold voltage V_th. Transparent state  can be obtained by reorienting the LC droplet director along the applied electric field as a result of  the refractive indices of the LC droplets and  the polymer binder match, [16,17]. Transparent state of the device is shown in Figure 2(b). The electro-optical property of PDLC film is depend on PDLC droplet size [15-17]. The threshold voltage is inversely proportional of the size of the droplet. The droplet size depends on the intensity of UV light during fabrication  [15-16].

Figure 1: Fabrication process of PDLC switchable windows (a) Injected PDLC precursor (b) UV exposure

Figure 2: Working principle of patterned  PDLC transparent display (a) V=0, (b) V > V_th.

For the fabrication of PDLC film a mixture of 5CB liquid crystal and liquid monomer NOA65 (Norland) have been used. A device has been fabricated using the two indium tin oxide (ITO) coated fused silica substrates as shown in Figure 1 (a). Both the substrates have been bonded together by a NOA61 polymer with a spacer of 20 μm. Later, the precursor in the isotropic state is infiltrated into the a 20 μm thick cell made with two ITO fused silica substrates. The fabricated substrate has been kept on a hot plate at temperature (60-70°C) during the infiltration procedure. This process makes precursor is in its isotropic phase and reduces the viscosity. The cell has been allowed to uniform UV exposure for 20 minutes using UV lamp (wavelength 365 nm) to initiate the polymerization as shown in Figure 1 (b). Dose of UV light decides the size of LC droplet. Consequently, a cured polymer of PDLC is obtained by the above process. Electrical connection is attached to ITO electrodes by using conducting tape. Voltage is applied between the ITO electrodes using electrical wire on ITO electrodes. The snap of fabricated PDLC device is shown Figure 3.

The device was characterized with the help of LED source, optical power meter and DC power supply. An LED light source of 1mW optical power incident on the device from one side and power measured on other side of the film with the help of optical power meter. We have measured the transmittance of the device at various voltages shown in Table 1. A graph between transmittance and applied voltage is shown in  Figure 4.  Figure 4.  shows that the transmittance is very small up to V = 36 volt and started to increase after V=36 volt, which shows that threshold voltage of PDLC is ~ 36 volt. This is because light is scattered due to mismatched of refractive indices of the LC and the polymer in the cured PDLC film below the threshold voltage. When voltage is further increased beyond V_th transmittance of PDLC increases significantly and reached  up to 0.86.  This is because all liquid crystal molecule in the droplets are aligned parallel to the electric field, and incident light go throughout PDLC film without scattering.

 

Figure 3: Image of IND at different applied voltages (a) V= 0 V; (b) V=50 V > V_th

To show the device work as electrically switchable windows, an printout of image IND is kept below the device. In the absence of electric field i.e. V=0, the IND is barely visible i.e. nothing can be seen through the device as shown in  Figure 3(a). This is due to strong scattering of light. When an uniform electric field (i.e.  V=V_th,) is applied to the device a hazy image of alphabet just started to visible. This is because all the LC droplet start to orient along the electric field. As the applied voltage is further increased up to V=50 V > V_th, the image of alphabet is clearly visible as shown in Figure 3(b). This is because high electric field makes the all LC droplet in PDLC aligned along the field, resulting in the transparent state. 

Table 1 : Measured transmittance at various voltage of the device ( incident  power - 1 mw)

Sl. No.	Voltage (volt)	Transmitted power (mW)	Transmittance
1	0	0.04	0.04
2	4	0.04	0.04
3	8	0.04	0.04
4	12	0.04	0.04
4	16	0.04	0.04
6	20	0.04	0.04
7	24	0.05	0.05
8	28	0.05	0.05
9	32	0. 06	0. 06
10	36	0.08	0.08
11	40	0.2	0.2
12	44	0.5	0.5
13	48	0.66	0.66
14	52	0.78	0.78
15	56	0.84	0.84
16	60	0.87	0.87

 

Figure 4: Measured V-T curves of the device. 

Figure 3 show that the PDLC film can be used in shutter mode as a smart window as the proposed device can operate in the scattering state or transparent state by controlling the applied voltage. The device can also be used in electrically controlled semi transparent mode in smart window.

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