3.2.3. Electro-Optical Characteristics and Bending Test Ability

In general, PDLC can be switched between a light-scattering to a transparent state by applying an external electric field. The effect results from a mismatch or match of refractive indices between the LC molecules and the polymer matrix and is due to the LC birefringence and the ability of an applied voltage to re-orient the LC molecules inside the droplets in order to match the LC's refractive index to that of the polymer matrix. The electro-optical characteristics were measured by an optical setup, shown in Figure 5d. In these measurements, the polarizer and analyzer were removed from the setup, since the polymer defines the polarization state of the PDLC film.

The typical transmittance dependence of the assembled AZO/PET PDLC device as a function of the applied voltage is shown in Figure 7a. Without an applied voltage, the LC molecules are randomly oriented in the droplets, which causes light scattering when the light passes through the structure. As a result, the "scattering" state appears. When the voltage is applied, the electric field aligns the LC's nematic director to the direction of the electric field, allowing light to pass through the droplets. As a result, a "transparent" state appears. Thus, the intensity of the transmitted light through the PDLC structure can be controlled by the application of an external voltage.

**Figure 7.** (**a**) Voltage-transmittance behavior of AZO/PET PDLC device and (**b**) the response time.

For the AZO/PET device shown in Figure 7a, we defined (i) the threshold voltage *V*th as a value of the applied voltage necessary to reach 10% of the maximum transmittance, e.g., to "turn on" the PDLC cell (measured *V*th ~ 6.1 V) and (ii) the saturation voltage *V*sat, defined as the value of applied voltage required to reach 90% of maximum transmittance T (*V*sat ~18 V). As seen, at the saturation state, the PDLC structure becomes transparent (the "BAS LOGO" image pattern become clear).

In addition, the response time of the assembled PDLC device for switching between "off" and "on" states was measured and is presented in Figure 7b. The measured response time and the fall time values were ~68 and ~88 ms, respectively. These values are very similar to the reported values for PDLC devices using other transparent contacts [31,32].

To sum up, the implementation of AZO layers as transparent conductive electrodes in LC and PDLC devices requires the synthesis of high uniformity, conformal and compact layers by an appropriate process such as ALD. The AZO films obtained by ALD technique enable low sheet resistance and high optical transparency through the use as transparent electrodes on the top surfaces of both the selected rigid and flexible substrates.
