3.2.1. AZO as Transparent Conductive Layer in Rigid LC Display

Several AZO/glass samples were selected for transparent electrodes in the LC cell assembly, hereafter called an AZO-based LC rigid display. First, a Polyvinyl Alcohol (PVA) layer was spin-coated, baked at 90 ◦C for 30 min and post baked at 120 ◦C for 30 min on AZO/glass samples. Next, the PVA-coated AZO/glass substrates were mechanically rubbed for LC molecule alignment. We used nematic LC (NLC type E7, Merck, Kenilworth, NJ, USA). The cell was arranged by gluing the two rubbed layers in an anti-parallel configuration with the alignment directions facing each other. A 12-μm Mylar spacer was used to keep the same thicknesses in the fabricated cells. The liquid crystal was injected via the capillary method into the empty cell. Finally, the arranged cell was sealed with ultraviolet (UV) glue and exposed with UV light to stabilize it. Copper tape wires were used for electrical connections (see Figure 5a,b). In addition, a reference LC cell using ITO/glass was prepared following the same fabrication procedure and thickness for comparison.

For electro-optical modulation characteristic measurements, the LC cells were placed between a pair of crossed polarizers. The incident beam was polarized at the angle of 45◦ with respect to the nematic director (a dimensionless unit vector n, which represents the direction of the preferred orientation of LC molecules). An alternating voltage (*f* = 1 kHz) with varying amplitude was applied across to the LC cell to orient the LC director. A helium–neon laser (He–Ne) emitting λ = 633 nm was used to probe the changes in the transmitted intensity of LC devices under different amplitudes of driving voltage. The power of the transmitted beam was monitored by a power meter. The light transmission was measured as a function of the applied root mean square (RMS) alternating current (AC) voltage with a 1-kHz frequency. The transmittance changes were detected by positioning a photodetector behind the device (Figure 5d).

**Figure 5.** Schematic diagrams of: (**a**) AZO/glass LC cell; (**b**) ITO/glass LC cell and (**c**) AZO/PET polymer-dispersed liquid crystal (PDLC) cell and (**d**) experimental setup to measure the electro-optical modulation behavior. Legend: lens (L), aperture (Ap), polarizer (P), analyzer (A), amplifier (Am), function generator (FG), data acquisition card (DAQ). Note: for PDLC cell measurements, the P and A are removed from the setup.

The basic operation principles of LC displays rely on the electro-optically controlled birefringence of the LC molecules [24,25]. Upon application of an electric field, since the LC molecules have a different polarizability along their long and short axis, an induced dipole moment arises and all the molecules start to reorient towards the direction of the applied field. Because of the LC's anisotropy, the nematic LC layer acts as the birefringent material, characterized by different refractive indices for a beam polarized along the long or short molecular axis.

Figure 6a,b shows the transmitted light intensity dependence of the applied voltage for an LC cell using AZO as transparent electrode. A reference cell, assembled using a commercial ITO electrode is shown for comparison. The transmittance–voltage behavior follows the typical sinusoidal function of the amplitude of applied voltage, governed by the electrical response of the liquid crystal molecules, as expressed by [25].

$$T = \frac{1}{2} \sin^2 \frac{\Gamma}{2} = 1/2 \sin^2 \left[ \frac{\pi d (n\_\text{e} - n\_0)}{\lambda} \right] \tag{1}$$

where Γ = <sup>2</sup><sup>π</sup> <sup>λ</sup> (*n*<sup>e</sup> − *n*0)*d* is the phase retardation due to the birefringence Δ*n* (Δ*n* = *n*<sup>e</sup> − *n*0) modulation, λ—the wavelength, *d*—the thickness of the LC layer and *n*<sup>e</sup> and *n*<sup>0</sup> are the refractive indices for extraordinary and ordinary waves, respectively.

As shown in Figure 6a, the transmitted intensity follows a series of maxima and minima (so-called Fréedericksz transition), which correspond to the phase retardation, as presented in Figure 6b.

The low sheet resistance of the AZO layer has a large impact on the modulation characteristics of assembled LC cells. As seen from Figure 6a, the modulation behavior of the LC cell using AZO contacts is very similar and competitive with the LC cell using ITO. The results are also supported by the calculated Haacke figure of merit (*FOM*) factor of performance, expressed by the ratio between the optical transmittance (*T*av) and the sheet resistance (*R*s) values [26]:

> FOM <sup>=</sup> *Tav*<sup>10</sup> *Rs*

(2)

**Figure 6.** (**a**) Transmittance–voltage characteristics of AZO/glass and ITO/glass (reference) LC cells and (**b**) phase retardation for both LC cells.

Using the transmittance data (Figure 2) and measured sheet resistance values of AZO/glass, we obtained FOM (AZO) <sup>=</sup> 1.05 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>Ω</sup>−1. Similar values have been calculated for (commercially available) ITO/glass, FOM (ITO) <sup>=</sup> 2.52 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>Ω</sup>−1. These values are in good confirmation with those previously reported [27,28] and demonstrate the high potential of AZO films as transparent electrodes for ITO-free LCD devices (Table 1).

**Table 1.** Haacke figures of merit (FOM).


3.2.2. AZO as Transparent Conductive Layer in Flexible Display (Smart Structures)

PDLC-based structures have attracted increasing attention for applications as outdoor displays, switchable privacy glasses, energy saving windows, light shutters, projection displays, and so on [29,30]. Here, the PDLC film was prepared using a polymerization-induced phase separation method, based on the phase separation of the LC (E7, Merck, *n*<sup>0</sup> = 1.521 and *n*<sup>e</sup> = 1.72) and photo-curable adhesive polymer matrix (NOA65, Norland, Cranbury, NJ, USA, *n* = 1.524) with a 3:7 weight ratio. During the fabrication of our flexible PDLC structures, supported by AZO/PET substrates, we perform the following steps: first, an empty cell was prepared by gluing two AZO/PET substrates with 12-μm Mylar spacers between them. Then, the LC/monomer mixture was injected into the empty cell and, finally, the cell was exposed with ultraviolet light ((λ = 365 nm) with an intensity of 60 mW/cm2 for 15 min) to polymerize NOA65. As a result, using the polymerization-induced phase separation method, randomly dispersed liquid crystal droplets (usually in micrometer size) are formed within a transparent polymer matrix. Figure 5c shows the schematic structure of the AZO/PET PDLC flexible device.
