**1. Introduction**

Liquid crystal displays (LCDs) are extensively utilized in daily life for items such as notebook computers, smartphones, and portable televisions due to their lightweight, low power consumption, and good mobility. Up to the present day, many studies have revealed several LC modes of display to achieve high picture qualities, e.g., in-plane switching (IPS) [1], pixel-patterned vertical alignment (VA) [2], and fringe-field switching (FFS) [3]. In an LCD, LC molecules located between two electrodes are moved by the applied electric field to transmit light, thereby displaying information.

Because of the growing interest in display applications, many studies on transparent electrodes have been recently conducted, focusing on electron transport and high transparency. In previous studies, metal oxides have been investigated as transparent electrodes because they display low resistance and high transparency [4,5]. Indium tin oxide (ITO) material has been commonly used for the last several decades as a transparent electrode thin film owing to its high visible transparency, excellent electrical conductivity, and availability of deposition on glass and plastic substrates. However, its high production cost and fragile characteristics limit its applicability for flexible electronic devices [6,7]. Also, the total reflection at the glass/ITO interface and the low adhesion to organic substrates lower the display performance of LC devices containing ITO transparent electrodes [8,9]. Many electrode materials, such as organic carbon nanotubes (CNTs) [10,11], metallic nanowires [12,13], metal thin films [14], graphene [15,16], and conductive polymers [17,18], have been studied as alternatives to ITO.

The single-walled carbon nanotube (SWNT) with a few nanometer radius is an emerging and especially promising material as alternative transparent electrodes. Several approaches have been utilized to generate SWNT films such as transfer printing [19], direct chemical vapor deposition (CVD) growth [20], vacuum filtration [21], and rod coating [22]. These SWNT thin films showed fairly high optical transmittance and good conducting properties which are similar to those of the conventional ITO-coated poly(ethylene terephthalate) (PET; 50–200 Ω resistance and ~83% transmission at 550 nm) substrate [23]. However, these methods tend to exhibit some problems such as poor film surface, optoelectronic performance, high cost, and a complex process [24]. Transfer printing, one of the most-used thin film techniques, results in difficult large-area fabrication and relatively brittle SWNT thin films. Therefore, in this study, we focused on the transparent electrode properties of SWNT by the layer-by-layer (LBL) process. Compared with other CNT thin film methods, the LBL technique provides the precise control needed to make thin films with a few nanometer thickness.

LBL assembly is one of the wet coating methods that has been widely studied in last 20 years due to its simple process and high controllability under ambient conditions [25,26]. This process can be used as a method of producing the thin films suitable for a variety of materials, which may include nanoparticles, nanotubes, nanowires, nanoplatelets, dyes, dendrimers, proteins, and viruses. The LBL process enables the production of homogeneous multifunctional multilayers with a controlled method. Moreover, it can be applied to a variety of substrates (e.g., glass and quartz slides, silicon wafers, and polymeric films) to evaluate the performance of electrical conductivity [27,28], sensing [29,30], and energy harvesting [31].

In this study, VA LCD devices with a polyimide (PI) alignment layer on SWNTs and poly (diallyldimethylammonium chloride) (PDDA) layer-by-layer electrodes were fabricated and characterized to evaluate their potential for transparent organic electrode materials in an LC display. Firstly, PDDA was used as the positively charged polyelectrolyte in conjunction with a negatively charged surfactant to deposit SWNTs onto a PDDA layer. The alternate dipping in positively and negatively charged solutions resulted in SWNT/polymer thin film deposition onto the substrate. The LC test devices were fabricated by filling the LCs into the inner parts of the PI layer by spin coating, and their device structures have been reported in the literature [32,33]. The electro-optical (E-O) properties of the prepared VA LCD devices containing the LBL transparent electrodes were examined and compared to those of LCD devices with conventional ITO electrodes.

#### **2. Materials and Methods**
