**1. Introduction**

Transparent conducting materials (TCMs) possess simultaneously high electrical conductivity and optical transparency (i.e., effective transmission of light in the visible spectrum), with these particular characteristics being achieved on processing the materials as thin films on transparent substrates. This specific combination of properties makes TCMs very interesting both from a fundamental viewpoint and for a large variety of applications. The principal factor that has stimulated the research on TCM syntheses and their processing is definitely the development of optoelectronic materials and devices in which the principles of actuation involve an application of electric current or voltage to control the emission or passage of light, the most notable examples being display devices [1,2]. Moreover, other applications that require TCM, such as smart windows (based on electrochromic or polymer-dispersed liquid crystalline materials) and photovoltaic systems, have attained enormous relevance in the present context of environmentally important energy efficiency and clean energies, further prompting the scientific and technical developments in TCMs, as evidenced in the intensive and continuing research in this field [3].

The most studied and practically used TCMs are oxides, referred to as transparent conducting oxides (TCOs). TCOs are usually made of high band gap (> 3 eV) oxides which are intrinsically or extrinsically doped to reach a very low resistivity (~10−<sup>4</sup> <sup>Ω</sup>·cm). The basic TCO materials include indium oxide (In2O3), tin oxide (SnO2), and zinc oxide (ZnO). They can be degenerately n-type doped with tin (In2O3:Sn, also known as ITO), fluorine (SnO2:F) or Al (ZnO:Al), as popular examples. The usage of TCO films depends on the application. In liquid crystal displays (LCDs), light-emitting diodes (LEDs), or transparent displays, these films are used as electrodes, while they are used as touch sensors for resistive and capacitive touch panels. Transparent electrodes are essential in high-impact technological areas such as photovoltaics, flat panel displays and touch screens, as well as in emerging areas such as smart sensors or organic electronics (organic light-emitting devices (OLEDs), organic photovoltaics) as well as transparent field-effect transistors (FETs) [4]. The most relevant properties for all these applications are their high conductivity combined with high transparency in the visible spectral region. Furthermore, key TCO performance factors, depending on the particular application, are their high chemical and thermal stability or the possibility of tuning their work function. Especially important for flexible electronics are the mechanical properties, high stretchability and bendability, and low-contact resistance with organic materials. Transparent conducting films are estimated to reach a market of value of US\$ 8.46 billion by 2026 [5]; intensive research is therefore important to discover superior materials, new substrates, and new ways to enhance light transmission, to increase the electric conductivity, to add flexibility, and to decrease costs.

ITO is the most widely used material for the fabrication of TCOs; however, ITO has several critical shortcomings such as generally high processing cost and unsuitability for flexible devices. In particular, for large-area touch screens, the resistivity is too high for the rapid touch sensing response; in addition, ITO is brittle and therefore inadequate for applications in flexible electronics such as flexible touch screen displays and solar cells. Aluminum-doped zinc oxide (AZO) is an affordable, non-toxic and robust transparent conductive oxide (TCO) [6]. AZO films have high transmission in the visible region and useable transmission to IR wavelengths as long as ~12 μm. In contrast, the more commonly known TCO, ITO, reflects IR at wavelengths longer than ~2 μm. IR transmission is very important because increasing the long-wavelength response is an approach to enhance the efficiency of some solar devices [7]. The higher stability of AZO in reducing atmospheres may also be an advantage for future applications [8]. A substantial cost saving is possible with AZO materials compared to ITO and other TCOs. Due to these practical advantages, AZO films are considered as ideal replacements for ITO films in applications such as transparent electrodes for solar cells, flat panel displays, LCD electrodes, touch panel transparent contacts and IR windows [9–11]. AZO thin films can be deposited by several techniques such as sol–gel [12], chemical spray [13], thermal evaporation [14], pulsed laser deposition [15], DC and RF magnetron sputtering [16], reactive mid-frequency magnetron sputtering using dual magnetrons [17] and atomic layer deposition (ALD) [18]. The desired properties of a good TCO—transparency, conductivity, and surface texture—depend significantly on the preparation technique and the growth parameters.

ALD is a growth technique that has recently become very popular since it provides uniform and conformal coverage and control of the thin film by atomic layer precision [19]. Although the growth rate of the ALD system is relatively low, the uniformity, conformality and the compactness of the film cross-section achieved from the ALD technique are superior to those from other techniques [20].

In this work, we demonstrate the application of AZO thin films as a substitute for ITO electrodes in LC and PDLC display devices. Using the ALD technique, AZO films are deposited on glass and PET substrates and their structural, optical and electric properties are measured and discussed. AZO layers on PET substrates show good flexing properties, as evidenced by their stable sheet resistance over 1000 bending cycles.

The article is structured as follows: we first report on the ALD deposition of AZO films, followed by brief characterizations of the layers, including structural, optical and electrical properties. The next step includes the measurement of the bending ability of the AZO layer deposited on a flexible PET substrate. In the final section, on the basis of the above characteristics, we present two applications of AZO as a transparent and conductive layer, respectively, in (i) a liquid crystal (LC) display using AZO/glass and (ii) flexible polymer-dispersed liquid crystal (PDLC) devices using AZO/PET. The obtained results show the great potential of AZO for integration into next-generation ITO-free flexible and stretchable devices.
