**1. Introduction**

Transparent electrodes (i.e., thin films based on transparent conductive (TC) materials) are some of the most important parts of many optoelectronic devices, such as touch panels, organic light-emitting diodes (OLEDs), optical sensors, and solar cells [1–5].

Nowadays, transparent electrodes based on Sn-doped In2O3 (ITO) present outstanding optoelectronic performance and have been widely used in various commercial domains [1,6]. However, the wide use of ITO transparent electrodes in optoelectronic devices is gradually pushing up the cost of ITO electrodes because indium is not abundant on Earth. Moreover, with the rapid development of new types of display systems, sensors, and solar energy, new requirements for transparent electrodes are emerging from device developers, in addition to their transparency and conductivity. It is getting harder for the traditional ITO electrodes to meet the new requirements. Therefore, alternative materials should be developed.

A variety of ITO replacements have been investigated, including doped wide-bandgap oxides with high transmittance, such as SnO2 [7], ZnO [8,9], and TiO2 [10]. However, these oxides were found to have lesser performance than ITO, combining both electrical and optical properties. As alternatives to ITO, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT–PSS) [11], graphene [12], carbon nanotubes [13], and metal nanowires and meshes [14,15] have been proposed. However, each alternative solution is affected by one or more drawbacks that prevent their widespread use [16,17].

In order to keep a low resistance and conversely maintain high optical transmittance, oxide–metal–oxide multilayered structures have recently received renewed interest as a highly promising route towards the production of flexible large area OLEDs and solar cells [3,18–20]. In this case, Ag is the optimal metal because of its low resistivity (approximately 1.6 <sup>×</sup> 10−<sup>6</sup> <sup>Ω</sup>·cm) and relatively low cost [21], whereas Ga-doped ZnO (GZO) is the optimal oxide due to its abundance, low cost, superior optical features, and rather high stability [22,23].

Various deposition techniques have been used to produce oxide–metal–oxide structures, including thermal evaporation [24], electron beam evaporation [25], spray pyrolysis [26], sol–gel methods [27], ion beam sputtering [28], and magnetron sputtering [18–20,28]. Low-cost wet chemical methods are usually the starting point and benchmark for most academic and industrial processes that require a thin and uniform coating, but the transparent electrodes obtained by these techniques have resulted in inferior electrical features compared with those deposited by ion plasma methods [25,29,30]. In this sense, it appears that DC magnetron sputtering is the most promising technique in terms of the industrial deposition of uniform films at a proper deposition rate [18]. From the point of view of the deposition of transparent electrodes on flexible substrates covering a large area, it is also very important to achieve TC films with good performance stability by using low-temperature processes [28,31].

In this article, symmetric GZO/Ag/GZO (GAG) multilayered structures were sequentially formed on glass substrates by room temperature DC magnetron sputtering under a pure Ar atmosphere. The uniqueness and novelty of this work resides in having found the process conditions that provide the optimal trade-off between low resistivity and high optical transmittance and are applicable for TC thin film formation on polymer substrates. The thicknesses of GZO and Ag layers were parametrized to get the optimal optical and electrical properties of the superstructures. The deposited multilayers were characterized and tested for their structural, electrical, optical, and adhesive properties.
