Towards AZO Thin Films for Electronic and Optoelectronic Large-Scale Applications

Abstract

Transparent conductive oxides (TCOs) have become essential components in a broad range of modern devices, including smartphones, flat-panel displays, and photovoltaic cells. Currently, indium tin oxide (ITO) is used in approximately 90% of these devices. However, ITO prices continue to rise due to the limited supply of indium (In), making the development of alternative materials for TCOs indispensable. Therefore, this study highlights the latest advances in creating new, affordable materials, with a focus on aluminum-doped zinc oxide (AZO). Over the last few decades, this material has been widely studied to improve its physical properties, particularly its low electrical resistivity, which can affect the performance of various devices. Now, it is close to replacing ITO due to several advantages including cost-effectiveness, stability under hydrogen plasma, low processing temperatures, and lack of toxicity. Besides that, in comparison to other TCOs such as IZO, IGZO, or IZrO, AZO achieved a low electrical resistivity (10⁻⁵ ohm cm) while maintaining a high transparency across the visible spectrum (over 85%). Additionally, due to the increasing development of technologies utilizing such materials, it is essential to develop more effective techniques for producing TCOs on a larger scale. Additionally, due to the increasing development of technologies utilizing such materials, it is essential to develop more effective techniques for producing TCOs on a larger scale. This review emphasizes the potential of AZO as a cost-effective and scalable alternative to ITO, highlighting key advancements in deposition techniques such as pulsed laser deposition (PLD).

1. Introduction

Transparent conducting oxides (TCOs) are intensively studied due to their role as essential components in the fabrication of modern electronic and optoelectronic devices that require large-area transparent electrodes, such as photovoltaic cells, displays, touch screens, etc. [1]. The origin of TCO dates back to 1907, when Badeker observed that thin films of Cd metal could be oxidized to become transparent while maintaining a low electrical resistivity (1.2 × 10⁻³ Ω∙cm) [2,3]. This study marked the start of the TCO era, in which several materials, including indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), were developed [4]. However, with the emergence of the display industry around 1970, the optoelectronic devices industry recorded significant progress. Since then, ITO has become the dominant material used as TCO in various applications [3,5,6].

TCOs are notable for their unique combination of high transparency in the visible spectrum and good electrical conductivity, properties which are essential for the efficient operation of the previously mentioned devices (Figure 1). Furthermore, such materials are employed in technologies that require a combination of physical properties, including low electrical resistivity (10⁻³–10⁻⁴ Ω cm) coupled with high transparency (>80%) in the visible range of the electromagnetic spectrum [2,7]. Generally, these materials are binary or ternary compounds containing one or two metallic elements characterized by a wide bandgap ranging from 3 to 3.37 eV [8]. Among the various materials used for TCOs, ITO exhibits high performance with electrical resistivity values lower than 10⁻⁴ Ω cm [9] combined with an optical transmittance that can exceed 90% in the visible region for film thicknesses of approximately 100 nm [8]. The main drawback of ITO is its increasing cost, which is caused by the growing demand for displays and photovoltaic cells on the market. As a result, its market price continues to rise due to the limited availability of In. Another concern is related to the chemical instability and toxic nature of indium [10,11]. Furthermore, a significant issue is the diffusion of indium into the active region of the photovoltaic cell, which causes degradation of the active layer and compromises the device’s performance. Moreover, ITO exhibits poor stability in hydrogen plasma, which is incompatible with an a-Si:H solar cell [12]. Therefore, research has focused on cost-effective alternatives that can offer properties comparable to, or even better than, those of ITO. Thus, new classes of doped semiconductors have been explored, including Al:ZnO (AZO), Ga:ZnO (GZO); In, Ga:ZnO (IGZO), F:SnO₂ (FTO), In:ZnO (IZO), Zr:In₂O₃ (IZrO), among others [11]. The earliest reports on transparent electrodes based on ZnO and Al-doped ZnO were made between 1971 and 1979 and Ssince then, ZnO-based TCOs have received considerable attention from the scientific community, primarily due to their low cost, non-toxicity, wide band gap (3.37 eV), and low resistivity of 10⁻⁴ Ω cm [13]. While ITO dominates the market in display production, alternative TCOs, such as fluorine-doped tin oxide (FTO), are preferred in applications like thin-film solar cells or low-emissivity glass due to their efficiency in preventing heat loss, which is ideal for applications involving low temperatures [14]. In low-emissivity (low-E) windows, TCOs are preferred due to their high free carrier concentration, which reflects infrared radiation for wavelengths longer than the plasma wavelength. In solar cells, TCOs have two essential roles: (1) they act as front contact, allowing the light to enter the absorber layer where the charge carrier generation occurs, and (2) after the charge carriers are generated in the absorber layer, TCOs function as an electrode, collecting the generated carriers [12]. In flat-panel displays, TCOs act similarly to those in solar cells, serving as an optical window and driving each pixel in FPDs. In electromagnetic shielding applications, TCOs are used to block the unwanted signals entering the window. In such applications, a TCO must have a high transmittance, while the resistivity can be moderate (200 Ω/sq.) [2,15].

2. Optical and Electrical Properties

To understand the combination of optical transparency and high electrical conductivity characteristics of TCOs, it is essential to discuss how free carriers are generated in these materials. Ideally, such a material should provide transparency in the near-UV, visible (VIS), and near-infrared (NIR) regions (see Figure 2). However, two characteristic transmission edges typically appear in the UV and NIR regions. In the UV region, the principal mechanism limiting transmission is the fundamental band gap, as photons with energies higher than Eg are absorbed due to interband electronic transition. However, in the near-infrared (NIR) region, highly doped transparent electrodes, known as degenerate semiconductors, exhibit a drop in transmittance. This occurs because their plasma frequency (which represents the collective oscillation of free electrons located in the conduction band in response to interaction with an electromagnetic field) is in the NIR region of the spectrum [16]. In turn, a second transmission edge appears in the NIR region due to free-carrier absorption governed by the plasma frequency (Equation (1)), resulting in a drop in transmittance. In contrast, in the VIS region, TCOs act as an optical window, allowing high transparency [17]. Therefore, for frequencies higher than the plasma frequency, the electrons cannot respond, and the material acts as a dielectric, allowing light to pass through. In the case of a frequency below the plasma frequency, TCOs absorb and reflect the incident radiation [15,16,18].

$${\mathsf{\omega }}_{\mathrm{p}}=\sqrt{{\displaystyle \frac{n e^2}{m^{*} {\epsilon }_r{\epsilon }_0}}}$$

(1)

TCOs are degenerate semiconductors that exhibit good electrical conductivity, in the range 10²–10⁶ S cm⁻¹, mainly due to controlled doping or native defects [20]. For an oxide to become conductive, it must exhibit stoichiometric deviation, either through the presence of native defects or extrinsic doping. In unintentionally doped materials, electrical conductivity arises from the presence of charge carriers primarily generated by native defects, which can act as donors or acceptors, resulting in either n-type or p-type behavior. For an n-type TCO, the free carriers are generated by interstitial cations or anion vacancies that create shallow donor states close to the conduction band. Additionally, at room temperature, electrons can be thermally activated from these levels, contributing to conduction. However, undoped TCOs exhibit low electrical conductivity due to the low number of free carriers. Additionally, the electrical conductivity can be improved through the controlled doping of the material with metal ions, which increases the charge carrier concentration [19]. A relevant example of TCO is zinc oxide (ZnO). In the undoped state, it exhibits n-type conductivity, which can be attributed to native defects such as interstitial zinc atoms (Zni) and oxygen vacancies (Vo) [21]. However, when it is extrinsically doped with metal ions such as Al³⁺, which substitutes for Zn²⁺ in the lattice (AZO), the carrier concentration increases, enhancing the electrical conductivity of AZO-based TCOs.

An essential property of TCOs is the optical band gap, which represents the lowest energy required for an allowed optical transition. At high carrier concentrations, electrons occupy energy states within the conduction band close to its edge, thereby pushing the Fermi level deeper into the conduction band, to higher energy states. Consequently, optical transitions can only occur between an occupied energy level and a higher, unoccupied or partially occupied energy level, leading to an apparent increase in the optical band gap (Figure 3), a phenomenon known as the Moss–Burstein effect. In Equation (2), E_g represents the fundamental band gap, which corresponds to the lowest energy required to move an electron from the valence band to the conduction band [7].

E_g = E^CB − E^VB

(2)

In highly doped semiconductors, the measured optical band gap, E_g^opt, appears larger than the fundamental band gap (E_g) due to the high electron concentration typically induced by donor doping or intrinsic defects, known as the Moss–Burstein effect (E_g^MB). This surplus of carriers fills lower energy states into the conduction band. Consequently, a direct optical transition from the maximum of the valence band (E^VB) into the minimum of the conduction band (E^CB) is not possible, as the lower states are already occupied. As a result, the photons require more energy to promote electrons to higher, unoccupied (or partially occupied) states in the conduction band. This causes a shift of the absorption edge to higher energies within the conduction band [7,8]. In such degenerate semiconductors, the Fermi E_F level is located in the conduction band (see Figure 2).

E_g^opt = E_g^MB + E_g = E_F − E^VB

(3)

E_g^MB = E_g − E^CB

(4)

The electrical conductivity of TCOs is determined by the number of free carriers in the material and their mobility. As previously discussed, the free carriers in TCOs are supplied by dopant elements. In the absence of doping, TCOs would behave similarly to [18] insulators, exhibiting a resistivity of approximately 10¹⁰ Ω cm, since the conduction band cannot be thermally populated at RT (kT~0.03 eV).

We can express the electrical conductivity as directly proportional to the charge density in the conduction band and the mobility of the charge carriers [7,22].

σ = neµ

(5)

where σ is the electrical conductivity, µ is the charge carrier mobility, n is the charge density, and e represents the electric charge. The charge carrier mobility can be expressed as follows [7,22]:

$$\mathrm{\mu }={\displaystyle \frac{\mathsf{\tau }\mathrm{e}}{{\mathrm{m}}^{\mathrm{*}}}}$$

(6)

where τ represents the average time between collisions, and m* represents the effective mass of the electrons. Knowing the electrical conductivity, we can express the resistivity as follows [7,22]:

$$\mathsf{\rho }={\displaystyle \frac{1}{\mathsf{\sigma }}}$$

(7)

$$\alpha ={\sigma }_c \ast n$$

(8)

Figure 4. A suggestive classification of some insulating, semiconductors, TCO, or conductive materials, based on their electrical conductivity (adapted from Ref. [23]).

Figure 4. A suggestive classification of some insulating, semiconductors, TCO, or conductive materials, based on their electrical conductivity (adapted from Ref. [23]).

To serve properly as a TCO, the material must exhibit high mobility (μ = 50–70 cm² V⁻¹ s⁻¹) [24] with low resistivity (ρ = 10⁻⁴–10⁻⁵ Ω cm), while maintaining a carrier concentration below 2 × 10²¹ cm⁻³, to minimize the unwanted optical absorption [25]. The charge carrier mobility depends on the charge carrier density differently for different classes of materials (Figure 4). In semiconductors, the mobility has high values at low carrier densities, as can be seen in Figure 5. The presence of a donor level near the conduction band leads to an apparent optical band gap widening due to the Burstein–Moss effect. This effect results in a shift of the optical absorption edge to higher photon energies [25]. Thus, the absorption coefficient becomes dependent on the electron density, as indicated in Equation (8).

Unfortunately, electrical conductivity in TCOs is limited since the electron density and mobility cannot be increased independently at higher carrier concentrations. For high electron densities, charge transport is affected by carrier scattering on ionized impurities and grain boundaries [26,27]. Therefore, a high doping level causes a decrease in mobility, while optical transmission decreases, particularly in the near-infrared region [7,8].

When discussing transparent and conductive oxides, we must consider the sheet resistance (Rsq), which is one of the most important parameters for evaluating the electrical properties of these materials. Sheet resistance is defined as the inverse of sheet conductivity (σs) and is typically determined using measurement techniques such as the Van der Pauw method or the 4-probe method [28].

$$R_s={\displaystyle \frac{\rho }{t}}={\displaystyle \frac{1}{{\sigma }_{s}t}}$$

(9)

As previously mentioned, an optimized TCO must address two contradictory requirements: low electrical resistivity and high optical transparency. Achieving this balance involves doping the material to enhance the electrical conductivity. However, excessive doping may result in a drop in transmittance in the NIR region, thereby limiting its application in solar cells. In this context, the first definition of figure of merit (FoM) for transparent conducting oxides was introduced by Fraser and Cook, aiming to describe the performance of such a material. The FoM is expressed as the ratio between the transmittance and the sheet resistance.

$$F_{TC}={\displaystyle \frac{T}{R_s}}=\sigma t{e}^{-\alpha t}$$

(10)

where T represents the optical transmission average around 550 nm, Rsq is the sheet resistance, t is the thickness, and α is the absorption coefficient. However, the equation becomes an analytical function depending on thickness, with t_max = 1/α, so the maximum figure of merit occurs at a film thickness that reduces the optical transmission to 37% [29]. According to the practical requirements for TCO, Haacke modified the FoM in favor of transmittance by raising it to the power of 10. In this formulation, the maximum FoM occurs at transmittance around 90%, which is optimal for practical applications. Equation (10) is the most used for the calculation of FoM [30,31].

$$F_{TC}={\displaystyle \frac{T^{10}}{R_s}}=\sigma t{e}^{-10\alpha t}$$

(11)

3. Indium Tin Oxide (ITO): Properties and Applications

ITO is an n-type semiconductor that exhibits two crystallographic structures: rhombohedral and cubic, the latter being the one stabilized to achieve both low resistivity and high optical transparency in the visible region [32]. The transparent nature of ITO arises from the Sn⁴⁺ dopants inserted into the In₂O₃ matrix. Furthermore, these dopants contribute with free electrons, increasing the carrier concentration. This surplus of electrons leads to a higher optical band gap, due to the Moss–Burstein effect, since the lowest states in the conduction band become filled. In this circumstance, only the electrons with enough energy can overcome the band gap and transition into the conduction band. This is the main mechanism responsible for the high optical transparency of ITO in the visible part of the electromagnetic spectrum [33]. On the other hand, the conductive nature of ITO arises from two primary conduction mechanisms: (1) oxygen vacancies; and (2) doping with Sn⁴⁺ ions. In the crystalline structure of In₂O₃, oxygen vacancies are typically surrounded by In³⁺ cations. The formation of these vacancies results in two weakly bound electrons, which are free to move within the structure, thereby increasing conductivity [33].

$${\mathrm{I}\mathrm{n}}_2{\mathrm{O}}_3\mathrm{ }\to \mathrm{ }{\mathrm{I}\mathrm{n}}_{2-\mathrm{x}}^3\mathrm{ }\mathrm{ }{\left({\mathrm{I}\mathrm{n}}^{3+}\mathrm{ }•\mathrm{e}\right)}_x\mathrm{ }{\mathrm{O}}_{\mathrm{ }3-\mathrm{ }\mathrm{x}}^{2-}\mathrm{ }+{\displaystyle \frac{\mathrm{x}}{2}}\mathrm{ }{\mathrm{O}}_2$$

(12)

Doping ITO with Sn generates shallow donor levels, providing a carrier population at room temperature. This occurs because Sn⁴⁺ substitutes for In³⁺, which acts as a one-electron donor and provides free carriers [17].

Regarding the outstanding performance recorded for ITO, Daxue Du et al. developed a device fabrication process that successfully integrated a bilayer ITO film into the SHJ solar cell configuration. This approach was necessary because several studies have demonstrated that high sputtered power reduces the power conversion efficiency (PCE) in SHJ solar cells due to damage to the microcrystalline silicon layer [34]. While speaking in terms of ITO properties, it has been demonstrated that increasing the sputtering power during ITO deposition enhances the electrical resistivity of the film. Thus, the authors designed a bilayer ITO configuration, where the buffer layer was prepared at low power and low O₂ concentration, followed by an ITO layer prepared at high power and high O₂ concentration. This results in an improved efficiency of 25.36% with a gain of 0.11% [35]. Significant progress has been recorded for perovskite solar cells by Wang Li and co-authors. They prepared ITO using a reactive sputtering method to serve as a buffer layer for inverted PSCs, achieving one of the highest certified efficiencies for wide-bandgap inverted PSCs. The device maintained more than 90% of its initial efficiency after 1023 h of thermal aging at 88 °C [36]. However, an alternative technique for producing TCOs with low damage for sensitive layers and interface in organic and perovskite solar cells is PLD. Yury Smirnov et al. prepared ITO via the PLD technique on the wafer-scale substrate for SHJ solar cells. In this paper, they achieved an SHJ efficiency > 21% with 75 nm-thick PLD ITO. Furthermore, the study demonstrated that efficiencies above 22% can be obtained with a 45 nm thick ITO layer when combined with TiOx for optical compensation [37]. Kassio P. S. Zanoni et al. prepared ITO with an industrial-scale PLD tool for thin film semi-transparent solar cells. The authors obtained ITO thin films with an absorbance of less than 10% in the visible range and a sheet resistance of 25 Ω/sq. They targeted a power conversion efficiency (PCE) exceeding 18% using ITO prepared using PLD [38].

However, ITO is extensively used as a TCO in organic solar cells (OSCs). Outstanding efficiencies of up to 20% were achieved in a conventional structure (ITO/PEDOT: PSS/active layer/PDINN/Ag), with ITO serving as the TCO (Figure 6). This represents the highest efficiency reported at that time for single-junction solar cells [39]. Other remarkable results include an efficiency of 19.47% (certified efficiency: 18.97%) for an inverted OPV, with a retention of 81.5% after 7724 h [40], as well as efficiencies of 14.5% reported for OPVs with active areas larger than 200 cm² [41]. In the touch screen panel (TSP) industry, ITO is frequently used due to its high optical transparency in the visible region and low electrical resistance. A TSP typically consists of a TCO deposited on glass or a flexible substrate and detects the presence of a finger through either capacitive or resistive sensing modes [42]. Several studies conclude that there is a mismatch in the coefficient of linear thermal expansion (CTE) between the polyethylene terephthalate polymer substrate and the ITO films, leading to cracking during the process. The instability of ITO at high temperatures leads to the restriction of its use in various devices [43]. Since various studies have demonstrated that one of the main drawbacks of amorphous ITO used as a flexible TCO is its tendency to crack [44], Chia-Ching Wu conducted a study in which a flexible and transparent amorphous/crystalline a-ITO/Ag/c-ITO triple-layer structure was prepared as an electrode for capacitive-type touch screen panels (TSPs) [45]. The structure a-ITO/Ag/c-ITO exhibited low sheet resistance combined with a high optical transparency. After the 30,000 bending cycles, the resistance change (ΔR) was 4.21%.

Metals, metal powders, and conductive polymers represent the most commonly employed materials in the fabrication of electromagnetic interference (EMI) shielding devices. However, their use is limited by their incompatibility with applications requiring optical transparency, such as transparent electronic devices. Another important limitation is their susceptibility to corrosion in the presence of humidity or other chemical agents, which can lead to performance degradation over time [46,47]. In a recent study, Bofei Fen et al. developed an ITO/Ag/ITO multilayer for EMI applications, achieving a high shielding effectiveness (EMI SE) of 36.5 dB, along with a light transmittance of 87.5% at 550 nm [47].

4. Fluorine-Doped Tin Oxide (FTO): Properties and Applications

Fluorine-doped tin oxide (F:SnO₂—FTO) is a wide band gap (~3.60 eV) [20,48] semiconductor that exhibits high optical transparency along with low electrical resistivity. FTO has a rutile structure with a tetragonal unit cell, identical to that of SnO₂, in which each Sn atom is coordinated by six O atoms [49]. The low resistivity can be attributed to the high carrier concentration induced by both oxygen vacancies and substitutional doping with fluorine (F) atoms [50]. When discussing the conductive nature of FTO, it is helpful to start with undoped SnO₂, which can be considered an insulator or an intrinsic semiconductor due to its low carrier concentration. To enhance the electrical resistivity, SnO₂ can be doped with F, Cl, Sb, Ni, and Cu. When F dopes SnO₂, the F atoms substitute oxygen atoms in the lattice, releasing charge carriers to the conduction band. Moreover, this substitution occurs easily because the ionic radius of F⁻ (RF- = 1.36 Å) is close to that of O²⁻ (RO₂⁻ = 1.40 Å), allowing for substitution with minimal structural distortion of the SnO₂ lattice [51,52]. Other advantages of FTO include chemical stability [48], high resistance to physical abrasion, low costs, and thermal resistance [53]. FTO is utilized in various device technologies, such as thin-film solar cells [54], dye-sensitized solar cells (DSSCs) [48,55], organic light-emitting diodes OLEDs [56], transparent field-effect transistors (FETs) and liquid crystal displays (LCDs) [57].

However, one issue related to FTO is the high processing temperature required (>360 °C) to achieve both optical transparency and low electrical resistivity. This limits its application in devices that use a flexible, transparent substrate, such as plastic liquid crystal displays or flexible solar cells [48]. In the OSC field, ITO remains the most used TCO despite its drawbacks. Several studies have been conducted to replace ITO with FTO, as FTO has been found to have higher thermal stability [58]. Many deposition techniques are used to prepare FTO, including magnetron sputtering, pulsed laser deposition (PLD), sol–gel, and spray pyrolysis [59]. However, spray pyrolysis using tin chlorides as a precursor remains the most frequently used method for preparing FTO films [60]. A study on the scalability of FTO was conducted by Yan Wang et al. using the offline atmospheric pressure chemical vapor deposition (APCVD) process. They prepared FTO films on a large-area glass substrate (1245 mm × 635 mm × 3 mm), with a sheet resistance of between 8 and 11 Ω/sq. and a direct transmittance of more than 83%. Furthermore, they compared the offline FTO coating with a commercially available FTO prepared via online CVD in a tandem amorphous silicon (a-Si:H) thin-film solar cell. The prepared FTO exhibited excellent performance with high quantum efficiency for an a-Si:H solar cell [61]. Although glass substrates are frequently used in many optoelectronic and electronic devices, they are not suitable for flexible devices such as portable components or smart cards. Also, another problem could appear in large-area applications due to the weight of the glass and its rigidity. To overcome this issue, H. Kim et al. conducted a study in which transparent electrodes based on FTO are prepared with PLD on a flexible polyethersulfone plastic substrate. They obtained as-deposited films with low electrical resistivity of 1–7 mΩ × cm and high optical transmittance of 80–90% in the visible region of the electromagnetic spectrum [62]. FTO prepared using spray pyrolysis was reported in [63] with an electrical resistivity of 4.3 × 10⁻⁴ Ω cm along with a transmission of 86% in the visible region.

5. Aluminum-Doped Zinc Oxide (AZO): Properties and Applications

While ITO has been extensively studied and debated, zinc oxide (ZnO) has emerged as a promising alternative in the TCO industry thanks to its outstanding physical properties and affordable fabrication. ZnO is classified as an n-type semiconductor within the II-VI group of compounds exhibiting crystalline structures such as wurtzite, zinc mix and rock salt [64]. This material offers a direct band gap (3.37 eV at room temperature) along with good optical and electrical performances. Also, low toxicity, and mechanical and chemical stability make this material a good competitor for replacing ITO [65,66]. Additionally, ZnO possesses a high dielectric constant and a strong exciton binding energy of 60 meV, which confers a long lifetime at room temperature [11,67,68]. Additionally, due to its high binding energy, ZnO is suitable for fabricating exciton-related devices, such as short-wave light emitters [69]. Based on its properties, ZnO is utilized in various devices, including transparent electrodes in solar cells, liquid crystal displays (LCDs), and plasma display panels (PDPs) [70]. Alternative applications of ZnO can include lithium-ion battery anodes [71], UV photodetectors [72], and gas sensors [73]. However, the electrical resistivity of ZnO films is high due to their low carrier concentration.

Typically, conductivity can be enhanced by doping the material with extrinsic donors, such as indium (In), gallium (Ga), or aluminum (Al) [74]. This process promotes free electrons by replacing the zinc cation with a trivalent ion [52,75]. Among these, Al-doped ZnO films (AZO) stood out due to their ease of processing, accessibility of raw materials, nontoxicity, stability in H2 plasma [52,76,77] and capability to produce on a large scale. In ZnO films, we found donor-type defects that are associated with oxygen vacancy (V_O), zinc antistites (ZnO), zinc interstitial (Z_ni), and acceptor-type defects like zinc vacancy (V_Zn) and oxygen interstitial (O_i). From these, the conduction mechanism in ZnO films is generally dominated by electrons generated from intrinsic donors like oxygen vacancy (O²⁻) and Zn interstitial (Z_ni), which act as shallow donors. However, doping the ZnO with Al is expected to improve the conductivity by introducing additional electrons due to the higher valence and the smaller radius of Al ions (Al³⁺, 0.53 Å) than that of Zn ions (Zn²⁺, 0.74 Å), leading to a contraction of the ZnO lattice [78,79]. Upon doping, the Al³⁺ ions substitute the Zn²⁺ ions, creating shallow donor states that enhance the electrical conductivity of the AZO films [20,80]. However, one of the main challenges for AZO is to reduce its electrical resistivity to levels comparable to those of ITO, without compromising optical transmittance in the visible region [54].

AZO films were successfully produced by a variety of deposition techniques, including RF sputtering [81], sol–gel [82], atomic layer deposition [78], spray pyrolysis [83], electron beam evaporation [84], chemical vapor deposition [85], and pulsed laser deposition [86]. Among them, the sol–gel method involves low processing temperatures and reduced costs. However, a major drawback of this technique is the much higher resistivity of AZO films compared to those obtained by other deposition methods [87]. Chen W. et al. reported a resistivity of 1.94 × 10⁻² Ω·cm for an AZO film synthesized via the sol–gel technique [82]. Magnetron sputtering has proven to be the most efficient method for obtaining AZO thin films, and it is the technique commonly used in the large-area manufacturing of TCOs. This method offers several advantages, including low process temperature, good adhesion to the substrate, and high film density [87]. Additionally, the deposition parameters can be easily controlled, enabling the optimization of the functional properties of the films. Lennon et al. demonstrated that the resistance of AZO films deposited by magnetron sputtering depends on the post-deposition thermal treatment. They reported that the film resistivity decreased from 3.79 × 10⁻³ Ω·cm to 7.19 × 10⁻⁴ Ω·cm, while achieving an optical transmittance of 86.9% up from 35% after 350 h of rapid thermal annealing (RTA) in a nitrogen atmosphere [88]. Another significant result was reported by Fang et al., who achieved a resistivity on the order of 10⁻⁴ Ω·cm and an optical transmittance of 90% for AZO films deposited by magnetron sputtering, following a thermal process at 400 °C for 2 h [89]. Additionally, there are studies that demonstrated that the properties of AZO films are influenced by working pressure during the deposition. A study in this direction was conducted by Michel Chaves et al. The results of the study showed a good resistivity of 2.8 × 10⁻³ Ω cm, obtained for the sample deposited at 0.16 Pa. Also, they conclude that the working pressure does not significantly affect optical transmission, all samples exhibiting 80% [90].

Here, we can review some important reports on AZO films prepared using PLD equipment. Studies involving the fabrication of AZO thin films by pulsed laser deposition (PLD) were carried out by M. Girtan et al. An excimer laser (λ = 248 nm) with a fluence of 1.5 J/cm² was used to deposit AZO (2 wt% Al₂O₃) onto a plastic substrate (HIFI PMX 739, 175 µm thick). The resulting films exhibited a resistivity of 1.3 × 10⁻³ Ω·cm and an optical transmission greater than 85% in the visible region of the spectrum [91]. Another significant study on the pulsed laser deposition of AZO films was conducted by Hideaki Agura et al. They used an ArF excimer laser (λ = 193 nm) to deposit AZO with Al₂O₃ doping levels of 1 wt% and 2 wt% onto glass substrates heated to 230 °C. For a film thickness of approximately 280 nm, they achieved a resistivity of 8.54 × 10⁻⁵ Ω·cm and an optical transmission exceeding 88% in the visible range [92]. This electrical resistivity value is extremely close to the lowest value recorded for ITO, namely 7.7 × 10⁻⁵ Ω·cm [93]. An interesting study was conducted by V.O. Anyanwu et al., who reported resistivities as low as 6.8 × 10⁻⁵ Ω·cm and an average optical transmission of approximately 83% in the UV–VIS–NIR region for AZO films deposited at a substrate temperature of 150 °C [94]. Several studies show that AZO electrodes cause a deterioration of the fill factor (FF) when used in amorphous silicon (a-Si)-based solar cells with a-SiC:H or a-Si:H as window layer. This effect can be explained by the appearance of a potential barrier at the interface between AZO/p a-SiC:H, which obstructs the movement of the charge carriers, leading to a decrease in FF [95]. It has been reported that a low conductivity is the principal factor that impedes the improvement of electromagnetic shielding performance in EMI applications. Several studies have reported that introducing a layer of high-conductivity metal to achieve a dielectric-metal configuration improves electrical conductivity [96]. Thus, Ag/AZO films were prepared in [97] with a highest FoM value of 0.0313 Ω⁻¹, low sheet resistance of R_sh = 4.95 Ω/sq. and an average electromagnetic interference shielding effectiveness of 27.1 dB in 1–10 GHz [97]. AZO prepared for thin film transistors was investigated by [98]. The study showed the effect of different Al concentration in AZO/ZnO structure prepared with the ALD technique. It was obtained that the optimal concentration of Al is 2% due to the good crystalline orientation and the reduction of oxygen vacancies (V_o).

Towards Large-Area Deposition of Al:ZnO as Transparent Conducting Oxide (TCO)

Transparent and conductive oxides are prepared using a wide range of deposition techniques, including sol–gel, spray pyrolysis, chemical vapor deposition, magnetron sputtering, and pulsed laser deposition. However, magnetron sputtering is the predominant method for manufacturing TCOs due to its ease of implementation in industrial processes dedicated to mass production in optoelectronic devices. This technique has garnered attention at the industrial level due to its advantages, including a high growth rate for TCOs, as well as good uniformity on large-area substrates and reproducibility [13,14]. Other benefits of this technique include good film adhesion, high film density, and low processing temperatures [99,100]. Several studies have demonstrated that the transparency and resistivity of sputtered AZO films strongly depend on deposition parameters, such as substrate temperature, sputtering power, and gas pressure. A recent study exploiting the properties of large-area AZO films was conducted by Amol C. Badgujar et al. The study aims to optimize the oxygen flux required during the DC sputtering process to achieve both high optical transparency and the desired conductivity without heating during the process. They obtained sputtered AZO films with an electrical sheet resistance of 8.8 Ω/sq. and a light transmittance of 78.5% in the VIS region. Meanwhile, uniformity of the film deposited on a 300 mm × 300 mm glass substrate exceeded 95%. Furthermore, the AZO film was used as a transparent electrode for CIGS solar cells, yielding a power conversion efficiency of 11.8%. They assumed that as the oxygen flux increases, a passivation of oxygen vacancies is expected; Al reacts with O₂ to form Al₂O₃, leading to a decrease in Al3+ substitution and, consequently, a decrease in the charge carrier concentration [101]. Studies on the effects of sputtering power and vacuum annealing on AZO films prepared at room temperature on large-area (2-inch) glass substrates were performed by Fatiha Challali et al. They observed a preferential c-axis orientation in AZO films sputtered at 50 W. Furthermore, the electrical resistivity increases with increasing sputtering power, with the lowest resistivity value of 1.65 × 10⁻³ Ω cm being obtained at 50 W, corresponding to an average visible transmittance greater than 75%. They assumed that the electrical conductivity in the case of films sputtered at RT is mainly due to the mobility, as well as to the good crystallization of the film at a power of 50 W. A large crystallite size leads to a decrease in carrier diffusion at the grain boundaries, accompanied by an increase in carrier lifetime. In addition, after a vacuum annealing treatment at 400 °C, an improvement in transmittance of up to 85% is recorded, along with a decrease in resistivity to 5.75 × 10⁻⁴ Ω cm [87]. Md. Amzad Hossain et al. conducted an essential study on the deposition of AZO on a large-area glass substrate (100 mm), where the effect of rotating the AZO target during the deposition process was investigated. The study showed that AZO films prepared in a rotational mode (40 rpm) exhibited a more uniform and smoother surface [102]. Confocal magnetron sputtering is used to prepare Al-doped ZnO at room temperature from two targets (2-inch size) of ceramic ZnO and metallic Al simultaneously on large-area (25 × 17 mm²) Si and glass substrates. The study, conducted by Fatiha Challal et al., aimed to investigate the effect of Al content on the structural, optical, and electrical properties of AZO films. The properties of AZO films were investigated both as-deposited and after performing annealing treatments in a controlled argon atmosphere. The results of the study indicate that all sputtered AZO films exhibit a wurtzite crystalline structure, with a decrease in crystalline quality for an Al content exceeding 5 at%. Moreover, a high figure of merit (FoM) was obtained for ZnO films with an Al content of 3.6 at.%, after performing an annealing treatment at 300° [103]. Reactive magnetron sputtering has been widely used to develop transparent AZO-based electrodes for a-Si:H thin-film photovoltaic cells. The authors prepared AZO films on glass substrates with dimensions of 1000 × 600 × 3 mm at temperatures below 200 °C and different total pressures. They observed the existence of two regions where the resistivity of AZO films increases. A rise in the resistivity appears at high gas pressures due to the oxidation of dopants, resulting in the incorporation of Al₂O₃ into the film. At the same time, a second region where the resistivity increased was observed at low gas pressures, due to the formation of substoichiometric films with poor structural properties. The lowest resistivity, around 270 μΩ, was achieved at a total pressure of 900 mTorr and a substrate temperature of 150 °C, with a sheet resistance homogeneity on float glass better than ±6% [104]. Another interesting study was conducted by Sanjay R. Dhage et al., who investigated the electrical and optical properties of AZO films prepared using cylindrical rotating DC magnetron sputtering over a large-area glass substrate (300 mm × 300 mm). They found that all the prepared samples exhibit high uniformity, greater than 97% across the entire area. They showed that for a sample prepared at 473 K, with a sputter power of 4000 W and a gas flow of 255 sccm, a low electrical resistivity (4.07 × 10⁻⁴ Ωcm) coupled with a good optical transmittance (84%) can be achieved [105]. The deposition of AZO on PET substrate over a large area (12 × 40 cm²) was investigated in [106]. They used the DC sputtering technique with a power of 2 W/cm² and a working pressure of 4 × 10⁻¹ Pa. Using these parameters, they could obtain a thickness between 0.2 and 1.1 μm with a deposition rate of approximately 2 nm/s. In terms of optical properties, they achieved a transmittance up to 90% in the visible spectrum for the thinnest layer. Also, they observed that with increasing film thickness, the electrical resistivity decreases from 2 mΩcm for the thinner sample to 1.6 mΩcm for the thicker one.

Spray pyrolysis is frequently used to prepare conductive oxides due to its low production costs and simplicity, making it suitable for large-area deposition. Spray pyrolysis was used to prepare AZO on a large-area glass substrate (75 × 25 mm²) at a temperature of 350 °C. The study reported a significant improvement in the electrical conductivity of the AZO film compared to undoped ZnO [107]. AZO for heater applications was produced on a 50 × 75 mm² glass substrate at 400 °C and annealed under forming gas for 90 min using ultrasonic spray pyrolysis. The 750 nm-thick AZO layer exhibited a good sheet resistance of 38.7 Ω/sq., along with a transmittance of up to 83% in the visible region. The low sheet resistance can be attributed to the removal of oxygen from the AZO lattice during forming gas annealing, leading to the formation of vacancies that act as donors. Additionally, the increased carrier concentration may result from the Al³⁺ substitution in Zn²⁺ sites [108].

Atomic layer deposition (ALD) is a technique used to prepare a wide range of thin films of various materials in the vapor phase. It is mainly used in emerging semiconductor and power conversion technologies [109]. Furthermore, this technique enables the production of thin films with high uniformity and precise thickness, controlled by adjusting the number of ALD cycles [110,111]. Additionally, the ALD method is preferred due to its low deposition temperature and scalability to large areas [112]. AZO was fabricated over a 4-inch area using the ALD technique to serve as a transparent and conductive electrode for OLED applications. AZO-based OLEDs exhibit a current density 5 times higher than ITO-based ones, with an AZO film resistivity of 6.24 × 10⁻⁴ Ω cm and a sheet resistivity of nearly 54 Ω/sq., confirmed by 4-probe measurements on a 4-inch wafer, demonstrating good uniformity of the deposited layer [113]. Seungsin Baek et al. conducted a study on interface engineering at the AZO and p-type a-SiC:H interface to obtain a better solar cell performance without loss in FF. They inserted a silicon buffer layer between AZO and a-SiC:H solar cell, observing an enhancement in the FF of up to 73% and in the efficiency of up to 8.18% [114].

AZO films with different Al contents were prepared using the ALD method on a 4″ (100) Czochralski silicon (Cz-Si) wafer and glass. The morphology and electrical properties of the AZO films exhibited a strong dependence on the Al content present in the grown film. Samples with a concentration higher than 3% were more disordered, and the electrical resistivity tended to increase rather than decrease. This effect was attributed to several factors, including the clustering of Al in its nonconducting form of Al2O3 due to crystalline disorder, in which excess Al behaved more as a charge trap than as a donor. In addition, excess Al induced zinc (Zn) interstitials and zinc vacancies (Vzn). In contrast, AZO films with a concentration of 2–3% showed electrical and optical properties suitable for integration as transparent electrodes in photovoltaic cells [112]. Studies targeting large-area AZO produced by ALD were conducted to assess whether their optical and electrical properties meet the criteria necessary for the integration into electromagnetic interference shielding (EMI) devices. The study reveals a low electrical resistivity of 5.876 × 10⁻⁴ Ω cm along with a transmittance of up to 85.93% in the visible region. The average value of the EMI-SE increased from 1.1 dB for the 121 nm-thick undoped ZnO film to 6.5 dB for the 131 nm-thick ZnO:Al (19:1) film [110].

In contrast, few studies have reported the use of Pulsed Laser Deposition (PLD) for large-area AZO thin films, despite its significance in the fabrication of TCOs. The advantages of this technique include: (I) versatility in the choice of ablated materials, including ceramics or complex compounds; (II) excellent adhesion of the film to the substrate, due to the high kinetic energies of the ablated species; (III) a good structural quality of the prepared films; (IV) the pulsed nature of the laser, which allows precise control over the film thickness by adjusting the number of pulses; (V) placement of the laser source outside the reaction chamber, which offers a flexibility of materials and the geometric configuration of the system including the variation of parameters such as background pressure or substrate temperature; and (VI) highly localized material evaporation, restricted to the laser focus area, which minimizes contamination and enables targeted deposition, as in Jing Yu et al. [115] and Lorenz et al.’s work [116]. However, one of the main drawbacks of this technique is its limited applicability in industry, as it has traditionally been considered a method used primarily in scientific research. The primary concern regarding the industrial applicability of the PLD method is related to the non-uniformity of the films over a large area. This limitation restricts the possibility of using a substrate larger than 1 cm², preventing its applicability in the mass production of electronic and optoelectronic devices [111]. One major limitation of PLD in the mass-production technologies of micro- and nano-electronic device fabrication lies in achieving uniform thickness on substrates with a large surface (more than 100 mm) since the area of the laser spot (1–5 mm²) on the surface of the ablated target is about 10 times smaller than the substrate area. This arises because, in the large-scale production of micro- and nanoelectronics devices, the standard for film nonuniformity is approximately 5%, excluding a 5 mm-edge region. As a result, achieving uniform films on large-area substrates (greater than 100 mm) remains one of the major challenges of this technique, considering that the size of the focused laser spot on the target is typically 1–5 mm², significantly smaller than the area of the substrate. Other problems are related to the appearance of particles or droplets on the film surface, as well as some deviations from stoichiometric transfer that can influence controlled doping. However, in recent years, significant advances have been made, including the development of larger deposition chambers, optimization of laser sources, and the use of advanced target materials. These advances are helping to expand the potential of PLD for industrial-scale applications [117,118]. A study on scalable TCOs was conducted by Joost W. C. Reinders et al., where AZO films were obtained using PLD with excellent morphological, optical, and electrical properties (Figure 7 and Figure 8). The deposition was performed on a large area (9 inches), with both thickness and sheet resistance showing spatially homogeneous variation, the variation being less than 3.5% over the analyzed area [118].

Table 1. Some electrical and optical properties of AZO films prepared on a large area using different deposition methods.

Deposition Method	ρ (Ω cm)	TVIS (%)/k	Scalability	Applications	Ref.
Magnetron Sputtering	0.9 1 × 10−3	75.8	300 × 300 mm	Used as transparent electrode in CIGS solar cell (PCE—11.8%)	[101]
	5.75 × 10−4 (after annealing treatment)	86 (after annealing treatment)	Large area glass substrate	-	[87]
	3.6 × 10−4	86	100 mm glass substrate	-	[102]
	3.9 × 10−3 (after annealing treatment)	85	2.5 × 1.7 mm2 silicon and glass substrate	-/prepared as TCO. They reported a figure of merit of 43.6 × 10−4 sq.Ω−1 for a sample with 3.6 % Al content, after an annealing treatment.	[103]
	4.07 × 10−4	84	300 mm × 300 mm glass substrate	-	[105]
	2 × 10−3	90	12 × 40 mm2 on PET	-	[106]
Spray Pyrolysis	2.9 × 10−3	83	50 × 75 mm2 glass substrates	Heater applications	[108]
ALD	6.24 × 10−4	-	4-inch glass wafer	OLED (Current density at 20 V—0.25 A/cm2)	[113].
	6.33 × 10−3	-	4″ (100) Czochralski silicon wafer	-	[112]
	5.876 × 10−4	85.93	Silicon wafers and glass substrates	EMI (SE-1.1 dB for 121 nm undoped ZnO, and 6.5 dB for 131 nm Al-doped ZnO sample)	[110]
PLD	5.5 × 10−4	63.3	Circular area of 500 cm2	Employed in three different configurations of p-i-n PSC as superstrate (bottom illumination-PCE of 18.5%), semitransparent (bottom illumination-PCE of 17.2%), and semitransparent (top illumination-PCE of 18.9%-best performance)	[118]

presents a comprehensive comparison of some physical properties of AZO films achieved on a large area.

6. Other TCOs

Regarding the dopants used to enhance the physical properties of ZnO, Indium (In) stands out as one of the most promising candidates. Indium is considered an excellent dopant for ZnO due to its low chemical reactivity and higher oxidation resistance compared to aluminum [119,120]. Indium-doped ZnO (IZO) films are transparent across most of the solar spectrum, making them suitable for photovoltaic applications. Moreover, compared to ITO, IZO exhibits superior stability. Studies have shown that IZO possesses favorable electrical and optical properties, similar to those of AZO, which are highly dependent on the deposition technique and the precise control of processing parameters. Hector Eduardo Silva-Lopez et al. conducted a study on IZO thin films deposited by RF magnetron sputtering. The deposition was performed using a working power of 20 W, at a target-to-substrate distance of 3.5 cm and a gas pressure of 1.33 × 10⁻¹ Pa. The substrates used were PET, PEN, and glass. Among these, the IZO sample deposited on a glass substrate exhibited the best performance, with a resistivity of 1.8 ± 0.3 × 10⁻³ Ω cm and an optical transmission of 80% in the range 400−1000 nm [120]. A study on the deposition of IZO films using the PLD technique was conducted by Cristina Beslega et al. In this study, IZO films were prepared using a KrF laser (λ = 248 nm) in an oxygen atmosphere (1 Pa), at a distance of 5 cm between the source and the substrate. The IZO sample had a thickness of 50 nm, exhibiting a resistivity of 4.8 × 10⁻⁴ Ω cm and an optical transmission of approximately 80%. The thin film was subsequently employed as a transparent electrode in polymer-based photovoltaic cells, utilizing a 1:1 blend of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). The devices demonstrated a promising quantum conversion efficiency [119].

Another material explored as a TCO is indium-doped zirconium oxide (IZrO), where high carrier concentration and mobility result from the combination of Zr⁴⁺ dopants and oxygen deficits [121]. However, another assumption can be due to the similar Zr⁴⁺ ionic radius to that of In³⁺ [122]. Yury Smirnov et al. developed a room-temperature PLD process on 4-inch wafers for the fabrication of IZrO thin films. This film was suitable for integration into semi-transparent perovskite solar cells, achieving a stabilized efficiency of 15.1%. Another study on IZrO transparent electrodes was carried out by Monica Morales-Masis et al., to serve as front contact in SHJ solar cells. The prepared electrode showed good properties with low free carrier absorption and high lateral conductivity. The carrier density was up to 2.5–3 × 10²⁰ cm⁻³ combined with an electron mobility of 100 cm²/Vs, resulting in a sheet resistance around 25 Ω/sq. for a film thickness of 200 nm. Furthermore, when used as the front contact in SHJ, it results in a significant increase in current density due to reduced parasitic absorption in the UV and IR regions [123].

A TCO widely recognized for its applications in thin-film transistors (TFTs), p-n junction rectifiers, solar cells, transparent field-effect transistors (FETs), and flexible display technology is indium–gallium–zinc-oxide (IGZO) [124,125]. Moreover, IGZO satisfies the specific requirements for transparent electrodes, such as high electron mobility and a good transmission ratio in the VIS spectrum [126]. Based on this, Chanmin Hwang et al. conducted a study in which the properties of the IGZO/Ag/IGZO (IAI) layer were investigated. The device presented an average transmittance of 85% in the visible range and a lowest sheet resistance of 6.03 Ω/sq after annealing at 500 °C for 60 s. In the solar cell simulation, the photo-generated short-circuit current (Jsc) absorbed in the Si substrate was analyzed, and the highest Jsc obtained was 40.73 mA/cm [127]. Studies on IGZO/Ag/IGZO triple layers were performed by Zhan-Sheng Yuan et al. using RF magnetron sputtering deposition techniques. They showed that the deposition time of the Ag plays a significant role in the main properties of the film. As the deposition time of the Ag layer increases, the transmission maximum slightly decreases while the mobility and carrier concentration increase [128]. Another study involving IGZO is reported in [129]. Here, this material is used as an electron transport layer (ETL) for a perovskite solar cell.

Table 2. Physical properties of various TCOs prepared using different deposition techniques.

	ρ (Ω cm)	Rs (Ω/sq.)	TVIS (%)	A (%)	k	Deposition Method	Applications	Ref.
ITO	4.87 × 10−4	46.1%	88.71%	-	0.009	PVD	SHJ (PCE-25.36%)	[35]
	-	60 Ω/sq.	-	Less than 5%	-	PLD	SHJ (PCE->22%)	[37]
	4 × 10−4	25 Ω/sq.	More than 70%	Below 10%	-	PLD	Semitransparent PSC (PCE-18%)	[38]
	-	6.4 Ω/sq.	87.5%	-	-	Sputtering	TSPs	[45]
	-	4.01 Ω/sq.	87.4%	-	-	Sputtering	EMI (SE-36.5 dB)	[47]
AZO	1.94 × 10−2	-	>85%	-	-	Sol-gel	CIGS solar cell	[82]
	7.1 × 10−4	-	86.9%	-	-	Sputtering	-	[88]
	1.3 × 10−3	-	>85%	-	-	PLD	Electrode for plastic solar cell	[91]
	8.54 × 10−5	-	88%	-	-	PLD	-	[92]
	6.8 × 10−5	-	83%	-	-	PLD	-	[94]
	$~$1.5 × 10−5	4.95 Ω/sq.	82.9%	-	-	magnetron sputtering	EMI (SE-27.1 Db)	[97]
	$~$10−4	-	Above 90%	-	-	Magnetron Sputtering	-	[89]
	2.8 × 10−3	-	80%	-	-	Magnetron Sputtering	-	[90]
FTO	-	10 Ω/sq.	83%	-	-	APCVD	Tandem amorphous silicon (a-Si:H) tandem solar cell	[61]
	1.3 × 10−3	40 Ω/sq.	89%			PLD	Transparent electrodes for flexible devices	[62]
	4.3 × 10−4	-	86%			Ultrasonic spray pyrolysis	-	[63]
	Others TCO
IZO	1.8 × 10−4	-	80%	-	-	Rf magnetron sputtering	-	[120]
	4.8 × 10−4	-	80%	-	-	PLD	Electrodes in a polymer-based photovoltaic cell	[119]
IZrO	-	21 Ω/sq.	$~$80%	Less than 10%	-	PLD	Used as a rear electrode in semi-transparent PSC (High stabilized efficiency of 15.1%)	[130]
	-	25 Ω/sq.	$~$80%	Less than 10%	$~$0	Rf sputtering	Used as front electrode in SHJ (PCE-23.4%)	[123]
IGZO	-	6 Ω/sq.	85%	-	-	Rf sputtering	Solar cell (JSC-40 mA/cm)	[127]
	5 × 10−5	2.5 Ω/sq.	89.5%	-	-	Rf magnetron sputtering	-	[128]
	-	-	-	-	-	PLD	PSC device (efficiency-15.11%)	[129]

shows a detailed comparison between AZO, ITO, FTO, and other TCOs, including IZO, IZrO, and IGZO, across the principal properties they have achieved in different studies.

7. Conclusions

Transparent and conductive oxides are widely used in numerous electronic and optoelectronic devices, such as flat panel displays, photovoltaic cells, and electromagnetic radiation shielding devices. Since their discovery in 1907 by Badeker, they have been extensively studied to enhance their thermal and chemical stability, resistivity, optical transparency, and to lower production costs. They can be produced using various deposition methods, but only a few are suitable for large-area coatings used in the electronics and optoelectronics industries. Replacing ITO remains the main current challenge due to the rising cost of In. As previously mentioned, AZO has shown properties comparable to those of ITO, with the added advantage of being cost-effective. The study aimed to summarize recent advancements in various applications, as well as improvements in optical transparency and electrical resistivity.