Recent Progress in Dielectric/Ag/Dielectric Transparent Electrodes on Flexible Substrates

Abstract

Dielectric/Ag/dielectric (DAD) multilayer thin-film transparent electrode features high visible-light transmittance, low sheet resistance, good mechanical flexibility, and low haze. The fabrication techniques are compatible with large-scale integrated circuits, and the materials are cheap. These advantages make the DAD transparent electrodes a promising alternative to indium tin oxide (ITO) electrodes for flexible devices. This review summarizes recent advances in DAD transparent electrodes on flexible substrates, mainly focusing on the opto-electrical performance improvement due to damping of the localized surface resonance (LSPR) of Ag nanoparticles (AgNPs). It begins with an analysis of the performance-limiting factors of DAD transparent electrodes, elucidating the importance of damping the LSPR of AgNPs. Subsequently, the state-of-the-art fabrication methods for Ag ultrathin films of weak LSPR and the dielectric material optimization are reviewed. It concludes with perspectives on future research.

1. Introduction

Flexible transparent electrode (FTE) is one of the essential components of flexible optoelectronic devices such as flexible displays, solar cells, and wearable electronic devices [1,2,3]. The most commonly used transparent electrode is indium oxide tin-doped (ITO) thin film. The ITO transparent electrode is of high visible-light transmittance, low resistivity, and good corrosion resistance, but poor flexibility and low storage of indium in the earth make it unsuitable for flexible optoelectronic devices. To overcome these limitations, several alternative FTEs [4], such as graphene [5], carbon nanotubes [6], conductive polymers [7], metal nanowires, metal nano grids [8,9], and dielectric/metal/dielectric multilayer thin film (DMD), have been developed [10]. Among them, the DMD transparent electrode, a layer of metal sandwiched by two dielectric layers s, using the metal layer to improve the flexibility and electrical conductivity, exhibits high electrical conductivity, excellent mechanical flexibility, high visible-light transmittance, and good environmental stability. Furthermore, the DMD transparent electrode can be prepared by vacuum coating, which is compatible with the manufacturing process of ultra-large-scale integrated circuits and thus suitable for mass production [11,12]. Also, the work function of the DMD transparent electrodes can be controlled via dielectric selection, allowing them to be used as a cathode–anode or even as an intermediate electrode in tandem solar cells [13]. Last but not least, the component materials of DMD are cheaper than ITO and most other flexible transparent electrodes. These advantages make the DMD transparent electrodes the most promising alternative to ITO electrodes used in flexible devices, and, therefore, they have received widespread attention in recent years [2,4,11,12,13,14,15,16,17,18,19,20].

The metal layer of DMD transparent electrodes determines the electrical–optical properties and the mechanical flexibility of the electrodes. Its thickness should be large enough to ensure the conductivity of the layer and, at the same time, be as small as possible so as not to impede the required transparency. Typically, highly ductile and conductive metals such as Au, Ag, Al, and Cu are employed, with Ag being the most prevalent choice [11,14,16,17,19] due to its superior electrical conductivity, center color [17], and stability (Ag-based DMD denoted as DAD). However, the Ag layer not only reflects but also absorbs incident light when the thickness of the Ag layer is smaller than 10 nm, resulting in low transmittance of DAD. When the refractive indices and thicknesses of the dielectric layers and the Ag layer match each other, the reflected light from each interface between the dielectric layer and the Ag layer undergoes coherent cancellation, such that the reflection can be minimized. However, the transmittance of the DAD electrode is still low and severely limits its applications.

Since 2008, Jeong et al. [11], for the first time, prepared Al₂O₃/Ag/Al₂O₃ flexible transparent electrodes on polyethylene terephthalate (PET) substrates using magnetron sputtering and vacuum evaporation deposition techniques; great progress has been made to enhance the optical transmittance of DAD. Many good reviews have been presented. However, the low transmittance is in fact caused by large absorbance induced by the localized surface plasmon resonance (LSPR) of silver nanoparticles (AgNPs), of which the Ag layer is composed when the thickness of the Ag layer is smaller than 10 nm. Herein, we review the recent advances of the DAD transparent electrode from the Ag layer and the dielectric layer [9,10,11,12,13,14,15,16,17,18,19], mainly focusing on the recent advances in the optic–electric performance improvement from the point of view of damping the LSPR of AgNPs. The detailed principle of LSPR is discussed in Section 2.

2. Research Progress of the Ag Layer

2.1. Analysis of the Limitation Factors for the Optical–Electric Performance of DAD Transparent Electrodes

As mentioned above, coherent cancellation can minimize optical loss caused by the reflection from the DAD, but the transmittance of the DAD is still not as high as expected. It is wavelength-related, and the maximal intensity is around wavelength of 550 nm with an average transmittance (T_av) smaller than 85%. This low transmittance is, in fact, caused by the absorbance, not the scattering of the Ag layer.

The surface energy of Ag thin film is about 1.2–1.4 J/cm² [21], which is much larger than that of dielectric materials, which are normally smaller than 1.0 J/cm² [22]; therefore, Ag thin film grown on dielectric layers follows Volmer–Weber (island) growth mode [23]. Firstly, the atoms reaching the substrate form nuclei on the surface. Then, the subsequently arrived Ag atoms join the existing nuclei and promote the nuclei to grow into islands (nanoparticles), as shown in Figure 1a. When the nanoparticles connect to each other, a continuous conductive thin film is formed. The minimum average thickness that forms a conductive thin film is defined as the percolation threshold. Here, we refer to it as conductive critical thickness (t_CT). t_CT depends on the nucleation density. The greater the nucleation density is, the smaller the mean size of the AgNPs is, and thus the smaller the t_CT is.

The nucleation density depends on the materials of thin films and substrates, and the surface physical and chemical environment of the substrate [23]. When Ag thin film is deposited on clean glass substrates using vacuum evaporation techniques, t_CT is about 10 nm [24,25,26]. The mean size of AgNPs is smaller than 50 nm, as shown in Figure 1b [27]. AgNPs in this size range possess strong LSPR [28]. LSPR refers to the resonance of the free electrons contained in the AgNPs with the incident electric field (Figure 1c) [29]. As a result, the AgNPs strongly absorb the incident light (Figure 1d) [30]. The LSPR peaks of isolated AgNPs are around 340 nm, but mutual coupling of adjacent nanoparticles broadens this resonance wavelength to the entire visible light range (Figure 1e) [31]. Therefore, although the absorbance of the Ag layer is small on a macroscopic scale, it is tremendously large on a nanoscale due to the LSPR of AgNPs. This strong absorbance sharply reduces the transmittance of the DAD transparent electrodes.

Figure 1. The introduction of LSPR. (a) The AFM image of AgNPs [28]. (b) The FESEM image of AgNPson Si substrate [27]. (c) The principal sketch of LSPR [29]. (d) Sketch showing the strong absorbance of metal nanoparticles caused by LSPR [30]. (e) The LSPR spectrum tuned by changing the deposition time of silver [31].

Figure 1. The introduction of LSPR. (a) The AFM image of AgNPs [28]. (b) The FESEM image of AgNPson Si substrate [27]. (c) The principal sketch of LSPR [29]. (d) Sketch showing the strong absorbance of metal nanoparticles caused by LSPR [30]. (e) The LSPR spectrum tuned by changing the deposition time of silver [31].

The mean size of AgNPs is smaller than 50 nm when the thickness of the Ag layer in DAD is near t_CT [24,29]. According to the calculation provided by ref. [32], the scattering from AgNPs is much weaker than the LSPR absorbance. For example, the scattering is less than 10% of the absorbance when the AgNPs’ size is smaller than 50 nm. Therefore, it is the LSPR absorbance, not the scattering, that is responsible for the low transmittance of the DAD transparent electrode [33].

The LSPR of AgNPs can be weakened through (1) reducing the AgNPs’ size and (2) suppressing the AgNPs’ agglomeration [30,33,34]. Low agglomeration means that the nanoparticles are flatter and cover a larger area than high agglomeration, as shown in Figure 2. This means that the wettability is high [35]. It is obvious that reducing the AgNPs’ size and suppressing the AgNPs’ agglomeration make the Ag layer dense and smooth; therefore, the t_CT and the surface roughness are reduced.

On the other hand, many papers attributed the low transmittance of DAD to the scattering caused by the surface roughness and to the large t_CT of DAD. The main methods to reduce the surface roughness and the t_CT are reducing the AgNPs’ size and suppressing the AgNPs’ agglomeration [36,37]. However, scattering is less than 10% of the absorbance in the interesting size range [32]; therefore, those methods developed to reduce scattering are, in fact, playing roles by damping the LSPR to reduce absorbance.

At present, nanoparticle size reduction can be achieved via directly breaking down large nanoparticles into small ones through Ar plasma irradiation [27] or by increasing the nucleation density in the early stage of silver thin-film growth [23]. The main techniques to increase the nucleation density and thus reduce the size of AgNPs include (1) introducing impurities such as doping [38], (2) introducing defects on the substrate surface in the initial stage of the Ag thin-film growth by Ar plasma irradiation [39,40], or by pre-depositing seed layers [41,42]. Suppressing nanoparticle agglomeration is mainly realized by increasing the surface energy of the substrate or reducing the interface energy of the Ag layer and the substrates [43] by gas-assisted deposition [44] or by active plasma irradiation [39]. A complex schematic diagram conceived to express the mechanisms is shown in Figure 3. The progress of the opto-electrical performance of DAD transparent electrodes based on these techniques is reviewed in detail in the following sections. They are plasma irradiation, gas-assisted deposition, metal doping, and pre-deposited seed layers.

2.2. Plasma Irradiation

Plasma irradiation refers to optimizing the DAD performance by inert Ar or active oxygen plasma via modifying the surface morphology of the Ag layer or the physical and chemical state of the Ag layer and the bottom dielectric layer of DAD electrodes. Through the irradiation of the Ag layer by Ar plasma [29], large AgNPs can be broken into small ones to reduce the nanoparticles’ size and thus increase the density of the Ag layer. Irradiation of the surface of the bottom dielectric layer by Ar plasma can produce defects to increase the nucleation density of the AgNPs. Irradiation by oxygen plasma can change the surface energy to prevent the agglomeration of AgNPs [39,43].

In 2015, our group, using Ar plasma irradiation of the Ag layer of the TiO₂/Ag/TiO₂ electrode, initiated the study of improving the optoelectronic performance of DAD electrodes by plasma produced by RF glow discharge for the first time [27]. As shown in Figure 4, after 10 s irradiation by Ar plasma, the T_av of DAD electrodes in the visible range reached 94%, with the sheet resistance (R_s) as low as 6 Ω/sq and the figure of merit (FOM =${\displaystyle \frac{T^{10}}{R_s}}$, [45]) value reaching 112 × 10⁻³ Ω⁻¹.

Following our study, in 2021, Lee et al. [39] applied argon and oxygen plasma to the bottom ZnO layer of ZnO/Ag/ZnO transparent electrodes to regulate the opto-electrical performance. Ultrathin continuous Ag nanofilm with a threshold thickness less than 7.0 nm was prepared. The T_av of the ZnO/7.0 nm Ag/ZnO flexible transparent electrode on PI substrate reaches 92.3% in the visible light range, with a R_s of 7.14 Ω/sq and a FOM value of 62.85 × 10⁻³ Ω⁻¹. Figure 5a compares the effects of ArP (RF 30 W, 2.6 Pa for 10 min) and OP (RF 50 W, 2.6 Pa for 10 min) on modulating the surface morphologies of the Ag layer. As shown in the sketch in Figure 5b, the sizes of AgNPs on the unmodified and the OP-irradiated ZnO surface are larger than those on the ArP-irradiated surface. By employing combined molecular dynamics (MD)/force-bias Monte Carlo (fbMC) simulations, the authors suggested that a large number of oxygen vacancies were formed on the ZnO surface that was irradiated by ArP. Ag atoms reaching the surface were adsorbed on the oxygen vacancies and bounded to the internal Zn atoms, which greatly impeded the diffusion of silver atoms and increased the nucleation density. Since the size of the AgNPs was reduced, the threshold thickness was reduced (Figure 5c), and at the same time, the LSPR was weakened, resulting in higher transmittance of the sample modified by ArP (Figure 5d–f).

In 2022, Choi et al. [40] irradiated the surfaces of amorphous TiO_x with Ar plasma (unirradiated is NM-TiO_x and irradiated is SM-TiO_x). Small surface roughness and t_CT smaller than 6.4 nm were achieved (Figure 6a). Significant improvement of the wettability of the Ag thin film simultaneously increased both the electric conductivity and the visible-light transmittance of the TiO_x/Ag/ZnO (TAZ) on the PET substrate. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis revealed that the surface of SM-TiO_x had a large number of oxygen vacancies (O_II, Figure 6b), providing nucleation sites and increasing the nucleation density (Figure 6c). The T_av of the TiO_x/6.4 nm Ag/ZnO (TAZ) transparent electrode in the visible light range reached 96.5%. The R_s was as low as 6.0 Ω/sq and the FOM value was as high as 116.7 × 10⁻³ Ω⁻¹.

The irradiation process and its parameters play an important role. For instance, OP could not enhance nucleation density, as shown in Figure 5 [40], while it could have acted in this way in a different condition. In 2021, Jeong et al. [43] discovered that irradiation of the bottom ZnO layer using OP could also improve the wettability of the Ag thin film. The mechanism is shown in Figure 7. Driven by thermodynamics, excessive O atoms were doped into the lattice near the ZnO surface. These O atoms migrated from the ZnO lattice to the Ag thin film. Some were doped into the Ag lattice, increasing the nucleation sites and thus reducing the nanoparticle size; some were combined with Ag atoms to form Ag₂O, reducing the interfacial energy between the Ag layer and ZnO layer and thus preventing the agglomeration of the AgNPs.

In 2023, Vo et al. [46] fabricated ZnO/5 nm Cu/ZnO transparent electrodes. By irradiation of the bottom ZnO using ArP (RF discharge for 2 min, Ar 20 sccm), it was found that the irregularity at the interface of ZnO/Cu was significantly reduced, as shown in Figure 8a (The yellow dashed line marks the interface between ZnO layer and Cu layer). The resultant FOM was 63 × 10⁻³ Ω⁻¹, which is 53% greater than that of the unaltered, otherwise identical, structure. In 2024, Lim et al. [47] irradiated the bottom TiO₂ layer of the TiO₂/Cu/ZnO transparent electrode using Ar plasma (RF 20 W, Ar 20 sccm, 2.7 Pa for 40 min, AP40-TiO_x). The surface roughness of the TiO_x layer was gradually reduced from 0.69 to 0.35 nm, and the surface roughness of the Cu layer was reduced, forming ultra-smooth TiO_x/Cu and Cu/ZnO interfaces (Figure 8b). A FOM value as high as 113 × 10⁻³ Ω⁻¹ was achieved, which is about 200% higher than that of the unmodified structure. These findings about ArP on the bottom dielectric layer of dielectric/Cu/dielectric transparent electrode can help us to better understand the opto-electrical performance improvement of DAD induced by ArP.

According to the above studies, plasma irradiation is very efficient in improving the optoelectronic performance of DAD. The FOM values can be higher than 100 × 10⁻³ Ω⁻¹.

2.3. Gas-Assisted Deposition

Gas-assisted deposition technology refers to introducing a trace amount of O₂ [48,49,50,51,52,53,54,55] or N₂ [44,56,57] in the early stage or during the whole process of Ag thin-film growth. On the one hand, the incorporated O and N atoms diffuse into the Ag lattice and produce additional nucleation sites; on the other hand, an AgO_x interfacial layer is formed, which reduces the surface energy difference between the Ag thin film and the dielectric surface. The former reduces the AgNPs’ size, and the latter suppresses the AgNPs’ agglomeration. In addition, oxygen in the Ag lattice can increase the refractive index of the Ag and thus can suppress the reflection of incident light. These combined effects simultaneously weaken the LSPR and lower the t_CT.

In 2014, Wang et al. [48], for the first time, prepared ZnO/AgO_x/ZnO flexible transparent electrodes on PET substrates by introducing a small amount of oxygen (O/Ag = 3.4 at%) during the vacuum sputtering deposition of Ag thin films. The Ag thin film was transformed from a three-dimensional granular state to a two-dimensional AgO_x thin film (Figure 9a–g) with a t_CT smaller than 6 nm. In addition, the surface roughness of the 8 nm Ag thin film decreased from 5.3 nm to 3.5 nm (Figure 9h). The T_av of the ZnO/8 nm AgO_x/ZnO transparent electrode in the visible light range reached 91% (Figure 9i), with a R_s of 20 Ω/sq, resulting in a FOM value of 19.47 × 10⁻³ Ω⁻¹. In 2016, Zhang et al. [49] improved the continuity of the Ag thin film by introducing oxygen of 2%–3% O₂/(Ar + O₂) during vacuum sputtering. The fabricated 50 nm ZnO/10 nm AgO_x/50 nm ZnO flexible transparent electrode on a PET substrate demonstrated outstanding optoelectronic performance with a T_av of 95.90% at 570 nm, a R_s of 8.11 Ω/sq, and a FOM value of 81.13 × 10⁻³ Ω⁻¹. t_CT smaller than 6 nm was achieved, as reported by refs [50,51].

The most critical issue of this technique lies in the precise control of the added gas-to-argon ratio. It is necessary to increase the nucleation sites through gas diffusion, or to change the surface energy by slight oxidization of the Ag thin film, but it is also essential to avoid the formation of excessive oxides or nitrides to keep the conductivity [48]. In 2023, Zapata et al. [53] found that O₂ flow rate could regulate the growth mode, oxidation state, and photoelectric properties of Ag thin films, identifying three reaction regimes based on oxygen concentration: low-flow regime (0% ≤ O₂ ≤ 4%), medium-flow regime (10% ≤ O₂ < 20%), and high-flow regime (O₂ ≥ 20%). In the low-flow regime, O₂ promotes the flattening of AgNPs, increases the nanoparticle density, and delays coalescence effects; at a medium flow rate, O₂ adsorption dominates, Ag oxidation is slowly increased, and the film resistance starts to rise; and at a high flow rate, the Ag forms amorphous Ag₂O compounds, and the resistance increases greatly. The increased resistance limits the improvement of the FOM value.

Zhang et al. [54] introduced O₂ only in the initial stage of sputtering to create a AgO_x layer between Ag film and dielectric substrate and studied the influence of oxygen flow rate on the morphology of Ag layer and the transmittance. As shown in Figure 10, at 15% oxygen concentration, the surface roughness is the smallest (Figure 10d), and the T_av reached 83.03% with a R_s of 17.77 Ω/sq. Reducing oxygen content to 5% (Figure 10b) significantly reduced the R_s (12.21 Ω/sq) with a marginal decrease in T_av (80.95%).

In 2024, Li et al. [55] introduced Ag₂O directly into ZnO/Ag/ZnO (ZAZ) films by magnetron sputtering. As shown in Figure 11, through tuning the sputtering power of the forming Ag₂O, the R_sh and T_av of ZnO/Ag-Ag₂O/ZnO (ZAAZ) thin films reached 6.5 Ω/sq (red in Figure 11a) and 96.6% (red line in Figure 11b) corresponding to sputtering power of 40 W, resulting in a FOM value as high as 104.1 × 10⁻³ Ω. The FOM value was much higher than those where oxygen was added in the initial or throughout the whole deposition process.

For N₂ gas-assisted deposition, it was found that nitrogen can lower the conductivity threshold thickness more significantly than oxygen. In 2018, Zhao et al. [56], for the first time, introduced a small amount of N₂ during sputtering of the Ag thin films with 45 sccm Ar and 16 sccm N₂, and the obtained AgN_x thin films had a t_CT smaller than 5.4 nm. The ZnO/AgN_x/ZnO flexible transparent electrodes prepared on PET substrates had a FOM value of 45.95 × 10⁻³ Ω⁻¹. As shown in Figure 12, when the thickness of the AgN_x thin film was 0.7 nm (Figure 12c), the AgN_x thin film had a nucleation density larger than the undoped Ag thin film (Figure 12a). When the thickness was increased to 1.5 nm, the neighboring islands in the AgN_x thin film (Figure 12d) began to form irregular bridge-like connections, but did not do so in the undoped Ag thin film (Figure 12b). This demonstrates that N₂ gas addition during the sputtering process can promote smooth and flat Ag surface formation, as shown by the sketch in Figure 12e [44].

Nitrogen content is a sensitive parameter. In 2022, Kim et al. [44] prepared AZO/AgN_x/AZO transparent electrodes on PI substrates by introducing a small amount of N₂ during sputtering to deposit Ag thin films. As shown in Figure 12f,g, when the ratio of N₂ to Ar was 0.07%, the T_av of the AZO/AgN_x/AZO transparent electrode was 82.4%, the R_s was 8.5 Ω/sq, and the FOM value was 16.98 × 10⁻³ Ω⁻¹, which is a significant improvement to be achieved in the optoelectrical performance compared with the AZO/Ag/AZO transparent electrode.

It is worth noting that normally, when t_CT of the metal film decreases, the conductivity of the thin film will be elevated; however, this is not necessarily the case for the addition of nitrogen during the preparation of the metal film. In 2024, Zapata et al. [57] deposited Ag thin films on amorphous SiO₂ using magnetron sputtering. With the addition of N₂ to Ar, the t_CT of the Ag thin film was drastically reduced, but the electrical conductivity of the Ag thin film was decreased significantly (Figure 13a). It was found that there was almost no chemical reaction between N₂ and Ag. Activated N species formed by ionizing nitrogen during the deposition process were dynamically adsorbed on the silver thin film. This dynamically adsorbed species reduces the surface energy of the Ag thin film. More nuclei were formed (Figure 13b), and thus the t_CT was reduced. At the same time, it leads to competition between the Ag (111) and Ag (100), causing structural changes in the Ag thin film, for example, generating grain boundaries which hinder electron transport and affect the electric resistivity. Therefore, the control of the gas amount is rather difficult, and the mechanism is more complex than expected.

Overall, O₂ or N₂ gas can be introduced in the initial or during the whole deposition process to reduce t_CT and improve the optical performance. It is better to introduce O₂ in the early stages. The flow rate or the ratio of the added gas to Ar is an important parameter. The added O₂ or N₂ can increase the nucleation density such that the t_CT is reduced, but too much added gas will reduce the conductivity of the Ag layer due to oxide formation or grain boundary generation. No chemical reaction between N₂ and Ag takes place. The activated N species is only dynamically adsorbed on the silver thin film and reduces the surface energy of the Ag thin film.

2.4. Metal Doping

Metal doping is usually achieved through co-sputtering in the deposition of Ag thin film. Heterogeneous metals (M) of high binding energy with oxygen are doped into the Ag thin film. An M-O bond is formed due to it having a stronger bonding strength than the Ag-O bond, lowering the surface energy and thus improving the wettability and preventing the agglomeration of the Ag thin film; or the dopant atoms serve as additional nucleation sites, resulting in increased nucleation density and reduced Ag nanoparticle size. Commonly used metals are Al [38,58,59,60], Cr [61], Cu [62], and Ni [63].

In 2014, Zhang and Gu et al. [58,59] doped Al into Ag by co-sputtering to prepare Ag:Al thin films on SiO₂ substrate. As shown in Figure 14a,b, the surface roughness of 9 nm Ag:Al thin films was dramatically reduced from 10.8 to 0.82 nm. This is because the co-sputtered Al atoms increased the nucleation sites of Ag:Al thin films and prevented the agglomeration of Ag thin films. Compared with the 9.0 nm Ag thin film, the T_av of the 9.0 nm Ag:Al thin film at 550 nm was increased from 55% to 80%, and the R_s was reduced from 19.0 to 14.2 Ω/sq. However, the transmittance of the Ag:Al film in the visible light range was reduced when the atom ratio of Al to Ag was larger than 10%. When Ag:Al thin films were applied to DAD, FOM values were increased. For example, in 2015, Zhao et al. [60] applied Ag:Al thin films to Ta₂O₅/Ag:Al/ZnO transparent electrodes. The Ta₂O₅/7 nm Ag:Al/ZnO transparent electrodes prepared on PET substrates showed a transmittance of 96% at 550 nm, a R_s of 23.1 Ω/sq, and a FOM value of 28.78 × 10⁻³ Ω⁻¹. In 2017, Zhang et al. [38] applied Ag:Al thin films to configure TiO₂/Ag:Al/TiO₂/MgF₂ transparent electrodes, and high and flat transmittance spectra in the visible light and near-infrared wavelengths (400–1000 nm) with an T_av of 92.4%, a R_s of 20 Ω/sq, and a FOM value of 22.68 × 10⁻³Ω⁻¹ were achieved.

Other metals, such as Cr [61], Cu [62], and Ni [63], have been doped into the Ag layer of DAD and have been demonstrated to improve the wettability of Ag thin films. The results are listed in

Table 1. The parameters of Ag layer doping and applications in DAD.

	T/Tav (%)	Rs (Ω/sq)	FOM Value (×10−3 Ω−1)	tCT (nm)	Surface Roughness	Ref.
Ag:Al	80	14.2			0.82	[59]
Ta2O5/7 nm Ag:Al/ZnO	96.0 at 550 nm	23.1	28.78			[60]
TiO2/Ag:Al/TiO2/MgF2	92.4	20	22.68			[38]
TiO2/10 nm Ag:Cr/TiO2	94.2 at 550 nm		57.15			[61]
6 nm Ag:Cu	76	14.1		6	0.19	[62]
Ag: Ni	80	18.92		7	0.57	[63]
AZO/6 nm Ag:SnOx/AZO	88	10.8	25.79	6		[65]
Zn:SnOx/10 nm ATC/Zn:SnOx	89	7.8	41.58	6		[64]
ZnO/Cu:Ag/ZnO	89					[66]

. Two-metal doping was also tried. In 2021, Jang et al. [64] co-sputtered and prepared Ag:Ti:Cr (ATC) thin films. The co-deposited Ti and Cr atoms limited the agglomeration of the Ag thin film, reducing the t_CT to below 6.0 nm. The Zn:SnO_x/10 nm ATC/Zn:SnO_x transparent electrode prepared on a PET substrate has a T_av of 89% in the visible light range, a R_s of 7.8 Ω/sq, and a FOM of 41.58 × 10⁻³ Ω⁻¹. On the other hand, SnO_x has also been doped. In 2020, Wang et al. [65] doped SnO_x into an Ag thin film by co-sputtering to prepare an Ag:SnO_x thin film. The t_CT was as low as 6.0 nm. The T_av of AZO/6 nm Ag:SnO_x/AZO transparent electrode prepared on PI substrate reached 88% in the visible light range, with a Rs of 10.8 Ω/sq. But the FOM value was only 25.79 × 10⁻³ Ω⁻¹. In a related study, Li et al. [66] employed magnetron co-sputtering to incorporate Ag into the Cu layer to produce ZnO/Cu:Ag/ZnO (ZCAZ) electrodes. The T_av was 89%. When integrated into flexible CZTS solar cells (CSCs), these ZCAZ electrodes improved both device flexibility and photovoltaic performance.

According to these studies, by doping metals into the Ag layer, the roughness can be reduced with high efficiency, and the optic-electrical performance of DAD can be improved, but the FOM values are seldom higher than 60 × 10⁻³ Ω⁻¹. The main reason is that the doped metal elements not only absorb incident light, resulting in low transmittance, but also increase the resistivity.

The difference between metal doping and gas additives was investigated by Zhao et al. [67] in 2025. They compared the effects of gaseous additives (O, N) and metallic additives (Cu, Zn) on the formation of ultrathin Ag layers on oxide substrates and found that the atomic-level O additive is the most effective in promoting the early cluster-to-layer transition of Ag, optimizing the optoelectronic properties and thermal stability. The experiments showed that the O additive can make the T_av of a 6–7 nm-thick Ag layer exceed 85%, while maintaining a low resistance of 22 Ω/sq with excellent thermal stability. In contrast, the metal additives (e.g., Cu, Zn) can improve the wettability, but high concentration leads to an increase in optoelectronic loss (Figure 15). DFT calculations show that O inhibits 3D agglomeration and promotes 2D layer growth by reducing Ag surface and interface free energy, which is better than the interfacial free energy modulation alone that is offered by metal additives.

2.5. Pre-Deposited Seed Layer

The pre-deposited seed layer technique is the deposition of a thin seed layer prior to Ag film growth. The seed layer material is selected for either high intrinsic surface energy or strong bonding affinity with Ag atoms to reduce the interfacial energy mismatch between the dielectric substrate and Ag thin film, preventing the agglomeration of the Ag layer, or the seed layer acts as nucleation sites to prevent the diffusion of Ag atoms. An Ag thin film with a small t_CT, a low surface roughness, and a high electrical conductivity can be fabricated.

The commonly employed seed layers include Ge [36,41,68,69], Ni [69], Ti [70,71,72], Cu [67,68,69,70,73,74,75,76,77,78,79], Si [73], Cr [80], Au, etc. In 2009, Logeeswaran et al. [41] prepared Ge/15 nm Ag samples on a SiO₂ surface using a Ge seed layer. As illustrated in Figure 16a, the addition of a Ge seed layer dramatically reduced the surface roughness from 6.1 to 0.8 nm, with nearly all of the improvement coming from the first 0.5 nm of Ge (red dot is data, blue dot line is guidance). This remarkable improvement originates from the fact that the Ge seed layer acts as a high-nucleation-density template, increasing the nucleation density of the silver atoms deposited on it, and thus it reduces the AgNPs’ size and the surface roughness of the silver thin films.

Liu et al. [68] compared the effects of the seed layer of Ge and Ni using 2 nm Ge/52.72 nm Ag and 2 nm Ni/56.84 nm Ag multilayer structures on Si substrates. As shown in Figure 16b, Ge is better than Ni, and the surface roughness of Ag nanofilm decreased from 8.09 to 0.71 nm.

Formica et al., in 2013 [73], compared the effects of Cu, Si, and Ti seed layers. The surface morphologies of Cu/Ag, Si/Ag, and Ti/Ag samples (1 nm seed layer/6 nm Ag) on the SiO₂ surface are shown in Figure 16c. Among these, the 1 nm Cu/6 nm Ag sample exhibited superior wettability, with a surface roughness smaller than 0.5 nm lower than that of Si/Ag and Ti/Ag. This is because Cu has a larger surface energy than Ag and Si, and the Ag-Cu bond energy is larger than that of Ag-Ag, which suppresses the migration of Ag atoms. This was demonstrated by Li et al. [74], who utilized a 1 nm Cu seed layer to enhance Ag film wettability, and t_CT smaller than 5 nm with an ultra-low surface roughness of 0.24 nm was obtained.

Zhao et al. compared the effect of the Ge seed layer with that of oxygen addition [81] ( the T_av of ZnO/Ge/Ag/ZnO is lower than that of ZnO/AgO_x/Ag/ZnO) and with Al and Cu doping [37]. As shown in Figure 16d,e, the surface of Ag on Ge is the smoothest, and is better than oxygen addition and Al and Cu doping. By MD simulation, they suggested that the atomic-level Ge interlayer can actively migrate and mix into Ag and SiOₓ substrates during the early stage of Ag growth, which more efficiently promotes the cluster-to-layer transition than Cu and Al [37].

The core role of the Ge interlayer is to suppress cluster merging by decreasing the diffusion ability of Ag clusters on the surface, to reduce the rearranging dynamics of the tiny Ag clusters, and to reduce the interfacial and surface free energy through the inherent low-surface-energy characteristics of Ge [37]. These synergistic effects enable the fabrication of a continuous Ag layer with minimal surface roughness. This mechanism is obviously different from the commonly accepted mechanism of Ge seed layers increasing the nucleation density of the silver atoms deposited on them.

Ji et al. [70] recently revealed the regulation mechanism of the sub-nanometer Ti wetting layer on Ag thin-film growth over silicon oxide substrate through experiments and density functional theory (DFT) calculations. It is shown that the 0.7 nm-thick Ti wetting layer undergoes strong interfacial reactions with the substrate and forms a Ti-O-Si mixed-oxide interface. The non-stoichiometric TiO_x formed by partial oxidation of the Ti layer enhances the binding force of Ag and Ti through electronic interactions, which induces the growth of the Ag layer in a two-dimensional (2D) laminar pattern, resulting in the formation of a continuously smooth Ag layer. An ultrathin Ag layer electrode with a T_av of 93.8% and an R_s as low as 8.29 Ω/sq has been prepared.

However, Ge and Ti seed layers are seldom applied to DAD. Among various seed layers, the Cu seed layer is the best choice. In 2023, Bernède et al. [75] applied a Cu layer to DAD transparent electrodes for the first time. The optical transmittance spectrum of the 20 nm ZnO/4 nm Cu/6 nm Ag/MoO₃ transparent electrode on a glass substrate was significantly broadened. A T_a of 81%, a R_s of 7.5 Ω/sq, and a FOM value of 16 × 10⁻³ Ω⁻¹ was achieved. In 2015, Mouchaal et al. [76] investigated the influence of the thickness of Cu seed layer on the transmittance of ZnS (50 nm)/Cu (x nm)/Ag (9 nm)/ZnS (50 nm) and found that 3 nm is the best. A T_av of 85%, a R_s of 4.8 Ω/sq, and a FOM of 41.02 × 10⁻³ Ω⁻¹ were achieved (Figure 17a). The shining feature is that the transmittance between 450 and 700 nm is nearly the same (green line in Figure 17a). In 2021, Liu et al. [77] prepared MoO_x/Cu/Ag/MoO_x (MCAM) transparent electrodes on glass to optimize the Cu seed layer thickness. The T_av of the MCAM transparent electrode reached the maximum value when the thickness of Cu was 1 nm (blue line in Figure 17b). Further decreasing the thickness of the Cu seed layer significantly reduced the conductivity of the transparent electrode. For example, as shown in Figure 17c, when the thickness decreases from 1 nm to 0.8 nm and then to 0.6 nm, the R_s of the MCAM transparent electrode increases from 9.45 Ω/sq to 20.8 Ω/sq and then to 33.2 Ω/sq. Overall, the MCAM transparent electrode has the highest optic-electric performance when the thickness of the Cu seed layer is 1 nm. The T_av in the visible light range is 62.32%, the R_s is 9.45 Ω/sq, and the highest FOM value is 9.35 × 10⁻³ Ω⁻¹.

In the same year, our group [24] prepared TiO₂/0.5 nm Cu/Ag/TiO₂ (TCAT) transparent electrodes on glass substrates. Using a 0.5 nm Cu seed layer, combined with regulating deposition parameters of the Ag thin film, TCAT transparent electrodes achieved transmittance spectra nearly parallel to the x-axis in the wavelength range of 400–800 nm, with an average transmittance of 92.5% ± 0.5% (Figure 18a, red line). The R_s was as low as 6.2 Ω/sq (Figure 18b), and the FOM was as high as 71.51 × 10⁻³ Ω⁻¹. HRTEM and XPS analyses [78] revealed that the epitaxial-like growth of AgNPs on the Cu seed layer, enabled by their similar lattice constants, suppressed the LSPR of AgNPs (Figure 18c, dot-dash line). Figure 18d,e shows the schematic principal diagrams of silver and copper nanoparticles (average thickness: 1 nm) on glass substrates. Both exhibit columnar growth morphology. However, the copper nanoparticles are flatter than the silver nanoparticles; this means that the silver nanoparticles are more agglomerated than the copper nanoparticles. When silver was deposited on the surface of copper nanoparticles (denoted as 1.0 nm CuNPs-1.0 nm AgNPs, Figure 18f), it grew in a highly wettable manner. The agglomeration of AgNPs and their LSPR effect were effectively suppressed. Therefore, damping the LSPR by lattice matching is also one of the important mechanisms to improve the optical transmittance of the DAD transparent electrode. This principle has been further validated by Zhang et al. [54], who demonstrated that lattice mismatch at the interface between AZO and AgO_x/Ag decreased with the increase in oxygen content, which helped to increase the wettability of the Ag thin films on the substrate surface. However, little attention has been paid to lattice match.

To develop high-performance flexible DAD transparent electrodes for chalcogenide solar cell applications, Jiang et al. [79] prepared an AZO/Cu/Ag/AZO structure on a PET substrate. Their study (Figure 19a) revealed that the optimal 0.1 nm Cu seed layer achieved average visible transmittance of 90.77%, R_s of 9.9 Ω/sq (Figure 19b), and a FOM value of 38.35 × 10⁻³ Ω⁻¹ (Figure 19c).

According to the reports mentioned above, the Cu seed layer can help the DAD to obtain excellent optical performance; not only is the transmittance large, but also the line of the transmittance between 400 and 800 nm can be a line parallel to the X-axis. This cannot be obtained using a Ge or Ti seed layer. As shown in Figure 20, for the Ge seed layer, according to the optical transmittance spectra (Figure 20a–d) provided by refs [36], the reflectivity (Figure 20b) of DAD using Ge as a seed layer is large, and the sheet resistance (Figure 20d) is also around 20 Ω, which is relatively larger than Cu as a seed layer. For the Ti seed layer, according to the optical transmittance spectra provided by refs [71,72], the reflectivity (Figure 20f) is relatively high; and also the average absorbance is about 5% (Figure 20h), indicating that the LSPR damping effect of Ti is not strong, therefore, high transmittance in the wavelength range of 650–800 nm can’t be obtained. Among various seed layers, the Cu seed layer is the best choice. Aside for its effectiveness, the fact that it can be deposited by simple resistance-heating is also a big advantage.

In summary, a Ge seed layer is the best to reduce the surface roughness of Ag thin film, but it is unsuitable to be applied in the DAD electrode, due to the large reflectivity and large resistance. A Cu seed layer is the best choice, and a FOM value as high as 71.51 × 10⁻³ Ω⁻¹ has been achieved when the thickness of the Cu seed layer is 0.5 nm and TiO₂ is the dielectric layer. Further reducing the thickness of the Cu seed layer decreases the light absorption of the Cu seed layer, but it reduces the wettability of the Ag thin film, which seriously degrades the optoelectronic performance of the DAD transparent electrodes. Lattice match between the Ag layer and the substrate promotes the wettability of the Ag layer.

3. Progress on the Dielectric Layers of DAD Transparent Electrodes

Too many dielectric layer materials, such as ZnO, SnO₂, Nb₂O₅, TiInZnO, MoO₃, WO₃, Al-doped ZnO, Al₂O₃, ZrON, ZnSnO, etc., have been developed. The ones that are commonly studied are ZnO, Al (ZAO or AZO), TiO₂, and MoO₃ because they are very stable, their bandgaps are wide, and their energy levels are suitable for solar cells, organic light-emitting diodes (OLEDs), touch screens, etc. For example, the high work function between 4.7 and 6.9 eV of MoO₃ makes it an efficient hole-selective interfacial layer in flexible opto-electro devices. AZO thin films, both transparent and conductive, are used as alternative electrodes to ITO in many cases. TiO₂ has a bandgap of 3.2 eV, a high refractive index (~2.7), and good chemical stability. It is the main material of DSSC and perovskite solar cells. The TiO₂/Ag/TiO₂ transparent electrode [82,83] is of high transmittance and low Rs, and has drawn wide interest. This part just lists several examples to indicate the progress on the dielectric layers of DAD transparent electrodes.

3.1. Progress in DAD with Same Dielectric Layer Materials

In 2013, Abachi et al. [84] prepared MoO₃/Ag/MoO₃ (MAM) transparent electrodes on PET substrates using the vacuum evaporation deposition technique. Combined with the regulation of the deposition rate of Ag, the electrodes exhibited a T_av of 72%, comparable to that of ITO electrodes, while achieving a lower R_s of 13 Ω/sq (vs. ~50 Ω/sq for ITO) and a FOM value of 2.90 × 10⁻³ Ω⁻¹. In 2021, by varying the ratio of Ar/O₂ and the sputtering power, the transmittance of MAM at 550 nm reached 72%, and the FOM value reached 5.70 × 10⁻³ Ω⁻¹ [85]. The introduction of O₂ gas during sputtering suppressed the formation of MoO_x (x < 3), eliminating strong visible-light absorption caused by Mo⁵⁺ [86]. In 2022, Goetz et al. [87] fabricated MTO/Ag/MTO on a PET substrate using a magnetron sputtering technique, where the dielectric layer consisted of TiO₂-doped MoO_x (MTO). By adjusting the MTO thickness, they achieved a transmittance at 550 nm of 87.8%. The FOM value reached 45.4 × 10⁻³ Ω⁻¹. Meanwhile, the transparent electrode exhibited exceptional stability: When immersed in water for 600 s, the MoO_x sample was basically completely dissolved, while the MTO sample was only dissolved by about 5%. This is due to the fact that Ti⁴⁺ reduces the cationic charge in the MoO₃ film layer and lowers the surface acidity, thus suppressing the hydrolysis process [88].

In 2018, Yan et al. [89] used AlH₃-doped ZnO (HAZO) as the dielectric layer and prepared HAZO/Ag/HAZO flexible transparent electrodes on PEN substrates using the magnetron sputtering technique. A transmittance of 88.2% at 550 nm, R_s of 6.37 Ω/sq, and FOM value of 44.7 × 10⁻³ Ω⁻¹ were achieved. In 2021, Ekmekcioglu et al. [90] fabricated ZTO/Ag/ZTO (ZAZ) transparent electrodes on both PET and PC substrates. The ZAZ structure (green dashed line in Figure 21a) exhibited exceptional optical performance with transmittance exceeding 100% in the 500–600 nm range, surpassing that of PC substrates (green solid line in Figure 21a). This was attributed to the fact that the refractive index and the thickness of the ZTO were well matched with the optical parameters of the Ag thin film, and the light reflections at the interfaces of the ZAZ were completely suppressed. The electrode achieved an unprecedented FOM value of 83.00 × 10⁻³ Ω⁻¹. In 2025, Li et al. [91] prepared BMZO (B-Mg co-doped ZnO)/15 nm Ag/BMZO multilayer films. The T_av of the as-prepared BMZO/Ag/BMZO composite film was increased to 90%, and the resistivity was 4.55 × 10⁻⁴ Ω cm.

In 2021, Hrostea et al. [92] prepared TNO/Ag/TNO flexible transparent electrodes on PET substrates using Nb-doped TiO₂ (TNO) dielectric layers. The TNO-based electrodes demonstrated superior performance compared to the undoped TiO₂/Ag/TiO₂ structure, achieving 84.17% transmittance at 550 nm (vs. 81.83% for the undoped counterpart, Figure 21b). These enhancements stem from two effects of Nb doping: (1) widening of the optical bandgap and modification of the refractive index, which minimizes interfacial light reflection; (2) introduction of Nb⁵⁺ as an n-type dopant in the TNO dielectric layer, which improves electrical conductivity. Consequently, TNO/Ag/TNO exhibited a very low R_s of 11.9 Ω/sq, nearly 70 times lower than the undoped TiO₂/Ag/TiO₂ electrodes (833.3 Ω/sq). In 2022, Goetz et al. [93] used DC magnetron sputtering from a conductive oxide target that allowed for high deposition rates, resulting in transparent TNO films even in an inert Ar atmosphere and without substrate heating. The combination with ultrathin Ag layers yields flexible DAD electrodes on a PET substrate with visible transmittance above 70% and sheet resistance below 10 Ω/sq.

3.2. Research Progress on DAD Flexible Transparent Electrodes with Different Dielectric Layer Materials

In 2014, Chiu et al. [94] prepared TiO₂/Ag/SiO₂ transparent electrodes on glass substrates through thickness optimization (25 nmTiO₂/10 nm Ag/70 nm SiO₂). An average transmittance of 89% in the range of 400–700 nm and a FOM value of 47.97 × 10⁻³ Ω⁻¹ was achieved. Among many electrode structures, the transmittance of asymmetric dielectric design TiO₂/Ag/SiO₂ with different dielectric materials on both sides is significantly higher than that of symmetric configurations of TiO₂/Ag/TiO₂ and ZnO/Ag/ZnO with the same dielectric materials (Figure 22a). This is because of the high refractive index of the bottom TiO₂ layer and the low refractive index of the top SiO₂ layer, which significantly suppress interfacial light losses.

In 2015, Formica et al. [95] prepared a 34 nm TiO₂/8 nm Ag/40 nm AZO flexible transparent electrode on an ultrathin glass flexible substrate. The T_av in the range of 400–700 nm reached 87.7%, and the FOM value reached 42.72 × 10⁻³ Ω⁻¹. In 2019, Kinner et al. [96] prepared 27 nm TiO₂/10 nm Ag/51 nm AZO flexible transparent electrodes on PET flexible substrates, and the T_av in the range of 400–700 nm was increased to 94%, and the FOM value was as high as 100.23 × 10⁻³ Ω⁻¹. Combined with the Cu seed layer technique, in 2020, Ji et al. [97] prepared 24 nm ZnO/6.5 nm Ag-Cu/56 nm Al₂O₃ flexible transparent electrodes on PET substrates. The T_av in the range of 400–700 nm was higher than that of the substrate, indicating T_av exceeding 100% (Figure 22b). In 2023, Rani et al. [98] evaluated the optical performance of 64 different combinations of DAD transparent electrodes based on ZnO, AZO, TiO₂, MoO₃, WO₃, NiO, SnO₂, and ZnS dielectric layers by using transfer matrix simulations. The study fixed the Ag interlayer thickness at 9 nm and maintained equal thickness for both dielectric layers. As shown in Figure 23, among the many combinations, the TiO₂/Ag/AZO transparent electrode had the highest optical performance, with a T_av of 93.3% in the range of 400–700 nm.

Figure 22. (a) Effect of the materials of dielectric layers on the transmittance of DMD. Comparison of transmittance spectra of DMD structures with different metal and dielectric layers [94]. (b) Transmittance over 100% is predicted. Calculated (red solid curve) and measured (blue dashed curve) absolute transmittance from 400 to 700 nm of the designed DMD transparent electrode PET substrate/24 nm ZnO/6.5 nm Cu-doped Ag/56 nm Al₂O₃. Inset presents the configuration of the designed DMD electrode [97].

Figure 22. (a) Effect of the materials of dielectric layers on the transmittance of DMD. Comparison of transmittance spectra of DMD structures with different metal and dielectric layers [94]. (b) Transmittance over 100% is predicted. Calculated (red solid curve) and measured (blue dashed curve) absolute transmittance from 400 to 700 nm of the designed DMD transparent electrode PET substrate/24 nm ZnO/6.5 nm Cu-doped Ag/56 nm Al₂O₃. Inset presents the configuration of the designed DMD electrode [97].

Figure 23. Graphical representation of the calculated average visible transparency (AVT) of various possible combinations of DMD transparent electrodes. Different shapes in the legend represent the bottom oxide layer, whereas the X-axis shows the corresponding overcoat oxide layer in the DMD stack [98].

Figure 23. Graphical representation of the calculated average visible transparency (AVT) of various possible combinations of DMD transparent electrodes. Different shapes in the legend represent the bottom oxide layer, whereas the X-axis shows the corresponding overcoat oxide layer in the DMD stack [98].

In addition to common dielectric layers, M. Socol et al. [99] investigated the preparation and performance optimization of reduced graphene oxide (RGO)-based multilayer transparent electrodes, RGO/Ag/ZnO. By adjusting the parameters of RGO concentration and laser pulse deposition, the T_av of RGO/Ag/ZnO multilayered electrodes reached 82%–85%, and the R_s was as low as 10.6 Ω/sq, which was about 50% lower than that of ZnO/Ag ZnO/Ag/ZnO multilayer electrode.

4. Summary and Outlook

DAD transparent electrodes based on ultrathin metal Ag film possess high optical transmittance, high electrical conductivity, high chemical stability, and excellent mechanical flexibility, and they will have a wide range of applications in flexible photovoltaic devices such as solar cells, light-emitting diodes, electrochromic devices, and transparent heating devices.

The key to improving the optical properties of DAD is to dampen the LSPR of AgNPs, and the key to improving the electrical properties is to reduce the t_CT. Strategies based on preventing silver atom agglomeration and reducing the size of AgNPs not only dampen the LSPR but also reduce the t_CT. The four kinds of techniques that were developed were plasma irradiation, gas-assisted deposition, metal doping, and seed layer technology. Metal doping dopes metal elements during the whole deposition of the Ag layer, and seed layer technology deposits a thin layer of metal before the deposition of the Ag layer. The two are highly efficient in reducing the t_CT and the surface roughness, but due to the absorbance or reflectivity of metal elements, the optical performance is not as good as expected, resulting in low FOM values. We suggest that seed layer slight oxidation is promising for future developments [100].

Gas-assisted deposition introduces oxygen or nitrogen in the initial stage or during the whole deposition stage to change the chemical state of AgNPs so that the agglomeration of AgNPs can be lowered. Good results have been achieved; however, controlling the amount of gas during gas-assisted deposition is difficult. Specifically, the effect of damping the LSPR of AgNPs is much weaker than metal doping and seed layer technology. Plasma irradiation modifies the surface of the Ag layer or the dielectric layers, demonstrating the most effective approach, with FOM values larger than 100 × 10⁻³ Ω⁻¹, which are much higher than the other three approaches; this has significant potential for future research. The pros and cons of different approaches are summarized in

Table 2. The pros and cons of different approaches.

Method	Main Advantages	Main Drawbacks
Plasma irradiation	Reducing the size of AgNPs by Ar plasma or increasing the nucleation density by producing defects on the dielectric surface, or reducing the interface energy between the Ag layer and the dielectric layer by oxygen plasma; high efficiency damping LSPR, and high FOM. Improved interfacial contact.	Equipment/parameters are more complex (power, atmosphere, duration, etc.)
Gas-assisted deposition	Reducing the interface energy, or increasing nucleation density to damp LSPR; reduced tCT	Challenging in precise control of gas flow rate/partial pressure.
Metal doping	Increasing the nucleation density to damp LSPR significantly reduced tCT and roughness.	Absorbance of doped metals leading to degraded transmittance.
Seed layer	Increasing the nucleation density for damping LSPR effectively reduced tCT and roughness.	Absorbance or reflectance of the metal seed layer itself leading to reduced transmittance.

.

The research on the dielectric layer has been relatively complete and effective. We suggest future efforts could focus on integrating additional functional coatings, such as anti-reflection or self-cleaning layers, into DAD electrodes. The exploration of new dielectric materials, such as graphene-enhanced layers, also holds promise.

We end this review with some broader suggestions about future research directions. Firstly, damping the LSPR of AgNPs using a metal layer with a lattice that is matched with that of the silver layer is also promising. Based on this mechanism, more techniques can be developed. Secondly, good results can be achieved by combining two kinds of approaches for damping the LSPR of AgNPs, or one of the four kinds of technologies combined with dielectric layer materials [101]. Thirdly, computational simulation based on MD is very important. On the one hand, it helps to disclose the mechanisms behind phenomena and deepens our understanding; on the other hand, through simultaneous optimization of chemical stability and compatibility between the dielectric layers and the Ag thin film, it can advance DAD electrode performance. Finally, application in real devices is very exciting and is waiting to be explored.