Transparent conducting oxides (TCOs) can be used as transparent electrodes for various optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic solar cells (OSCs).220.127.116.11.6.7.8.–9 Indium tin oxide (ITO) is widely used as a TCO due to its high optical transmittance (OT ) and good conductivity (sheet resistance ).1011.–12 However, ITO film has limitations due to its high growth temperature, high cost, and poor mechanical flexibility.13,14 Furthermore, ITO films are obtained using sputtering techniques, which can damage the underlying layers.15,16 Thus, significant research has been directed toward finding a suitable replacement for ITO. Nanostructured conducting materials such as Ag nanowires,17,18 graphenes,19 poly(3, 4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS),20 metal grids,2122.23.24.–25 ITO nanobranch,26,27 and ZnO-doped 28 have been evaluated as flexible electrodes to overcome the shortcomings of ITO. Metal nanowires show high transmittance () and relatively low sheet resistance (), but poor thermal stability and rough surface morphology causing poor efficiency.29,30 Graphene, PEDOT:PSS, and ITO nanobranch have superior flexibility, but are limited by low conductivity and lack of uniformity in large areas.20,26,31 Metal grids have also been used for transparent electrodes, with excellent conductivity and flexibility. However, fabrication procedures are high cost and complex.21
One way to produce a transparent electrode is to incorporate a thin metal electrode. Metal/dielectric and dielectric/metal/dielectric structures combine the characteristics of high electrical conductivity of metals and good mechanical ductility of thin films.318.104.22.168.–37 However, these structures have relatively low OT . Ultrathin Ag electrodes are promising for this function because the OT and of these electrodes are comparable to or better than those of ITO electrodes.3822.214.171.124.126.96.36.199.47.48.–49 In addition, their mechanical properties are appropriate for use in flexible devices.40 This review highlights recent progress in the fabrication of nanoscale thin Ag electrodes. It includes an introduction on the relevant theory and detailed discussion of the characteristics of ultrathin Ag film. To clarify the trends in ultrathin Ag electrode development, we organize them according to the methods used to form ultrathin Ag films.
Ag Electrodes for Organic Electronics
Several studies have considered applications of Ag film to the electrode of organic electronics. Ag has the highest OT and lowest among metals, and is ductile.50,51,52 However, even though Ag has a high OT compared to other metals, a thin film of Ag () has a low OT compared to ITO film.53 Therefore, most Ag electrodes are used as semitransparent electrodes. Use of a semitransparent electrode can cause a microcavity effect, which improves the optical property of devices.50,54 Destructive interferences within the microcavity reduce the internal reflection of OLEDs. Ultrathin Ag electrodes are promising for this function because of OTs and s that are comparable to or better than those of ITO electrodes.50 The optical enhancement ratio varies according to metal film and dielectric material [Fig. 1(a)]. To maximize the microcavity effect, the thickness of each layer should be designed optically.54 Thin Ag electrodes can achieve an OSC performance that exceeds those of OSCs with ITO electrodes.50 The thickness of the active layer can be adjusted to maximize the electric field within the active layer [Fig. 1(b)]. The resulting microcavity resonant condition amplifies the electric field within the cell to give a higher performance than the ITO-based devices. Although microcavity effects can increase the device efficiency, its OT is degraded due to reflection and absorption by the Ag electrodes. Therefore, a microcavity is not appropriate for transparent displays, lighting, or solar cells. The irradiance that has been absorbed is given by the difference between the irradiance incident on the thickness element and the irradiance that emerges from the exit side.55 The irradiance absorbed by a layer is given by5527
To maximize the OT of the Ag film, it must be as thin as possible. Generally, an OLED with an Ag electrode thick shows a narrow emission spectrum due to the strong cavity effect, and a weak intensity contribution in the undesigned wavelength region. Furthermore, emission color changes with the viewing angle. Therefore, Ag electrodes thick are not good for broad wavelength range applications such as white OLEDs. Several approaches to use an ultrathin () Ag electrode have been reported.38188.8.131.52.184.108.40.206.47.48.–49 As the OT of the electrode increases, its emission spectrum broadens, in agreement with theoretical expectations for a weak microcavity.56 Using a weak microcavity also increases the angular color stability and the device efficiency.44
Fundamental Principle on Conduction and Transmittance of Thin Ag Film
Agglomeration (growth of “islands”) is a transport process that reduces the overall energy of a system.57 In most heteroepitaxial systems, films grown on a foreign substrate energetically favor island growth and thus are subject to dewetting instability.58 This means that the interaction between substrate and Ag atoms is an important factor to determine the growth mode. Depending on the interaction energies of substrate atoms and film atoms, any of three growth modes (Frank-van der Merwe mode, Volmer-Weber mode, Stranski-Krastanov mode) can occur (Fig. 2).59 Layers of material grow one on top of another: this is called layer-by-layer growth (Frank-van der Merwe mode). Interaction between substrate and atoms in a film is greater than the distance between adjacent atoms in the film (Fig. 2). Ag layers exhibit three-dimensional (3-D) island growth (Volmer-Weber mode) as a result of the poor wettability of Ag on the substrate. Ag films have very low adhesion energy at the metal/oxide interface:60 this means that the Ag atoms interact more strongly with each other than with the substrate. Therefore, Ag films tend to form islands on the substrates; this process yields a rough and discontinuous surface.
A discrete Ag film that consists of islands strongly absorbs and scatters light because of localized surface plasmon resonance (LSPR).39,61 LSPR results from the coupling between incident light and the free electron gas at the surface of a metal island structure.62 Coupling between light and free electron causes some light to be absorbed and some to be scattered. This coupling occurs at specific frequencies for each kind of metal. Ag has its LSPR frequency in the visible wavelength region. Glass/Ag samples show specific dips related to LSPR at wavelengths near 500 nm [Fig. 3(a)]. Simulations and experimental results did not match at a low Ag thickness because Ag film thick has many isolated clusters and voids. The of Ag on the bathocuproine increases as Ag thickness decreases [Fig. 3(b)] due to island growth of the Ag film, and is infinite at . The isolated metallic grains provide no sufficient conductive paths through the layer. However, after the percolation threshold is reached, separate metal islands start to connect and form a continuous layer: this process causes to decrease rapidly.63 To obtain a highly conductive and transparent Ag film, it must be continuous and smooth.
Recent Methods for Formation of Ultrathin Ag Film
The island growth of Ag occurs because the substrate has low surface energy compared to Ag ().64 The introduction of seed layers reduces the difference in between the substrate and Ag. Wetting of Ag layer on the substrate is significantly improved by adding seed materials such as Au, Al, Ca, Ge, Ni, , or .38220.127.116.11.43.44.45.–46 A Ca seed layer with , and an Al seed layer with reduce the difference in between the substrate and Ag.39 Schubert et al.39 obtained a continuous and smooth 7-nm thick Ag film using an Au seed layer. Because Au has a higher than does Ag, the attachment of Ag atoms to the Au surface is favored over agglomeration [Fig. 4(a)]. For a nominal thickness , glass/Ag samples have a low OT and high [Fig. 4(b)].38 The use of an Au seed layer causes the morphology of the Ag film to become very smooth and homogeneous, even for total thickness of the Ag layer . Compared to ITO, the Ag film with an Au seed layer has a similar OT and lower [Fig. 4(b)].
Seed layers such as Ge and promote nucleation of Ag.46,66 Williams et al. used a Ge seed layer to achieve a continuous Ag film.65 Although the Ge deposition on the substrate also caused island growth, the density of the Ge nuclei was much larger and the islands were significantly smaller than those for Ag deposited without a seed layer on the substrate. This surface provided a heterogeneous nucleation site for the deposited Ag atoms. The activation energy for Ag diffusion is on a Ge surface and on a surface.67 Therefore, using a Ge seed layer before depositing Ag reduces the surface diffusion and mass transportation of Ag, and results in a smooth and continuous Ag film. Compared to the topology between rough and smooth [Fig. 4(c), inset], without the layer of Ge, the deposited Ag formed distinct polycrystalline granular clusters with irregular shapes and a significant density of pinholes, which made the film electrically discontinuous. However, a Ge seed layer with thickness leads to a suppression of Ag island growth, thereby smoothing the film. The smoother Ag thin films also had a lower sheet resistance (, ).65 Metal and oxide can be used as good wetting layers in all device structures.
The growth of Ag islands can also be reduced by derivatizing the substrate with a layer of molecules designed to enhance Ag nucleation by interacting strongly with both the substrate and the incoming Ag atoms.68 Zou et al.69 improved the Ag wetting property by applying a double-end functionalized 11-mercapto-undecanoic acid (MUA) self-assembled monolayer (SAM) on the substrate as a molecular binder to covalently attach Ag to the substrate. In the absence of a seed layer, discontinuous Ag islands grow on a glass substrate [Fig. 5(a)], but MUA can interact with the incident Ag atoms to form an ester linkage which minimizes surface diffusion and enhances Ag nucleation, so the Ag film is very smooth with a low root mean square roughness and reduced [Fig. 5(b)]. However, lauric acid SAM has an inert terminal group which leads to a significantly rough surface due to the growth of Ag islands [Fig. 5(c)]. To maximize the adhesion between the substrate and Ag, the appropriate SAM material must be chosen.
Temperature can affect the growth of Ag islands. Even dewetting has been observed when the temperature exceeds 102°C.70,71 Ultrathin continuous Ag films are produced at low temperatures. A low substrate temperature generally reduces surface diffusion of Ag atoms, and thus alters the nucleation processes.72 Presland et al.73 defined induction time as the time at which the void density increases in the film during an isothermal anneal for a given film thickness; it is related to the activation energy of void formation as74 Therefore, to reduce Ag agglomeration, should be increased and should be decreased. Sergeant et al.75 optimized several deposition parameters to minimize the growth of Ag islands by controlling the evaporation rate and substrate temperature. The best results were obtained at a deposition rate of 5.5 to onto a substrate cooled to . Increasing the evaporation rates and reducing the substrate temperature caused a reduction in the surface diffusion of Ag atoms: this means that sufficient nucleation sites are created to enhance the lateral growth of Ag films. Although a low-temperature process may be useful for the growth of ultrathin Ag layers, the process cost is high due to the requirement of the equipment needed to maintain a low temperature.
In a nonwetting system, flat ultrathin Ag films tend to transform to 3-D islands.76 This dewetting tendency of pure Ag becomes more obvious at elevated temperatures.57,77 Zhang et al.48 and Gu et al.49 used Al doping to fabricating ultrathin (3 nm) Ag-based thin films with high thermal stability on (100) substrates. Al doping yielded smaller and denser nuclei [Fig. 6(a)] than those in pure Ag films [Fig. 6(b)]: this difference reveals that Al doping causes an increase in the nuclei density of the films. Al-O bonds are much stronger than Ag-O bonds60 so Al atoms are immobilized on the substrate and can hinder the diffusion of Ag atoms; therefore, Al-doping causes increased spatial density of nuclei and reduced size of particles. Figures 6(c) and 6(e) show the SEM images of 15 nm as-deposited pure Ag and Al-doped Ag films. Due to dewetting, pure Ag film easily changes to isolated islands after annealing at 300°C [Fig. 6(d)]. In contrast, an Al-doped Ag film shows an ultrasmooth surface morphology that is very similar to that of the as-deposited surface [Fig. 6(f)]. These results show that Al doping is a very effective method to form an ultrathin Ag layer with high thermal stability. Wang et al.47 used oxygen doping to achieve an ultrathin Ag-based film. The oxygen-doped Ag () layer was 6-nm thick and was completely continuous and smooth. Oxygen doping of the ultrathin Ag layer improved its OT without any noticeable degradation of electrical conductivity.
Application and Developments
Organic Light-Emitting Diode Lighting and Organic Solar Cell with a High Performance
Monochromatic top-emitting OLEDs based on thin metal electrodes show high efficiency due to their microcavity structure. However, as a result of spectral narrowing and change of emission color with viewing angle, white OLEDs show low efficiency and color quality.78,79 To achieve highly efficient white OLEDs, the OT of the transparent electrode must be increased. Schwab et al.38 used an ultrathin Ag electrode to improve the characteristics of the top electrode, and achieved better color quality and color stability compared to those of an optimized white OLED based on an ITO electrode. The highly transparent ultrathin Ag electrode increased the spectral width of the microcavity [Fig. 7(a), red line] and increased the luminous efficacy by 25% compared to a standard Ag electrode.
Wang et al.47 demonstrated a highly transparent ultrathin Ag electrode for flexible inverted OSCs, and it achieved a higher power conversion efficiency (PCE) and external quantum efficiency (EQE) than did conventional ITO-based OCSs [Fig. 7(b)]. The PCE and EQE enhancements using an ultrathin oxygen doped electrode were mainly the result of its increased OT [Fig. 7(b), inset]. The electrode provided an at 620 nm: this is the highest value achieved by any flexible OSC fabricated on polymer substrates.
OSCs fabricated on a thin metal electrode show enhanced PCEs due to the optical cavity effect. The method of tuning the optical cavity in a device is to change the thickness of the layers between reflective and semitransparent metal electrodes, with the objective of changing the length of the optical path.54 Some groups have tuned the optical cavity to improve the performance of OSCs. To maximize the photocurrent, the thickness of the hole transport layer (HTL) can be optimized to the optical cavity so that the resonance frequency occurs in a spectral range in which light absorption by the absorber is weak.75 This tuning increased the optical characteristics such as high fill factor, open circuit voltage, and PCE by up to 4.4%, so the HTLs are attractive replacements for ITO electrodes in OSCs.75 A 7 nm Al-doped Ag-based device with a resonance cavity in the active layer between the reflective anode and the semitransparent ultrathin Al-doped Ag cathode has a spectrum peak that is close to the absorption edge of a photoactive absorber.48
Flexible Device Application
ITO-based flexible optoelectronic devices show a dramatically decreased performance during bending. A flexible OSC that uses an Ag electrode with a seed layer showed stable performance even after bending cycles ().50 Wang et al. demonstrated a flexible OSC that uses an ultrathin electrode [Fig. 8(a)].47 The percentage change in of the ITO electrode was 35% higher than that of the electrode at the same bending radius as a result of the formation and propagation of microscopic cracks in the ITO [Fig. 8(b), inset]. However, the of the ultrathin Ag electrode changed little under bending stress, and formed no cracks on the film surface. Figure 8(c) shows the device architecture of the flexible OSC using an ultrathin Ag electrode. As the bending radius decreased, the PCE of the ITO-based OSC greatly decreased, but that of the ultrathin Ag-based OSC was not severely affected [Fig. 8(d)]. Compared to ITO-based OSCs, the higher flexibility of the ultrathin Ag electrode ensured the superior structural durability of OSCs.
Ultrathin Ag electrodes are promising candidates as next-generation flexible transparent electrodes. Compared with other transparent electrodes, these ultrathin metal-based transparent electrodes have the highest OT, flexibility, and low . However, growth of 3-D Ag islands causes strong absorption and scattering of incident light: because of LSPR, and these phenomena degrade its transparency. Smooth and ultrathin Ag layers () could be produced by introducing a seed layer, by using surface treatment, by low-temperature deposition, and by doping. The high OT of the ultrathin Ag electrode can improve the color quality and color stability of white OLEDs that use a weak microcavity structure.38,40,44 Ultrathin Ag electrodes have been used to achieve the highest EQE of any flexible OSC fabricated on polymer substrates.47 Ultrathin Ag-based electrodes provide opportunities to demonstrate highly flexible and transparent optoelectronic applications. However, several disadvantages of ultrathin Ag electrodes remain to be overcome; these include light absorption by the seed layer, a trade-off between OT and when changing the thickness of Ag films and low thermal stability.
This research was financially supported by the National Research Foundation (NRF) of Korea grant funded by the Korean government (MEST) (No. NRF-2013R1A2A2A01069237).
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Illhwan Lee received his BS degree in materials science and engineering from Hanyang University in 2009 and his MS degree in materials science and engineering from POSTECH in 2011. He is currently a PhD candidate in Professor Jong-Lam Lee's research group at POSTECH. His research interests include design of flexible transparent electrodes and architecture of nanostructured light out-coupling layers for optical devices.
Jong-Lam Lee received a PhD degree in materials science and engineering from KAIST in 1985. He joined POSTECH in 1996. He is heading the Department of Materials Science and Engineering and was selected as a POSTECH fellow. His research covers architecture of nanostructures for photon extraction, flexible substrates for printing processes of flexible displays, and design of photonic devices. He published more than 300 papers in academic journals.