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Abstract
We propose a four-layer WO3/Ag/PEI/CuSCN laminated transparent electrode with a PEI (polyethyleneimine) seed layer. The optical properties of the WO3/Ag/CuSCN electrode were simulated by a transfer matrix theory. Its optimal structure was WO3 (35 nm)/ Ag (9 nm)/CuSCN (47 nm), and the optical transmittance reached 92.7% at a wavelength of 550 nm. The transmittance decreased with the increase of the Ag thickness (> 9 nm). The WO3/Ag/PEI/CuSCN laminated electrode was prepared by a solution method and a vacuum evaporation technique. The quality of an ultra-thin Ag film can be improved via the PEI seed layer in this electrode so that the ultra-thin Ag film has formed a uniform and continuous film at a thickness of 9 nm. The flexible electrode WO3 (35 nm)/Ag (9 nm)/PEI/CuSCN (47 nm) shows a sheet resistance of 10.2 Ω/sq, an optical transmittance of 90% and a surface root mean square roughness of 4.4 nm. The resistance of the electrode remained stable after 1000 times of bending test at a radius of 1 mm, and it has a good mechanical property.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent electrode is an important part of organic optoelectronic devices, including organic solar cells, organic light-emitting diode, touch screen and intelligent window. In recent years, with the gradual commercialization of flexible electronic products and the rapid development of flexible solar cells, flexible sensors and flexible display screens, the need for flexible electrodes is becoming more and more [1]. At present, the most mature and widely used transparent electrode material is indium tin oxide (ITO). However, the inherent brittleness of ITO and the scarcity of indium resources make it unable to meet the strict requirements of future optoelectronic devices for flexibility, lightness and green environmental protection [2]. Therefore, through the joint efforts of many researchers, a variety of flexible transparent electrode materials have been developed like silver nanowires [3], graphene [4], noble metal [5], carbon nanotubes [6] and metal oxides [7]. Nevertheless, these electrode materials have some defects such as a high surface roughness or a poor conductivity, which make them unable to be replaced completely ITO thin films. Thus, it needs to be further researched in transparent electrodes.
The multi-layers film based on the dielectric/metal/dielectric multilayers structure of the ultra-thin metal film is a new transparent electrode. Its conductivity mainly depends on the intermediate metal layer, and the optical transmittance is closely related to the intermediate metal layer and the dielectric layer [8]. To improve the transmittance of the electrode, on the one hand, the multi-layers electrode, such as WO3/Ag/WO3 [9], MoO3/Ag/MoO3 [10], ZnO/Ag/ZnO [11], TiO2/Ag/TiO2 [12], is prepared by selecting the dielectric material with a high refractive index and a high transparency. In particular, compared with the commonly used MoO3, ZnO and TiO2, WO3 has a smaller band gap and a larger light absorption range. [8–11]. However, the improvement of its transmittance is limited. On the other hand, the preparation of the high quality ultra-thin metal is the most widely used in multi-layer electrodes because of the best conductivity of the metal Ag and the smallest optical absorption coefficient in visible light region. Nevertheless, because of the island growth of the ultra-thin Ag film [13,14], it do not form continuous films when the thickness is 10 nm, resulting in a strong light scattering and a low transmittance. When the Ag film forms a continuous film, the thickness is thicker. And there is a strong light reflection, which makes the optical transmittance of the electrode lower. To make the ultra-thin Ag films form uniform and continuous films, the concept of a seed layer is presented. The seed layer of several nanometer or even sub-nanometer films was prepared before the deposition of the ultra-thin Ag films to improve the film forming quality of the Ag film. At present, the seed layers of Au [15], Cu [16], Ni [17] and a polymer [18–21] have been proposed. However, they have little been used in multi-layer transparent electrodes.
Herein, we propose a WO3/Ag/PEI/CuSCN multi-layer transparent electrode of a four-layer structure. The optical properties of the electrode WO3/Ag/CuSCN were simulated by a transfer matrix theory. The optimal structure of the electrode was WO3 (35 nm)/ Ag (9 nm)/CuSCN (47 nm), and the optical transmittance reached 92.7% at a wavelength (λ) of 550 nm. The transmittance decreased with the increase of the Ag layer thickness (> 9 nm). PEI (polyethyleneimine) was a seed layer of the Ag layer, which was used to improve the quality of the ultra-thin Ag layer [20]. CuSCN was a material with a high transmittance (98%) and a high refractive index (2.15), which can be used to improve the optical transmittance of the electrode. PEI and CuSCN can be prepared by a solution method. While Ag and WO3 can be deposited by a vacuum evaporation technique, which is the current mainstream preparation method of organic optoelectronic devices. It can greatly simplify the electrode deposition process.
2. Simulation and experimental details
2.1 Transfer matrix method
The simple physical model of the multi-layer electrode is shown in Fig. 1. Each layer of the material in the electrode is regarded as a homogeneous medium, then each layer of the medium can be represented by a transfer matrix. And the elements in the matrix are a function of an incident angle, a refractive index and a thickness of the medium. Thus the matrices M1, M2, M3 can be obtained. These matrices are multiplications in turn according to the order of the media layer, and then you can get the transmission matrix of the whole multi-layer media system, as shown in the following formula:
In a multi-layer optical film system, the first layer and the last layer of the mediums are usually semi-infinite mediums. In the first layer of the mediums, there are reflected light waves. For the last layer, there are only transmitted light waves. Thus,
In this formula, Ei and Er represents the components of incident and reflected electric fields. Hi and Hr represents the components of incident magnetic fields and reflected magnetic fields, respectively. ${\boldsymbol{\theta}}_i$ is the incident angles. After a series of calculations, the transmission coefficient of the final multi-layer homogeneous mediums can be expressed as follows:
From the above Eq. (4), P1 is the external environment (air) parameters above the first layer of the transparent electrode, and Pn is the parameter of the external environment (substrate) contacted by the bottom layer of the transparent electrode. Then the whole optical transmittance of the multi-layer transparent electrode can be calculated by using the substrate as the incoherent layer [22].
2.2 Simulation details
In the multi-layer transparent electrode, the thickness of each layer of the dielectric has a very important influence on the optical-electrical characteristic of the multi-layer electrode. Therefore, how to determine the thickness of each layer is the most important part in the multi-layer transparent electrode. The thickness of the intermediate ultra-thin metal layer and the film quality determine the conductivity of the electrode. Because of the size effect of the ultra-thin metal [23,24], the resistivity of the ultra-thin metal is difficult to estimate, and there is a big error. Therefore, the conductivity of the electrode is ignored and only the optical properties of the electrode are analyzed in the numerical simulation. In our study, the transmission matrix method (TMM) [25,26] is used to analyze the optical transmittance of the electrode. Because it can calculate the transmittance of the multi-layer optical thin film system simply, quickly and efficiently through the optical constant of each layer (refractive index (n), dielectric thickness (d), extinction coefficient (k), etc.). The optical transmission of different composite electrodes are simulated, as shown in Fig. 2. Figure 2(a) presents the multi-layer transparent electrode model with different material combinations. From the Fig. 2(b), it can be seen that compared with the Ag film and Ag/CuSCN composite film, both WO3/Ag/PEI/CuSCN (WAPC) and WO3/Ag/CuSCN (WAC) have a good transmittance, and it is almost the same. The transmittance reaches the maximum when there is a dielectric layer on both sides of the Ag layer. Meanwhile, the transmittance of the electrode was not affected by the addition of the PEI seed layer under the Ag layer.
Fig. 2. (a) Multi-layer transparent electrode models with different material combinations. (b) Optical transmittances of different composite multi-layer electrodes. (c) Transmittances of different dielectric layer thickness of the electrode. (d) Transmittance of different Ag thickness.
To further obtain the thickness of each layer of the dielectric in the laminated electrode, the thickness of the ultra-thin Ag film is fixed at 9 nm. Figure 2(c) shows that the optimal thickness of CuSCN and WO3 is 47 nm and 35 nm, respectively, and the corresponding average transmittance is about 90%. Taking the conclusion of Fig. 2(c) as a reference, Fig. 2(d) presents that with the increase of the thickness of the Ag layer, the transmittance of the WAC laminated electrode is increased first and then decreases. Its optimal thickness is 9 nm, and the optical transmittance reaches 92.7% at a wavelength of 550 nm. The average transmittance of the whole visible light is 90%. It indicates that the WAC electrode has the potential to obtain a high transmittance. Thus, the WAC multi-layers electrode optimal structure is WO3 (35 nm)/Ag (9 nm)/ PEI/CuSCN (47 nm) according to the above analysis.
2.3 Samples preparation
WAPC multi-layers transparent electrode samples were prepared on the surface of the ultra-thin (25 μm) colorless polyimide (cPI) substrate. The schematic diagram is shown in Fig. 3.
Dissolved a certain amount of CuSCN in diethyl sulfide (DES) and was configured as a CuSCN solution of 20 mg/ml, as shown in Fig. 4(a). A certain amount of PEI (Sigma-Aldrich, 99%) was dissolved in anhydrous ethanol and configured into 2 mg/ml PEI ethanol solutions [see Fig. 4(b)]. CuSCN and PEI were prepared by a rotating coating process, as shown in Fig. 5. The cleaned cPI ultra-thin substrate was placed on the glass substrate. The CuSCN solution was coated on the cPI surface at the rotating speed of 1.3k rpm/min, and then the CuSCN thin film with a thickness of 47 nm was obtained by a high temperature annealing at 120 ℃. After that, the PEI film were prepared by coating the PEI solution on the CuSCN film at the rotating speed of 2k rpm/min for 40 s. At last, the cPI film with CuSCN and PEI were put into a vacuum evaporation equipment, and the ultra-thin Ag film and WO3 film were deposited at the evaporation rates of 0.1 Å/s and 0.2 Å/s, respectively.
Fig. 4. (a) CuSCN chemical structure and configuration solutions. (b) PEI chemical structure and configuration solutions.
2.4 Characterization technique
The surface morphology of the WAPC electrode and the ultra-thin Ag film was measured by a scanning electron microscope (SEM) (TESCAN MIRA3) and an atomic force microscope (MFP-3d-bio asylum research), respectively. For the transmittance of the WAPC electrode, an ultraviolet visible spectrophotometer (UV-vis spectrophotometer UV-2450/2550) was used to obtain it. The thickness of each layer of the electrode was tested through using an ellipsometer (ME- L wide spectral ellipsometer) and a step instrument (xp-100). The square resistance was gained by means of a four-probe measuring instrument (RTS- 8 four-probe).
3. Results and discussion
It is well known that the coating quality and the thickness of the ultra-thin Ag film are the main factors affecting the transmittance of multi-layers transparent electrodes. Because of the island growth of the ultra-thin Ag film, it is difficult for the Ag film to obtain continuous and uniform conductive film below 10 nm. Thus, it is necessary to introduce the seed layer to improve the forming quality of the Ag film. In many seed layers, such as the transition metal Au, Cu, Ni and so on, these materials can absorb visible light, which leads to the decrease of the transmittance. Through the theoretical analysis of section 2.1, it is found that the PEI thickness of 5 nm has no effect on the transmittance of the multi-layers transparent electrode. And then, we configure different concentrations of the PEI solution and spin coating on the CuSCN film. The effect of the PEI film thickness on the optical transmittance is investigated experimentally. As shown in Fig. 6(a), with the increase of the PEI concentration, the optical transmittance of the PEI/CuSCN composite film has no decrease, indicating that PEI does not cause the decrease of the optical transmittance when it as a seed layer.
Fig. 6. (a) Transmittances of PEI/CuSCN composite thin films with different PEI concentrations. (b) Optical-electrical characteristics of Ag/PEI/CuSCN composite films with different PEI concentrations, and the inset is the sheet resistance of different PEI concentration. (c) SEM diagram of Ag/PEI/CuSCN composite thin films. (d) SEM diagram of Ag/CuSCN composite thin films.
After discussing the effect of the PEI concentration (or thickness) on the transmittance of the CuSCN thin film, we prepared 9 nm ultra-thin Ag films on top of the PEI film, and tested their optical-electrical characteristics, as shown in Fig. 6(b). It can be seen that when the concentration of PEI is 2 mg/ml, the Ag/PEI/CuSCN composite films have a low square resistance. Compared with the films without PEI (PEI concentration is 0 mg/ml), the square resistance of Ag/PEI/CuSCN (with PEI) composite films is reduced by 16 Ω, reaching 10 Ω/sq [ohms per square, see the inset of Fig. 6(b)]. While the optical transmittance of the composite films at λ=500 nm is 71%, which is 8% higher than that of the films without PEI (0 mg/ml). After adding PEI layer, the Ag/PEI/CuSCN composite films have a high optical transmittance. From this result, it can be seen that the introduction of the PEI seed layer improves the forming quality of the ultra-thin Ag film, so that it can form uniform and continuous thin film (9 nm). Figures 6(c) and 6(d) present the surface morphology of the ultra-thin Ag film on PEI/CuSCN (with PEI) and CuSCN (without PEI) composite films, respectively. It can be seen that compared with the film without PEI, the Ag particles in the film with PEI are much smaller. The film is more uniform and smooth, and has a good conductivity and a high optical transmittance. It proves that the PEI plays an important role in the film forming process of the Ag.
The optical-electrical characteristics of Ag/PEI/CuSCN composite films are better than those of Ag/CuSCN films because the evaporated Ag atoms are deposited on PEI and form coordinate-covalent bond with the PEI [20], as shown in Fig. 7. Due to the interaction of the coordinate-covalent bond between Ag and PEI, Ag atoms are uniformly fixed on the PEI surface without the migration and the aggregation. Ag atoms can evaporate uniformly on the PEI surface to form the ultra-thin and continuous film. The conductivity and the optical transmittance of ultra-thin Ag films are closely related to the PEI concentration (thickness) [see Fig. 6(b)]. When the PEI has a low concentration, the ultra-thin continuous films may not be formed on the surface of CuSCN. The ultra-thin Ag films have a poor optical-electrical characteristic. On the contrary, when the PEI has a high concentration and a thick film. Because of the large gap between the polymer atoms and the interaction between the amine groups in the inner layer on the Ag atom, the Ag atom diffuses into the PEI. As a result, the film has a poor quality, which shows that the optical-electrical characteristic of the composite film decrease with the increase of the PEI concentration.
In the previous theoretical simulation, we obtain that the optimal thickness of the ultra-thin Ag layer is 9 nm. Therefore, we fixed the thickness of WO3 and CuSCN to 35 nm and 47 nm, respectively. The effect of the thickness of the ultra-thin Ag layer on the optical-electrical characteristic of WAPC multi-layers transparent electrode was discussed, as shown in Fig. 8(a). In the figure, the square resistance of the electrode decreases with the increase of the Ag layer thickness. When the thickness of the Ag layer is 9 nm, the square resistance of the Ag layer is about 10.2 Ω/sq [see the inset of Fig. 8(a)]. In terms of the optical performance of the electrode, the optical transmittance is increased first and then decreased. When the thickness of the Ag layer is 9 nm, the transmittance reaches the maximum, 90% optical transmittances at a wavelength of 550 nm and 85% average transmittances of the visible light. After that, with the increase of the thickness of the Ag layer, the optical transmittance is decreased gradually. The experimental results show that the structure of the WAPC multi-layers electrode is WO3 (35 nm)/Ag (9 nm)/PEI (2 mg/ml)/CuSCN (47 nm), which is in accordance with the simulation results. Although the actual transmittance is smaller than the theoretical value. This is because the surface roughness of the ideal interface in theoretical simulation is 0. While in the actual process, there will be certain surface fluctuations and film formation inhomogeneities, resulting in the smaller optical transmittance than the theoretical value. Figure 8(b) presents the image of the WAPC electrode. From the figure, it can be seen that the WAPC multi-layers electrode has an excellent optical transmittance [see the left of Fig. 8(b)]. And it can also be seen that LED (Light Emitting Diode) can emit light normally, indicating that the current can pass through the electrode smoothly, and the electrode has a good conductivity [see the right of Fig. 8(b)].
Fig. 8. (a) Optical-electrical characteristics of the WAPC laminated transparent electrode. (b) Image of the WAPC electrode (15mm*15 mm).
Wearable devices, as the next generation of optoelectronic devices, have attracted widely attentions, among which it is very important that electronic devices need to have the excellent mechanical properties. And the transparent electrodes are an indispensable role of this kind of optoelectronic devices. This makes the electrode has a good bending characteristic has become more and more important. Before testing the flexibility of the electrode, the adhesion between the electrode and the substrate is measured. The method of the testing is to stick the tape on the surface of the electrode, then tear it off hard and do it many times. In our study, the electrode on the cPI substrate has been repeatedly posted and tested for 50 times, and the square resistance of the electrode has remained stable. The result indicates that there is a strong adhesion between the electrode and the cPI substrate. And it also shows that there is an excellent adhesion between the WO3/Ag/PEI/CuSCN/cPI stack, which will be helpful to the mechanical properties of the electrode to a certain extent. The fixing bending times of the electrode are 1000 times, and Fig. 9(a) is obtained as the bending radius (rc) decreases. It can be seen that the WAPC multi-layers transparent electrode has undergone thousands of bending tests larger than the bending radius of 1 mm, and its relative resistance (R/R0) has hardly changed. While the square resistance of the WAC multi-layers electrode without PEI has increased 4 times under the 1 mm bending radius, indicating that the thin layer PEI improves the mechanical properties of the WAC transparent electrode. When the bending radius of the WAPC multi-layers electrode decreases to 0.5 mm, the square resistance begins to change obviously and increase. It demonstrates that the bending radius limit of the WAPC laminated electrode is about 1 mm. It also shows that the WAPC laminated electrode has the excellent mechanical properties. In terms of the mechanical properties of the WAPC electrode, in addition to the selection of materials, the ultra-thin cPI substrate also play an important role. Furthermore, the surface roughness of the electrode was measured. The surface of the WAPC multi-layers transparent electrode prepared on the cPI film by Fig. 9(b), is very smooth. And the surface root mean square (RMS) roughness is 4.4 nm, which indicates that the WAPC electrode has a good surface roughness.
Fig. 9. (a) WAPC laminated transparent electrode flexibility test, and the influence of the bending radius (rc) on the relative resistance (R/R0). (b) The surface roughness test of the transparent electrode.
4. Conclusions
To improve the optical-electrical characteristic of the WO3/Ag/PEI/CuSCN multi-layers transparent electrode, a four-layer electrode structure with a seed layer was proposed. The optical properties of the electrode WO3/Ag/CuSCN were simulated by a transfer matrix theory. The optimal multi-layers structure was WO3 (35 nm)/ Ag (9 nm)/CuSCN (47 nm), and its optical transmittance at a wavelength of 550 nm reached 92.7%. The electrode was prepared by a solution method and a vacuum evaporation technique. The WO3 (35 nm)/ Ag (9 nm)/PEI (2 mg/ml)/CuSCN (47 nm) multi-layers transparent electrode prepared on the cPI thin film showed 90% optical transmittances at λ=550 nm, 10.2 Ω/sq and 4.4 nm surface root mean square roughness. Moreover, the adhesion and mechanical properties of the electrode were tested. After 50 times of the posting and the tearing experiments, the square resistance remained stable. Under the maximum bending radius of 1 mm for 1000 times, the square resistance also remained stable, indicating that the electrode has excellent mechanical properties. The experimental results show that the four-layer transparent electrode with the seed layer proposed can obtain better comprehensive performances. We believe that the high-performance WO3/Ag/PEI/CuSCN multi-layers transparent electrode can provide a new idea for the subsequent electrode research and as a potential flexible transparent electrode alternative.
Funding
National Natural Science Foundation of China (61271059); Key Project of Application Development Plan of Chongqing City (cstc2019jscx-fxyd0018); Fundamental Research Funds for the Central Universities (2018CDGFHG0012).
Disclosures
The authors declare no conflicts of interest.
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