Highly Conductive Transparent and Flexible Electrodes Including

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Highly Conductive Transparent and Flexible Electrodes including Double Stacked Thin Metal Films for Transparent Flexible Electronics Jun Hee Han, Do-hong Kim, Eun Gyo Jeong, Tae-Woo Lee, Myung Keun Lee, Jeong Woo Park, Hoseung Lee, and Kyung Cheol Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Highly Conductive Transparent and Flexible Electrodes including Double Stacked Thin Metal Films for Transparent Flexible Electronics Jun Hee Han†, Do-hong Kim†, Eun Gyo Jeong†, Tae-Woo Lee†, Myung Keun Lee†, Jeong Woo Park†, Hoseung Lee† and Kyung Cheol Choi†,*

†

School of Electrical Engineering, the Korea Advanced Institute of Science and Technology,

Yuseong-gu, Daejeon 34141, Republic of Korea

KEYWORDS. Electrode, Transparent, Flexible, organic light emitting diodes (OLEDs), Pattern

ACS Paragon Plus Environment

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ABSTRACT.

In order to keep pace with the era of transparent and deformable electronics, the electrode functions should be improved. In this paper, an innovative structure is suggested to overcome the tradeoff between optical and electrical properties that commonly arises with transparent electrodes. The structure of double stacked metal films showed high conductivity (< 3 Ω/sq.) and high transparency (~90 %) simultaneously. A proper space between two metal films led to high transmittance by an optical phenomenon. The principle of parallel connection allowed the electrode to have high conductivity. In situ fabrication was possible because the only materials composing the electrode were silver and WO3, which can be deposited by thermal evaporation. The electrode was flexible enough to withstand 10,000 bending cycles with a 1 mm bending radius. Furthermore, a few µm scale patterning of the electrode was easily implemented by using photolithography, which is widely employed industrially for patterning. Flexible organic light emitting diodes and a transparent flexible thin film transistor were successfully fabricated with the proposed electrode. Various practical applications of this electrode to new transparent flexible electronics are expected.

1. INTRODUCTION Electrodes are indispensable components of most electronics. Among the various characteristics that electrodes should have, high conductivity is the most basic but essential requirement because it enhances the power efficiency of the electronics, preventing unnecessary energy consumption, and also makes the device drive stably by transmitting equivalent current and power through


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whole device. Recently, with greater demand for transparent and deformable electronics, high transmittance and flexibility have also become important characteristics that electrodes should have. However, it is difficult to satisfy desired optical, mechanical, and electrical properties at once. Many studies have been carried out to develop electrodes that satisfy those three conditions. Graphene and carbon nanotubes are among the candidates for future electrodes. They have high transmittance and flexible characteristics, but the problem of low conductivity has not been solved yet.1–4 Conducting polymers have also resolved optical and mechanical properties, but low conductivity has been a problem.5,6 In metal nanowires or metal mesh electrodes, the use of large amounts of nanowires for high conductivity or narrowing the spacing of the meshes can lead to loss of transmittance.7–13 This problem also occurs even in the case of thin metal films electrodes. In order to achieve high transmittance by reducing the thickness of the films, the conductivity is lowered, and to enhance the conductivity by increasing the thickness of the films, the transmittance is drastically decreased.14–23 Although it is possible to fabricate electrodes that meet all of the required optical, mechanical, and electrical characteristics, practical use of such electrodes without a proper patterning method would be challenging. This is because the purpose of the electrode is not to produce it, but to transmit electric power to a designated place by patterning of the electrode. For example, hybrid type electrodes that combine more than two technologies such as graphene and Ag nanowires have been studied recently, achieving high conductivity and transmittance. However, while they satisfied desired optical and mechanical properties, the difficulty of patterning has yet to be resolved.24–28 This is not only a problem of hybrid electrodes but also for various transparent flexible electrodes that have been studied.1–6,16,29,30


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In this paper, an innovative design using double stacked metal films was studied for a highly conductive transparent flexible electrode. The proposed electrode is easy to fabricate and can be patterned. Metal is suitable for the flexible electrode because of its ductility. However, when metal is used as a transparent electrode, a tradeoff between optical and electrical properties arises.15–18,31 The double stacked metal films suggested in this paper can be a solution for this tradeoff. It is possible to obtain transmittance that is as high as that of a thin metal film by an optical effect with a proper space between the two metal films and optimized layer thicknesses. Furthermore, high conductivity can be attained by the use of two metal films. We fabricated a double stacked metal electrode (DME) composed of only Ag and WO3 using thermal evaporation alone, and it showed low sheet resistance of less than 3 Ω/sq. while maintaining a high transmittance (~90 %) in the visible range. In addition, flexibility was guaranteed based on the negligibly changed characteristics of the device after a 10,000 cycle bending test with a bending radius of 1mm. A few µm scale pattern was also implemented. The potential for application of the DME in transparent flexible electronics was experimentally verified with a DME based flexible organic light emitting diode (OLED) and a transparent flexible thin film transistor (TFT).

2. EXPERIMENTAL SECTION 2.1 Fabrication of the Devices The suggested DME was manufactured on a cleaned glass substrate and a PET substrate. All substrates were cleaned with isopropyl alcohol in an ultrasonic bath. The thickness of the glass was 700 µm and that of the PET was 125 µm. The 50 µm thick PET was also used for the


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bending test. The WO3 (1–4 mm pcs 4N, TASCO) and Ag (3–5 mm granule 4N, TASCO) were deposited by thermal evaporation with a vacuum pressure of less than 7 × 10−6 Torr. The OLED device was manufactured on a cleaned glass substrate and a PET substrate. The substrates were loaded into a thermal evaporation chamber, where the multilayer stack was deposited sequentially in a vacuum pressure of less than 7 x 10-6 Torr: Ag (100 nm)/aluminum (Al) (5 nm)/lithium quinolate (Liq) (1 nm)/Bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2): Tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3) (8 wt%, 70 nm)/N,N′Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (60 nm)/WO3 (5 nm)/Ag (12 nm)/WO3 (105 nm)/Ag (12 nm)/WO3 (50 nm). In order to pattern the DME, a thin film mask was used while WO3 and Ag were deposited in a thermal evaporator. When the DME was patterned by photolithography, a negative photoresist (NR9-3000PY, FUTURREX INC.) was spin-coated on cleaned glass. After the spin-coating step, the photoresist was soft-baked at 150 °C for 3 minutes inside an oven. The samples were exposed to ultraviolet light (UV) with a 360 mJ/cm2 exposure dose. The photoresist was then baked again at 110 °C inside the oven for 2 minutes. After the baking process, the photoresist was developed for 17 seconds in an AZ300MIF developer. After the photoresist was patterned, multilayers were deposited on the patterned photoresist sample, and then the photoresist was stripped by acetone sonication cleaning for 10 minutes. The a-IGZO TFT with a top gate bottom contact was fabricated on a PET (250 µm) substrate coated with ITO. The ITO (150 nm) on PET was deposited by a RF sputtering system with 5 mTorr total pressure and 20 sccm argon (Ar) flow. The power was 100 W. Source/drain electrodes were defined by PR patterning and a wet etching process using ITO etchant. A 20 nm


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thick a-IGZO layer was deposited by a RF sputtering system. The RF power was 100 W. The total pressure and oxygen partial pressure were 5 mTorr and 5% (Ar:O2 = 32:2 sccm), respectively. Next, a 9 nm thick Al2O3 layer was deposited as a gate insulator by thermal atomic layer deposition (THALD). Trimethylaluminum (TMA) and H2O were used to deposit the Al2O3. The active region was defined using PR patterning and a wet etching step with a diluted hydrofluoric acid. Annealing was then conducted at 180 ˚C for 8 hours in air. The 31 nm thick Al2O3 was deposited by THALD. The annealing step was carried out again at 180 ˚C for 5 hours in air. The Al2O3 was etched with heated phosphoric acid for a contact via. The gate electrode was defined by a lift off process followed by the deposition of a 100 nm ITO layer by RF sputtering or the DME by thermal evaporation. Finally, the last annealing step was carried out at 180 ˚C for 5 hours with the ITO gate device, or 150 ˚C for 10 hours with the DME gate device, respectively. 2.2 Device characterization The transmittance spectrum of the DME at normal incidence was measured by a UV-Vis spectrometer (UV-2550, SHIMADZU). The ratio of the light transmitted through the incident light from the normal direction was measured. The sheet resistance of the DME was measured by a 4-point probe (FFP-2400, DASOLENG).32,33 A bending test was conducted using a bending machine (Bending machine, SCIENCETOWN). A sourcemeter (2400, Keithley) and a spectroradiometer (CS-2000, Konica Minolta) were used to measure the electrical and optical characteristics of the OLED. The refractive indices of Ag and WO3 were measured experimentally by a spectroscopic ellipsometer (M2000D). TFT characteristics were measured by means of a HP 4156C semiconductor parameter analyzer in an ambient atmosphere.


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2.3 Simulation Calculations of the transmittance and optimized layer thickness were performed using MATLAB (MathWorks) based on the characteristic matrix method.

3. RESULTS AND DISCUSSION


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Figure 1. Design overview of suggested transparent flexible electrode. (a) Structure of the transparent flexible DME. (b) Schematic diagrams of the energy band structure of WO3 and Ag. (c) Graphical representation of the electric and magnetic field at the boundaries in the DME. (d) Calculation result to obtain the optimized thicknesses of WO3 films for high transparency by MATLAB simulation. (e) Scanning electron microscope (SEM) images of DME in cross section view. The thicknesses of each layer are indicated on the SEM images.


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Table 1. The optimized layer thicknesses of DME.

Material

1st WO3

Ag

2nd WO3

Ag

3rd WO3

Thickness

50 nm

12 nm

105 nm

12 nm

50 nm

Figure 1a shows the structure of a DME. As shown in Figure 1a, the DME structure consists of two thin Ag films surrounded by WO3 layers. In contrast with previous studies that employed a single thin metal layer for the transparent electrode, two metal layers were used in this study. In order to attain a low sheet resistance with a single metal film, it is necessary to make utilize a thick metal film. However, there is a tradeoff between the transmittance and the sheet resistance because a thick metal film has a problem of low transmittance.15–18,31 Double stacked metal films could provide a solution to this problem. As is well known, connecting resistors in parallel lowers the overall resistance. Based on this principle, the DME can have low sheet resistance. In order to take advantage of low sheet resistance for the application of the DME, there should be a specific film that can electrically connect two Ag films. In this study, a WO3 film was inserted between two Ag films, because WO3 has a similar work function to that of Ag, and hence an ohmic contact can occur between Ag and WO3, as Figure 1b shows. For this reason, WO3 acts as an electrical bridge of electrons through an ohmic contact, and it helps the two Ag layers become electrically connected when the DME is applied to a real device such as an OLED.34–38 It was experimentally confirmed that the two Ag films could be electrically connected through the WO3 film (Figure S1, Figure S2).


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Although an electrode composed of two metal films can be considered to be opaque, it has a transmittance as high as that of a thin metal film electrode by using an optical effect. Light has a property of reflection and transmission at the boundary between two materials with different physical properties. Since DME is composed of several layers, this phenomenon occurs in a complex manner, as shown in Figure 1c.37,39,40 In order to enhance the transmittance of the DME, there should be a constructive interference when the light passes through the final boundary. One of the factors affecting light interference is phase, and it is determined by the optical path length which is related to the thicknesses and the materials of each films in the DME. In addition, the boundary should be in the light path at an appropriate location. Therefore, considering the phase difference, optimized layer thicknesses should be considered for higher transmittance.41 The total amount of light passing through the DME and reflected light can be easily calculated by an optical formula that is the characteristic matrix.37,39,42 The MATLAB was used to calculate the characteristic matrix. The optimized layer thicknesses were determined when the calculated transmittance had the highest value by changing the thickness of each WO3 layer. The outermost WO3 films that had optimized thicknesses were necessary to enhance the transmittance because they worked as anti-reflection films controlling the interference of the light. Furthermore, the outermost layer reduced the difference of the optical admittance between the electrode and the incident medium.19,37,43,44 In order to obtain two-fold improved conductivity compared to that of previous studies, the thickness of Ag was fixed to 12 nm while the simulation and experiment were conducted.16,19 Figure 1d shows simulation results, and Table 1 summarizes the optimized thickness of each layer.


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A DME device was fabricated based on the simulation results. Figure 1e shows a crosssectional scanning electron microscope (SEM) image of the fabricated DME device. Compared with Table 1, it can be seen that the thicknesses of layers were matched to the optimized value. Since both WO3 and the Ag thin film can be deposited by thermal evaporation method, the DME device can be fabricated in situ without breaking the vacuum. This in situ fabrication for DME is useful for large area fabrication and easy patterning of DME.


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Figure 2. Optical, mechanical, and electrical properties of the DME. (a) Photographs of the realized DMEs. (b) Transmittance of the DME with optimized layer thicknesses. The dot line is the calculated data by the simulation. The red colored rectangles are experimental data before bending. The blue open triangles are experimental data after 10,000 bending cycles with 1 mm bending radius. (c) The ratio of sheet resistance change to initial sheet resistance of ITO (150 nm), and the DME as a function of applied strain. The sheet resistance data were obtained after 10,000 bending cycles with specific strain. The inset image shows the initial sheet resistance of the DME and the schematic of the 4-point probe. (d) ∆RS/RS0 of the DME as a function of specific bending cycle at a 1 mm bending radius. The inset image shows a photograph of a bent DME with a 1 mm bending radius. (e) Comparison of sheet resistance and transmittance plot from other references: green triangles for dielectric/metal/dielectric,16 green stars for ITO,12,45 pink triangles for graphene,1 purple diamonds for carbon nano tubes,4 blue circles for Ag NWs.12,13

Figure 2a shows photographs of the fabricated DME devices. As Figure 2a shows, the DME is transparent enough to look through the background or landscape. In particular, the photo at the center shows a DME element with a size of 70 mm × 70 mm, which is the largest electrode that can be fabricated with thermal evaporation equipment in our laboratory. Depending on the size of the equipment, a larger size DME can be fabricated, and this implies that DME can be applied to large electronic devices to be developed in the future. Figure 2b shows the calculated and measured transmittance data. The data showed high transmittance in the visible light region and high transmittance of over 90% at the 530 nm


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wavelength where the human eye is sensitive. The results of the characteristic matrix simulation and the measured transmittance data of the DME device were similar to each other. These results demonstrated that the high transmittance of the DME device was due to the optical phenomenon through the appropriate film thickness. After the DME device was fabricated on polyethylene terephthalate (PET), it was bent 10,000 times with a bending radius of 1 mm. As figure 2b shows, the transmittance of the DME remained high without losing the optical characteristic. The DME has a low sheet resistance of less than 3 Ω/sq. (Figure 2c), and it is about twofold smaller than that of the conventional transparent electrode with a 12 nm Ag layer.16,19 This confirms that the DME has two-fold higher conductivity than the conventional thin single metal film electrode. This high conductivity is expected to be used for portable devices where it is necessary to minimize the power consumption by lowering the unnecessary energy loss. In addition, even in large devices such as large area displays, the DME could be used to deliver uniform power throughout the device. Figure 2c shows the ratio of the changed sheet resistance value to that of the original value after 10,000 bending cycles for each strain. Analysis of the graph shows that the DME exhibits good bending properties without a significant change of the sheet resistance even under bending of 2.5% strain which takes at 1 mm bending radius (Figures 2c and 2d). On the other hand, the sheet resistance of indium tin oxide (ITO), which is a synonym of a transparent electrode, increased significantly under small strain (Figure 2c). The reason for the barely changed sheet resistance might be the good ductility of the Ag layer, which mainly determines the sheet resistance of the DME.46 Furthermore, the overall ductility of the device is thought to be enhanced by the double stacked Ag layers. The experimentally proven bending properties of DME could be sufficient for application as electrodes in future transparent flexible and foldable devices.


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Figure 2e shows the relationship between the sheet resistance and the transmittance of the transparent electrodes studied previously. Various studies have been carried out for transparent electrodes and there have been many attempts to achieve low sheet resistance while maintaining high

transmittance.1,4,12,13,16,45

Compared

with

other

studies,

especially

with

dielectric/metal/dielectric (DMD) electrodes having a similar structure, the DME developed in this study demonstrates high conductivity while maintaining high transmissivity.

σ DC 188.5 = σ Op (λ ) Rs ⋅ (T (λ ) −1/2 − 1)

(1)

The ratio of DC conductivity to optical conductivity, σ DC / σ Op (λ ) , which is the figure of merit (FOM), was also used to compare the characteristics of the DME with other transparent electrodes.13,47 In eq. 1, σ Op (λ ) is the optical conductivity ( λ = 550 nm), σ DC is the DC conductivity of the electrode, RS is the sheet resistance of the electrode, and T (λ ) is the transmittance ( λ = 550 nm). The value of the FOM for a graphene-based film and a carbon nanotube was 0.5 and 25, respectively, in previous reports.48,49 The ITO, silver nanowires, and DMD structure showed higher values of 350, 500, and 600,13,16,32 respectively. However, the DME has a FOM value of over 1100 ( RS = 2.85 Ω/sq., T (550 nm ) = 89.5 %) which is about twice higher than that of previous reports, especially compared with a DMD structure. The higher value of the FOM means it is a more transparent electrode. This result indicates that the DME has better performance as a transparent electrode than other transparent electrodes.


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Figure 3. The performance of OLEDs with the DME. (a) Schematic device structure of the red phosphorescence top emission OLED with the transparent flexible DME. (b) Photograph of a flexible OLED with the DME in operation. (c) Current density and luminance versus voltage characteristics of the OLED on PET and glass substrates. Red circles are the characteristics of the OLED on the PET substrate. Blue rectangles are the characteristics of the OLED on the glass substrate. (d) Current efficiency versus luminance characteristics of the OLED on PET and glass substrates. Red circles are the characteristics of the OLED on the PET substrate. Blue rectangles are the characteristics of the OLED on the glass substrate. Inset images are the pixel images of the OLED in operation on glass and PET substrates. (e) Normalized electroluminescence spectra of OLED with DME. The peak value is at 630 nm wavelength.


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In order to confirm the application of the DME to actual devices, a red phosphorescence top emission OLED was fabricated with the DME as an anode. The OLED device was fabricated with

the

structure

of

Ag/aluminum

(Al)/lithium

quinolate

(Liq)/Bis(10-

hydroxybenzo[h]quinolinato)beryllium (Bebq2): Tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3)/N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB)/DME as shown in Figure 3a. As mentioned previously, the DME could be fabricated in situ without breaking the vacuum by a thermal evaporation method. Therefore, it is a major advantage to fabricate an OLED containing the DME by thermal evaporation without an additional process. Through this process, it is possible to easily manufacture an OLED with the DME regardless of the bottom emission or top emission structure. In this study, a top emission OLED, which has been studied for high resolution displays, was fabricated to investigate the applicability of the DME. Considering the change of optical and electrical properties of the DME when it was applied to the OLED, the thickness of WO3 contacting NPB was modified to 5 nm (Figure S3, Figure S4). A photograph of the fabricated device is shown in Figure 3b. Even though the OLED was harshly bent, it operated well because the DME is flexible. Figure 3c shows the current density and luminance characteristics for the voltage. Figure 3d shows the current efficiency measurement for the luminance and the inset shows the pixel images of OLEDs on PET and glass substrates. The same results were obtained regardless of the substrates with current efficiency of about 6.6 cd/A at 1000 cd/m2. The performance of the OLEDs fabricated using the DME showed similar values of the red phosphorescence reference with ITO as the anode and the results suggest that the DME is fully utilizable in flexible OLEDs without reducing device efficiency.50 The normalized electroluminescence spectrum of the OLED device including the DME was also almost the same as the red phosphorescence reference with ITO as an anode.50 Furthermore, the angular property


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of an OLED device with the DME was measured and the results showed that the device had a Lambertian emission profile, and the electroluminescence spectra barely changed by the viewing angle (Figure S5). This verified that the DME has transparency comparable to that of the conventional transparent electrode.


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Figure 4. Patterning of the DME and its applications. (a) SEM image of a cross section view of deposited DME on the patterned negative photoresist. (b) SEM image of a cross section view of patterned DME. Negative photoresist was stripped by acetone cleaning in a sonication bath. (c) SEM image of a surface view of patterned DME. (d) Schematic device structure of a-IGZO TFT with DME top gate. The inset image shows a microscope image of the fabricated device. (e) Transfer curves of TFTs with ITO gate electrode and DME gate electrode. Blue open circles are the graph for the TFT with ITO gate electrode. Red solid diamonds are the graph for the TFT with DME gate electrode. The inset image shows a photograph of the transparent flexible TFT with the DME gate electrode. (f) Photograph of an OLED with the DME as electrode and wiring. (g) The equivalent circuit of (f).

Table 2. Performances of TFTs using DME and ITO as gate electrodes.

Mobility [cm2/V·s]

S.S [mV/dec]

On/off

Vth [V]

DME

2.71

115

4.12+E10

1.2

ITO

3.74

106

9.32+E09

0.3

In order to use the proposed transparent flexible electrode in real industry, easy patterning technology as well as an easy fabrication process is necessary. In this respect, an easy patterning process is another advantage of the DME. Previously studied transparent flexible electrodes had many limitations in terms of easy implementation of the desired patterns because of chemistry or fabrication reasons. However, the DME can be easily patterned by using thin metal film masks or photolithography. This is because both WO3 and Ag constituting the DME can be deposited


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by thermal evaporation, and they do not react with acetone, which is used as a photoresist stripper in photolithography. It was experimentally confirmed that the optical and electrical properties of the DME were not affected by acetone (Figure S6). The fabrication method, that is, thermal evaporation, was suitable for patterning with photolithography because it afforded poor step coverage. Figure 4a shows the DME deposited on the patterned negative photoresist. Figures 4b and 4c show the cross section view and the surface view of the patterned DME after removing the negative photoresist using acetone in a sonication bath. In this study, a line pattern of about 5 µm was realized. However, since the patterning scale of the photoresist has been developed to the extent of nm, it is considered that patterning on a nm unit scale for the DME is possible. A TFT requiring a finely pattern electrode can be easily fabricated with the DME by using a photolithography patterning method. Figure 4d shows the structure of a transparent flexible indium gallium zinc oxide (IGZO) TFT using the DME as a gate electrode and a microscope image of the fabricated device. The transfer curves of IGZO TFTs with ITO and DME as gate electrodes at the saturation region are shown in Figure 4e, and Table 2 summarizes the performance of each TFT. As can be seen in Figure 4e and Table 2, there were no significant differences in performance between the two TFT devices, and this shows the possibility of utilizing the DME in transparent flexible electronics. The slight difference in mobility is thought to be a reason for the low annealing temperature. The TFT using the DME was annealed at 150 o

C due to the thermal stability of the flexible substrate. In addition, the shift of Vth was caused by

the difference in the work function between WO3 and ITO.51 The inset image in Figure 4e is a photograph of a fabricated TFT device.


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Furthermore, it is experimentally confirmed that the DME can be used for wiring. As shown in Figure 4f, four pixels could be illuminated with just one contact point of the cathode and the anode because the DME acted as wiring to connect each pixel. The equivalent circuit is depicted in Figure 4g, and the resistor in the circuit is the DME wiring. The low sheet resistance, the ability of wiring function, and the possibility of fine patterning demonstrated in Figure 4 verify the strong possibility of utilizing the DME in electronic devices.

4. CONCLUSION In this paper, we demonstrate a highly transparent flexible electrode using double stacked metal films. High conductivity (

Highly Conductive Transparent and Flexible Electrodes Including

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