Thin Solid Films 550 (2014) 435–443

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Electromechanical properties of indium–tin–oxide/ poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) hybrid electrodes for flexible transparent electrodes Sunghoon Jung a, Kyounga Lim a, Jae-Wook Kang b, Jong-Kuk Kim a, Se-In Oh c, Kyoungtae Eun c, Do-Geun Kim a,⁎, Sung-Hoon Choa c,⁎⁎ a b c

Functional Coatings Research Group, Korea Institute of Materials Science (KIMS), 797, Changwon daero, Changwon, Gyeongnam 641-831, Republic of Korea Department of Flexible and Printable Electronics, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju 561-756, Republic of Korea Graduate School of NID Fusion Technology, Seoul National University of Science and Technology, Gongneun-Dong, Nowon-Gu, Seoul 139-743, Republic of Korea

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 24 September 2013 Accepted 25 September 2013 Available online 5 October 2013 Keywords: Transparent flexible electrode ITO/PEDOT:PSS hybrid electrode Electromechanical properties

a b s t r a c t We investigated an indium–tin–oxide (ITO)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) hybrid electrode as a potential flexible and transparent electrode. In particular, the mechanical integrity of an ITO/PEDOT:PSS hybrid electrode deposited onto a polyethylene terephthalate (PET) substrate was investigated via outer/inner bending, twisting, stretching, and adhesion tests. A PEDOT:PSS layer was inserted between ITO and PET substrate as a buffer layer to improve the flexibility and electrical properties. When a PEDOT:PSS layer was inserted, the sheet resistance of the 20 nm-thick ITO film decreased from 270 Ω/square to 57 Ω/square. Notably, the ITO/PEDOT:PSS hybrid electrode had a constant resistance change (ΔR/R0) within an outer and inner bending radius of 3 mm. The bending fatigue test showed that the ITO/PEDOT:PSS hybrid electrode can withstand 10,000 bending cycles. Furthermore, the stretched ITO/PEDOT:PSS hybrid electrode showed a fairly constant resistance change up to 4%, which is more stable than the resistance change of the ITO electrode. The ITO/PEDOT:PSS electrode also shows good adhesion strength. The superior flexibility of the ITO/PEDOT:PSS hybrid electrode is attributed to the existence of a flexible PEDOT:PSS layer. This indicates that the hybridization of an ITO and PEDOT:PSS layer is a promising electrode scheme for nextgeneration flexible transparent electrodes. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Recently, there has been a tremendous demand for the production of flexible electronic devices, such as flexible displays, flexible solar cells, and flexible circuits [1–4]. A transparent conductive electrode (TCE) is a core component in many optoelectronic devices, such as liquid crystal displays, organic light-emitting diodes, e-papers, touch screens and solar cells. The selection of a TCE is the most important factor to consider because it determines the flexibility and reliability of the devices. Indium tin oxide (ITO) has been the most dominant transparent conductor material due to its high optical transparency and electrical conductivity. ITO electrodes deposited on flexible substrates at room temperature without intentional heating, however, generally show a high resistivity due to the limitations of process temperature and the difficulty of

⁎ Corresponding author. ⁎⁎ Corresponding author. Fax: +82 2 972 2202. E-mail address: [email protected] (S.-H. Choa).

dopant activation. In addition, the rising costs of indium due to its limited availability and the deposition process have led to significant issues for this transparent electrode. Furthermore, the brittleness of the ITO film easily results in the formation and propagation of cracks when it is bent or deformed [5,6]. For these reasons, various flexible transparent electrodes, such as silver nanowire, thin metal film, conductive polymer, carbon nanotube, and graphene, have been suggested as alternatives to flexible ITO electrodes [7–13]. Flexible oxide–metal-oxide multilayer electrodes, including In–Zn–O(IZO)/Ag/IZO, ITO/Ag/ITO, In–Zn–Sn– O(IZTO)/Ag/IZTO, and Zn–Sn–O(ZTO)/Ag/ZTO structures, are another way to improve flexibility [14–16]. Even though it has been reported that graphene, due to its high conductivity and flexibility, could be an alternative to ITO electrodes, it is too difficult to obtain highquality graphene on sizable substrates because the manufacturing process is complicated. As a conductive polymer, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), has many merits as an alternative TCE application. Its polymeric film could improve flexibility and can be prepared by a simple solution process. However, PEDOT:PSS has relatively poor electrical conductivity and

0040-6090/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.075

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Table 1 Sheet resistance and transmittance as a function of thickness for ITO/PET, PEDOT:PSS/PET, and ITO/PEDOT:PSS(150 nm)/PET samples.

integrity of the ITO/PEDOT:PSS hybrid electrodes on the PET substrate was investigated via outer/inner bending, twisting, and stretching test systems, as well as adhesion tests. 2. Experimental

PEDOT:PSS/PET

ITO/PET

ITO/PEDOT:PSS(150 nm)/PET

PEDOT :PSS thickness (nm)

Rsheet (Ω/sq.)

T(%)

ITO Thickness (nm)

Rsheet (Ω/sq.)

T(%)

86

204±8

94

20

274±3

96

147

62±4

87

50

105±1

85

212

33±1

81

100

24±1

87

261

18±1

77

ITO Thickness (nm)

Rsheet (Ω/sq.)

T(%)

20

57±2

90

100

28±1

83

instability. In order to improve the electrical conductivity, several methods have been suggested, such as addition of solvent additives and post-treatment [17–21]. Current researches on flexible electronics have been concentrated mainly on the characteristics of the electrical and optical properties of the films. Research regarding the mechanical integrity of these films is still lacking. Furthermore, researches on the reliability of flexible electronics have primarily been concentrated on simple bending tests [22,23]. Flexible electronics will be subjected to various mechanical deformations, such as bending, compression, stretching, and twisting, depending on their application, fabrication, and handling processes. Therefore, a more detailed understanding of the durability and reliability of flexible electronics under various types of mechanical deformation is important for commercialization and mass production. In this paper, we have introduced a hybrid TCE that consists of an aITO (amorphous ITO) and PEDOT:PSS layer. A hybrid TCE compensates for the brittleness of ITO and poor conductivity of PEDOT:PSS, resulting in good flexibility and high electrical conductivity. The conductive polymer PEDOT:PSS as a buffer layer was coated on a polyethylene terephthalate (PET) substrate. We deposited a high quality of a-ITO onto the PEDOT:PSS layer using a radio frequency (RF)-superimposed direct current (DC) magnetron sputtering technique. The mechanical

The PEDOT:PSS (PH1000) film was deposited on the PET substrate by the spray coating technique. O2 plasma pre-treatment was performed over 5 min under an RF power of 500 W in order to enhance the adhesion between the PEDOT:PSS film and PET substrate. To increase conductivity of the PEDOT:PSS, 5 wt.% of dimethyl sulfoxide (DMSO) was added to the PEDOT:PSS solution. The thicknesses of the PEDOT:PSS layer and the PET substrate were 150 nm and 125 μm, respectively. After preparing the PEDOT:PSS layer, post-annealing was performed for 15 min at 120 °C on a hotplate. Subsequently, the 20 nm-thick ITO film was deposited onto the PEDOT:PSS layer by a magnetron sputtering technique mixed with RF-superimposed DC power. It is well known that poor sheet resistance is obtained at room temperature for the single RF magnetron sputtering or DC magnetron sputtering method because both techniques have difficulty in crystallizing ITO at room temperature [24,25]. On the while, it is reported that the ITO film deposited by RF superimposed DC magnetron sputtering showed good electrical properties and slight improvement of mechanical durability by reducing the damage to the films during deposition process [26,27]. In this study, ITO was deposited using the magnetron sputtering technique with applied RF-superimposed DC power. A 4-inch In2O3:SnO2 target (10 wt.% SnO2) was used for ITO film deposition, and the distance from target to substrate was 11 cm. The base pressure and working pressure were kept at 0.32 × 10− 3 Pa and 0.15 Pa, respectively. The ITO film deposition was performed in a gas mixture atmosphere of Ar:O2 (100:1). The Ar flow rate was 30 sccm, and the O2 flow rate was 0.3 sccm. During the deposition, an RF power of 50 W and a DC current of 0.5 A were applied, and no bias was applied to the substrate for ITO deposition. The thicknesses of the PEDOT:PSS and ITO films were measured using the alpha-step surface profiler (Tencor P-11). The electrical and optical properties of each film were examined using a four-point probe and a UV/visible spectrometer (Cary 5000). The mechanical integrity of the ITO/PEDOT:PSS hybrid electrode deposited onto the PET substrate was evaluated via outer/inner bending, twisting, stretching, and cyclic fatigue tests. The lab-made testers were used to conduct various tests, such as outer/inner bending and stretching and fatigue tests. The four-point probe station was used to measure changes in electrical resistance, and an optical microscope (OM) was mounted on the probe station to observe the existence of cracks in the films. The bending test can be performed with two different approaches, depending on the stress on the films. The outer bending test induces tensile stress on the film, while the inner bending test induces compressive stress. The bending radius (r) is calculated using the equation below [28] Bending radius ðr Þ ¼

L sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dL π 2 h2s − 2π L 12L2

where L, dL/L, and hs denote the initial length, the applied strain, and the substrate thickness, respectively. The nominal bending strain can also be calculated using the following equation: Strain ¼

Fig. 1. Photograph of a sample of the fabricated ITO/PEDOT:PSS hybrid electrode with high flexibility and transparency.

hs : 2r

The twisting durability of the electrode was measured using a labmade twisting test machine. The two RC servo motors rotated in opposite directions. The sample size of the mechanical integrity test was 25 mm × 25 mm.

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Fig. 2. (a) Outer bending reliability tests with decreasing bending radius. (b) Optical microscope images of the ITO/PEDOT:PSS hybrid electrode at outer bending radii of ∞ (flat) and 3.5 mm, respectively.

Adhesion between the ITO/PEDOT:PSS electrodes and the PET substrate could be an important issue that affects the mechanical properties of the ITO and PEDOT:PSS electrode and the development of a suitable handling process for fabrication. Therefore, the adhesion of the ITO/PEDOT:PSS electrode on the PET substrate was qualitatively estimated by a peel-off test using adhesive tape (3 M Tape™), as well as a nano-scratch test. Scratch tests were performed with a Nano Scratch Tester (CSM Instruments), using a 2 μm-radius spherical indenter that scanned over a 5 mm track at a scan speed of 5 μm/s. For the scratch test, the samples were glued on glass microscope slides and mounted to the scratch tester's stage. The load was ramped up from the initial 0.3 mN to the final load of 20 mN. The failure modes that occur at certain critical loads were confirmed using an OM and field emission scanning electron microscope (FESEM; Hitachi S-4800). The operating voltage was 15 kV.

3. Results and discussion Table 1 summarizes the optical transmittance at a 550 nm wavelength and the sheet resistance of ITO, PEDOT:PSS, and ITO/PEDOT:PSS electrodes as a function of film thickness. In the case of the PEDOT:PSS electrode, the sheet resistance decreased from 204 Ω/square to 18 Ω/square when increasing the thicknesses from 86 nm to 261 nm. In particular, the optical transmittance reduced drastically from 94% to 77%. For the ITO electrode, the sheet resistance and optical transmittance abruptly decrease from 274 Ω/square to 24 Ω/square and from 96% to 87%, respectively, when the thickness of ITO increased from 20 nm to 100 nm. In order to improve the flexibility of the TCE, a PEDOT:PSS layer having 150 nm in thickness was introduced between the ITO and the PET substrates as a buffer layer. When the PEDOT:PSS layer was inserted, the sheet resistance of the 20 nmthick ITO film decreased from 274 Ω/square to 57 Ω/square, and the

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Fig. 3. Outer bending fatigue reliability at an outer bending radius of 10 mm.

optical transmittance slightly decreased from 96% to 90%. It is noteworthy that a highly conductive PEDOT:PSS layer was more effective in reducing the sheet resistance of the ITO/PEDOT:PSS hybrid film when a thinner ITO film (20 nm) was used, whereas it had no influence on the sheet resistance of ITO films above 100 nm in thickness. The average optical transmittance of the ITO/PEDOT: PSS hybrid electrode was 84.8%, which was slightly lower than that of a single-layer ITO electrode. Fig. 1 shows a sample of the fabricated ITO/PEDOT:PSS hybrid electrode with high flexibility and transmittance. Fig. 2 shows the results of the outer bending test of the hybrid ITO (20 nm)/PEDOT:PSS(150 nm) electrodes and single-layer ITO (20 nm) electrodes with a decreasing outer bending radius. The change in resistance of the flexible electrode was expressed as ΔR(= R − R0) / R0, where R0 is the initially measured resistance and R is the measured value after substrate bending. The outer bending test results showed that the electrical resistance of the ITO/PEDOT:PSS hybrid electrodes did not change until it was bent to a bending radius of 3 mm

Fig. 4. (a) Inner bending reliability tests with decreasing bending radius. (b) Optical microscope images of the ITO/PEDOT:PSS hybrid electrode at inner bending radii of ∞ (flat) and 3.5 mm, respectively.

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Fig. 5. SEM images with enlarged images of the cracks generated on the ITO/PEDOT:PSS hybrid electrodes after the (a) outer bending and (b) inner bending test. (c) Schematic drawing of PEDOT:PSS layer delamination and cracks in the ITO layer after the inner bending test.

which corresponds to a strain of 2.1% as shown in Fig. 2(a). However, at a specific bending radius of 3.5 mm, cracks began to initiate on the ITO/PEDOT:PSS hybrid electrode. After the cracks were initiated and propagated on the film, the broken film either lost its functionality or led to the damage of the whole device structure, even though there was no change in the electrical resistance of the film. On the while, in the case of the 20 nm-thick ITO electrode, cracks were initiated at the bending radius of 5.5 mm. Fig. 2(b) represents the OM images of the ITO/PEDOT:PSS hybrid electrode. At the initial state, the ITO/PEDOT:PSS hybrid electrode showed a smooth surface without cracks. However, at a bending radius of 3.5 mm, cracks were observed to initiate on the ITO/PEDOT:PSS hybrid electrode. Due to the contact of the clamp and the substrate thickness effect, decreasing the bending radius below 3 mm was impossible in our system. Fig. 3 shows the outer bending fatigue tests of the ITO/PEDOT:PSS hybrid electrodes. The bending radius was fixed at 10 mm, which corresponded to a strain of 0.6%.

439

There was no change in resistance throughout the 10,000 bending cycles. Fig. 4 reveals the results of the inner bending test with a decreasing inner bending radius. The ITO/PEDOT:PSS hybrid electrodes showed constant resistance with a decreasing inner bending radius up to 3 mm. However, at the bending radius of 3.5 mm, cracks began to initiate, much like in the outer bending test. For the ITO electrode, cracks were initiated at a bending radius of 6 mm. As opposed to the outer bending test, cracking in the ITO/PEDOT:PSS hybrid electrode during the inner bending test was more gradual, and the occurrence of cracks did not lead to changes in the electrical resistance. The difference between the points of crack initiation and changes in electrical resistance implied that there must be some path of conduction after crack formation. SEM images confirmed the formation of the cracks in the ITO/PEDOT:PSS hybrid electrodes after outer and inner bending tests, as shown in Fig. 5(a) and (b), respectively. In the outer bending test, shown in Fig. 5(a), the ITO/PEDOT:PSS hybrid electrode was completely separated due to the formation of cracks. On the other hand, after the inner bending test, the delamination of the ITO/PEDOT:PSS hybrid electrode from the PET substrate was observed, as shown in Fig. 5(b). The ITO/PEDOT:PSS hybrid electrode was locally released from the PET substrate, and deflected upward as a result of the compressive stress applied during the inner bending test. Furthermore, with a decreasing bending radius, the ITO film on the PEDOT:PSS layer was ripped off and overlapped under the compressive stress. Fig. 5(c) shows the schematic drawing of PEDOT:PSS layer delamination and cracks in the ITO layer during inner bending. Even though the PEDOT:PSS layer was delaminated from the substrate and the ITO film was partially cracked, the change in electrical resistance was very small because the overlapped ITO films and locally delaminated PEDOT:PSS layer formed the conducting path. The superior flexibility of the ITO/PEDOT:PSS hybrid electrode indicates that the ITO/PEDOT:PSS hybrid electrode is a desirable flexible electrode material for flexible electronics. The robustness of the ITO/PEDOT:PSS hybrid electrode is attributed to the existence of the PEDOT:PSS buffer layer between the ITO layer and the substrate. Fig. 6 reveals the uniaxial stretching test results of the ITO/ PEDOT:PSS hybrid electrodes and ITO electrodes. Cracks were not generated until a PET substrate strain of 4% was induced. It was observed that the cracks started at a strain of 4% and that the electrical resistance also increased sharply. On the while, ITO electrodes showed sharp increase in resistance and cracks at the strain of 2%, which are similar to the results reported by Sierros et al. [29], for which the change in electrical resistance began at a 2% strain. The ITO/PEDOT:PSS hybrid electrode showed superior stretchability when compared to the ITO electrode. Fig. 6(b) shows the OM image of the transverse surface cracks of the ITO/PEDOT:PSS hybrid electrode running perpendicular to the stretching direction after the stretching test. Fig. 7(a) shows the twisting reliability test results for the ITO/ PEDOT:PSS hybrid electrodes. It is noteworthy that the ITO/ PEDOT:PSS hybrid electrode showed a constant electrical resistance, even as it twisted to 50°. However, crack formations were observed at a twisting angle of 46°, as shown in the OM image. In the case of the ITO electrode, cracks began at a twisting angle of 30°. Fig. 8 represents the OM and SEM images of the cracks generated after the twisting test. Like the inner bending test, the cracks overlapped one another, and the slight delamination of the PEDOT:PSS layer was observed. Therefore, it was thought that the ITO/ PEDOT:PSS electrode made the conduction path and led to constant resistance when it was twisted. So far, no uniform standard has been used to measure the absolute value of adhesion strength between the hard brittle film and the polymer substrate. In this work, the adhesion of the ITO/PEDOT:PSS

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Fig. 6. (a) Stretchability test results with uniaxial stretching tests. (b) Optical microscope images of the ITO/PEDOT:PSS hybrid electrode at strains of 0% and 6%.

electrode on the PET substrate was qualitatively estimated via a peeloff test using adhesive tape (3 M Tape™) and a nano-scratch test. The adhesive tape was firmly pressed onto the ITO/PEDOT:PSS hybrid electrode and was slowly peeled off from the electrode at 180°. The ITO/PEDOT:PSS hybrid electrode did not peel off, and no damage or delamination was observed on the surface of the ITO/PEDOT:PSS electrode. Subsequently, the adhesion strength of the ITO/PEDOT:PSS electrode on the PET substrate was examined using a nano-scratch test. Nanoscratch tests of 20 nm-thick and 100 nm-thick ITO electrodes on a PET substrate were also performed for comparison purposes. Fig. 9 shows a panoramic OM and SEM image of the total scratch length of each sample, with the direction of sliding occurring from left to right. Two failure modes were observed. Film cracking is the first failure mechanism. The second mechanism is film buckling delamination. Both mechanisms are commonly observed in hard film–soft substrate systems, such as ITO film on PET [30–32]. For the ITO/PEDOT:PSS electrode, initial film failure

was observed at the applied load of 1.4 mN in the form of film thickness cracking as a result of the ITO/PEDOT:PSS electrode bending into the scratch track due to the deformation of the underlying PET substrate. As the normal load increased, the ITO film buckled ahead of the indenter due to the plowing of the underlying substrate, and this subsequently led to interfacial failure and delamination from the substrate. The buckling delamination of the ITO/PEDOT:PSS electrode was observed at around 2.1 mN. For the 20 nm-thick and 100 nmthick ITO electrodes shown in Fig. 9(b) and (c), cracking failure was observed at 0.5 mN and 1.9 mN, respectively, and buckling delamination was observed at 1.5 mN and 2.8 mN, respectively. Table 2 shows the summary of the failure load of adhesion strength results for each sample. It was observed that the adhesion strength of the ITO/PEDOT:PSS electrode was higher than that of the 20 nm-thick ITO electrode, but lower than that of the 100 nm-thick ITO electrode. However, it was known that as the thickness of the film increases, the adhesion strength increases

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Fig. 7. (a) Twistability test results with increasing twisting angle. (b) Optical microscope images of the ITO/PEDOT:PSS hybrid electrodes at twisting angles of 0° and 46°.

[31,33]. Therefore, considering the thickness effects on adhesion strength, it is thought that the ITO/PEDOT:PSS electrode has good adhesion strength. 4. Conclusions In this study, we proposed a highly flexible TCE with PEDOT:PSS as a buffer layer between the ITO film and PET substrate. The mechanical integrity of a flexible ITO/PEDOT:PSS hybrid electrode deposited onto a PET substrate was investigated via outer/inner bending, twisting, stretching, and adhesion tests. The ITO/PEDOT:PSS hybrid electrode enhanced not only the mechanical properties but also the electrical properties when compared to the ITO electrode. When a PEDOT:PSS layer was inserted, the sheet resistance of the 20 nm-thick ITO film decreased from 274 Ω/square to 57 Ω/square. The failure bending radius of the ITO/PEDOT:PSS electrode in outer and inner bending tests was 3.5 mm, which is lower than that of the ITO electrode

due to the buffering effect of PEDOT:PSS film. The differences between the points of crack initiation and the changes in electrical resistance were attributed to the flexible PEDOT:PSS layer and the existence of a conducting path due to overlapping films after crack generation. The outer bending fatigue test showed that the ITO/PEDOT:PSS hybrid electrode can withstand 10,000 bending cycles. For the stretching test, the PEDOT:PSS electrode also showed superior stretchability when compared to the conventional ITO film. The ΔR/R0 value of the PEDOT:PSS electrode was very small, and no cracks or delamination occurred, even though the substrate was stretched to a strain of 4.0%. For the twisting test, the ΔR/R0 value was small until the twisting angle became 50°. The cracks of the PEDOT:PSS electrode were observed at a twisting angle of 46°. The ITO/PEDOT:PSS electrode on the PET substrate also shows good adhesion strength. This indicates that the flexible ITO/PEDOT:PSS electrode is a viable alternative to the ITO electrode for flexible electronic devices.

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References

Fig. 8. SEM images with enlarged images of the cracks generated on the ITO/PEDOT:PSS hybrid electrodes after the twisting test at a twisting angle of 46°.

Acknowledgment This work was supported by a grant from the cooperative R&D Program (B551179-10-01-00) funded by the Korea Research Council Industrial Science and by the Fundamental Research Program of the Korean Institute of Materials Science (KIMS).

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Fig. 9. Panoramic view of the scratched ITO/PEDOT:PSS hybrid electrode and ITO electrodes, and the major scratch failure mechanisms of film cracking and film buckling delamination: (a) ITO (20 nm)/PEDOT:PSS(150 nm) hybrid electrode. (b) 20 nm-thick ITO electrode. (c) 100 nm-thick ITO electrode.

Table 2 Critical normal loads for cracking and buckling delamination during nano-scratch testing.

Cracking failure point Buckling delamination failure point

ITO/PEDOT:PSS electrode

20 nm-thick ITO electrode

100 nm-thick ITO electrode

1.4 mN 2.1 mN

0.5 mN 1.5 mN

1.9 mN 2.8 mN