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A transparent conductive oxide electrode with highly enhanced flexibility achieved by controlled crystallinity by incorporating Ag nanoparticles on substrates
Journal of Alloys and Compounds 620 (2015) 340–349
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A transparent conductive oxide electrode with highly enhanced flexibility achieved by controlled crystallinity by incorporating Ag nanoparticles on substrates Ross E. Triambulo a, Hahn-Gil Cheong a, Gun-Hwan Lee b, In-Sook Yi c, Jin-Woo Park a,⇑ a b c
Department of Materials Science and Engineering, Yonsei University, Seoul, Republic of Korea Advanced Thin Film Research Group, Korea Institute of Materials Science (KIMS), Changwon, Republic of Korea R&D Center, InkTec Co., Ltd., Ansan, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 June 2014 Received in revised form 16 September 2014 Accepted 21 September 2014 Available online 30 September 2014 Keywords: Hybrid TCO Indium tin oxide (ITO) Ag nanoparticle Flexible organic light emitting diode (OLED) Interface
a b s t r a c t We report the synthesis of highly flexible indium tin oxide (ITO) on a polymer substrate whose surface was engineered by oxide-coated Ag nanoparticles (AgNPs) smaller than 20 nm in diameter. Polyimide (PI) substrates were spin coated with Ag ion ink and were subsequently heat treated to form AgNP coatings. The Ag oxide was formed by O2 plasma treatment to reduce the light absorbance by AgNPs. ITO was dc magnetron sputter-deposited atop the AgNPs. The ITO on the AgNPs was crystalline grown primarily with (2 2 2) growth orientation. This contrasts to the typical microstructure of ITO grown on the polymer, which is that growing c-ITO nucleates are embedded in an amorphous ITO (a-ITO) matrix like a particulate composite. The surface roughness of ITO on AgNPs was as small as the ITO on PI without AgNPs. The crystalline nature of the ITO on the AgNP-coated polymer resulted in the decrease of electric resistivity (q) by 65% compared to that of ITO on the bare PI. Furthermore, an electric resistivity change (Dq) of the ITO on the AgNPs was only 8% at a bending radius (rb) down to 4 mm, whereas the ITO on the non-coated polymer became almost insulating at an rb of 10 mm, owing to a drastic increase in the number of cracks. To validate the potential application in the displays, flexible organic light emitting diodes (f-OLEDs) were fabricated on the ITO on AgNPs and the performances was compared with the f-OLED on ITO on the bare PI. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As wearable and flexible electronics have been focused on for various applications, flexible polymer films coated with a transparent conductive oxide (TCO) electrode have taken considerable attention as substrates [1,2]. Particularly, ITO has been the most extensively used TCO in various flexible opto-electronic devices due to its superior functional properties to other TCO materials [3]. However, other transparent conductive materials to replace ITO have been extensively studied due to the issues of indium scarcity and the inherent brittleness of ITO, limiting the flexibility of the devices [4]. Nanostructured materials with a high surface-to-volume ratio, such as nanowires (NWs), graphene, carbon nanotubes (CNTs), and nanoparticles (NPs), have superior conductivity and optical transmittance compared to conventional materials [5–8]. These ⇑ Corresponding author. Tel.: +82 221235834; fax: +82 231235375. E-mail address: [email protected] (J.-W. Park). http://dx.doi.org/10.1016/j.jallcom.2014.09.159 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
materials are also highly flexible and are considered to be promising candidates for the replacement of ITO, as electrodes for flexible displays and energy devices [3,9]. However, ITO is still used, particularly as the anode in OLEDs for large area flexible displays by major display companies because the ITO films can be reproducibly prepared over large areas at low cost [9,10], that is, on a large scale by a roll-to-roll process [10]. According to our previous studies, the mechanical stability of ITO on polymer substrates is greatly enhanced as the degree of crystallinity increases [11,12]. As amorphous ITO is generally grown on polymer substrates without thermal annealing, thinmetal interlayers were used to act as crystalline seed layers [13,14]. However, the thickness of the interlayers should be greater than a certain threshold to attain good mechanical properties, and the optical transmittance decreases with increasing thickness [15]. When the thin metal interlayers with oxides formed on the surface were inserted between TCOs, reduced reflection on the metal surfaces in the center leads to improved optical properties [16]. However, the film production requires multiple deposition steps
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R.E. Triambulo et al. / Journal of Alloys and Compounds 620 (2015) 340–349 Table 1 Sample descriptions in detail. Sample name
ITO thickness (nm)
100ITO (reference) 50ITO (reference) 100ITO-NP 50ITO-NP 50ITO-ptNP
100 50 100
Spin coating at 3500 rpm
Heat treatment
O2 plasma
Ag ink concentration (wt%)
Temp. (°C)
Time (min)
Power (W)
Time (sec)
–
–
–
–
–
–
–
1.0
120
6 70
30
50
including oxygen plasma treatment, which still limits its application to large-scale coating processes. In our previous investigation [17]. Ag nano-particles (AgNPs), instead of continuous films, were coated onto the surfaces of polymer substrates, and the crystalline ITO with two major crystalline orientations were grown. The crystalline ITO grown on AgNPs along one direction, (2 2 2), formed a domain-like structure embedded in the matrix ITO with the other crystalline orientation, (4 0 0) [17]. The flexibility of ITO was highly improved as the propagating cracks were blocked by the domains. However, optical transmittance (T) of the ITO was degraded due to the increased absorption by AgNPs at the interfaces [17]. The decreased T could be improved by forming oxides on the surface of AgNPs using O2 plasma treatment [18]. However, the major drawback of the hybrid-structured ITO is the surface roughness (Rrms) increased by more than an order compared to ITO grown on bare substrate [17,18]. The particle diameter was 40 nm on average, and the surface roughness of the coated polymer substrate was reflected on the ITO surfaces [17]. Although the AgNPs were partially embedded into the polymer substrates by hot pressing, the roughness was little reduced [17,18]. The highly improved electrical and mechanical properties of the hybrid-structured ITO enhance the device performance and flexibility, which was validated in the previous study by fabricating f-OLEDs on the hybrid ITO [18]. However, the surface chemical and physical status of ITO such as Rrms significantly affects the device long-term reliability as it hinders good interfacial contact with organic layers coated atop ITO in devices [3,19]. The contact
problem becomes worse when the device is bent. Furthermore, the protrusions can cause electric short in the devices [10]. In this study, we proposed the use of Ag ink and a controlled process to reduce Ag ions to fine AgNPs (with a particle diameter of 20 nm on average) onto a polymer substrate. The spin-coating and post-thermal heat treatment processes were optimized for the AgNPs to be uniformly distributed on the substrate without significant agglomeration. Compared to the previous studies [17,18], the particle size was highly reduced; hence, the hot-pressing step could be removed. In addition, using an O2 plasma treatment, a silver oxide layer was formed on the surfaces of the AgNPs to further improve the optical transmittance of the ITO deposited atop AgNPs [5]. ITO was deposited using a DC magnetron sputtering system. The Rrms and film microstructures were analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HR TEM), respectively. The mechanical stability of the ITO was evaluated using a bending test, and f-OLEDs were fabricated on the hybrid ITO to validate the applicability of this nanostructured material as a transparent electrode. 2. Experimental procedures Commercial Ag ion ink (Inktec Co., Ltd.) was employed as the ion source to reduce to AgNPs. As-received 10 wt% Ag ion content ink was diluted to 0.1, 0.5, and 1.0 wt% using isopropyl alcohol. Then, the diluted ink was spin coated at 3,500 rpm onto 200lm-thick colorless PI substrates (L-3430, Mutsubishi Gas Chemical Co., Inc.) inside a glove box under a N2 atmosphere. The coated ink was then heat treated at 120 °C for 6 min to reduce the Ag ions to AgNPs with a diameter of 20–30 nm and to completely evaporate any residual solvent. The Ag ions in the ink are stabilized by organic
Fig. 1. Schematic diagrams showing the fabrication process of hybrid-structured ITO with AgNPs.
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Fig. 2. FE-SEM images showing the distribution of AgNPs on a PI substrate using (a) 0.1, (b) 0.5, and (c) 1.0 wt% Ag ion ink spin casted at 3500 rpm and subsequently heat treated at 120 °C for 6 min; FE-SEM surface images after the application of shear stress in the adhesion test of (d) AgNP-coating from the 1.0 wt% Ag ion ink and (e) the AgNPs whose surface was O2 plasma-treated. In (d) and (e), the applied vertical load of the cotton swabs were increased 33.8 g. Under the shear load less than 33.8 g, plasma-treated AgNPs were not removed from the PI surface.
Fig. 3. AFM analysis results for (a) ITO without an AgNP coating, (b) ITO on a 0.1 wt% AgNP coating, (c) ITO on a 0.5 wt% AgNP coating, and (d) ITO on a 1.0 wt% AgNP coating, where the number in parentheses corresponds to Rrms and, the thickness of ITO is 100 nm. complexes to make soluble and stable in organic solvents. During the coating process, heat is used to evaporate the organic complexes and solvent, and the organic complexes (not Ag ions) are oxidized at the same time. The oxidized complexes donate electrons to reduce Ag ions into neutral metal atoms. Colorless PI was chosen as the substrate because of its relatively high glass transition temperature (Tg), approximately 300 °C, which is suitable for the heat treatment temperatures employed in this study. The AgNP coatings were treated
with O2 plasma at 70 W for 30 s (Table 1) to decrease the optical absorbance by forming a silver oxide layer on the surface of the NPs, thus improving T. Only for conditions of 70 W and 30 s, T was improved while maintaining similar electrical resistivity (q) and mechanical properties of the hybrid ITO. Based on our experimental results for process optimization, other combinations of plasma power and treatment time degraded either the electrical conductivity or the mechanical strength.
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Fig. 4. (a) Variation in the optical transmittance of 50ITO-ptNP in comparison to 50ITO-NP within visible ranges, (b) changes in the optical absorbance spectra of the AgNPs before and after O2 plasma treatment, and (c) results from wide- and narrow-scan XPS analysis of the O2-plasma-treated AgNPs compared with untreated AgNPs.
AgNPs act as the crystalline seed layer of ITO; hence, the chemical and microstructural statuses of the Ag oxides formed on the surface of AgNPs will significantly affect the microstructure of ITO that determines both the electrical conductivity and mechanical strength of ITO. The power and time of the O2 plasma treatment seem to be the major factors for the chemical and physical statuses of the Ag oxides. The experimental verification of the relationship between the thickness, stoichiometry, composition, and microstructure of the Ag oxides and the treatment conditions is one of our on-going and future works. ITO thin films with thicknesses of 50 and 100 nm were deposited on bare PI (as reference samples) and AgNP-coated PI substrates using a DC magnetron sputtering system at a substrate temperature of 75 °C. The ITO sputtering target (Advanced Nano Product Corp., Daejun, Korea) has 99.99% purity and the composition of In2O3:SnO2 is 90:10. The sample descriptions are summarized in Table 1, and the fabrication process of the hybrid ITO is schematically described in Fig. 1. 100ITO-NP and 50ITO-NP in Table 1 are the 100 nm and 50 nm-thick ITO on AgNPs without O2 plasma treatment, respectively. 50ITO-ptNP is the 50 nm-thick ITO grown on O2 plasma-treated AgNPs. Electrical properties of the samples such as q values and charge carrier density were measured with a four-point probe and Hall measurement, respectively. The spectra of T and absorbance of the thin films were measured and a UV–Visible spectrophotometer. In addition to the absorbance data, the formation of silver oxide
on the AgNPs after O2 plasma treatment was assessed by comparing the XPS analysis results of the untreated and treated AgNP coatings. The mechanical stability of ITO was tested with multiple specimens per each sample by outward bending at a decreasing radius to 4 mm by placing the samples between two vertical plates with a separation distance that varied at a rate of 5 mm s 1. The bending stability of the ITO thin films was evaluated by the ratio of the change in electrical resistivity (Dq) after bending to q before bending (q0). While handling the AgNP-coated substrates, the density and uniformity of the AgNPs may change unless the adhesion with the substrate is significantly low. To test the adhesion of AgNPs and O2 plasma- treated AgNPs, a simple test was designed. Samples fixed on a glass plate were moved at a constant speed under cotton tips of swabs fixed by a holder. Lowering the height of the holding clip, the vertical load applied onto the sample surface was increased and measured using a microgram scale. After wiping the surface of the sample with the cotton tip, the changes in the morphologies of the AgNPs were observed by FE-SEM. AFM and field-emission scanning electron microscopy (FE-SEM) were used to evaluate the surface morphology. The microstructures of the homogeneous ITO and the ITO with the AgNP interlayer were determined using X-ray diffractometry (XRD) and HR TEM. The crystallographic textures of the ITO without and with AgNPs were determined by conducting a fast Fourier transform (FFT) on the HR TEM lattice images using Gatan Digital Micrograph™ software. Then, from the
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Table 2 Electrical resistivity (q0) and optical transmittance (T) values. Samples
Thickness of ITO (nm)
Electrical properties Resistivity, q0, (10
100ITO (reference) 50ITO (reference) PI (1.0 wt% AgNP-coated) 100ITO-NP 50ITO-NP PI (1.0 wt%, plasma-treated AgNP-coated) 50ITO-ptNP a
100 50 – 100 50 – 50
1.11 1.16 Too large/insulator 1.16 1.23 Too high/insulator 0.43
3
X cm)
Sheet resistance, Rs, (X/sq.) 111 232 Too large/insulator 116 246 Too high/insulator 86
Maximum optical transmittancea, Tmax, within visible range (%) 88 92 63 66 65 94 74
Tmax are the values that do not take the T of bare PI (90%) into account.
FFT diffraction patterns, the ratios of the principal spot spacing and the corresponding separation angles were calculated and measured, respectively. By comparing the ratios and angles with standard indexed diffraction patterns, the plane orientations were identified. Red f-OLEDs were fabricated on selected ITO samples to evaluate device operation using the flexible ITO reported herein. The f-OLED structure consists of the ITO thin film (which acted as the anode), a hole injection layer (HIL), an emission layer (EML), and an Al thin-film cathode. Commercially available PEDOT:PSS (Heraeus Clavios™ PH 500) and active polymer (Merck liviluxÒ Polymer red) were used as the HIL and EML, respectively. The detailed fabrication processes of the f-OLEDs can be found elsewhere [18,20]. The current density–voltage (J–V) curve characteristics of the OLEDs were measured using a Kiethley 2400 sourcemeter.
3. Results and discussion The FE-SEM images in Fig. 2a–c compare the distribution of AgNPs spin-coated onto the PI substrate, which was then heat treated, as Ag ink concentration was increased from 0.1 to 1.0 wt%. As shown in Fig. 2a, the AgNPs were grouped into domains at an ink concentration of 0.1 wt%. The inset in Fig. 2a illustrates the networked AgNPs (located inside the white spots shown in the low-magnification FE-SEM image). Increasing the ink concentration resulted in a decreased distance between the AgNP domains, as shown by the transition between Fig. 2a and b. Nearly uniform AgNP distributions were achieved using 1.0 wt% ink (Fig. 2c), as indicated by there being no detectable AgNP domains (unlike in Fig. 2a) or black spots (unlike in Fig. 2b). Compared to the distribution of larger AgNPs (>40 nm in diameter) dried from Ag Paste (0.36 wt% AgNPs) in the previous study [17], more uniform distribution even without local agglomeration was attained with the AgNPs reduced from the ink of higher concentration (1.0 wt%). As shown in Fig. 2c, the particle size is less than 20 nm, which is less than a half the average particle size of the AgNPs dried from the paste in the previous study [17]. The results of the adhesion tests by applying shear loads are presented in Fig. 2d and e. The tests and FE-SEM analysis were repeated, increasing the applied load. The load was increased until the plasma treated AgNPs were removed, which was 33.8 g. The adhesion strength of the plasma-treated AgNPs was stronger than untreated particles (Fig. 4e). Less than 33.8 g, the untreated AgNPs were detached as presented in Fig. 2d. However, the density and uniformity of the coated particles were not changed during the sample handling processes as shown in Fig. 2d and e. Under 33.8 g, the treated AgNPs were removed from the PI surface, but the PI surface was wrinkled and torn off perpendicular to the shearing direction, which may indicate that the adhesion between the treated AgNPs and PI is as high as the cohesion strength of PI. As crystalline seed layers, we selected the AgNP coating of 1.0 wt% ink over the other two concentrations based on the atomic force microscopy (AFM) results shown in Fig. 3. The surface morphology and Rrms (i.e., the values given in parentheses below the Ag concentrations) of the ITO deposited on PI with and without the AgNP coating are compared in Fig. 3a–d. The AFM analysis in Fig. 3a–d suggest that the ITO surface was smoother when it was
deposited on AgNPs resulting from spin casting with 1.0 wt% ink than was the ITO in the absence of AgNPs. Compared to the Rrms of ITO on AgNPs of 40 nm in diameter [17], Rrms of the ITO on AgNPs reduced from 1 wt% ink (Fig. 3d) is reduced by 80%. As shown in Fig. 3b and c, black pits were observed in the AFM scan results of the ITO for Ag ion ink concentrations of 0.1 and 0.5 wt%, and these pits seem to contribute to the larger Rrms. For the samples 50ITO-NP and 50ITO-ptNP (Table 1), the same AFM analysis as in Fig. 3 was done. The Rrms of ITO on the untreated AgNPs (50ITO-NP) is 2.418 nm. Compared to 100ITO-NP in Fig. 3d, the surface of 50ITO-NP is rougher because the degree of the reflection of the AgNP-coating surfaces increases with decreasing the ITO film thickness. The O2 plasma treatment on the AgNPs seemed not to affect the surface roughness of ITO. The Rrms of 50ITO-ptNP is 2.608 nm. Compared to the Rrms of 50ITO-NP, the difference is almost negligible. According to the results in Table 2, there were negligible differences between the q0 values of 100ITO, 50ITO and 100ITO-NP, 50ITO-NP, respectively. However, the maximum T (Tmax) within the visible range decreased almost by 25–29% for the ITO on AgNPs (100ITO-NP and 50ITO-NP) compared to the ITO on bare PI (100ITO and 50ITO) because the Tmax of the AgNP-coated PI decreased by 30% compared to that of bare PI (Table 2). The q0 value of the AgNP coating was omitted in Table 2 because the experimental values were beyond the limit of the four-point probe used to measure the property of this material, which indicates that the NP coatings are discontinuous. To improve the optical properties of the ITO on AgNPs, O2 plasma treatment of AgNPs was performed to form a silver oxide layer on the AgNPs, thus decreasing the optical absorbance [5]. As shown in Table 2, Tmax for 1.0 wt%, plasma treated (pt) -AgNPcoated PI was improved by 27% compared to 1.0 wt% AgNP-coated PI. Also, 50ITO-ptNP was improved considerably compared to that for 50ITO-NP throughout the visible ranges as presented in Fig. 4a. The absorbance of ITO and Tmax on the AgNPs also highly improved when the AgNPs had been treated with O2 plasma, as shown in Fig. 4b and Table 2. In Fig. 4b, the optical absorbances of pristine and plasmatreated AgNPs under the conditions of sample 50ITO-ptNP are compared. Fig. 4b shows a significant decrease in the optical absorbance owing to the plasma treatment. Fig. 4c shows wide- and narrow-scan XPS results for the plasma-treated AgNPs on PI. For the wide-scan XPS results of the plasma-treated AgNPs shown at the top of Fig. 4c, the Ag3d and O1s spectra were analyzed in detail and compared with those of the untreated AgNPs, as shown in the narrow-scan peaks at the bottom of Fig. 4c. The Ag3d spectra (bottom left of Fig. 4c) of the AgNP coatings indicate a 0.5-eV shift toward lower binding energies as a result of the plasma treatment. This result agrees well with previous studies [5], in which the formation of silver oxide on AgNPs resulted in a peak shift in the Ag3d spectrum toward a lower binding energy. Using Spectral Data Processor software (XPS
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Fig. 5. The outward bending test results (averaged values with testing multiple specimens for each sample) for (a) 100ITO and 100ITO-NP, (b) 50ITO and 50ITO-NP, and (c) 50ITO-ptNP.
Fig. 6. XRD analysis results for (a) the 100 nm-ITO samples described in Table 1, (b) the 50 nm-ITO samples described in Table 2, (c) AgNPs without and with O2 plasmatreatment (the concentration of Ag ion ink for spin-coating was intentionally increased to 5 wt% to increase the peak intensities).
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Fig. 7. Bright field images and HR TEM analysis results for (a) 50ITO and (b) 50ITO-ptNP; (c) a STEM image and EDS analysis results of 50ITO-ptNP.
International, Inc.), the O1s spectra were found to be composed of contributions from several oxygen species, as shown at the bottom right side of Fig. 4c. The contributions to the O1s spectra at 530.96–531.13 eV and 531.87–532.08 eV were attributed to the oxygen atoms in Ag2CO3 and absorbed CO2 and dissolved oxygen, respectively. In addition, the peaks at approximately 529.5 eV correspond to the oxygen in Ag2O. Our results are in agreement with results from previous studies [5]. As shown in the O1s narrow-scan data at the bottom right side of Fig. 4c, silver oxide was present on the NPs prior to
O2 plasma treatment. After plasma treatment, the Ag2O content increased relative to the initial amount and relative to the levels of the other compounds present, decreasing the optical absorbance of the AgNPs in the visible range. The thickness of Ag oxides formed on the surface of AgNPs could not be measured. However, the thickness of the oxide layers is estimated to be less than 5 nm based on the following two facts that: (1) the observation depth of the XPS machine we used to analyze Ag oxides is 5 nm from the top surface of the sample materials; (2) according to the XPS analysis results in Fig. 4c, the peaks of Ag as well as Ag oxide were attained.
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Fig. 7 (continued)
The q0 and Rs of the 50ITO-ptNP also decreased significantly in comparison to other samples, as shown in Table 2. As the trends in q0 of 100ITO and 50ITO and of 100ITO-NP and 50ITO-NP shows in Table 2, the increase in q0 with decreasing hf is a general trend for thin films [21]. However, q0 of 50ITO-ptNP is less than a half of q0 of 100ITO and 100ITO-NP, and is even smaller than those of 50ITO and 50ITO-NP by more than 65%. Based on Hall measurement results, the improved electrical property seems to result from the increased charge density for 50ITO-ptNP compared to the ITO on bare PI. The sheet charge concentration of 50ITO-ptNP was 9.86 1015 /cm2 according to Hall measurement results, which is more than five times greater than that of 100ITO (1.85 1015 /cm2). The effect of the AgNPs on the bending reliability of ITO was evaluated by measuring Dq/q0. q0 measured for the samples are compared in Table 2. The bending test results for 100ITO and 100ITO-NP are presented in Fig. 5a. When the samples were bent outward (as schematically displayed in the inset image in Fig. 5a), 100ITO-NP had a significantly larger bending stability than 100ITO. The q0 of 100ITO changed by two orders of magnitude at an rb of 8 mm and became almost insulating as rb decreased further. In contrast, the q0 value of the 100ITO-NP sample only changed by 15.5% at an rb of 4 mm. The commercial standard for the minimum bending radius of TCO on flexible substrates without a drastic increase in q0 is 5 mm, which has not been achieved with any TCO [22,23]; thus, the value of 15.5% at 4 mm clearly confirms the important role of the AgNP interlayers in enhancing the flexibility of ITO.
Furthermore, it should be noted that the standard deviations were smaller for the 100ITO-NP sample than for 100ITO at all values of rb. When the thickness (hf) of the ITO layer was reduced to 50 nm (50ITO and 50ITO-NP in Table 1), the change in the q0 of 50ITO-NP was only 11.8% for an rb of 4 mm, whereas a change of two orders of magnitude was observed for 50ITO at the larger rb of 8 mm. The bending stability of ITO was further enhanced by Ag oxide formation on the AgNPs. As shown in Fig. 5c, the change in q0 at a bending radius of 4 mm is only by 8.3%. 50ITO-ptNP shows the highest bending stability among the samples in Table 1. According to the XRD analysis of the film microstructures (Fig. 6a and b), the crystallinity seems to be one of the major factors for the highly improved bending stability of the ITO grown on the AgNPs. In our previous studies, the effect of an increased degree of crystallinity on enhancing the coherent strength of ITO was experimentally proven [24]. As shown in Fig. 6a and b, the ITO samples have four major preferred growth orientations in common, and (2 2 2) is the most preferred direction, corresponding to the highest peak intensity. The electrical and mechanical properties of ITO improve as the (2 2 2) growth orientation becomes more dominant [25,26]. The XRD results in Fig. 6a demonstrate that the peak intensities of 100ITO-NP were greater than those of 100ITO in all orientations. The degree of crystallinity of the 50ITO-NP sample also increased, particularly in the (2 2 2) and (4 0 0) preferred orientations, compared to 50ITO (Fig. 6b). The XRD results in Fig. 6b also reveal that 50ITO-ptNP grew as crystalline ITO, but with an almost uniform preferred growth
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Fig. 8. (a) Flexible OLEDs fabricated on 100ITO and 50ITO-ptNP samples (flat and bent during device operation). The inset image on top shows the schematic structure of the f-OLED on the ITO with AgNPs (50ITO-ptNP), (b) J–V curves for the OLEDs in (a).
orientation, (2 2 2), whereas the other samples exhibited more than four orientations, as shown in Fig. 6a and b. The difference in the film microstructure of 50ITO-ptNP compared to the other samples in Table 1 was confirmed by Fig. 6c and TEM analysis in Fig. 7a–c. As presented in Fig. 6c, the major crystalline orientation of AgNPs is (1 1 1). Two other peaks of the orientations, (2 0 0) and (2 2 0), also appeared, but the intensities were negligible compared to (1 1 1). After the O2 plasma-treatment, the most preferred growth orientation of the Ag2O formed on the surface of AgNPs was found to be (1 1 1). According to Fig. 7a, the 50ITO has a composite-like structure in which crystalline ITO domains are embedded in predominantly amorphous ITO. The selected area diffraction pattern (SADP) for a bright field plan-view TEM image (Region A in Fig. 7a) confirms the composite-like structure of 50ITO. The domains (Region B in Fig. 7a) have various crystalline orientations as demonstrated by the HR TEM image (Region C in Fig. 7a) and fast Fourier transformed (FFT) SADP shown in Fig. 7a. In contrast, random network-like microstructures were observed in the plan-view TEM images of 50ITO-ptNP, as presented in Region A in Fig. 7b and a scanned TEM image (STEM) in Fig. 7c. Based on the bright field TEM image (Regions A and B in Fig. 7b) and the HR TEM analysis (Regions C and D shown in Fig. 7b), both the dark and white networks have a primarily (2 2 2) crystalline orientation. According to the STEM image and the energy dispersive X-ray spectroscopy (EDS) analysis results in Fig. 7c, the white and black networked regions that correspond to the black and white regions, respectively, in the bright-field image (Fig. 7b) are
the ITO grown on plasma treated (pt)-AgNPs and the ITO on PI, respectively. It should be noted that ITO grown from PI in the 50ITO-ptNP sample also have a (2 2 2) texture as shown in Fig. 7b, which suggests that (2 2 2) domains grown on pt-AgNPs also promote the growth of the same texture between two separated pt-AgNPs. This trend may be attributed to the fact that the AgNPs are sufficiently close to each other, where the separation distance is smaller than the diameter of the NPs. This mostly (2 2 2) texture resulting in the highly enhanced flexibility without sacrificing the functional properties could not be achieved in ITO on the continuous Ag interlayers [15]. Also, the smaller sizes of the crystallized domains grown on pt-AgNPs than the grains of homogeneous ITO contribute to the improved strength and flexibility of the hybrid ITO. In Fig. 8a, the f-OLEDs fabricated on PI with the 100ITO and 50ITO-ptNP electrodes are shown in the flat and bent configurations used during operation. The schematic image at the top of Fig. 8a displays the structure of the OLED. As described in section II, 16 cells were constructed for each f-OLED as shown in Fig. 8a, and we confirmed that all of the cells functioned normally. The J–V curves shown in Fig. 8b were constructed from averaged values for the 16 cells in the flat configuration. The cells on 50ITO-ptNP functioned in the same manner when bent, as indicated in Fig. 8a, while the OLEDs on 100ITO were not lit on bent. As clearly shown in Fig. 8b, the J–V characteristics of OLED 50ITO-ptNP are equivalent to or better than those of the f-OLED on 100ITO (the conventional, commercially available ITO for
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f-OLEDs) at higher values of V even though the ITO thickness of 50ITO-ptNP is half the 100ITO. The turn-on voltages (Von) of f-OLEDs on 50ITO-ptNP were measured to be almost similar to those on 100ITO, which were 3.0 V at 1 mA/cm2 and 2.1 V at 1 cd/m2. However, J of the f-OLEDs on 50ITO-ptNP increased faster and reached its maximum at lower voltages than that of f-OLEDs on 100ITO, as presented in Fig. 8b. 4. Conclusions In this study, we constructed AgNPs and ITO hybrid-type transparent conductive electrode (TCE) with a significantly greater bending stability than conventional homogeneous ITOs and the previously developed hybrid-structured ITO on AgNPs. The diameter of the AgNPs was as small as 20 nm, but uniformly distributed AgNP coatings without significant agglomeration could be attained on PI by optimizing the concentration of the ink (Ag ion sources) and coating processes. The surface roughness was even smoother than the ITO without AgNPs. The degradation of optical transmittance by the metal nano-particles, which is a critical limitation of using AgNPs in TCEs, was resolved by O2 plasma treatment of the powder surfaces under optimized conditions. The treatment increased the formation of Ag oxides on the powder surfaces, thus decreasing the absorbance of the AgNPs. By combining the O2 plasma treatment of AgNPs with our optimized DC sputtering conditions, we also decreased the ITO film thickness to 50 nm without sacrificing the functional and mechanical properties of the 100-nm-thick ITO. The flexibility and electrical resistivity of the 50-nm-thick ITO on the plasma-treated AgNPs substantially improved in comparison to conventional 100-nm-thick ITO and previously developed hybrid ITO with AgNPs. This result was validated by testing the performance of flexible OLEDs on the above-mentioned 50-nm-thick ITO. The f-OLED in the bent state was also comparable to that in the flat state, confirming the potential applications of these hybrid films as electrodes in various flexible electronics. Acknowledgments This research was supported by a Grant from the Fundamental R&D Program for the Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy, Republic of Korea and a Grant of Business for Cooperative R&D between Industry, Academy, and Research Institute (Grants No. C0188503) funded by Korea Small and Medium Business Administration in 2014. References [1] C.-T. Chi, I.C. Cheng, J.-Z. Chen, Bandgap tuning of MgZnO in flexible transparent n(+)-ZnO:Al/n-MgZnO/p-CuAlOx:Ca diodes on polyethylene terephthalate substrates, J. Alloys Comp. 544 (2012) 111–114. [2] Y. Hu, H. Zhu, J. Wang, Z. Chen, Synthesis of layered birnessite-type manganese oxide thin films on plastic substrates by chemical bath deposition for flexible transparent supercapacitors, J. Alloys Comp. 509 (2011) 10234–10240.
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[3] D. Langley, G. Giusti, C. Mayousse, C. Celle, D. Bellet, J.-P. Simonato, Flexible transparent conductive materials based on silver nanowire networks: a review, Nanotechnology 24 (2013) 1–20. [4] Z. Chen, B. Cotterell, W. Wang, E. Guenther, S.-J. Chua, A mechanical assessment of flexible optoelectronic devices, Thin Solid Films 394 (2001) 201–205. [5] Y. Han, R. Lupitskyy, T.-M. Chou, C.M. Stafford, H. Du, S. Sukhishvili, Effect of oxidation on surface-enhanced Raman scattering activity of silver nanoparticles: a quantitative correlation, Anal. Chem. 83 (2011) 5873–5880. [6] J.-Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Solution-processed metal nanowire mesh transparent electrodes, Nano Lett. 8 (2008) 689–692. [7] N.O. Weiss, H. Zhou, L. Liao, Y. Liu, S. Jiang, Y. Huang, X. Duan, Graphene: An emerging electronic material, Adv. Mater. (Weinheim Ger.) 24 (2012) 5782– 5825. [8] M. Layani, S. Magdassi, Flexible transparent conductive coatings by combining self-assembly with sintering of silver nanoparticles performed at room temperature, J. Mater. Chem. 21 (2011) 15378–15382. [9] G. Jo, M. Choe, S. Lee, W. Park, Y.H. Kahng, T. Lee, The application of graphene as electrodes in electrical and optical devices, Nanotechnology 23 (2012) 1–19. [10] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 808–816. [11] J.-W. Park, G. Kim, S.-H. Lee, E.-H. Kim, G.-H. Lee, The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates, Surf. Coat. Technol. 205 (2010) 915–921. [12] J.-W. Park, S.-H. Lee, C.-W. Yang, Investigation of the interfacial adhesion of the transparent conductive oxide films to large-area flexible polymer substrates using laser-induced thermo-mechanical stresses, J. Appl. Phys. 114 (2013). 063513-1–063513-8. [13] B. Sim, E.-H. Kim, J. Park, M. Lee, Highly enhanced mechanical stability of indium tin oxide film with a thin Al buffer layer deposited on plastic substrate, Surf. Coat. Technol. 204 (2009) 309–312. [14] J.-I. Han, Stability of Externally Deformed ITO Films, in: G.P. Crawford (Ed.), Flexible Flat Panel Displays, John Wiley & Sons, West Sussex, England, 2005, pp. 121–132. [15] C.-W. Yang, J.-W. Park, The cohesive crack and buckle delamination resistances of indium tin oxide (ITO) films on polymeric substrates with ductile metal interlayers, Surf. Coat. Technol. 204 (2010) 2761–2766. [16] W. Wang, M. Song, T.-S. Bae, Y.H. Park, Y.-C. Kang, S.-G. Lee, S.-Y. Kim, D.H. Kim, S. Lee, G. Min, G.-H. Lee, J.-W. Kang, J. Yun, Transparent ultrathin oxygendoped silver electrodes for flexible organic solar cells, Adv. Funct. Mater. 24 (2014) 1511–1561. [17] R.E. Triambulo, J.-H. Kim, M.-Y. Na, H.-J. Chang, J.-W. Park, Highly flexible, hybrid-structured indium tin oxides for transparent electrodes on polymer substrates, Appl. Phys. Lett. 102 (2013). 241913-1–241913-5. [18] R.E. Triambulo, H.-G. Cheong, H. Zhou, G.-H. Lee, J.-W. Park, Hybrid-structured indium tin oxide with Ag nanoparticles as crystalline seeds for transparent electrode with enhanced flexibility and its application to organic light emitting diodes, Jpn. J. Appl. Phys. 53 (2014). 05FB13-01–05FB13-07. [19] Y.-F. Wu, C.-H. Chen, Effect of passive film on electrochemical surface treatment for indium tin oxide, J. Alloys Comp. 550 (2013) 263–267. [20] H.-G. Cheong, R.E. Triambulo, G.-H. Lee, I.-S. Yi, J.-W. Park, Silver nanowire network transparent electrodes with highly enhanced flexibility by welding for application in flexible organic light-emitting diodes, ACS Appl. Mater. Interfaces (2014) 7846–7855. [21] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809–817. [22] K. Baek, K. Jang, Y.J. Lee, K. Ryu, W. Choi, D. Kim, J. Yi, Improvement of mechanical reliability by patterned silver/Indium–Tin–Oxide structure for flexible electronic devices, Thin Solid Films 531 (2013) 349–353. [23] J.W. Lim, S.I. Oh, K. Eun, S.H. Choa, H.W. Koo, T.W. Kim, H.K. Kim, Mechanical flexibility of ZnSnO/Ag/ZnSnO films grown by roll-to-roll sputtering for flexible organic photovoltaics, Jpn. J. Appl. Phys. 51 (2012) 115801. [24] E.H. Kim, C.W. Yang, J.W. Park, The crystallinity and mechanical properties of indium tin oxide coatings on polymer substrates, J. Appl. Phys. 109 (2011). 043511-1–043511-8. [25] Y. Shigesato, D.C. Paine, A microstructural study of low-resistivity tin-doped indium oxide prepared by DC magnetron sputtering, Thin Solid Films 238 (1994) 44–50. [26] Y. Shigesato, S. Takaki, T. Haranoh, Electrical and structural properties of low resistivity tin-doped indium oxide-films, J. Appl. Phys. 71 (1992) 3356–3364.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A transparent conductive oxide electrode with highly enhanced flexibility achieved by controlled crystallinity by incorporating Ag nanoparticles on substrates Ross E. Triambulo a, Hahn-Gil Cheong a, Gun-Hwan Lee b, In-Sook Yi c, Jin-Woo Park a,⇑ a b c
Department of Materials Science and Engineering, Yonsei University, Seoul, Republic of Korea Advanced Thin Film Research Group, Korea Institute of Materials Science (KIMS), Changwon, Republic of Korea R&D Center, InkTec Co., Ltd., Ansan, Republic of Korea
a r t i c l e
i n f o
Article history: Received 30 June 2014 Received in revised form 16 September 2014 Accepted 21 September 2014 Available online 30 September 2014 Keywords: Hybrid TCO Indium tin oxide (ITO) Ag nanoparticle Flexible organic light emitting diode (OLED) Interface
a b s t r a c t We report the synthesis of highly flexible indium tin oxide (ITO) on a polymer substrate whose surface was engineered by oxide-coated Ag nanoparticles (AgNPs) smaller than 20 nm in diameter. Polyimide (PI) substrates were spin coated with Ag ion ink and were subsequently heat treated to form AgNP coatings. The Ag oxide was formed by O2 plasma treatment to reduce the light absorbance by AgNPs. ITO was dc magnetron sputter-deposited atop the AgNPs. The ITO on the AgNPs was crystalline grown primarily with (2 2 2) growth orientation. This contrasts to the typical microstructure of ITO grown on the polymer, which is that growing c-ITO nucleates are embedded in an amorphous ITO (a-ITO) matrix like a particulate composite. The surface roughness of ITO on AgNPs was as small as the ITO on PI without AgNPs. The crystalline nature of the ITO on the AgNP-coated polymer resulted in the decrease of electric resistivity (q) by 65% compared to that of ITO on the bare PI. Furthermore, an electric resistivity change (Dq) of the ITO on the AgNPs was only 8% at a bending radius (rb) down to 4 mm, whereas the ITO on the non-coated polymer became almost insulating at an rb of 10 mm, owing to a drastic increase in the number of cracks. To validate the potential application in the displays, flexible organic light emitting diodes (f-OLEDs) were fabricated on the ITO on AgNPs and the performances was compared with the f-OLED on ITO on the bare PI. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction As wearable and flexible electronics have been focused on for various applications, flexible polymer films coated with a transparent conductive oxide (TCO) electrode have taken considerable attention as substrates [1,2]. Particularly, ITO has been the most extensively used TCO in various flexible opto-electronic devices due to its superior functional properties to other TCO materials [3]. However, other transparent conductive materials to replace ITO have been extensively studied due to the issues of indium scarcity and the inherent brittleness of ITO, limiting the flexibility of the devices [4]. Nanostructured materials with a high surface-to-volume ratio, such as nanowires (NWs), graphene, carbon nanotubes (CNTs), and nanoparticles (NPs), have superior conductivity and optical transmittance compared to conventional materials [5–8]. These ⇑ Corresponding author. Tel.: +82 221235834; fax: +82 231235375. E-mail address: [email protected] (J.-W. Park). http://dx.doi.org/10.1016/j.jallcom.2014.09.159 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
materials are also highly flexible and are considered to be promising candidates for the replacement of ITO, as electrodes for flexible displays and energy devices [3,9]. However, ITO is still used, particularly as the anode in OLEDs for large area flexible displays by major display companies because the ITO films can be reproducibly prepared over large areas at low cost [9,10], that is, on a large scale by a roll-to-roll process [10]. According to our previous studies, the mechanical stability of ITO on polymer substrates is greatly enhanced as the degree of crystallinity increases [11,12]. As amorphous ITO is generally grown on polymer substrates without thermal annealing, thinmetal interlayers were used to act as crystalline seed layers [13,14]. However, the thickness of the interlayers should be greater than a certain threshold to attain good mechanical properties, and the optical transmittance decreases with increasing thickness [15]. When the thin metal interlayers with oxides formed on the surface were inserted between TCOs, reduced reflection on the metal surfaces in the center leads to improved optical properties [16]. However, the film production requires multiple deposition steps
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R.E. Triambulo et al. / Journal of Alloys and Compounds 620 (2015) 340–349 Table 1 Sample descriptions in detail. Sample name
ITO thickness (nm)
100ITO (reference) 50ITO (reference) 100ITO-NP 50ITO-NP 50ITO-ptNP
100 50 100
Spin coating at 3500 rpm
Heat treatment
O2 plasma
Ag ink concentration (wt%)
Temp. (°C)
Time (min)
Power (W)
Time (sec)
–
–
–
–
–
–
–
1.0
120
6 70
30
50
including oxygen plasma treatment, which still limits its application to large-scale coating processes. In our previous investigation [17]. Ag nano-particles (AgNPs), instead of continuous films, were coated onto the surfaces of polymer substrates, and the crystalline ITO with two major crystalline orientations were grown. The crystalline ITO grown on AgNPs along one direction, (2 2 2), formed a domain-like structure embedded in the matrix ITO with the other crystalline orientation, (4 0 0) [17]. The flexibility of ITO was highly improved as the propagating cracks were blocked by the domains. However, optical transmittance (T) of the ITO was degraded due to the increased absorption by AgNPs at the interfaces [17]. The decreased T could be improved by forming oxides on the surface of AgNPs using O2 plasma treatment [18]. However, the major drawback of the hybrid-structured ITO is the surface roughness (Rrms) increased by more than an order compared to ITO grown on bare substrate [17,18]. The particle diameter was 40 nm on average, and the surface roughness of the coated polymer substrate was reflected on the ITO surfaces [17]. Although the AgNPs were partially embedded into the polymer substrates by hot pressing, the roughness was little reduced [17,18]. The highly improved electrical and mechanical properties of the hybrid-structured ITO enhance the device performance and flexibility, which was validated in the previous study by fabricating f-OLEDs on the hybrid ITO [18]. However, the surface chemical and physical status of ITO such as Rrms significantly affects the device long-term reliability as it hinders good interfacial contact with organic layers coated atop ITO in devices [3,19]. The contact
problem becomes worse when the device is bent. Furthermore, the protrusions can cause electric short in the devices [10]. In this study, we proposed the use of Ag ink and a controlled process to reduce Ag ions to fine AgNPs (with a particle diameter of 20 nm on average) onto a polymer substrate. The spin-coating and post-thermal heat treatment processes were optimized for the AgNPs to be uniformly distributed on the substrate without significant agglomeration. Compared to the previous studies [17,18], the particle size was highly reduced; hence, the hot-pressing step could be removed. In addition, using an O2 plasma treatment, a silver oxide layer was formed on the surfaces of the AgNPs to further improve the optical transmittance of the ITO deposited atop AgNPs [5]. ITO was deposited using a DC magnetron sputtering system. The Rrms and film microstructures were analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HR TEM), respectively. The mechanical stability of the ITO was evaluated using a bending test, and f-OLEDs were fabricated on the hybrid ITO to validate the applicability of this nanostructured material as a transparent electrode. 2. Experimental procedures Commercial Ag ion ink (Inktec Co., Ltd.) was employed as the ion source to reduce to AgNPs. As-received 10 wt% Ag ion content ink was diluted to 0.1, 0.5, and 1.0 wt% using isopropyl alcohol. Then, the diluted ink was spin coated at 3,500 rpm onto 200lm-thick colorless PI substrates (L-3430, Mutsubishi Gas Chemical Co., Inc.) inside a glove box under a N2 atmosphere. The coated ink was then heat treated at 120 °C for 6 min to reduce the Ag ions to AgNPs with a diameter of 20–30 nm and to completely evaporate any residual solvent. The Ag ions in the ink are stabilized by organic
Fig. 1. Schematic diagrams showing the fabrication process of hybrid-structured ITO with AgNPs.
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Fig. 2. FE-SEM images showing the distribution of AgNPs on a PI substrate using (a) 0.1, (b) 0.5, and (c) 1.0 wt% Ag ion ink spin casted at 3500 rpm and subsequently heat treated at 120 °C for 6 min; FE-SEM surface images after the application of shear stress in the adhesion test of (d) AgNP-coating from the 1.0 wt% Ag ion ink and (e) the AgNPs whose surface was O2 plasma-treated. In (d) and (e), the applied vertical load of the cotton swabs were increased 33.8 g. Under the shear load less than 33.8 g, plasma-treated AgNPs were not removed from the PI surface.
Fig. 3. AFM analysis results for (a) ITO without an AgNP coating, (b) ITO on a 0.1 wt% AgNP coating, (c) ITO on a 0.5 wt% AgNP coating, and (d) ITO on a 1.0 wt% AgNP coating, where the number in parentheses corresponds to Rrms and, the thickness of ITO is 100 nm. complexes to make soluble and stable in organic solvents. During the coating process, heat is used to evaporate the organic complexes and solvent, and the organic complexes (not Ag ions) are oxidized at the same time. The oxidized complexes donate electrons to reduce Ag ions into neutral metal atoms. Colorless PI was chosen as the substrate because of its relatively high glass transition temperature (Tg), approximately 300 °C, which is suitable for the heat treatment temperatures employed in this study. The AgNP coatings were treated
with O2 plasma at 70 W for 30 s (Table 1) to decrease the optical absorbance by forming a silver oxide layer on the surface of the NPs, thus improving T. Only for conditions of 70 W and 30 s, T was improved while maintaining similar electrical resistivity (q) and mechanical properties of the hybrid ITO. Based on our experimental results for process optimization, other combinations of plasma power and treatment time degraded either the electrical conductivity or the mechanical strength.
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Fig. 4. (a) Variation in the optical transmittance of 50ITO-ptNP in comparison to 50ITO-NP within visible ranges, (b) changes in the optical absorbance spectra of the AgNPs before and after O2 plasma treatment, and (c) results from wide- and narrow-scan XPS analysis of the O2-plasma-treated AgNPs compared with untreated AgNPs.
AgNPs act as the crystalline seed layer of ITO; hence, the chemical and microstructural statuses of the Ag oxides formed on the surface of AgNPs will significantly affect the microstructure of ITO that determines both the electrical conductivity and mechanical strength of ITO. The power and time of the O2 plasma treatment seem to be the major factors for the chemical and physical statuses of the Ag oxides. The experimental verification of the relationship between the thickness, stoichiometry, composition, and microstructure of the Ag oxides and the treatment conditions is one of our on-going and future works. ITO thin films with thicknesses of 50 and 100 nm were deposited on bare PI (as reference samples) and AgNP-coated PI substrates using a DC magnetron sputtering system at a substrate temperature of 75 °C. The ITO sputtering target (Advanced Nano Product Corp., Daejun, Korea) has 99.99% purity and the composition of In2O3:SnO2 is 90:10. The sample descriptions are summarized in Table 1, and the fabrication process of the hybrid ITO is schematically described in Fig. 1. 100ITO-NP and 50ITO-NP in Table 1 are the 100 nm and 50 nm-thick ITO on AgNPs without O2 plasma treatment, respectively. 50ITO-ptNP is the 50 nm-thick ITO grown on O2 plasma-treated AgNPs. Electrical properties of the samples such as q values and charge carrier density were measured with a four-point probe and Hall measurement, respectively. The spectra of T and absorbance of the thin films were measured and a UV–Visible spectrophotometer. In addition to the absorbance data, the formation of silver oxide
on the AgNPs after O2 plasma treatment was assessed by comparing the XPS analysis results of the untreated and treated AgNP coatings. The mechanical stability of ITO was tested with multiple specimens per each sample by outward bending at a decreasing radius to 4 mm by placing the samples between two vertical plates with a separation distance that varied at a rate of 5 mm s 1. The bending stability of the ITO thin films was evaluated by the ratio of the change in electrical resistivity (Dq) after bending to q before bending (q0). While handling the AgNP-coated substrates, the density and uniformity of the AgNPs may change unless the adhesion with the substrate is significantly low. To test the adhesion of AgNPs and O2 plasma- treated AgNPs, a simple test was designed. Samples fixed on a glass plate were moved at a constant speed under cotton tips of swabs fixed by a holder. Lowering the height of the holding clip, the vertical load applied onto the sample surface was increased and measured using a microgram scale. After wiping the surface of the sample with the cotton tip, the changes in the morphologies of the AgNPs were observed by FE-SEM. AFM and field-emission scanning electron microscopy (FE-SEM) were used to evaluate the surface morphology. The microstructures of the homogeneous ITO and the ITO with the AgNP interlayer were determined using X-ray diffractometry (XRD) and HR TEM. The crystallographic textures of the ITO without and with AgNPs were determined by conducting a fast Fourier transform (FFT) on the HR TEM lattice images using Gatan Digital Micrograph™ software. Then, from the
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Table 2 Electrical resistivity (q0) and optical transmittance (T) values. Samples
Thickness of ITO (nm)
Electrical properties Resistivity, q0, (10
100ITO (reference) 50ITO (reference) PI (1.0 wt% AgNP-coated) 100ITO-NP 50ITO-NP PI (1.0 wt%, plasma-treated AgNP-coated) 50ITO-ptNP a
100 50 – 100 50 – 50
1.11 1.16 Too large/insulator 1.16 1.23 Too high/insulator 0.43
3
X cm)
Sheet resistance, Rs, (X/sq.) 111 232 Too large/insulator 116 246 Too high/insulator 86
Maximum optical transmittancea, Tmax, within visible range (%) 88 92 63 66 65 94 74
Tmax are the values that do not take the T of bare PI (90%) into account.
FFT diffraction patterns, the ratios of the principal spot spacing and the corresponding separation angles were calculated and measured, respectively. By comparing the ratios and angles with standard indexed diffraction patterns, the plane orientations were identified. Red f-OLEDs were fabricated on selected ITO samples to evaluate device operation using the flexible ITO reported herein. The f-OLED structure consists of the ITO thin film (which acted as the anode), a hole injection layer (HIL), an emission layer (EML), and an Al thin-film cathode. Commercially available PEDOT:PSS (Heraeus Clavios™ PH 500) and active polymer (Merck liviluxÒ Polymer red) were used as the HIL and EML, respectively. The detailed fabrication processes of the f-OLEDs can be found elsewhere [18,20]. The current density–voltage (J–V) curve characteristics of the OLEDs were measured using a Kiethley 2400 sourcemeter.
3. Results and discussion The FE-SEM images in Fig. 2a–c compare the distribution of AgNPs spin-coated onto the PI substrate, which was then heat treated, as Ag ink concentration was increased from 0.1 to 1.0 wt%. As shown in Fig. 2a, the AgNPs were grouped into domains at an ink concentration of 0.1 wt%. The inset in Fig. 2a illustrates the networked AgNPs (located inside the white spots shown in the low-magnification FE-SEM image). Increasing the ink concentration resulted in a decreased distance between the AgNP domains, as shown by the transition between Fig. 2a and b. Nearly uniform AgNP distributions were achieved using 1.0 wt% ink (Fig. 2c), as indicated by there being no detectable AgNP domains (unlike in Fig. 2a) or black spots (unlike in Fig. 2b). Compared to the distribution of larger AgNPs (>40 nm in diameter) dried from Ag Paste (0.36 wt% AgNPs) in the previous study [17], more uniform distribution even without local agglomeration was attained with the AgNPs reduced from the ink of higher concentration (1.0 wt%). As shown in Fig. 2c, the particle size is less than 20 nm, which is less than a half the average particle size of the AgNPs dried from the paste in the previous study [17]. The results of the adhesion tests by applying shear loads are presented in Fig. 2d and e. The tests and FE-SEM analysis were repeated, increasing the applied load. The load was increased until the plasma treated AgNPs were removed, which was 33.8 g. The adhesion strength of the plasma-treated AgNPs was stronger than untreated particles (Fig. 4e). Less than 33.8 g, the untreated AgNPs were detached as presented in Fig. 2d. However, the density and uniformity of the coated particles were not changed during the sample handling processes as shown in Fig. 2d and e. Under 33.8 g, the treated AgNPs were removed from the PI surface, but the PI surface was wrinkled and torn off perpendicular to the shearing direction, which may indicate that the adhesion between the treated AgNPs and PI is as high as the cohesion strength of PI. As crystalline seed layers, we selected the AgNP coating of 1.0 wt% ink over the other two concentrations based on the atomic force microscopy (AFM) results shown in Fig. 3. The surface morphology and Rrms (i.e., the values given in parentheses below the Ag concentrations) of the ITO deposited on PI with and without the AgNP coating are compared in Fig. 3a–d. The AFM analysis in Fig. 3a–d suggest that the ITO surface was smoother when it was
deposited on AgNPs resulting from spin casting with 1.0 wt% ink than was the ITO in the absence of AgNPs. Compared to the Rrms of ITO on AgNPs of 40 nm in diameter [17], Rrms of the ITO on AgNPs reduced from 1 wt% ink (Fig. 3d) is reduced by 80%. As shown in Fig. 3b and c, black pits were observed in the AFM scan results of the ITO for Ag ion ink concentrations of 0.1 and 0.5 wt%, and these pits seem to contribute to the larger Rrms. For the samples 50ITO-NP and 50ITO-ptNP (Table 1), the same AFM analysis as in Fig. 3 was done. The Rrms of ITO on the untreated AgNPs (50ITO-NP) is 2.418 nm. Compared to 100ITO-NP in Fig. 3d, the surface of 50ITO-NP is rougher because the degree of the reflection of the AgNP-coating surfaces increases with decreasing the ITO film thickness. The O2 plasma treatment on the AgNPs seemed not to affect the surface roughness of ITO. The Rrms of 50ITO-ptNP is 2.608 nm. Compared to the Rrms of 50ITO-NP, the difference is almost negligible. According to the results in Table 2, there were negligible differences between the q0 values of 100ITO, 50ITO and 100ITO-NP, 50ITO-NP, respectively. However, the maximum T (Tmax) within the visible range decreased almost by 25–29% for the ITO on AgNPs (100ITO-NP and 50ITO-NP) compared to the ITO on bare PI (100ITO and 50ITO) because the Tmax of the AgNP-coated PI decreased by 30% compared to that of bare PI (Table 2). The q0 value of the AgNP coating was omitted in Table 2 because the experimental values were beyond the limit of the four-point probe used to measure the property of this material, which indicates that the NP coatings are discontinuous. To improve the optical properties of the ITO on AgNPs, O2 plasma treatment of AgNPs was performed to form a silver oxide layer on the AgNPs, thus decreasing the optical absorbance [5]. As shown in Table 2, Tmax for 1.0 wt%, plasma treated (pt) -AgNPcoated PI was improved by 27% compared to 1.0 wt% AgNP-coated PI. Also, 50ITO-ptNP was improved considerably compared to that for 50ITO-NP throughout the visible ranges as presented in Fig. 4a. The absorbance of ITO and Tmax on the AgNPs also highly improved when the AgNPs had been treated with O2 plasma, as shown in Fig. 4b and Table 2. In Fig. 4b, the optical absorbances of pristine and plasmatreated AgNPs under the conditions of sample 50ITO-ptNP are compared. Fig. 4b shows a significant decrease in the optical absorbance owing to the plasma treatment. Fig. 4c shows wide- and narrow-scan XPS results for the plasma-treated AgNPs on PI. For the wide-scan XPS results of the plasma-treated AgNPs shown at the top of Fig. 4c, the Ag3d and O1s spectra were analyzed in detail and compared with those of the untreated AgNPs, as shown in the narrow-scan peaks at the bottom of Fig. 4c. The Ag3d spectra (bottom left of Fig. 4c) of the AgNP coatings indicate a 0.5-eV shift toward lower binding energies as a result of the plasma treatment. This result agrees well with previous studies [5], in which the formation of silver oxide on AgNPs resulted in a peak shift in the Ag3d spectrum toward a lower binding energy. Using Spectral Data Processor software (XPS
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Fig. 5. The outward bending test results (averaged values with testing multiple specimens for each sample) for (a) 100ITO and 100ITO-NP, (b) 50ITO and 50ITO-NP, and (c) 50ITO-ptNP.
Fig. 6. XRD analysis results for (a) the 100 nm-ITO samples described in Table 1, (b) the 50 nm-ITO samples described in Table 2, (c) AgNPs without and with O2 plasmatreatment (the concentration of Ag ion ink for spin-coating was intentionally increased to 5 wt% to increase the peak intensities).
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Fig. 7. Bright field images and HR TEM analysis results for (a) 50ITO and (b) 50ITO-ptNP; (c) a STEM image and EDS analysis results of 50ITO-ptNP.
International, Inc.), the O1s spectra were found to be composed of contributions from several oxygen species, as shown at the bottom right side of Fig. 4c. The contributions to the O1s spectra at 530.96–531.13 eV and 531.87–532.08 eV were attributed to the oxygen atoms in Ag2CO3 and absorbed CO2 and dissolved oxygen, respectively. In addition, the peaks at approximately 529.5 eV correspond to the oxygen in Ag2O. Our results are in agreement with results from previous studies [5]. As shown in the O1s narrow-scan data at the bottom right side of Fig. 4c, silver oxide was present on the NPs prior to
O2 plasma treatment. After plasma treatment, the Ag2O content increased relative to the initial amount and relative to the levels of the other compounds present, decreasing the optical absorbance of the AgNPs in the visible range. The thickness of Ag oxides formed on the surface of AgNPs could not be measured. However, the thickness of the oxide layers is estimated to be less than 5 nm based on the following two facts that: (1) the observation depth of the XPS machine we used to analyze Ag oxides is 5 nm from the top surface of the sample materials; (2) according to the XPS analysis results in Fig. 4c, the peaks of Ag as well as Ag oxide were attained.
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Fig. 7 (continued)
The q0 and Rs of the 50ITO-ptNP also decreased significantly in comparison to other samples, as shown in Table 2. As the trends in q0 of 100ITO and 50ITO and of 100ITO-NP and 50ITO-NP shows in Table 2, the increase in q0 with decreasing hf is a general trend for thin films [21]. However, q0 of 50ITO-ptNP is less than a half of q0 of 100ITO and 100ITO-NP, and is even smaller than those of 50ITO and 50ITO-NP by more than 65%. Based on Hall measurement results, the improved electrical property seems to result from the increased charge density for 50ITO-ptNP compared to the ITO on bare PI. The sheet charge concentration of 50ITO-ptNP was 9.86 1015 /cm2 according to Hall measurement results, which is more than five times greater than that of 100ITO (1.85 1015 /cm2). The effect of the AgNPs on the bending reliability of ITO was evaluated by measuring Dq/q0. q0 measured for the samples are compared in Table 2. The bending test results for 100ITO and 100ITO-NP are presented in Fig. 5a. When the samples were bent outward (as schematically displayed in the inset image in Fig. 5a), 100ITO-NP had a significantly larger bending stability than 100ITO. The q0 of 100ITO changed by two orders of magnitude at an rb of 8 mm and became almost insulating as rb decreased further. In contrast, the q0 value of the 100ITO-NP sample only changed by 15.5% at an rb of 4 mm. The commercial standard for the minimum bending radius of TCO on flexible substrates without a drastic increase in q0 is 5 mm, which has not been achieved with any TCO [22,23]; thus, the value of 15.5% at 4 mm clearly confirms the important role of the AgNP interlayers in enhancing the flexibility of ITO.
Furthermore, it should be noted that the standard deviations were smaller for the 100ITO-NP sample than for 100ITO at all values of rb. When the thickness (hf) of the ITO layer was reduced to 50 nm (50ITO and 50ITO-NP in Table 1), the change in the q0 of 50ITO-NP was only 11.8% for an rb of 4 mm, whereas a change of two orders of magnitude was observed for 50ITO at the larger rb of 8 mm. The bending stability of ITO was further enhanced by Ag oxide formation on the AgNPs. As shown in Fig. 5c, the change in q0 at a bending radius of 4 mm is only by 8.3%. 50ITO-ptNP shows the highest bending stability among the samples in Table 1. According to the XRD analysis of the film microstructures (Fig. 6a and b), the crystallinity seems to be one of the major factors for the highly improved bending stability of the ITO grown on the AgNPs. In our previous studies, the effect of an increased degree of crystallinity on enhancing the coherent strength of ITO was experimentally proven [24]. As shown in Fig. 6a and b, the ITO samples have four major preferred growth orientations in common, and (2 2 2) is the most preferred direction, corresponding to the highest peak intensity. The electrical and mechanical properties of ITO improve as the (2 2 2) growth orientation becomes more dominant [25,26]. The XRD results in Fig. 6a demonstrate that the peak intensities of 100ITO-NP were greater than those of 100ITO in all orientations. The degree of crystallinity of the 50ITO-NP sample also increased, particularly in the (2 2 2) and (4 0 0) preferred orientations, compared to 50ITO (Fig. 6b). The XRD results in Fig. 6b also reveal that 50ITO-ptNP grew as crystalline ITO, but with an almost uniform preferred growth
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Fig. 8. (a) Flexible OLEDs fabricated on 100ITO and 50ITO-ptNP samples (flat and bent during device operation). The inset image on top shows the schematic structure of the f-OLED on the ITO with AgNPs (50ITO-ptNP), (b) J–V curves for the OLEDs in (a).
orientation, (2 2 2), whereas the other samples exhibited more than four orientations, as shown in Fig. 6a and b. The difference in the film microstructure of 50ITO-ptNP compared to the other samples in Table 1 was confirmed by Fig. 6c and TEM analysis in Fig. 7a–c. As presented in Fig. 6c, the major crystalline orientation of AgNPs is (1 1 1). Two other peaks of the orientations, (2 0 0) and (2 2 0), also appeared, but the intensities were negligible compared to (1 1 1). After the O2 plasma-treatment, the most preferred growth orientation of the Ag2O formed on the surface of AgNPs was found to be (1 1 1). According to Fig. 7a, the 50ITO has a composite-like structure in which crystalline ITO domains are embedded in predominantly amorphous ITO. The selected area diffraction pattern (SADP) for a bright field plan-view TEM image (Region A in Fig. 7a) confirms the composite-like structure of 50ITO. The domains (Region B in Fig. 7a) have various crystalline orientations as demonstrated by the HR TEM image (Region C in Fig. 7a) and fast Fourier transformed (FFT) SADP shown in Fig. 7a. In contrast, random network-like microstructures were observed in the plan-view TEM images of 50ITO-ptNP, as presented in Region A in Fig. 7b and a scanned TEM image (STEM) in Fig. 7c. Based on the bright field TEM image (Regions A and B in Fig. 7b) and the HR TEM analysis (Regions C and D shown in Fig. 7b), both the dark and white networks have a primarily (2 2 2) crystalline orientation. According to the STEM image and the energy dispersive X-ray spectroscopy (EDS) analysis results in Fig. 7c, the white and black networked regions that correspond to the black and white regions, respectively, in the bright-field image (Fig. 7b) are
the ITO grown on plasma treated (pt)-AgNPs and the ITO on PI, respectively. It should be noted that ITO grown from PI in the 50ITO-ptNP sample also have a (2 2 2) texture as shown in Fig. 7b, which suggests that (2 2 2) domains grown on pt-AgNPs also promote the growth of the same texture between two separated pt-AgNPs. This trend may be attributed to the fact that the AgNPs are sufficiently close to each other, where the separation distance is smaller than the diameter of the NPs. This mostly (2 2 2) texture resulting in the highly enhanced flexibility without sacrificing the functional properties could not be achieved in ITO on the continuous Ag interlayers [15]. Also, the smaller sizes of the crystallized domains grown on pt-AgNPs than the grains of homogeneous ITO contribute to the improved strength and flexibility of the hybrid ITO. In Fig. 8a, the f-OLEDs fabricated on PI with the 100ITO and 50ITO-ptNP electrodes are shown in the flat and bent configurations used during operation. The schematic image at the top of Fig. 8a displays the structure of the OLED. As described in section II, 16 cells were constructed for each f-OLED as shown in Fig. 8a, and we confirmed that all of the cells functioned normally. The J–V curves shown in Fig. 8b were constructed from averaged values for the 16 cells in the flat configuration. The cells on 50ITO-ptNP functioned in the same manner when bent, as indicated in Fig. 8a, while the OLEDs on 100ITO were not lit on bent. As clearly shown in Fig. 8b, the J–V characteristics of OLED 50ITO-ptNP are equivalent to or better than those of the f-OLED on 100ITO (the conventional, commercially available ITO for
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f-OLEDs) at higher values of V even though the ITO thickness of 50ITO-ptNP is half the 100ITO. The turn-on voltages (Von) of f-OLEDs on 50ITO-ptNP were measured to be almost similar to those on 100ITO, which were 3.0 V at 1 mA/cm2 and 2.1 V at 1 cd/m2. However, J of the f-OLEDs on 50ITO-ptNP increased faster and reached its maximum at lower voltages than that of f-OLEDs on 100ITO, as presented in Fig. 8b. 4. Conclusions In this study, we constructed AgNPs and ITO hybrid-type transparent conductive electrode (TCE) with a significantly greater bending stability than conventional homogeneous ITOs and the previously developed hybrid-structured ITO on AgNPs. The diameter of the AgNPs was as small as 20 nm, but uniformly distributed AgNP coatings without significant agglomeration could be attained on PI by optimizing the concentration of the ink (Ag ion sources) and coating processes. The surface roughness was even smoother than the ITO without AgNPs. The degradation of optical transmittance by the metal nano-particles, which is a critical limitation of using AgNPs in TCEs, was resolved by O2 plasma treatment of the powder surfaces under optimized conditions. The treatment increased the formation of Ag oxides on the powder surfaces, thus decreasing the absorbance of the AgNPs. By combining the O2 plasma treatment of AgNPs with our optimized DC sputtering conditions, we also decreased the ITO film thickness to 50 nm without sacrificing the functional and mechanical properties of the 100-nm-thick ITO. The flexibility and electrical resistivity of the 50-nm-thick ITO on the plasma-treated AgNPs substantially improved in comparison to conventional 100-nm-thick ITO and previously developed hybrid ITO with AgNPs. This result was validated by testing the performance of flexible OLEDs on the above-mentioned 50-nm-thick ITO. The f-OLED in the bent state was also comparable to that in the flat state, confirming the potential applications of these hybrid films as electrodes in various flexible electronics. Acknowledgments This research was supported by a Grant from the Fundamental R&D Program for the Technology of World Premier Materials (WPM) funded by the Ministry of Knowledge Economy, Republic of Korea and a Grant of Business for Cooperative R&D between Industry, Academy, and Research Institute (Grants No. C0188503) funded by Korea Small and Medium Business Administration in 2014. References [1] C.-T. Chi, I.C. Cheng, J.-Z. Chen, Bandgap tuning of MgZnO in flexible transparent n(+)-ZnO:Al/n-MgZnO/p-CuAlOx:Ca diodes on polyethylene terephthalate substrates, J. Alloys Comp. 544 (2012) 111–114. [2] Y. Hu, H. Zhu, J. Wang, Z. Chen, Synthesis of layered birnessite-type manganese oxide thin films on plastic substrates by chemical bath deposition for flexible transparent supercapacitors, J. Alloys Comp. 509 (2011) 10234–10240.
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