- Email: [email protected]
Low temperature Solution-Processed ZnO film on flexible substrate
Materials Science in Semiconductor Processing 47 (2016) 20–24
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Low temperature Solution-Processed ZnO film on flexible substrate JeongSoo Hong a, Hajime Wagata b, Ken-ichi Katsumata a, Nobuhiro Matsushita c,n a b c
Photocatalysis International Research Center, Tokyo University of Science, Chiba 278-8510, Japan Department of Environmental Science and Technology, Shinshu University Nagano 380-8533, Japan Department of Chemistry and Material Science, Tokyo Institute of Technology, Tokyo 152-8550, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 8 December 2015 Received in revised form 8 February 2016 Accepted 16 February 2016
Transparent conductive ZnO films were directly deposited on unseeded polyethersulfone (PES) substrates with a spin-spray method using aqueous solution at a low substrate temperature of 85 °C. All ZnO films were crystalline with wurtzite hexagonal structure and impurity phases were not detected. ZnO films deposited without citrate ions in the reaction solution had a rod array structure. In contrast, ZnO films deposited with citrate ions in the reaction solution had a continuous, dense structure. The transmittance of the ZnO films was improved from 11.9% to 85.3% as their structure changed from rod-like to continuous. After UV irradiation, the ZnO films with a continuous, dense structure had a low resistivity of 9.1 10 3 Ω cm, high carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Flexible ZnO Solution PES Bending test
1. Introduction Flexible electronics possess merits such as light weight and portability, and are attractive for various applications such as electronic paper, digital signage and organic light-emitting diodes (OLEDs) [1–4]. Deposition of transparent conductive oxide (TCO) films on plastic substrates to produce flexible electronics is receiving much attention because TCOs are fundamental components of many devices. Various kinds of oxide materials such as tungsten oxide (WO3), zinc oxide (ZnO) were studied as an alternative to indium tin oxide (ITO) [5,6]. In the case of WO3 film, it can achieve high electrical conductivity and transmittance by dielectric/metal/dielectric (DMD) multilayer [7]. However, establishment of optimum condition for each layer is difficult and it make complicated fabrication process. In addition, formed thin dielectric layer degrade the conductivity [8]. On the other hand, ZnO has wide band gap of 3.4 eV and large exciton binding energy of 60 meV, and these properties mean that ZnO can play various roles in different devices [9]. For example, in the case of one-dimensional ZnO nanostructured films, ZnO provides high surface area and can be used as a photoelectrode in dye-sensitized solar cells [10]. Meanwhile, ZnO films with continuous structure have been used in solar cells, OLEDs and thin film transistors as an electrode or channel layer [11–13]. Many dry processes to deposit ZnO films on plastic substrates n
Corresponding author. E-mail address: [email protected] (N. Matsushita).
http://dx.doi.org/10.1016/j.mssp.2016.02.010 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
have been developed including sputtering, pulsed-laser deposition, and ion plating. However, these methods are energy-expensive and complicated. As a result, solution-based deposition methods have been investigated as an attractive alternative because of their simplicity and low cost. However, solution processes such as the sol-gel method, filtration method, chemical-bath deposition and spray pyrolysis require a long reaction period and seed layer for good adhesion to the substrate [14–18]. Additionally, a high substrate temperature and/or post annealing process may be necessary for crystallization, which can cause the substrate to warp and the film to peel off. These factors make it difficult to deposit ZnO films on plastic substrates using conventional solution processes. Therefore, a method that can deposit stable ZnO films on plastic substrates is needed. One possibility is the spin-spray method, which is an environmentally friendly process using aqueous solution that can deposit high-quality crystalline ZnO films without requiring a high substrate temperature or post-annealing process. Additionally, the spin-spray method can achieve good film adhesion without a seed layer [19,20]. Plastic substrates such as polyether-sulfone (PES), polyethylene-terephthalate, polycarbonate, polyimide, polymethylmethacrylate and polyethylene-naphthalate, have been intensively studied for use in flexible electronic devices [21–27]. Among them, PES which has low absorption, good mechanical property, high thermal stability and optical transmittance, has been widely studied in flexible electronic devices [28,29]. In this study, ZnO films are deposited on PES substrates by the spin-spray method without using a seed layer, and the properties of the resulting films are investigated as a transparent electrode for flexible electronic devices.
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
21
2. Experimental details 2.1. Deposition process of ZnO films Before deposition, the PES plastic substrates (40 30 0.3 mm) were ultrasonically cleaned in deionized water and ethanol for 5 min each to remove impurities from their surfaces. The substrates were dried for 10 min in a conventional oven at 60 °C to remove the residual moisture adsorbed onto their surfaces. The PES substrates were then subjected to plasma treatment for 3 min using discharge plasma to increase the hydrophilicity of their surfaces. Plasma treatment caused the contact angle to the substrates to decrease to 5.9°, as shown in Fig. 1. The source solution was prepared by dissolving Zn(NO3)2∙6H2O in 2.0 L of Millipore deionized water to give a concentration of Zn(NO3)2∙6H2O of 10 mmol. The reaction solution was prepared by dissolving 120 mL of NH3 solution in 1.88 L of Millipore deionized water. Sodium citrate was added to the reaction solution to give concentrations of 0 and 2 mmol. NH3 solution was used as a pH adjuster and sodium citrate was used as a surfactant to fabricate ZnO films with a continuous structure [30]. The source and reaction solutions were simultaneously and continuously sprayed onto the rotating substrates for 10 min at 85 °C, as shown in Fig. 2.
Fig. 2. Schematic diagram of the spin-spray method.
2.2. Measurements Contact angles and transmittance were measured by a contact angle meter (DMs-400, Kyowa Interface Science, Japan) and UV– vis spectrometer (Lambda 35, Perkin Elmer, Japan), respectively. The surface morphology of ZnO films was observed by scanning electron microscopy (SEM; S4000, Hitachi, Japan) performed at 15 kV. Structural properties were analyzed by X-ray diffraction (XRD; Rint2000, Rigaku, Japan) with CuKα radiation (λ ¼1.5418 Å). Electrical properties were measured using a Hall Effect measurement system (HL5500, Nanometrics).
3. Results and discussion 3.1. ZnO films on plastic substrates ZnO films were successfully deposited on PES substrates without a seed layer by the spin-spray method at a low substrate temperature of 85 °C. The surface morphology and crystallographic properties of the as-deposited ZnO films were evaluated
Fig. 3. XRD pattern of a ZnO film on a PES substrate. Inset are top-view and crosssectional SEM images of the same sample.
by SEM and XRD; the results are shown in Fig. 3. The XRD pattern of the as-deposited ZnO films revealed the films had a preferred orientation along the c-axis. All of the diffractions peaks were consistent with a hexagonal wurtzite ZnO crystal structure; no impurity phases were observed. The dominant (002) peak was observed at 34.4°, which is close to that of a standard ZnO crystal (JCPDS 36-1451). The full width at half-maximum (FWHM) of this peak was measured to evaluate the crystallinity of the sample. The FWHM of the (002) peak was 0.25°, which is lower than those reported for ZnO films fabricated using a seed layer on ITO and Si (100) substrates [31]. The crystallite size in the ZnO film calculated using the Scherrer equation [32,33] was 33.3 nm. The transmittance of the as-deposited ZnO films on PES substrates was very low. As shown in Fig. 4, a ZnO film deposited on a PES substrate showed 11.9% transmittance in the visible range (400–700 nm) because of scattering caused by its rod array structure. 3.2. Transparent conductive ZnO films on plastic substrates
Fig. 1. Variation of contact angle of PES substrates induced by discharge plasma treatment.
Transparent, continuous ZnO films can be fabricated by adding citrate to the reaction solution [30]. As shown in the SEM images in the inset of Fig. 5, a ZnO film deposited on a PES substrate by the spin-spray method from a reaction solution containing citrate clearly possessed different morphology to that deposited from a reaction solution lacking citrate. The structure of the ZnO films changed from a rod array to continuous structure by adding citrate to the reaction solution. The continuous ZnO film had a smooth surface. An XRD pattern of this film (Fig. 5) revealed that the preferred orientation changed from (002) to (101) because of the
22
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
Fig. 4. Transmittance of a ZnO film on a PES substrate. Inset is a photograph of the same sample.
Fig. 5. XRD pattern of a transparent ZnO film on a PES substrate. Inset are top-view and cross-sectional SEM images of the same sample.
presence of citrate ions in the reaction solution. Citrate ions are selectively adsorbed onto the (001) plane of ZnO, which induced a change of crystal growth. As a result, the intensity and FWHM of the (002) peak and the thickness were decreased because of the suppression of c-axis growth. The surface morphology of the film also became continuous [20,30], which improved its optical properties. As shown in Fig. 6, the ZnO film deposited from a reaction solution containing citrate exhibited high transparency (above 80%) in the visible range and its optical band gap calculated by the Tauc and Davis-Mott models was 3.46 eV [34,35]. A transparent electrode needs to possess high conductivity, smooth surface and high transparency, although the most important factor is conductivity. The ZnO films deposited with citrate in the reaction solution had continuous structure and high transmittance. However, these films exhibited low conductivity because of the presence of citrate. To increase their conductivity, the ZnO films were irradiated with UV light from a black light blue lamp (wavelength ¼ 300–400 nm, intensity¼0.8 mW/cm2) for 24 h. We expected that UV exposure would decompose any organic substances in the ZnO film by photocatalytic reaction and as a result, conductivity would be improved by photo-induced ion substitution with C and/or H [20,36]. To confirm that UV exposure affected the conductivity of the ZnO film, its surface resistance was measured. As shown in Fig. 7,
Fig. 6. Transmittance of an as-deposited ZnO film on a PES substrate. Inset are a photograph and the optical band gap energy of the same sample.
Fig. 7. Decrease in the surface resistance of a ZnO film on a PES substrate caused by UV illumination.
the as-deposited ZnO film had a high surface resistance of several hundred Ω before UV exposure. The surface resistance of the film decreased considerably to 62 Ω during UV exposure. The resistivity of the film was also determined by Hall Effect measurements. The ZnO film exhibited a resistivity of 9.1 10 3 Ω cm after UV exposure with a carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1. The improved conductivity of the film was attributed to the increase in carrier concentration originating from the decomposition of organic substances such as carboxyl groups under UV illumination [20,36]. 3.3. Bending test The mechanical flexural strength of TCO films deposited on flexible substrates is an important factor affecting the lifetime of such devices. Multilayer films are often used to improve mechanical flexural strength. Generally, such multilayer films consist of three layers, such as TCO-metal-TCO. However, as mentioned above, fabrication of multilayers is complicated because the optimum conditions for each layer need to be established. A simpler option for flexible devices is to use a single layer that has strong flexural strength. The mechanical properties of transparent, continuous ZnO films deposited on PES substrates with a size of 25 50 mm were evaluated by bending testing, as shown in Fig. 8. Bending tests
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
23
Fig. 8. a. Schematic diagram of the bending test setup. b. Photographs of a sample before (left) and during (right) bending.
dominant (002) XRD peak. This film had poor conductivity and transparency. Addition of citrate to the reaction solution gave continuous ZnO films with smooth surface morphology and high transmittance above 80%. A UV-irradiated continuous ZnO film had a low resistivity of 9.1 10 3 Ω cm, high carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1.
References
Fig. 9. Variation of resistivity of a ZnO film on a PES substrate after bending testing.
with a small bending radius of 20 mm were carried out using cyclic bending equipment for duration of 5000 cycles. As shown in Fig. 9, during bending testing, the resistivity of the as-deposited films rapidly increased from 9.6 10 3 Ω cm to 618 kΩ cm. After 5000 cycles, the resistivity of the films could not be measured. The generation of cracks caused by broken inner and surface structures in the films increased their resistivity [37,38]. The insets in Fig. 9 reveal that the ZnO film bent 500 times did not contain any cracks and still had a low resistivity of 6.7 10 1 Ω cm. In contrast, ZnO films bent 2000 and 5000 times contained cracks in the film surfaces and exhibited very high resistivity. During bending testing, the strain in the films is increased, so the chance of crack generation is also increased, which may affect film resistivity.
4. Conclusion ZnO films with rod array and continuous structures were deposited on PES substrates without a seed layer by the spin-spray method at a low substrate temperature of 85 °C without and with citrate in the reaction solution, respectively. The rod array-structured ZnO film had a preferred orientation along the c-axis and
[1] H.C. You, Indium doping concentration effects in the fabrication of zinc-oxide thin-film transistors, Int. J. Electrochem. Sci. 8 (2013) 9773–9784. [2] J.J. Ho, C.Y. Chen, Power effects in indium-zinc oxide thin films for OLEDs on flexible applications, J. Electrochem. Soc. 152 (2005) G57–G61. [3] J.W. Kang, W.I. Jeong, J.J. Kim, H.K. Kim, D.G. Kim, G.H. Lee, High-performance flexible organic light-emitting diodes using amorphous indium zinc oxide anode, Electrochem. Solid-State Lett. 10 (2007) J75–J78. [4] D.J. Rogers, F. Hosseini Teherani, E.V. Sandra, M. Razeghi, ZnO thin films & nanostructures for emerging optoelectronic applications, Proc. SPIE 7605 (2010) 76050K(1)–76050K(11). [5] J.S. Hong, N. Matsushita, K.H. Kim, Effects of dopants and thermal treatment on properties of Ga-Al-ZnO thin films fabricated by hetero targets sputtering system, Thin Solid Films 531 (2013) 238–242. [6] J. Zhu, S. Wei, M.J. Alexander, T.D. Dang, T.C. Ho, Z. Guo, Enhanced electrical switching and electrochromic properties of poly(p-phenylenbenzobisthiazole) thin films embedded with Nano-WO3, Adv. Funct. Mater. 20 (2010) 3076–3084. [7] K. Jeon, H. Youn, S. Kim, S. Shin, M. Yang, Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes, Nanoscale Res. Lett. 7 (2012) 253(1)–253(7). [8] H. Cho, C. Yun, J. Park, S. You, Highly flexible organic light-emitting diodes based on ZnS/Ag/WO3 multilayer transparent electrodes, Org. Electron. 10 (2009) 1163–1169. [9] Z. Guo, S. Wei, B. Shedd, R. Scaffaro, T. Pereira, H.T. Hahn, Particle surface engineering effect on the mechanical, optical and photoluminescent properties of ZnO/vinyl-ester resin nanocomposites, J. Mater. Chem. 17 (2007) 806–813. [10] C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong, Improved dye-sensitized solar cells with a ZnO-nanoflower photoanode, Appl. Phys. Lett. 90 (2007) 263501 (1)–263501(3). [11] J.S. Hong, K.W. Jang, Y.S. Park, H.W. Choi, K.H. Kim, Preparation of ZnO based thin films for OLED anode by facing targets sputtering system, Mol. Cryst. Liq. Cryst. 538 (2011) 103–111. [12] J.S. Hong, S.M. Kim, S.J. Park, H.W. Choi, K.H. Kim, Preparation of In2O3-ZnO (IZO) thin film on glass substrate for organic light emitting device (OLED), Mol. Cryst. Liq. Cryst. 520 (2010) 19–27. [13] H.J. Bong, W.H. Lee, D.Y. Lee, B.J. Kim, J.H. Cho, K.W. Cho, High-mobility lowtemperature ZnO transistors with low-voltage operation, Appl. Phys. Lett. 96 (2010) 192115(1)–192115(3). [14] N.S. Ridhuan, K.A. Razak, Z. Lockman, A.A. Aziz, Structural and morphology of ZnO nanorods synthesized using ZnO seeded growth hydrothermal method
24
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
and its properties as UV sensing, PloS One 7 (2012) e50405(1)–e50405(15). [15] X. Yan, Z. Li, R. Chen, W. Gao, Template growth of ZnO nanorods and microrods with controllable densities, Cryst. Growth Des. 8 (2008) 2406–2410. [16] Q. Yu, W. Fu, C. Yu, H. Yang, R. Wei, Y. Sui, S. Liu, Z. Liu, M. Li, G. Wang, C. Shao, Y. Liu, G. Zou, Structural, electrical and optical properties of yttrium-doped ZnO thin films prepared by sol–gel method, J. Phys. D Appl. Phys. 40 (2007) 5592–5597. [17] M. Krunks, T. Dedova, E. Karber, V. Mikli, I. Oja Acik, M. Grossberg, A. Mere, Growth and electrical properties of ZnO nanorod arrays prepared by chemical spray pyrolysis, Physica B 404 (2009) 4422–4425. [18] Y. Li, H. Zhu, H. Gu, Z. Fang, N.J. Weadock, Z. Guo, L. Hu, Strong transparent magnetic nanopaper prepared by immobilization of Fe3O4 nanoparticle in a nanofibrillated cellulose network, J. Mater. Chem. A 1 (2013) 15278–15283. [19] H. Wagata, N. Ohashi, T. Taniguchi, A.K. Subramani, K. Katsumata, K. Okada, N. Matsushita, Single-step fabrication of ZnO rod arrays on a nonseeded glass substrate by a spin-spray technique at 90 °C, Cryst. Growth Des. 10 (2010) 3502–3507. [20] J.S. Hong, H. Wagata, K. Katsumata, K. Okada, N. Matsushita, Effects of thermal treatment on crystallographic and electrical properties of transparent conductive ZnO films deposited by spin-spray method, Jpn. J. Appl. Phys. 52 (2013) 110108(1)–110108(5). [21] P.H. Lei, C.M. Hsu, Y.S. Fan, Flexible organic light-emitting diodes on a polyestersulfone (pES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system, Org. Electron. 14 (2013) 236–249. [22] T. Gui, G. Dong, F. Gao, Y. Xiao, Q. Chen, X. Diao, High performance ZnO:Al films deposited on PET substrates using facing target sputtering, Appl. Surf. Sci., 282, (2013) 467–471. [23] Q. Shi, K. Zhou, M. Dai, S. Lin, H. Hou, C. Wei, F. Hu, Structural and opto-electric properties of and F codoped ZnO thin films on PC substrates, Vacuum 94 (2013) 81–83. [24] M. Matsumura, R.P. Camata, Pulsed laser deposition and photoluminescence measurements of ZnO thin films on flexible polyimide substrates, Thin Solid Films 476 (2005) 317–321. [25] T. Guo, G. Dong, Q. Liu, M. Wang, M. Wang, F. Gao F, Q. Chen, H. Yan, X. Diao, Optimization of oxygen and pressure of ZnO:Al films deposited on PMMA substrates by facing target sputtering, Superlattice Microstruct. 64 (2013) 552–562. [26] K. Tao, Y. Sun, H. Cai, D. Zhang, K. Xie, Y. Wang, Deposition Ga-doped ZnO films on PEN substrate at room temperature for thin film silicon solar cells, Appl.
Surf. Sci. 258 (2012) 5943–5946. [27] P.H. Lei, H.M. Wu, C.M. Hsu, Zinc oxide (ZnO) grown on flexible substrate using dual-plasma-enhanced metalorganic vapor deposition (DPEMOCVD), Surf. Coat. Technol. 206 (2012) 3258–3263. [28] J.J. Kim, J.H. Lee, J.Y. Lee, D.J. Lee, B.R. Jang, H.S. Kim, Characteristics of ZnO film deposited on plastic substrates at various RF sputtering powers, J. Korean Phys. Soc. 55 (2009) 1910–1914. [29] P. Lei, C. Hsu, Y. Fan, Flexible organic light-emitting diodes on polyestersulfone (pES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system, Org. Electron. 14 (2013) 236–249. [30] H. Wagata, N. Ohashi, T. Taniguchi, K. Katsumata, K. Okada, N. Matsushita, Control of the microstructure and crystalline orientation of ZnO films on a seed-free glass substrate by using a spin-spray method, Cryst. Growth Des. 10 (2010) 4968–4975. [31] R.B. Marcia del, J.P. Oscar, P.S. Surinder, A.J. Jose, A.A. Joaquin, P.H. Samuel, Improved low-temperature aqueous synthesis of ZnO nanorods and their use in SERS detection of 4-ABT and RDX, Mater. Sci. Appl. 4 (2013) 29–38. [32] G.K. Williamson, R.E. Smallman, III. Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1 (1956) 34–35. [33] B. Cllity B, Elements of X-Ray Diffraction, Addison-Wesley, Reading (1978), p. 102. [34] J. Tauc, Optical properties of amorphous semiconductors, Amorphous and Liquid Semiconductors, Plenum press, London, New york, 1974. [35] E.A. David, N.F. Mott, Conduction in non-crystalline systems V. conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag. 22 (1970) 903–922. [36] H. Wagata, N. Ohashi, K. Katsumata, H. Seagawa, Y. Wada, H. Yoshikawa, S. Ueda, K. Okada, N. Matsushita, Aqueous solution process and subsequent UV treatment for highly transparent conductive ZnO films, J. Mater. Chem. 22 (2012) 20706–20712. [37] H. Liu, W. Huang, J. Gao, K. Dai, G. Zheng, C. Liu, C. Shen, X. Yan, J. Guo, Z. Guo, Piezoresistive behavior of porous carbon nanotube-thermoplastic polyurethane conductive nanocomposites with ultrahigh compressibility, Appl. Phys. Lett. (2016) 011904. [38] H. Liu, Y. Li, K. Dai, G. Zheng, C. Liu, C. Shen, X. Yan, J. Guo, Z. Guo, Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications, J. Mater. Chem. C 4 (2016) 157–166.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Low temperature Solution-Processed ZnO film on flexible substrate JeongSoo Hong a, Hajime Wagata b, Ken-ichi Katsumata a, Nobuhiro Matsushita c,n a b c
Photocatalysis International Research Center, Tokyo University of Science, Chiba 278-8510, Japan Department of Environmental Science and Technology, Shinshu University Nagano 380-8533, Japan Department of Chemistry and Material Science, Tokyo Institute of Technology, Tokyo 152-8550, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 8 December 2015 Received in revised form 8 February 2016 Accepted 16 February 2016
Transparent conductive ZnO films were directly deposited on unseeded polyethersulfone (PES) substrates with a spin-spray method using aqueous solution at a low substrate temperature of 85 °C. All ZnO films were crystalline with wurtzite hexagonal structure and impurity phases were not detected. ZnO films deposited without citrate ions in the reaction solution had a rod array structure. In contrast, ZnO films deposited with citrate ions in the reaction solution had a continuous, dense structure. The transmittance of the ZnO films was improved from 11.9% to 85.3% as their structure changed from rod-like to continuous. After UV irradiation, the ZnO films with a continuous, dense structure had a low resistivity of 9.1 10 3 Ω cm, high carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Flexible ZnO Solution PES Bending test
1. Introduction Flexible electronics possess merits such as light weight and portability, and are attractive for various applications such as electronic paper, digital signage and organic light-emitting diodes (OLEDs) [1–4]. Deposition of transparent conductive oxide (TCO) films on plastic substrates to produce flexible electronics is receiving much attention because TCOs are fundamental components of many devices. Various kinds of oxide materials such as tungsten oxide (WO3), zinc oxide (ZnO) were studied as an alternative to indium tin oxide (ITO) [5,6]. In the case of WO3 film, it can achieve high electrical conductivity and transmittance by dielectric/metal/dielectric (DMD) multilayer [7]. However, establishment of optimum condition for each layer is difficult and it make complicated fabrication process. In addition, formed thin dielectric layer degrade the conductivity [8]. On the other hand, ZnO has wide band gap of 3.4 eV and large exciton binding energy of 60 meV, and these properties mean that ZnO can play various roles in different devices [9]. For example, in the case of one-dimensional ZnO nanostructured films, ZnO provides high surface area and can be used as a photoelectrode in dye-sensitized solar cells [10]. Meanwhile, ZnO films with continuous structure have been used in solar cells, OLEDs and thin film transistors as an electrode or channel layer [11–13]. Many dry processes to deposit ZnO films on plastic substrates n
Corresponding author. E-mail address: [email protected] (N. Matsushita).
http://dx.doi.org/10.1016/j.mssp.2016.02.010 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
have been developed including sputtering, pulsed-laser deposition, and ion plating. However, these methods are energy-expensive and complicated. As a result, solution-based deposition methods have been investigated as an attractive alternative because of their simplicity and low cost. However, solution processes such as the sol-gel method, filtration method, chemical-bath deposition and spray pyrolysis require a long reaction period and seed layer for good adhesion to the substrate [14–18]. Additionally, a high substrate temperature and/or post annealing process may be necessary for crystallization, which can cause the substrate to warp and the film to peel off. These factors make it difficult to deposit ZnO films on plastic substrates using conventional solution processes. Therefore, a method that can deposit stable ZnO films on plastic substrates is needed. One possibility is the spin-spray method, which is an environmentally friendly process using aqueous solution that can deposit high-quality crystalline ZnO films without requiring a high substrate temperature or post-annealing process. Additionally, the spin-spray method can achieve good film adhesion without a seed layer [19,20]. Plastic substrates such as polyether-sulfone (PES), polyethylene-terephthalate, polycarbonate, polyimide, polymethylmethacrylate and polyethylene-naphthalate, have been intensively studied for use in flexible electronic devices [21–27]. Among them, PES which has low absorption, good mechanical property, high thermal stability and optical transmittance, has been widely studied in flexible electronic devices [28,29]. In this study, ZnO films are deposited on PES substrates by the spin-spray method without using a seed layer, and the properties of the resulting films are investigated as a transparent electrode for flexible electronic devices.
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
21
2. Experimental details 2.1. Deposition process of ZnO films Before deposition, the PES plastic substrates (40 30 0.3 mm) were ultrasonically cleaned in deionized water and ethanol for 5 min each to remove impurities from their surfaces. The substrates were dried for 10 min in a conventional oven at 60 °C to remove the residual moisture adsorbed onto their surfaces. The PES substrates were then subjected to plasma treatment for 3 min using discharge plasma to increase the hydrophilicity of their surfaces. Plasma treatment caused the contact angle to the substrates to decrease to 5.9°, as shown in Fig. 1. The source solution was prepared by dissolving Zn(NO3)2∙6H2O in 2.0 L of Millipore deionized water to give a concentration of Zn(NO3)2∙6H2O of 10 mmol. The reaction solution was prepared by dissolving 120 mL of NH3 solution in 1.88 L of Millipore deionized water. Sodium citrate was added to the reaction solution to give concentrations of 0 and 2 mmol. NH3 solution was used as a pH adjuster and sodium citrate was used as a surfactant to fabricate ZnO films with a continuous structure [30]. The source and reaction solutions were simultaneously and continuously sprayed onto the rotating substrates for 10 min at 85 °C, as shown in Fig. 2.
Fig. 2. Schematic diagram of the spin-spray method.
2.2. Measurements Contact angles and transmittance were measured by a contact angle meter (DMs-400, Kyowa Interface Science, Japan) and UV– vis spectrometer (Lambda 35, Perkin Elmer, Japan), respectively. The surface morphology of ZnO films was observed by scanning electron microscopy (SEM; S4000, Hitachi, Japan) performed at 15 kV. Structural properties were analyzed by X-ray diffraction (XRD; Rint2000, Rigaku, Japan) with CuKα radiation (λ ¼1.5418 Å). Electrical properties were measured using a Hall Effect measurement system (HL5500, Nanometrics).
3. Results and discussion 3.1. ZnO films on plastic substrates ZnO films were successfully deposited on PES substrates without a seed layer by the spin-spray method at a low substrate temperature of 85 °C. The surface morphology and crystallographic properties of the as-deposited ZnO films were evaluated
Fig. 3. XRD pattern of a ZnO film on a PES substrate. Inset are top-view and crosssectional SEM images of the same sample.
by SEM and XRD; the results are shown in Fig. 3. The XRD pattern of the as-deposited ZnO films revealed the films had a preferred orientation along the c-axis. All of the diffractions peaks were consistent with a hexagonal wurtzite ZnO crystal structure; no impurity phases were observed. The dominant (002) peak was observed at 34.4°, which is close to that of a standard ZnO crystal (JCPDS 36-1451). The full width at half-maximum (FWHM) of this peak was measured to evaluate the crystallinity of the sample. The FWHM of the (002) peak was 0.25°, which is lower than those reported for ZnO films fabricated using a seed layer on ITO and Si (100) substrates [31]. The crystallite size in the ZnO film calculated using the Scherrer equation [32,33] was 33.3 nm. The transmittance of the as-deposited ZnO films on PES substrates was very low. As shown in Fig. 4, a ZnO film deposited on a PES substrate showed 11.9% transmittance in the visible range (400–700 nm) because of scattering caused by its rod array structure. 3.2. Transparent conductive ZnO films on plastic substrates
Fig. 1. Variation of contact angle of PES substrates induced by discharge plasma treatment.
Transparent, continuous ZnO films can be fabricated by adding citrate to the reaction solution [30]. As shown in the SEM images in the inset of Fig. 5, a ZnO film deposited on a PES substrate by the spin-spray method from a reaction solution containing citrate clearly possessed different morphology to that deposited from a reaction solution lacking citrate. The structure of the ZnO films changed from a rod array to continuous structure by adding citrate to the reaction solution. The continuous ZnO film had a smooth surface. An XRD pattern of this film (Fig. 5) revealed that the preferred orientation changed from (002) to (101) because of the
22
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
Fig. 4. Transmittance of a ZnO film on a PES substrate. Inset is a photograph of the same sample.
Fig. 5. XRD pattern of a transparent ZnO film on a PES substrate. Inset are top-view and cross-sectional SEM images of the same sample.
presence of citrate ions in the reaction solution. Citrate ions are selectively adsorbed onto the (001) plane of ZnO, which induced a change of crystal growth. As a result, the intensity and FWHM of the (002) peak and the thickness were decreased because of the suppression of c-axis growth. The surface morphology of the film also became continuous [20,30], which improved its optical properties. As shown in Fig. 6, the ZnO film deposited from a reaction solution containing citrate exhibited high transparency (above 80%) in the visible range and its optical band gap calculated by the Tauc and Davis-Mott models was 3.46 eV [34,35]. A transparent electrode needs to possess high conductivity, smooth surface and high transparency, although the most important factor is conductivity. The ZnO films deposited with citrate in the reaction solution had continuous structure and high transmittance. However, these films exhibited low conductivity because of the presence of citrate. To increase their conductivity, the ZnO films were irradiated with UV light from a black light blue lamp (wavelength ¼ 300–400 nm, intensity¼0.8 mW/cm2) for 24 h. We expected that UV exposure would decompose any organic substances in the ZnO film by photocatalytic reaction and as a result, conductivity would be improved by photo-induced ion substitution with C and/or H [20,36]. To confirm that UV exposure affected the conductivity of the ZnO film, its surface resistance was measured. As shown in Fig. 7,
Fig. 6. Transmittance of an as-deposited ZnO film on a PES substrate. Inset are a photograph and the optical band gap energy of the same sample.
Fig. 7. Decrease in the surface resistance of a ZnO film on a PES substrate caused by UV illumination.
the as-deposited ZnO film had a high surface resistance of several hundred Ω before UV exposure. The surface resistance of the film decreased considerably to 62 Ω during UV exposure. The resistivity of the film was also determined by Hall Effect measurements. The ZnO film exhibited a resistivity of 9.1 10 3 Ω cm after UV exposure with a carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1. The improved conductivity of the film was attributed to the increase in carrier concentration originating from the decomposition of organic substances such as carboxyl groups under UV illumination [20,36]. 3.3. Bending test The mechanical flexural strength of TCO films deposited on flexible substrates is an important factor affecting the lifetime of such devices. Multilayer films are often used to improve mechanical flexural strength. Generally, such multilayer films consist of three layers, such as TCO-metal-TCO. However, as mentioned above, fabrication of multilayers is complicated because the optimum conditions for each layer need to be established. A simpler option for flexible devices is to use a single layer that has strong flexural strength. The mechanical properties of transparent, continuous ZnO films deposited on PES substrates with a size of 25 50 mm were evaluated by bending testing, as shown in Fig. 8. Bending tests
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
23
Fig. 8. a. Schematic diagram of the bending test setup. b. Photographs of a sample before (left) and during (right) bending.
dominant (002) XRD peak. This film had poor conductivity and transparency. Addition of citrate to the reaction solution gave continuous ZnO films with smooth surface morphology and high transmittance above 80%. A UV-irradiated continuous ZnO film had a low resistivity of 9.1 10 3 Ω cm, high carrier concentration of 2.7 1020 cm 3 and mobility of 2.5 cm2 V 1 s 1.
References
Fig. 9. Variation of resistivity of a ZnO film on a PES substrate after bending testing.
with a small bending radius of 20 mm were carried out using cyclic bending equipment for duration of 5000 cycles. As shown in Fig. 9, during bending testing, the resistivity of the as-deposited films rapidly increased from 9.6 10 3 Ω cm to 618 kΩ cm. After 5000 cycles, the resistivity of the films could not be measured. The generation of cracks caused by broken inner and surface structures in the films increased their resistivity [37,38]. The insets in Fig. 9 reveal that the ZnO film bent 500 times did not contain any cracks and still had a low resistivity of 6.7 10 1 Ω cm. In contrast, ZnO films bent 2000 and 5000 times contained cracks in the film surfaces and exhibited very high resistivity. During bending testing, the strain in the films is increased, so the chance of crack generation is also increased, which may affect film resistivity.
4. Conclusion ZnO films with rod array and continuous structures were deposited on PES substrates without a seed layer by the spin-spray method at a low substrate temperature of 85 °C without and with citrate in the reaction solution, respectively. The rod array-structured ZnO film had a preferred orientation along the c-axis and
[1] H.C. You, Indium doping concentration effects in the fabrication of zinc-oxide thin-film transistors, Int. J. Electrochem. Sci. 8 (2013) 9773–9784. [2] J.J. Ho, C.Y. Chen, Power effects in indium-zinc oxide thin films for OLEDs on flexible applications, J. Electrochem. Soc. 152 (2005) G57–G61. [3] J.W. Kang, W.I. Jeong, J.J. Kim, H.K. Kim, D.G. Kim, G.H. Lee, High-performance flexible organic light-emitting diodes using amorphous indium zinc oxide anode, Electrochem. Solid-State Lett. 10 (2007) J75–J78. [4] D.J. Rogers, F. Hosseini Teherani, E.V. Sandra, M. Razeghi, ZnO thin films & nanostructures for emerging optoelectronic applications, Proc. SPIE 7605 (2010) 76050K(1)–76050K(11). [5] J.S. Hong, N. Matsushita, K.H. Kim, Effects of dopants and thermal treatment on properties of Ga-Al-ZnO thin films fabricated by hetero targets sputtering system, Thin Solid Films 531 (2013) 238–242. [6] J. Zhu, S. Wei, M.J. Alexander, T.D. Dang, T.C. Ho, Z. Guo, Enhanced electrical switching and electrochromic properties of poly(p-phenylenbenzobisthiazole) thin films embedded with Nano-WO3, Adv. Funct. Mater. 20 (2010) 3076–3084. [7] K. Jeon, H. Youn, S. Kim, S. Shin, M. Yang, Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes, Nanoscale Res. Lett. 7 (2012) 253(1)–253(7). [8] H. Cho, C. Yun, J. Park, S. You, Highly flexible organic light-emitting diodes based on ZnS/Ag/WO3 multilayer transparent electrodes, Org. Electron. 10 (2009) 1163–1169. [9] Z. Guo, S. Wei, B. Shedd, R. Scaffaro, T. Pereira, H.T. Hahn, Particle surface engineering effect on the mechanical, optical and photoluminescent properties of ZnO/vinyl-ester resin nanocomposites, J. Mater. Chem. 17 (2007) 806–813. [10] C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong, Improved dye-sensitized solar cells with a ZnO-nanoflower photoanode, Appl. Phys. Lett. 90 (2007) 263501 (1)–263501(3). [11] J.S. Hong, K.W. Jang, Y.S. Park, H.W. Choi, K.H. Kim, Preparation of ZnO based thin films for OLED anode by facing targets sputtering system, Mol. Cryst. Liq. Cryst. 538 (2011) 103–111. [12] J.S. Hong, S.M. Kim, S.J. Park, H.W. Choi, K.H. Kim, Preparation of In2O3-ZnO (IZO) thin film on glass substrate for organic light emitting device (OLED), Mol. Cryst. Liq. Cryst. 520 (2010) 19–27. [13] H.J. Bong, W.H. Lee, D.Y. Lee, B.J. Kim, J.H. Cho, K.W. Cho, High-mobility lowtemperature ZnO transistors with low-voltage operation, Appl. Phys. Lett. 96 (2010) 192115(1)–192115(3). [14] N.S. Ridhuan, K.A. Razak, Z. Lockman, A.A. Aziz, Structural and morphology of ZnO nanorods synthesized using ZnO seeded growth hydrothermal method
24
J. Hong et al. / Materials Science in Semiconductor Processing 47 (2016) 20–24
and its properties as UV sensing, PloS One 7 (2012) e50405(1)–e50405(15). [15] X. Yan, Z. Li, R. Chen, W. Gao, Template growth of ZnO nanorods and microrods with controllable densities, Cryst. Growth Des. 8 (2008) 2406–2410. [16] Q. Yu, W. Fu, C. Yu, H. Yang, R. Wei, Y. Sui, S. Liu, Z. Liu, M. Li, G. Wang, C. Shao, Y. Liu, G. Zou, Structural, electrical and optical properties of yttrium-doped ZnO thin films prepared by sol–gel method, J. Phys. D Appl. Phys. 40 (2007) 5592–5597. [17] M. Krunks, T. Dedova, E. Karber, V. Mikli, I. Oja Acik, M. Grossberg, A. Mere, Growth and electrical properties of ZnO nanorod arrays prepared by chemical spray pyrolysis, Physica B 404 (2009) 4422–4425. [18] Y. Li, H. Zhu, H. Gu, Z. Fang, N.J. Weadock, Z. Guo, L. Hu, Strong transparent magnetic nanopaper prepared by immobilization of Fe3O4 nanoparticle in a nanofibrillated cellulose network, J. Mater. Chem. A 1 (2013) 15278–15283. [19] H. Wagata, N. Ohashi, T. Taniguchi, A.K. Subramani, K. Katsumata, K. Okada, N. Matsushita, Single-step fabrication of ZnO rod arrays on a nonseeded glass substrate by a spin-spray technique at 90 °C, Cryst. Growth Des. 10 (2010) 3502–3507. [20] J.S. Hong, H. Wagata, K. Katsumata, K. Okada, N. Matsushita, Effects of thermal treatment on crystallographic and electrical properties of transparent conductive ZnO films deposited by spin-spray method, Jpn. J. Appl. Phys. 52 (2013) 110108(1)–110108(5). [21] P.H. Lei, C.M. Hsu, Y.S. Fan, Flexible organic light-emitting diodes on a polyestersulfone (pES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system, Org. Electron. 14 (2013) 236–249. [22] T. Gui, G. Dong, F. Gao, Y. Xiao, Q. Chen, X. Diao, High performance ZnO:Al films deposited on PET substrates using facing target sputtering, Appl. Surf. Sci., 282, (2013) 467–471. [23] Q. Shi, K. Zhou, M. Dai, S. Lin, H. Hou, C. Wei, F. Hu, Structural and opto-electric properties of and F codoped ZnO thin films on PC substrates, Vacuum 94 (2013) 81–83. [24] M. Matsumura, R.P. Camata, Pulsed laser deposition and photoluminescence measurements of ZnO thin films on flexible polyimide substrates, Thin Solid Films 476 (2005) 317–321. [25] T. Guo, G. Dong, Q. Liu, M. Wang, M. Wang, F. Gao F, Q. Chen, H. Yan, X. Diao, Optimization of oxygen and pressure of ZnO:Al films deposited on PMMA substrates by facing target sputtering, Superlattice Microstruct. 64 (2013) 552–562. [26] K. Tao, Y. Sun, H. Cai, D. Zhang, K. Xie, Y. Wang, Deposition Ga-doped ZnO films on PEN substrate at room temperature for thin film silicon solar cells, Appl.
Surf. Sci. 258 (2012) 5943–5946. [27] P.H. Lei, H.M. Wu, C.M. Hsu, Zinc oxide (ZnO) grown on flexible substrate using dual-plasma-enhanced metalorganic vapor deposition (DPEMOCVD), Surf. Coat. Technol. 206 (2012) 3258–3263. [28] J.J. Kim, J.H. Lee, J.Y. Lee, D.J. Lee, B.R. Jang, H.S. Kim, Characteristics of ZnO film deposited on plastic substrates at various RF sputtering powers, J. Korean Phys. Soc. 55 (2009) 1910–1914. [29] P. Lei, C. Hsu, Y. Fan, Flexible organic light-emitting diodes on polyestersulfone (pES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system, Org. Electron. 14 (2013) 236–249. [30] H. Wagata, N. Ohashi, T. Taniguchi, K. Katsumata, K. Okada, N. Matsushita, Control of the microstructure and crystalline orientation of ZnO films on a seed-free glass substrate by using a spin-spray method, Cryst. Growth Des. 10 (2010) 4968–4975. [31] R.B. Marcia del, J.P. Oscar, P.S. Surinder, A.J. Jose, A.A. Joaquin, P.H. Samuel, Improved low-temperature aqueous synthesis of ZnO nanorods and their use in SERS detection of 4-ABT and RDX, Mater. Sci. Appl. 4 (2013) 29–38. [32] G.K. Williamson, R.E. Smallman, III. Dislocation densities in some annealed and cold-worked metals from measurements on the X-ray debye-scherrer spectrum, Philos. Mag. 1 (1956) 34–35. [33] B. Cllity B, Elements of X-Ray Diffraction, Addison-Wesley, Reading (1978), p. 102. [34] J. Tauc, Optical properties of amorphous semiconductors, Amorphous and Liquid Semiconductors, Plenum press, London, New york, 1974. [35] E.A. David, N.F. Mott, Conduction in non-crystalline systems V. conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag. 22 (1970) 903–922. [36] H. Wagata, N. Ohashi, K. Katsumata, H. Seagawa, Y. Wada, H. Yoshikawa, S. Ueda, K. Okada, N. Matsushita, Aqueous solution process and subsequent UV treatment for highly transparent conductive ZnO films, J. Mater. Chem. 22 (2012) 20706–20712. [37] H. Liu, W. Huang, J. Gao, K. Dai, G. Zheng, C. Liu, C. Shen, X. Yan, J. Guo, Z. Guo, Piezoresistive behavior of porous carbon nanotube-thermoplastic polyurethane conductive nanocomposites with ultrahigh compressibility, Appl. Phys. Lett. (2016) 011904. [38] H. Liu, Y. Li, K. Dai, G. Zheng, C. Liu, C. Shen, X. Yan, J. Guo, Z. Guo, Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications, J. Mater. Chem. C 4 (2016) 157–166.