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AZO composite film as a transparent conductive material
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Preparation of silver nanowire/AZO composite film as a transparent conductive material ⁎
Quntao Tanga, Honglie Shena,b, , Hanyu Yaoa, Ye Jianga, Chaofan Zhenga, Kai Gaoa a College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China b Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Silver nanowire Transparent conductive film AZO Magnetron sputtering
In this paper, flexible, robustly adhesive, surface smooth and oxide-resistive AgNWs/AZO composite transparent conductive films (TCFs) were prepared by spin-coating solvothermal derived AgNWs followed by magnetron sputtering AZO protective layer, after which, annealing treatment at 200 ℃ for 20 min was adopted to further increase the performance of the composite film. The samples were characterized by means of X-ray diffraction, Raman spectroscopy, scan electron microscopy, scanning probe microscope, UV–Vis spectrophotometer and four point probe. The best result of AgNWs/AZO composite films was the one with a transmittance of 85% at 550 nm accompanied with a low sheet resistance of 19 Ω/□ by controlling the spincoating times fixed at three. The figure of merit for the composite TCFs was 10.4, which was larger than the value of 8.8 for commercial ITO film. These amazing results would render the AgNWs /AZO composite TCFs an ideal candidate to replace conventional ITO for its potential application in flexible devices including OPV and OLED.
1. Introduction Transparent conductive films (TCFs), especially flexible TCFs, have attracted tremendous attention for their wide range of applications including wearable sensors, transparent film heaters, organic light emitting diode (OLED) and so forth [1–3]. Typically, indium tin oxide (ITO) has been widely used as TCF due to its excellent transparency across the visible spectrum range, low sheet resistance, and the compatibility of its work function with most devices. However, two drawbacks accompanied with ITO have limited its further application for next-generation flexible devices: one was the rising cost due to indium scarcity, which rendered its difficulty to be used in low-cost, large-area electronics, and the other one was the fact that ITO was brittle and could easily crack when being used in applications where bending was involved [4,5]. Due to the high dc conductivity and optical transmittance of silver nanowires (AgNWs) random networks, AgNWs have been widely synthesized and studied by lots of research groups in the past few years for their potential ability to replace conventional ITO TCFs [6–8]. Although the performance of AgNWs TCFs was excellent, they were easy to suffer from oxidation when exposured to air, as a result of which their potential performance in practical applications was greatly limited. Therefore, a lot of protective materials such as
graphene [9,10], reduced graphene oxide (RGO) [11], nickel [12] and palladium [13] were adopted by different groups to strengthen the oxidation-resistive ability of AgNWs TCFs without degrading their original performance too much. Simultaneously, Al-doped zinc oxide (AZO) thin film, as an interesting transparent conductive oxide (TCO) material, has also been extensively investigated to substitute the widely used expensive ITO for its rich storage and non-toxicity [14–16]. It has been applied into various fields including flat-panel displays [17], thin film solar cells [18], gas sensors [19] and so on. However, in some applications the performance of the AZO thin films was inferior to that of ITO, which limited its further application. Therefore, films with multilayer structures have been proposed for transparent conducting materials in literatures [20,21]. It was believed that the combination of AgNWs and AZO film could not only improve the air-stability of AgNWs due to the protection of AZO capping layer but also enhanced the performance of the AZO layer for the superior conductivity of AgNWs, as a result of which the AgNWs/AZO composite film with excellent performance could be obtained. In this paper, AZO layer was deposited on spin-coated AgNWs TCFs by magnetron sputtering method to improve the performance of the whole film including surface roughness, thermal stability, adhesiveness towards the substrate and flexibility without sacrificing its original
⁎ Corresponding author at: College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China. E-mail address: [email protected] (H. Shen).
http://dx.doi.org/10.1016/j.ceramint.2016.10.048 Received 23 July 2016; Received in revised form 23 September 2016; Accepted 7 October 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tang, Q., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.10.048
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with different AgNWs coverage, the spin-coating procedure was repeated as wanted. After desired coating times, the AgNWs TCFs (SC1, SC2, SC3, SC4 and SC5, SC was short for spin coating, while the numeric number denoted the spin-coating time) were obtained. For the flexibility experiment, the AgNWs film was prepared on 6×6 cm2 flexible PET by Mayer bar method.
performance too much. The purpose of this investigation was to investigate the potential ability of the AgNWs /AZO composite films as a transparent conductive material for flexible electronic devices. 2. Experimental details 2.1. Synthesis of AgNWs
2.4. Sputtering of AZO protective layer Silver nanowires were synthesized by a solvothermal method. Typically, 0.8 g polyvinylpyrrolidone (PVP, MW=55000) and 2.2 mg NaCl were subsequently added into 25 ml ethylene glycol (EG) in the stirring process at 120 ℃ to obtain a solution A. Simultaneously, 0.27 g AgNO3 was dissolved into 15 ml ethylene glycol (EG) to obtain a solution B. Subsequently, the obtained A solution was dropwise added into solution B to get the final mixture solution C. A 40 ml mixture solution C was then transferred into a 50 ml autoclave. The autoclave was heated at 160 ℃ for 5 h and then naturally cooled down to room temperature. The obtained solution was centrifuged at 4000 rpm for 20 min to remove the small Ag nanoparticles and short Ag nanowires by discarding the supernatant after centrifugation. After being washed by ethanol and water alternatively several times, the resulted AgNWs were dispersed into ethanol to get a silver nanowire alcoholic solution with a concentration of 2 mg/ml for further use.
The AZO protective layer was sputtered on AgNWs TCFs by magnetron sputtering method to prepare AgNWs / AZO composite TCFs. AZO (Al2O3: 2% wt) target was used as the sputtering target. During the sputtering process, the substrate temperature was kept at room temperature with a chamber base pressure of 8.0×10−4 Pa and 20 sccm Ar flowing. The sputtering power and time used here were 150 W and 20 min, respectively. The thickness of the deposited AZO layer was estimated to be about 120 nm. SC1/AZO, SC2/AZO, SC3/AZO, SC4/ AZO, SC5/AZO below referred to the AgNWs/AZO composite TCFs without annealing treatment, while ASC1/AZO, ASC2/AZO, ASC3/ AZO, ASC4/AZO and ASC5/AZO were the AgNWs/AZO composite TCFs annealed at 200 ℃ for 20 min. The numeric number here all denoted the spin-coating time. For the flexibility experiment, the protective AZO was also deposited on AgNWs mesh on the surface of flexible PET substrate.
2.2. Substrate treatment 2.5. Characterization Substrate cleaning and hydrophilic treatment were two important procedures for the realization of the final uniform TCFs. The slide glass substrates used here were firstly cleaned by SC1 (Volume ratio: NH3· H2O:H2O2:H2O =1:1:5) and SC2 (Volume ratio: HCl:H2O2:H2O =1:1:5), respectively, after which it was rinsed by deionized (DI). Before the preparation of AgNWs TCFs, hydrophilic treatment was received by SPM (Volume ratio: H2SO4:H2O2:H2O =1:1:5) treatment followed by DI water rinsing. The hydrophilic treatment of PET flexible substrate was conducted by oxygen plasma treatment in magnetron sputtering equipment. The substrate temperature was kept at 25 ℃ with a chamber base pressure of 6.0×10−4 Pa and 30 sccm O2 flowing. The power and time used here were 100 W and 600 s, respectively.
The surface morphology of AgNWs and AgNWs/AZO composite TCFs were characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) and scanning probe microscope (SPM, Shimadz SPM9600). The phase and crystallographic information were obtained from the X-ray diffraction patterns (XRD, Rigaku Ultima-IV diffraction-meter with Cu–Kα radiation source) and Raman spectroscopy (Thermo Fisher DXR). The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS) attached to the SEM. Room-temperature transmission spectrum measurement in the range of 380–1000 nm and sheet resistance test were conducted by a Shimadzu UV3600 and four point probe (KDY-1), respectively.
2.3. Preparation of AgNWs TCFs
3. Results and discussion
The AgNWs TCFs were prepared by spin-coating method. Typically, 30 μl obtained AgNWs alcoholic solution was dropped onto a slide glass substrate with a size of 2×2 cm2, after which spinning process at 1300 rpm for 60 s was adopted to get the TCFs. In order to get TCFs
The schematic process for preparation of AgNWs/AZO composite TCFs was shown in Fig. 1. After centrifugation of the as-obtained solution by several times, an AgNWs alcoholic solution with a certain concentration could be obtained and was further used for the fabrica-
Fig. 1. Schematic illustration for the preparation of AgNWs/AZO composite TCFs.
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Fig. 2. SEM images of SC1 (a): low magnification, (e): high magnification; SC3 (b): low magnification, (f): high magnification; SC5 (c): low magnification, (g): high magnification and ASC5/AZO (d): low magnification, (h): high magnification. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. three-dimensional SPM images of SC5 sample (a) and ASC5/AZO sample (b).
coverage realized by varying spin-coating time. The average diameter and average length of the obtained AgNWs was about 97 nm and 78 µm, respectively. It was obvious that the AgNWs formed a uniform mesh without a significant nanowires density difference across the substrate. As the spin-coating time increased from (a) to (c), more silver nanowires covered the substrate surface and bridged with each other (the red dotted circle regions in Fig. 2(e), (f) and (g)), and thus fewer voids between AgNWs were left for the passage of light, as a
tion of AgNWs TCFs by spin-coating method. Subsequently, magnetron sputtering method, as a developed technology for the commercial process of transparent conductive oxide (TCO) film, was adopted to sputter an AZO protective layer onto AgNWs mesh. Finally, in order to decrease the junction resistance between AgNWs, an annealing treatment was conducted to further increase the performance of the AgNWs/AZO composite TCFs [22]. Fig. 2 showed SEM images of AgNWs TCFs with different AgNWs 3
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composite film before and after annealing process, phase characterizations including XRD and Raman measurements were conducted. Fig. 4(a) showed the XRD patterns of as-prepared and annealed AgNWs /AZO composite film. Both films showed a dominant peak centered at around 2θ=34° which corresponded to the (002) crystal plane of AZO film, indicating a preferential orientation of c-axis perpendicular to the substrate surface. Simultaneously, the increased intensity of (002) peak due to annealing process indicated the improved crystallinity of the annealed composite film. The magnified XRD patterns around (002) peak was shown in Fig. 4(b). It could be clearly seen that annealing treatment shifted the (002) peak from 2θ=34.09° to 2θ=34.19°, indicating the decrease of the interplanar spacing along the c axis. The decrease in lattice parameters of the annealed composite film probably occurred by the substitutional incorporation of Al3+ ions into Zn2+ sites due to the smaller radius of Al3+(0.53 Å) than that of Zn2+(0.72 Å) during annealing process [24]. In order to further understand the structure change before and after annealing process, Raman spectra were shown in Fig. 4(c). For both samples, E2-high and A1-LO mode could be observed. The appearance of the E2 mode suggested the preferred wurtzite structure of the AZO grains, which agreed well with the XRD observation shown in Fig. 4(a). Normally, E2 peak was at 438 cm−1 for pure AZO film. Annealing treatment shifted the E2 peak from 451.6 cm−1 to 438.3 cm−1, indicating the reduction of residual stress in the as-prepared composite film [25]. The presence of A1-LO mode was always attributed to oxygen vacancy (Vo) [26]. It could be seen that the thermal treatment reduce the peak intensity due to the occupation of Vo by hydrogen atoms (forming Ho). As the XRD peaks of AgNWs could not be observed due
result of which both the surface resistivity and optical transmittance decreased. After the deposition of AZO protective layer, the pre-formed AgNWs mesh was buried under the AZO capping layer without disturbing the networks of nanowires (as shown in Fig. 2(d)). Compared with the bare AgNWs, the diameter of the AZO sputtered nanowires was larger (as shown in Fig. 2(h)), which was caused by the coverage of the AZO layer on AgNWs. In the practical application of electronic devices, surfaces with high roughness could cause shorting through each layer of the device, and this was especially problematic for devices using AgNWs as the lower electrode, leading to low shunt resistances, high dark current and poor device efficiency. Therefore, the surface topography and roughness of the AgNWs TCFs were of significant importance for their compatibility with high-efficiency devices, as a result of which SPM images of the samples before and after AZO deposition were shown in Fig. 3. The root mean square (RMS) for both samples was 79.3 nm and 53.6 nm, respectively. Though the surface roughness was not low enough for compatibility with high-performance electronic devices, the surface roughness of the ASC5/AZO composite TCF was considerably reduced compared to the surface roughness of the bare AgNWs mesh. It is expected that the surface of the composite film could become smoother through a mechanical pressing technique. Generally, a post annealing process was always adopted to fuse the junction between the nanowires. Thus the contact resistance between the nanowires could be reduced and the conductivity of the film would be enhanced. Herein, a 200 ℃ annealing treatment was used to further improve the TCF’s performance according to the results reported in the literatures [23]. In order to know the phase change of the AgNWs /AZO
Fig. 4. (a) XRD patterns of as-prepared and annealed AgNWs/AZO composite film; (b) magnified view of plot (a) around the (002) peak; (c) Raman spectra of as-prepared and annealed AgNWs/AZO composite film and (d) EDS data of annealed AgNWs/AZO composite film, the inset in plot (d) showed the scanning region during EDS test.
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Fig. 5. Optical transmittance of unannealed AgNWs TCFs (a) and unannealed AgNWs/AZO composite TCFs (b) measured by UV–Vis spectrometer with the substrates subtracted, and the photos (c) (d) below the spectra showed the macroscopic appearance of the corresponding samples.
of the arrows revealed the changing trend of the transmittance at 550 nm and of sheet resistance of the films after different treatment. The points circled in Fig. 6(a) and (b) referred to the films with one and three spin-coating times, respectively. From the point of view for application, the samples in the upper left region of Fig. 6(a) and (b) were more desirable due to their high transmittance and high conductivity. The particular data of all the samples were listed in Table 1. The sample numbers in the table represented the spin-coating time. In general, the transmittance at 550 nm and sheet resistance of all the samples tended to decrease with increasing the spin-coating time. They were resulted from the fact that the relatively low transmittance of the as-sputtered AZO layer [27] accompanied with AZO’s ability to bind or tie [28,29] the original loosely connected AgNWs led to the decrease of transmittance at 550 nm and of sheet resistance for the composite films, respectively. Interestingly, the transmittance at 550 nm slightly rose accompanied with further lowering their sheet resistance after annealing treatment. The promotion in the transmittance at 550 nm of the composite film was caused by the diffusion of Al into ZnO during annealing process [27] as demonstrated by XRD shown in Fig. 4(a), while the further reduction of its sheet resistance should be due to the partial fusion and closer contact of AgNWs [30]. Figure of merit ΦTC defined by Haacke [31] was often adopted as an index to estimate the quality of the obtained transparent conductive film:
to its relatively low signal intensity, EDS test was conducted to demonstrate the presence of Ag element in the composite film (as shown in Fig. 4(d)). As transmittance was a very important parameter for TCFs, optical transmittance of unannealed AgNWs TCFs (Fig. 5(a)) and unannealed AgNWs/AZO composite TCFs (Fig. 5(b)) was represented. With the increase of spin-coating time, the transmittance of both unannealed AgNWs TCFs and unannealed AgNWs/AZO composite TCFs showed a decreasing tendency in which fewer voids existed with more AgNWs covering the substrate surface. Compared with the unannjjealed AgNWs TCFs, the transmittance of unannealed AgNWs/AZO composite TCFs with the same AgNWs coverage was slightly lower due to the presence of AZO protective layer. Photographs (Fig. 5(c) and (d)) below the spectra showed the macroscopic appearance of all the samples through which the transmittance of different samples could be roughly determined. It was obvious that all the TCFs in Fig. 5(c) and (d) were transparent so that the logo of our university could be clearly seen through all these films. Fig. 6(a) showed the relationship between transmittance at 550 nm and sheet resistance for AgNWs TCFs and unannealed/annealed AgNWs/AZO TCFs, while Fig. 6(b) revealed the magnified view of the circled region in Fig. 6(a). The five symbols in a same line referred to the samples with different AgNWs coverages, and the AgNWs coverage increased from right to left for the same line. Arrows were used to interconnect symbols enclosed by the dotted circles, and the direction 5
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Fig. 6. The relationship between transmittance at 550 nm and sheet resistance for AgNWs film and un/annealed composite film (a) (b) and figure of merit for AgNWs film and un/ annealed composite film with different spin-coating time (c).
and firmly anchored on the substrates. The oxide-resistive ability of AgNWs TCFs and AgNWs/AZO TCFs were tested by putting the two samples on a hotplate with its temperature set at 80 ℃ in the air atmosphere. The plot of measured sheet resistance versus time was shown in Fig. 7(a). The sheet resistance of annealed AgNWs/AZO film did not show a significant rise, while the annealed AgNWs lost lots of its conductance over treating time, which demonstrated the relatively robust oxide resistivity of our composite film. The reason for this phenomenon was due to the fact that the capped thin AZO layer prevented the direct contact between oxygen in air and the surface of AgNWs. As adhesion was also an important parameter in practical applications, a tape test was adopted to confirm adhesion ability of our composite film towards substrate. As clearly shown in Fig. 7(b), the AgNWs film was vulnerable to detachment during test process, whereas the composite film remained intact, indicative of strong adhesion, which was resulted from the protection of AZO layer. Another important requirement of TCFs for flexible devices including flexible photovoltaic devices and flexible supercapacitors was its ability to be flexible. In order to prove the flexibility of our composite film, a bending test was performed during which the testing film (T % at 550 nm is 81% and Rs is 39 Ω/□) coated on PET substrate was repeatedly bent with the radius of curvature fixed at 3 mm (Fig. 8). The inset (up-left and up-right) in Fig. 8 showed the transparency and flexibility of the composite film and the middle schematics showed the bending test condition. Specifically, relative sheet resistance variation ΔR/R was adopted to demonstrate the stability of the composite film under bending test, where ΔR and R referred to the sheet resistance difference of the tested film before and after bending test and initial
Table 1 Transmittance at 550 nm and sheet resistance of as-prepared AgNWs film, unannealed AgNWs/AZO film and annealed AgNWs/AZO film. AgNWs film
Unannealed AgNWs/ AZO film
Annealed AgNWs/ AZO film
Samples
T (%)
Rs (Ω/□)
T (%)
Rs (Ω/□)
T (%)
Rs (Ω/□)
1 2 3 4 5
95 92 88 85 84
3268 90 28 24 14
91 87 84 80 79
1123 74 23 22 13
92 88 85 83 80
557 69 19 15 17
ΦTC =
T10 Rs
(1)
where T and Rs referred to the transmittance at 550 nm and sheet resistance, respectively. According to the data in Fig. 6(c), the circled points referred to the samples with the ΦTC bigger than the commercially used ITO (ΦTC≈8.8) among which the specific values of our annealed composite films were 10.4 and 10.3 respectively. The highest ΦTC for our composite film was much larger than that of AZO (0.53) and AgNWs film (5.9) reported in the literatures [32,33]. This result paved a road for our annealed composite films to replace the commercially available ITO film. One advantage of our AgNWs/AZO TCFs was its excellent oxideresistive ability (Fig. 7(a)) and mechanical robustness against adhesion (Fig. 7(b)), since the AgNWs was protected by the capped AZO layer 6
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Fig. 7. (a) Oxide-resistance test: the sheet resistance value of annealed AgNWs film and annealed AgNWs/AZO composite film varied with time at 80 ℃ in air atmosphere; (b) photograph of AgNWs film (left) and AgNWs/AZO composite film (right) after tape test.
surface resistance of 19 Ω/□. In addition, the composite TCFs were oxide-resistive, robustly adhesive towards substrates and mechanically bendable. The figure of merit for composite TCFs was larger than that of commercial ITO film, which demonstrated its potential ability to replace ITO in flexible device areas. Acknowledgements This work has been financially supported by National Nature Science Foundation of China (61176062), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Funding of Jiangsu Innovation Program for Graduate Education KYLX15_0304 (the Fundamental Research Funds for the Central Universities), and the research fund of Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology (SKLPSTKF201506). References Fig. 8. Resistance change of AgNWs/AZO TCFs under bending test; the insets show the bending test sample (left), bending test condition (middle) and bending testing process (right).
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4. Conclusions In this work, we successfully prepared AgNWs/AZO composite TCFs with excellent performance by combining spin-coating AgNWs and magnetron sputtering AZO. The scan electron microscopy and scanning probe microscope showed that the surface roughness could be improved by sputtering AZO protective layer without disturbing the networks of nanowires underneath. The X-ray diffraction and Raman spectroscopy revealed that a post annealing treatment at 200 ℃ accelerated the substitutional incorporation of Al3+ into the Zn2+ sites accompanied with the improvement of AZO crystallinity and release of residual stress in the as-prepared composite film, which demonstrated an improved conductivity and transmittance of the composite film after the annealing process. The UV–Vis spectrophotometer and four point probe test showed that the pure AgNWs film with three spin-coating times exhibited a good transmittance of 88% at 550 nm and a low surface resistance of 28 Ω/□, while the corresponding annealed composite TCFs showed a transmittance of 85% at 550 nm and a low 7
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Preparation of silver nanowire/AZO composite film as a transparent conductive material ⁎
Quntao Tanga, Honglie Shena,b, , Hanyu Yaoa, Ye Jianga, Chaofan Zhenga, Kai Gaoa a College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China b Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Changzhou University, Changzhou 213164, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Silver nanowire Transparent conductive film AZO Magnetron sputtering
In this paper, flexible, robustly adhesive, surface smooth and oxide-resistive AgNWs/AZO composite transparent conductive films (TCFs) were prepared by spin-coating solvothermal derived AgNWs followed by magnetron sputtering AZO protective layer, after which, annealing treatment at 200 ℃ for 20 min was adopted to further increase the performance of the composite film. The samples were characterized by means of X-ray diffraction, Raman spectroscopy, scan electron microscopy, scanning probe microscope, UV–Vis spectrophotometer and four point probe. The best result of AgNWs/AZO composite films was the one with a transmittance of 85% at 550 nm accompanied with a low sheet resistance of 19 Ω/□ by controlling the spincoating times fixed at three. The figure of merit for the composite TCFs was 10.4, which was larger than the value of 8.8 for commercial ITO film. These amazing results would render the AgNWs /AZO composite TCFs an ideal candidate to replace conventional ITO for its potential application in flexible devices including OPV and OLED.
1. Introduction Transparent conductive films (TCFs), especially flexible TCFs, have attracted tremendous attention for their wide range of applications including wearable sensors, transparent film heaters, organic light emitting diode (OLED) and so forth [1–3]. Typically, indium tin oxide (ITO) has been widely used as TCF due to its excellent transparency across the visible spectrum range, low sheet resistance, and the compatibility of its work function with most devices. However, two drawbacks accompanied with ITO have limited its further application for next-generation flexible devices: one was the rising cost due to indium scarcity, which rendered its difficulty to be used in low-cost, large-area electronics, and the other one was the fact that ITO was brittle and could easily crack when being used in applications where bending was involved [4,5]. Due to the high dc conductivity and optical transmittance of silver nanowires (AgNWs) random networks, AgNWs have been widely synthesized and studied by lots of research groups in the past few years for their potential ability to replace conventional ITO TCFs [6–8]. Although the performance of AgNWs TCFs was excellent, they were easy to suffer from oxidation when exposured to air, as a result of which their potential performance in practical applications was greatly limited. Therefore, a lot of protective materials such as
graphene [9,10], reduced graphene oxide (RGO) [11], nickel [12] and palladium [13] were adopted by different groups to strengthen the oxidation-resistive ability of AgNWs TCFs without degrading their original performance too much. Simultaneously, Al-doped zinc oxide (AZO) thin film, as an interesting transparent conductive oxide (TCO) material, has also been extensively investigated to substitute the widely used expensive ITO for its rich storage and non-toxicity [14–16]. It has been applied into various fields including flat-panel displays [17], thin film solar cells [18], gas sensors [19] and so on. However, in some applications the performance of the AZO thin films was inferior to that of ITO, which limited its further application. Therefore, films with multilayer structures have been proposed for transparent conducting materials in literatures [20,21]. It was believed that the combination of AgNWs and AZO film could not only improve the air-stability of AgNWs due to the protection of AZO capping layer but also enhanced the performance of the AZO layer for the superior conductivity of AgNWs, as a result of which the AgNWs/AZO composite film with excellent performance could be obtained. In this paper, AZO layer was deposited on spin-coated AgNWs TCFs by magnetron sputtering method to improve the performance of the whole film including surface roughness, thermal stability, adhesiveness towards the substrate and flexibility without sacrificing its original
⁎ Corresponding author at: College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China. E-mail address: [email protected] (H. Shen).
http://dx.doi.org/10.1016/j.ceramint.2016.10.048 Received 23 July 2016; Received in revised form 23 September 2016; Accepted 7 October 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Tang, Q., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.10.048
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with different AgNWs coverage, the spin-coating procedure was repeated as wanted. After desired coating times, the AgNWs TCFs (SC1, SC2, SC3, SC4 and SC5, SC was short for spin coating, while the numeric number denoted the spin-coating time) were obtained. For the flexibility experiment, the AgNWs film was prepared on 6×6 cm2 flexible PET by Mayer bar method.
performance too much. The purpose of this investigation was to investigate the potential ability of the AgNWs /AZO composite films as a transparent conductive material for flexible electronic devices. 2. Experimental details 2.1. Synthesis of AgNWs
2.4. Sputtering of AZO protective layer Silver nanowires were synthesized by a solvothermal method. Typically, 0.8 g polyvinylpyrrolidone (PVP, MW=55000) and 2.2 mg NaCl were subsequently added into 25 ml ethylene glycol (EG) in the stirring process at 120 ℃ to obtain a solution A. Simultaneously, 0.27 g AgNO3 was dissolved into 15 ml ethylene glycol (EG) to obtain a solution B. Subsequently, the obtained A solution was dropwise added into solution B to get the final mixture solution C. A 40 ml mixture solution C was then transferred into a 50 ml autoclave. The autoclave was heated at 160 ℃ for 5 h and then naturally cooled down to room temperature. The obtained solution was centrifuged at 4000 rpm for 20 min to remove the small Ag nanoparticles and short Ag nanowires by discarding the supernatant after centrifugation. After being washed by ethanol and water alternatively several times, the resulted AgNWs were dispersed into ethanol to get a silver nanowire alcoholic solution with a concentration of 2 mg/ml for further use.
The AZO protective layer was sputtered on AgNWs TCFs by magnetron sputtering method to prepare AgNWs / AZO composite TCFs. AZO (Al2O3: 2% wt) target was used as the sputtering target. During the sputtering process, the substrate temperature was kept at room temperature with a chamber base pressure of 8.0×10−4 Pa and 20 sccm Ar flowing. The sputtering power and time used here were 150 W and 20 min, respectively. The thickness of the deposited AZO layer was estimated to be about 120 nm. SC1/AZO, SC2/AZO, SC3/AZO, SC4/ AZO, SC5/AZO below referred to the AgNWs/AZO composite TCFs without annealing treatment, while ASC1/AZO, ASC2/AZO, ASC3/ AZO, ASC4/AZO and ASC5/AZO were the AgNWs/AZO composite TCFs annealed at 200 ℃ for 20 min. The numeric number here all denoted the spin-coating time. For the flexibility experiment, the protective AZO was also deposited on AgNWs mesh on the surface of flexible PET substrate.
2.2. Substrate treatment 2.5. Characterization Substrate cleaning and hydrophilic treatment were two important procedures for the realization of the final uniform TCFs. The slide glass substrates used here were firstly cleaned by SC1 (Volume ratio: NH3· H2O:H2O2:H2O =1:1:5) and SC2 (Volume ratio: HCl:H2O2:H2O =1:1:5), respectively, after which it was rinsed by deionized (DI). Before the preparation of AgNWs TCFs, hydrophilic treatment was received by SPM (Volume ratio: H2SO4:H2O2:H2O =1:1:5) treatment followed by DI water rinsing. The hydrophilic treatment of PET flexible substrate was conducted by oxygen plasma treatment in magnetron sputtering equipment. The substrate temperature was kept at 25 ℃ with a chamber base pressure of 6.0×10−4 Pa and 30 sccm O2 flowing. The power and time used here were 100 W and 600 s, respectively.
The surface morphology of AgNWs and AgNWs/AZO composite TCFs were characterized by field emission scanning electron microscope (SEM, Hitachi S-4800) and scanning probe microscope (SPM, Shimadz SPM9600). The phase and crystallographic information were obtained from the X-ray diffraction patterns (XRD, Rigaku Ultima-IV diffraction-meter with Cu–Kα radiation source) and Raman spectroscopy (Thermo Fisher DXR). The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS) attached to the SEM. Room-temperature transmission spectrum measurement in the range of 380–1000 nm and sheet resistance test were conducted by a Shimadzu UV3600 and four point probe (KDY-1), respectively.
2.3. Preparation of AgNWs TCFs
3. Results and discussion
The AgNWs TCFs were prepared by spin-coating method. Typically, 30 μl obtained AgNWs alcoholic solution was dropped onto a slide glass substrate with a size of 2×2 cm2, after which spinning process at 1300 rpm for 60 s was adopted to get the TCFs. In order to get TCFs
The schematic process for preparation of AgNWs/AZO composite TCFs was shown in Fig. 1. After centrifugation of the as-obtained solution by several times, an AgNWs alcoholic solution with a certain concentration could be obtained and was further used for the fabrica-
Fig. 1. Schematic illustration for the preparation of AgNWs/AZO composite TCFs.
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Fig. 2. SEM images of SC1 (a): low magnification, (e): high magnification; SC3 (b): low magnification, (f): high magnification; SC5 (c): low magnification, (g): high magnification and ASC5/AZO (d): low magnification, (h): high magnification. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 3. three-dimensional SPM images of SC5 sample (a) and ASC5/AZO sample (b).
coverage realized by varying spin-coating time. The average diameter and average length of the obtained AgNWs was about 97 nm and 78 µm, respectively. It was obvious that the AgNWs formed a uniform mesh without a significant nanowires density difference across the substrate. As the spin-coating time increased from (a) to (c), more silver nanowires covered the substrate surface and bridged with each other (the red dotted circle regions in Fig. 2(e), (f) and (g)), and thus fewer voids between AgNWs were left for the passage of light, as a
tion of AgNWs TCFs by spin-coating method. Subsequently, magnetron sputtering method, as a developed technology for the commercial process of transparent conductive oxide (TCO) film, was adopted to sputter an AZO protective layer onto AgNWs mesh. Finally, in order to decrease the junction resistance between AgNWs, an annealing treatment was conducted to further increase the performance of the AgNWs/AZO composite TCFs [22]. Fig. 2 showed SEM images of AgNWs TCFs with different AgNWs 3
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composite film before and after annealing process, phase characterizations including XRD and Raman measurements were conducted. Fig. 4(a) showed the XRD patterns of as-prepared and annealed AgNWs /AZO composite film. Both films showed a dominant peak centered at around 2θ=34° which corresponded to the (002) crystal plane of AZO film, indicating a preferential orientation of c-axis perpendicular to the substrate surface. Simultaneously, the increased intensity of (002) peak due to annealing process indicated the improved crystallinity of the annealed composite film. The magnified XRD patterns around (002) peak was shown in Fig. 4(b). It could be clearly seen that annealing treatment shifted the (002) peak from 2θ=34.09° to 2θ=34.19°, indicating the decrease of the interplanar spacing along the c axis. The decrease in lattice parameters of the annealed composite film probably occurred by the substitutional incorporation of Al3+ ions into Zn2+ sites due to the smaller radius of Al3+(0.53 Å) than that of Zn2+(0.72 Å) during annealing process [24]. In order to further understand the structure change before and after annealing process, Raman spectra were shown in Fig. 4(c). For both samples, E2-high and A1-LO mode could be observed. The appearance of the E2 mode suggested the preferred wurtzite structure of the AZO grains, which agreed well with the XRD observation shown in Fig. 4(a). Normally, E2 peak was at 438 cm−1 for pure AZO film. Annealing treatment shifted the E2 peak from 451.6 cm−1 to 438.3 cm−1, indicating the reduction of residual stress in the as-prepared composite film [25]. The presence of A1-LO mode was always attributed to oxygen vacancy (Vo) [26]. It could be seen that the thermal treatment reduce the peak intensity due to the occupation of Vo by hydrogen atoms (forming Ho). As the XRD peaks of AgNWs could not be observed due
result of which both the surface resistivity and optical transmittance decreased. After the deposition of AZO protective layer, the pre-formed AgNWs mesh was buried under the AZO capping layer without disturbing the networks of nanowires (as shown in Fig. 2(d)). Compared with the bare AgNWs, the diameter of the AZO sputtered nanowires was larger (as shown in Fig. 2(h)), which was caused by the coverage of the AZO layer on AgNWs. In the practical application of electronic devices, surfaces with high roughness could cause shorting through each layer of the device, and this was especially problematic for devices using AgNWs as the lower electrode, leading to low shunt resistances, high dark current and poor device efficiency. Therefore, the surface topography and roughness of the AgNWs TCFs were of significant importance for their compatibility with high-efficiency devices, as a result of which SPM images of the samples before and after AZO deposition were shown in Fig. 3. The root mean square (RMS) for both samples was 79.3 nm and 53.6 nm, respectively. Though the surface roughness was not low enough for compatibility with high-performance electronic devices, the surface roughness of the ASC5/AZO composite TCF was considerably reduced compared to the surface roughness of the bare AgNWs mesh. It is expected that the surface of the composite film could become smoother through a mechanical pressing technique. Generally, a post annealing process was always adopted to fuse the junction between the nanowires. Thus the contact resistance between the nanowires could be reduced and the conductivity of the film would be enhanced. Herein, a 200 ℃ annealing treatment was used to further improve the TCF’s performance according to the results reported in the literatures [23]. In order to know the phase change of the AgNWs /AZO
Fig. 4. (a) XRD patterns of as-prepared and annealed AgNWs/AZO composite film; (b) magnified view of plot (a) around the (002) peak; (c) Raman spectra of as-prepared and annealed AgNWs/AZO composite film and (d) EDS data of annealed AgNWs/AZO composite film, the inset in plot (d) showed the scanning region during EDS test.
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Fig. 5. Optical transmittance of unannealed AgNWs TCFs (a) and unannealed AgNWs/AZO composite TCFs (b) measured by UV–Vis spectrometer with the substrates subtracted, and the photos (c) (d) below the spectra showed the macroscopic appearance of the corresponding samples.
of the arrows revealed the changing trend of the transmittance at 550 nm and of sheet resistance of the films after different treatment. The points circled in Fig. 6(a) and (b) referred to the films with one and three spin-coating times, respectively. From the point of view for application, the samples in the upper left region of Fig. 6(a) and (b) were more desirable due to their high transmittance and high conductivity. The particular data of all the samples were listed in Table 1. The sample numbers in the table represented the spin-coating time. In general, the transmittance at 550 nm and sheet resistance of all the samples tended to decrease with increasing the spin-coating time. They were resulted from the fact that the relatively low transmittance of the as-sputtered AZO layer [27] accompanied with AZO’s ability to bind or tie [28,29] the original loosely connected AgNWs led to the decrease of transmittance at 550 nm and of sheet resistance for the composite films, respectively. Interestingly, the transmittance at 550 nm slightly rose accompanied with further lowering their sheet resistance after annealing treatment. The promotion in the transmittance at 550 nm of the composite film was caused by the diffusion of Al into ZnO during annealing process [27] as demonstrated by XRD shown in Fig. 4(a), while the further reduction of its sheet resistance should be due to the partial fusion and closer contact of AgNWs [30]. Figure of merit ΦTC defined by Haacke [31] was often adopted as an index to estimate the quality of the obtained transparent conductive film:
to its relatively low signal intensity, EDS test was conducted to demonstrate the presence of Ag element in the composite film (as shown in Fig. 4(d)). As transmittance was a very important parameter for TCFs, optical transmittance of unannealed AgNWs TCFs (Fig. 5(a)) and unannealed AgNWs/AZO composite TCFs (Fig. 5(b)) was represented. With the increase of spin-coating time, the transmittance of both unannealed AgNWs TCFs and unannealed AgNWs/AZO composite TCFs showed a decreasing tendency in which fewer voids existed with more AgNWs covering the substrate surface. Compared with the unannjjealed AgNWs TCFs, the transmittance of unannealed AgNWs/AZO composite TCFs with the same AgNWs coverage was slightly lower due to the presence of AZO protective layer. Photographs (Fig. 5(c) and (d)) below the spectra showed the macroscopic appearance of all the samples through which the transmittance of different samples could be roughly determined. It was obvious that all the TCFs in Fig. 5(c) and (d) were transparent so that the logo of our university could be clearly seen through all these films. Fig. 6(a) showed the relationship between transmittance at 550 nm and sheet resistance for AgNWs TCFs and unannealed/annealed AgNWs/AZO TCFs, while Fig. 6(b) revealed the magnified view of the circled region in Fig. 6(a). The five symbols in a same line referred to the samples with different AgNWs coverages, and the AgNWs coverage increased from right to left for the same line. Arrows were used to interconnect symbols enclosed by the dotted circles, and the direction 5
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Fig. 6. The relationship between transmittance at 550 nm and sheet resistance for AgNWs film and un/annealed composite film (a) (b) and figure of merit for AgNWs film and un/ annealed composite film with different spin-coating time (c).
and firmly anchored on the substrates. The oxide-resistive ability of AgNWs TCFs and AgNWs/AZO TCFs were tested by putting the two samples on a hotplate with its temperature set at 80 ℃ in the air atmosphere. The plot of measured sheet resistance versus time was shown in Fig. 7(a). The sheet resistance of annealed AgNWs/AZO film did not show a significant rise, while the annealed AgNWs lost lots of its conductance over treating time, which demonstrated the relatively robust oxide resistivity of our composite film. The reason for this phenomenon was due to the fact that the capped thin AZO layer prevented the direct contact between oxygen in air and the surface of AgNWs. As adhesion was also an important parameter in practical applications, a tape test was adopted to confirm adhesion ability of our composite film towards substrate. As clearly shown in Fig. 7(b), the AgNWs film was vulnerable to detachment during test process, whereas the composite film remained intact, indicative of strong adhesion, which was resulted from the protection of AZO layer. Another important requirement of TCFs for flexible devices including flexible photovoltaic devices and flexible supercapacitors was its ability to be flexible. In order to prove the flexibility of our composite film, a bending test was performed during which the testing film (T % at 550 nm is 81% and Rs is 39 Ω/□) coated on PET substrate was repeatedly bent with the radius of curvature fixed at 3 mm (Fig. 8). The inset (up-left and up-right) in Fig. 8 showed the transparency and flexibility of the composite film and the middle schematics showed the bending test condition. Specifically, relative sheet resistance variation ΔR/R was adopted to demonstrate the stability of the composite film under bending test, where ΔR and R referred to the sheet resistance difference of the tested film before and after bending test and initial
Table 1 Transmittance at 550 nm and sheet resistance of as-prepared AgNWs film, unannealed AgNWs/AZO film and annealed AgNWs/AZO film. AgNWs film
Unannealed AgNWs/ AZO film
Annealed AgNWs/ AZO film
Samples
T (%)
Rs (Ω/□)
T (%)
Rs (Ω/□)
T (%)
Rs (Ω/□)
1 2 3 4 5
95 92 88 85 84
3268 90 28 24 14
91 87 84 80 79
1123 74 23 22 13
92 88 85 83 80
557 69 19 15 17
ΦTC =
T10 Rs
(1)
where T and Rs referred to the transmittance at 550 nm and sheet resistance, respectively. According to the data in Fig. 6(c), the circled points referred to the samples with the ΦTC bigger than the commercially used ITO (ΦTC≈8.8) among which the specific values of our annealed composite films were 10.4 and 10.3 respectively. The highest ΦTC for our composite film was much larger than that of AZO (0.53) and AgNWs film (5.9) reported in the literatures [32,33]. This result paved a road for our annealed composite films to replace the commercially available ITO film. One advantage of our AgNWs/AZO TCFs was its excellent oxideresistive ability (Fig. 7(a)) and mechanical robustness against adhesion (Fig. 7(b)), since the AgNWs was protected by the capped AZO layer 6
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Fig. 7. (a) Oxide-resistance test: the sheet resistance value of annealed AgNWs film and annealed AgNWs/AZO composite film varied with time at 80 ℃ in air atmosphere; (b) photograph of AgNWs film (left) and AgNWs/AZO composite film (right) after tape test.
surface resistance of 19 Ω/□. In addition, the composite TCFs were oxide-resistive, robustly adhesive towards substrates and mechanically bendable. The figure of merit for composite TCFs was larger than that of commercial ITO film, which demonstrated its potential ability to replace ITO in flexible device areas. Acknowledgements This work has been financially supported by National Nature Science Foundation of China (61176062), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Funding of Jiangsu Innovation Program for Graduate Education KYLX15_0304 (the Fundamental Research Funds for the Central Universities), and the research fund of Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology (SKLPSTKF201506). References Fig. 8. Resistance change of AgNWs/AZO TCFs under bending test; the insets show the bending test sample (left), bending test condition (middle) and bending testing process (right).
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4. Conclusions In this work, we successfully prepared AgNWs/AZO composite TCFs with excellent performance by combining spin-coating AgNWs and magnetron sputtering AZO. The scan electron microscopy and scanning probe microscope showed that the surface roughness could be improved by sputtering AZO protective layer without disturbing the networks of nanowires underneath. The X-ray diffraction and Raman spectroscopy revealed that a post annealing treatment at 200 ℃ accelerated the substitutional incorporation of Al3+ into the Zn2+ sites accompanied with the improvement of AZO crystallinity and release of residual stress in the as-prepared composite film, which demonstrated an improved conductivity and transmittance of the composite film after the annealing process. The UV–Vis spectrophotometer and four point probe test showed that the pure AgNWs film with three spin-coating times exhibited a good transmittance of 88% at 550 nm and a low surface resistance of 28 Ω/□, while the corresponding annealed composite TCFs showed a transmittance of 85% at 550 nm and a low 7
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