1. Introduction

Transparent electrodes with high light transmittance (usually in the visible wavelength range) and low sheet resistance are one of the critical elements for many optoelectronic applications [1], such as light emitting diodes (LEDs), touch panels, liquid crystal displays, and solar cells. Tin-doped indium oxide (ITO) is currently dominating the transparent electrode market. However, indium is scarce, making its compound, ITO, increasingly expensive. ITO cannot work well at high temperatures (e.g., > 500 °C) with degradation of the electrical performance, which is mainly due to the variation of the grain-boundary potential [2]. Other impurity-doped metal oxides, e.g., aluminium-doped zinc oxide, fluorine-doped tin oxide, etc., have been developed to replace ITO, though they unfortunately suffer from brittleness (like ITO) [3]. Recently, some emerging materials have been proposed, such as conducting polymers [4, 5 ], carbon nanotubes [6, 7 ], graphene [8–11 ], and metallic networks [12–31 ]. Among them, carbon-based materials still cannot rival commercial ITO in terms of transparency and conductivity, while metallic networks are more promising, especially silver nanowire (Ag NW) networks [1, 32–34 ]. Bulk silver material is a very good conductor, with a very small resistivity of 1.5 μΩ•cm, very high carrier concentration of 5.8 × 10²² cm⁻³, and very high electron mobility of 72 cm²V⁻¹s⁻¹ [1]. Its small plasma wavelength (0.14 μm) will not induce too much absorption in the visible wavelength range. The structure of nanowire networks, which can be fabricated easily via various methods, is more favorable for light scattering and thus transmission enhancement. Therefore, Ag NW networks have attracted wide attention among the research community. Much effort has been made to improve nanowire network surface roughness and adhesiveness [12–15 ], wire-to-wire contact resistance [16–18 ], and chemical, thermal and mechanical stability [15–19 ]. However, for practical applications, Ag NWs must be uniformly distributed on a large substrate (at least 4” in diameter), which is still a research topic far from mature.

With solution-synthesized Ag NWs [35, 36 ], Ag NW networks are mainly fabricated by drop-casting [13, 20 ], spin-coating [14–17, 21 ], spray deposition [22], dry-transfer [19, 23 ], Meyer rod coating [18, 24 ], etc. Due to the poor adhesion between Ag NWs and the glass substrate, it is not easy to distribute Ag NWs uniformly in a large area by either drop casting or spin-coating. Additional substrate surface treatment with poly-l-lysine is necessary, but it is still difficult to get a uniform coverage on an entire 2.5 cm × 2.5 cm glass substrate [21]. Spray deposition is also not suitable to fabricate large-area uniform Ag NWs because the nanowires are prone to concentrate in the center [22]. The size of the final sample fabricated by dry-transfer is limited by the size of the initial template covered with Ag NWs, a large number of which must remain on the template and cannot be used any more [19, 23 ]. For Meyer rod coating, the uniformity of the networks depends on the concentration of the Ag NW suspension. A high concentration of Ag NWs is preferred in order to achieve uniform networks, but the light transmittance will be adversely affected [18, 24 ]. Without Ag NWs at hand, electron-beam lithography (EBL) can be employed to define uniform metal grids or fractures [25, 26 ]. Instead of EBL, which is too expensive to be used in mass-production, electrospun polymer fibers [27] and naturally cracked gel films [28] have been proposed as templates for evaporation or sputtering of metals in high vacuum to form random metallic nano-/micro-structure networks. Bio-inspired networks based on chemically extracted leaf venation systems [29, 30 ] and spider's silk web [29] have also been proposed to fabricate transparent electrodes for further optoelectronic applications. The performances and the uniformities of those electrodes greatly depend on the templates, especially for the bio-inspired networks [29, 30 ]. Ag NW networks can also be formed through microwave or furnace sintering of Ag nanoparticles. However, substrates with textured surfaces are required [31].

In this paper, we propose a very simple and easy way to produce uniform Ag NW networks with high transmittance and low sheet resistance in a very large area (4” in diameter). Based on spin-coating, our method is assisted with PMMA, in which solution-synthesized Ag NWs are suspended. Since PMMA can be uniformly spin-coated on any substrate, big or small, rigid or flexible, Ag NWs can also be deposited uniformly wherever PMMA can be applied and removed. We believe that our method is very versatile and promising to be widely applied in various optoelectronic devices. It is known that PMMA can also be applied as a sacrificial material to transfer graphene [11, 37 ] or carbon nanotubes [38] onto a target substrate. In those methods, the to-be-transferred graphene or carbon nanotubes are not carried inside the PMMA film but are sticked to the film surface, which will be mechanically peeled off and be attached to the target substrate. Therefore, the adhesion between the PMMA and the graphene or carbon nanotubes should be sufficiently strong, which is, however, not a very strict requirement for our method. The distinction of our method is that PMMA serves both as a carrier where Ag NWs are suspended and as a scrificial material to be removed ultimately. The full-solution process is without any mechanical techniques and therefore much easier to be conducted.

2. Experimental procedure

Ag NWs with an average length of 10 μm and diameters of 60 nm were purchased from Blue Nano with a concentration of 10 mg/mL in ethanol. The Ag NW suspension was first concentrated to 20 mg/mL and then mixed with PMMA 679.04 at a specific volume ratio. This is a critical parameter, named “X” for easy reference, for us to tune the density of the Ag NWs to be deposited and consequently the optical and electrical performances of the fabricated electrode, which will be explained in detail later. The Ag NW-PMMA mixture was sufficiently mixed by using a VORTEX-GENIE mixer, uniformly suspending the Ag NWs in the PMMA. The suspension was spin-coated on a glass substrate that had been carefully cleaned (or a PDMS film for a flexible substrate), first at 2000 rpm for 3 s and then at another speed for 59 s. In the first rotating phase, the suspension dropped on the substrate can be evenly spread out, while the coating thickness is mainly determined by the speed of the second rotating phase, named “Y” for easy reference. Since the performance of the Ag NW network depends on the suspension thickness, which ultimately determines the total amount of Ag NWs on the substrate, Y is also a very critical parameter to be tuned and optimized. This will be explained in detail below. To remove the PMMA, we immersed the suspension-coated substrate in acetone for two hours, which was long enough for the PMMA to be completely dissolved. During this time, the PMMA gradually dissolved, and the Ag NWs originally embedded in it fell on top of the substrate, forming a random nanowire network. After being taken out of the solution, the sample with the Ag NW network was dried in air, followed by systematical characterizations. From the above fabrication processes, it is seen that PMMA serves as a carrier, taking Ag NWs wherever it goes. Since PMMA can be easily and uniformly spin-coated onto any substrate, Ag NWs can also be uniformly distributed onto any substrate. The morphology of the deposited Ag NW network was inspected with a scanning-electron microscope (SEM, Carl Zeiss Utral 55). Its sheet resistance (R_sh) was measured by our home-built four-probe measurement system, and its transmission spectrum was measured by a spectrometer based on an integrating sphere.

3. Uniform Ag NW networks on rigid substrates

As shown in Fig. 1(a) , a transparent electrode based on a random Ag NW network was successfully fabricated on a glass substrate as large as 4” in diameter, when the key process parameters were chosen as X = 1/2.5 and Y = 5000 rpm for our PMMA-assisted spin-coating method. It will be shown later that such parameter configurations result in optimal optical and electrical performances. The good uniformity can also be seen from the infrared image taken by an infrared camera (Mikron Infrared M7500) shown in Fig. 1(b), where the big blue circle indicates the position of the 4” Ag NW network transparent electrode and the four supports around it are clearly seen. The low-magnification SEM image in Fig. 1(c) shows that the Ag NWs are randomly distributed on the glass substrate with good uniformity. The two tilted SEM images with increasing magnifications shown in Figs. 1(c) and 1(d) demenstrate that both Ag NWs and the substrate are very clean without any residuals or contaminations on them. Therefore, good contacts are allowed among Ag NWs and between the network and the substrate.

 

Fig. 1 (a) A photo and (b) An infrared image of Ag NW network on a 4” glass substrate. SEM images of the Ag NW network on the glass substrate with different magnifications: (b) top view, (c, d) tilted view at 45°. The scale bars are all 1 μm in the three SEM images.

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Its transmission spectra and sheet resistance, R_sh, were measured every 1 cm from the center to the edge of the sample. The transmission spectra at different distances were normalized to the transmission spectrum of a bare glass, as shown in Fig. 2(a) . From this figure, a high transmittance of ~0.91 can be observed in a very broad wavelength range from 380 nm to 1000 nm, which is better than that of a 80 nm thick ITO typically applied to solar cells (as listed in Table 1 ). As the distance increases from the center to the edge, the transmission spectrum varies very little, indicating a high level of uniformity over a large area with a diameter of 4”. The value of R_sh has a narrow range of 4.4 Ω/sq from 12.8 to 17.2 Ω/sq as the distance from the center increases, which is much better than that of a 80 nm thick ITO with R_sh ≈60 Ω/sq as listed in Table 1. For further comparison, another 4” transparent electrode was fabricated by directly spin-coating the diluted Ag NW suspension in ethanol onto the glass substrate. The concentration was only 1.5 mg/mL. The rotary speed was 200 rpm for the first 5 s and then 800 rpm for another 59 s, much slower than our PMMA-assisted method. In this case, sufficient Ag NWs, which have very poor affinity to glass, could remain on the substrate without being discarded with the ethanol. The values of R_sh at the different distances along the radius are included in Fig. 2(b) for comparison with those of the sample fabricated by the PMMA-assisted spin-coating method. Although its transmission spectra shown in Appendix Fig. 7 are very close to each other throughout the whole sample, its R_sh varies widely over a range of 14.5 Ω/sq from 11.9 to 26.4 Ω/sq. In contrast, our PMMA-assisted spin-coating method shows high superiority to the traditional one without the assistance of PMMA in terms of both lower average sheet resistance and better uniformity of Ag NWs on the 4” glass substrate. The error bars shown in Fig. 2(b) also indicate better reproducibility of our PMMA-assisted spin-coating method. We attribute such advantages of our method to the much better adhesiveness of PMMA to the substrate than that of ethanol. From Table 1, where the performances of some typical transparent electrodes based on Ag NW networks are included, it is also seen that Ag NW networks fabaricated by our PMMA-assisted spin-coating method have equivalent or even better optical and electrical perforamncs in comparison with those fabricated by previously-reported methods [13, 18, 21, 22 ], which, however, are inferior in terms of uniformity over a large area, as mentioned before.

 

Fig. 2 (a) Transmission spectra at different distances from the center to the edge of the 4” transparent electrode fabricated by the PMMA-assisted spin-coating method. Comparison between the spin-coating methods with and without the assistance of PMMA in terms of (b) sheet resistance, R_sh, at different distances from the center to the edge and (c) sheet resistance variation, ΔR_sh (normalized to the initial value), after being stored in air for a certain period of time (where the grey solid line and the grey dashed line indicate the average ΔR_sh of 5.98% and 13.7% for samples fabricated by the spin-coating method assisted with and without PMMA, respectively).

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Table 1. Comparison of the optical and electrical performances of our work, ITO, as well as some typical Ag NW networks fabricated by different methods.

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Another significant advantage of our PMMA-assisted spin-coating method is that the transparent electrode fabricated by this method is much more stable than that fabricated by the method without PMMA as the solvent. It is seen in Fig. 2(c) that the values of R_sh for the former electrode degrade by only 5.98% on average after being left in air at room temperature for 150 days, which is much better than those for the latter electrode with degradation of as large as 13.7% on average after being stored in the same condition for only 51 days. We hypothesized that there might be a very thin layer of PMMA left coating the Ag NWs and protecting them from oxidation in air. However, no carbon (the main element of PMMA) was detected by the energy dispersive X-ray spectroscopy in conjunction with the high-resolution SEM, as shown in Appendix Fig. 8. The reason for the long-term stability remains unclear, but such long-term stability is of great importance for practical applications.

In order to understand the mechanism of our PMMA-assisted spin-coating method, we conducted a series of experiments to investigate the effects of its critical processing parameters, including the volume ratio, X, and the second rotary speed, Y, on the optical and electrical performances of the fabricated Ag NW networks. According to the procedure, the volume ratio, X, of Ag NWs mixed with PMMA is the first parameter to prepare, which determines how many Ag NWs can be deposited onto the substrate. With other parameters fixed, more Ag NWs can be deposited onto the substrate if X becomes larger. In this case, the conductivity of the network can be definitely enhanced, but the transmission will be degraded because of the strong reflective scattering of light and the absorption of Ag NWs. This is clearly demonstrated in Fig. 3(a) (where Y = 4000 rpm). Considering that high transmission and low R_sh are preferred for transparent electrodes, we chose X = 1/2.5, where its transmission spectrum is sufficiently high (> 0.9 on average) and its R_sh is sufficiently low (~20 Ω/sq), to conduct the following investigations.

 

Fig. 3 The effects of various parameters of the PMMA-assisted spin-coating method on the optical transmission and sheet resistance, R_sh, of the fabricated Ag NW networks on glass substrates. (a) Transmission spectra and R_sh (black dashed curve) for different volume ratio, X, of the Ag NW suspension in PMMA at the second rotary speed, Y = 4000 rpm. (b) Transmission spectra and (c) R_sh (black dashed curve) for different second rotary speed, Y, with X = 1/2.5. In Fig. (c), the corresponding thicknesses of the Ag NW-PMMA film (red solid curve) before the PMMA is removed are also shown by the right axis.

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With an optimal volume ratio, i.e., X = 1/2.5, both the transmission spectrum and the sheet resistance of the fabricated Ag NW network can be further improved by optimizing the rotary speed of the spin-coating process, particularly the second rotary speed, Y, as mentioned above. Figure 3(b) shows that the transmission spectrum is slightly enhanced as Y is increased from 3000 to 8000 rpm, except when Y = 7000 rpm. This might be due to errors during the fabrication processes, which however is acceptable. Its average transmittance still remains close to 0.9. Interestingly, R_sh does not follow the same trend. It drops first and then rises with its minimum of around 12.8 Ω/sq occurring at Y = 5000 rpm. It is easy to understand that when Y is very high, e.g., Y > 5000 rpm, the suspension film becomes very thin as shown in Fig. 3(c), leading to less Ag NWs depositing on the substrate and thus higher R_sh and higher transmission, as shown in Figs. 3(b) and 3(c). In Fig. 3(c), it is seen that the suspension film thickness increases almost linearly with decreasing Y. However, R_sh also increases unexpectedly. From the SEM pictures of the Ag NW networks fabricated at different second rotary speeds shown in Fig. 4 , it is seen that the densities of the Ag NWs deposited at Y = 3000 and 4000 rpm seem comparable to those of the samples fabricated at Y = 7000 and 8000 rpm, even though the suspension films for the former two cases are thicker than those for the latter two cases. Obviously, all of the Ag NW densities in the four cases are lower than those of the samples fabricated at Y = 5000 and 6000 rpm, whose values of R_sh are thus much lower as shown in Fig. 3(c). To further comfirm this, we fabricated another Ag NW network by replacing PMMA 679.04 with densier PMMA 669.06 and obtained a thicker suspension film with thickness of about 520 nm at Y = 4000 rpm. After removing PMMA, the Ag NW density however seems almost the same as that for the case fabricated with PMMA 679.04 and Y = 5000 rpm (Appendix Fig. 9). Looking closer, we found that the thicker suspension film left a lot of residuals or contaminations coating Ag NWs and the substrate (Appendix Fig. 9), resuling in extremely high R_sh. In contrast, carried by the thinner Ag NW-PMMA 679.04 suspension film, much cleaner Ag NW network can be deposited as shown in Figs. 1(d) and 1(e). Considering this, we speculate that some of the Ag NWs embedded in the thick film flow away with the PMMA dissolved in acetone. The remaining Ag NWs loosely accumulate on the substrate, resulting in higher wire-to-wire contact resistance and thus higher R_sh. The light incident on the loose Ag NW network is strongly scattered and localized within it. Consequently, the transmission spectrum becomes lower. Meanwhile, some residuals or contaminations may also appear and adversely affect the optical and electrical properties of Ag NW networks fabricated with lower second rotary speeds (e.g., Y = 3000 and 4000 rpm). Here, the critical film thickness is about 236.6 nm when Y = 5000 rpm and enough Ag NWs can be deposited onto the substrate to form a sufficiently compact network without too many contaminations.

 

Fig. 4 SEM images of Ag NW networks fabricated at different second rotary speeds, i.e., Y = (a) 3000 rpm, (b) 4000 rpm, (c) 5000 rpm, (d) 6000 rpm, (e) 7000 rpm, and (f) 8000 rpm, respectively.

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4. Flexible transparent electrodes

By utilizing the versatile spin-coating method, Ag NWs taken by PMMA in our PMMA-assisted spin-coating method can be deposited onto any substrate. Here in this work, we successfully demonstrated that our method could be easily extended to a flexible substrate, i.e., PDMS (polydimethylsiloxane). The details about the fabrication process are as follows. The PDMS elastomer (Sylgard 184 Silicone Elastomer, Dow Corning) was spin-coated on a clean silicon substrate with an initial rotary speed of 100 rpm for 5 s and a subsequent rotary speed of 200 rpm for 59 s. The sample was thermally annealed at 80 °C for 1 hour so that the top PDMS layer was solidified and became a thin film of about 0.5 mm in thickness. Then, the solidified PDMS film was peeled off from the Si substrate. In order to increase the surface affinity of the film to Ag NWs, the prepared PDMS film was etched in oxygen for 10 s using our inductively coupled plasma etcher (STS ICP). Limited by the sample holder size of the etcher, the flexible PDMS substrate can be as large as 5 cm × 5 cm to be coated with random Ag NWs based on the same procedure used to distribute Ag NW networks onto glass substrates. Since the PDMS surface affinity to Ag NWs was different from glass, the process parameters were slightly tuned. In detail, the 20 mg/mL Ag NWs in ethanol was mixed with PMMA 679.04 at a volume ratio of X = 1/4. This Ag NW-PMMA suspension was spin-coated onto the PMDS film at 2000 rpm for 3 s and then Y = 4000 rpm for 59 s. Afterwards, the sample was immersed in acetone for 2 hours and then dried in air. The fabricated network based on the Ag NWs is uniformly distributed onto the flexible PDMS substrate as large as 5 cm × 5 cm, which is shown in Figs. 5(a) and 5(b) . Good uniformity of the Ag NW network is demonstrated in Fig. 5(c) by the measured transmission spectra (~0.88) and the values of R_sh (16.4 – 22.5 Ω/sq) at different distances from the center to the edge. R_sh is much better than that of an ITO with thickness of 80 nm [20, 25 ] as listed in Table 1, which also indicates that our method is still very competitive to the dry-transfer method [23] in terms of higher transmittance though a little bit higher R_sh is observed. Without any treatment, our Ag NW network shows poor flexibility. A large variation in resistance has been detected before and after bending (Appendix Fig. 10). Especially for convex bending, the wire-to-wire contacts are adversely influenced by the extended film. In spite of this, we will show later that such variation will not affect its application in a circuit as a conducting material. Fortunately, such defects can be removed significantly through appropriate thermal, chemical, or mechanical treatment [14–16 ].

 

Fig. 5 (a) and (b) Photos of Ag NW networks on a flexible PDMS substrate with an area of 5 cm × 5 cm. (c) Transmittance and R_sh (black dashed line) at different distances from the center to the edge of the transparent electrode on the flexible PDMS substrate.

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5. Electrical verification in practical applications

From the above characterization, it is seen that the Ag NW networks fabricated by our PMMA-assisted spin-coating method have not only good uniformity over a large area, but also have high transmisstion and high conductivity, no matter what kind of substrate they are distributed. In order to verify its high conductivity in practical applications, we conducted a simple experiment, where a light-emitting diode (LED) was connected in a circuit between the Ag NW network and a battery as shown in Fig. 6(a) . Only when the Ag NW network is conductive, will the LED become bright. From Figs. 6(b) and 6(c), it is seen that the Ag NW networks work very well on both rigid glass and flexible PDMS substrates, respectively. Even though the flexibility of the Ag NW network on PDMS substrate is not very good, bright light can still be observed from the LED after convex and concave bending of the substrate, as demonstrated in Figs. 6(d) and 6(e), respectively. It is confirmed again that our full-solution processing method is of great potential for mass production of Ag NW networks, which can be employed in practical applications and even replace ITO as transparent electrodes.

 

Fig. 6 Practical application of the conducting Ag NW networks fabricated by our PMMA-assisted spin-coating method in a LED-driving circuit: (a) the circuit in use; Ag NW networks are distributed on (b) the rigid glass substrate, (c) the flat PDMS substrate before bending, (d) the PDMS substrate after convex bending, and (e) the PDMS substrate after concave bending.

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6. Conclusion

In conclusion, we have proposed a full-solution spin-coating method for distributing large-area uniform Ag NW networks on both rigid and flexible substrates with the assistance of PMMA. With this method, a Ag NW network transparent electrode on a glass substrate has been demonstrated with high optical transmittance (~0.91, much better than that of a 80 nm thick ITO typically used in solar cells), good electrical conductivity (R_sh ≈15 Ω/sq on average, also better than that of ITO) and long-term stability. The mechanism has been presented through the systematic investigation of the effects of two key processing parameters, namely, the volume ratio of the 20 mg/mL Ag NWs in ethanol to the PMMA 679.04, and the second rotary speed of the spin-coating process. Furthermore, our method can be easily extended to flexible substrates. A Ag NW network transparent electrode as large as 5 cm × 5 cm has been fabricated on a flexible PDMS substrate with good uniformity. The practical application demonstration confirms again that our method is very robust and promising to be applicable to mass production of Ag NW-based transparent electrodes that are likely to replace commercial ITO.

Appendix

 

Fig. 7 Transmission spectra at different distances from the center to the edge of the 4” transparent electrode fabricated by the conventional spin-coating method without the assistance of PMMA.

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Fig. 8 No carbon is detected by the energy-dispersive X-ray spectroscopy in conjunction with the scanning electron microscopy. O, Na, Al, Si and K are mainly from the glass substrate.

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Fig. 9 SEM images of Ag NW networks fabricated with (a) PMMA 679.04 and Y = 5000 rpm; (b, c) PMMA 669.06 and Y = 4000 rpm.

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Fig. 10 The resistances of one typical sample of our Ag NW network on the PDMS substrate (a) before bending, (b) after convex bending and (c) after concave bending. Silver paste was applied at the opposite ends of the sample to make good electrical contacts to connect to the multimeter through electrical cables.

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Acknowledgments

The authors thank Yichao Liu for his kind help with the infrared image measurement. This work was partially supported by the National Natural Science Foundation of China (Nos. 91233208, 91233119, 61307078, and 61178062), the Program of Zhejiang Leading Team of Science and Technology Innovation (No. 2010R50007), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130101120134) and the Fundamental Research Funds for the Central Universities.

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