Hybrid transparent electrodes of silver nanowires and carbon nanotubes: a low-temperature solution process
© Tokuno et al.; licensee Springer 2012
Received: 14 February 2012
Accepted: 31 May 2012
Published: 31 May 2012
Hybrid transparent electrodes with silver nanowires (AgNWs) and single-walled carbon nanotubes (SWCNTs) were fabricated on plastic films by a low-temperature solution process. The hybrid transparent electrodes exhibited a sheet resistance of 29.2 Ω/sq with a transparency of 80% when 6 wt.% of SWCNTs was mixed with AgNWs. This sheet resistance was less than one-fourth that of the AgNW transparent electrodes that were prepared using the same method. This reduction in sheet resistance is because the SWCNTs formed bridges between the AgNWs, thus, resulting in high conductivity of the hybrid transparent electrodes. The hybrid electrodes formed on plastic films exhibited high conductivity as well as excellent stability in sheet resistance when tested using a repeated bending test.
PACS: 62.23.Hj; 61.48.De; 81.15.-z.
Flexible transparent electrodes are critical components of flexible electronic devices such as touch screens, organic solar cells, and organic light-emitting diodes (OLEDs). Considerable efforts have been made to develop flexible transparent electrodes [1, 2], for example, carbon nanomaterials such as single-walled carbon nanotubes (SWCNTs) [3–10] and graphenes [11–14] and random networks of silver nanowires (AgNWs) [15–27]. Compared with rare and expensive indium tin oxide (ITO) films, SWCNT and graphene transparent electrodes may have significantly lower costs owing to the abundant availability of source materials and coating processes. However, the sheet resistances of SWCNT and graphene transparent electrodes are 100 to 500 Ω/sq with a transparency of 80% to 95% [1, 2, 6, 14]. This value is too high for use with organic solar cells and OLEDs, which require sheet resistances of approximately 10 to 50 Ω/sq . Thus, a substantial improvement in the sheet resistance of SWCNT and graphene electrodes is a necessity. Among these materials, AgNW transparent electrodes exhibit excellent electrical conductivity because silver has the lowest resistivity (as low as 1.59 × 10−6 Ω·cm) among metals as well as good stability and high corrosion stability [15–27]. Moreover, because the AgNW electrodes can be coated by cost-effective and scalable roll-to-roll manufacturing, a variety of electrical devices deposited on AgNW electrodes have been reported including touch screens , organic solar cells [16, 19–21, 25, 27], and OLEDs [22, 23].
Although AgNW electrodes have several excellent features, there is a problem that inhibits their fabrication on heat-sensitive substrates: a heat treatment at approximately 200°C is usually required to achieve high electrical conductivity. The heat treatment fuses the AgNWs such that tight connections are created between them leading to high conductivity [18, 25]. However, AgNW electrodes cannot be fabricated on heat-sensitive substrates because heat-sensitive plastic films are easily deformed by heat treatments at such high temperatures (approximately 200°C). Specifically, plastic films are deformed by heat because they undergo tension in roll-to-roll processes, when the process temperature is higher than the glass transition point of the plastic films. Therefore, processing at lower temperature is desirable for roll-to-roll processes, and room temperature processing is ideal. The key requirement for fabricating AgNW transparent electrodes with low sheet resistance is that the AgNWs should be electrically connected. Certain techniques for the low-temperature processing of AgNW electrodes have been proposed [17, 25–27]. In our previous study, AgNW electrodes on plastic films were dipped into water and ethanol to eliminate insulating polyvinylpyrrolidone (PVP) and then mechanically pressed to connect the junctions between the AgNWs at room temperature . The conductivity of the AgNW transparent electrodes did not increase by simply eliminating the PVP because in this case there contacts between AgNWs were still too weak. The rinsed AgNW transparent electrodes were mechanically pressed to achieve high conductivity at room temperature by connecting the AgNWs. Unfortunately, the use of mechanical pressing is limited in certain cases. For example, mechanical pressing cannot be applied to brittle layers or substrates. Garnett et al. have connected junctions of AgNWs on plastic films by a light-induced plasmonic nanowelding technique . The nanoscale gap between two crossed AgNWs enables effective light concentration and heating at the point where the wires are to be joined together. Zhu et al. have fabricated hybrid transparent electrodes with AgNWs and titanium dioxide (TiO2) on plastic films at a low temperature of 80°C . Tighter contacts between AgNWs were achieved by volume reduction of the TiO2 solution and the capillary force induced by solvent evaporation, which largely improved the conductivity. Although hybrid transparent electrodes of AgNWs and other materials are very useful, only few additional attempts toward their preparation have been reported.
To achieve low sheet resistance at low temperatures, the authors have attempted to incorporate other conductive materials into AgNWs to create conductive bridges between them. Coated AgNW electrodes exhibit high sheet resistance because there are weak contacts, and small spaces exist between the AgNWs [25, 27]. Conductive nanomaterials which form conductive bridges between the AgNWs are expected to enhance the conductivity of AgNW transparent electrodes. The added materials should have high conductivity, high transparency, a suitable shape for creating bridges between the AgNWs, and compatibility with respect to solution processing. Recently, it was reported that CNTs acted as conductive bridges in hybrid transparent electrodes of graphenes, and they enhance the electric conductivity of the electrodes [28–31]. CNTs have high conductivity [32–34] and high transparency owing to their nanoscale diameter. Moreover, only a single coating is required when using a suspension of mixed AgNWs and CNTs because CNTs can be uniformly dispersed into ethanol which is the solvent used in our AgNW suspension. For these reasons, CNTs were selected as the additional material for preparing the hybrid transparent electrodes by a coating method.
In this study, the authors have fabricated transparent electrodes at a low temperature by adding SWCNTs to AgNWs. The hybrid transparent electrodes on plastic films exhibited low sheet resistance, high transparency, and excellent flexibility. These studies on hybrid transparent electrodes demonstrate the potential for the fabrication of electrical devices on plastic films by continuous roll-to-roll processes using a simple, inexpensive, and scalable process.
AgNWs and SWCNTs
AgNWs with an average diameter of approximately 70 nm and length of 5 to 20 μm were synthesized by the reduction of silver nitrate in the presence of PVP in ethylene glycol . Sixteen grams of FeCl3 solution (6 × 10−4 mol/L in ethylene glycol), 1.08 g of silver nitrate, and 0.98 g of PVP (average molecular weight of 360 k in terms of monomeric units) were added to 125 g of ethylene glycol. After the mixture was heated at 150°C for 1.5 h, 30 mL of the resulting suspension was filtered out through a cellulose acetate membrane filter (Y100A047A, Toyo Roshi Kaisha, Ltd., Tokyo, Japan). The filtered AgNWs were then dispersed in ethanol. The concentration of the AgNWs was approximately 4.0 mg/mL. The SWCNTs had a diameter of approximately 1.4 nm and length of 1 to 5 μm. An SWCNT suspension in ethanol (SWNT SO, Meijo Nano Carbon Co. Ltd., Aichi, Japan) was diluted to approximately 0.7 mg/mL. The SWCNT and AgNW suspensions were simply mixed to form hybrid suspension. By varying mixture ratios of AgNW and SWCNT suspensions, hybrid suspensions with different weight percentage were formed. Then, hybrid electrodes were fabricated using these hybrid suspensions. The weight fraction of the SWCNTs varied from 0% to 100%. Fabrication of hybrid transparent electrodes with AgNWs and SWCNTs.
The hybrid suspension was drop-coated onto glass or polyethyleneterephthalate (PET) substrates, and the resulting electrodes were heated at 60°C for 3 min to prevent peel off during the subsequent rinsing process. The heated hybrid electrodes were dipped into a mixed solution of water/ethanol (1:1 w/w) for 3 min and then into ethanol for 10 s. Finally, the hybrid electrodes were heated at 60°C for 20 min. Transparent electrodes only with transparency of 80.0% ± 1.0% were selected to compare the sheet resistance. The hybrid electrodes were fabricated with a weight percentage of SWCNTs ranging from 0 to 100 wt.%. AgNW and SWCNT electrodes and hybrid electrodes with 6 wt.% of SWCNTs with a transparency over 80% were fabricated using diluted suspension.
Characterization of transparent electrodes
Optical transparency at the wavelength of 550 nm was measured using a UV/visible/near-infrared spectrophotometer (V670, JASCO Corp., Tokyo, Japan) with a glass or PET substrate as a reference. The hybrid electrodes on the substrates were observed using a field-emission scanning electron microscope (FE-SEM) (JSM-6700F, JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 5.0 kV and a working distance of 7 to 8 mm. The sheet resistances of the electrodes in a 20 × 20 mm2 were measured using the four-probe method (LorestaGP T610, Mitsubishi Chemical Analytech Co. Ltd., Mie, Japan). The hybrid electrodes on PET films were repeatedly bended by placing them on the convex side of a cylinder with a curvature radius of 5 mm. After the bending test, the sheet resistances were measured again.
Results and discussion
Hybrid transparent electrodes of AgNWs and SWCNTs were fabricated on plastic films by a low-temperature solution process. The hybrid electrodes exhibited a sheet resistance of 29.2 Ω/sq with a transparency of 80%. The SWCNTs formed bridges between the AgNWs and filled in the surrounding spaces, thus, resulting in high conductivity of the hybrid transparent electrodes. Furthermore, the hybrid electrodes on PET films exhibited excellent stability in sheet resistance under repeated application of the bending test with a curvature radius of 5 mm. The current studies on hybrid transparent electrodes demonstrate the potential for the fabrication of electronic devices such as organic solar cells and OLEDs by continuous roll-to-roll processes using a relatively simple, inexpensive, and scalable process.
TT is a doctoral student, MN is an associate professor, JJ is a post-doctoral researcher, and KS is a professor at The Institute of Scientific and Industrial Research (ISIR), Osaka University.
The authors acknowledge Meijo Nano Carbon Co. Ltd. for providing suspension of single-walled carbon nanotubes.
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