Interfacial Engineering of Flexible Transparent Conducting Films

3.1. Modulation of the sheet resistance of SWCNT-based TCFs by silane sol

In this study, we investigated the effect of the interfacial tension between bare SWCNT network films and a top-coating of passivation materials on the Rs of the film. We demonstrated that the Rs of the SWCNT film can be affected by a thermal expansion coefficient (CTE) mismatch between the substrate and the SWCNT film.

The spray-coated SWCNT films have porous structures on a scale of tens of nanometers. The Rs and transmittance are related by [7].

where σ_DC and σ_Op are the DC and optical conductivities, respectively.

The conductivity, σ_DC, of the disordered nanotube films depends on the number density of the network junctions, N_j, which in turn scales with the network morphology through the film fill-factor, V_f, the mean diameter, <D>, of the bundles, and the mean junction resistance, <R_J> [8, 9, 10, 11],

Here, K is the proportionality factor that scales with the bundle length. Therefore, if we can reduce <R_J> and V_f, the sheet resistance of the SWCNT films can be improved. To realize this, the SWCNT films were coated with silane sols by considering their surface energy. Considering the interfacial tension between the SWCNT film and silane sols, two top-coating materials such as a tetraorthosilicate (TEOS) sol with silanol groups and methyltrimethoxysilane (MTMS) sol with hydrophobic methyl groups were used. It is worth noting that top-coating with TEOS sol unexpectedly decreased the Rs of the film to less than 80% of the Rs of the as-prepared film. However, the Rs values of MTMS sol-coated SWCNT films gradually increased. This large disparity between MTMS and TEOS sols can be explained by a change in the contact resistance between the bundles. Hydrophilic TEOS sol can densify the hydrophobic SWCNT networks, while MTMS sol, having methyl groups, can penetrate the hydrophobic SWCNT networks, resulting in an increase of the contact resistance of SWCNTs. This interfacial tension effect was minimized by deposition of gold chloride solution onto the SWCNT film (Figure 5b) to make it hydrophilic, as shown in Figure 5c.

Figure 6 shows the Rs change after heating at 130°C and cooling. Bare PET, hard-coated PET, and glass substrates were used to illustrate the CTE mismatch effect on the Rs changes of SWCNT films. Interestingly, the Rs of the SWCNTs on bare PET substrates increased by 40% relative to the initial values, while the Rs increase was suppressed in bare SWCNT films on hard-coated PET and glass. These results imply that the CTE value should be considered in order to obtain highly stable SWCNT TCFs on plastic substrates. To illustrate this phenomenon, a Raman spectroscopic study was performed, and the G+ and G− peak positions related to the strain of SWCNTs were compared. The G-band frequencies for SWCNT films on bare PET were up-shifted by 1–2 cm⁻¹ after heating at 130°C and cooling, which corresponds to a compressive strain of ~0.1%. This compressive strain may cause the increase of the Rs of the SWCNT film on bare PET.

3.2. Self-passivation of SWCNT films on plastic substrates by nanowelding

Plastic substrates are generally used to fabricate flexible TCFs by deposition of CNTs or metal nanowires. In particular, the electrical properties of SWCNT network films are sensitive to humidity and temperature. In this context, top-coating with passivation materials or hybridization with binder materials are applicable for improving the stability of TCFs. Another way to passivate TCFs is welding or embedding in plastic substrates by chemical or thermal treatments. Figure 7 shows the Rs change of the SWCNT films after deposition of solvents. To investigate the solvent effects, we used solvents with optimal polarity and affinity for the PET substrate. Moreover, the presence of electron-donating and electron-withdrawing groups in the solvent molecules can affect the electronic structure of the SWCNTs. Thus, nonpolar solvents were selected. In particular, aromatic hydrocarbon, benzene, and toluene can swell the PET substrate. Most interestingly, deposition of toluene or benzene decreased the Rs of the SWCNT films. After doping with gold chloride, the Rs and transmittance of the film were measured to be 85 Ω/sq and 90%, respectively. Moreover, I-V plots measured after solvent deposition show clearly that the electrical conductivity of the SWCNT films was enhanced after toluene deposition, which can swell PET substrates.

Figure 8 shows that large SWCNT bundles were welded, and small bundles were embedded on the PET substrate after spraying aromatic hydrocarbons, while spraying cyclohexane did not trigger welding. The strain induced on the SWCNT networks during network formation on the substrate may cause an initial high resistance in the SWCNT network film. Thus, solvent-induced chemical welding of the SWCNT film can release their strain. The recovery of the G band in the Raman spectra of the SWCNT films demonstrates strain relaxation via chemical welding.

Thermal treatment is an alternative way to produce SWCNT film-substrate welding without any chemicals. In particular, fast selective heating of CNTs on plastic substrates can provide an interesting opportunity for thermal welding [14, 15]. Microwaves irradiate the SWCNT films inside the rectangular waveguide microwave applicator, within which the microwave electric field is well defined and controlled. The microwave mode in the applicator is a fundamental transverse electric (TE10) mode (Ez = 0) with a frequency of 2450 MHz, so the microwave electric field (Ey) is sinusoidally distributed along the x- and z-axes and constant along the y-axis. Immediate flash Ohmic heating with an energy conversion of greater than 99% can be realized because the microwave electric field is parallel to the overall SWCNT film and can efficiently induce a fast oscillating current in the film. The amplitude of the conduction current density, Js, induced on the CNT film by the microwave electric field intensity, EMW, may be described as follows [16]:

where σCNT is the electric conductivity of the SWCNT film.

Figure 9a shows the surface temperature and Rs changes of the SWCNT film by varying the irradiation time. The surface temperature of the SWCNT film is dramatically increased after 7 s irradiation at 40 W without heat deflection. Of interest is that the Rs decreased after 7 s of irradiation, due to the occurrence of chemical welding. The Raman spectra in Figure 9e show the strain relaxation of the SWCNT network. The SEM image also shows clearly that the SWCNTs are welded or embedded in the plastic substrate. Importantly, the MW-irradiated SWCNT networks are protected by a self-passivation layer that protects the nanotubes from water molecules. The Rs values of the SWCNT films increase by less than 10% at 80°C and 90% relative humidity, despite embedding of the nanotubes in the plastic substrates.

3.3. CNT-induced migration of AgNW networks into plastic substrates

AgNW-based TCFs are not very environmentally stable without some form of passivation. If the AgNW network can be welded onto a thermoplastic substrate, it can be self-passivated, as was accomplished with SWCNT film. However, the surface tension of AgNWs (~500 mN/m of liquid silver in air) is much different from that of the hydrophobic PC substrate (~34.2 mN/m), which prevents the AgNWs from completely embedding in the plastic substrate, as illustrated in Figure 10. This surface tension mismatch can be solved by deposition of SWCNTs onto the AgNW network because of the low surface tension of CNTs (40–80 mN/m). Therefore, SWCNTs can trigger the migration of AgNWs into plastic substrates by thermal or chemical treatment. Moreover, the high thermal electrical conductivity of the SWCNT can promote the self-passivation of AgNWs by stable Joule heating of the film with an applied DC voltage. Figure 11 shows the surface morphology of the SWCNT-overcoated AgNW film after a voltage of 20 V was applied. In stark contrast to AgNWs in AgNW film shown in Figure 10a, AgNWs were fully embedded in the plastic substrate by electrical heating. Atomic force microscopy (AFM) height profiles also demonstrate the embedding of the AgNW–SWCNT network in the plastic substrate. This self-passivation of AgNW networks assisted by SWCNTs with electrical heating improved the mechanical and hydrothermal stability of the film.

3.4. Interfacial engineering with GO for AgNW TCFs

In terms of the applications of metal nanowire networks, interfacial engineering is an important step to improve their performance with respect to electrical conductivity, environmental stability, surface roughness, and work function modulation. In particular, interfacial engineering of AgNW film can affect the opto-electrical performance because of junction formation in the network. In this study, HOGO nanosheets were utilized for efficient thermal joining of AgNW networks on thermoplastic substrates (Figure 12a). Figure 12b shows the Rs changes of the AgNW network films on bare PC, GO-modified PC, and glass after heating at 150°C with increasing exposure time. The Rs was dramatically reduced by thermal treatment via a junction joining of the networks. Importantly, the Rs decrease of the AgNW film was more efficient on GO-modified PC than on bare PC and glass. Interestingly, the changed Rs of AgNW films on PC was stable even after heating for 180 min, while the Rs of the AgNW film on glass gradually increased, even after 30 min, due to air oxidation. This result provides an opportunity to obtain high-performance AgNW TCFs by a combination of thermal welding and junction joining of AgNW networks. SEM and AFM images in Figure 12 show clearly that on GO nanosheets, limited embedding or welding of AgNWs was observed. This demonstrates the more efficient reduction of Rs of AgNWs on the GO-modified PC.

4.1. Graphene oxide-modified SWCNT-based TCFs

SWCNT-based TCFs with a low haze value are suitable for highly transparent opto-electronic devices. However, for achievement of a low Rs value of the films, one challenge is the development of an efficient and stable dopant. In addition, their high porosity and hydrophobic surface properties are a drawback as an electrode material in opto-electronic devices. In this context, we introduced easily deformable GO nanosheets containing electron-withdrawing groups on the basal plane and edges, which can give a p-type doping effect on the SWCNT film. Figure 13 shows that the Rs of the SWCNT film can be dramatically reduced by up to 40% compared to the as-prepared SWCNT film by deposition of GO solution onto the film by spraying. The efficiency of Rs reduction depends on the lateral sizes of the GO nanosheets. Small-sized GO nanosheets prepared by decanting the first supernatant (S1) by centrifugation were more efficient than larger GO nanosheets. As shown in Figure 14, the SWCNT bundles are easily wrapped with small GO nanosheets, while larger GO nanosheets can be freestanding between SWCNT networks. This means that densification of the SWCNT network is more efficient using small GO than large GO. The reduction of porosity and junction resistance of the SWCNT network can have a positive effect on the decrease of Rs. Moreover, the effect of p-type doping by GO is clearly shown in Raman spectra (Figure 14c and d). An upshift of 3.5 cm⁻¹ in the G+ band for the semiconducting SWCNTs by small GO nanosheets (S1) demonstrates p-type doping of the SWCNTs from the GO nanosheets via a charge transfer mechanism.

To evaluate the opto-electrical performance of a GO-SWCNT electrode on PET, organic photovoltaic (OPV) cells with a PET/GO-SWCNT/PEDOT-PSS/active layer/LiF/Al structure were fabricated (Figure 15). For fabrication of the layered structure of the OPV cells, the wettability of the electrode on the upper loaded aqueous PEDOT:PSS solution is important. As shown in Figure 15a, the hydrophobic SWCNT film was converted to hydrophilic by deposition of hydrophilic GO nanosheets. Moreover, importantly, the work function of the SWCNT film changed from 4.7 to 5.05 eV by deposition of S1-GO nanosheets, which induces a facile hole injection from the HOMO of P3HT (5.0 eV) to the electrode. The resultant device performance with the GO-modified SWCNT anodes shows a significant enhancement in overall photovoltaic performance compared to devices fabricated on pristine SWCNT electrodes, as shown in Figure 15d.

4.2. Electrically stable SWCNT/AgNW hybrid TCFs

Under high current flow, metal NW scan be disrupted by Joule heating at the junction due to a relatively high junction resistance between metal NWs. Self-joining of NW network junctions can solve this problem via post-treatment. Another approach is to interconnect the NWs with other conducting materials or metal oxides. For more efficient processing of metal NW-based TCFs, we need to exclude additional steps, such as irradiation with light, heating at high temperatures, and the removal of surfactant molecules after the deposition of AgNWs or AgNW hybrid materials. Thus, we suggest that a small amount of SWCNTs can stabilize the AgNW networks under current flow without post-treatment. To realize this, the major challenge is the fabrication of a stable dispersion of SWCNTs in liquid medium without dispersant molecules that can be removed after deposition. To solve this issue, the SWCNTs were functionalized with quadruple hydrogen bonding (QHB) motifs of 2-ureido-4[1H]pyrimidinone (UHP) moieties through a previously reported sequential coupling reaction [21]. The AgNW/SWCNT mixture solution was easily prepared by direct mixing of the aqueous AgNW solution with a paste of SWCNTs functionalized with UHP (UHP-SWCNTs) by shaking, as shown in Figure 16a. The spray-coated AgNW/SWCNT hybrid film has an Rs value of ~20 Ω/sq. and T > 90% and was used to fabricate transparent film heaters to investigate the effect of SWCNTs on the electrical stability of the AgNW films under current flow. Notably, the breaking up of AgNWs at junctions was observed at 9 V (Figure 17a), which might have been induced by rapid joule heating at the junctions because of the high junction resistance of the AgNWs (R₁₁ ≈ 10³–10⁹ Ω). In stark contrast, after hybridization with SWCNTs, a new current pathway through the AgNW-SWCNT junction may be formed because of the relatively low contact resistance between the AgNW and SWCNT (R₁₂ ≈ 10³ Ω) when compared to R₁₁, resulting in the formation of stable network films even at 15 V. Moreover, a very small work function difference between AgNW and UHP-SWCNTs, based on the Φ values of AgNW (4.1 eV) and UHP-SWCNTs (4.3 eV), can promote the current pathway through the AgNW-SWCNT junction (Figure 18).