Highly flexible transparent electrodes using a silver nanowires-embedded colorless polyimide film via chemical modification

Abstract

Conducting metal oxides such as indium tin oxide (ITO) and Al-doped ZnO have been used for application in transparent conducting electrodes (TCEs), but they are not suitable for flexible transparent electrodes due to their brittleness. Silver nanowires (AgNWs) are one of the most promising alternatives to conducting metal oxides because of their high electrical conductivity and superior ductility, which are essential criteria for flexible optoelectronics. In order to develop high-temperature resistant flexible electrodes, we combined colorless polyimide (CPI) as a backbone matrix substrate with the AgNW network. In this work, we firstly demonstrate the hybrid transparent electrodes of AgNWs-embedded CPI encapsulated by a ZnO layer (ZnO/AgNWs-embedded CPI) via KOH treatment based chemical modification of the CPI film. The ZnO/AgNWs-embedded CPI substrate exhibited a low sheet resistance of 24 Ω sq⁻¹ while maintaining a high optical transparency of 81%. More importantly, the hybrid transparent electrodes exhibited superior thermal stability (heat treatment for 24 hours at 230 °C) with negligible conductivity decay and outstanding bending stability (stable for 10 000 bending cycles). ZnO/AgNWs-embedded CPI showed relatively low roughness (RMS, 9.6 nm) compared with non-embedded AgNWs on CPI (RMS, 82 nm). Embedding of AgNWs on top of the CPI film offers a good potential for application in flexible TCEs with thermal stability.

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Introduction

As flexible optoelectronic devices become increasingly popular, there is a great demand for flexible transparent conducting electrodes (TCEs), which are essential components for optoelectronic devices.^1,2 Indium tin oxide, which has high optical transparency and low sheet resistance, has been traditionally used for transparent electrodes for a variety of applications such as solar cells, LEDs, and touch screens. However, ITO is not appropriate for flexible transparent electrodes because it is brittle.³ Therefore, a number of candidate materials have been examined to replace ITO, including carbon nanotubes (CNTs), graphene, conducting polymers, and metal nanowires.⁴ Recently, random networks of silver nanowires (AgNWs)^5–9 on flexible substrates have attracted intense research interest as an alternative to ITO due to their facile fabrication, high ductility, as well as the fact that Ag has the highest electrical conductivity (6.3 × 10⁷ S m⁻¹) among metals. However, in spite of all these advantages, three significant problems remain in applying AgNWs to flexible transparent electrodes. Firstly, AgNWs can be easily detached from the substrate by external stimuli or flexure, thereby leading to lack of long-term stability and flexibility. Secondly, they are vulnerable to damage from heat, oxygen, and sulfur. Lastly, the inherent irregular surface morphology of AgNWs leads to poor cell efficiency when they are applied to solar cells.^7,9 To overcome these problems, over the last few years, several studies have been devoted to embedding AgNWs into flexible substrates. The dominant flexible substrates used for TCEs are transparent polymers. Kim et al. selected a UV-curable adhesive photopolymer for embedding of AgNWs.¹⁰ Xu et al. prepared composite electrodes of AgNWs-embedded PDMS.¹¹ Also, Jin et al. adopted glass-fabric reinforced UV-curable resin as a flexible substrate in which AgNWs are embedded.¹² However, the desired level of thermal stability and durability are not achieved for those flexible substrates. Very recently, colorless polyimide (CPI) has been suggested as a potential flexible polymer substrate.¹³ The CPI has the feature of high transmittance as polymer of the imide groups with low yellow index due to a lower intra- and/or intermolecular charge transfer complexes formation in the molecules.¹⁴ Recently, flexible TCEs using CPI substrate have been reported. However, they have been fabricated through layer-by-layer process to realize buried structure of electrodes.^15–20

In this paper, as highly flexible and durable transparent electrodes, we introduce AgNWs films with ZnO layer embedded in CPI via a unique process which have low yellow index and high transmittance with the original advantages of polyimide, thereby making CPI suitable as a flexible substrate of transparent electrodes. We also demonstrate the method of embedding AgNWs uniformly onto the surface of CPI.

Experimental

Synthesis of CPI

Polyamic acid (PAA) solution, which is the precursor of CPI, was prepared by dissolving 2.0365 g of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 1.018 g of 3,3′-diaminodiphenyl sulfone (APS) in 4 g of dimethylformamide (DMF) solvent. For complete dissolution, this solution was stirred at 500 rpm with a magnetic stirrer for 5 hours at room temperature. The 50 μm thick homogeneously dissolved PAA solution was coated on glass substrates (2 cm × 2 cm) with a doctor's blade. Afterward, PAA was imidized to CPI by heat treatment, which was performed at 100 °C, 200 °C, and 230 °C for 1 hour, sequentially in a box furnace.^25,26

Synthesis of AgNWs

AgNWs were synthesized by reducing AgNO₃ in the presence of polyvinylpyrrolidone (PVP) in ethylene glycol (EG).^27–29 First, in a three-neck flask, 6.68 g of PVP was added in 200 ml of EG. A solution of EG with PVP was heated up to 170 °C for 1 hour with constant magnetic stirring at 200 rpm for stabilization. Under stirring, 0.5 g of AgCl was added to the mixture to nucleate seeds of Ag nanoparticles. After 1 hour, the Ag source, 2.2 g of AgNO₃, which was completely dissolved in 5 ml of EG, was slowly added to the mixture (5 ml hour⁻¹) by a syringe pump. After the injection, the reaction mixture was heated for additional 2 hours and then cooled to room temperature. To remove precipitates such as PVP, Ag particles, and Ag rods, the reaction mixture was centrifuged for 30 minutes four times in de-ionized water (2000 rpm) and additional four times in ethanol (2000 rpm). Finally, the remaining AgNWs (about 0.01 g) after centrifugation were dispersed in 50 ml of methanol.

Fabrication of ZnO-coated AgNWs–CPI composite

To embed AgNWs in CPI film, CPI film coated on glass was immersed in KOH solution (1.0 M) for 5 minutes. During this process, CPI film is naturally detached from the glass and becomes freestanding film. AgNWs dispersed in methanol were filtered through the nylon membrane by a vacuum filtration machine. AgNWs were transferred from the nylon filter membrane to surface-modified CPI layer (potassium polyamate) and then the AgNWs were embedded in potassium polyamate film by uniaxial pressing machine with 30 kg cm⁻² of pressure at room temperature. Finally, AgNWs-embedded potassium polyamate was reduced to AgNWs embedded CPI by heat treatment performed at 235 °C for 2 hours. To form a 20 nm thick ZnO layer on AgNWs embedded CPI film, ZnO sputtering was carried out with a power of 100 W for 6 minutes at room temperature.

Measurement

The transmittance was measured using a UV-visible spectrometer (Shimadzu UV-3100) with glass as a reference sample. The sheet resistance was measured using a four-point probe. The crystal structure of Ag was investigated by X-ray diffractometer (D/MAX-RB (12 kW) and D/MAX-RC (12 kW), RIGAKU). All scanning electron microscopy (SEM) images were taken using Phillips (FEI). Transmission electron microscopy (TEM) images were taken using Tecnai F30 S-Twin (FEI). The bonding of polyimide was analyzed by Fourier transform infrared (FT-IR) (IFS66v/s & Hyperion3000, Bruker). The roughness measurement was carried out by atomic-force microscopy (AFM) (SFX100, KLA-Tencor).

Result and discussion

As illustrated in Fig. 1, for facile and homogeneous embedding of AgNWs network within the top surface of CPI film, we employed potassium hydroxide (KOH) solution treatment to modify the structural characteristic of the CPI surface.^21,22 When the CPI is immersed in KOH solution, the surface of the CPI film is hydrolyzed to form potassium polyamate which breaks imide rings and makes the surface of CPI soft. We embedded AgNWs in soft potassium polyamate surface by applying uniaxial pressure. Then, broken imide rings of CPI were fully recovered through re-imidization step via high temperature heating at 230 °C for 1 hour. Finally, we coated a ZnO layer using sputtering on top of AgNWs-embedded CPI substrate to improve the thermal and mechanical stability of AgNWs.

Fig. 1 Schematic illustration of fabricating process for AgNWs-embedded CPI: (a) synthesized AgNWs are transferred onto filter, (b) polyamic acid is coated on glass substrate and then imidized to CPI via heat treatment, (c) CPI is immersed in KOH solution to be potassium polyamate, (d) AgNWs are embedded in potassium polyamate. Then, potassium polyamate is reduced to CPI.

AgNWs were synthesized by polyol process using EG which functions as solvent and, at the same time, a reducing agent. PVP passivates (100) face of silver nanoparticles, thereby inducing the anisotropic growth of the silver nanoparticles into the 〈100〉 direction, which finally leads to the formation of silver nanowires.²³ The length and diameter of synthesized AgNWs were 25–50 μm and 35–50 nm, respectively (Fig. 2a). The SEAD pattern of AgNWs confirmed successful synthesis of the crystalline AgNWs (Fig. 2b). Also, the X-ray diffraction (XRD) pattern revealed that as-synthesized AgNWs had pure face-centered cubic crystal structure (Fig. 2c) and no other element was detected except for Ag (Fig. 2d).

Fig. 2 (a) TEM image of AgNWs (b) SEAD pattern of AgNWs (c) SEM image of AgNWs networks (d) XRD analysis of AgNWs.

In this work, CPI film was prepared from diamine and anhydride monomers. The formation of high purity CPI substrate was confirmed by examining the FT-IR spectra. In addition, the formation of potassium polyamate and re-imidized polyimide, which was serially reacted by chemical and thermal reaction from original polyimide, was observed. The spectra of CPI in three cases are shown in Fig. 3a–c, and the molecular structure in each case is shown in Fig. 3d–f. In Fig. 3a, there are three C=O peaks at 718 cm⁻¹ (C=O bending), 1723 cm⁻¹ (symmetrical C=O stretching), and 1785 cm⁻¹ (asymmetrical C=O stretching), and they indicate the formation of imide rings of CPI. The presence of characteristic peak at 1366 cm⁻¹ corresponding to C–N–C stretching is also an evidence of imide groups of CPI (Fig. 3a). When the original CPI is immersed in KOH, as shown in Fig. 3b, new peaks from 1550 cm⁻¹ to 1650 cm⁻¹ were formed. These peaks come from N–H bonding in potassium polyamate.²⁴ The intensity of characteristics peaks at 1723 cm⁻¹ and 1366 cm⁻¹ was relatively reduced due to the hydrolysis of CPI in KOH (Fig. 3b). It is important to note that the intensity variations of these peaks are not so significant because only the surface of CPI film turns to potassium polyamate. Fig. 3c shows the FT-IR spectra of CPI film after re-imidization of potassium polyamate through heat treatment. The peaks from 1550 cm⁻¹ to 1650 cm⁻¹ disappeared, and original peaks of imide groups were recovered (Fig. 3c).

Fig. 3 FT-IR graphs of (a) original polyimide, (b) potassium polyamate, (c) re-imidized polyimide.

Fig. 4 shows photographs of CPI film on a glass substrate (Fig. 4a) and CPI film detached from a glass substrate after KOH treatment (Fig. 4b). The transmittance of bare CPI film with 300 μm thickness is 90.2%, as shown in Fig. 4c, without yellow color, as shown in Fig. 4b. Moreover, the CPI film is easily bent without brittleness and cracks. For comparison, we also prepared reference sample, which is fabricated by transferring AgNWs onto the surface of CPI film (non-embedded AgNWs). The tilted cross-sectional SEM image of non-embedded AgNWs exhibits the morphology of AgNW networks in which AgNWs lied upon one another (Fig. 5a). On the other hand, the tilted cross-sectional SEM image of ZnO/AgNWs-embedded CPI shows tracks of AgNWs, which are mostly buried within the top surface of CPI (Fig. 5b). To produce the AgNWs–CPI composite, vacuum-filtered AgNWs were imprinted into the surface of CPI by applying uniaxial pressure. As a result, approximately 60–70% of the AgNWs were embedded in CPI, while others were exposed on the surface. The embedded and exposed AgNWs were well-connected, providing interconnected networks for fast electron transport. The cross-sectional STEM image clearly shows that AgNWs were embedded within CPI (Fig. 5c). In addition, ZnO layer was further coated on top of AgNWs-embedded CPI.

Fig. 4 Photographs of (a) CPI film on a glass substrate, (b) CPI film detached from a glass substrate after KOH treatment, (c) transmittance spectrum of CPI film.

Fig. 5 Tilted cross-sectional SEM images of (a) non-embedded AgNWs, (b) AgNWs-embedded CPI, (c) cross sectional TEM image of ZnO/AgNWs-embedded CPI.

One of the main advantages of our hybrid film is highly improved electrical properties. This phenomenon can be explained by the ZnO overcoated layer and welding effect at the junction of AgNWs during high temperature re-imidization process. The sheet resistance of ZnO/AgNWs-embedded CPI with different loading amount of AgNWs was measured at three stages during the fabrication of the hybrid films (Fig. 6a). Black squares represent sheet resistance, which was measured after embedding of AgNWs into potassium polyamate surface. In this state, the sheet resistance value is similar with that of non-embedded AgNWs. When very thin ZnO layer with the thickness of 20 nm was coated by RF-sputtering onto the AgNWs-embedded potassium polyamate, the sheet resistance decreased by a small extent over all AgNWs loading amount (red circles). According to the previous report, the ZnO over-coated layer fills the voids between AgNWs and tightens the AgNWs junction contacts, thereby lowering the sheet resistance of the hybrid film.²⁵ After heat treatment for re-imidization which returns potassium polyamate to CPI, the sheet resistance decreased by about 20% in every AgNWs loading amount (blue triangles). Since the heat treatment for re-imidization process was conducted at 235 °C for 2 hours, the contacts between AgNWs are welded together, leading to further decrease in sheet resistance (Fig. 6b and c). This result indicates that, with the same loading amount of AgNWs, ZnO/AgNWs-embedded CPI showed lower sheet resistance than non-embedded AgNWs samples. Fig. 6a shows optical transmittance of each transparent electrode with different AgNWs loading amount. As expected, higher loading of AgNWs led to less transparent and more conductive electrodes. Generally, the transparent electrodes are recommended to possess high optical transmittance (>80%) and low sheet resistance (<20 Ω sq⁻¹). The transparent electrodes with AgNWs loading amount of 200 μL and 300 μL could meet the requirements and 300 μL of AgNWs loading seems better because much lower sheet resistance was observed at similar transmittance (22 Ω sq⁻¹ at 81%).

Fig. 6 (a) Sheet resistance versus AgNWs loading amount according to each procedures, (b) SEM image of welded AgNWs contacts of ZnO/AgNWs-embedded CPI after heat treatment for reimidization, (c) tilted SEM image of welded AgNWs contacts of ZnO/AgNWs-embedded CPI after heat treatment for re-imidization.

In order to evaluate the thermal stability of ZnO/AgNWs-embedded CPI films, we carried out aging test at 230 °C (Fig. 7a). The sheet resistance of three types of transparent electrodes, i.e., AgNWs-embedded CPI, ZnO/AgNWs-embedded CPI, and non-embedded AgNWs, with the same AgNWs loading amount was measured every 1 hour. Non-embedded AgNWs showed a dramatic increase of sheet resistance because AgNWs were partially melted, coalesced after 21 hours, as shown in the inset of SEM image of Fig. 7c. In contrast, AgNWs-embedded CPI without ZnO layer showed much lower increase compared with non-embedded AgNWs. More importantly, ZnO/AgNWs-embedded CPI exhibited negligible sheet resistance change for 24 hours. After the aging test, most non-embedded AgNWs were broken and coalesced (Fig. 7c) while AgNWs in ZnO/AgNWs-embedded CPI maintained their original shapes without any morphological changes (Fig. 7d). This result confirms that embedding of AgNWs within thermally stable CPI layer is very effective route for protecting AgNWs from heat and oxygen, thereby preventing AgNWs from oxidation and sulfurization. The ZnO overcoated layer further reinforces the passivation effect, leading to exceptionally high thermal stability of ZnO/AgNWs-embedded CPI electrodes. Furthermore, chemical stability of AgNWs is another importance issue to be addressed. Ag is easily sulfonated upon exposure to sulfur-containing compounds, which makes AgNWs no longer conductive. AgNWs-embedded CPI, ZnO/AgNWs-embedded CPI, and non-embedded AgNWs were exposed to sulfur atmosphere for 80 min at room temperature and sheet resistance of three specimens was measured every 10 minutes (Fig. 7b). The result shows similar trend with the aging test results. The sheet resistance of non-embedded AgNWs sharply increased, while the sheet resistances of AgNWs-embedded CPI and ZnO/AgNWs-embedded CPI were maintained. Thus, AgNWs-embedded CPI could provide high thermal and environmental stability against oxygen and sulfur. In addition, the ZnO overcoated layer enhances the passivation effect.

Fig. 7 (a) Sheet resistance of hybrid films during heat treatment at 230 °C for 21 hours, (b) sheet resistance of hybrid films in sulfur atmosphere, (c) SEM image of non-embedded AgNWs after aging test for 21 hours, (d) SEM image of ZnO/AgNWs-embedded CPI after aging test for 21 hours.

In order to investigate mechanical stability of ZnO/AgNWs-embedded CPI films, we conducted a bending test using a lab-made bending test tool with a bending radius of 2.5 mm. Sheet resistance was measured as a function of bending cycles (Fig. 8a). During the bending test, the sheet resistance of non-embedded AgNWs increased steeply, whereas that of ZnO/AgNWs embedded CPI maintained nearly the same sheet resistance for up to 10 000 bending cycles. Without a ZnO overcoated layer, the AgNWs-embedded CPI showed slightly worse flexibility than that of ZnO/AgNWs-embedded CPI. The adhesion strength between AgNWs and the CPI substrate are remarkably improved due to the tight embedding of AgNWs within the top surface of CPI and sputtered ZnO layer further improved the mechanical stability by preventing the detachment of AgNWs from CPI substrate. Fig. 8b shows SEM image of non-embedded AgNWs after the bending test in which AgNWs were detached and broken. SEM image of AgNWs embedded and ZnO/AgNWs embedded samples after bending test are shown in Fig. 8c and d which prove that embedding protects nanowires from detachment and breaking leading to desired mechanical stability.

Fig. 8 (a) Sheet resistance change as hybrid films are bent up to 10 000 cycles, tilted cross-sectional SEM image of (b) non-embedded AgNWs, (c) AgNWs embedded CPI, and (d) ZnO/AgNWs embedded CPI samples after bending test.

Embedding of AgNWs firstly reduce surface area of AgNWs that are exposed to air thus hinder oxidation process. At the same time, AgNWs are fixed tightly in polyimide matrix which provides excellent bending stability compared to non-embedded samples in which AgNWs were detached from the surface and sheet resistance raise rapidly. Thirdly, surface roughness is reduced significantly after embedding and smooth surface is essential for TCE application to prevent short circuit. ZnO acts as, on the one hand, oxygen barrier to slow down oxidation, and on the other hand, surface smoothing layer to further reduce roughness of our hybrid film.

Fig. 9 shows surface AFM images of three different types of films with a root mean square (RMS) roughness value. The roughness of non-embedded AgNWs was 82 nm (Fig. 9a); that of AgNWs-embedded CPI was 14 nm (Fig. 9b); and that of ZnO/AgNWs-embedded CPI was 9.6 nm (Fig. 9c). This result might be attributed to the fact that many parts of AgNWs were embedded on the substrate and ZnO overcoated layer filled the void between AgNWs, flattening them even more. The faint lines in the tilted images of Fig. 9b and c are embedded AgNWs.

Fig. 9 AFM images of (a) non-embedded AgNWs, (b) AgNWs-embedded CPI, (c) ZnO/AgNWs-embedded CPI.

Conclusion

In this work, we prepared AgNWs network which is tightly immobilized within the top surface layer of CPI film for the fabrication of flexible transparent electrode and subsequently deposited a ZnO layer on AgNWs for passivation purpose. After the surface of CPI was hydrolyzed to potassium polyamate by dipping in KOH solution, embedding of AgNWs was performed by uniaxial pressing. The embedding process and ZnO layer contribute to the desired electrode performance in several ways. Firstly, during heat treatment for re-imidization, contacts between AgNWs are welded together, leading to improved electrical conductivity. Furthermore, AgNWs are passivated from heat and sulfur by highly temperature-resistant CPI and the ZnO layer. In particular, the ZnO/AgNWs-embedded CPI shows high bending stability, which means the transparent electrodes are highly flexible. In addition, the roughness of the hybrid film is reduced significantly. From a practical point of view, these advantages of our flexible transparent electrode give opportunities for broad applications such as solar cells, touch panel screen, OLED, etc.

Acknowledgements

This work was also funded by the Ministry of Science, ICT & Future Planning as Biomedical Treatment Technology Development Project (2015M3A9D7067418). The authors thank KOLON Corporation, Korea, for providing funding for this research through the KOLON-KAIST Lifestyle Innovation Center Project (LSI14-MAKID0001).

Notes and references

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