Fabrication of Flexible Transparent Electrode with Enhanced

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Fabrication of Flexible Transparent Electrode with Enhanced Conductivity from Hierarchical Metal Grids Linjie Li, Bo Zhang, Binghua Zou, Ruijie Xie, Tao Zhang, Sheng Li, Bing Zheng, Jiansheng Wu, Jiena Weng, Weina Zhang, Wei Huang, and Fengwei Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12298 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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ACS Applied Materials & Interfaces

Fabrication of Flexible Transparent Electrode with Enhanced Conductivity from Hierarchical Metal Grids Linjie Li,1 Bo Zhang,

1

Binghua Zou,

1

Ruijie Xie,

1

Tao Zhang,

1

Sheng Li,

1

Bing Zheng,

1

Jiansheng Wu, 1 Jiena Weng, 1 Weina Zhang, 1 Wei Huang* 1, 2 and Fengwei Huo* 1 1

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China 2

Key Laboratory for Organic Electronics and Information Displays (KLOEID), Institute of

Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing 210046, P. R. China * Corresponding author: [email protected]; [email protected]

KEYWORDS: Transparent Electrodes; Photolithography; Hierarchical Structure; Flexible Electronics; Metal Grids

ACS Paragon Plus Environment

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ABSTRACT

Flexible transparent conductive electrodes (FTCEs) are essential components for numerous optoelectronic devices. In this work, we have fabricated the hierarchical metal grids (HMG) FTCEs by a facile and low-cost near-field photolithography strategy. Compared to normal metal grids (MG), the HMG structure can provide distinctly increased conductivity of the electrode yet without obvious reduction of the optical transmittance. This HMG sample possesses excellent optoelectronic performance and high mechanical flexibility, making it a promising component for practical applications.

Over the past decades, there have been growing interests in flexible transparent conductive electrodes (FTCEs) as they will play important roles in the next generation electronic devices, such as flexible displays, flexible touch panel screens, organic light-emitting diodes (OLEDs), wearable photovoltaic, and smart windows.1 Indium tin oxide (ITO) has excellent optoelectronic properties (low sheet resistance and high optical transmittance) which can be used as transparent electrodes. However, its brittleness cannot be ignored, and the high cost of high-quality ITO also limits its successful demonstrating of the applications in flexible electronics and wearable electronics. Therefore, the exploring emerging materials with good mechanical flexibility, low sheet resistance (Rs) and high optical transmittance (T) has great significance. Recently, advances in alternative materials research provide the possibility of commercialization of FTCEs with these materials. Several functional conductive materials such as carbon-based conductors based on carbon nanotubes (CNTs), graphene, or conducting polymers can be employed as conductive electrodes for charge transport after being assembled


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on elastomeric substrates.2 However, the conductivities of these alternative conductors are very sensitive to the materials fabrication technologies.3 Another family of transparent conductors is metal nanowires which has shown great potential in optoelectronic devices because of their intrinsically high conductivity, transparency and flexibility.4-6 However, metal nanowire networks also have several problems, such as difficulties for uniform distribution on the substrate, too much roughness of the networks on the surface of the substrates and the contact resistance among the nanowire networks, which may cause power loss in many optoelectronic devices.7-9 Transparent electrodes based on metal grids (MG) have also been a viable option because both their Rs and T can be easily controlled by tuning the grids widths, spaces, and thickness.10-16 The most significant matter for MG electrodes is to improve the combination properties of sheet resistance and transmittance (Rs-T) performance as the large space among the connected metal lines will reduce the current transportation efficiency.17-19 Some researchers also created a lot of different composite materials, such as reduced graphene oxide (rGO)/metal nanowire, poly (3, 4-ethylenedioxythiophene)–poly (styrene sulfonate) (PEDOT:PSS)/metal nanowires.20-25 These composite materials can realize the increase of the conductivity or the optical transmittance. However, for these methods, the post-treatments of nanowires junction are still necessary for reducing the resistances, such as thermo, electrochemical or nanoplasmonic welding; even so, the contact resistance between the hybrid materials still exists. Cui’s group introduced a mesoscale metal-wire concept in conjunction with metal nanowire networks to realize an order of magnitude reduction in Rs at a given T.26 But for this fabrication route, the annealing step is still necessary and the three different steps for producing this hybrid material made the whole procedure sophisticated. Therefore, it is necessary to search some general


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methods to fabricate the FTCEs with good performance, in spite of the significant application in flexible electronics. Herein, we report a facile strategy to prepare the FTCEs by structuring the orderly hierarchical metal grids (HMG) patterns on a flexible Polyethylene glycol terephthalate (PET) substrate. The HMG patterns are fabricated by employing the near-field photolithography with transparent polydimethylsiloxane (PDMS) trenches as photomasks. Near-field photolithography is a facile and low-cost way to produce materials with nanopatterns, which combines the mass production capability and sub-diffraction resolution,27 and we can make many types of nanopatterns on a vary series of substrates through this method. The prepared HMG patterns can be grown orderly on the substrate as an integral, and there will not be any contact resistance among the nanowires. In addition, further treatments such as thermo or electrochemical annealing of the products are not necessary here, which cannot be avoided by other preparation methods. The Rs-T performance of the HMG electrodes can be facilely tuned through different coarse grids’ widths and spaces, as well as the size of nanowire arrays. The optimized transparent electrodes in this work could achieve a T of 83.1% at 550 nm and a Rs of 9.8 Ω/sq. Besides, the FTCEs based on the HMG pattern exhibited excellent mechanical flexibility and these samples were successfully used as electrode materials in the light-emitting diodes (LED). Scheme 1 depicts the detailed fabrication steps for producing the flexible transparent electrodes with the HMG structures. This structure is composed of two parts: 1) metal nanowire arrays with widths from 100 to 400 nm and gaps of a few micrometers; and 2) the coarse grids in HMG patterns with widths from 5 µm to 15 µm and spaces around 50 micrometers to hundreds


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micrometers. First, the PET substrate coated photoresist was exposed through PDMS stamp and chromium (Cr) mask, then the substrate was immersed into the developer. After the thermal evaporation and lift-off, the HMG patterns can be obtained on the substrate. MG products were also prepared as controlled samples under exactly the same condition only without the metal nanowire arrays (Fig. S1). Due to the fact that the metal nanowire arrays fill in the space of the coarse grids and the widths of these arrays are just one hundred or two hundred nanometers, there are distinct conductivity increases for the HMG in comparison with the MG samples but with less transmittance loss.

Scheme 1. Schematic of fabrication for the flexible transparent electrode with HMG structures. When we made the MG samples, we did not do the first exposure step.

Transparent electrodes with HMG structures have been successfully developed by near-field photolithography. The widths of coarse grids in HMG sample (5 µm to 10 µm) are one order of magnitude larger than metal nanowire arrays (100 nm to 400 nm), the spaces of coarse grids here


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are from 100 µm to 200 µm, whereas the gap of metal nanowire arrays is 5 µm. Due to the facile and convenience of the near-field photolithography, we can vary our HMG with different widths and spaces. The optical microscope image of the fabricated Ag FTCEs with uniform HMG structures and a large area on the PET substrate was observed, and the width and space of coarse grids here are 10 µm and 100 µm, separately (HMG (10 µm /100 µm)) (Fig. 1A). The SEM images of the HMG (10 µm /100 µm) with well-defined cross connecting structures was observed (Fig. 1B and C). Fig. 1D illustrates the metal nanowire arrays were in good order and the width of line was about 210 nm. In order to maintain adequate Rs and T for different applications, the gaps for certain width of metal nanowire arrays is generally kept on certain values. In Fig. S2, different series of HMG are also prepared, illustrating coarse grids’ space and width are adjustable.


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Figure 1. A) OM image of the hierarchical metal grids (HMG) on PET. B, C, D) Low- and high-magnification SEM images of the HMG and the metal nanowire arrays. The electrodes with metal grids (MG) structures without the metal nanowire arrays also be made (Fig. S1).

In order to investigate the conductivity performance of the HMG and MG, we use the fourpoint probe technique to measure their Rs. We studied the Rs of these samples, of which the coarse grids’ widths are all 10 µm and grids’ spaces are 100 µm, 150 µm and 200 µm (Fig. 2A). The HMG and MG share the same coarse grids’ size, but for HMG, the Rs are 9.8 Ω/sq, 16.6 Ω/sq, 24.5 Ω/sq, respectively in comparison with MG, which are 21.5 Ω/sq, 33.4 Ω/sq, 49.7 Ω/sq, respectively. We also studied Rs of HMG and MG with the same coarse grids’ space of 100 µm, but different grids’ widths (5 µm, 7.5 µm and 10 µm) (Fig. 2B). Their Rs have a similar trend, as 45.2 Ω/sq, 26.7 Ω/sq, 21.5 Ω/sq for MG and 20.8 Ω/sq, 13.7 Ω/sq, 9.8 Ω/sq for HMG, respectively. It can be concluded that the HMG samples provide a distinct conductivity increase compared to the MG samples. This is because in the metal nanowire arrays there are more electrons channels in HMG samples. Meanwhile, the lengths of electrons transportation paths have been significantly shortened in HMG samples. In addition, the electrons can be efficiently collected by metal nanowire arrays which filled the open area of the coarse grids, and then transport to the coarse grids.


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Figure 2. A, B) Sheet resistance of MG and HMG, A) grids width is 10 µm, grid spaces are 100 µm, 150 µm and 200 µm, respectively, B) grid widths are 5 µm, 7.5 µm and 100 µm, respectively, grids space is 100 µm. C) Optical transmittance of HMG. D) Optical /electrical performance of MG and HMG.

In order to demonstrate the transparency of our HMG electrodes, the UV spectrophotometer was used to measure the visible light transmittance of the HMG electrodes we fabricated (Fig. 2C). There was no doubt the metal nanowire arrays in the HMG would affect the transmittance more or less, so we measure the light transmittance of MG for comparison (Fig. S3). Transparent electrodes need to get a balance between optical transmittance and conductivity, thus the Rs-T


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performance is the most significant property of FTCEs. The Rs-T performance were illustrated about the HMG and MG at 550 nm (Fig. 2D). For MG (10 µm /100 µm) and HMG (10 µm /100 µm), the T changed from 85.4% to 83.1%, T of MG (10 µm /150 µm) and HMG (10 µm /150 µm) from 89.5% to 87.4%, MG (10 µm /200 µm) and HMG (10 µm /200 µm) from 91.8% to 89.8%, MG (7.5 µm /100 µm) and HMG (7.5 µm /100 µm) from 88% to 86.5%, MG (5 µm /100 µm) and HMG (5 µm /100 µm) from 91.3% to 88.5%, respectively. For each size of MG, the corresponding HMG has no significant transmittance decrease and a distinct conductivity increase. Generally, the Rs-T performance of a transparent conductor is simply determined by a figure of merit of σDC/σOp value, which is calculated by equation (1) and quoted at λ = 550 nm. This expression has been widely used by other workers and previously been shown to accurately describe films of carbon nanotubes.11, 14, 28

ܶ(ߣ) = ቀ1 +

૚ૡૡ.૞ ఙೀ೛ ିଶ ோೞ

ఙವ಴

ቁ

(1)

For instance, this expression could be seen in Fig. S4, in which the transparent electrode HMG (10 µm /100 µm) illustrated a Rs of 9.8 Ω/sq when the T is up to 83.1% at 550 nm. The corresponding σDC/σOp value of this sample was as high as 198.3. This is an optimum value of our samples and it is extremely large for a transparent electrode. It is noted that in general, the σDC/σOp values for ITO are 120 to 240 as transparent electrodes. In most cases, the transmittance in the visible light range should be at least 80% to meet the requirements of present electronic devices. The outstanding transmittance could also be evidenced in Fig. 3A, the pattern could be clearly observed under the HMG (10 µm /100 µm) FTCE sample (T up to 83.1% at 550 nm).


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Figure 3. A) Photographic of the flexible transparent electrode based on the HMG. B) Metal cover percent of MG and HMG of different size, and their influence to the σDC/σOp of the transparent electrodes. C), Photographic of the flexible transparent electrode under obvious bending. D) Sheet resistance change of flexible transparent electrode based on the HMG (10 µm /100 µm) during 1000 bending cycles.


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The Rs-T performance of the HMG electrode described by σDC/σOp shown in Table 1 is superior to many other reported transparent electrodes from SWCNTs, graphene, ITO, Ag NWs, MG, RGO/metal grids composite and hybrid metal NWs.2-3, 8, 11, 19, 26, 29 Compared to modeled ITO, SWCNTs and RGO/metal grids, the HMG electrode has smaller Rs and higher T, so the HMG electrode has better Rs-T performance. When it comes to graphene, silver NWs and metal grids, electrodes based on these materials have better optical transmittance, but their conductivities are not as good as the HMG electrode. Therefore, the HMG electrode has a bigger σDc/σOp value than these materials. Another hybrid metal NWs in the literature also have low Rs and high T, but the fabrication process is very complicated. It can be concluded that among these literatures, our work has achieved a remarkable performance (σDc/σOp) with a simpler preparation method.

Table 1. Rs-T Performance Parameters of FTCEs

Electrode

Sheet resistance (Ω/sq)

Transmittance (%)

σDc/σOp

Ref.

ITO (modeled)

10

80

159.7

29

Graphene

30

90

116.2

3

SWCNTs

200

80

8

2

Silver NWs

30

86

80.2

8

Metal grids

97

96

45.2

11

RGO/metal grids

18

80

88.7

19


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Hybrid metal NWs

0.36

92

12299.4

26

HMG (10 µm /100 µm)

9.8

83.1

198.3

This work

We calculated the metal cover percent of MG and HMG samples with different grids’ sizes, and investigated their influence on the σDC/σOp of the transparent electrodes (Fig. 3B). Compared to MG samples, the electrodes based on HMG structures got obviously larger values of σDC/σOp, in a relatively same metal cover percentage range. As to MG electrodes, in table S1, the Rs and T are in linear relation of the metal used amount. In contrast, the performances of Rs-T were enhanced dramatically with the increase of metal usage. Compared to MG electrodes, with a metal addition of 19.9%, the Rs of HMG electrodes decreased up to 54.4%. Taking the MG and HMG electrodes with different coarse grids’ size into consideration, it could be found that HMG electrodes with less metal could achieve better conductivities than MG electrodes. For instance, the Rs of HMG (10 µm /150 µm) was 16.6 Ω/sq with 16.35% metal cover, while that of MG (10 µm /100 µm) was 21.5 Ω/sq but 18.2% metal cover. For the HMG electrodes, the T decrease which were resulted from the metal nanowire arrays was not as large as expected. This is because that the metal nanowire arrays we made were just about one hundred nanometers or two hundred nanometers, and there would be light diffraction when the visible light transmits through the metal nanowire arrays. It could be found that a little metal nanowire arrays can make considerable conductivity increase with a negligible optical transmittance loss. Besides the high electrical conductivity in the normal state, FTCEs for flexible electronics need to maintain their conductivity under mechanical bending. Thus, the flexibility of FTCE is another significant property. The Rs of the FTCE based on the HMG (10 µm /100 µm) was


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measured during repeated bending cycles under a bending radius of 3 mm, and the Rs only changes less than 10% over the whole 1000 cycles (Fig. 3D). The standard requirement for the change of Rs should be less than 30%, and it varies for different device applications. The electrical properties of the electrode were recovered almost completely, and the electrode under bending state could be found in Fig. 3C. We also checked the metal lines after 1000 cycles bending by SEM. We found the metal lines did not peel off (Fig. S5). The transparent electrode based on the HMG structures was then used in LEDs (Fig. 4A). The basic architecture of the LED is consisted of four layers: an HMG transparent electrode, an electroluminescence ZnS:Cu layer, an dielectric layer, and an Cu counter electrode. We also did a bending test of the optoelectronic device based on this flexible electrode. The device showed good flexibility and was still lighting up after bending. (Fig. 4B). This proved that the FTCEs based on the HMG structures could offer a good choice for preparing efficient flexible display and lighting devices to replace the costly and brittle devices based on ITO electrodes. In order to demonstrate the performance of the HMG electrode, we did the IV test and the optoelectronic properties of the LEDs and referenced with ITO electrode sample (Fig. S7). The HMG sample had higher luminescent intensity than the ITO sample.


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Figure 4. A) LED based on our HMG electrode. B) After bending, the LED light is still on.

In conclusion, we developed FTCEs based on a novel structure of HMG by a facile and low-cost near-field photolithography method with PDMS stamps as photomasks, for achieving high-performance flexible optoelectronic devices. The Rs-T performance could be easily enhanced or tuned through different coarse grids’ widths, spaces, and the sizes of metal nanowire arrays. The resulting HMG electrodes exhibited a high optical transmittance of 83.1%, with only a sheet resistance of 9.8 Ω/sq; meanwhile a remarkable flexibility has also been achieved. The as-prepared electrode could also be successfully used as components in electronic devices, such as LEDs. Supporting Information. Experimental details, more SEM images, UV spectrophotometer images and a table of the analysis data as described in the text. AUTHOR INFORMATION


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Corresponding Author Prof. Fengwei Huo, Email: [email protected] Prof. Wei Huang, Email: [email protected]

Author Contributions L.J. Li, B. Zhang, and B.H. Zou designed experiments, performed, analyzed the results, and drafted the manuscript. R.J. Xie and J.N Weng gave assistants on experiments. T. Zhang helped for part of SEM characterization. W.N Zhang, S. Li, B. Zheng, J.S. Wu and Professor W. Huang were responsible for part of the interpretation of results and helped to revise the manuscript. Professor F.W. Huo supervised the project, helped design the experiments, and revised the manuscript. All authors contributed to the analysis of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The project was supported by the National Science Foundation for Distinguished Young Scholars (21625401), the National Natural Science Foundation (21574065, 21504043, 21604038, 21604040), the Jiangsu Provincial Founds for Natural Science Foundation (BK20160975, BK20160981), the Jiangsu Specially-Appointed Professor, the Jiangsu Provincial Founds for Distinguished Young Scholars (BK20140044), the Program for Outstanding Young Scholars from the Organization Department of the CPC Central Committee, and the National Key Basic Research Program of China (2015CB932200).


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Fabrication of Flexible Transparent Electrode with Enhanced

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