Summary

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Published: June 23, 2017
doi:

Summary

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Abstract

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 Ω/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) – up to 1.5 x 104 – which are amongst the best values in the published literature.

Introduction

Worldwide, studies are being conducted to look for replacements for rigid transparent conductive oxides (TCOs), such as indium tin oxide and fluorine-doped tin oxide (FTO) films, in order to fabricate flexible/stretchable TEs to be used in future flexible/stretchable optoelectronic devices1. This necessitates novel materials with new fabrication methods.

Nanomaterials, such as graphene2, conducting polymers3,4, carbon nanotubes5, and random metal nanowire networks6,7,8,9,10,11, have been studied and have demonstrated their capabilities in flexible TEs, addressing the shortcomings of existing TCO-based TEs, including film fragility12, low infrared transmittance13, and low abundance14. Even with this potential, it is still challenging to attain high electrical and optical conductance without deterioration under continuous bending.

In this framework, regular metal meshes15,16,17,18,19,20 are evolving as a promising candidate and have accomplished remarkably high optical transparency and low sheet resistance, which can be tunable on demand. However, the extensive use of metal mesh-based TEs has been hindered due to numerous challenges. First, fabrication often involves the expensive, vacuum-based deposition of metals16,17,18,21. Second, the thickness may easily cause electrical short-circuiting22,23,24,25 in thin-film organic optoelectronic devices. Third, the weak adhesion with the substrate surface results in poor flexibility26,27. The abovementioned limitations have created a demand for novel metal mesh-based TE structures and scalable approaches for their fabrication.

In this study, we report a novel structure of flexible TEs that contains a metal mesh completely embedded in a polymer film. We also describe an innovative, solution-based, and low-cost fabrication approach that combines lithography, electrodeposition, and imprint transfer. FoM values as high as 15k have been achieved on sample EMTEs. Due to the embedded nature of EMTEs, remarkable chemical, mechanical, and environmental stability were observed. Furthermore, the solution-processed fabrication technique established in this work can potentially be used for the low-cost and high-throughput production of the proposed EMTEs. This fabrication technique is scalable to finer metal-mesh linewidths, larger areas, and a range of metals.

Protocol

CAUTION: Please pay attention to electron beam safety. Please wear the correct protective glasses and clothes. Also, handle the all flammable solvents and solutions carefully. 1. Photolithography-based Fabrication of the EMTE Photolithography for fabricating the mesh pattern. Clean FTO glass substrates (3 cm x 3 cm) with liquid detergent using cotton swab. Rinse them thoroughly with deionized (DI) water using a clean cotton swab. Further clean them usin…

Representative Results

Figure 1 displays the schematic and fabrication flowchart of the EMTE samples. As presented in Figure 1a, the EMTE consists of a metal mesh fully embedded in a polymer film. The upper face of the mesh is on the same level as the substrate, displaying a generally smooth platform for subsequent device production. The fabrication technique is schematically explained in Figure 1b-e. Afte…

Discussion

Our fabrication method can be further modified to allow for scalability of the feature sizes and areas of the sample and for the use of various materials. The successful fabrication of sub-micrometer-linewidth (Figure 3a-3c) copper EMTEs using EBL proves that EMTE structure and key steps in LEIT fabrication, including electroplating and imprint transfer, can be reliably scaled down to a sub-micrometer range. Similarly, other large-area lithography processes, such as phase-shift photolith…

Disclosures

The authors have nothing to disclose.

Acknowledgements

This work was partially supported by the General Research Fund of the Research Grants Council of the Hong Kong Special Administrative Region (Award No. 17246116), the Young Scholar Program of the National Natural Science Foundation of China (61306123), the Basic Research Program-General Program from the Science and Technology Innovation Commission of Shenzhen Municipality (JCYJ20140903112959959), and the Key Research and Development Program from the Zhejiang Provincial Department of Science and Technology (2017C01058). The authors would like to thank Y.-T. Huang and S. P. Feng for their help with the optical measurements.

Materials

Acetone Sigma-Aldrich W332615 Highly flammable
Isopropanol Sigma-Aldrich 190764 Highly flammable
FTO Glass Substrates South China Xiang S&T, China
Photoresist  Clariant, Switzerland 54611L11 AZ 1500 Positive tone resist (20cP)
UV Mask Aligner  Chinese Academy of Sciences, China URE-2000/35
Photoresist Developer  Clariant, Switzerland 184411 AZ 300 MIF Developer
Cu, Ag, Au, Ni, and Zn Electroplating solutions Caswell, USA Ready to use solutions (PLUG N' PLATE)
Keithley 2400 SourceMeter Keithley, USA 41J2103
COC Plastic Films TOPAS, Germany F13-19-1 Grade 8007 (Glass transition temperature: 78 °C)
Hydraulic Press  Specac Ltd., UK GS15011 With low tonnage kit ( 0-1 ton guage)
Temperature Controller  Specac Ltd., UK GS15515 Water cooled heated platens and controller
Chiller  Grant Instruments, UK T100-ST5
Polymethyl Methacrylate (PMMA) Sigma-Aldrich 200336
Anisole Sigma-Aldrich 96109 Highly flammable
EBL Setup Philips, Netherlands FEI XL30 Scanning electron microscope equipped with a JC Nabity pattern generator  
Isopropyl Ketone  Sigma-Aldrich 108-10-1
Silver Paste Ted Pella, Inc, USA 16031
UV–Vis Spectrometer  Perkin Elmer, USA L950

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Cite This Article
Khan, A., Lee, S., Jang, T., Xiong, Z., Zhang, C., Tang, J., Guo, L. J., Li, W. Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh. J. Vis. Exp. (124), e56019, doi:10.3791/56019 (2017).

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