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

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 polymers^3,4, carbon nanotubes⁵, and random metal nanowire networks^6,

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

1. Photolithography for fabricating the mesh pattern.
   1. 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 using ultra-sonication (frequency = 40 kHz, temperature = 25 °C) in isopropyl alcohol (IPA) for 30 s before drying them with compressed air.
      ....

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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