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^{6,7,8,9,10,11}, 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 meshes^{15,16,17,18,19,20} 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 metals^16,17,18,21. Second, the thickness may easily cause electrical short-circuiting^22,23,24,25 in thin-film organic optoelectronic devices. Third, the weak adhesion with the substrate surface results in poor flexibility^26,27. 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

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.
      CAUTION: Handle compressed air carefully.
   2. Spincoat 100 µL of the photoresist on the cleaned FTO glass for 60 s at 4,000 rpm (approximately 350 x g for samples with a 2 cm radius) to get a 1.8 µm-thick, uniform film.
   3. Bake the photoresist film on a hotplate for 50 s at 100 °C.
   4. Expose the photoresist film through a photomask with a mesh pattern (3 µm linewidth, 50 µm pitch) using a UV mask aligner for a dose of 20 mJ/cm².
   5. Develop the photoresist by immersing the sample in the developer solution for 50 s.
   6. Rinse the sample in DI water and dry it with compressed air.
      CAUTION: Handle compressed air carefully.
2. Electrodeposition of metals.
   1. Pour 100 mL of copper aqueous plating solution in a 250 mL beaker.
      NOTE: Other aqueous plating solutions (e.g., silver, gold, nickel, and zinc) can be used for the fabrication of EMTEs with the respective metals.
      CAUTION: Pay attention to chemical safety.
   2. Connect the photoresist-covered FTO glass to the negative terminal of the two-electrode electrodeposition setup and immerse it in the plating solution as the working electrode.
   3. Connect the copper metal bar to the positive terminal of the two-electrode electrodeposition setup as the counter electrode.
   4. Supply a constant 5-mA current (current density: ~3 mA/cm²) using a voltage/current sourcing and measurement instrument (e.g. Sourcemeter) for 15 min to deposit the metal to a thickness of approximately 1.5 µm.
   5. Thoroughly rinse the photoresist-coated FTO glass sample with DI water and dry it with compressed air.
      CAUTION: Handle compressed air carefully.
   6. Place the photoresist-coated FTO glass sample in acetone for 5 min to dissolve the photoresist film, with the bare metal mesh on top of the FTO glass.
3. Thermal imprint transfer of the metal mesh to the flexible substrate.
   1. Place the metal mesh-covered FTO glass sample onto the electrically heated platens of the thermal imprinter and put a 100 µm-thick flexible cyclic olefin copolymer (COC) film on top of the sample, facing the metal mesh side.
   2. Heat the plates of the heated press to 100 °C.
   3. Apply 15 MPa of imprint pressure and hold it for 5 min.
      CAUTION: Pay attention to safety when using the heated press.
      NOTE: The imprint transfer can be done at a lower pressure; the pressure value (15 MPa) reported here is relatively high. This high pressure was used to ensure that the metal mesh was fully embedded in the COC film.
   4. Cool the heated platens to the demolding temperature of 40 °C.
   5. Release the imprint pressure.
   6. Peel off the COC film from the FTO glass, with the metal mesh entirely embedded in the COC film.

2. Fabrication of Sub-micron EMTEs

1. Fabrication of sub-micron EMTEs using electron beam lithography (EBL).
   1. Spincoat 100 µL of polymethyl methacrylate (PMMA) solution (15k M.W., 4 wt. % in anisole) on the cleaned FTO glass for 60 s at 2,500 rpm (approximately 140 x g for samples with a 2 cm radius) to achieve a 150 nm-thick, uniform film.
   2. Bake the PMMA film on a hotplate for 30 min at 170 °C.
   3. Turn on the EBL system and design the mesh pattern (400-nm linewidth, 5 µm pitch) using a pattern generator²⁹.
   4. Place the sample in a scanning electron microscope connected to the pattern generator and execute the writing process²⁹.
   5. Develop the resist for 60 s in a mixed solution of methyl isopropyl ketone and isopropanol at a 1:3 ratio.
   6. Rinse the sample with DI water and dry it with compressed air.
      CAUTION: Handle compressed air carefully.
   7. Place 100 mL of the copper aqueous plating solution in a medium-size beaker.
      NOTE: Other aqueous plating solutions (e.g., silver, gold, nickel, and zinc plating solutions) should be used for the fabrication of EMTEs with the respective metals.
   8. Attach the PMMA-coated FTO glass to the negative terminal of the two-electrode electrodeposition setup, dip it in the plating solution as the working electrode, and connect the copper metal bar to the positive terminal to complete the circuit.
      NOTE: Other metals bars (i.e., silver, gold, nickel, and zinc) should be used for the respective metal electrodepositions.
   9. Apply a suitable current, corresponding with a current density of approximately 3 mA/cm², to the mesh pattern region for 2 min to deposit metal to a thickness of approximately 200 nm (the actual thickness must be determined by SEM or AFM).
   10. Carefully wash the sample with DI water and place it in acetone for 5 min to dissolve the PMMA film.
   11. Put the metal mesh-covered FTO glass sample on the electrically heated platens of the thermal imprinter and place a COC film (100 µm-thick) on top of the sample.
   12. Heat the plates to 100 °C, apply a 15 MPa imprint pressure, and hold it for 5 min.
   13. Cool the heated platens to the demolding temperature of 40 °C and release the imprint pressure.
   14. Peel off the COC film from the FTO glass, along with thesub-micron metal mesh completely embedded in the COC film.

3. Performance Measurement of the EMTEs

1. Sheet resistance measurement.
   1. Spread silver paste on two opposite edges of the square sample and wait until it dries.
   2. Carefully place the four probes of the resistance measurement device on the silver pads, following the equipment instructions.
   3. Switch to the resistance measurement mode of the power source/measurement instrument and record the value on the display.
2. Optical transmission measurement.
   1. Turn on the UV-Vis measurement setup and calibrate the spectrometer (i.e., correlate the readings with a standard sample to check the accuracy of the instrument).
   2. Place the EMTE sample on the spectrometer sample holder and properly align the optical direction.
   3. Adjust the spectrometer for 100% transmittance.
      NOTE: All transmittance values presented here are normalized to the absolute transmittance through the bare COC film substrate.
   4. Measure the transmittance of the sample.
   5. Save the measurement and logout of the setup.

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