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.
The overall goal of this procedure is to use a solution-based fabrication process that combines lithography, electric deposition, and imprint transfer to produce a high performance, flexible, transparent conductive film with a self-anchored, fully embedded micro metal mesh. This mesh can help address key challenges facing future metal-mesh based flexible electronic devices, such as non-flat surface typography, low fabrication throughput, and high manufacturing cost. Embedded metal mesh provides several advantages, such as crucial self-smoothness, mechanical stability, and a high-burning stress, strong adhesion to the flexible substrate, and resistance against moisture, oxygen, and chemicals.
Our process will place this by based the metal deposition with solution-based electric deposition, and is simple for pursuing high throughput, like volume and low cost production. My group helped Dr.Wendy Lee's group to test the dimensional stabilities of metal-mesh fabrication process by patterning 400 nanometer metal-mesh with our home built, electro beam lithography system. My assistant Xiong Ze is going to demonstrate the electro beam patterning process.
To begin EMTE fabrication, clean a three centimeter by three centimeter piece of flooring-doped tin oxide coated glass with liquid detergent and a cotton swab. Thoroughly rinse the glass substrate with deionized water, and remove traces of detergent with another swab. Sonicate the FTO glass in isopropanol for 30 seconds at 40 kilohertz.
Then dry the clean glass with compressed air. Next, place the clean, dry FTO glass in a spin-coater and apply 100 microliters of positive photo-resist. Spin-coat the glass at 4, 000 RPM for 60 seconds to produce a 1.8 micron-thick film.
Bake the coated glass at 100 degrees Celsius for 50 seconds. Cover the coated glass with a mesh-pattern mask, and expose the photo-resist to sufficient UV light to achieve a radiant fluence of 20 millijoules per square centimeter. Then, immerse the coated glass in the appropriate developer for 50 seconds to remove the exposed photo-resist.
Rinse the sample in deionized water and dry it under a stream of compressed air. Next, place 100 miloliters of acquiesce copper electro-plating solution in a 250 milliliter beaker. Immerse the sample in the plating solution.
And connect it to the negative terminal of a two-electrode electro-deposition apparatus. Then, connect a copper metal bar to the positive terminal of the apparatus. Apply a constant five milliamp current to achieve a current density of three milliamps per square centimeter for 15 minutes, to deposit a 1.5 micron thick layer of copper on the sample.
The croqueting is the critical step in the fabrication. Current density and electroplating time effect the morphology of the metal mesh and the final performance, and should be tested and optimized with your own samples. Thoroughly rinse the electroplated sample with deionized water, and dry it under a stream of compressed air.
Immerse the sample in acetone for five minutes to dissolve the remaining photo resist to leave a bare metal mesh atop the FTO glass surface. Rinse and dry the sample with deionized water and compressed air. Next, place the sample on the platen of a hydraulic press with the metal mesh facing up.
Cover the sample with a 100 micron thick cyclic olefin copolymer film with a glass transition temperature of 78 degrees Celsius. Heat the platens to 100 degrees Celsius, and then apply 15 millipascals of imprint pressure to the sample for five minutes. Pull the platens to 40 degrees Celsius before releasing the imprint pressure.
Pressure and temperature are important prime interests in the imprint transfer step. Make sure your imprint and pressure is uniform and high enough for complete transfer. The temperature should be approximately 20 degrees higher than the substrate material glass transition temperature.
Carefully peel the polymer film with the embedded mesh from the FTO glass surface to obtain the EMTE. To begin preparing a submicron EMTE, clean a three centimeter by three centimeter piece of FTO glass with liquid detergent and deionized water followed by sonication in isopropanol. Place the clean, dry FTO glass in a spin-coater, and apply 100 microliters of 4%by weight PMMA in anastole.
Spin-coat the glass at 2500 RPM for 60 seconds to produce a 150 nanometer thick film. Bake the film at 170 degrees Celsius for 30 minutes, then start the electron beam lithography system and prepare a mesh pattern with a pattern generator. Place the sample in the electron beam lithography system and run the patterning process.
Develop the PMMA by immersion in a one to three mixture of methyl-isopropyl-ketone and isopropanol for 60 seconds. Rinse the patterned sample with deionized water and dry it under a stream of compressed air. Next, place the patterned sample in copper-electroplating solution and connect the sample to the negative terminal of a two electrode electrodeposition apparatus.
Connect the positive terminal to a copper, metal bar. Apply a constant current to achieve a current density of three milliamps per square centimeter for two minutes to plate 200 nanometers of copper on the sample. Rinse the sample with deionized water and immerse the sample in acetone for five minutes to dissolve the PMMA.
Next, place the sample on the platen of a hydraulic press. Cover the sample with a 100 micron thick cyclic olefin co-polymer film with a glass transition temperature of 78 degrees Celsius. Heat the platens to 100 degrees Celsius and apply 15 millipascals of imprint pressure for five minutes.
Cool the platens to 40 degrees Celsius before releasing the pressure. Carefully peel the film from the FTO glass to obtain the submicron EMTE. To begin sheet resistance measurements, first spread silver past on opposite edges of the EMTE and allow the paste to dry.
Place the four probes of the sheet resistance measurement device on the lines of silver paste per the device manufacturer's instructions. Measure and record the sheet resistance. To perform optical transmission measurements, first place the EMTE on the sample holder of a calibrated UV vis spectrophotometer set to 100%transmittance.
Align the sample perpendicular to the beam. Acquire a transmission spectrum of the EMTE to assess the electro-transparency. Copper EMTEs were fabricated with various grid patterns to evaluate the effect of the grid geometry on the electrode properties.
The ratio of electrical conductance to optical conductance for copper EMTEs at 550 nanometers was over 1.5 times 10 to the fourth. Thicker meshes corresponded to lower optical transmittance and sheet resistance. Larger pitches corresponded to greater sheet resistance and transmittance.
EMTEs were fabricated with various metals using a 50 micron pitch mesh, all of which showed flat, featureless transmittance spectra. With the same relationship between mesh thickness and transmittance, the transmittance and sheet resistance can first be tuned by adjusting the geometry and composition of the mesh. The sheet resistance of copper EMTEs was evaluated with respect to compressive and tensile bending tests.
No significant change was observed for four millimeter and five millimeter compressive bending tests. Sheet resistance is gradually increased with tensile bending tests. No degradation and sheet resistance was observed over 24 hours of exposure to water isopropanol or a hot and humid atmosphere.
New students can learn this technique within a few days. Once mastered, the whole fabrication process can be done in two to three hours and the equipment is ready. This technique paves the way for using scalable solution process fabrication methods to develop novel micro and nano structured devices, such as our self-anchored, high aspect ratio micro metal-mesh, embedded in flexible substrate.
Many applications such as touch panels, displace sensors, and solar cells can benefit from our high performance, embedded metal mesh transparent electrodes. After watching this video, you should have a good understanding of how to use this solution-based fabrication process to produce metal-mesh transparent actuals. Thanks for watching, we're open to collaborations.
