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

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. After spincoating a photoresist film on an FTO glass substrate, photolithography is used to create the mesh pattern in photoresist by UV exposure and development (Figure 1b), revealing the conductive surface of the glass in the trench. In the following step, the respective metal is grown inside the trenches by electrodeposition, which fills the trenches to form a regular metal mesh (Figure 1c). Then, the photoresist is carefully removed in acetone, resulting in a weakly attached metal mesh on the surface of the FTO glass (Figure 1d). Next, a polymer film is positioned on the sample and heated to a temperature higher than its glass transition temperature. Metal mesh is pushed into the softened polymer film through the application of a uniform pressure (Figure 1e). Finally, by cooling the stack to room temperature and peeling off the polymer film from the conductive glass, the metal mesh is transferred to the plastic film in fully embedded form (Figure 1f). The entire fabrication procedure is solution-based and is implemented in an ambient atmosphere; therefore, it can easily be adapted for mass production.

Figure 2 presents atomic force microscopy (AFM) and scanning electron microscopy (SEM) images of the morphology of the EMTE at different fabrication steps of the LEIT process. Figure 2a presents the trench images in the photoresist film made by photolithography. In this specific sample, the width of the photoresist trench is about 4 µm, while the depth is nearly 2 µm. Figure 2b shows the electroplated copper mesh on FTO glass. As apparent from the results, the copper mesh has a thickness and linewidth of about 1.8 and 4 µm, respectively. Figure 2c displays the transferred copper mesh on a COC film²⁸. The AFM images confirm that the surface roughness of the accomplished EMTE (1.8 µm thickness) is lower than 50 nm, confirming its embedded configuration. The LEIT method can be further studied by varying the electrodeposition time to make copper EMTEs of different thicknesses. The correlation of metal thickness and electrodeposition time is presented in Figure 2d.The curve shown in Figure 2d reveals that the thickness of metal changes nonlinearly with an increase in electrodeposition time. This is because of the non-rectangular cross-section of the photoresist trench (Figure 2a), which has a narrower bottom but a wider top. Thus, during electrodeposition (constant current), the rate of growth of metal thickness decreases with time. Hence, the mesh has a larger width at the higher part, which is advantageous for imprint transfer since it can be mechanically anchored in the plastic film.

Figure 3a-c demonstrates the structural characterization of EBL-patterned EMTE manufacturing at various steps of the LEIT process to validate its dimensional scalability. Figure 3a shows the AFM and SEM images of the trenches made in PMMA film via EBL. The trench depth and width are around 150 and 400 nm, respectively. Figure 3b shows the copper mesh electroplated on FTO glass, and Figure 3c presents the imprint-transferred copper mesh on a COC film. The metal mesh on the COC substrate is in completely embedded form,offering strong adhesion and stability with the plastic substrate.

Figure 4a shows the transmittance of copper EMTEs of 600 nm, 1 µm, and 2 µm thicknesses in the wavelength range of 300-850 nm. When the metal mesh thickness increased from 600 nm to 2 µm, only a minimal reduction in transmittance was detected, and this drop is attributed to the nonrectangular profile of the trench in the photoresist and the metal overplating. On the other hand, the sheet resistance of EMTEs can be significantly decreased when the metal thickness is increased, as demonstrated in Figure 4b. An exceptionally low sheet resistance of 0.07 Ω/sq has been recorded for the copper EMTE with a 2-µm thickness, with the optical transmittance still higher than 70%.

Figure 4b exhibits the ratio of electrical conductance to optical conductance (σ_dc/σ_opt), a FoM commonly used to compare the performance of TEs. The FoM values shown in Figure 4b were calculated for various EMTEs made in this work by applying the following commonly used expression^4,7,17,18:

where R_s is the sheet resistance and T is the optical transmittance at a 550-nm wavelength. The inset of Figure 4b displays the relationship between the FoM and the metal thickness. The given plot shows that the thickness of the metal has a significant influence on the sheet resistance and therefore on the value of the FoM by enhancing the conductivity of a thicker metal mesh without considerably losing transmittance.The prototype EMTEs attained FoM values higher than 1.5 x 10⁴, which are amongst the best values reported in the literature.

Figure 5a demonstrates the sheet resistance and UV-Vis spectra of a high transparent copper EMTE on COC film (5 x 5 cm²) with a pitch, linewidth, and thickness of 150, 4, and 1 µm, respectively, exhibiting the scalability of the total size of our EMTE structure and LEIT fabrication strategy. Due to the relatively large pitch, the sample displays higher optical transmittance (94%) while maintaining a lower sheet resistance (0.93 Ω/sq). Similarly, numerous arrangements of sheet resistance and optical transmittance can be attained for different devices by adjusting the key geometric characteristics of the EMTE.

Figure 5b shows the sheet resistance and optical transmittance spectra of EMTEs of various metals, including silver, gold, nickel, and zinc, to demonstrate the versatility of material choice with our EMTE. The transmittance spectra are almost flat and featureless over the whole visible range, which is beneficial for display devices and solar cell applications. Zinc-, silver-, and nickel-based EMTEs have comparable metal thicknesses, so all samples have approximately similar transmittances (nearly 78%), while sheet resistances are 1.02, 0.52, and 1.40 Ω/sq, respectively. Due to the different metal thicknesses, the gold- and copper-based EMTEs (nearly 2 µm and 600 nm, respectively) have sheet resistances of 0.20 and 0.70 Ω/sq and transmittances of 72% and 82%, respectively. The successful production of these EMTEs confirmed the material versatility, therefore satisfying diverse requirements for the chemical compatibility and work function of the conductor in various devices.

Figure 6a and b present the superior flexibility of our EMTEs by correlating the sheet resistance with the bending cycles for compressive and tensile loadings at radii of 3, 4, and 5 mm. The results shown in Figure 6a demonstrate that, for compressive bending with 4 and 5 mm radii, no obvious variation in sheet resistance (0.07 Ω/sq) occurs for 1,000 bendings. Also, the variation in sheet resistance is within 100% of its initial value (from 0.07 Ω/sq to 0.13 Ω/sq) for the 3 mm bending radius. Similarly, for tensile bending, variations in sheet resistance against the bending cycles are shown in Figure 6b, indicating that for 1,000 cycles of 3, 4, and 5 mm radii, the sheet resistances changed by almost 350%, 150%, and 30%, respectively. Figure 6c illustrates the environmental stability of the copper EMTEs after immersion in DI water and IPA and exposure to a hot and humid atmosphere (60 °C, 85% relative humidity). It is obvious from the results that after 24 h, the morphological structures and sheet resistances of the EMTEs remain unaffected.

Figure 1: Schematic Diagrams of the EMTE Structure and LEIT Fabrication Procedure. (a) An EMTE with a metal mesh embedded in a transparent plastic film. (b) Mesh patterns made in a resist film layered on a conductive glass substrate using lithography. (c) Electrodeposition of metal inside the trenches of the resist to fabricate a uniform metal mesh. (d) Dissolving the resist to achieve bare metal mesh. (e) Heating and pressing the metal mesh into a plastic film. (f) Separation of the plastic film and the metal mesh in a completely embedded form. This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.

Figure 2: Fabrication of Prototype 50 µm-pitch Copper EMTEs. (a–c) SEM (left, with the inset showing the zoomed-in image) and AFM (right) characterizations of a sample EMTE at different stages of LEIT: (a) Mesh pattern in the photoresist. (b) Copper mesh on the FTO glass after dissolving the photoresist. (c) Copper mesh completely embedded in a COC substrate. (d) Relationship between metal thickness and electrodeposition time at a constant electrodeposition current density (3 mA/cm²). Unsuccessful and successful cases following the imprint transfer are denoted by red and black colors, respectively.This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.

Figure 3: SEM (left) and AFM (right) Characterizations of a Prototype Sub-micrometer-linewidth EMTE at Various Stages of LEIT. (a) Nanomesh patterns made in a PMMA film using EBL. (b) Copper nanomesh on the FTO glass after dissolving the PMMA film. (c) Copper nanomesh completely embedded in a COC substrate. This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.

Figure 4: Performance Characterization of the Prototype 50 µm-pitch Copper EMTEs. (a) Optical spectra of the typical copper EMTEs. Inset: optical image of the flexible copper EMTE. (b) Relationship between transmittance and sheet resistance for copper EMTEs of various mesh thicknesses; the corresponding FoM values are displayed in the inset. This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.

Figure 5: Dimensional Scalability and Material Versatility of the Copper EMTEs. (a) Sheet resistance and optical spectra of a high transparent copper EMTE with a pitch of 150 µm on a large COC substrate (5 x 5 cm²). Inset: optical image of the large-area EMTE. (b) Sheet resistances and optical spectra of 50 µm-pitch EMTEs made of different metals. This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.

Figure 6: Mechanical and Environmental Stability of the Copper EMTEs. (a) Curve of changes in sheet resistance with repeated compressive bending cycles. (b) Curve of changes in sheet resistance with repeated tensile bending cycles. (c) Changes in sheet resistance in the environmental and chemical tests.Inset: SEM pictures after the tests. This figure has been modified from reference²⁹. Please click here to view a larger version of this figure.