Graphene-Based Grid Patterns Fabricated via Direct Ink Writing for Flexible Transparent Electrodes

Abstract

Graphene is considered one of the most promising flexible transparent electrode materials as it has high charge carrier mobility, high electrical conductivity, low optical absorption, excellent mechanical strength, and good bendability. However, graphene-based flexible transparent electrodes face a critical challenge in balancing electrical conductivity and optical transmittance. Here, we present a green and scalable direct ink writing (DIW) strategy to fabricate graphene grid patterns by optimizing ink formulation with sodium dodecyl sulfate (SDS) and ethanol. SDS eliminates the coffee ring effect via Marangoni flow, while ethanol enhances graphene flake alignment during hot-pressing, achieving a high conductivity of 5.22 × 10⁵ S m⁻¹. The grid-patterned graphene-based flexible transparent electrodes exhibit a low sheet resistance of 21.3 Ω/sq with 68.5% transmittance as well as a high stability in high-temperature and corrosive environments, surpassing most metal/graphene composites. This method avoids toxic solvents and high-temperature treatments, demonstrating excellent stability in harsh environments.

1. Introduction

Flexible transparent electrodes (FTEs) with high light transmittance, high electrical conductivity and excellent stretchability are key components of various flexible optoelectronic devices including flexible displays, solar cells, touch screens, and light-emitting diodes [1,2,3,4]. Indium tin oxide (ITO), the conventional transparent electrode, has good transparency and electrical conductivity but suffers from its brittleness and scarcity [5]. Metal nanowires (e.g., Ag, Cu) are promising candidates for fabricating FTEs since they have excellent electrical conductivity and can be synthesized by solution processing methods on a large scale [6,7]. Unfortunately, they are expensive, susceptible to oxidation, and chemically unstable in harsh conditions (acid/alkali environments, high temperature, etc.) [8].

Graphene has received increasing research interest as an alternative transparent electrode material over recent years due to its excellent physical, chemical and electronic properties such as high charge carrier mobility (2 × 10⁵ cm² V⁻¹ S⁻¹), high electrical conductivity, low optical absorption, excellent mechanical strength and bendability, and high stability [9,10,11]. Although the theoretical resistance of single-layer graphene is as low as 30 Ω/sq, graphene-based flexible transparent electrodes (GFTEs) exhibit high sheet resistance (600 Ω/sq–1000 Ω/sq), limiting their practical application [11]. This drastic deviation in electrical conductivity can be attributed to the interfacial charge transportation barriers between adjacent graphene flakes [12,13]. In recent years, many efforts have been made to improve the electrical conductivity of GFTEs. For example, Choe et al. fabricated high-quality graphene sheets with few defects and grain boundaries by chemical vapor deposition at 800 °C, which achieved a low sheet resistance of 1000 Ω/sq with a light transmittance of 87% [13]. Kim et al. prepared GFTEs using AuCl₃-doped graphene sheets to increase the charge carrier density of graphene, and the square resistance could be reduced to 150 Ω/sq with 87% light transmittance [14]. Liu et al. obtained p-type doped graphene by a liquid-phase reaction, improving its electrical conductivity to as high as 3.13 × 10⁴ S/m [15]. Specifically, to minimize the charge transportation barrier between multiple graphene sheets in the deposited electrode, graphene oxides (GOs) were used as the raw material since GOs can be well aligned in the deposited film due to the repulsive force between each flake. However, subsequent carbonization of GOs at high temperatures above 1000 °C was generally applied to achieve an electrical conductivity as high as 10⁶ S/m [16,17,18]. In conclusion, the current preparation methods for GFTEs have the disadvantages of high costs, unscalable production, and inconvenient post-treatment. Therefore, a versatile and efficient preparation method for GFTEs is required.

Direct ink writing (DIW) is one of the most popular 3D printing methods applied in energy or electronic device printing because of its low cost, material flexibility, high material utilization, simple operation process, low energy requirements, and environmental friendliness [19]. Usually, a DIW device includes a simple desktop 3D printer, heated bed, air-powered dispenser, and micro-nozzle, which are controlled by a computer to enable precise patterning of functional materials [20,21]. In principle, DIW techniques are based on the deposition of inks with a desired rheological behavior, e.g., nanoparticle-, colloid-, or organic-based inks, to build patterns in a layer-by-layer fashion [22]. After deposition, the patterns solidify through the evaporation of solvents, chemical changes (e.g., cross-linking of polymers), or cooling [23]. With the help of DIW technology, fine micro–nano patterns of conductive films can be prepared so as to precisely adjust the square resistance and light transmittance to meet the requirements of various devices [24]. However, there are still challenges in preparing GFTES via DIW techniques as the “coffee ring” effect occurs during evaporation, leading to the formation of graphene films with uneven thicknesses [25,26].

In this study, graphene-based grid patterns were fabricated by DIW to address the above issues. The composition of the conductive graphene ink was carefully manipulated. A water/ethanol mixed solvent was used to prepare the graphene ink, and sodium dodecyl sulfate (SDS) was added to the graphene ink. SDS induces Marangoni flow to eliminate non-uniform deposition, while ethanol enhances flake alignment during hot-pressing. The resulting grid-patterned GFTE achieves a low square resistance of 21.3 Ω/sq and a relatively high light transmittance of 68.5%. The overall optic and electronic performance exceeds most metal/graphene composite FTEs. More importantly, this DIW-based technique can be conducted without using toxic solvents or high-temperature steps, thus paving the way for scalable fabrication of flexible electronics.

2. Experimental Details

2.1. Materials

Transparent PET was purchased from Suzhou Aokai Polymer Material Co., Ltd. (Suzhou, China) (Thickness: 0.025 mm). Graphene suspension was purchased from xfnano (diameter: 1–5 μm; thickness: <10 nm, Suzhou, China). Anhydrous ethanol (C₂H₅OH, analytical purity, Chengdu Kolon Chemical Co., Ltd., Chengdu, China), sodium dodecyl sulfate (SDS, 92.5–100.5%, Aladdin Industries, Shanghai, China), and deionized water were used as received.

2.2. Preparation of Graphene Films by DIW

In this experiment, four different kinds of dispersion solvents were used, i.e., deionized water and anhydrous ethanol in four different ratios (0 ethanol, 10% ethanol, 20% ethanol, and 30% ethanol, all in volume fraction). The graphene suspension (0.3 g), dispersion solvent (10 mL), and sodium dodecyl sulfate (0.015 g) were put into a beaker and sonicated for 20 min to obtain the conductive graphene ink. Then, the conductive graphene ink was transferred into a syringe equipped with a gas pump. The PET substrate was fixed on the sliding operation table with a heating plate, and the distance between the needle and the substrate was controlled to be less than 5 mm. Next, the graphene film was directly written on the substrate under continuous heating (100 °C) using four types of conductive inks, denoted as G-0, G-10, G-20, and G-30 (with 0–30 vol.% ethanol in the solvent). The robotic arm's moving speed and extrusion flow rate were optimized to obtain graphene films with certain widths. The deposition process was repeated several times to obtain a graphene film with a certain thickness. After the full evaporation of the solvent, the as-printed graphene film was hot-pressed at 300 MPa for 5 min. The grid-patterned GFTE was fabricated by DIW following the same procedure. As shown in Figure 1, three types of grid patterns were fabricated with different transparent aperture sizes, i.e., 2 mm × 2 mm, 3 mm × 3 mm, and 4 mm × 4 mm (denoted as G-2 × 2, G-3 × 3, and G-4 × 4, respectively).

2.3. Characterizations and Testing Methods

The surface and cross-sectional morphology of as-printed and hot-pressed graphene film were characterized by field emission scanning electron microscopy (FESEM, Nova NanoSEM 450, FEI, Portland, OR, USA). The light transmittance of grid-patterned graphene films at 400–800 nm was recorded by a UV-Vis NIR spectrophotometer (SolidSpec UV-3700, Tokyo, Japan). The sheet resistance of the single-track graphene film and grid-patterned graphene film was measured using a Keithley 2450 source meter SMU (Tektronix Technology, Shanghai, China). The thickness of the graphene films was measured by a step meter (KLA-Tencor D120, Bruker DktakXT, Billerica, MA, USA). The average thickness was obtained by averaging the thickness measured at five different locations of each graphene film.

3. Results and Discussions

3.1. Characterization of Graphene Film Fabricated by DIW

A typical graphene film was fabricated by the DIW technique with a single trace and one layer. The appearance of the film is band-like, as shown in Figure 2. As can be seen in the optical image, the graphene film was continuous and the width was uniform, indicating good rheological behavior of the graphene ink [27]. More importantly, the “coffee ring” effect was absent in the as-prepared graphene film using the modified graphene ink, which will be further discussed in the following section.

The surface and cross-sectional morphologies of G-0 and G-20 graphene films after depositing 15 layers were analyzed by SEM, as presented in Figure 3. Figure 3a shows the flat and continuous surface of the G-20 film, indicating that the graphene flakes were uniformly distributed in the film. This can be attributed to the shear force induced by the DIW process and hot-pressing [28,29]. As can be seen in the cross-sectional morphology of the G-20 film shown in Figure 3b, graphene flakes were densely stacked without folds, voids, and fractures. In contrast, when pure water was used as the dispersion solvent, the graphene flakes were stacked messily, and folds and voids can be clearly observed in Figure 3c. Due to the difference in graphene flake stacking, the thicknesses of the graphene films were different. The thicknesses of G-0, G-10, G-20, and G-30 were measured by a step meter and are listed in Figure 3d and

Table 1. Thickness (μm) of graphene films with different numbers of layers.

	Number of Layers
	1	3	5	7	9	11	13	15	17	19
G-0	0.98 ± 0.05	1.25 ± 0.10	1.57 ± 0.10	3.39 ± 0.10	5.82 ± 0.15	6.96 ± 0.15	8.19 ± 0.15	10.30 ± 0.20	11.52 ± 0.20	12.83 ± 0.20
G-10	0.52 ± 0.05	1.02 ± 0.05	1.36 ± 0.10	1.82 ± 0.10	2.34 ± 0.10	2.86 ± 0.10	3.38 ± 0.10	4.00 ± 0.10	4.52 ± 0.15	4.96 ± 0.15
G-20	0.63 ± 0.05	0.92 ± 0.05	1.33 ± 0.10	1.87 ± 0.10	2.67 ± 0.10	2.94 ± 0.10	3.47 ± 0.10	3.83 ± 0.10	4.23 ± 0.15	4.87 ± 0.15
G-30	0.62 ± 0.05	0.92 ± 0.05	1.33 ± 0.10	1.82 ± 0.10	2.60 ± 0.10	2.86 ± 0.10	3.38 ± 0.10	3.60 ± 0.10	4.02 ± 0.15	4.38 ± 0.15

. As can be seen, the thickness of graphene film increased with increasing printed layers, which can be attributed to the accumulation of graphene flakes in the film. G-0 exhibited the largest thickness, while G-10, G-20, and G-30 had comparable thicknesses. A two-fold increase in thickness for G-0 was witnessed after 15 layers were printed on the substrate. In addition, the variation in thickness for the graphene films prepared with different solvent recipes was considered. As can be seen in Figure 3e,f, G-0 exhibited a large variation compared with G-20, indicating the more uniform thickness of graphene films prepared using a water/ethanol mixture solvent. Considering that all the graphene films, using different dispersion solvents, experienced the exact same process during fabrication, i.e., DIW, solvent evaporation, and hot-pressing, it is speculated that the addition of SDS and ethanol to the dispersion solvent contributed to the formation of densely packed and uniformly distributed graphene thin films.

Subsequently, the sheet resistances of G-0, G-10, G-20, and G-30 were measured by a resistance meter to evaluate their performance to be used as electrodes, and the results are listed in

Table 2. Sheet resistance (Ω/sq) of graphene films with different numbers of layers.

	Number of Layers
	1	3	5	7	9	11	13	15	17	19
G-0	129 ± 15	48.4 ± 10	15.0 ± 5	8.9 ± 1	2.1 ± 0.5	1.4 ± 0.5	0.93 ± 0.2	0.43 ± 0.1	0.35 ± 0.1	0.33 ± 0.1
G-10	153 ± 15	35.2 ± 10	20.0 ± 5	10.1 ± 1	7.1 ± 1	3.5 ± 0.5	2.01 ± 0.5	1.24 ± 0.1	1.12 ± 0.1	1.06 ± 0.1
G-20	85.1 ± 10	36.4 ± 10	10.0 ± 5	4.0 ± 1	2.0 ± 0.5	1.3 ± 0.5	1.03 ± 0.2	0.56 ± 0.1	0.46 ± 0.1	0.41 ± 0.1
G-30	108 ± 15	41.9 ± 10	17.0 ± 5	8.0 ± 1	4.1 ± 1	2.6 ± 0.5	1.85 ± 0.2	1.07 ± 0.1	0.91 ± 0.1	0.86 ± 0.1

. Initially, the sheet resistance decreases rapidly with an increase in the number of printed graphene layers. However, after 15 layers are printed, the resistance reduction plateaus. It is also worth noting that the lowest sheet resistance was achieved by the G-0 and G-20 films, while G-0 was thicker than G-20.

To better evaluate the electrical property of graphene films fabricated by DIW, the electrical conductivity of each film was calculated according to the following equation:

$$\mathsf{\sigma }={\displaystyle \frac{1}{\mathsf{\rho }}}$$

(1)

where $\mathsf{\sigma }$ is the conductivity in S/m and $\mathsf{\rho }$ is the resistivity in Ω∙m. The resistivity can be calculated according to the following equation:

$$\mathsf{\rho }={\displaystyle \frac{R·S}{L}}=Rs·w$$

(2)

where Rs (Ω/sq) is the sheet resistance and w (m) is the average thickness of each graphene film. The values of w and Rs for G-0, G-10, G-20 and G-30 with different numbers of layers are listed in Table 1 and Table 2, respectively, and were used to calculate the conductivity.

The calculated electrical conductivities of each graphene film with different numbers of layers are presented in Figure 4. It is obvious that using the water/ethanol mixture solvent led to a higher electrical conductivity, and the best performance was achieved by G-20. The highest conductivity of 5.22 × 10⁵ S/m was obtained after depositing 15 layers. This value is among the highest conductivities reported in other studies [30,31,32,33]. The main reason for the high conductivity obtained by G-20 can be attributed to the improved graphene flakes alignment using the water/ethanol mixture solvent. When >15 layers of graphene flakes were deposited, the electrical conductivity decreased. Considering the requirements for sheet resistance of GFTEs and the fabrication period/cost, 15 layers were deposited to fabricate the GFTEs in our study.

3.2. Formation Mechanism of Well-Aligned Graphene Flakes in DIW Films

By comparing the morphology and electrical conductivity of graphene films prepared with different dispersion solvents, it can be concluded that the addition of SDS and ethanol in the solvent plays an important role in obtaining a well-stacked and uniformly distributed graphene thin film with high conductivity. Therefore, the effects of SDS and ethanol are discussed in this section.

The uniform distribution of graphene flakes is crucial in affecting the electrical conductivity of as-deposited graphene conductive films. The presence of voids and other defects in the films would induce excessive resistance between adjacent graphene flakes. One of the main difficulties in forming uniformly distributed graphene flakes in the deposited films is the “coffee ring” effect during solvent evaporation [34]. The “coffee ring effect” refers to the phenomenon that when a droplet evaporates from a solid surface, the edge line of the droplet is pinned and the surface tension creates capillary flows toward the edge, which carry the suspended material to the edge of the droplet for deposition, resulting in uneven deposition of the material after evaporation [35]. The addition of small ionic surfactants such as sodium dodecyl sulfate (SDS) causes an increase in the surfactant concentration on the edge line so that the surface tension of the droplet is locally reduced. The gradient of surface tension triggers a Marangoni vortex toward the center of the droplet, which eventually leads to the homogeneous deposition of the remaining particles [36,37,38]. As can be seen from cross-sectional SEM images (Figure 3b) and step meter results (Figure 3d inset), the thickness of graphene conductive films along the lateral direction was uniform, indicating that the “coffee ring” effect was suppressed well with 0.5 wt.% SDS addition. Additionally, shear forces exerted during DIW printing can improve the alignment of graphene flakes, as shown in Figure 5.

The effect of ethanol is discussed as follows: When the deposited ink was heated and the temperature reached the boiling point of the water/ethanol mixture, ethanol molecules absorbed the thermal energy to form a reversible covalent bond with the graphene sheet in the interlayer. Conversely, water molecules absorbed the thermal energy to overcome intermolecular attraction with the graphene and escape from the graphene interlayer as vapor. The ethanol molecules that are covalently bonded to graphene flakes can improve the formability and flexibility of the deposited graphene film upon deformation [39]. Consequently, when the deposited graphene film was hot-pressed, the defects or voids formed due to solvent evaporation could be eliminated without changing the structure of graphene flakes and the connections between each flake could be improved (see Figure 5). As a result, a densely packed graphene film with fewer folds and pores, a smoother surface, and lower thickness was obtained.

The DIW strategy employed in this study, which involves the addition of SDS and ethanol, offers remarkable advantages over traditional methods of modifying graphene dispersions with surfactants alone when it comes to preparing graphene thin films. While conventional surfactant-modified graphene dispersions can somewhat improve graphene dispersion, they often fail to achieve a highly uniform distribution of graphene during complex deposition processes, thereby producing graphene films with wrinkles and voids between flakes due to the lack of structural optimization during film formation [40,41]. In this study, the synergistic effect of SDS and ethanol not only eliminates the coffee ring effect via Marangoni flow, resulting in a more uniform deposition of graphene on the substrate, but also enhances the alignment of graphene flakes during hot-pressing. This leads to a significant improvement in film uniformity and conductivity, with the highest conductivity reaching 5.22 × 10⁵ S/m, surpassing many traditional modification methods.

3.3. Fabrication and Performance Evaluation of Grid-Patterned Graphene Electrodes

For the application of these highly conductive graphene films in FTEs, it is also necessary to ensure that they have high light transmittance. However, the conductivity–transmittance “trade-off” has become a main limitation. Graphene films with a thickness larger than 1 μm exhibit limited optical transmittance, while transparent GO or rGO films usually present a low conductivity.

In some recent trials, grid patterns made of non-transparent but highly conductive metallic nanowires were prepared to overcome the conductivity–transmittance “trade-off”. By precisely tuning the aperture sizes of the grid pattern, a combination of high optical transmittance and high conductivity can be achieved [42]. DIW technology, due to its excellent process flexibility, excellent geometric controllability, and relatively low-cost and efficient processes, is promising for fabricating grid-patterned graphene electrodes since the width and position of each conductive line can be precisely adjusted to optimize the sheet resistance and light transmittance to meet the specific needs of GFTEs for different devices. As a demonstration, three different grid patterns with apertures of 2 mm × 2 mm, 3 mm × 3 mm, and 4 mm × 4 mm were designed and prepared by DIW using G-20 (see Figure 1b). The optical and electrical properties of these GFTEs were measured.

As shown in Figure 6, the transmittances of G-2 × 2, G-3 × 3, and G-4 × 4 grid-patterned graphene electrodes with 5 and 15 layers under visible light (400–800 nm) were determined by a UV-vis spectrometer to be 24.9%, 55.2%, and 68.5%, respectively. It was found that thickness had little effect on the transmittance of each pattern, indicating that the optical property was mainly regulated by the aperture size. However, the sheet resistances of G-2 × 2-15, G-3 × 3-15, and G-4 × 4-15 were much lower than those of G-2 × 2-5, G-3 × 3-5, and G-4 × 4-5. Figure 6b shows the overall optic and electronic performance comparison of our GFTEs with other FTEs reported in the literature. A low sheet resistance of 21.3 Ω/sq and a good transparency of 68.5% transmittance could be achieved by G-4 × 4-15, which was the best overall optic and electronic performance among the GFTEs prepared in this work. Even though lower sheet resistances of 7.0 Ω/sq and 15.3 Ω/sq could be achieved by G-2 × 2-15 and G-3 × 3-15, respectively, their light transmission properties (24.9% for G-2 × 2-15 and 55.2% for G-3 × 3-15) were unsuitable for application in FTEs. It can be seen that the overall optic and electronic performance of G-4 × 4-15 was close to that of metal/CNT or metal/graphene composite films [43,44,45]. The incorporation of metallic nanowires into the composite film decreases its resistance but might induce other problems such as sensitivity to oxidation and corrosion. Moreover, the overall optic and electronic performance of G-4 × 4-15 was superior to that of GFTEs prepared by pyrolyzing GO films at 1300 °C [46]. The thickness of such graphene films was around 30 nm to achieve a relatively high transparency but would inevitably result in a high resistance (>500 Ω/sq). In addition, GFTEs can be prepared by chemical vapor deposition and wet chemical doping to metals, which exhibit high transparency and low resistance, but the preparation method is costly [47]. In this work, by using the advantages of DIW technology like reproducibility and controllability, we were able to decrease sheet resistance by repeated ink writing while designing a suitable grid pattern to increase light transmission, therefore overcoming the conductivity–transmittance “trade-off”.

For practical application of GFTEs, the stability under different service environments such as high-temperature environments and acid–base corrosive environments needs to be taken into consideration. The thermal stability of our GFTEs was tested by placing G-4 × 4-15 in different high-temperature environments and measuring the change in its sheet resistance at regular time intervals, and the results are shown in Figure 7a. The increases in the sheet resistance of G-4 × 4-15 at 80 °C, 120 °C, and 160 °C were insignificant. After holding at 80 °C for 16 h, the sheet resistance of G-4 × 4-15 only increased by 4.5%, indicating its good thermal stability. The corrosion resistance of our GFTEs was tested by soaking G-4 × 4-15 in solutions with different pH values ranging from 1 to 14 for 1 min and measuring the change in its sheet resistance after rinsing and drying. As shown in Figure 7b, the sheet resistance increased by 3.8% in an acidic solution, which was the largest sheet resistance increase among different testing conditions. This result indicated that our GFTEs can work stably in corrosive environments. In conclusion, the GFTEs prepared in this work have good resistance to high temperatures and corrosion, which may be due to the superior stability of graphene over metallic nanowires.

4. Conclusions

This work demonstrates that a graphene conductive ink consisting of graphene flakes, sodium dodecyl sulfate, and ethanol/water solvent can be used to prepare grid-patterned GFTEs with good overall optical and electronic performance, including high electrical conductivity and relatively high transparency, by the DIW method. It is found that ethanol and SDS play a vital role in forming a well-ordered and densely stacked graphene flake thin film, which improves the electrical conductivity of graphene thin films without using toxic solvents and high-temperature post-treatment, rendering this process green, easy, and low-cost. In addition, the graphene grid patterns maintain good electrical conductivity even in high-temperature and corrosive environments. This strategy might be promising for fabricating GFTEs that can fulfill the requirements in applications like solar cells, flexible screens, and light-emitting diodes. The main conclusions of this study are listed below:

• A high electrical conductivity of 5.22 × 10⁵ S/m with 15 layers of graphene conductive ink deposited on a graphene thin film can be achieved due to the orderly packed and well-connected nature of graphene flakes in the film.
• An optimized microstructure of graphene films is obtained by manipulating the formulation of the ink solution, i.e., adding SDS and ethanol to the solvent to inhibit the coffee ring effect and improve formability during hot-pressing.
• Grid-patterned graphene electrodes with different aperture sizes were fabricated by DIW using the modified graphene conductive ink. A sheet resistance of 21.3 Ω/sq and a transmittance of 68.5% were achieved by G-4 × 4-15.