# 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 \(\times\) 10\({}^{5}\) S m\({}^{-1}\). The grid-patterned graphene-based flexible transparent electrodes exhibit a low sheet resistance of 21.3 \(\Omega\)/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. direct ink writing; transparent graphene electrode; grid pattern; electrical conductivity 2020 IEEE  
low as 30 \(\Omega\)/sq, graphene-based flexible transparent electrodes (GFTEs) exhibit high sheet resistance (600 \(\Omega\)/sq-1000 \(\Omega\)/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, [PERSON] et al. fabricated high-quality graphene sheets with few defects and grain boundaries by chemical vapor deposition at 800 \({}^{\circ}\)C, which achieved a low sheet resistance of 1000 \(\Omega\)/sq with a light transmittance of 87% [13]. [PERSON] et al. prepared GFTEs using AuCl\({}_{3}\)-doped graphene sheets to increase the charge carrier density of graphene, and the square resistance could be reduced to 150 \(\Omega\)/sq with 87% light transmittance [14]. [PERSON] et al. obtained p-type doped graphene by a liquid-phase reaction, improving its electrical conductivity to as high as 3.13 \(\times\) 10\({}^{4}\) 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 \({}^{\circ}\)C was generally applied to achieve an electrical conductivity as high as 10\({}^{6}\) 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].