# Graphene-Based Grid Patterns Fabricated via Direct Ink Writing for Flexible Transparent Electrodes
## 3 Results and Discussions
### 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 \(\upmu\)m exhibit limited optical transmittance, while transparent GO or GO 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 \(\times\) 2 mm, 3 mm \(\times\) 3 mm, and 4 mm \(\times\) 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 \(\times\) 2**, **G-3 \(\times\) 3**, and **G-4 \(\times\) 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 \(\times\) 2-15**, **G-3 \(\times\) 3-15**, and **G-4 \(\times\) 4-15** were much lower than those of **G-2 \(\times\) 2-5**, **G-3 \(\times\) 3-5**, and **G-4 \(\times\) 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 \(\Omega\)/sq and a good transparency of 68.5% transmittance could be achieved by **G-4 \(\times\) 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 \(\Omega\)/sq and 15.3 \(\Omega\)/sq could be achieved by **G-2 \(\times\) 2-15** and **G-3 \(\times\) 3-15**, respectively, their light transmission properties (24.9% for **G-2 \(\times\) 2-15** and 55.2% for **G-3 \(\times\) 3-15**) were unsuitable for application in FTEs. It can be seen that the overall optic and electronic performance of **G-4 \(\times\) 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 \(\times\) 4-15** was superior to that of GFTEsprepared by pyrolyzing GO films at 1300 \({}^{\circ}\)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 \(\Omega\)/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\".