www.nature.com/scientificreports OPEN Patterning of High-Viscosity Silver Paste by an Electrohydrodynamic- Jet Printer for Use in TFT Received: 21 November 2018 Accepted: 29 May 2019 Applications Published: xx xx xxxx Thi Thu Thuy Can, Tuan Canh Nguyen & Woon-Seop Choi Electrohydrodynamic (EHD) jet printing has a variety of benefts compared to conventional inkjet techniques, such as high resolution and the ability to work with high-viscosity pastes. In this work, Ag nanoparticles with 4000 cPs were chosen because they are printable on various substrates for electronic devices. The efects of additive on the high-viscosity Ag paste formulation were investigated, and pattern lines narrower than 100 μm were achieved by EHD-jet printing with an average sheet resistance of 0.027 Ω −1. Furthermore, solution-processed oxide TFTs were fabricated with EHD jet-printed Ag electrodes for the frst time. The electrical properties obtained were a current ratio of 1.5 × 106, a mobility of approximately 1 cm2 V−1 s−1, a threshold voltage of 21.5 V, and a subthreshold slope of 3.05 V dec−1. Direct-write techniques have been investigated extensively for making fne patterns in micro- or nano-scale. Tis technique is low cost, high speed, contact-free operation and very environmentally friendly manufacturing processes compared to conventional methods1,2. Among the large number of direct-write techniques, the inkjet method is widely known for fabricating printed electronic devices. Te benefts of inkjet printing are on-demand jetting and high throughput with a high jetting frequency of 10–100 kHz. However, because of the large size of droplets or patterns, it is not easy to meet the demanding requirements in case of high-density and conductive printed circuit boards or thin-flm transistors (TFTs)3,4. For printed electronics, silver nanoparticles attracted a great interest for the many applications because of their thermally and electrically conductive, solvent resistant, and adhesive to most materials. Many studies have concentrated on the direct printing of silver nanoparticles with very low viscosity for the fabrication of electrically functional microstructures5–7. Electrohydrodynamic (EHD) jet printing is a technique with very potentials for directly patterning func- tional materials onto many substrates. Its ability to make patterns and use in thin flm deposition can support the fabrication of the electronic devices through a single technique8. EHD jet printing has received attention as a non-contact printing technique with a unique ability to produce fne patterns with either a large-diameter nozzle or ultra-fne nozzle9–11. Some of our previous studies on EHD jet printing demonstrated high-resolution patterns compared to spin-coating and ink-jet techniques. Te TFT characteristics attained using an EHD jet printer were much bet- ter than those obtained with spin-coating technology. An EHD jet-sprayed TFT showed a threshold voltage of 7.17 V and a very high on-to-of ratio of 107 12. Moreover, the electrical properties of the pattern printed from the EHD jet printer surpassed those of the conventional ink jet technique. An EHD jet-printed ZTO TFT showed a feld-efect mobility of 9.82 cm2 V−1 s−1, on-to-of current ratio of 3.7 × 106, and threshold voltage of 2.36 V at 500 °C13. Just few researchers have examined multi-nozzle EHD jet printing techniques to overcome the difculties of the inkjet technique14,15. A three-nozzle-EHD inkjet printing head was used to fabricate conductive lines of silver colloidal ink on glass substrate simultaneously. However, the process was quite complex, and average printed track widths of 140 ± 5 μm were obtained16. In this study, silver line patterns from high-viscosity Ag paste formulations were produced using an EHD jet printer and applied in electronic devices. Sharp and continuous lines with a width of 100 μm or less were obtained for the frst time using a very inexpensive steel needle that required no treatments. ZTO TFTs with Department of Display Engineering, Hoseo University, Asan, Chungnam, 31499, Korea. Correspondence and requests for materials should be addressed to W.-S.C. (email: [email protected]) SCIENTIFIC REPORTS | (2019) 9:9180 | https://doi.org/10.1038/s41598-019-45504-5 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Confguration of experimental setup used for EHD jet printing. Figure 2. (a) Optical microscopy images of EHD-jetted Ag patterns with various distances between the substrate and nozzle tip of 10, 5 and 3 mm. (b) Optical microscopy images of EHD-jetted Ag patterns: without PGMEA (lef) and with PGMEA (right) (volume ratio of Ag nano-particles: silveray: PGMEA = 100:1:1). EHD-fabricated Ag source and drain electrodes were also successfully manufactured with acceptable electrical characteristics. Results and Discussion Efect of tip height on patterns. Te frst main factor that afects the quality of a pattern line is the dis- tance h between the nozzle tip and the substrate surface, which is called the tip height in Fig. 1. To investigate the dependence between h and the silver line characteristics, Ag paste was jetted using three tip heights of approxi- mately 3, 5, and 10 mm. At 10 mm, no pattern in the line shape can be obtained because the distance is large, as shown in Fig. 2a. Te multi-jet mode and/or ramifed-jet mode occurred, and there was no obvious change in the shape of the patterns. Te patterns did not seem to be continuous, and there were some small holes in the lines. With a tip height of 3 mm, the lines covered the substrate well, but there was still splashing on two sides of the patterns. When moving the tip closer to ITO surface, it is not safe for the tip and the substrate because of the increased electrostatic force between the nozzle and conductive stage. Stabilizing and optimizing tip height is critical for microscale applications in EHD jet printing. Tus, the tip height should be around 3 mm to obtain acceptable patterns. Efect of additive on patterns. In general, commercially nanoparticle precursor conductive inks with low viscosity are produced with low evaporation rate binder solvents in order to get rid of the blockage in the nozzle tips in printing technology. As a result, it requires long drying times for printed patterns which limits to mass production. In this work, silver paste with relatively high viscosity of 4,000 cPs can easily be accumulated at the narrow print-head. Terefore, fnding an additive to prevent material obstruction and allow for promoting evaporation rate as well is a key component in the process of preparing the electrically conductive ink. Many ingredients can be added to improve the quality of silver paste quality, such as a weak solvent, binder, stabilizer, and so on. We found that the desired neat patterns could not be obtained with only the original silver nanoparticle pastes (the lef image of Fig. 2b). To optimize the conductive paste for inkjet printing, several groups have used solvents with a high vapor pressure to allow for quick drying time, such as hexane and toluene17–19. Among many solvents used in commercial conductive inks, propylene glycol monomethyl ether acetate (PGMEA) is known as a solvent commonly used in inks, coating, and cleaner in the semiconductor industry, primarily for the application of surface adherents. It is featured by high vapor pressure of 2.8 mmHg at 20 °C, boiling point of 146 °C and low viscosity of 1.1 mPas. A fast drying-conductive ink was developed for inkjet printing conductive tracks, and the SCIENTIFIC REPORTS | (2019) 9:9180 | https://doi.org/10.1038/s41598-019-45504-5 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 3. (a) Optical microscopy images of EHD-jetted Ag patterns with various stage speeds from 2,200 to 3,500 μm s−1. (b) Pattern width was measured from 4 lines corresponding to diferent stage speeds with a ratio of original silver paste:silveray:PGMEA mixture of 100:1:1. solvent played a main role because of the relatively high vapor pressure compared to those of solvents used in commercial conductive inks20. Terefore, PGMEA was added to the silver nanoparticle paste in this research to obtain neat and smooth edges. A tiny amount of PGMEA enormously enhanced the silver patterns, as shown in the right image of Fig. 2b. Particularly, smaller width and good coverage of the substrate were achieved due to the adhesion between the paste and ITO glass. Efect of stage speeds and voltages on patterns. Te same conditions used to obtain neat silver wires were applied but with a faster stage speed ranging from 2000 to 3500 μm s−1. Te volume ratio of silver paste: silveray (solvent): PGMEA (additive) in the silver mixture was 100:1:1. Te width of the printed silver decreased when the stage velocity increased, as illustrated in Fig. 3a. Te reason is that the stage moved quickly and the resulted in much less time for silver paste approach surface. Te maximum speed should be lower than 3500 μm s−1 to obtain straight and sharp lines, as shown in Fig. 3b. Te smallest width achieved was 126 μm. Figure 4a illustrates the dependence of the pattern width on the voltage and stage speed. Te experiment was carried out with a material volume ratio of 100:1:1.5 (original silver paste:silverary:PGMEA), and the range of pressure was 80–90 kPa. With a satisfactory applied voltage, the paste from the nozzle tip forms a Taylor cone, and a fuid jet is emitted from the metallic tip (Fig. 4c). When no electric feld (0 kV) was applied to the tip, silver paste accumulated at the end of the tip due to repulsive pressure from the syringe.