## 2 Experimental  
The schematic structure of the bottom-gate, top-contact a-IGZO TFT with inkjet print Ag S/D electrodes is shown in Figure 1a. To prepare the device, a 300-nm-thick layer of Al:Nd alloy (3 wt % of Nd) was firstly sputtered on glass as gate electrode at room temperature. Then, the film was anodized in the electrolyte consisting of ammonium tartrate solution and ethylene glycol, accordingly forming a 200-nm-thick Al\({}_{2}\)O\({}_{3}\):Nd gate dielectric layer on the surface of Al gate. Next, a 25-nm-thick a-IGZO active layer was deposited on Al\({}_{2}\)O\({}_{3}\) by radio frequency (RF) magnetron sputtering in an Ar/O\({}_{2}\) gas mixture with a power of 80 W, and subsequently the sample was annealed at 400 \({}^{\circ}\)C for 1 h on a hotplate. Finally, the Ag S/D electrodes were directly printed onto the a-IGZO active layer.  
For the formation of top Ag S/D, a purely transparent commercial ink (TEC-IJ-010, INKTEC, Ansan-city, Kyungki-do, Korea), which consists of silver salt and ethanol-based solvent, is not formulated by nanoparticle structure and thus could be stably dissolved in the solvent without disperse stabilizer. It contains 15 wt % silver contents with a viscosity of 9-15 cps and a surface tension of 30-32 dynes/cm. The silver ink was patterned onto the a-IGZO via a piezoelectric inkjet printer (DMP2800, Fujifilm Dimatix, Santa Clara, CA, USA) installed with a 1 pL cartridge. An optimized waveform was set with a drop velocity of 5 m/s. The temperature of the cartridge and substrate were maintained at 60 \({}^{\circ}\)C during the printing process, meanwhile, droplets were deposited with a spacing of 25 \(\upmu\)m to obtain uniform Ag S/D electrodes on a-IGZO as shown in Figure 1b. Channel length/width measurement was 550/154 \(\upmu\)m. After printing, the device was sintered at 150 \({}^{\circ}\)C for 10 min on hotplate. Comparing devices fabricated with different types silver inks, the device with printed Ag S/D electrodes using a silver nanoparticle ink (30-35% in volume, DGP 40 TE-20C, Advanced Nano Products, Bugang-myeon, Sejong-Si, Korea) was further fabricated under optimized conditions. The droplet ejection velocity was 2 m/s, the substrate temperature was 60 \({}^{\circ}\)C, and the interdrop space was 35 \(\upmu\)m. After printing, the Ag electrodes were dried with UV equipment (Intelli-Ray 600, Uvitron, West Springfield, MA, USA) using 100% intensity for 300 s. Channel length/width measurement was 549/142 \(\upmu\)m.  
Figure 1: **(a)** Schematic of a-IGZO TFT with inkjet printing Ag S/D electrodes; **(b)** Optical microscope image of device finally fabricated.  
The size of channel was measured with a polarized optical microscope (Nikon, DeWitt, IA, USA). The structure of Ag film on glass substrate was obtained by scanning electron microscopy, scanning electron microscopy (SEM, NOVA NANOSEM 430, FEI, Eindhoven, The Netherlands), and X-ray diffraction (XRD, Empyrean, PANalytical, Almelo, The Netherlands). The electrical properties of devices were characterized in ambient air using a semiconductor parameter analyzer (Agilent 4155C, Santa Clara, CA, USA). The properties of the Ag/a-IGZO interface were analyzed by transmission electron microscope, (TEM, Bruker, Adlershof, Berlin, Germany) with an electron energy loss spectroscope (EELS, Gatan Enfinium ER Model 977, Pleasanton, CA, USA).