Fully Inkjet‐Printed Transparent Oxide Thin Film Transistors Using a Fugitive Wettability Switch

DOI: 10.1002/aelm.201500086 compatible with various fl exible substrates have appeared in the literature. [ 38,39 ] Several attempts have been made to use sol-gel routes to produce transparent conductive electrodes or semiconductors; [ 19,20 ] however, the authors of these reports have universally made use of blanket deposited layers and have not successfully realized fully printed transparent transistors. Indeed, to this point, the serious diffi culties associated with the formation of uniform fi lms via inkjet printing of liquid phase inks [ 40,41 ] have prevented the realization of fully printed devices. In this work, we demonstrate fully printed transparent oxide devices for the fi rst time. First, we demonstrate a complete family of printable metal-oxide inkjet inks, including inks for the electrodes (Sb-doped tin oxide (SnO 2 )), semiconductors (SnO 2 ), and insulators (zirconia (ZrO 2 )). As a unique innovation that allows for printing of thin sol-gel precursor fi lms, we develop a novel technology based on a fugitive wettability switch coupled with optimized ink formulations. Ultrathin organic wetting layers are used to facilitate the printing of uniform and well-controlled oxide fi lms; these fi lms are then burnt off using post-print heating. Thus, these fugitive layers ensure good wettability, but are largely removed from the fi nal device to ensure good device performance. At temperatures above 100 °C, the reduction of the spreading speed on the surface-energy-controlled ultrathin organic wetting layers due to the high-viscosity co-solvents in the ink formulation largely suppresses the coffeering effect, resulting in uniform structures. By exploiting these techniques in conjunction with the aforementioned materials, we demonstrate inkjetted transparent metal-oxide-based transistors that consist of an inkjetted bottom antimony-doped tin oxide (ATO) gate electrode, an inkjetted ZrO 2 insulator, an inkjetted ATO source/drain electrode, and an inkjetted SnO 2 semiconductor. The devices exhibit excellent electrical characteristics and deliver a saturation mobility of ≈11 cm 2 V −1 s, a 10 6 on/off ratio and a 0.18 V/decade sub-threshold slope while operating at low voltage (<5.0 V). We have previously reported the fabrication of spin-coated devices using sol-gel SnO 2 and ZrO 2 . [ 34 ] However, to this point, we have not demonstrated a fully printed transparent device fabricated using these materials. A critical missing piece is a printable transparent conductor material; we have used printed sol-gel ATO in the present work, formed by exploiting co-conversion of antimony and tin sol-gel precursors. The variation in resistivity of the ATO fi lms (≈100 nm thick) prepared using different SbCl 3 compositions and different air annealing conditions are summarized in Figure S1 and Table S1, Supporting Information. Optimized resistivity of the fi lms of approximately 0.04 ohm cm was achieved using 8 wt% SbCl 3 precursor, annealed at 500 °C for 15 min in air; the resistivity of the fi lms then increased slightly with increasing processing time. Over the last decade, intensive efforts have targeted the production of low-cost large-area systems such as displays, radio-frequency identifi cation (RFID) tags, sensors, and solar cells. [ 1–11 ]