Inkjet printing of oxide thin film transistor arrays with small spacing with polymer-doped metal nitrate aqueous ink

Abstract

Fabrication of oxide thin film transistor (TFT) arrays by inkjet printing with small spacing was investigated, aiming at printing high resolution display backplanes. Two processes are involved during the printing and drying of oxide ink drops; the initial ink drop spreads to a large size upon landing on a substrate and then shrinks to a small size after the solvent dries out. The spreading of the ink drop determines how close two ink drops can be placed and the shrinkage of the ink drop after drying determines the final thickness of the printed dot. It was found that by hydrophobic treatment of the substrate surface with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS) assembly, the minimum spacing (center-to-center distance) of the printed oxide dot array was reduced from over 80 μm to 45 μm (HMDS treated) and 35 μm (OTS treated), while the dot diameters simultaneously decreased from 70 μm to 20 μm (HMDS treated) and 15 μm (OTS treated). However, too much shrinking caused random movement of the ink drops and an increase of oxide layer thickness, both of which are detrimental to the performance of printed TFTs. To overcome these problems, the ink viscosity was adjusted by adding polyvinylpyrrolidone (PVP) to the ink. With the increase of ink viscosity, the random movement of the ink drops was suppressed and the smallest spacing of 50 μm was achieved for printed oxide TFT arrays. Printed IGZO-TFTs annealed at 350 °C exhibited the highest mobility of about 6.2 cm² V⁻¹ s⁻¹ and on-to-off current ratio of more than 10⁷.