Metal oxide (MO) semiconductors have attracted extensive attention for printed thin film transistors (TFTs) due to their higher performance than competing organic semiconductors. To achieve MO-based high performance circuits, forming unipolar circuits with n-type TFTs having different threshold voltages (Vt) is required owing to the lack of feasible p-type MO semiconductors. Several approaches have been reported to control the Vt of MO TFTs such as varying the active layer thickness and depositing metals on active layers. However, they have the disadvantages of degrading performance of TFTs in thicker film and being unsuitable for printing processes, respectively. This work addresses these challenges by selectively printing polymers on the back-channel of printed MO TFTs; by using printed polymers as back-channel control layers, TFT Vt can be controllably and selectively adjusted, thus allowing for the realization of printed NMOS logic circuits.
Polymers such as polyethyleneimine (PEI) and epoxy-based resists SU-8 were inkjet printed on the back-channel of the TFTs having printed indium oxide semiconductors and ITO source/drain electrodes on SiO2/Si wafers. These layers have been found to passivate the back-channel of the oxide semiconductor, thus controllably altering Vt. The average Vt of MO TFTs was negatively shifted from 3 V in bare TFTs to -15 V or -37 V in the case of printing of SU-8 single or SU-8/PEI double layer, respectively. The Vt could also be shifted back over a wide range using additional post-annealing below 150°C. Based on the results above, NMOS inverters with depletion-mode TFT loads and enhancement-mode TFT drivers were fabricated on glass substrates with solution-processed aluminum oxide dielectrics and ITO gate electrodes. The resulting inverters showed excellent performance, delivering a gain of 23 V/V and a propagation delay of 0.5 ms at a 10 V supply, this performance was achieved despite the use of relatively long printed channels of 220 μm channel length.
This work thus represents an effective methodology to adjust the Vt of MO TFTs by selective polymer printing, thereby presenting an efficient strategy for realizing printed oxide logic circuits.