Revealing Al evaporation-assisted functions in solution-processed ZnO thin film transistors

Metal oxide semiconductors based on a solution process have facilitated major breakthroughs in the emerging field of flexible and transparent electronic devices. In particular, enhanced output performance of metal oxide semiconductors obtained by a solution process is desirable, because they are easy to fabricate and cost effective at low temperatures. To date, a carbon-free method involving an aqueous zinc amine complex has been employed to generate metal oxide active layers that have outstanding electrical features despite the formation of a very thin active layer. However, manipulation of trap states induced by chronic weak bonding structures initially present during the solution process remains a challenge. In addition, a thin active layer is highly susceptible to the initial surface and interface charge traps, resulting in the deterioration of electron transport by unclear mechanisms. Therefore, intentional control of intrinsic defects arising from porosity and pinholes is becoming one of the key issues in the development of highly stable solution-processed metal oxide semiconductors. Here, we describe a generic metal evaporation approach to enhance the electrical performance of solution-processed ZnO TFTs. In particular, we do not use passivation or post-annealing processes. Based on systematic structural and electrical analyses, we propose a mechanism based on Al metal evaporation-driven reduction of trap states that convincingly explains the unique features of the solution-processed ZnO TFTs obtained in this study. We anticipate that these findings will spur progress toward the realization of solution-processed electronic devices.