Hysteresis in organic field-effect devices: Simulated effects due to trap recharging

Abstract

Current organic field-effect transistors feature predominantly undesirable hysteresis effects which appear also in the capacitance-voltage (CV) characteristics of organic metal-oxide-semiconductor (MOS) capacitors. So far, these effects are not sufficiently characterized experimentally and their origin is even less understood. In the literature one finds presumptions that they are caused by trap recharging or by mobile ions (in the organic semiconductor or in the insulator). In order to check the first of these presumptions, detailed numerical simulations have been carried out. Hysteresis in the field effect is studied at best in the MOS capacitor without the additional influence of the source/drain contacts of the transistor. Although there are differences between quasistatic and dynamic measurements, our simulations are done for the quasistatic CV characteristics since they already give the desired information for the understanding of the hysteresis effects. Organic semiconductors with different types of traps of different energetic positions, concentrations, and energetic and spatial distributions are considered and their parameters are varied in a wide range. Trap recharging is connected with the transport of the emitted (captured) carriers from (to) the traps during the voltage sweep. It is demonstrated that rather different types of hysteresis can occur thereby. All of the simulated shapes of the hysteresis are qualitatively different from that one which is typically observed experimentally. Therefore, it should be ruled out that the hysteresis observed in organic MOS devices is solely caused by trap recharging. The observed changes of the CV characteristics for a variation of different measuring conditions and parameters indicate that the hysteresis is caused by at least two different processes. We suppose that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., the direct polaron-bipolaron reaction or a complex formation reaction of polarons/bipolarons with counterions.