Unveiling the Effects of Hydroxyl-Induced Trap States on the Charge Transport in p- and n-Channel Organic Field-Effect Transistors through Variable-Temperature Characterization

Abstract

Trap states at the gate dielectric-organic semiconductor (OSC) interface are one of the main sources of extrinsic traps in organic field-effect transistors (OFETs). However, they are often overlooked and their effects on the charge transport are attributed to the exposure of devices to ambient air. Here a first variable-temperature transfer length method characterization of both p- and n-channel OFETs under full high vacuum conditions is reported. By comparing a hydroxylated aluminum oxide (Al2O3) gate dielectric with a hydroxyl-free, tetradecylphosphonic acid-functionalized Al2O3 dielectric, it is shown that hydroxyl-induced trap states reduce the charge carrier mobility in OFETs regardless of the channel type. This observation challenges the common belief that the hydroxyl-induced traps are affecting primarily the n-channel transport. The variable-temperature analysis yields a high activation energy of charge transport as the main effect of a hydroxylated gate dielectric. Moreover, the injection barrier at the interface between the source-drain electrodes and the OSC layer is significantly lower for devices with a hydroxyl-free dielectric and correlates with the activation energy of charge transport. This work identifies previously hidden limitations of charge transport in OFETs, opening opportunities for further improvements in device performance and potential device applications.