Charge Carrier Doping As Mechanism of Self-Assembled Monolayers Functionalized Electrodes in Organic Field Effect Transistors

Abstract

Coating electrodes with self-assembled monolayer (SAM) of polar molecules is known to reduce contact resistance (RC) in organic field effect transistors (OFETs). It is shown that the behavior of SAM in OFETs can be explained by considering a mechanism of interfacial doping in organic semiconductor by electrodes due to charge transfer during interface formation. The enhancement is analyzed in performance of pentacene OFETs with Cu electrodes, by coating Cu with SAM of pentafluorobenzenethiol or perfluorodecanethiol. It is found that application of either SAM leads to an increase in work function of Cu surface. However, work function shift due to SAM does not correlate with trends in RC and OFET drain current. Further, first principle calculations reveal a notable difference in delocalization of frontal orbitals with either SAM, an indicator of the difference in ease of charge transfer across interface. Based on the mechanism of interfacial doping, a semiconductor physics model is developed for estimating interface doping and injection barrier, and for predicting the consequent device characteristics. It is believed that the model and methodology developed in this study can be utilized beyond the SAM and semiconductor system used here.