Direct investigations of the interface impedance of organic field-effect transistors with self-assembled-monolayer-modified electrodes

Abstract

a b s t r a c tIt has been demonstrated that the modification of electrodes with self-assembled monolayers (SAMs) reduces the contact resistance and improves the device performances of organic field-effect transistors (OFETs). However, it has been difficult to judge if the contact resistance was reduced by the change in the electronic properties or by the change in the morphology of the metaleorganic interface caused by the SAM modification because they have been difficult to be separately assessed. We have directly investigated the local impedance and the potential difference at the electrodeechannel interfaces of the OFETs with and without modification of the electrodes by a pentafluorobenzenethiol SAM using frequencymodulation scanning impedance microscopy (FM-SIM). The potential profile measurement and the FM-SIM measurement at the interface showed that the improvement of the field-effect mobility in the SAM-modified OFET was caused by the reduction of the energy level mismatch, namely, the hole injection barrier at the sourceechannel interface, presumably with the reduction of the hole trap sites at the sourceechannel interface.

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

“…For dynamic measurements of device performance, impedance spectroscopy (IS) has proven to be a powerful tool for characterization . It is a frequency‐dependent method of measuring the linear response of the transistor: devices are configured in a DC steady‐state (for instance, with V GS and V DS held constant) while a time‐varying perturbation is added to an input.…”

“…PFBT is a popular SAM incorporated in p‐type OFETs. It is also a fluorine‐substituted SAM but containing five –F atoms on the benzene structure, and it can increase the work function of gold by up to 0.8 eV . Other heavily fluorinated SAMs include 4‐(tri‐ fluoromethyl)‐benzenethiol (TFBT) and 2,3,5,6‐tetrafluoro‐4‐(trifluoromethyl)‐ benzenethiol (TTFP), which were shown to increase the work function, lower contact resistance, and improve OFET performance, as seen in Figure d .…”

Review—Charge Carrier Mobility of Organic Thin Film Transistor: Intrinsic and Extrinsic Influencing Factors Based on Organic Semiconducting Materials

“…152 However, injection resistance and barriers influence the flow of carriers, thus affecting the mobility of OTFTs. The charge injection mechanism at the metal/OSM depends on various factors such as non-ideal electrode/OSM semiconductor interface, 153 the existence of considerable dipole barriers at metal/ OSM interfaces, [154][155][156] growth orientation of the π-π stacking of OSM parallel to the electrodes, 157 the position of the energy levels between the electrode and organic semiconductor, 158,159 different in the work function of S/D electrodes and HUMO (or LUMO) level of the organic molecules, 160 exposure of the metal interface to air 161 and energetic mismatch between the respective frontier electronic states. 162 Typical contact resistance (R c ) values for OTFTs are usually larger than 10 2 Ω cm, in contrast to Si-based metal oxide semiconductor FETs (MOSFETs) with Rc less than 0.1 Ω cm.…”

Effect of contact resistance in organic field‐effect transistors

“…Since alkanethiols and perfluorinated alkanethiols have opposite dipoles, they can be used to decrease [ 72,77,96,105 ] and increase [ 72,93,97,98,105–107 ] the WF of metal electrodes, respectively. Besides their ability to tune the WF, recently the interface dipole formed by SAMs was proved effective to reduce the trap sites at the metal/organic interface in OFETs [ 108 ] and to improve the crystallinity and grain sizes of the semiconductor films. [ 95,97,109 ] Thus, SAM‐modified electrodes would significantly influence the value of R C in OFETs.…”