Organic metal engineering for enhanced field-effect transistor performance

Abstract

A key device component in organic field-effect transistors (OFETs) is the organic semiconductor/metal interface since it has to ensure efficient charge injection. Traditionally, inorganic metals have been employed in these devices using conventional lithographic fabrication techniques. Metals with low or high work-functions have been selected depending on the type of semiconductor measured and, in some cases, the metal has been covered with molecular self-assembled monolayers to tune the work function, improve the molecular order at the interface and reduce the contact resistance. However, in the last few years, some approaches have been focused on utilizing organic metals in these devices, which have been fabricated by means of both evaporation and solution-processed techniques. Higher device performances have often been observed, which have been attributed to a range of factors, such as a more favourable organic/organic interface, a better matching of energy levels or/and to a reduction of the contact resistance. Further, in contrast to their inorganic counterparts, organic metals allow their chemical modification and thus the tuning of the Fermi level. In this perspective paper, an overview of the recent work devoted to the fabrication of OFETs with organic metals as electrodes will be carried out. It will be shown that in these devices not only is the matching of the HOMO or LUMO of the semiconductor with the metal work-function important, but other aspects such as the interface morphology can also play a critical role. Also, recent approaches in which the use of organic charge transfer salts as buffer layers at the metal contacts or on the dielectric or as doping agents of the organic semiconductors that have been used to improve the device performance will be briefly described.

Tuning Crystal Ordering, Electronic Structure, and Morphology in Organic Semiconductors: Tetrathiafulvalenes as a Model Case

“…The mobility and also the contact resistance were found to change with the metal workfunction. The organic metal resulted in the lower contact resistance devices, which was attributed to small potential shift on the organic/organic interface compared with the organic/metal interface . In agreement with these results, OFETs based on single crystals of hexamethylene‐tetrathiafulvalene (HM‐TTF) exhibited a mobility of 0.02 cm 2 V −1 s −1 when Au was employed as source‐drain but reached a mobility exceeding 10 cm 2 V −1 s −1 when gold was replaced by (TTF)(TCNQ) …”

Light‐Modulation of the Charge Injection in a Polymer Thin‐Film Transistor by Functionalizing the Electrodes with Bistable Photochromic Self‐Assembled Monolayers

“…After a thorough study of the optical, energetic, and electrical properties of Au planar surfaces functionalized with thiolated‐DAE SAMs, DAE monolayers were chemisorbed on gold source and drain electrodes of OTFTs and their optical switching behavior in the devices was explored. To maximize the effective injection area, we utilized a top‐gate bottom‐contact device geometry . P(NDI2OD‐T2), a well‐known n‐type polymer semiconductor, was employed because it exhibited high field‐effect mobilities when integrated into top‐gate OTFTs .…”

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

“…Organic metals have gained attention as alternatives to conventional metals due to their low‐cost processing and better interface morphology that they can yield given the similarities in composition, hence better compatibility with the organic semiconductor. They include charge transfer complexes (CTs), PEDOT‐derivatives, carbon nanotubes (CNTs), graphene and its derivatives . For example, dibenzotetrathiafulvalene OFETs with contacts consisting of tetrathiafulvalene–7,7,8,8‐tetracyanoquinodimethane (TTF‐TCNQ) ( Figure a) yielded significantly lower contact resistance and higher mobility than those with Au contacts, in spite of a higher Schottky barrier estimated from the value of the work functions.…”

“…We limited our discussion to several examples based on CTs; a review on charge transfer complexes was recently published by Goetz et al, and one dedicated specifically to OFET electrodes consisting of these materials by Pfattner et al…”