Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors

James W Borchert,Boyu Peng,Karin Zojer,Hagen Klauk,Joachim N Burghartz,Florian Letzkus,Paddy K L Chan,Sabine Ludwigs

doi:10.1038/s41467-019-09119-8

Abstract

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V.

Highlights

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs
The reason is that the contact resistance is a key limiting factor determining the transit frequency of organic TFTs4,5., and when the TFT dimensions are in the range required for megahertz operation at low voltages, the contact resistance is more limiting to the transit frequency than the intrinsic carrier mobility[2]
This is the optimum semiconductor-layer thickness that we have previously identified for TC organic TFTs based on DPh-DNTT37

Summary

Introduction

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. BC organic TFTs has motivated investigations into other molecules capable of forming monolayers, those with a large number of fluorine atoms to induce a work function shift beyond that obtained with PFBT24. Despite these efforts to improve the contact-semiconductor interface of BC organic TFTs, their contact resistance is still largely inferior to that of the best TC organic TFTs1. Recent drift-diffusion-based simulations performed by Zojer et al predict that BC organic TFTs may exhibit lower contact resistances than otherwise equivalent TC organic TFTs11,30, provided the energy barrier between the source contact and the organic semiconductor is sufficiently low and the gate dielectric is sufficiently thin. Given the importance of the contact resistance for the dynamic TFT performance[1,2], this is a potentially critical finding, but an experimental study to confirm the impact of the gate-dielectric thickness on the contact resistance has to our knowledge not yet been performed, investigations into the effects of the gate-dielectric thickness on other organic-TFT-performance parameters are abundant[31,32,33,34]

Methods

Results

Conclusion