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.

The primary driver for the development of organic thin-film transistors (TFTs) over the past few decades has been the prospect of electronics applications on unconventional substrates requiring low-temperature processing. A key requirement for many such applications is high-frequency switching or amplification at the low operating voltages provided by lithium-ion batteries (~3 V). To date, however, most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances. Here, we present flexible low-voltage organic TFTs with record static and dynamic performance, including contact resistance as small as 10 Ω·cm, on/off current ratios as large as 1010, subthreshold swing as small as 59 mV/decade, signal delays below 80 ns in inverters and ring oscillators, and transit frequencies as high as 21 MHz, all while using an inverted coplanar TFT structure that can be readily adapted to industry-standard lithographic techniques.

He received his B.A. degree in physics from Rutgers University and M.S.E. degree in materials science and engineering from the University of Pennsylvania. He then worked as a staff scientist at Innova Dynamics in San Francisco, before completing his Ph.D. in materials science at the Max Planck Institute for Solid State Research and the University of Stuttgart, where he developed flexible high-frequency organic transistors with record-low contact resistance. His current research interests include light-matter interaction in organic semiconductors and interfaces in organic electronic devices. R. Thomas Weitz received his diploma in physics at the University of Heidelberg and his Ph.D. from the Max Planck Institute for Solid State Physics in Stuttgart. After a postdoc at Harvard University and the MPI in Stuttgart, he went into industrial research at BASF SE, Ludwigshafen. After having been a professor at the Ludwig-Maximilians University, Munich, he is currently a full professor at the 1st Institute of Physics at the Georg August University of Göttingen. His research is focused on quantum transport in low-dimensional materials and organic electronics.