On the minimization of contact resistance in organic thin-film transistors

Abstract

Organic semiconductors have been implemented in a variety of electronic devices ranging from organic light-emitting diodes, organic photovoltaic devices, and organic transistors. In all of these devices, the efficient injection and/or extraction of charges across interfaces with conducting contacts is an essential requirement for device performance. The high contact resistance in organic transistors, which limits their usefulness in high-frequency electronics applications, has been a particularly challenging problem to solve. The contact resistance can depend strongly on various parameters, including the transistor architecture and the mismatch between the contact work function and the transport levels of the organic semiconductor. In this work, it is shown that using a thin gate-dielectric layer (around 5 nm) in a thin-film transistor (TFT) in combination with contacts modified using a chemisorbed monolayer to tune the contact-semiconductor interface properties yields record-low contact resistance in organic transistors (as small as 10 Ωcm). This approach was then extended to small-scale TFTs and circuits leading to additional record results in the dynamic performance, including voltage-normalized transit frequency of 7 MHz/V. Finally, strong evidence is shown that Fermi-level pinning limits the effectiveness of tuning the contact work function with chemisorbed monolayers to improve the contact resistance further.