Organic thin-film transistors with inkjet-printed electrodes on hydrophobic Teflon-AF gate dielectric with reversible surface properties

Abstract

Printed organic thin-film transistors (OTFTs) are promising for flexible, low-cost electronics; however, a major challenge is that successive processing steps of multiple solution-processed layers can interfere with each other. In particular, hydrophobic fluoropolymers are a promising group of materials to fabricate gate dielectrics with low charge trap density and properties such as good thermal stability, chemical inertness, low dielectric constant, and water repellency. However, the main difficulty in incorporating hydrophobic fluoropolymers in printed electronic devices is their low surface energy. Plasma treatment is a common method for surface modification that renders these hydrophobic layers wettable to print subsequent layers. This plasma processing can also affect the interface with the organic semiconductor (OSC) and transistor performance, which is studied here for the first time. The morphological and surface chemical properties of Teflon amorphous fluoropolymer (Teflon-AF) films change after plasma treatment and gradually reverse after post-annealing of subsequent layers as demonstrated here. Here, we fabricate solution-processed OTFTs with Teflon-AF as the gate dielectric. We report OTFTs with inkjet-printed source and drain electrodes with a minimum channel length of 20 μm on Teflon-AF for the first time. We show that the annealing of the inkjet-printed electrodes changes the morphology and the surface chemistry of the Teflon-AF gate dielectric underneath, thus reversing the effect of the plasma. We show that this electrode post-annealing step can improve the transistor performance by improving the interface between the Teflon-AF gate dielectric and the OSC in terms of smoothness and hydrophobicity. The transistor parameters such as mobility, on/off current ratio, and threshold voltage all follow a trend explained here by the properties of the Teflon-AF films. Increasing the post-annealing temperature to just below complete surface reversibility decreases surface roughness and trap-sites created after plasma treatment leading to optimized device performance. We characterize the Teflon-AF surface with contact angle measurement, roughness measurement, Fourier transform infrared spectroscopy (FTIR), and x-ray photoelectron spectroscopy (XPS). This understanding of the reversibility of the Teflon-AF surface helps achieve optimal performance of devices and systems incorporating fluoropolymers. • The challenge of processing OTFT layers on hydrophobic dielectrics is addressed. • 20 μm channel OTFTs with inkjet-printed electrodes on hydrophobic dielectric are achieved. • Teflon-AF surface changes after plasma and reverses after post-annealing. • Electrode annealing changes the morphology and surface chemistry of the dielectric. • Electrode annealing can improve the gate dielectric/organic semiconductor interface.