Development of Molecular-Pore-Containing Polymer Semiconductors via Thermal Side-Chain Cleavage for Enhanced Alcohol Vapor Sensing in Organic Thin-Film Transistors

Abstract

This work presents the development of a low detection-limit ethanol vapor sensor, operating as an organic thin-film transistor (OTFT). OTFTs have garnered much attention for their use in gas sensing applications; owing to their low-cost, relatively simple fabrication and ability to be deployed as miniaturized and wearable devices. As such, a series of polythiophenes were synthetized in this work with the aim of being the semiconductive channel material in ethanol vapor sensors. The materials were synthesized with various functionalized side chains – either thermally cleavable or stable in nature. The thermally cleavable sidechains (TCSs) are ester functionalities which can be removed and converted to carboxylic acids upon high temperature post-processing of the devices. The content of TCSs / thermally stable side chains within the polymers in the series were systematically altered to investigate their effect on sensing performance. It was found that complete side chain removal (owing to 100% use of TCSs) totally inhibits sensing performance due to collapse of the film morphology after post-processing. However, including thermally stable side chains in the polymer structure acts as a molecular scaffold and preserves film morphology after TCS removal. This imparts porosity into the thin-film, which facilitates analyte vapor diffusion into the sensing layer and consequently enhances the ethanol vapor sensitivity. A sensitivity increase of ~26% is observed after side chain removal in polymers containing molecular scaffolded structures, proving the formation of stable pores into the polymer films.