Organic electrochemical transistors (OECTs) offer significant advantages in electrophysiological applications, primarily due to their ability to facilitate ionic-to-electronic conversion and establish a direct interface with the surrounding aqueous environments by using organic mixed ionic-electronic conductors. This study employs a side-chain free n-type conducting polymer, poly(benzodifurandione) (PBFDO), as the channel material in OECTs to scrutinize the interplay between various ion concentrations in electrolytes and the conjugated polymer and to assess their subsequent impact on device performance. Our findings reveal that PBFDO-based OECTs demonstrate superior transfer characteristics, attributed to their high conductivity and remarkable stability in aqueous solutions. Interestingly, the ion concentration does not alter the electronic band structure of PBFDO during the doping process, but a high-salt-concentration electrolyte could accelerate the electrochemical process compared to its counterparts. Furthermore, the diluted solution significantly enhances the surface roughness and decreases the crystalline coherence length of the film compared with concentrated solutions. A quantitative analysis utilizing an electrochemical quartz crystal microbalance discloses that the electrolyte ions penetrate the PBFDO film, inducing the absorption of a fraction of water molecules, which is pronounced in diluted solutions and negligible in their concentrated counterparts. This notable swelling of the polymer in diluted solutions potentially hampers the transport of charge carriers, consequently diminishing the OECT performance. This research elucidates a direct correlation between microstructure alterations and device performance during operation, paving the way for the optimization of ionic and electronic conductivity in polymers to foster the development of high-performance organic electronic devices.
Keywords: electrochemical doping; electrolyte concentrations; ions-conjugated polymers coupling; n-type organic electrochemical transistors; side-chain free conjugated polymer.