Polymer Crystallinity Controls Water Uptake in Glycol Side-Chain Polymer Organic Electrochemical Transistors

We study poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT), a new polythiophene derivative with ethylene glycol-based side chains, as a promising semiconducting polymer for accumulation-mode organic electrochemical transistors (OECTs) with figures of merit comparable to state-of-the-art materials. By characterizing the OECT performance of P3MEEMT transistors as a function of anion, we find that large, hydrophobic anions lower the threshold voltage. We find that, compared to poly(3-hexylthiophene-2,5-diyl) (P3HT), P3MEEMT shows faster anion injection rates, which we attribute to the hydration of the P3MEEMT crystal lattice. We study P3MEEMT-based OECT and organic field-effect transistor (OFET) performance as a function of film crystallinity, and show that changing the crystallinity of the polymer by thermal annealing increases the OFET mobility, yet decreases the OECT mobility. We attribute this difference to the fact that, unlike OFETs, OECTs operate in an aqueous environment. To pro...