Distinctive Behavior of Field-Effect and Redox Electrolyte-Gated Organic Transistors

Electrolyte-gated transistors (EGTs) have recently garnered significant attention due to their extensive applications, mainly as electronic transducers for biosensing devices and in neuromorphic computing. Nonetheless, our understanding of interfacial and bulk doping effects, particularly related to the interplay between liquid-based electrolytes and hydrophobic-polymer-based EGTs, remains limited. In this study, we conducted simultaneous in operando electrical and optical characterizations in regioregular poly-(3-hexylthiophene-2,5-diyl) (rr-P3HT) EGTs, focusing on elucidating the influence of the electrolyte nature on the EGTs mechanism of operation. We showed that perchlorate-based electrolytes, regardless of the solvent employed, promote a 4-fold modulation of the P3HT electrical conductivity, together with an 80% absorbance bleaching at 510 nm, indicating high doping efficiency. Consequently, the modulation of transistor drain current aligns with the accumulation-mode operation of organic electrochemical transistors (OECTs), which is attributed to bulk doping effects. In contrast, chloride-based electrolytes, within the same voltage range operation, displayed minimal enhancement in bulk conductivity and no discernible change in absorbance spectra. In this context, the transistor's operational mode is solely based on interfacial field-effect phenomena, as in electrolyte-gated organic field-effect transistors (EGOFETs). Moreover, a direct correlation between the film's electrical conductivity (sigma) and the product of carrier mobility (mu) and capacitance (C) was observed. In summary, our findings enhance our comprehension of the operational mechanisms of EGTs, highlighting that a thoughtful choice of electrolyte composition can facilitate either interfacial (field-effect) or volumetric (electrochemical) doping. This intriguingly allows hydrophobic polymers to exhibit effective ion uptake when interfaced with aqueous electrolytes, thereby enabling the usage of a great pallet of water-insoluble polymers for application in organic bioelectronics as well as in water-based electrochemical devices.