Real-Time Gravimetric and Spectroelectrochemical Analysis of Mixed Ionic-Electronic Conductive Materials Electrosynthesis

Organic electrochemical transistors (OECTs) are electrolyte-gated devices that permit large signal amplification, ionic-to-electronic signal transduction, and controlled variable resistance. OECTs offer increased detection sensitivity compared to other electrolyte-gated devices because their electroactive channel materials are mixed ionic-electronic conductors (MIEC) that enable volumetric faradaic interactions with the adjacent electrolyte.

As a device class, OECTs are still in an early stage of development, but have demonstrated utility in applications such as neuromorphic computing platforms and bioelectronic devices. However, the vast majority of OECT studies have employed spin-coated polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) as the active material. While PEDOT:PSS exhibits many desirable qualities for an OECT channel material (e.g., biocompatibility, high transconductance, relatively low mechanical modulus, and responsivity to aqueous ions and bioanalytes), the commercially available formulation is deposited via spin-coating, operates in depletion mode, and has pre-determined physical characteristics (e.g., molecular weight, counterion type, dopant-to-monomer ratio) that preclude investigation of varied material configurations. In addition, the material has general susceptibility to changes in ionic strength of the solution it is measuring, which limits its utility for ion-specific detection and systems for which changes in ionic strength accompany the events/changes that the OECT seeks to detect or influence (i.e. neurotransmitter detection).

As the magnitude of signal amplification in OECTs is directly proportional to the channel transconductance, strategies are needed to increase control over material properties that influence both electronic and ionic conductivity. Electrodeposition, in conjunction with concurrent gravimetric and spectroelectrochemical analyses, provides a convenient method to evaluate the origin and development of electronic/ionic transport properties during materials synthesis. Although electropolymerization of MIEC polymers like polypyrrole, polythiophene, and polyaniline is well-studied, only a few isolated reports^1,2 have evaluated the benefits of electropolymerization for OECT channel fabrication and performance (i.e., compared to spin-coating or vapor-phase polymerization).

In this report, we characterize the effect of electrosynthesis parameters on channel material characteristics and device performance (i.e., total channel capacity, transconductance, ionic/electronic conductivity, and overall electrochemical impedance). We examine the influence of two electrodeposition methods (cyclic voltammetry versus differential pulse voltammetry) on the total degree of polymerization, and the dopant used during electrosynthesis (comparing small, labile dopants such as chloride or tosylate, versus larger semi-labile dopants such as dodecyl benzene sulfonate, versus large immobile polymer dopants such as PSS). Our results suggest that electropolymerization provides several advantages for OECT channel fabrication:

(1) permit more tailored fabrication of channels with diverse characteristics and dopants;
(2) increase control over material properties that influence both electronic and ionic conductivity;
(3) produce channels that operate in either accumulation or depletion mode, depending on the counterion used during synthesis.