Investigating Doping/Dedoping Processes in Electropolymerized Organic Mixed Ionic-Electronic Conductors

Organic electrochemical transistors (OECTs) based on organic mixed ionic-electronic conductors (OMIECs), which possess both electronic and ionic transport capabilities, have shown promising application potentials in bioelectronics.¹ OECTs utilize the reversible doping/dedoping properties of active OMIEC channel materials by applying a gate voltage across the electrolyte to modulate the channel conductance.² Although there has been extensive research to understand the working principle which involves mixed ionic and electronic transport, the study on the fundamental understanding of ionic transport and injection/ejection is still limited. For example, in one of the model OMIECs, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with organic polyanions poly(styrenesulfonate) (PSS), the dedoping mechanism is believed to be mainly due to the insertion of cations from the electrolyte to the PEDOT:PSS film. However, when small anion-based dopants, such as ClO₄^- and BF₄^-, are used in the PEDOT system, the anions may be released from the material to the electrolyte.² The doping/dedoping mechanisms are therefore becoming more complicated. To better understand these doping/dedoping processes, we utilized an <i>in situ</i> film fabrication method, electropolymerization, to prepare PEDOT-based films with various dopants, such as PSS^-, ClO^- and BF₄^-. This approach allows the material properties to be readily adjusted by controlling the dopant type, dopant concentration, and other electropolymerization parameters³. The polymer films were deposited on the source and drain electrodes and grow gradually to cover the gap in between. The effects of dopant type, concentration, and deposition time on the performance of OECTs were systematically studied. Next, the electrochemical quartz crystal microbalance (EQCM), which can monitor the mass exchange between the film and electrolyte, was incorporated to explore the doping/dedoping behavior. Our work contributes to a deeper understanding of the ionic charge transport in small anion-doped OMIECs and the working mechanism of corresponding OECTs.<br/><br/>References:<br/>(1) Nawaz, A.; Liu, Q.; Leong, W. L.; Fairfull-Smith, K. E.; Sonar, P. Organic Electrochemical Transistors for In Vivo Bioelectronics. <i>Adv Mater </i><b>2021</b>, <i>33</i> (49), e2101874. DOI: 10.1002/adma.202101874.<br/>(2) Rivnay, J.; Inal, S.; Salleo, A.; Owens, R. M.; Berggren, M.; Malliaras, G. G. Organic electrochemical transistors. <i>Nature Reviews Materials </i><b>2018</b>, <i>3</i> (2). DOI: 10.1038/natrevmats.2017.86.<br/>(3) Heinze, J.; Frontana-Uribe A. B.; Ludwigs, S. Electrochemistry of Conducting Polymers-Persistent Models and New Concepts. Chemical Reviews <b>2010</b>, 110 (8). DOI: 10.1021/cr900226k.