Doctoral Dissertation Defense
PROGRAM | Materials Science & Engineering
Electrochemical Fabrication and Characterization of Organic Electrochemical Transistors
By: Junghyun Lee Chair: David Martin
ABSTRACT
Organic electrochemical transistors (OECTs) have attracted considerable interest in various advanced applications such as biosensors and biomedical devices due to their distinct advantages including high transconductance, low operating voltage, and availability to operate in the liquid phase. The performance of OECTs is largely determined by their employed active materials, which are typically organic mixed ionic and electronic conductors (OMIECs). As OMIECs, the conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped by poly(styrene sulfonate) (PSS) (PEDOT:PSS) has been widely used thanks to its high chemical stability, thermostability, biocompatibility, and ionic/electronic conductivities. Instead of using commercial PEDOT:PSS aqueous suspension, an alternative convenient fabrication method to form PEDOT films is to use electrochemical polymerization to synthesize PEDOT directly from the monomer, 3,4-ethylenedioxythiophene (EDOT). This approach has several advantages in that the PEDOT can be deposited directly onto the metallic electrode and the thickness of PEDOT films can be precisely controlled by total applied charge density. Furthermore, the properties and surface morphologies of electrodeposited PEDOT films can be tailored and modified with different monomers, additives, or counterions. In spite of these features, the uses of electrochemically polymerized and deposited PEDOT for the fabrication and characterization of OECTs have not yet been well investigated.
Here, we have examined the use of electrochemical polymerization and deposition methods for using PEDOT, as the active material of OECTs, with three different counterions chosen by their molecular sizes and chemical structures. Films with the large counterion, PEDOT:PSS were relatively smooth, while PEDOT films with smaller counterions, PEDOT:pTS and PEDOT:LiClO4, developed much rougher and irregular surfaces. The PEDOT films with pTS and PSS, having similar chemical structures, grew along the substrate surface (in-plane direction) much faster than with PEDOT:LiClO4, confirming the dependence of the growth properties of PEDOT on the chemical structure of counterions. OECTs with PEDOT films were successfully fabricated and the maximum transconductance (gm,max) was 46 mS with PEDOT:pTS.
We systematically investigated the influences of PEDOT channel thickness, counterion composition, and applied voltage control on the performance of electrochemically fabricated OECTs including stability and sensitivity. While the device performance was not significantly changed over the examined range of channel thicknesses (110 ~ 800 nm), the device stability was enhanced with larger sized counterions (in this case PEDOT:PSS), thinner channel films (~110 nm), and lower applied drain voltages (~-0.1 V) with the fixed range of applied gate voltage (0 V to 1 V). These insights were used to fabricate and investigate OECT-based label-free glucose sensors using glucose oxidase (GOx) immobilized in PEDOT films. Glucose sensors with PEDOT:PSS showed a higher normalized response (~0.45 at 10 mM) than PEDOT:pTS (~0.2 at 10 mM).
Our established experimental protocols were also extended to investigate other monomers, especially maleimide-functionalized EDOT (EDOT-MA) which can be modified efficiently through “click-chemistry”, to fabricate and characterize OECTs. Electrochemically polymerized and deposited PEDOT-MA showed a nanofibrillar network structure with typical lateral dimensions of ~25 nm, and pore sizes of ~100 to 900 nm, rather than the bumpy and nodular surfaces typically seen in normal PEDOT. The average fibril width of ~25 nm was not significantly changed with increasing applied charge density, while the average pore size (perimeter) increased with increasing film thickness. Also, PEDOT-MA presented a non-linear thickness trend versus applied charge density (the estimate of t = 0.88 q0.43), probably due to this nanofibril morphology. The best performance of OECTs with PEDOT-MA was exhibited from 0.2 C/cm2 of the applied charge density (corresponding to a nominal thickness of ~500 nm), with maximum transconductance (gm,max) and operating voltage (VG(gm,max)) were 0.2 S and 0.1 V, respectively.
These results pave the way to fabricate OECTs more efficiently and effectively and can provide new insights to modify active materials and enhance device performances for further advanced applications such as biosensors or biological devices.
