Recent Technological Advances in Fabrication and Application of Organic Electrochemical Transistors

Abstract

The classical organic semiconducting material used for OECTs is an ionomer mixture called poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), comprising of a conjugated PEDOT phase and a deprotonated PSS phase. Holes in the PEDOT phase is compensated by the negatively charged sulfonate ions in the PSS phase, making the PEDOT phase conducting. In the absence of a gate bias (V GS = 0 V), the flow of mobile holes within the polymer maintains a hole current when a drain voltage (V DS) is applied (ON state). By applying a positive gate voltage (V GS > 0 V), cations from the electrolyte are injected into the channel and PSS anions are charge compensated, initiating the dedoping of PEDOT, reducing the hole density and thus lowering the drain current (OFF state). Such mode of operation is known as depletion mode; the OECT is ON in the absence of a gate bias. It is also possible to operate the OECT in accumulation mode if the conjugated polymer does not consist of any native dopants. Accumulatedmode OECT is typically in the OFF state and application of a negative gate voltage leads to injection of anions into the semiconducting polymer, initiating an electrochemically driven hole accumulation in the channel, leading to the ON state. Recently, naphthalene diimides (NDIs)-based polymers, [15] ladder-type polymers such as poly(benzimidazobenzophenanthro-line) (BBL), [16] fullerene derivatives with glycolated side chains, [17] and random copolymer with glycol side chain such as P-90 [9,18] were reported to be promising n-type materials to develop accumulation mode OECTs with stable operation in aqueous environments. Another class of promising p-type materials for accumulation mode OECTs is semiconducting polymers based on benzodithiophene or bithiophene core with glycol side chains, [19,20] glycolated thiophene (g2T-T) polymers, [21] poly(3hexylthiophene) (P3HT), [22,23] conjugated polyelectrolytes based on poly(6-(thiophene-3-yl)hexane-1-sulfonate) tetrabutylammonium (PTHS), [24] polythiophene derivative with ethylene glycolbased side chains (P3MEEMT), [12] and poly(6-(thiophen-3-yl) hexane-1-sulfonate) tetrabutylammonium (PTHS:TEA). [25] Generally, the material must have simultaneously an aggregated, interconnected structure that supports efficient hole transport, and an amorphous phase to hydrate, and thus enable the transport of ions. The in-depth introduction on active materials for OECTs can be found in several review papers. [26-30] The key feature of OECT is that the transition of doping/ dedoping states occurs over the entire channel volume, as Organic electrochemical transistors (OECTs), where conjugated polymers undergo doping/dedoping from an electrolyte, are widely studied for applications ranging from switching elements, artificial synapses to transducers for biological sensing. The concurrent transport of electronic and ionic charges within the channel provide OECTs with excellent amplification capability and efficient ion-to-electron transduction at low operating voltages (<1 V). Herein, the la...

Molecular design strategies for high‐performance organic electrochemical transistors

“…Besides, the liquid environment, softness of organic semiconductors, and solution processing method make OECTs ideal for biocompatible devices. 4 With the characteristics mentioned above, OECTs are widely studied in the field of chemical and biological sensors, [5][6][7][8] biological interfaces, 9,10 and artificial synapses. [11][12][13][14] The first OECT is developed by Wrighton and co-workers in 1984 using polypyrrole (PPy) as the semiconducting layer.…”

Organic Electrochemical Transistors: An Emerging Technology for Biosensing

“…In addition, OECT device fabrication and processing techniques are constantly evolving, yielding unique devices with amplified performance characteristics. [ 117,143–146 ] Until now, synthetic endeavors toward OMIEC materials specially designed as sensor channel layers remains scarcely explored. However, one could envisage the design of functionalized OMIECs with pendant groups possessing strong affinities toward certain biomarkers enabling sensing within the channel as opposed to conventional devices which often rely on modulation of ion transport and subsequent alteration of the drain output.…”

“…Despite these advances, the depletion mode (de‐doping) nature of PEDOT:PSS leads to poor bistability (stability in both the ON and OFF regimes) and limits device memory retention. [ 117 ] To combat this, an artificial synapse was developed, comprised of a postsynaptic electrode PEDOT:PSS layer which was partially reduced with poly(ethylenimine) (PEI), interfaced with a presynaptic PEDOT:PSS electrode, via an intermediate electrolyte ( Figure ). [ 118 ] Upon the application of a positive potential at the presynaptic PEDOT:PSS gate electrode, cations flowed through the electrolyte to the postsynaptic electrode.…”

Organic Electrochemical Transistors for In Vivo Bioelectronics

“…Moreover, the use of various organic solvents, such as photoresist developers and acetone, can be damaging to the polymer channel layer. [ 95 ] For the application of OECTs in bioelectronics, where high‐resolution recordings and detection need to be performed, new techniques that allow fine control over nano/microscale patterning of the polymer layer are continuously being researched and developed. Recently, Li et al used the mold‐guided drying technique to pattern PEDOT:PSS directly from its water suspension and fabricate OECTs with narrow channels.…”