The organic electrochemical transistor (OECT) represents a versatile and impactful electronic building block in the areas of printed electronics, bioelectronics, and neuromorphic computing. ...Significant efforts in OECTs have focused on device physics, new active material design and synthesis, and on preliminary implementation of individual transistors as proof-of-concept components for sensing and computation. However, as most of the current studies are based on single devices, the integration of OECTs into circuits or high-level systems has lagged. In this review, we focus on recent efforts to incorporate individual OECTs into digital, analog, and neuromorphic circuits, and lay out important considerations relevant for (hybrid) systems integration. We summarize the operation principles and the functions of OECT-based circuits and discuss the approaches for wireless power and data transmission for practicality in biological and bio-inspired applications. Finally, we comment on the future directions and challenges facing OECT circuits from both a fundamental and applied perspective.
•Organic electrochemical transistor (OECT)-based digital, analog and neuromorphic circuit for biological applications.•Wireless power and data transmission for OECT-based circuits.•Novel fabrication techniques and materials enable new circuit concepts.•Promising potential of OECT-based circuit in the field of bioelectronics.
We study poly(3-{2-(2-methoxyethoxy)ethoxymethyl}thiophene-2,5-diyl) (P3MEEMT), a new polythiophene derivative with ethylene glycol-based side chains, as a promising semiconducting polymer ...for accumulation-mode organic electrochemical transistors (OECTs) with figures of merit comparable to those of state-of-the-art materials. By characterizing the OECT performance of P3MEEMT transistors as a function of the anion, we find that large hydrophobic anions lower the threshold voltage. We find that, compared to poly(3-hexylthiophene-2,5-diyl) (P3HT), P3MEEMT has faster anion injection rates, which we attribute to the hydration of the P3MEEMT crystal lattice. We study P3MEEMT-based OECT and organic field-effect transistor (OFET) performance as a function of film crystallinity and show that changing the crystallinity of the polymer by thermal annealing increases the OFET mobility yet decreases the OECT mobility. We attribute this difference to the fact that, unlike OFETs, OECTs operate in aqueous environments. To probe how hydration affects the operation of OECTs, we investigate the role of water in electrochemical doping using electrochemical quartz microbalance (EQCM) gravimetry. We find that steady-state hydration and hydration dynamics under electrochemical bias differ dramatically between the crystalline and amorphous P3MEEMT films. These results suggest that the presence of water reduces the electronic connectivity between the crystalline regions of P3MEEMT, thus lowering the mobility in solution. Overall, our study highlights the importance of the role of polymer hydration and nanoscale morphology in elucidating design principles for OECT operation.
N‐type conjugated polymers as the semiconducting component of organic electrochemical transistors (OECTs) are still undeveloped with respect to their p‐type counterparts. Herein, we report two rigid ...n‐type conjugated polymers bearing oligo(ethylene glycol) (OEG) side chains, PgNaN and PgNgN, which demonstrated an essentially torsion‐free π‐conjugated backbone. The planarity and electron‐deficient rigid structures enable the resulting polymers to achieve high electron mobility in an OECT device of up to the 10−3 cm2 V−1 s−1 range, with a deep‐lying LUMO energy level lower than −4.0 eV. Prominently, the polymers exhibited a high device performance with a maximum dimensionally normalized transconductance of 0.212 S cm−1 and the product of charge‐carrier mobility μ and volumetric capacitance C* of 0.662±0.113 F cm−1 V−1 s−1, which are among the highest in n‐type conjugated polymers reported to date. Moreover, the polymers are synthesized via a metal‐free aldol‐condensation polymerization, which is beneficial to their application in bioelectronics.
Two new n‐type semiconducting polymers, PgNaN and PgNgN, bearing oligo(ethylene glycol) (OEG) side chain are developed with a fully conformationally locked backbone and deep‐lying LUMO energy level. As a result, the polymer of PgNaN exhibits a good performance on OECT devices with a maximum dimensionally normalized transconductance of 0.212 S cm−1 and a product μC* of 0.662±0.113 F cm−1 V−1 s−1.
Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of ...reactive side‐products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox‐active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side‐reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high‐performance, state‐of‐the‐art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2O2), a reactive side‐product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox‐active organic semiconductors based on donor–acceptor copolymers that prevents the formation of H2O2 during device operation. This study elucidates the previously overlooked side‐reactions between redox‐active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte‐gated devices in application‐relevant environments.
Faradaic side‐reactions of redox‐active materials, that can produce harmful side‐products, should be minimized when employed in bioelectronic devices for studying biological systems. This work sheds light on side‐reactions with oxygen in state‑of‑the‑art materials for electrochemical transistors, forming hydrogen peroxide (H2O2), and provides design rules toward high‐performance materials that prevent adverse reactions by tailoring the energy levels of the redox‐active material.
Novel p-type semiconducting polymers that can facilitate ion penetration, and operate in accumulation mode are much desired in bioelectronics. Glycol side chains have proven to be an efficient method ...to increase bulk electrochemical doping and optimize aqueous swelling. One early polymer which exemplifies these design approaches was p(g2T-TT), employing a bithiophene-co-thienothiophene backbone with glycol side chains in the 3,3′ positions of the bithiophene repeat unit. In this paper, the analogous regioisomeric polymer, namely pgBTTT, was synthesized by relocating the glycol side chains position on the bithiophene unit of p(g2T-TT) from the 3,3′ to the 4,4′ positions and compared with the original p(g2T-TT). By changing the regio-positioning of the side chains, the planarizing effects of the S–O interactions were redistributed along the backbone, and the influence on the polymer’s microstructure organization was investigated using grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The newly designed pgBTTT exhibited lower backbone disorder, closer π-stacking, and higher scattering intensity in both the in-plane and out-of-plane GIWAXS measurements. The effect of the improved planarity of pgBTTT manifested as higher hole mobility (μ) of 3.44 ± 0.13 cm2 V–1 s–1. Scanning tunneling microscopy (STM) was in agreement with the GIWAXS measurements and demonstrated, for the first time, that glycol side chains can also facilitate intermolecular interdigitation analogous to that of pBTTT. Electrochemical quartz crystal microbalance with dissipation of energy (eQCM-D) measurements revealed that pgBTTT maintains a more rigid structure than p(g2T-TT) during doping, minimizing molecular packing disruption and maintaining higher hole mobility in operation mode.
Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different ...electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.
A series of fully fused n-type mixed conduction lactam polymers
, systematically increasing the alkyl side chain content, are synthesized via an inexpensive, nontoxic, precious-metal-free aldol ...polycondensation. Employing these polymers as channel materials in organic electrochemical transistors (OECTs) affords state-of-the-art n-type performance with
recording an OECT electron mobility of 1.20 × 10
cm
V
s
and a μ
* figure of merit of 1.83 F cm
V
s
. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1
-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for
, with a maximum electrical conductivity of 7.67 S cm
and a power factor of 10.4 μW m
K
. These results are among the highest reported for n-type polymers. Importantly, while this series of fused polylactam organic mixed ionic-electronic conductors (OMIECs) highlights that synthetic molecular design strategies to bolster OECT performance can be translated to also achieve high organic thermoelectric (OTE) performance, a nuanced synthetic approach must be used to optimize performance. Herein, we outline the performance metrics and provide new insights into the molecular design guidelines for the next generation of high-performance n-type materials for mixed conduction applications, presenting for the first time the results of a single polymer series within both OECT and OTE applications.
Organic mixed ionic and electronic conductors are of significant interest for bioelectronic applications. Here, three different isoindigoid building blocks are used to obtain polymeric mixed ...conductors with vastly different structural and electronic properties which can be further fine‐tuned through the choice of comonomer unit. This work shows how careful design of the isoindigoid scaffold can afford highly planar polymer structures with high degrees of electronic delocalization, while subtle structural modifications can control the dominant charge carrier (hole or electron) when probed in organic electrochemical transistors. A combination of experimental and computational techniques is employed to probe electrochemical, structural, and mixed ionic and electronic properties of the polymer series which in turn allows the derivation of important structure–property relations for this promising class of materials in the context of organic bioelectronics. Ultimately, these findings are used to outline robust molecular‐design strategies for isoindigo‐based mixed conductors that can support efficient p‐type, n‐type, and ambipolar transistor operation in an aqueous environment.
A new molecular design criteria for isoindigo‐based semiconducting polymers is presented and their versatility is showcased for the development of mixed conductors relevant to organic bioelectronic applications. Through rational choice of the isoindigoid motif and an appropriate comonomer, efficient hole‐ and electron‐transport materials are developed and strategies for further optimization are identified, including the development of ambipolar mixed conductors.
On-site signal amplification for bioelectronic sensing is a desirable approach to improving recorded signal quality and to reducing the burden on signal transmission and back-end electronics. While ...organic electrochemical transistors (OECTs) have been used as local transducers of bioelectronic signals, their current output presents challenges for implementation. OECT-based circuits offer new opportunities for high-performance signal processing. In this work, we introduce an active sensing node based on cofacial vertical OECTs forming an ambipolar complementary inverter. The inverter, which shows a voltage gain of 28, is composed of two OECTs on opposite side walls of a single active area, resulting in a footprint identical to a planar OECT. The inverter is used as an analog voltage preamplifier for recording electrocardiogram signals when biased at the input voltage corresponding to peak gain. We further demonstrate compatibility with nontraditional fabrication methods with potential benefits for rapid prototyping and large-area printed electronics.
Organic electrochemical transistors (OECTs) are devices with broad potential in bioelectronic sensing, circuits, and neuromorphic hardware. Their unique properties arise from the use of organic mixed ...ionic/electronic conductors (OMIECs) as the active channel. Typical OMIECs are linear polymers, where defined and controlled microstructure/morphology, and reliable characterization of transport and charging can be elusive. Semiconducting two‐dimensional polymers (2DPs) present a new avenue in OMIEC materials development, enabling electronic transport along with precise control of well‐defined channels ideal for ion transport/intercalation. To this end, a recently reported 2DP, TIIP, is synthesized and patterned at 10 µm resolution as the channel of a transistor. The TIIP films demonstrate textured microstructure and show semiconducting properties with accessible oxidation states. Operating in an aqueous electrolyte, the 2DP‐OECT exhibits a device‐scale hole mobility of 0.05 cm2 V–1 s–1 and a µC* figure of merit of 1.75 F cm–1 V–1 s–1. 2DP OMIECs thus offer new synthetic degrees of freedom to control OECT performance and may enable additional opportunities such as ion selectivity or improved stability through reduced morphological modulation during device operation.
Semiconducting two‐dimensional (2D) polymers feature synthetically tailored porosity and thus show both efficient electronic and ion transport. It is shown that two‐dimensional polymers (2DPs) can be patterned on the micrometer scale, and can be used as the active material for organic electrochemical transistors.
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