Organic Electrochemical Transistors Performance Research Articles

Ultrastable N-Type Semiconducting Fiber Organic Electrochemical Transistors for Highly Sensitive Biosensors.

Organic electrochemical transistors (OECTs) have attracted increasing attention due to their merits of high transconductance, low operating voltage, and good biocompatibility, ideal for biosensors. However, further advances in their practical applications face challenges of low n-type performance and poor stability. Here, it is demonstrated that wet-spinning the commercially available n-type conjugated polymer poly(benzimidazobenzophenanthroline) (BBL) into highly aligned and crystalline fibers enhances both OECT performance and stability. Although BBL is only soluble in high-boiling-point strong acids, it can be wet-spun into high-quality fibers with adjustable diameters. The BBL fiber OECTs exhibit a record-high area-normalized transconductance (gm,A ) of 2.40 µS µm-2 and over 10 times higher figure-of-merit (µC*) than its thin-film counterparts. More importantly, these fiber OECTs exhibit remarkable stability with no noticeable performance attenuation after 1500 cycles over 4h operation, outperforming all previously reported n-type OECTs. The superior performance and stability can be attributed to shorter π-π stacking distance and ordered molecular arrangement in the fibers, endowing the BBL fiber OECT-based biosensors with outstanding sensitivity while keeping a miniaturized form factor. This work demonstrates that, beyond new material development, developing new fabrication technology is also crucial for addressing the performance and stability issues in n-type OECTs.

Unveiling the Impact of the Electrolyte's Counter Ions on Organic Electrochemical Transistor Performance

AbstractThe effect of the electrolyte's counter‐ion in organic electrochemical transistors is often neglected. the influence of anions (i.e., counter ions) is investigated on organic electrochemical transistors (OECTs) with a PEDOT:PSS‐like semiconductor through device simulations. The study examined the effects of mobile anions on OECT performance under two scenarios: when anions are blocked by the semiconductor and when they can penetrate it. In each case, the OECT's ON and OFF states are analyzed. The findings show that when anions can penetrate the semiconductor, the ON/OFF ratio of the OECT remains unchanged while the transconductance significantly increases. In the ON state, the case of blocked anions is observed that the current is predominantly surface‐current, whereas it becomes volumetric only when anions can penetrate the semiconductor. Furthermore, the extreme case is explored in which anions remain stationary within the electrolyte. In this scenario, achieving a reasonable ON/OFF ratio necessitates an ion density within the electrolyte that is two orders of magnitude higher than the dopant density of the semiconductor. This work underscores the substantial influence of counter anions on OECT performance, highlighting their critical role in shaping device behavior.

Eutectogels as a Semisolid Electrolyte for Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are signal transducers offering high amplification, which makes them particularly advantageous for detecting weak biological signals. While OECTs typically operate with aqueous electrolytes, those employing solid-like gels as the dielectric layer can be excellent candidates for constructing wearable electrophysiology probes. Despite their potential, the impact of the gel electrolyte type and composition on the operation of the OECT and the associated device design considerations for optimal performance with a chosen electrolyte have remained ambiguous. In this work, we investigate the influence of three types of gel electrolytes-hydrogels, eutectogels, and iongels, each with varying compositions on the performance of OECTs. Our findings highlight the superiority of the eutectogel electrolyte, which comprises poly(glycerol 1,3-diglycerolate diacrylate) as the polymer matrix and choline chloride in combination with 1,3-propanediol deep eutectic solvent as the ionic component. This eutectogel electrolyte outperforms hydrogel and iongel counterparts of equivalent dimensions, yielding the most favorable transient and steady-state performance for both p-type depletion and p-type/n-type enhancement mode transistors gated with silver/silver chloride (Ag/AgCl). Furthermore, the eutectogel-integrated enhancement mode OECTs exhibit exceptional operational stability, reflected in the absence of signal-to-noise ratio (SNR) variation in the simulated electrocardiogram (ECG) recordings conducted continuously over a period of 5 h, as well as daily measurements spanning 30 days. Eutectogel-based OECTs also exhibit higher ECG signal amplitudes and SNR than their counterparts, utilizing the commercially available hydrogel, which is the most common electrolyte for cutaneous electrodes. These findings underscore the potential of eutectogels as a semisolid electrolyte for OECTs, particularly in applications demanding robust and prolonged physiological signal monitoring.

Development of Synthetically Accessible Glycolated Polythiophenes for High‐Performance Organic Electrochemical Transistors

AbstractFour glycolated polythiophene‐based organic mixed ionic‐electronic conductors (OMIECs), PE2gTT, PE2gT, PT2gTT, and PT2gT are prepared by atom‐efficient direct arylation polymerization, avoiding the need for toxic organometallic precursors. PE2gT, PT2gTT, and PT2gT are operable in p‐type accumulation mode organic electrochemical transistors (OECTs), with PT2gT displaying the best device performance with a µC* product figure‐of‐merit of 290 F cm−1 V−1 s−1. A record volumetric capacitance among p‐type glycolated polythiophene OMIECs of 313 F cm−3 is observed for PE2gT, ascribed to the high proportionality of polar components in its materials design. The good OECT performance of PE2gT with µC* = 84.2 F cm−1 V−1 s−1, comparable with state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) devices, coupled with its synthetic accessibility and favorable accumulation mode operation makes PE2gT an ideal glycolated alternative to PEDOT:PSS in bioelectronics. PE2gT with the least negative threshold voltage also displays the best OECT operational cycling stability, linked to better resistance of its oxidized state against parasitic redox side reactions . Shelf life stability of OECTs stored (without bias) is observed to be better for materials with a more negative threshold voltage and higher average molecular weight (PT2gT), that are less susceptible to ambient auto‐oxidation and film delamination.

Cleanroom-Free Direct Laser Micropatterning of Polymers for Organic Electrochemical Transistors in Logic Circuits and Glucose Biosensors.

Organic electrochemical transistors (OECTs) are promising devices for bioelectronics, such as biosensors. However, current cleanroom-based microfabrication of OECTs hinders fast prototyping and widespread adoption of this technology for low-volume, low-cost applications. To address this limitation, a versatile and scalable approach for ultrafast laser microfabrication of OECTs is herein reported, where a femtosecond laser to pattern insulating polymers (such as parylene C or polyimide) is first used, exposing the underlying metal electrodes serving as transistor terminals (source, drain, or gate). After the first patterning step, conducting polymers, such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), or semiconducting polymers, are spin-coated on the device surface. Another femtosecond laser patterning step subsequently defines the active polymer area contributing to the OECT performance by disconnecting the channel and gate from the surrounding spin-coated film. The effective OECT width can be defined with high resolution (down to 2µm) in less than a second of exposure. Micropatterning the OECT channel area significantly improved the transistor switching performance in the case of PEDOT:PSS-based transistors, speeding up the devices by two orders of magnitude. The utility of this OECT manufacturing approach is demonstrated by fabricating complementary logic (inverters) and glucose biosensors, thereby showing its potential to accelerate OECT research.

Face‐on Orientation Matches Vertical Organic Electrochemical Transistors for High Transconductance and Superior Non‐Volatility

AbstractThe recently developed vertical structure of organic electrochemical transistors (OECTs) can integrate volatile and non‐volatile functions into one reconfigurable device, making it highly promising. However, comparing with the conventional planar OECT (c‐OECT), the understanding of vertical OECT (v‐OECT) working principles and device engineering strategies is still lacking, impeding rational optimization. Since a major difference between c‐ and v‐OECTs is their charge transport directionality, which is highly influenced by crystallite orientations, the orientation–device structure match thus becomes an important yet outstanding topic for OECTs. Herein, using an n‐type small molecule IDIC‐MEG, investigate how much impact such match can have on OECT performance. The IDIC‐MEG c‐OECT fails to work due to the seriously hindered in‐plane electron transport by face‐on orientation. Surprisingly, simply changing the device structure from planar to vertical allows the resultant v‐OECT to exhibit the highest reported transconductance (46.3 mS) among all small‐molecule OECTs, thanks to the match between face‐on orientation and the vertical structure. Such match also leads to excellent non‐volatility, including highly predictable programmability and good operational stability. This work, for the first time, explicitly demonstrates the significance of orientation–device structure match for OECT optimization, establishing new guidelines for achieving high‐performance volatile and non‐volatile OECTs.

Influence of Interaction between Electrolyte with Side-Chain Free Conjugated Polymer on the Performance of Organic Electrochemical Transistors

Influence of Interaction between Electrolyte with Side-Chain Free Conjugated Polymer on the Performance of Organic Electrochemical Transistors

Carboxyl-Alkyl Functionalized Conjugated Polyelectrolytes for High Performance Organic Electrochemical Transistors.

Contemporary design principles for organic mixed ionic electronic conductors (OMIECs) are mostly based on the ethylene glycol moiety, which may not be representative of the OMIEC class as a whole. Furthermore, glycolated polymers can be difficult to synthesize and process effectively. As an emerging alternative, we present a series of polythiophenes functionalized with a hybrid carboxyl-alkyl side chain. By variation of the alkyl spacer length, a comprehensive evaluation of both the impact of carboxylic acid functionalization and alkyl spacer length was conducted. COOH-functionalization endows the polymer with preferential intrinsic low-swelling behavior and water processability to yield solvent-resistant conjugated polyelectrolytes while retaining substantial electroactivity in aqueous environments. Advanced in situ techniques, including time-resolved spectroelectrochemistry and Raman spectroscopy, are used to interrogate the materials' microstructure, ionic-electronic coupling, and operational stability in devices. To compare these materials' performance to state-of-the-art technology for the design of OMIECs, we benchmarked the materials and demonstrated significant application potential in both planar and interdigitated organic electrochemical transistors (OECTs). The polythiophene bearing carboxyl-butyl side chains exhibits greater electrochemical performance and faster doping kinetics within the polymer series, with a record-high OECT performance among conjugated polyelectrolytes ([μC*]pOECT = 107 ± 4 F cm-1 V-1 s-1). The results provide an enhanced understanding of structure-property relationships for conjugated polyelectrolytes operating in aqueous media and expand the materials options for future OMIEC development. Further, this work demonstrates the potential for conjugated polymers bearing alkyl-COOH side chains as a path toward robust OMIEC designs that may facilitate further facile (bio)chemical functionalization for a range of (bio)sensing applications.

Study of the doping of PEDOT:PSS films and the reversibility in organic electrochemical transistors

In this work, the effect in the performance of organic electrochemical transistors (OECTs) due to the doping with a secondary dopant solvent of poly(3,4ethylenedioxythiophene): poly (styrene sulfonic acid) (PEDOT:PSS) films) has been studied. We found an improvement in the performance of the OECTs related to the increase of conductivity of the PEDOT:PSS film due to the doping with ethylene glycol (EG). We found that PEDOT:PSS films doped with EG showed higher conductivity due to the improvement of conducting paths between the PEDOT+ chains, which causes a higher transconductance and response of the OECTs. However, a higher concentration of EG is detrimental to the OECT performance due to the removal of PSS- chains caused by the EG secondary doping, which was corroborated by X-ray diffraction.We propose that the good reversibility between low-resistance state (ON) to high-resistance state (OFF), is caused by the injection/removal of cations in the polymeric film to maintain the electroneutrality. We found applying more negative gate voltage pulses in the transfer characteristics of the OECTs is necessary to reach the complete oxidation of the PEDOT:PSS film, as lower negative voltages only partially dedope the film.

Assessment of the Testing Methods for Evaluating the Performance of Organic Electrochemical Transistors

Assessment of the Testing Methods for Evaluating the Performance of Organic Electrochemical Transistors

Organic Electrochemical Transistor Based on Hydrophobic Polymer Tuned by Ionic Gels

AbstractHydrophobic conjugated polymers have poor ionic transport property, so hydrophilic side chains are often grafted for their application as organic electrochemical transistors (OECTs). However, this modification lowers their charge transport ability. Here, an ionic gel interfacial layer is applied to improve the ionic transport while retaining the charge transport ability of the polymers. By using the ionic gels comprising gel matrix and ionic liquids as the interfacial layers, the hydrophobic polymer achieves the OECT feature with high transconductance, low threshold voltage, high current on/off ratio, short switching time, and high operational stability. The working mechanism is also revealed. Moreover, the OECT performance can be tuned by varying the types and ratios of ionic gels. With the proposed ionic gel strategy, OECTs can be effectively realized with hydrophobic conjugated polymers.

Organic Electrochemical Transistor Based on Hydrophobic Polymer Tuned by Ionic Gels.

Hydrophobic conjugated polymers have poor ionic transport property, so hydrophilic side chains are often grafted for their application as organic electrochemical transistors (OECTs). However, this modification lowers their charge transport ability. Here, an ionic gel interfacial layer is applied to improve the ionic transport while retaining the charge transport ability of the polymers. By using the ionic gels comprising gel matrix and ionic liquids as the interfacial layers, the hydrophobic polymer achieves the OECT feature with high transconductance, low threshold voltage, high current on/off ratio, short switching time, and high operational stability. The working mechanism is also revealed. Moreover, the OECT performance can be tuned by varying the types and ratios of ionic gels. With the proposed ionic gel strategy, OECTs can be effectively realized with hydrophobic conjugated polymers.

Acceptor Functionalization via Green Chemistry Enables High‐Performance n‐Type Organic Electrochemical Transistors for Biosensing, Memory Applications

AbstractThe organic electrochemical transistor (OECT) is one of the most versatile building blocks within the bioelectronics device toolbox. While p‐type organic semiconductors have progressed as OECT channel materials, only a few n‐type semiconductors have been reported, precluding the development of advanced sensor‐integrated OECT‐based complementary circuits. Herein, green aldol polymerization is uses to synthesize lactone‐based n‐type conjugated polymers. Fluorination of the lactone‐based acceptor endows a fully locked backbone with a low‐lying lowest unoccupied molecular orbital, facilitating efficient ionic‐to‐electronic charge coupling. The resulting polymer has a record‐high n‐type OECT performance with a high product of mobility and capacitance (µC* = 108 F cm−1 V−1 s−1), excellent mobility (0.912 cm2 V−1 s−1), low threshold voltage (0.02 V), and fast switching speed (τON, τOFF = 336 µs,108 µs). This work demonstrates two types of device architectures and applications enabled by the high performance of this n‐type OECT, i.e., an artificial synapse and a complementary amplifier for detecting α‐synuclein, a potential biomarker of Parkinson's disease. This study shows that materials that enable high gain and fast speed n‐type OECTs can be developed via a green polymerization route, and the diverse form factors that these devices take promise for exploration of other application areas.

Enhanced Organic Electrochemical Transistor Performance of Donor-Acceptor Conjugated Polymers Modified with Hybrid Glycol/Ionic Side Chains by Postpolymerization Modification.

Emergent bioelectronic technologies are underpinned by the organic electrochemical transistor (OECT), which employs an electrolyte medium to modulate the conductivity of its organic semiconductor channel. Here we utilize postpolymerization modification (PPM) on a conjugated polymer backbone to directly introduce glycolated or anionic side chains via fluoride displacement. The resulting polymers demonstrated increased volumetric capacitances, with subdued swelling, compared to their parent polymer in p-type enhancement mode OECTs. This increase in capacitance was attributed to their modified side chain configurations enabling cationic charge compensation for thin film electrochemical oxidation, as deduced from electrochemical quartz crystal microbalance measurements. An overall improvement in OECT performance was recorded for the hybrid glycol/ionic polymer compared to the parent, owing to its low swelling and bimodal crystalline orientation as imaged by grazing-incidence wide-angle X-ray scattering, enabling its high charge mobility at 1.02 cm2·V-1·s-1. Compromised device performance was recorded for the fully glycolated derivative compared to the parent, which was linked to its limited face-on stacking, which hindered OECT charge mobility at 0.26 cm2·V-1·s-1, despite its high capacitance. These results highlight the effectiveness of anionic side chain attachment by PPM as a means of increasing the volumetric capacitance of p-type conjugated polymers for OECTs, while retaining solid-state macromolecular properties that facilitate hole transport.

N‐Type Glycolated Imide‐Fused Polycyclic Aromatic Hydrocarbons with High Capacity for Liquid/Solid‐Electrolyte‐based Electrochemical Devices

AbstractCurrently, n‐type small‐molecule mixed ionic‐electronic conductors remain less explored and their molecular design rules are not mature enough. Herein, two n‐type glycolated imide‐fused polycyclic aromatic hydrocarbons (IPAHs), d‐gdiPDI and t‐gdiPDI, are developed to probe the effects of molecular conformation on the electronic, electrochemical, morphological, and coupled ionic‐electronic transport properties. It is found that the highly twisted scaffold in d‐gdiPDI, compared to the nearly planar one of t‐gdiPDI, has a strong positive effect on the charge storage properties and thus the performance of organic electrochemical transistors (OECTs). d‐gdiPDI exhibits a volumetric capacitance of 657 F cm−3, obviously outperforming that of t‐gdiPDI (261 F cm−3), which is the highest value reported to date for small‐molecule OECT materials. Moreover, a high charge‐storage capacity of up to 479 F g−1 is observed for d‐gdiPDI. Arising from such high ionic‐electronic coupling characteristic, d‐gdiPDI‐based OECTs present a ≈2 × times higher geometry‐normalized transconductance (gm,norm) of 105.3 mS cm−1 relative to that of t‐gdiPDI counterparts. Significantly, further application of d‐gdiPDI in solid‐electrolyte OECTs delivers a gm,norm of 142.4 mS cm−1. These findings indicate that IPAHs are very promising candidates for n‐type small‐molecule OECTs and highlight the superiority of twisting conformation manipulation in materials design toward high‐performance electrochemical devices.

N-Type semiconductors for organic electrochemical transistor applications

Over the last decade, the organic electrochemical transistor (OECT) has appeared as a powerful platform for developing organic bioelectronic applications such as electronic biosensors and neuromorphic devices. The rapidly growing interest in this field has spurred the development of new active bioelectronic materials that are tailor-made to fulfil the mixed ionic-electronic conduction requirements of the OECT. While p-type (hole-transporting) organic semiconductors quickly appeared with impressive mixed conduction properties, their electron-transporting counterparts, n-type organic semiconductors have lagged severely in terms of OECT performance metrics. Here, we review recent progress on the development of n-type organic semiconductors, including both small-molecule and polymer systems, for OECT applications and discuss the bioelectronic applications that have emerged from the materials development.

N-Type Organic Electrochemical Transistors with High Transconductance and Stability

An n-type conjugated polymer based on diazaisoindigo (AIID) and fluorinated thiophene units is introduced. Combining the strong electron-accepting properties of AIID with backbone fluorination produced gAIID-2FT, leading to organic electrochemical transistors (OECTs) with normalized values of 4.09 F cm–1 V–1 s–1 and a normalized transconductance (gm,norm) of 0.94 S cm–1. The resulting OECTs exhibit exceptional operational stability and long shelf-life in ambient conditions, preserving 100% of the original maximum drain current after over 3 h of continuous operation and 28 days of storage in the air. Our work highlights the advantages of integrating strong electron acceptors with donor fluorination to boost the performance and stability of n-type OECTs.

Effect of ionic conductivity of electrolyte on printed planar and vertical organic electrochemical transistors.

Conducting polymers with mixed electronic/ionic transport are attracting a great deal of interest for applications in organic electrochemical transistors (OECTs). Ions play a crucial role in OECT performance. The concentration and mobility of ions in the electrolyte influence the current flow in the OECT and its transconductance. This study examines the electrochemical properties and ionic conductivity of two semi-solid electrolytes, iongels, and organogels, with diverse ionic species and properties. Our results indicate that the organogels exhibited higher ionic conductivities than the iongels. Furthermore, the geometry of OECTs plays an important role in determining their transconductance. Thus, this study employs a novel approach for fabricating vertical-configuration OECTs with significantly shorter channel lengths planar devices. This is achieved through a printing method that offers advantages, such as design versatility, scalability, expedited production time, and reduced cost relative to traditional microfabrication methods. The transconductance values obtained for the vertical OECTs were significantly (approximately 50 times) higher than those of the planar devices because of their shorter channel lengths. Finally, the impact of different gating media on the performance of both planar and vertical OECTs was studied, and devices gated by organogels demonstrated improved transconductance and switching speed (almost two times higher) than those gated by iongels.

A guide for the characterization of organic electrochemical transistors and channel materials.

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Improved stability of organic electrochemical transistor performance with a low swelling mixed conducting polymer: a comparative study with PEDOT:PSS

OECTs soaked in an electrolyte for up to 40 days produced with a low swelling mixed conductor, poly[3-(6-hydroxy)hexylthiophene] (P3HHT), showed enhanced stability in their electrochemical performance in comparison to PEDOT:PSS-based OECT.