Organic Mixed Conductors Research Articles

Ambipolar blend-based organic electrochemical transistors and inverters

CMOS-like circuits in bioelectronics translate biological to electronic signals using organic electrochemical transistors (OECTs) based on organic mixed ionic-electronic conductors (OMIECs). Ambipolar OECTs can reduce the complexity of circuit fabrication, and in bioelectronics have the major advantage of detecting both cations and anions in one device, which further expands the prospects for diagnosis and sensing. Ambipolar OMIECs however, are scarce, limited by intricate materials design and complex synthesis. Here we demonstrate that judicious selection of p- and n-type materials for blend-based OMIECs offers a simple and tunable approach for the fabrication of ambipolar OECTs and corresponding circuits. These OECTs show high transconductance and excellent stability over multiple alternating polarity cycles, with ON/OFF ratios exceeding 103 and high gains in corresponding inverters. This work presents a simple and versatile new paradigm for the fabrication of ambipolar OMIECs and circuits with little constraints on materials design and synthesis and numerous possibilities for tunability and optimization towards higher performing bioelectronic applications.

Synthesis and Properties of Electron-Deficient and Electron-Rich Redox-Active Ionic π-Systems.

There is growing interest towards the design and synthesis of organic redox-active systems, which exist in ionic form. Multi- redox systems entail life-sustaining processes like photosynthesis and cellular respiration. The significant challenge for material scientists is to rationally design complex molecular materials that can store and transfer multiple electrons at low operational potentials and are stable under ambient conditions. Also, important are the designed ionic π-systems that combine efficient electron and ion transport. Here, we discuss the synthesis of ionic π-systems which exist in the closed-shell form. Firstly, different classes of ionic arylenediimides and viologens with different π-linkers are discussed from the synthetic, structural and redox perspective. These ionic π-systems are based on the electron deficient π-scaffolds, and are shown to accumulate upto six electrons. We then discuss electron-rich ionic arylenediimides which can exist in anionic form or zwitterionic form. The anionic electron donors have absorption extending to the near Infrared (NIR) region and can be stabilized in aqueous solution. We also discuss the effect of the electron accumulation on the aromaticity and non-aromaticity of the naphthalene and the imide rings of the naphthalenediimides. We finally discuss in brief, the applications related to the organic mixed ionic-electronic conductors.

Cyano-Functionalized n-Type Polymer with High Electron Mobility for High-Performance Organic Electrochemical Transistors.

n-Type organic mixed ionic-electronic conductors (OMIECs) with high electron mobility are scarce and highly challenging to develop. As a result, the figure-of-merit (µC*) of n-type organic electrochemical transistors (OECTs) lags far behind the p-type analogs, restraining the development of OECT-based low-power complementary circuits and biosensors. Here, two n-type donor-acceptor (D-A) polymers based on fused bithiophene imide dimer f-BTI2 as the acceptor unit and thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. The cyanation of TVT enables polymer f-BTI2g-TVTCN with simultaneously enhanced ion-uptake ability, film structural order, and charge-transport property. As a result, it is able to obtain a high volumetric capacitance (C*) of 170±22F cm-3 and a record OECT electron mobility (μe,OECT ) of 0.24 cm2 V-1 s-1 for f-BTI2g-TVTCN, subsequently achieving a state-of-the-art µC* of 41.3 F cm-1 V-1 s-1 and geometry-normalized transconductance (gm,norm ) of 12.8 S cm-1 in n-type accumulation-mode OECTs. In contrast, only a moderate µC* of 1.50 F cm-1 V-1 s-1 is measured for the non-cyanated polymer f-BTI2g-TVT. These remarkable results demonstrate the great power of cyano functionalization of polymer semiconductors in developing n-type OMIECs with substantial electron mobility in aqueous environment for high-performance n-type OECTs.

Mixed Ionic-Electronic Conduction, a Multifunctional Property in Organic Conductors.

Organic mixed ionic-electronic conductors (OMIECs) have gained recent interest and rapid development due to their versatility in diverse applications ranging from sensing, actuation and computation to energy harvesting/storage, and information transfer. Their multifunctional properties arise from their ability to simultaneously participate in redox reactions as well as modulation of ionic and electronic charge density throughout the bulk of the material. Most importantly, the ability to access charge states with deep modulation through a large extent of its density of states and physical volume of the material enables OMIEC-based devices to display exciting new characteristics and opens up new degrees of freedom in device design. Leveraging the infinite possibilities of the organic synthetic toolbox, this perspective highlights several chemical and structural design approaches to modify OMIECs' properties important in device applications such as electronic and ionic conductivity, color, modulus, etc. Additionally, the ability for OMIECs to respond to external stimuli and transduce signals to myriad types of outputs has accelerated their development in smart systems. This perspective further illustrates how various stimuli such as electrical, chemical, and optical inputs fundamentally change OMIECs' properties dynamically and how these changes can be utilized in device applications.

Adaptive Biosensing and Neuromorphic Classification Based on an Ambipolar Organic Mixed Ionic-Electronic Conductor.

Organic mixed ionic-electronic conductors (OMIECs) are central to bioelectronic applications such as biosensors, health-monitoring devices, and neural interfaces, and have facilitated efficient next-generation brain-inspired computing and biohybrid systems. Despite these examples, smart and adaptive circuits that can locally process and optimize biosignals have not yet been realized. Here, a tunable sensing circuit is shown that can locally modulate biologically relevant signals like electromyograms (EMGs) and electrocardiograms (ECGs), that is based on a complementary logic inverter combined with a neuromorphic memory element, and that is constructed from a single polymer mixed conductor. It is demonstrated that a small neuromorphic array based on this material effects high classification accuracy in heartbeat anomaly detection. This high-performance material allows for straightforward monolithic integration, which reduces fabrication complexity while also achieving high on/off ratios with excellent ambient p- and n-type stability in transistor performance. This material opens a route toward simple and straightforward fabrication and integration of more sophisticated adaptive circuits for future smart bioelectronics.

Synthetic Nuances to Maximize n-Type Organic Electrochemical Transistor and Thermoelectric Performance in Fused Lactam Polymers.

A series of fully fused n-type mixed conduction lactam polymers p(g7NCnN), 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 p(g7NC10N) recording an OECT electron mobility of 1.20 × 10–2 cm2 V–1 s–1 and a μC* figure of merit of 1.83 F cm–1 V–1 s–1. In parallel to high OECT performance, upon solution doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), the highest thermoelectric performance is observed for p(g7NC4N), with a maximum electrical conductivity of 7.67 S cm–1 and a power factor of 10.4 μW m–1 K–2. 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.

From p- to n-Type Mixed Conduction in Isoindigo-Based Polymers through Molecular Design.

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.

Influence of the molecular weight and size distribution of PSS on mixed ionic-electronic transport in PEDOT:PSS.

The commercially available polyelectrolyte complex poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is ubiquitous in organic and hybrid electronics. As such, it has often been used as a benchmark material for fundamental studies and the development of new electronic devices. Yet, most studies on PEDOT:PSS have focused on its electronic conductivity in dry environments, with less consideration given to its ion transport, coupled ionic-electronic transport, and charge storage properties in aqueous environments. These properties are essential for applications in bioelectronics (sensors, actuators), charge storage devices, and electrochromic displays. Importantly, past studies on mixed ionic-electronic transport in PEDOT:PSS neglected to consider how the molecular structure of PSS affects mixed ionic-electronic transport. Herein, we therefore investigated the effect of the molecular weight and size distribution of PSS on the electronic properties and morphology of PEDOT:PSS both in dry and aqueous environments, and overall performance in organic electrochemical transistors (OECTs). Using reversible addition-fragmentation chain transfer (RAFT) polymerization with two different chain transfer agents, six PSS samples with monomodal, narrow (Đ = 1.1) and broad (Đ = 1.7) size distributions and varying molecular weights were synthesized and used as matrices for PEDOT. We found that using higher molecular weight of PSS (M n = 145 kg mol-1) and broad dispersity led to OECTs with the highest transconductance (up to 16 mS) and [μC * ] values (~140 F·cm-1V-1s-1) in PEDOT:PSS, despite having a lower volumetric capacitance (C * = 35 ± 4 F cm-3). The differences were best explained by studying the microstructure of the films by atomic force microscopy (AFM). We found that heterogeneities in the PEDOT:PSS films (interconnected and large PEDOT- and PSS-rich domains) obtained from high molecular weight and high dispersity PSS led to higher charge mobility (μ OECT ~ 4 cm2V-1s-1) and hence transconductance. These studies highlight the importance of considering molecular weight and size distribution in organic mixed ionic-electronic conductor, and could pave the way to designing high performance organic electronics for biological interfaces.

Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic-Electronic Conductors for Organic Electrochemical Transistors.

Expanding the toolbox of the biology and electronics mutual conjunction is a primary aim of bioelectronics. The organic electrochemical transistor (OECT) has undeniably become a predominant device for mixed conduction materials, offering impressive transconduction properties alongside a relatively simple device architecture. In this review, we focus on the discussion of recent material developments in the area of mixed conductors for bioelectronic applications by means of thorough structure–property investigation and analysis of current challenges. Fundamental operation principles of the OECT are revisited, and characterization methods are highlighted. Current bioelectronic applications of organic mixed ionic–electronic conductors (OMIECs) are underlined. Challenges in the performance and operational stability of OECT channel materials as well as potential strategies for mitigating them, are discussed. This is further expanded to sketch a synopsis of the history of mixed conduction materials for both p- and n-type channel operation, detailing the synthetic challenges and milestones which have been overcome to frequently produce higher performing OECT devices. The cumulative work of multiple research groups is summarized, and synthetic design strategies are extracted to present a series of design principles that can be utilized to drive figure-of-merit performance values even further for future OMIEC materials.

Influence of Molecular Weight on the Organic Electrochemical Transistor Performance of Ladder-Type Conjugated Polymers.

Organic electrochemical transistors (OECTs) hold promise for developing a variety of high-performance (bio-)electronic devices/circuits. While OECTs based on p-type semiconductors have achieved tremendous progress in recent years, n-type OECTs still suffer from low performance, hampering the development of power-efficient electronics. Here, it is demonstrated that fine-tuning the molecular weight of the rigid, ladder-type n-type polymer poly(benzimidazobenzophenanthroline) (BBL) by only one order of magnitude (from 4.9 to 51kDa) enables the development of n-type OECTs with record-high geometry-normalized transconductance (gm,norm ≈ 11 S cm-1 ) and electron mobility × volumetric capacitance (µC* ≈ 26 F cm-1 V-1 s-1 ), fast temporal response (0.38ms), and low threshold voltage (0.15V). This enhancement in OECT performance is ascribed to a more efficient intermolecular charge transport in high-molecular-weight BBL than in the low-molecular-weight counterpart. OECT-based complementary inverters are also demonstrated with record-high voltage gains of up to 100V V-1 and ultralow power consumption down to 0.32 nW, depending on the supply voltage. These devices are among the best sub-1 Vcomplementary inverters reported to date. These findings demonstrate the importance of molecular weight in optimizing the OECT performance of rigid organic mixed ionic-electronic conductors and open for a new generation of power-efficient organic (bio-)electronic devices.

Top–Down Coarse-Grained Framework for Characterizing Mixed Conducting Polymers

Organic polymers that exhibit both ionic and electronic conduction are of interest for energy storage devices and emerging bioelectronic applications. Nevertheless, organic mixed conductors are at an early stage of development with nascent design rules and relatively few material chemistries having been experimentally characterized. Here we report a coarse-grained modeling framework that is sufficiently flexible to represent a range of mixed conducting chemistries while retaining the molecular physics necessary to interrogate structure-function relationships. A detailed overview of the framework is presented, accompanied by an applied study of the effect of hydration and oxidation levels on a representative mixed conductor. The model recapitulates experimental trends related to the macroscopic ionic and electronic conductivities, including the non-linear suppression of the electronic mobility with respect to oxidation level and the direct relationship between ionic mobility and hydration level, while revealing the complex interplay of polymer morphology, ionic-electronic coupling, and electrolyte distribution that govern these relationships. These results provide a validation of this framework for future applications in establishing structure-function relationships in this important materials class, and suggests several near-term opportunities for tailoring mixed conduction via side-chain design.

A Low-Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes.

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer-based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de-doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3-(6-hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side-chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2-(3,3'-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2'-bithiophen]-5-yl)thieno[3,2-b]thiophene) (p[g2T-TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by <10%while that of p(g2T-TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de-doping while p(g2T-TT) remains substantially swollen. The authors, thus, expand the materials-design toolbox for the creation of low-swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

In Situ Measurement of Breathing Strain and Mechanical Degradation in Organic Electrochromic Polymers.

Organic mixed ionic-electronic conductors (OMIECs) are an emerging family of materials crucial in the development of flexible, bio-, and optoelectronics. In electrochromic polymers, the cyclic redox reaction is associated with a mechanical breathing strain, which deforms the OMIECs and degrades the device reliability. We set forth an in situ nanoindentation approach to measure the breathing strain of a poly(3,4-propylenedioxythiophene) (PProDOT) thin film in a customized liquid cell during electrochromic cycles. A breathing volumetric strain of 12-25% is persistent in different sets of electrolytes of various solvents, salts, and salt molarities. The electrochemical conditioning, intermittence time, and cyclic protocol have minor effects on the mechanical response of PProDOT. The mechanical behavior and anion diffusivity measurement further infer the redox kinetics. Heavily cycled PProDOT films show reduced volumetric strain and accumulated mechanical damage of channel cracks and dysfunctional regions of slow and inhomogeneous electrochromic switching. This work is a systematic characterization of mechanical deformation and damage in a model OMIEC and informs the mechanical reliability of organic electrochromic devices.

Decoupling Critical Parameters in Large-Range Crystallinity-Controlled Polypyrrole-Based High-Performance Organic Electrochemical Transistors

Despite the importance of structure and properties in organic mixed conductors, there exist very few material systems where the effects of relative crystallinity, crystallite size, and doping conce...

Time-Resolved Structural Kinetics of an Organic Mixed Ionic-Electronic Conductor.

The structure and packing of organic mixed ionic-electronic conductors have an especially significant effect on transport properties. In operating devices, this structure is not fixed but is responsive to changes in electrochemical potential, ion intercalation, and solvent swelling. Toward this end, the steady-state and transient structure of the model organic mixed conductor, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), is characterized using multimodal time-resolved operando techniques. Steady-state operando X-ray scattering reveals a doping-induced lamellar expansion of 1.6 Å followed by 0.4 Å relaxation at high doping levels. Time-resolved operando X-ray scattering reveals asymmetric rates of lamellar structural change during doping and dedoping that do not directly depend on potential or charging transients. Time-resolved spectroscopy establishes a link between structural transients and the complex kinetics of electronic charge carrier subpopulations, in particular the polaron-bipolaron equilibrium. These findings provide insight into the factors limiting the response time of organic mixed-conductor-based devices, and present the first real-time observation of the structural changes during doping and dedoping of a conjugated polymer system via X-ray scattering.

Mixed Ionic and Electronic Conduction in Radical Polymers

Organic mixed ionic and electronic conductors typically have heterogeneous conjugated macromolecular backbones and ether-based pendant groups to transport ions and charges. Moving from this archety...

The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes.

Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addition to electronic transport in such systems. Here, we investigate three polythiophene homopolymers (P3MEET, P3MEEMT, and P3MEEET) having differently linked (without spacer and with methyl and ethyl spacer, respectively) diethylene glycol side chains. All the polymers were tested in organic electrochemical transistors (OECTs). They show drastic differences in the device performance. The highest μOECT C* product of 11.5 F/cm·V·s was obtained for ethyl-spaced P3MEEET. How the injection and transport of ions is influenced by the side-chain linkage was studied with electrochemical impedance spectroscopy, which shows a dramatic increase in volumetric capacitance from 80 ± 9 up to 242 ± 17 F/cm3 on going from P3MEET to P3MEEET. Thus, ethyl-spaced P3MEEET exhibits one of the highest reported volumetric capacitance values among p-type polymers. Moreover, P3MEEET exhibits in dry thin films an organic field-effect transistor (OFET) hole mobility of 0.005 cm2/V·s, highest among the three, which is one order of magnitude higher than that for P3MEEMT. The extracted hole mobility from OECT (oxidized swollen state) and the hole mobility in solid-state thin films (OFET) show contradictory trends for P3MEEMT and P3MEEET. In order to understand exactly the properties in the hydrated and dry states, the crystal structure of the polymers was investigated with wide-angle X-ray scattering (WAXS) and grazing incidence WAXS, and the water uptake under applied potential was monitored using electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD). These measurements reveal an amorphous state for P3MEET and a semicrystalline state for P3MEEMT and P3MEEEET. On the other hand, E-QCMD confirms that P3MEEET swells 10 times more than P3MEEMT in the oxidized state. Thus, the importance of the ethyl spacer toward crystallinity and mixed-conduction properties was clearly demonstrated, emphasizing the impact of side chain linkage of diethylene glycol. This detailed study offers a better understanding of how to design high-performance organic mixed conductors.

Organic mixed ionic–electronic conductors

Materials that efficiently transport and couple ionic and electronic charge are key to advancing a host of technological developments for next-generation bioelectronic, optoelectronic and energy storage devices. Here we highlight key progress in the design and study of organic mixed ionic-electronic conductors (OMIECs), a diverse family of soft synthetically tunable mixed conductors. Across applications, the same interrelated fundamental physical processes dictate OMIEC properties and determine device performance. Owing to ionic and electronic interactions and coupled transport properties, OMIECs demand special understanding beyond knowledge derived from the study of organic thin films and membranes meant to support either electronic or ionic processes only. We address seemingly conflicting views and terminology regarding charging processes in these materials, and highlight recent approaches that extend fundamental understanding and contribute to the advancement of materials. Further progress is predicated on multimodal and multi-scale approaches to overcome lingering barriers to OMIEC design and implementation.

Influence of Water on the Performance of Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) composed of organic mixed conductors can operate in aqueous, biological media and translate low-magnitude ionic fluctuations of biological origin into measurable electrical signals. The growing technological interest in these biotransducers makes the fundamental understanding of ion-to-electron coupling extremely important for the design of new materials and devices. One crucial aspect in this process that has been so far disregarded is the water taken up by the film during device operation and its effects on device performance. Here, using a series of the same electrolyte with varying ion concentrations, we quantify the amount of water that is incorporated into a hydrophilic p-type organic semiconductor film alongside the dopant anions and investigate structural and morphological changes occurring in the film upon electrochemical doping. We show that infiltration of the hydrated dopant ions into the film irreversibly changes the polymer structure and negatively...

A highly porous and conductive composite gate electrode for OTFT sensors.

Ionic/protonic to electronic transducers based on organic thin film transistors have shown great promise for applications in bioelectronic interface devices and biosensors, and development of materials that exhibit mixed ionic/electronic conduction are an essential part of these devices. In this work, we investigated the proton sensing properties of an all solid-state and low voltage operating organic thin film transistor (OTFT) that uses the organic mixed conductor poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) as the gate electrode. To address the limited sensitivity due to the lack of porosity in PEDOT:PSS base sensors, we proposed a composite gate electrode material composed of PEDOT:PSS and proton conducting mesoporous SO3H-Si-MCM-41 nanoparticles for improved proton sensitivity. The composite gate electrode doubles the proton sensitivity of the OTFT, indicating a clear advantage of adding SO3H-Si-MCM-41 in the PEDOT:PSS gate. Moreover, the OTFTs with the composite gate electrode maintained OTFT characteristics similar to that of the PEDOT:PSS gated OTFT. A detailed and systematic study of the effect of variation in the composition of PEDOT:PSS:SO3H-Si-MCM-41 on OTFT characteristics and sensing properties is carried out. Our results open up the possibility of combining inorganic nanomaterials with organic conductors in the development of highly efficient bioelectronic sensing platforms.