A limitation for polymeric mixed ionic/electronic conductors (MIECs) is the trade-off between ionic and electronic conductivity; changes made that improve one typically hinder the other. In order to address this fundamental problem, we investigated a common oligoethylene glycol side chain polymer by adjusting the oxygen atom content and position, providing structural insights for materials that better balanced the two conduction pathways. The investigated polymer series showed the prototypical conflict between ionic and electronic conduction for oxygen atom content, with increasing oxygen atom content increasing ionic conductivity, but decreasing electronic conductivity; however, by increasing the oxygen atom distance from the polymer backbone, both ionic and electronic conductivity could be improved. Following these rules, we show that poly(3-(methoxyethoxybutyl)thiophene), when blended with lithium bistrifluoromethanesulfonimide (LiTFSI), matches the ionic conductivity of a comparable MIEC [poly(3-(methoxyethoxyethoxymethyl)thiophene)], while simultaneously showing higher electronic conductivity, highlighting the potential of this design strategy. We also provide strategies for tuning the MIEC performance to fit a desired application, depending on if electronic, ionic, or balanced conduction is most important. These results have implications beyond just polythiophene-based MIECs, as these strategies for balancing backbone crystallization and coordinating group interconnectivity apply for all semicrystalline conjugated polymers.

Conjugated polymers with polar side chains are capable of mixed electronic and ionic conduction, making them attractive candidates for electrodes in electrochemical devices. Their electrochemical and redox properties can be tuned through choice of polymer backbone and side chain [1, 2] while the processible nature of the materials facilitates both manufacture and recycling.  When applied as electrodes in electrochemical energy storage devices, conjugated polymer electrodes show excellent charging and discharging rates, high coulometric efficiency and compatibility with simple salt-water electrolytes, whilst specific capacity and stability in ambient environments need to be improved [3]. In this work, we investigate the impact of the of side-chain and backbone structure on the redox properties, charging rate, specific capacity, mass uptake and cycling stability of the electrodes. We show that small changes of the side chain composition can significantly influence the degree of water uptake (and thereby the mechanical stability of the electrodes), the redox-stability of the materials in aqueous electrolytes, and the electrodes’ specific capacity. We use molecular dynamics simulations to understand how the side chain structure controls the internal microstructure of the polymer electrode and its response to  water and ion uptake. We also explore differences in the mechanisms of charging in n and p-type polymers. Finally, we consider different strategies for enhancing specific capacity. The findings can help to develop chemical designs for improved conjugated polymer electrodes for aqueous environments.

Organic electrochemical transistors (OECTs) are in a stage of rapid development as novel applications that use these versatile devices continue to emerge. OECTs are characterized by the coupling of both ionic and electronic inputs to modulate transistor channel conductance, which makes them ideal for interfacing electronics with biology. However, the current performance mismatch between p-type (hole-transporting) and n-type (electron-transporting) OECTs hinders the development of power-efficient complementary devices/circuits, essential to many (bio-)electronic applications. Here, we will summarize our effort to develop n-type mixed ionic-electronic conducting polymers for OECTs. We will discuss the impact of polymer backbone rigidity and molecular weight on the OECT performance. We will show large-area printing/integration of these devices and demonstrate neurosynaptic circuits capable of Hebbian learning.

Organic bioelectronics has experienced tremendous growth over the past two decades thanks to the expansion of the library of organic electronic materials available. Electron conducting (n-type) polymers are particularly suitable to translate biological events that involve the generation of electrons. However, n-type polymers that are stable when addressed electrically in aqueous media are relatively scarce, and the performance of existing ones lags behind their hole conducting (p-type) counterparts. Here, we report a new family of donor-acceptor type polymers based on naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene (NDI-T2) backbone where the NDI unit always bears an ethylene glycol (EG) side chain. We study how small variations in the side chains tethered to the acceptor as well as the donor unit affect the performance of the polymer films in the state-of-the-art bioelectronic device, the organic electrochemical transistor (OECT). First, we show that substitution of the T2 core with an electron-withdrawing group (i.e., methoxy) or an EG side chain leads to ambipolar charge transport properties and causes significant changes in film microstructure revealed by ex-situ X-ray scattering studies, which overall impairs the n-type OECT performance. We thus find that the best n-type OECT performer is the polymer that has no substitution on the T2 unit. Next, we evaluate the distance of the oxygen from the NDI unit as a design parameter by varying the length of the carbon spacer placed between the EG unit and the backbone. We find that the distance of the EG from the backbone affects the film order and crystallinity, and thus, the electron mobility. As such, we develop the best performing NDI-T2 based n-type OECT material to date. Our work provides new guidelines for the side chain architecture of n-type polymers for OECTs and insight on the structure-performance relationships for mixed ionic-electronic conductors, crucial for devices where the film operates at the aqueous electrolyte interface.

A very promising strategy to design highly efficient materials for organic mixed-ionic-electronic conductors, is based on the idea of using the backbone structure of “more traditional” conjugated polymers – already designed and optimised specifically for electronic transport – and to modify the chemical composition of their side-chains in order to achieve efficient ionic transport. However, it is well-known that side-chains play an essential role in determining the microstructure of polymer thin films, which in turns deeply affects the charge carrier mobilities of the material and the efficiency in devices. As a consequence, understanding the structural effects of exchanging alkyl for glycol side chains in conjugated polymers is of central importance in determining accurate structure-function relationships for this new class of materials.

In this talk I will demonstrate that by using our recently developed combination of vacuum electrospray deposition (ESD) and high-resolution scanning tunnelling microscopy (STM) [1], it is possible to image conjugated polymers used in organic electronic and bioelectronic applications at the (sub-)molecular scale, thereby revealing their conformation and assembly with unprecedented spatial resolution [2,3]

I will present results on the prototypical semicrystalline polythiophene polymer pBTTT and directly compare these with analogous results on the mixed-ionic-electronic conductors obtained by substituting the alkyl side-chains of pBTTT with ethylene glycol (EG) side-chains. Our high-resolution images identify clear differences in the tendency of assembling between the two types of polymers and show that this depends on the local polymer density. I will further demonstrate that glycol side-chain polymers are capable of interdigitating and use the observed structural characteristics to interpret X-ray diffraction measurements of 3D thin films [4].

Finally, I will show that our observations can be explained based on a simple model that accounts for the different intermolecular interactions between alkyl and EG chains and propose this as a general framework to rationalise the assembly of conjugated polymers with EG side-chains. Moreover, I will show that atomistic molecular dynamic simulations (Nelson group, Imperial College London) can reproduce very closely the experimentally observed polymer conformations and assembly patterns.

When combined with oxidase enzymes, the NDI-T2 based electron transporting (n-type) polymer led to high performance metabolite sensors, yet their working mechanism has been poorly understood.[1], [2] By monitoring oxygen, hydrogen peroxide, and pH changes in the electrolyte surrounding the n-type channel and gate as well as the potential of each electrical contact in the transistor, we shed light on the catalytic events occurring at the polymer-enzyme interface. We show that in its doped sate, the n-type film performs oxygen reduction reaction and that the n-OECT characteristics are sensitive to oxygen. We find a correlation between the amount of dissolved oxygen and the n-OECT sensor current generated during the metabolite oxidation and that using the n-type polymer at the gate electrode is critical for sensor operation. Our results show the importance of in operando analysis for understanding polymer-catalytic enzyme activity, as well as the importance of ambient oxygen in the operation of n-type devices.

Successful adoption of defect management and carrier confinement strategies in Ruddlesden−Popper (RP) perovskites has driven the impressive improvements to performance of perovskite-based light-emitting diodes (PeLEDs) seen to date. Although functional additives have been advantageous in mitigating defects, their influence over crystallization behavior of RP (L2Am−1PbmX3m+1) perovskites has yet to be fully studied. This is especially important for blue-emitting monohalide RP perovskites, where stringent control over m domain distribution is needed for efficient PeLEDs. Herein, we investigate the effect of triphenylphosphine oxide (TPPO) on crystallization behavior of blue RP (PBA2Csm−1PbmBr3m+1) perovskites. Despite TPPO addition, its absence in the resulting film eliminates its role as a passivating agent. Instead, TPPO acts as crystallization and phase distribution modulator promoting the formation of a narrow distribution of higher m domains with higher Br content. In doing so, an enhancement of ∼35% was noted with the champion device yielding efficiency of 3.8% at λ of 483 nm.

Organic mixed ionic/electronic conductors (OMIECs) have gained considerable interest in bioelectronics, power electronics, circuits and neuromorphic computing. These organic, often polymer-based, semiconductors rely on a combination of ionic transport, electronic transport, and high volumetric charge storage capacity. Growth in this field has produced a vast library of new materials and an expanding suit of new or improved device implementation strategies. In this talk, we highlight recent synthetic and processing approaches used to tailor electrochemical device properties and stability, as well as new device concepts enabled by such advances. Control of ionic transport and trapping for example presents a promising avenue towards the development of non-volatile electrochemical transistors. We show that additives in prototypical conducting polymers can lead to neuromorphic function which can mimic basic neural function. The need for deeper fundamental studies to solve persistent barriers in OMIECs is also addressed.

The field of organic neuromorphic electronics has been recently emerged for biology-oriented applications as well as for the implementation of on-chip learning circuitry. Various neuromorphic devices will be presented that are based on organic-mixed conductors (ionic-electronic), materials that are traditionally used in organic bioelectronics. A prominent example of a device in bioelectronics that exploits mixed conductivity phenomena is the organic electrochemical transistor (OECT). Devices based on OECTs show volatile, non-volatile and tunable dynamics suitable for the emulation of synaptic plasticity functions. Finally, small-scale organic neuromorphic circuits allow for local learning in robotics and the realization of on-chip neuronal dynamics that are biophysically realistic.

Mechanical deformability underpins many of the advantages of semiconducting and stimuli-responsive polymers in applications from flexible solar cells to wearable devices for healthcare and virtual touch. The mechanical properties of these materials are, however, diverse, and the molecular characteristics that permit deformability while retaining function remain poorly understood. In this talk, I describe the ways in which molecular structure and solid-state packing structure govern the mechanical properties of organic semiconductors, especially of π-conjugated polymers. In particular, I describe how low modulus, good adhesion, and absolute extensibility prior to fracture enable robust performance. I will also present my group’s recent work on the intersection between the science of soft materials and the science of touch. This field, which we have named “organic haptics,” combines active polymers, contact mechanics, and psychophysics. We are beginning to understand the ways in which stick slip friction, adhesion, and capillary forces between planar surfaces and human skin affect the ways materials produce tactile objects in consciousness as mediated by the sense of touch. This work, which combines human subject experiments, laboratory mockups of human skin, and analytical models accounting for friction, has led to several important observations. In particular, we have elucidated the mechanism by which humans can differentiate hydrophilic from hydrophobic surfaces when bulk parameters such as hardness, roughness, and thermal conductivity are held constant. We have taken the insights from these psychophysical experiments to design new electroactive and ionically conductive materials to produce haptic biomaterials whose goal is to produce realistic sensations for applications in tactile therapy, instrumented prostheses, education and training, and virtual and augmented reality.

H₂O₂ plays a significant role in a vast range of physiological processes in aerobic environments where it is produced by cells and performs vital tasks in redox signaling. The sensitivity of many biological signaling pathways to H₂O₂ opens up a unique direction in the development of bioelectronics devices to control levels of reactive-oxygen species (ROS). Here we present a microfabricated ROS modulation device which relies on controlled faradaic reactions. We report a concentric pixel arrangement of a peroxide-evolving cathode surrounded by an anode ring which decomposes the peroxide, resulting in localized peroxide delivery. We exploit the conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT, as the cathode. PEDOT selectively catalyzes the oxygen reduction reaction resulting in the production of hydrogen peroxide (H₂O₂) via a two-electron pathway. To complete the pixel, a palladium anode is used to catalytically consume peroxide. Using electrochemical and optical assays, combined with computational modeling, we benchmark the performance of the devices. The concentric pixels generate tunable gradients of peroxide and oxygen concentrations. We prototype the faradaic pixel devices by successfully modulating human H₂O₂-sensitive Kv7.2/7.3 (M-type) channels expressed in a single-cell model (Xenopus laevis oocytes). The Kv7 ion channel family is responsible for regulating action potential firing and neuronal excitability in various tissues as the heart, brain, and vascular smooth muscles, making it an ideal testing platform for faradaic ROS stimulation. We present also a wireless, light-driven version of this device. Our results demonstrate the potential of PEDOT to act as a H₂O₂ delivery system, paving the way to novel ROS-based organic bioelectronics.

Though electrochemistry of conducting polymers is a rather old topic[1], only recently conducting polymers have received renewed attention as inherently mixed-ionic-electronic conductors for a number of emerging switchable technologies including actuators, electrochromic displays and electrically switchable metasurfaces.

One of the work-horses of the community remains poly(ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS). While typically known as “synthetic metal” with application as transparent flexible electrodes, the material is a mixed conductor[2] and shows ionic conductivity which is strongly affected by humidity.[3] The humidity dependence of the PSS polyelectrolyte phase together with the electroactive nature of the PEDOT can be used to create multifunctional and multiresponsive materials. A recent example from my group is the preparation of “intelligent” humidity-triggered bilayer actuators whose bending behavior (curvature) can be nicely explained by the humidity-dependent mechanical behavior of the constituents.[4] In collaboration with nanooptics experts the electrochemical stimuli were further used to switch nanoantennas “on” and “off” between the metallic and the insulating state.[5] In this context advanced electrochemical techniques based on coupling of cyclic voltammetry with in-situ transmission spectroscopy,  spectroscopic ellipsometry and conductivity measurements help to shed light on the dependence of  optical constants, conductivity and swelling behavior as function of the redox state.

Conjugated polymers have promising applications in electronics and energy storage due to the polymer’s tunable conductivity and redox activity. For example, the conductivity of poly(3-hexylthiophene) (P3HT) is heavily dependent upon the doping level and the dopant type. This feature becomes especially important when considering P3HT or similar conjugated polymers for devices that require switching between electronic states (conductive vs insulating). In this study, the mechanism of mixed ion-electron transfer studied using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is discussed. During cyclic voltammetry and galvanostatic charge-discharge experiments, the mass change of a P3HT film is monitored in real time. Distinct mass transfer regions are quantified as a function of doping level and potential, which are then correlated to changes with in situ conductance and spectroelectrochemical response. To identify the time scale at which the doping reaction transitions from kinetic to diffusion control, electrochemical impedance spectroscopy is coupled with EQCM-D. This work gives valuable insight into the nature of mixed ion-electron transfer, including its time scale, as it relates to the electronic properties of P3HT