Mixed Ionic–Electronic Conduction, a Multifunctional Property in Organic Conductors

Abstract

consumption and sensing as well as high-energy-density batteries for energy storage. Recently, interest in developing systems that interface seamlessly with the human body (e.g., wearables, braincomputer interfaces, and soft robots) has driven the development of soft, organic, and biomimetic materials that emulate the functions of their inorganic counterparts. Beyond emulation, these materials are unique because they provide an opportunity to embody such multifunctional properties within the material itself, rather than relying on device design. These multifunctional properties are intrinsic to organic mixed ionic electronic conductors (OMIECs), that is, once synthesized and processed, OMIECs can serve as the active component of multiple devices (be it transistors, sensors, energy-storage devices, etc.), where essentially the material is the device.OMIECs generally consist of a conjugated backbone for electronic conduction as well as sidechains to facilitate ionic intercalation from the operational electrolyte and to aid in solvation in processing solvents. [1] Organic chemistry provides a large toolbox in the molecular design of the backbone, side chains, and other additives, resulting in an almost infinite design space for the corresponding materials properties: energy levels, electronic and ionic conductivity, optical, volume, and moduli. Additionally, one or more of these properties can be modified during device operation, thereby transducing an input (e.g., ionic) into an output (e.g., electronic), allowing OMIECs to be used for a variety of applications including sensors, transistors, optoelectronic devices, energy-storage electrodes, and actuators. The multifunctionality of OMIECs is illustrated in Figure 1 to highlight their versatility in design and their ability to respond to a variety of stimuli.The underlying and unifying phenomena behind these property changes arise from the large modulation in electronic and ionic charge density in the bulk of the OMIEC. This modulation in turn results in second-order effects such as modulations in electrochemical potential (electron energy levels), electronic and ionic transport, capacitance, free volume, optical bandgap, and modulus. Tuning these properties throughout the bulk of the material enables new design parameters that were previously untapped in traditional electronic devices where 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...

Organic Mixed Ion-Electron Conductivity in Polymer Hybrid Systems

“…By fine-tuning of conjugated backbone of OMEIC, the entire color wheel ( Figure 11 ) can be synthesized. 21 Such devices are useful in energy-saving-colored windows, full-color bistable displays, and dimmable goggles, glasses, and visors. A donor–acceptor approach is used to afford a myriad of colors, and backbone engineering strategies comprise tuning the core heterocycle choice as well as copolymerization.…”

“… Illustration of color control in OMIECs through backbone and side chain engineering strategies. [Reprinted with permission from ref ( 21 ).] …”

“…20 In addition, copolymeric OMIECs containing an n-type NDI core, with a p-type building block (for eample, polythiophene) are drawing much attention as electrons are effectively injected and delocalized along the copolymer chain, especially when ionically conductive side chains are utilized. 21 Recently, a hybrid of N,N-di((S)-1-carboxylethyl)-3, 4:9,10 perylene tetra carboxyl diimide (PTCDA, n-type semiconductor) with polyaniline (PANI, p-type semiconductor) made from dilute acetic acid medium at a 13 wt % concentration of PTCDA has been reported. The I−V characteristic curve exhibits signature OMEIC properties under both dark and illuminated conditions.…”

Towards organic electronics that learn at the body-machine interface: A materials journey

“…Organic mixed ionic-electronic conductors (OMIECs), also classified as ionpermeable organic semiconductors (OSCs), have thus emerged in recent years as excellent channel materials for OECTs owing to their compatibility with both ions and electrons, mechanical matching (compared to inorganic counterparts), biocompatibility, and structural tenability. [23] This multifunctionality enables unprecedented set of properties such as energy-efficient sensing and learning/computing. [23,24] These conductors, which will be discussed in more details in later sections, are mostly conjugated polymers with polar sidechains that can interface with electrolytic environments and translate ionic signals into electrical signals in terms of their conductance change upon ionic insertion.…”

“…[23] This multifunctionality enables unprecedented set of properties such as energy-efficient sensing and learning/computing. [23,24] These conductors, which will be discussed in more details in later sections, are mostly conjugated polymers with polar sidechains that can interface with electrolytic environments and translate ionic signals into electrical signals in terms of their conductance change upon ionic insertion. Considering recent advances in the fields of neuromorphics and bioelectronics, OMIEC-based transducing devices are envisioned to complement the body in the form of smart prosthetics (e.g., monitoring, sensing, nerve regeneration).…”