Shun Watanabe

This roadmap presents key advances and future directions in the field of the conducting polymer polyethylene dioxythiophene (PEDOT), with a focus on its role as multifunctional material in emerging electronic applications. It covers a broad spectrum of topics, including self-healing materials, plant bioelectronics, energy harvesting and storage, and neural interfaces. The roadmap also addresses recent progress in the synthesis and functionalization of PEDOT derivatives, as well as insights from molecular dynamics simulations and transport physics. Emphasis is placed on a range of processing techniques, such as electropolymerization, vapor phase polymerization, and 3D printing, and their implications for device performance. Specific applications explored include thermoelectrics, biosensors, organic electrochemical transistors, and advanced bioelectronic platforms for neuromodulation, stretchable electronics, and neuromorphic computing. This collective effort aims to serve as a comprehensive resource outlining the current state of PEDOT research and identifying challenges and opportunities for its integration across diverse electronic technologies.

A highly periodic electrostatic potential and coherent band transport can emerge in organic molecular crystals, despite weak van der Waals interactions. Although charge carrier mobility in single-crystalline organic semiconductors (OSCs) reaches 10 square centimeters per volt per second (cm ² V ⁻¹ s ⁻¹ ), it is predominantly limited by molecular vibrations excited at room temperature. The extent to which mobility in single-crystalline OSCs can be increased remains a central question. Here, we demonstrate charge transport in a clean two-dimensional hole gas (2DHG) in uniaxially strained, single-crystalline OSCs at cryogenic temperatures, with minimized lattice vibrations. Hall effect measurements reveal a mobility of 117 cm ² V ⁻¹ s ⁻¹ at 2 kelvin under 2.8% compressional strain, with an extraordinarily large piezoresistive effect and low sheet resistivity of 550 ohms, one-fifth of the lowest resistivity in unstrained samples. These clean systems offer opportunities to explore intrinsic strain-induced charge transport physics, where condensed matter phenomena, characterized by weakly bonded molecular orbitals, combine electronic correlation and lattice degrees of freedom.

Solution processing of polymeric semiconductors provides a facile way to fabricate functional diodes. However, energy barriers at metal-semiconductor interfaces often limit their performance. Here, we report rectifying polymer diodes with markedly modified energy-level alignments. The gold electrode surface was treated with a dimeric metal complex, which resulted in a shallow work function of 3.7 eV by forming a monolayer-thick ionized donor layer. When a polymeric semiconductor was coated on the treated electrode, most of the ionized donors remained at the metal-semiconductor interface. The confined ionized donors with the ideal thickness enabled fabrication of a polymer diode with a forward current density of over 100 A cm ⁻² . Furthermore, a power conversion efficiency of 7.9% was observed for rectification at a microwave frequency of 920 MHz, which is orders of magnitude higher than that reported for organic diodes. Our findings will pave a way to solution-processed high-frequency and high-power devices.

Thin-film single crystals of organic semiconductors (OSCs) enable simple and low-cost fabrication of high-mobility organic field-effect transistors (OFETs) via solution processing. However, injection barriers in OFETs limit material selection and device performance. Researchers typically use high-work-function noble metals as contact electrodes for p-type OFETs owing to transport levels well below −5.0 eV for ambient stable OSCs. This raises questions regarding the economic and environmental advantages of OSC based printed electronics. This study demonstrates high-mobility OFETs with copper contact electrodes using doped interlayers. A poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) interlayer was laminated on the OSC single crystals. The PTAA interlayer was chemically doped using aqueous doping solutions, followed by evaporation of copper contact electrodes. Our OFET showed a proper p-type operation with a mobility of 5.0 cm²V⁻¹ s⁻¹ and onoff ratio of approximately 10⁴. This is in contrast to the case without the doped interlayer showing poor performance and a low mobility of 0.2 cm²V⁻¹ s⁻¹. This study provides new opportunities for designing devices with a high performance, low cost, and material sustainability.

Transistors fabricated from thin‐film single crystals of organic semiconductors (OSCs) have exhibited high mobility exceeding 10 cm² V⁻¹ s⁻¹ and show compatibility with low‐cost solution processing. However, their carrier mobility is limited by the molecular vibrations in their soft lattices. This study establishes a practical method for applying compressive strain to single‐crystal OSCs to enhance mobility and transistor performance. In this method, a polymer film substrate is bent to mechanically stretch its surface. Organic single‐crystal transistors are then laminated onto the stretched surface of substrate. Releazing the stretch by recovering the flat surface of the substrate allowed the transistors to be compressed by up to 3%. This resulted in a 52% increase in mobility, reaching 26.4 cm² V⁻¹ s⁻¹. X‐ray diffraction measurements confirmed lattice strain in the OSC single crystals. Moreover, carrier mobility and cutoff frequency increased in MHz‐operating short‐channel transistors, demonstrating applicability for high‐frequency devices. The mobility increase is maintained even three years after introducing the 1% compressive strain, possibly owing to the flexible, molecularly thin characteristics of OSC single crystals. The proposed strain management methods may provide new avenues to enhance the performance of high‐mobility and high‐frequency electronic devices based on OSC thin‐film single crystals.

Exciton dissociation into free electron-hole carriers is a central issue in optoelectronic devices. Despite recent progress in free-carrier generation due to an admixture of organic compounds, strategies to prolong the lifetime of photo-induced carriers are limited. Herein, we present a proof-of-concept of efficient exciton dissociation via heterogeneous device engineering; an insulating polymer layer adjacent to organic semiconductors (OSCs) in conjunction with an application of electrical voltage can capture photogenerated electrons selectively, and the resulting semipermanent built-in electric field gives rise to extraordinarily persistent photo-induced carriers, which lasts for more than half a year in single-crystalline OSCs.

Strong electron correlation is the essential mediator that creates various exotic phases in two-dimensional electronic systems which has been continuously intriguing in modern condensed-matter physics. Such electronic states as Mott insulators, charge orders, and high-temperature superconductivity would be simply Fermi-degenerated metals unless the strong correlation plays essential roles. However, how it emerges, particularly to overcome screening effects upon doping band insulators, has not been experimentally studied. In this study, we report evolution of a strongly correlated electron system from a band-insulating organic semiconductor. Carriers are continuously doped via electric double layers up to a density of 10¹⁴cm⁻². Notably, significant deviations from a simple metallic system are observed even at far from half-filled band, possibly due to charge-order instability. The findings reveal that off-site Coulomb energy can compete with Thomas–Fermi screening. This competition enables the emergence of strongly correlated exotic phases, even in systems distant from Mott insulators.

Thin-film devices that transduce the chemical activity of ions into electronic signals are essential components in various applications, including healthcare diagnostics and environmental monitoring. Combinations of organic semiconductors (OSCs) and ion-selective materials have been explored for developing solution-processable ion sensors. However, the necessity of reference electrodes (REs) and operational stability in ion-permeable OSCs have posed questions regarding whether reliable measurements with thin-film components are attainable with OSCs. Herein, we report electric double-layer transistors (EDLTs) with OSCs in single-crystal forms for ion sensing. Our EDLTs demonstrated high operational stability, with a one-to-one relationship between the source electrode potential and device resistance, and served as quasi-REs (qRE). When our EDLT is served as qRE, its drift was as small as 0.5 mV/h and comparable to that of commonly employed REs. In our system, the semiconductor–electrolyte interface is self-passivated by the alkyl chains of OSCs in single-crystal structures, with the two-dimensional transport layer appearing unaltered upon gating. EDLT arrays with ion-selective and nonselective liquid junctions enable ion concentration sensing without a conventional RE. These findings provide opportunities to develop thin-film devices based on OSCs for easy integration and reliable measurements.

Notorious zinc dendrite growth and hydrogen precipitation reactions disrupt the galvanic/stripping process at the electrolyte/electrode interface, which seriously affects the cycling stability of zinc anodes in aqueous zinc ion batteries. To improve the stability and reversibility of zinc anodes, we report an artificial SEI consisting of hydrophobic carbon nanocrystals and highly conductive carbon nanotube networks. This interfacial hydrophobicity effectively excludes free water from the surface of the zinc anode, which prevents water erosion and reduces the interfacial side reactions, resulting in a significant improvement in the cycling stability and coulombic efficiency of Zn plating/stripping. Benefiting from the reversible proton storage and fast desolvation kinetic behavior of the CNC/CNT interlayer, the stable cycling time of Zn/Zn symmetric batteries exceeds 700 h even at a high current density of 5 mA cm⁻². The assembled CNC/CNT@Zn‖V₂O₅ full cell maintains a high capacity of 101.1 mA h g⁻¹ after 5000 cycles (1.0 mA g⁻¹). This study opens up a new area for expanding the use of organic compounds in zinc anode protection and offers a promising strategy for accelerating the development of aqueous zinc-ion batteries.