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When designing organic semiconductors, side-chain engineering is as important as modifying the conjugated backbone, which has a significant impact on molecular ordering, morphology, and thus electronic device performance. We have developed three dicyanovinyl-end-capped donor-acceptor diketopyrrolopyrrole-based n-type small molecules (C2C9CN, SiC4CN, and EH4PCN) bearing an identical length of alkyl spacer yet different end-functionalized side chains (i.e., alkyl-, siloxane-, and phosphonate-end pendants). The effects of the end-functionalized side chains on the intrinsic molecular properties, microstructure, and charge transport of the small-molecule series were investigated. In comparison with the alkyl-end side chains, incorporating siloxane-end side chains into the backbone facilitates 2D edge-on oriented high intergrain connectivity/crystallinity and compatibility with the substrate surface, whereas the phosphonate-end analogues have an adverse effect on the film-forming quality due to high polarity. Thereby, an organic field-effect transistor fabricated by SiC4CN shows the best electron mobility up to 1.59 × 10-1 cm2 V-1 s-1 along with a high current on/off ratio >105. This study contributes to our understanding of the role of the end-functionalized side chains (e.g., the effects of polarity and bulkiness of the end groups) for the development of high-performance semiconductors.

One-dimensional (1D) organic chiral supramolecules have received a great deal of attention for their promising applications in chiral recognition systems, chemical sensors, catalysts, and optoelectronics. Compared to modifications at the imide position of a perylene diimide (PDI), few studies have explored bay substitution of chiral PDIs and their self-assemblies into 1D nanomaterials. Herein, we describe the synthesis of three bay-substituted PDIs and explore the effects of bay substitution on supramolecular chirality by examining circular dichroism spectra and the optoelectronic performance of chiral PDI nanomaterials in phototransistors. Among the three fabricated self-assemblies, nanomaterials based on (R)-CN-CPDI-Ph exhibited the highest electron mobility of 0.17 cm2 V-1 s-1, a low threshold voltage of -1 V, and enhanced optoelectronic performance. For example, the photoresponsivity and external quantum efficiency of (R)-CN-CPDI-Ph assemblies were 4-fold higher than those of (R)-2Br-CPDI-Ph and (R)-2F-CPDI-Ph. All three nanomaterials exhibited fast switching speeds compared with previously reported N-substituted PDIs, suggesting that bay substitution can be an effective means of achieving rapid photoswitching. A comprehensive study using density functional theory calculations and crystal analyses revealed that the enhanced optoelectronic performance of (R)-CN-CPDI-Ph nanomaterials is related to the substitution of CN at the bay position of PDI. This minor change provides simultaneous improvements in electron injectability and structural order. Our findings demonstrate that bay substitution can significantly impact the self-assembly, supramolecular chirality, and optoelectronic properties of PDI nanomaterials.

Chiral supramolecular metal–organic frameworks (SMOFs), i.e., AlaNDI-Ca micro-/nanocrystals have been synthesized via solvothermal reaction using alanine-based naphthalene diimide (NDI) ligands and calcium nitrate. Interestingly, chiral self-discrimination is observed upon mixing two enantiomeric ligands with calcium nitrate due to the thermodynamic preference of heterochiral SMOFs over homochiral analogs. Upon irradiation with UV light, the conductivity of SMOFs micro-/nanocrystals significantly increases and their color changes from yellow to dark brown, due to the electron-transfer-related chemical processes inside the SMOF and/or photo-induced radical formation in NDI ligands, while they are not responsive to visible light. Heterochiral AlaNDI-Ca micro-/nanocrystals with a large band gap exhibit excellent photoswitching properties, due to the reversible generation and depletion of radicals upon exposure to UV light or in the dark, respectively. To our knowledge, photodetecting MOFs for optoelectronics are extremely rare, and herein we have firstly quantitatively characterized single crystalline SMOF photodiodes with redox NDI ligands with regard to their visible-blind UV photodetecting properties with high photoresponsivity and detectivity, which exhibit superior performances in comparison with precedent MOF-based photodetectors. These findings pave the way for applications of multifunctional micro-/nanoscaled MOFs in next-generation optoelectronics.

Organic phototransistors (OPTs) have been widely used in biomedical sensing, optical communications, and imaging. Charge-trapping effect has been utilized as an effective strategy for enhancing their photoresponsivity by effectively decreasing the dark current. The combination of organic semiconductors (OSCs), especially chiral OSCs, with insulating polymers has rarely been carried out for optoelectronic applications. Here, we fabricated OPTs containing both enantiopure and racemic air-stable n-type perylene diimide derivatives, CPDI-CN2-C6, and insulating biopolymer polylactide (PLA) and evaluated their photoresponsive properties. The PLA-blended systems exhibited greatly enhanced optoelectronic performances owing to the intense charge-trapping effect. Interestingly, the racemic system showed 3 times higher electron mobility and 12 times higher specific detectivity (1.3 × 1013 jones) compared with the enantiopure systems due to the more aggregated morphologies and larger grains, indicating that chiral composition can be used as a tuning parameter in optoelectronic devices. Our systematic study provides a feasible and effective method for producing high-performance n-type OPTs under ambient conditions.

Chiral organic optoelectronics using circularly polarized light (CPL) as the key element in the photonic signal has recently emerged as a next-generation photonic technology. However, it remains challenging to simultaneously achieve high polarization selectivity and superior optoelectronic performance. Supramolecular two-dimensional (2D) chiral organic single crystals may be good candidates for this purpose due to their defect-free nature, molecular diversity, and morphologies. Here, quasi-2D single crystals of chiral perylene diimides with parallelogram and triangle/hexagon morphologies have been selectively fabricated

Two-dimensional layered transition metal dichalcogenides (TMDs) have been investigated intensively as next-generation semiconducting materials. However, conventional TMD-based devices exhibit large contact resistance at the interface between the TMD and the metal electrode because of Fermi level pinning and the Schottky barrier, which results in poor charge injection. Here, we present enhanced charge transport characteristics in molybdenum diselenide (MoSe

The understanding of the two-phase flow phenomena in porous media is a critical issue for debris bed coolability assessment in severe nuclear power plant accidents. Hence, two-phase-flow friction force modeling has been extensively attempted, based on pressure-loss measurement data. However, most previous two-phase flow pressure loss experiments were conducted at a restricted void-fraction range up to approximately 0.6, which is considerably lower than the dryout condition. Therefore, in this research, two-phase flow tests in packed particle beds are conducted to gather two-phase pressure loss data in the high-void-fraction region. The test beds are constructed by packing mm-scale particles, 3–7 mm in diameter. The experimental data are analyzed and compared with the predicted values of the previous two-phase friction models in term of not only the pressure gradient but also the interfacial friction force between the liquid and gas phases. As there was no valid data for the high-void-fraction region during the previous model development processes, an appropriate model for the interfacial friction force at void fractions greater than approximately 0.6 is unavailable. However, the interfacial friction force plays a significant role in the coolability assessment of the ex-vessel debris bed because it determines the water ingression rate under a cocurrent flow condition. The obtained experimental results in this research indicate the necessity to further improve the model in order to decrease the uncertainty in severe accident risk quantification.

Soft electronic materials are key elements for realizing wearable, attachable, and stick-on electronics. The development of deformable and stretchable electrodes is a key research area as they are one of the most important components for soft electronic devices. Recently, significant progress in the development of deformable and stretchable electrodes has been achieved with organic materials offering electrical tunability, simple mechanical implementation, and desirable chemical and optical properties. In this review, we present recent progress in the design of stretchable electronics based on deformable conducting materials, including their fabrication and conductivity properties and the methods that are employed to enhance performance. In addition, we review the development status of organic- and carbon- based conductive materials and their hybrid composites being used for electronic applications, including carbon nanotubes, graphene, metal composites, conductive polymers, hybrid composites, and ion gel composites. The structural aspects, such as wavy or mesh configurations, of stretchable electrodes and other high-performance conducting materials are investigated intensively. Many stretchable electrodes show great potential for use in future electronics such as electronic skin (e-skin) and stretchable displays, which require reversible deformation and a high degree of operational stability.

Multifunctional hydrogels with properties including transparency, flexibility, self-healing, and high electrical conductivity have attracted great attention for their potential application to soft electronic devices. The presence of an ionic species can make hydrogels conductive in nature. However, the conductivity of hydrogels is often influenced by temperature, due to the change of the internal nano/microscopic structure when temperature reaches the sol-gel phase transition temperature. In this regard, by introducing a novel surface-capacitive sensor device based on polymers with lower critical solution temperature (LCST) behavior, near-perfect stimulus discriminability of touch and temperature may be realized. Here, we demonstrate a multimodal sensor that can monitor the location of touch points and temperature simultaneously, using poly(N-isopropylacrylamide) (PNIPAAm) in hybrid poly(vinyl alcohol) (PVA) and sodium tetraborate decahydrate cross-linked hydrogels doped with poly(sodium acrylate) (SA) [w/w/w = 5:2.7:1-3]. This multimodal sensor exhibits a response time of 0.3 s and a temperature coefficient of resistance of -0.58% K-1 from 20 to 40 °C. In addition, the LCST behavior of PNIPAAm-incorporated PVA/SA gels is investigated. Incorporation of LCST polymers into high-end hydrogel systems may contribute to the development of temperature-dependent soft electronics that can be applied in smart windows.

Textile-based electronic devices should be not only bendable but also highly stretchable for human-friendly wearable electronic applications. Herein, a highly stretchable and mechanically durable fiber transistor was designed and prepared by combining all-stretchable electronic components. First, electrically conductive and stretchable electrodes were fabricated by simple prestraining-then-buckling of graphene/silver hybrid fibers on a highly elastic thermoplastic polyurethane (TPU) monofilament, which maintained a low resistivity (∼30 Ω cm) at a high stretching strain (∼70%) and under many stretch/release cycles (∼1,000). A highly stretchable active channel was prepared by directly blending semiconducting poly(3-hexylthiophene) (P3HT) and viscoelastic TPU without using pre-grown P3HT nanofibrils. The P3HT/TPU blend (5/5 w/w blend ratio) active layer possessed reasonably high mobility (1.46 × 10−3 cm2 V−1 s−1) and adequate mechanical strength (21 MPa) with a high elongation strain (∼120%). In addition, a stretchable dielectric layer and gate electrode were prepared using an elastic ion-gel film and a liquid metal composite, respectively. The assembled fiber transistor exhibited excellent stretchability (∼50%) while maintaining good electrical properties (average charge carrier mobility of 1.74 cm2 V−1 s−1, on/off current ratio of 104) and showed outstanding electrical stability up to 1,000 cycles of stretch/release testing. To the best of our knowledge, these superior stretchability and stability have not been reported elsewhere in the area of fiber-type transistors. We believe that our work can serve as an important step toward the development of core components in wearable devices, such as wearable displays, computers, and biomedical sensors.

Hybrid photovoltaics (HPVs) incorporating both organic and inorganic semiconducting materials have attracted much attention as next-generation photovoltaics because of their advantage of combining both materials. The hybridization of ZnO nanowires (NWs) and organic semiconductors is expected to be a suitable approach to overcome the limited exciton diffusion length and low electron mobility associated with current organic photovoltaics. The use of ZnO NWs allows researchers to tune nanoscale dimensions more precisely and to achieve rod-to-rod spacing below 10 nm. However, the perfect incorporation of organic semiconductors into densely packed ZnO NW arrays has yet to be achieved. In this study, we report the fabrication of ZnO NW arrays and various organic heterojunction-based HPVs using the feasible and effective vacuum-assisted double coating (VADC) method, achieving full coverage of the organic semiconductors on the compact ZnO NW arrays. The newly proposed VADC method ensures perfect infiltration and full coverage of the organic semiconductors on the densely packed NW arrays. Compared with the conventional single spin-coating process, the use of the VADC method led to 11 and 14% increases in the power conversion efficiency of P3HT:PCBM- and PBDTTT-C-T:PC71BM-based HPVs, respectively. Our studies provide a feasible method for the fabrication of efficient HPVs.

Chiral self-sorting has great potential for constructing new complex structures and determining chirality-dependent properties in multicomponent mixtures. However, it is still of great challenge to achieve high fidelity chiral self-discrimination. Besides, the researches on the coordination polymers or metal-organic frameworks for micro/nanooptoelectronics are still rare due to their low conductivity and difficulty in developing a rapid and simple scale-up synthetic method. Here, heterochiral supramolecular coordination networks (SCNs) were synthesized by the solvothermal reaction of naphthalene diimide enantiomers and cadmium iodide, using the chirality as a synthetic tuning parameter to control the morphologies. Intriguingly, heterochiral micro/nanocrystals exhibited photochromic and photodetecting properties. Furthermore, we also developed a simple and efficient doping method to enhance the conductivity and photoresponsivity of micro/nanocrystals using hydrazine. From experimental and theoretical studies, the mechanism was suggested as follows: the radicals in the singly occupied molecular orbital level of the ligands provide charge carriers that can undergo "through-space" transport between π-π stacked ligands and the electron transfer from adsorbed hydrazine to the SCNs results in reduction of energy gap, leading to increased conductivity. Our findings demonstrate a simple and powerful strategy for implementing coordination networks with redox ligands for micro/nanooptoelectronic applications.

Herein, we report the effects of chalcogen atoms on organic field-effect transistors (OFETs) by studying a series of furan-flanked diketopyrrolopyrrole copolymers (PFDPPF-Si, PFDPPT-Si, and PFDPPS-Si) with different chalcogenophene comonomers (furan, thiophene, and selenophene), where the siloxane-terminated chains are used as solubilizing groups. The optical/electrochemical properties, microstructural analyses, and charge transport characteristics of the resulting copolymers are discussed as a function of the different chalcogenophenes. A larger heteroatom size led to a bathochromic shift in the absorption profiles and narrowing of the band gaps. Besides, as the heteroatomic size increased from O to S and Se, the degree of intramolecular steric hindrance observed in the density functional theory calculations followed the trend of deviating from the coplanar backbone conformations. Consequently, the morphological and crystalline features of PFDPPF-Si were shown to be pronounced with a highly crystalline structure and dense π–π stacking, which led to the best hole mobility of 2.48 cm2 V−1 s−1. In addition, we demonstrated a complementary-like inverter based on PFDPPS-Si with a high gain of ∼54. The systematic characterization of heteroaromatics enables the design of conjugated copolymers, which provides a clear understanding of their structure–property behaviors on OFET characteristics.

Molecules capable of producing zero-field circularly polarized phosphorescence (CPP) are highly valuable for chiroptoelectronic applications that rely on triplet exciton. However, the paucity of tractable molecular design rules for obtaining CPP emission has inhibited full utilization. We report amplification of CPP by the formation of helical co-assemblies consisting of achiral square planar cycloplatinated complexes and small fractions of homochiral cycloplatinated complexes. The latter has a unique Pfeiffer effect during the formation of superhelical co-assemblies, enabling versatile chiroptical control. Large dissymmetry factors in electronic absorption (gabs, 0.020) and phosphorescence emission (glum, 0.064) are observed from the co-assemblies. These values are two orders of magnitude improved relative to those of individual molecules. In addition, photoluminescence quantum yields (PLQY) also increase by a factor of ten. Our structural, photophysical, and quantum chemical investigations reveal that the chiroptical amplification is attributable to utilization of both the magnetically allowed electronic transition and asymmetric coupling of excitons. The strategy overcomes the trade-off between glum and PLQY which has frequently been found for previous molecular emitters of circularly polarized luminescence. It is anticipated that our study will provide new insight into the future research for the exploitation of the full potential of CPP.

Supramolecular chirality has drawn a great deal of attention due to promising applications in chiral recognition, sensing, catalysis, and device design. Here, we studied the influence of the side chains of chiral perylene diimides (PDIs) on the supramolecular chirality and optoelectronic performance. PDIs with various N-substituted alkyl chains were synthesized and self-assembled into one-dimensional nanostructures. PDI nanowires (NWs) with less bulky alkyl chains (CPDI-Cy) exhibited the maximum electron mobility of 0.67 cm2 V−1 s−1 and showed better optoelectronic performance than those with bulkier substituents (CPDI-Naph), with three orders of magnitude improvements in photoresponsivity, photosensitivity, external quantum efficiency, and detectivity (D*). Particularly, the D* of CPDI-Cy NWs was two or three orders of magnitude higher than those of other chiral PDI nanomaterials and PDI-based polymers, and is one of the highest among chiral organic semiconductors. Judging from density functional theory calculations and molecular dynamics simulations, the higher optoelectronic performance of CPDI-Cy NWs mainly originated from their denser packing and favorable molecular arrangements for charge transport. These findings demonstrate the impact of side chain engineering on tuning the self-assembly process of PDI molecules, supramolecular chirality, and their consequent device performance.

Soft electronic devices are key components of flexible or wearable smart electronics. Chemical sensors based on organic field-effect transistors (OFETs) show great promise because of their high sensitivity, low cost, fast response, flexible applications, and signal amplification via gate voltage tuning. In addition, the sensor response can be made highly selective by incorporating specific functional groups or recognition elements. The response magnitude of an OFET sensor depends strongly on the surface area of the active layer that interacts with the analyte. In this study, two strategies including nanostructuring and surface functionalization have been explored to enhance the performance and sensitivity of OFET sensors. We describe the fabrication of highly flexible, electrospun poly(3,3-didodecylquaterthiophene)/calix[8]arene (PQT-12)/C[8]A sensors employing semiconducting polymer nanofibers (NFs) as the active layer. The resulting bendable and flexible PQT-12/C[8]A OFET sensors show high sensitivity to ethanol (192%) and toluene (229%) and moderate sensitivity to n-hexane (121%) with a minimum detectable level of 10 ppm. In addition, the PQT-12/C[8]A NF sensors are electrically stable under typical operating conditions with a bending radius down to 0.5 mm. This work demonstrates a viable approach for the fabrication of flexible OFETs for use as wearable sensors and health-monitoring devices.

In addition to attaining strong charge transport capability, field-effect transistors based on conjugated polymer thin films should be processed using non-chlorinated solvents for device fabrication, to avoid adverse impacts on human health and the environment. Although many high-performance diketopyrrolopyrrole (DPP) based conjugated polymers have been studied, few DPP based polymers that can be processed in non-chlorinated solvents have been reported to date. In this work, ambipolar polymeric semiconductors based on a DPP backbone that can be solution-processed using non-chlorinated solvents are reported. The DPP backbone combined with asymmetric conjugated segments results in high solubility and processability in non-chlorinated solvents, such as toluene and o-xylene, without disruption in the conjugation and molecular ordering of the main backbone. The polymer films fabricated with a high-boiling-point additive show ambipolar charge transport characteristics and highly enhanced electrical performance due to greatly enhanced intermolecular interactions and the resulting aggregated nanostructures. The combination of non-chlorinated solution processing with the green additive, along with ambipolar characteristics, enables high-performance environmentally friendly complementary logic circuit integration to be achieved.

Organic neuromorphic devices and sensors that mimic the functions of chemical synapses and sensory perception in humans have received much attention for next-generation computing and integrated logic circuits. Despite recent advances, organic artificial synapses capable of detecting both neurotransmitters in liquid environments and light are not reported. Herein, inspired by hippocampal synapses, a dual-gate organic synaptic transistor platform with a photoconductive polymer semiconductor, a ferroelectric insulator of P(VDF-TrFE), and an extended-gate electrode functionalized with boronic acid is developed to simultaneously detect the neurotransmitter dopamine and light. The developed synaptic transistor enables memory consolidation upon repetitive exposure to dopamine and polychromatic light, exhibiting effectively modulated postsynaptic currents. This proof-of-concept hippocampal-synapse-mimetic organic neuromorphic system combining a chemical sensor and a photosensor opens new possibilities for developing low-power organic artificial synaptic multisensors and light-induced memory consolidative artificial synapses, and can also contribute to the development of human-machine interfaces.

Despite great challenges, the development of new molecular structures with multiple and even conflicting characteristics are eagerly pursued for exploring advanced applications. To develop high-performance chiral organic semiconducting molecules, a distorted π-system is required for strong coupling with circularly polarized light (CPL), whereas planar π-stacking systems are necessary for high charge-carrier mobility. To address this dilemma, in this work, we introduce a skeleton merging approach through distortion of a perylene diimide (PDI) core with four fused heteroaromatics to form an ortho-π-extended PDI double-[7]heterohelicene. PDI double helicene inherits a high dissymmetry factor from the helicene skeleton, and the extended π-planar system concurrently maintains a high level of charge transport properties. In addition, ortho-π-extension of the PDI skeleton brings about near-infrared (NIR) light absorption and ambipolar charge transport abilities, endowing the corresponding organic phototransistors with high photoresponsivity of 450 and 120 mA W−1 in p- and n-type modes respectively, along with a high external quantum efficiency (89%) under NIR light irradiations. Remarkably, these multiple characteristics enable high-performance broadband CPL detections up to NIR spectral region with chiral organic semiconductors., In organic semiconducting molecules materials, distorted π-systems enable strong coupling with circular polarized light while planar π-stacking systems are necessary for high charge-carrier mobility. Here, the authors address this dilemma by introducing a skeleton merging approach through distortion of a perylene diimide core with four fused heteroaromatics to form a π-extended double helicene.

Hybrid materials in optoelectronic devices can provide synergistic effects that complementarily enhance the properties of each component. Here, flexible high-performance graphene hybrid photodetectors (PDs) are developed by introducing gold nanostars (GNSs) and perovskites for strong light trapping with hot electron transfer and efficient light harvesting characteristics, respectively. While pristine graphene PDs do not exhibit discernible photodetection properties due to the very low photon absorption and ultrafast charge carrier recombination, graphene PDs functionalized with GNSs and a densely covered perovskite layer exhibit outstanding photoresponsive properties with a photoresponsivity (R) of 5.90 × 104 A W−1 and a specific detectivity of 1.31 × 1013 Jones, the highest values among those reported for perovskite-functionalized graphene PDs thus far. Moreover, we fabricated a flexible 10 × 10 PD array that shows well-resolved spatiotemporal mapping of light signals with excellent operational and mechanical stabilities at a bending radius down to 3 mm and in repeated bending tests for over 1000 cycles. Comprehensive analyses using finite-difference time-domain (FDTD) theoretical calculations, scanning near-field optical microscopy, and photoluminescence mapping reveal the effective light trapping effect of GNSs and the charge carrier transfer between the perovskite and graphene. This work provides a new design platform for flexible and high-performance photodetection systems. Flexible photodetectors that convert light into electric signals for applications including low-light photography can be improved using star-shaped flecks of gold. Single-atom thin graphene sheets have proven useful in many bendable photodetectors due to their favorable mechanical and electronic properties. South Korean researchers led by Hyunhyub Ko from the Ulsan National Institute of Science and Technology and Joon Hak Oh at Seoul National University have now developed a technique to improve graphene’s response to light. By tweaking a typical nanoparticle synthesis method using a sulfur-based buffering agent, the team produced gold nanostars that trap light using localized field-enhanced regions around their spiky tips. Prototype devices, prepared by putting nanostar-coated graphene electrodes onto plastic substrates, could be flexed thousands of times while retaining sufficient light sensitivity for imaging purposes. Flexible high-performance hybrid graphene photodetectors have been developed by introducing both perovskite materials and gold nanostars. The developed photodetectors exhibit significantly enhanced optoelectronic performances due to the synergistic effects of hybrid system including high light absorption of perovskites and strong plasmonic effects of gold nanostars with high mechanical stability, which extends the range of their practical applications.

Bioelectronics for healthcare that monitor the health information on users in real time have stepped into the limelight as crucial electronic devices for the future due to the increased demand for "point-of-care" testing, which is defined as medical diagnostic testing at the time and place of patient care. In contrast to traditional diagnostic testing, which is generally conducted at medical institutions with diagnostic instruments and requires a long time for specimen analysis, point-of-care testing can be accomplished personally at the bedside, and health information on users can be monitored in real time. Advances in materials science and device technology have enabled next-generation electronics, including flexible, stretchable, and biocompatible electronic devices, bringing the commercialization of personalized healthcare devices increasingly within reach, e.g., wearable bioelectronics attached to the body that monitor the health information on users in real time. Additionally, the monitoring of harmful factors in the environment surrounding the user, such as air pollutants, chemicals, and ultraviolet light, is also important for health maintenance because such factors can have short- and long-term detrimental effects on the human body. The precise detection of chemical species from both the human body and the surrounding environment is crucial for personal health care because of the abundant information that such factors can provide when determining a person's health condition. In this respect, sensor applications based on an organic-transistor platform have various advantages, including signal amplification, molecular design capability, low cost, and mechanical robustness (e.g., flexibility and stretchability). This Account covers recent progress in organic transistor-based chemical sensors that detect various chemical species in the human body or the surrounding environment, which will be the core elements of wearable electronic devices. There has been considerable effort to develop high-performance chemical sensors based on organic-transistor platforms through material design and device engineering. Various experimental approaches have been adopted to develop chemical sensors with high sensitivity, selectivity, and stability, including the synthesis of new materials, structural engineering, surface functionalization, and device engineering. In this Account, we first provide a brief introduction to the operating principles of transistor-based chemical sensors. Then we summarize the progress in the fabrication of transistor-based chemical sensors that detect chemical species from the human body (e.g., molecules in sweat, saliva, urine, tears, etc.). We then highlight examples of chemical sensors for detecting harmful chemicals in the environment surrounding the user (e.g., nitrogen oxides, sulfur dioxide, volatile organic compounds, liquid-phase organic solvents, and heavy metal ions). Finally, we conclude this Account with a perspective on the wearable bioelectronics, especially focusing on organic electronic materials and devices.

Improving the charge-carrier mobility of conjugated polymers is important for developing high-performance, solution-processed optoelectronic devices. Although [1]benzothieno[3,2-b]benzothiophene (BTBT) has been frequently used as a high-performance p-type small molecular semiconductor and employed a few times as a building block for p-type conjugated polymers, it has never been explored as a donor moiety for high-performance n-type conjugated polymers. Here, BTBT has been conjugated with either n-type perylene diimide (PDI) or naphthalene diimide (NDI) units to generate a donor–acceptor copolymer backbone, for the first time. Charge-transport measurements of organic field-effect transistors show n-type dominant behaviors, with the electron mobility reaching ∼0.11 cm2 V–1 s–1 for PDI–BTBT and ∼0.050 cm2 V–1 s–1 for NDI–BTBT. The PDI–BTBT mobility value is one of the highest among the PDI-containing polymers. The high π–π stacking propensity of BTBT significantly improves the charge-carrier mobility in thes...

Polymer-based pressure sensors play a key role in realizing lightweight and inexpensive wearable devices for healthcare and environmental monitoring systems. Here, conductive core/shell polymer nanofibers composed of poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP)/poly(3,4-ethylenedioxythiophene) (PEDOT) are fabricated using three-dimensional (3D) electrospinning and vapor deposition polymerization methods, and the resulting sponge-like 3D membranes are used to create piezoresistive-type pressure sensors. Interestingly, the PEDOT shell consists of well-dispersed spherical bumps, leading to the formation of a hierarchical conductive surface that enhances the sensitivity to external pressure. The sponge-like 3D mats exhibit a much higher pressure sensitivity than the conventional electrospun 2D mats due to their enhanced porosity and pressure-tunable contact area. Furthermore, large-area, wireless, 16 × 10 multiarray pressure sensors for the spatiotemporal mapping of multiple pressure points and wearable bands for monitoring blood pressure have been fabricated from these 3D mats. To the best of our knowledge, this is the first report of the fabrication of electrospun 3D membranes with nanoscopically engineered fibers that can detect changes in external pressure with high sensitivity. The developed method opens a new route to the mass production of polymer-based pressure sensors with high mechanical durability, which creates additional possibilities for the development of human–machine interfaces. A flexible material for creating wearable pressure sensors has been manufactured by scientists in Korea using a simple and cost-effective nanofiber-spinning technique. Wearable electronics require pliable materials that also conduct electricity. Joon Hak Oh and co-workers from Pohang University of Science and Technology (POSTECH), South Korea, developed a pressure-sensitive material using electrospinning and vapor deposition polymerization. Conventionally, electrospinning forces a polymer solution through a narrow nozzle to create a two-dimensional network of fibers. These layers are then stacked to create a three-dimensional structure, but the small gaps between layers makes it less sensitive to forces applied from the top. The researchers manufactured a sponge-like three-dimensional nanofiber mat, which they coated with a spherical bump-shape conducting polymer. This three-dimensional membrane was nearly three times more sensitive to pressure changes than structures prepared using two-dimensional electrospinning. PVDF-HFP/PEDOT core/shell nanofibers with well-dispersed spherical bumps have been prepared using 3D electrospinning and vapor deposition polymerization methods, and their sponge-like 3D membranes are used to create piezoresistive-type pressure sensors. Owing to the enhanced porosity and pressure-tunable contact area, the sensors exhibit outstanding mechanoelectrical performance. Furthermore, large-area, wireless, 16 × 10 multi-array pressure sensors for spatiotemporal mapping of multiple pressing points and wearable bands for monitoring blood pressure have been fabricated from these 3D mats. The developed method opens up a new route for the mass production of polymer-based sensors, which is promising for biomedical diagnostics and environmental monitoring systems.

Emulating essential synaptic working principles using a single electronic device has been an important research field in recent years. However, achieving sensitivity and energy consumption comparable to biological synapses in these electronic devices is still a difficult challenge. Here, we report the fabrication of conjugated polyelectrolyte (CPE)-based artificial synapse, which emulates important synaptic functions such as paired-pulse facilitation (PPF), spike-timing dependent plasticity (STDP) and spiking rate dependent plasticity (SRDP). The device exhibits superior sensitivity to external stimuli andlow-energy consumption. Ultrahigh sensitivity and low-energy consumption are key requirements for building up brain-inspired artificial systems and efficient electronic-biological interface. The excellent synaptic performance originated from (i) a hybrid working mechanism that ensured the realization of both short-term and long-term plasticity in the same device, and (ii) the mobile-ion rich CPE thin film that mediate migration of abundant ions analogous to a synaptic cleft. Development of this type of artificial synapse is both scientifically and technologically important for construction of ultrasensitive highly-energy efficient and soft neuromorphic electronics.

Despite having unique structural features, e.g., high co-planarity and a strong polar bicyclic lactam structure, thiophene bipyrrolylidene-2,2′(1H,1′H)-dione (TBPD) has been less explored as a dye, mainly due to the quite low yield in its synthesis via lactone-to-lactam conversion. We reported an efficient methodology for synthesizing TBPD in high yield using p-toluenesulfonic acid monohydrate and a catalytic amount of 4-dimethylaminopyridine in the chloroform solvent. From the newly synthesized series of TBPD-based donor–acceptor-type polymers, we fabricated organic field-effect transistors (OFETs), which were subjected to a systematic study on the relationship between film microstructure and charge transport. Among them, the annealed PTBPD-Th film revealed a more ordered lamellar packing with the highest number of interlayers and preferential edge-on orientation, yielding the best hole mobility (up to 0.46 cm2 V−1 s−1). The improved synthesis of TBPD and our findings concerning related polymers could promote further research and development associated with the TBPD unit.

Health self-monitoring systems that can automatically recognize health information and respond in real-time through wearable sensors and artificial intelligence are one of the major development issues of the fourth industrial revolution. To realize such health self-monitoring systems, it is essential to develop wearable sensors that can detect biophysical signals from the human body. Organic sensors based on organic semiconductors or conductive materials are receiving a lot of attention as wearable sensors for health self-monitoring systems due to the advantages of their flexibility, stretchability, low cost, and light weight. Here, we review recent developments in organic sensors for health monitoring and methods for improving their performance.

Air-stable and heating-free doping processes are highly desirable for realizing advanced flexible electronics on plastic substrates. A synthetic protocol for a high-performance n-type organic dopant using an organic cationic dye, Pyronin B, via a simple chemical reduction method with NaBH4 in various organic non-polar solvents is developed, in which the cationic dye Pyronin B is transformed into its leuco form. The reduced Pyronin B (rPyB, leuco form) spontaneously interacts with carbon nanomaterials, such as graphene and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), via hydride-mediated electron transfer doping, which is confirmed by electron spin resonance (ESR) analysis. After coating rPyB onto graphene, the non-polar solvents are removed by simple evacuation (∼1 × 10−3 Torr) without an annealing step. The work function of graphene is reduced from 4.52 eV to 3.98 eV after doping. Multiple coatings of rPyB on graphene field-effect transistors (FETs) result in air stability for 90 days under ambient conditions. Selective n-type doping using a polydimethylsiloxane (PDMS) stamp and rPyB has also been performed on a large-area 16 × 16 graphene FET array. Our findings demonstrate a viable methodology for the cost-effective synthesis of an n-type dopant and its application in soft electronics.

The poor oxidative stability of tin-based perovskites has been an obstacle to their widespread implementation in high-performance solar cells. Herein, quasi-two-dimensional (quasi-2D) tin-based perovskite solar cells are fabricated with significantly improved performance and stability, by introducing an additional formamidinium thiocyanate (FASCN) additive into quasi-2D tin-based perovskites. The incorporation of the FASCN additive greatly prevents quasi-2D tin-based perovskites from oxidation during film formation, through strong chemical interactions with the tin component (Sn2+). Moreover, it results in a coarser perovskite grain and a higher degree of crystallinity in the out-of-plane direction, leading to enhanced optoelectronic performance of quasi-2D tin-based perovskites. The best-performing tin-based perovskite solar cell shows an efficiency of 8.17% under reverse scan with a steady-state efficiency of 7.84% at a maximum power point (MPP), while retaining over 90% of its initial efficiency after 1000 hours in a glovebox filled with nitrogen. These results demonstrate a versatile, yet simple methodology that can be applied to other lead-free perovskites suffering from poor oxidative stability.

Summary With the advent of the Internet of Things (IoT) era, flexible sensors are regarded as one of the most important technologies for the development of human-friendly wearable devices. Organic field-effect transistors (OFETs) based on conjugated polymers or small molecules are promising sensor platforms because they have various advantages, including high sensitivity, mechanical flexibility, and low-cost fabrication processes. OFET-based sensors enable continuous monitoring of external stimuli or target analytes with superior detection capabilities. This review describes the working principles and sensing mechanisms of various OFET-based sensors, including chemical, biological, photo, pressure, and temperature sensors, and introduces the recent progress in this field. In addition, the technical challenges and future outlook of OFET-based sensors for next-generation flexible electronics are briefly discussed.