A Hippocampus-Inspired Dual-Gated Organic Artificial Synapse for Simultaneous Sensing of a Neurotransmitter and Light.

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.

Understanding of Fluorination Dependence on Electron Mobility and Stability of Naphthalenediimide-Based Polymer Transistors in Environment with 100% Relative Humidity.

A family of copolymers (P(NDIOD-T2Fx)) based on naphthalenediimide (NDI) and 2,2'-bithiophene (T2) units with different amounts of 3,3'-difluoro-2,2'-bithiophene (T2F) decoration were synthesized, characterized, and used in n-type organic field-effect transistors (OFETs). With increasing T2F content in the backbone, we observe increased melting and crystallization transitions, blue-shifted absorptions, and deeper-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels, together with improved hydrophobicity. The highest electron mobility of 4.48 × 10-1 cm2 V-1 s-1 was obtained for P(NDIOD-T2F0) without a T2F unit, which is attributed to the larger domain grains and crystallites, as well as a more tightly packed and oriented crystalline structure, as evidenced from the morphological study. In contrast, P(NDIOD-T2F100) with the highest T2F content has superior air stability, showing greater than 25% electron mobility retention after 30 days in wet conditions of 100% relative humidity without encapsulation. Even P(NDIOD-T2F100) is able to operate normally after 30 min of immersion in water, which is due to the synergistic contributions from the deep HOMO/LUMO levels and improved hydrophobicity. This study advances our fundamental understanding of how the morphology/crystallinity, device performance, and device stability of n-type copolymers are tuned by incorporating different concentrations of T2F in the backbone, shedding light on an important modification for air- and water-stable n-type materials for future OFET applications.

Organic Transistor-Based Chemical Sensors for Wearable Bioelectronics.

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.

Toward Environmentally Robust Organic Electronics: Approaches and Applications.

Recent interest in flexible electronics has led to a paradigm shift in consumer electronics, and the emergent development of stretchable and wearable electronics is opening a new spectrum of ubiquitous applications for electronics. Organic electronic materials, such as π-conjugated small molecules and polymers, are highly suitable for use in low-cost wearable electronic devices, and their charge-carrier mobilities have now exceeded that of amorphous silicon. However, their commercialization is minimal, mainly because of weaknesses in terms of operational stability, long-term stability under ambient conditions, and chemical stability related to fabrication processes. Recently, however, many attempts have been made to overcome such instabilities of organic electronic materials. Here, an overview is provided of the strategies developed for environmentally robust organic electronics to overcome the detrimental effects of various critical factors such as oxygen, water, chemicals, heat, and light. Additionally, molecular design approaches to π-conjugated small molecules and polymers that are highly stable under ambient and harsh conditions are explored; such materials will circumvent the need for encapsulation and provide a greater degree of freedom using simple solution-based device-fabrication techniques. Applications that are made possible through these strategies are highlighted.

High-Performance Furan-Containing Conjugated Polymer for Environmentally Benign Solution Processing.

Developing semiconducting polymers that exhibit both strong charge transport capability via highly ordered structures and good processability in environmentally benign solvents remains a challenge. Given that furan-based materials have better solubility in various solvents than analogous thiophene-based materials, we have synthesized and characterized furanyl-diketopyrrolopyrrole polymer (PFDPPTT-Si) together with its thienyl-diketopyrrolopyrrole-based analogue (PTDPPTT-Si) to understand subtle changes induced by the use of furan instead of thiophene units. PTDPPTT-Si films processed in common chlorinated solvent exhibit a higher hole mobility (3.57 cm2 V-1 s-1) than PFDPPTT-Si films (2.40 cm2 V-1 s-1) under the same conditions; this greater hole mobility is a result of tightly aggregated π-stacking structures in PTDPPTT-Si. By contrast, because of its enhanced solubility, PFDPPTT-Si using chlorine-free solution processing results in a device with higher mobility (as high as 1.87 cm2 V-1 s-1) compared to that of the corresponding device fabricated using PTDPPTT-Si. This mobility of 1.87 cm2 V-1 s-1 represents the highest performances among furan-containing polymers reported to the best of our knowledge for nonchlorinated solvents. Our study demonstrates an important step toward environmentally compatible electronics, and we expect the results of our study to reinvigorate the furan-containing semiconductors field.

Chemically Robust Ambipolar Organic Transistor Array Directly Patterned by Photolithography.

Organic ambipolar transistor arrays for chemical sensors are prepared on a flexible plastic substrate with a bottom-gate bottom-contact configuration to minimize the damage to the organic semiconductors, for the first time, using a photolithographically patternable polymer semiconductor. Well-balanced ambipolar charge transport is achieved by introducing graphene electrodes because of the reduced contact resistance and energetic barrier for electron transport.

Requirements for Forming Efficient 3-D Charge Transport Pathway in Diketopyrrolopyrrole-Based Copolymers: Film Morphology vs Molecular Packing.

To achieve extremely high planarity and processability simultaneously, we have newly designed and synthesized copolymers composed of donor units of 2,2'-(2,5-dialkoxy-1,4-phenylene)dithieno[3,2-b]thiophene (TT-P-TT) and acceptor units of diketopyrrolopyrrole (DPP). These copolymers consist of a highly planar backbone due to intramolecular interactions. We have systematically investigated the effects of intermolecular interactions by controlling the side chain bulkiness on the polymer thin-film morphologies, packing structures, and charge transport. The thin-film microstructures of the copolymers are found to be critically dependent upon subtle changes in the intermolecular interactions, and charge transport dynamics of the copolymer based field-effect transistors (FETs) has been investigated by in-depth structure-property relationship study. Although the size of the fibrillar structures increases as the bulkiness of the side chains in the copolymer increases, the copolymer with the smallest side chain shows remarkably high charge carrier mobility. Our findings reveal the requirement for forming efficient 3-D charge transport pathway and highlight the importance of the molecular packing and interdomain connectivity, rather than the crystalline domain size. The results obtained herein demonstrate the importance of tailoring the side chain bulkiness and provide new insights into the molecular design for high-performance polymer semiconductors.

Use of heteroaromatic spacers in isoindigo-benzothiadiazole polymers for ambipolar charge transport.

Inspired by the outstanding charge-transport characteristics of poly(isoindigo-alt-benzothiadiazole) (PIIG-BT) in our previous study, herein we present two new polymers (PIIG-DTBT and PIIG-DSeBT) involving IIG and BT blocks constructed using five-membered heteroaromatic spacers such as thiophene (T) and selenophene (Se) and investigate the effects of the spacer groups on the optical, electrochemical, and charge-transport properties. As a consequence of the red-shifts induced by the more extended conjugation and enhanced intramolecular charge transfer (ICT), both PIIG-DTBT and PIIG-DSeBT show smaller bandgaps compared to PIIG-BT. Interestingly, the LUMO energy levels (-3.57 eV) for the two polymers are the same, but the HOMO levels (-5.39 and -5.26 eV for PIIG-DTBT and PIIG-DSeBT, respectively) clearly vary as a function of the structural modification of the spacers. In addition to the changes in their optical properties and energy levels induced by the incorporation of the spacers, ambipolar charge transport behaviors with hole and electron mobilities of up to 7.8 × 10(-2) and 3.4 × 10(-2) cm(2) V(-1) s(-1), respectively, are observed for PIIG-DTBT films with highly ordered lamellar packing. This represents the second example of IIG-based polymers exhibiting ambipolar charge transport in OFETs reported to date.

Acceptor-acceptor type isoindigo-based copolymers for high-performance n-channel field-effect transistors.

Two acceptor-acceptor (A-A) type copolymers (PIIG-BT and PIIG-TPD) with backbones composed exclusively of electron-deficient units are designed and synthesized. Both copolymers show unipolar n-type operations. In particular, PIIG-BT shows electron mobility of up to 0.22 cm(2) V(-1) s(-1). This is a record value for n-type copolymers based on lactam cores.