such as light or pressure as inputs. [7,[9][10][11][12][13] Little effort has been made to develop organic artificial synapses that can detect chemicals, especially analytes in a liquid medium.Among the platforms of organic synapses, electrolyte-based organic synaptic transistors have been widely studied because of their high biocompatibility required for future applications of artificial sensory synaptic system from in vitro to in vivo. [14][15][16][17] In order to optimize the synaptic performance and expand the functionality of electrolyte-based synaptic transistors, state-of-the-art organic artificial synapse studies are attempting to implement two or more types of synapse-driving mechanisms in a single device or devise new synaptic device structure. For example, a ferroelectric/electrochemical synapse successfully supplemented conventional electrochemical transistors with longer persistent nonvolatile plasticity and unique threshold switching properties. [18] Also, parallel programming of organic synapses has been demonstrated by connecting a gate terminal of an organic redox transistor to a diffusive memristor. [19] Therefore, in order to expand functionality of various organic synapses, it is necessary to explore organic synaptic transistors with various operation mechanisms and device structures and, as a result, combine their advantageous characteristics.In a biological synapse, which is a principal emulation target of neuromorphic devices, excitation or inhibition occurs via the release or reception of neurotransmitters. Therefore, it is of great benefit to develop artificial sensory synaptic system which recognizes neurotransmitter signals and modulates postsynaptic currents including potentiation/depression, paired-pulse facilitation, and Hebbian learning. [20][21][22] Among various neurotransmitters, dopamine plays an important role in learning and memory consolidating functions of hippocampal memory, affecting plasticity, synaptic transmission, and network activity in the hippocampal circuitry. [23][24][25] Recently, a biohybrid synapse with dopaminergic PC-12 cells coupled to an organic neuromorphic device has been reported to enable dopamine signaling in artificial synapses. [26] This approach was based on continuous exposure of both sensing and signal processing parts to the wet environment, cell incubation, and immobilizing process. Thus, it will also be interesting to develop a dopaminergic signaling 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 d...

Because of the limited availability of synthetic strategies and strong acceptor units, constructing new types of low-band-gap donor–acceptor-type copolymers for use in multiple functional applications remains a big challenge. Herein, we report the synthesis, characterization, and optoelectronic applications (i.e., organic field-effect transistors (OFETs) and organic phototransistors (OPTs)) of a novel class of ultralow-band-gap copolymers (PDAP–Fu, PDAP–Th, and PDAP–Se) on the basis of the unique, interesting, yet rarely researched bicyclic 2,5-diazapentalene (DAP) strong acceptor in conjugation with chalcogenophene donors (furan (Fu), thiophene (Th), or selenophene (Se)). All of the copolymers exhibit broad near-infrared (NIR) absorption and optical band gaps as low as ∼1.0 eV. The effects of the actual chalcogen atoms on the geometry, optical properties, energy levels, and film organization are carefully determined for OFET and OPT applications. Regarding the OFET studies, all of the copolymers show unipolar transport behavior in bottom-gate and top-contact OFETs, and PDAP–Se exhibits the highest hole mobility of 4.76 × 10–1 cm2 V–1 s–1. Besides, investigations of the OPTs indicate that a high photoresponse is achieved for all of the copolymers at a wavelength of 1060 nm in the NIR spectral region combined with an excellent external quantum efficiency (η) and photodetectivity (D*). This is particularly true for PDAP–Se (η = 6.56 × 104% and D* = 1.80 × 1012 Jones). Thus, such ultralow-band-gap copolymers are promising candidates for use in integrated circuits and optoelectronic devices.

A series of highly soluble copolymers (EH4P-Th, EH4P-Se, EH4P-TT, and EH4P-BT) based on phosphonate chain-end functionalized diketopyrrolopyrrole monomer and four different counterpart comonomers with varied electron-donating strength and conjugation length have been synthesized, characterized, and used in p-channel organic field-effect transistors (OFETs). It was found that introducing different counterpart comonomers into the main backbone alters the copolymers’ intrinsic properties, including absorption, frontier energy levels, molecular microstructure, and charge transport in OFETs. In OFETs fabricated on n-octadecyl­trimethoxysilane (OTS)-treated silicon (Si)/silicon dioxide (SiO2) surfaces, the copolymers exhibit good hole transport with maximum hole mobility (μh) of 1.46 × 10–1 cm2 V–1 s–1 in EH4P-TT, which is attributed to edge-on packing, fibrillar intercalating networks, and large crystalline π-stacking. More intriguing is the fact that high solubility and polarity of the resulting copolymers are induced via polar and bulky phosphonate chain-end groups, allowing for proper OFET operation using not only OTS-untreated Si/SiO2 substrates but also an eco-friendly 2-methyltetra­hydrofuran solution process. These results demonstrate promising applications of phosphonate chain-end groups in the design of conjugated polymers for various purposes.

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.

The effects of film thickness on the dipolar relaxation of ferroelectric PbTiO3 thin films were investigated in the microwave-frequency range. The real and imaginary dielectric constants (ε′–iε″) were measured up to 30GHz using interdigital capacitors on high-quality SiO2. As the polycrystalline PbTiO3 film thickness increased from 42 to 407nm, the dipolar-relaxation frequency reduced with increasing grain size. The observed relaxation behavior for ε′–iε″ was explained in terms of the convolution of Debye relaxation. The relaxation frequency in the thin films was higher than the previous values reported in bulk PbTiO3, due to the smaller grain size of the thin films.