Stretchable conductors for stretchable field-effect transistors and functional circuits.

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Organic Transistors Based on Highly Crystalline Donor-Acceptor π-Conjugated Polymer of Pentathiophene and Diketopyrrolopyrrole.

Oligomers and polymers consisting of multiple thiophenes are widely used in organic electronics such as organic transistors and sensors because of their strong electron-donating ability. In this study, a solution to the problem of the poor solubility of polythiophene systems was developed. A novel π-conjugated polymer material, PDPP-5Th, was synthesized by adding the electron acceptor unit, DPP, to the polythiophene system with a long alkyl side chain, which facilitated the solution processing of the material for the preparation of devices. Meanwhile, the presence of the multicarbonyl groups within the DPP molecule facilitated donor-acceptor interactions in the internal chain, which further improved the hole-transport properties of the polythiophene-based material. The weak forces present within the molecules that promoted structural coplanarity were analyzed using theoretical simulations. Furthermore, the grazing incidence wide-angle X-ray scanning (GIWAXS) results indicated that PDPP-5Th features high crystallinity, which is favorable for efficient carrier migration within and between polymer chains. The material showed hole transport properties as high as 0.44 cm2 V-1 s-1 in conductivity testing. Our investigations demonstrate the great potential of this polymer material in the field of optoelectronics.

Preparation of Dye Semiconductors via Coupling Polymerization Catalyzed by Two Catalysts and Application to Transistor.

Organic dye semiconductors have received increasing attention as the next generation of semiconductors, and one of their potential applications is as a core component of organic transistors. In this study, two novel diketopyrrolopyrrole (DPP) dye core-based materials were designed and separately prepared using Stille coupling reactions under different palladium catalyst conditions. The molecular weights and elemental compositions were tested to demonstrate that both catalysts could be used to successfully prepare materials of this structure, with the main differences being the weight-average molecular weight and the dispersion index. PDPP-2Py-2Tz I with a longer conjugation length exhibited better thermodynamic stability than the counterpart polymer PDPP-2Py-2Tz II. The intrinsic optical properties of the polymers were relatively similar, while the electrochemical tests showed small differences in their energy levels. The polymers obtained with different catalysts displayed similar and moderate electron mobility in transistor devices, while PDPP-2Py-2Tz I possessed a higher switching ratio. Our study provides a comparison of such dye materials under different catalytic conditions and also demonstrates the great potential of dye materials for optoelectronic applications.

Multi-Stimuli-Responsive Polymer/Inorganic Janus Composite Nanoparticles.

Multi-stimuli-responsive Janus composite nanoparticles (JNPs) of poly(N-isopropylacrylamide)-Fe3O4-poly(2-(dimethylamino)ethyl methacrylate)) (PNIPAM-Fe3O4-PDMEAMA) are synthesized by sequential reversible addition-fragmentation chain-transfer grafting of the polymer PNIPAM and atom-transfer radical polymerization grafting of the polymer PDMEAMA from the corresponding sides of modified Fe3O4 nanoparticles of ∼10 nm size. The hydrophilic/amphiphilic/hydrophobic reversible transition of the JNP can be triggered by pH and temperature since the wettability of the two polymers on the opposite sides is tunable accordingly. At a high pH value and a low surrounding temperature, applying near-infrared irradiation will induce the amphiphilic/hydrophobic transition owing to the photothermal effect of Fe3O4 NPs. The JNP can serve as a responsive solid emulsifier, and the stability and microstructure of the emulsions can be easily controlled by external stimuli such as the pH, temperature, and magnetic field.

Design of Novel Functional Conductive Structures and Preparation of High-Hole-Mobility Polymer Transistors by Green Synthesis Using Acceptor-Donor-Acceptor Strategies.

The design of novel acceptor molecular structures based on classical building blocks is regarded as one of the efficient ways to explore the application of organic conjugated materials in conductivity and electronics. Here, a novel acceptor moiety, thiophene-vinyl-diketopyrrolopyrrole (TVDPP), was envisioned and prepared with a longer conjugation length and a more rigid structure than thiophene-diketopyrrolopyrrole (TDPP). The brominated TVDPP can be sequentially bonded to trimethyltin-containing benzo[c][1,2,5]thiadiazole units via Suzuki polycondensation to efficiently prepare the polymer PTVDPP-BSz, which features high molecular weight and excellent thermal stability. The polymerization process takes only 24 h and eliminates the need for chlorinated organic solvents or toxic tin-based reagents. Density functional theory (DFT) simulations and film morphology analyses verify the planarity and high crystallinity of the material, respectively, which facilitates the achievement of high carrier mobility. Conductivity measurements of the polymeric material in the organic transistor device show a hole mobility of 0.34 cm2 V-1 s-1, which illustrates its potential for functionalized semiconductor applications.

Donor-Acceptor-Based Organic Polymer Semiconductor Materials to Achieve High Hole Mobility in Organic Field-Effect Transistors.

Organic polymer semiconductor materials are conveniently tuned to energy levels because of their good chemically modifiable properties, thus enhancing their carrier transport capabilities. Here, we have designed and prepared a polymer with a donor-acceptor structure and tested its potential as a p-type material for organic field-effect transistor (OFET) applications using a solution-processing method. The conjugated polymers, obtained via the polymerization of the two monomers relying on the Stille coupling reaction, possess extremely high molecular weights and thermodynamic stability. Theoretical-based calculations show that PDPP-2S-Se has superior planarity, which is favorable for carrier transport within the main chain. Photophysical and electrochemical measurements systematically investigated the properties of the material and the energy levels with respect to the theoretical values. The maximum hole mobility of the PDPP-2S-Se-based OFET device is 0.59 cm2 V-1 s-1, which makes it a useful material for potential organic electronics applications.

Rational Design of Novel Conjugated Terpolymers Based on Diketopyrrolopyrrole and Their Applications to Organic Thin-Film Transistors.

Organic polymer semiconductor materials, due to their good chemical modifiability, can be easily tuned by rational molecular structure design to modulate their material properties, which, in turn, affects the device performance. Here, we designed and synthesized a series of materials based on terpolymer structures and applied them to organic thin-film transistor (OTFT) device applications. The four polymers, obtained by polymerization of three monomers relying on the Stille coupling reaction, shared comparable molecular weights, with the main structural difference being the ratio of the thiazole component to the fluorinated thiophene (Tz/FS). The conjugated polymers exhibited similar energy levels and thermal stability; however, their photochemical and crystalline properties were distinctly different, leading to significantly varied mobility behavior. Materials with a Tz/FS ratio of 50:50 showed the highest electron mobility, up to 0.69 cm2 V-1 s-1. Our investigation reveals the fundamental relationship between the structure and properties of materials and provides a basis for the design of semiconductor materials with higher carrier mobility.

Synergistic Use of All-Acceptor Strategies for the Preparation of an Organic Semiconductor and the Realization of High Electron Transport Properties in Organic Field-Effect Transistors.

The development of n-type organic semiconductor materials for transporting electrons as part of logic circuits is equally important to the development of p-type materials for transporting holes. Currently, progress in research on n-type materials is relatively backward, and the number of polymers with high electron mobility is limited. As the core component of the organic field-effect transistor (OFET), the rational design and judicious selection of the structure of organic semiconductor materials are crucial to enhance the performance of devices. A novel conjugated copolymer with an all-acceptor structure was synthesized based on an effective chemical structure modification and design strategy. PDPPTT-2Tz was obtained by the Stille coupling of the DPPTT monomer with 2Tz-SnMe3, which features high molecular weight and thermal stability. The low-lying lowest unoccupied molecular orbital (LUMO) energy level of the copolymer was attributed to the introduction of electron-deficient bithiazole. DFT calculations revealed that this material is highly planar. The effect of modulation from a donor-acceptor to acceptor-acceptor structure on the improvement of electron mobility was significant, which showed a maximum value of 1.29 cm2 V-1 s-1 and an average value of 0.81 cm2 V-1 s-1 for electron mobility in BGBC-based OFET devices. Our results demonstrate that DPP-based polymers can be used not only as excellent p-type materials but also as promising n-type materials.

Facile Synthesis of Conductive Metal-Organic Frameworks Nanotubes for Ultrahigh-Performance Flexible NO Sensors.

Cu-benzenehexathiol (Cu-BHT) has attracted significant attention due to its record high electrical conductivity and crystal defects Cu2c . However, the nonporous structure and small specific surface area of Cu-BHT with two-dimensional kagome lattice invariably limit its practical application in sensing and catalysis. In this work, Cu-BHT nanotubes (Cu-BHT-NTs) are designed and prepared via a facile homogeneous reaction to solve these problems. Compared with the traditional nanorod-like structure, the Cu-BHT-NTs not only have a higher specific surface area but also possess a higher proportion of crystal defects (66.6%). The successfully configured DPPTT/Cu-BHT-NTs heterostructure organic field-effect transistor (OFET)-based sensor exhibits excellent sensitivity as high as 13 610%, a minimum detection limits down to 5 ppb, and exceptional selectivity to nitric oxide (NO) toxic gases. Theoretical analysis systematically shows that Cu2c sites in the Cu-BHT-NTs increase the number of electrons transferred from the heterostructure to NO molecules, confirming that the high sensitivity and selectivity result from the high binding between Cu-BHT-NTs and NO molecules. Furthermore, a fully flexible device based on the heterojunction OFET sensor is prepared to ensure the convenience of wearing and carrying gas sensors, opening up a new avenue for the next generation of wearable intelligent electronics.

Chain-Extending Polymerization for Significant Improvement in Organic Thin-Film Transistor Performance.

To achieve high-performance polymer semiconductors, it is crucially important to explore novel and effective synthesis strategies. Here, chain-extending polymerization as a synthesis strategy to design polymer semiconductors is introduced. Furthermore, we demonstrate its superiority over a conventional synthesis strategy─one-pot polymerization. Diketopyrrolopyrrole-thieno[3,2-b]thiophene-containing polymers (PDPPTT and PDPPTT-vinylene) are used in this study. PDPPTT and PDPPTT-vinylene are synthesized through one-pot polymerization and chain-extending polymerization, respectively. The utilization of this novel strategy enhances the hole/electron mobilities of PDPPTT-vinylene to up to 3.70/2.96 cm2 V-1 s-1 (compared to 2.71/0.63 cm2 V-1 s-1 for PDPPTT), thereby achieving the required performance for organic circuits like inverters and ring oscillators. The significant improvement in the transistor performance of PDPPTT-vinylene is attributed to the introduced vinylene linking units during the polymerization process, which can fine-tune the electronic structure, expand π-conjugation, and induce stronger intermolecular π-π interactions with more significant crystallization. These results demonstrate that chain-extending polymerization is an effective synthesis strategy for developing high-performance polymer semiconductors.

Ultrafast In Situ Synthesis of Large-Area Conductive Metal-Organic Frameworks on Substrates for Flexible Chemiresistive Sensing.

The widespread use of electrically conductive metal-organic frameworks (EC-MOFs) in high-performance devices is limited by the lack of facile methods for synthesizing large-area thin films on the desired substrates. Herein, we propose a spin-coating interfacial self-assembly approach to in situ synthesize high-quality centimeter-sized copper benzenehexathiol (Cu-BHT) MOFs on diverse substrates in only 5 s. The film thickness (ranging from 5 to 35 nm) and surface morphology can be precisely tuned by controlling the reaction time. The gas sensor based on the 10 nm thick Cu-BHT film exhibits a low limit of detection (0.23 ppm) and high selectivity value (>30) in sensing NH3 at ultralow driving voltages (0.01 V). Moreover, the Cu-BHT films retain their initial sensor performance after 1000 repetitive bending cycles at a bending radius of 3 mm. Density functional theory calculations suggest that Cu2c sites induced by crystal particles on the film surface can improve the sensing performance. This facile and ultrafast approach for in situ synthesis of large-area EC-MOF films on diverse substrates with tunable thickness on a nanometer scale should facilitate application of EC-MOFs in flexible electronic device arrays.

Urchin-like non-precious-metal bifunctional oxygen electrocatalysts: Boosting the catalytic activity via the In-situ growth of heteroatom (N, S)-doped carbon nanotube on mesoporous cobalt sulfide/carbon spheres.

We successfully design and construct urchin-like non-precious-metal bifunctional oxygen electrocatalysts via a two-step pyrolysis process, where nitrogen, sulfur co-doped carbon nanotube frameworks are grafted onto mesoporous cobalt sulfide/nitrogen, sulfur co-doped carbon spheres. The urchin-like structure grants large electrochemically active area, good electron and mass transfer capability, as well as excellent structural stability. Nitrogen, sulfur co-doped carbon can synergistically enhance the catalytic activity of cobalt sulfide sites, and also contribute to the exposure of heteroatom-induced active sites, such as, pyridinic N, graphitic N, and C-S-C. Hence, benefiting from the unique architecture and efficient catalytic sites, the resulting catalysts demonstrate excellent bifunctional catalytic activities with a positive half-wave potential of 0.860 V vs. RHE for oxygen reduction reaction and low overpotential of ∼390 mV at the current density of 10 mA cm-2 for oxygen evolution reaction in alkaline medium, which can rank them among one of the most promising cobalt-based bifunctional oxygen electrocatalysts reported previously.

Triple Acceptors in a Polymeric Architecture for Balanced Ambipolar Transistors and High-Gain Inverters.

The exploration of novel molecular architectures is crucial for the design of high-performance ambipolar polymer semiconductors. Here, a "triple-acceptors architecture" strategy to design the ambipolar polymer DPP-2T-DPP-TBT is introduced. The utilization of this architecture enables DPP-2T-DPP-TBT to achieve deep-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of -5.38/-4.19 eV, and strong intermolecular interactions, which are favorable for hole/electron injection and intermolecular hopping through π-stacking. All these factors result in excellent ambipolar transport characteristics and promising applications in complementary-like circuits for DPP-2T-DPP-TBT under ambient conditions with high hole/electron mobilities and a gain value of up to 3.01/3.84 cm2 V-1 s-1 and 171, respectively, which are among the best performances in ambipolar polymer organic thin-film transistors and associated complementary-like circuits, especially in top-gate device configuration with low-cost glass as substrates. These results demonstrate that the "triple-acceptors architecture" strategy is an effective way for designing high-performance ambipolar polymer semiconductors.

Flexible small-channel thin-film transistors by electrohydrodynamic lithography.

Small-channel organic thin-film transistors (OTFTs) are an essential component of microelectronic devices. With the advent of flexible electronics, the fabrication of OTFTs still faces numerous hurdles in the realization of highly-functional, devices of commercial value. Herein, a concise and efficient procedure is proposed for the fabrication of flexible, small-channel organic thin-film transistor (OTFT) arrays on large-area substrates that circumvents the use of photolithography. By employing a low-cost and high-resolution mechano-electrospinning technology, large-scale micro/nanofiber-based patterns can be digitally printed on flexible substrates (Si wafer or plastic), which can act as the channel mask of TFT instead of a photolithography reticle. The dimensions of the micro/nanochannel can be manipulated by tuning the processing parameters such as the nozzle-to-substrate distance, applied voltage, and fluid supply. The devices exhibit excellent electrical properties with high mobilities (∼0.62 cm2 V-1 s-1) and high on/off current ratios (∼2.47 × 106), and they are able to maintain stability upon being bent from 25 mm to 2.75 mm (bending radius) over 120 testing cycles. This electrohydrodynamic lithography-based approach is a digital, programmable, and reliable alternative for easily fabricating flexible, small-channel OTFTs, which can be integrated into flexible and wearable devices.

π-Extended Isoindigo-Based Derivative: A Promising Electron-Deficient Building Block for Polymer Semiconductors.

The exploration of novel electron-deficient building blocks is a key task for developing high-performance polymer semiconductors in organic thin-film transistors. In view of the situation of the lack of strong electron-deficient building blocks, we designed two novel π-extended isoindigo-based electron-deficient building blocks, IVI and F4IVI. Owing to the strong electron-deficient nature and the extended π-conjugated system of the two acceptor units, their copolymers, PIVI2T and PF4IVI2T, containing 2,2'-bithiophene donor units, are endowed with deep-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels and strong intermolecular interactions. In comparison to PIVI2T, the fluorinated PF4IVI2T exhibits stronger intra- and intermolecular interactions, lower HOMO/LUMO energy levels up to -5.74/-4.17 eV, and more ordered molecular packing with a smaller π-π stacking distance of up to 3.53 Å, resulting in an excellent ambipolar transporting behavior and a promising application in logic circuits for PF4IVI2T in ambient with hole and electron mobilities of up to 1.03 and 1.82 cm2 V-1 s-1, respectively. The results reveal that F4IVI is a promising and strong electron-deficient building unit to construct high-performance semiconducting polymers, which provides an insight into the structure-property relationships for the exploration and molecular engineering of excellent electron-deficient building blocks in the field of organic electronics.

D-A1-D-A2 Copolymer Based on Pyridine-Capped Diketopyrrolopyrrole with Fluorinated Benzothiadiazole for High-Performance Ambipolar Organic Thin-Film Transistors.

A novel donor-acceptor-donor-acceptor (D-A1-D-A2) π-conjugated copolymer (PDBPyDT2FBT) has been prepared by Stille coupling reaction. It is found that PDBPyDT2FBT exhibits low LUMO energy level mainly because of multiple electron-deficient units and donor-acceptor interaction, which is favorable to obtain more efficient electron injection and transport in organic thin-film transistors (OTFTs). Moreover, introducing two electron-deficient moieties into the thiophene-containing copolymer increases the length of conjugated main chain and enhances the coplanarity of the backbone, which may be beneficial for promoting the molecular crystallinity and improving molecular ordering capability at low temperatures. High electron and hole mobilities up to 0.65 and 0.24 cm(2) V(-1) s(-1) were obtained at relatively low annealing temperatures of 100 and 80 °C, respectively, implying that PDBPyDT2FBT is a promising ambipolar polymer semiconductor applied in low-cost and large-area manufacturing of OTFTs.

Design of High-Mobility Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic Thin-Film Transistors.

Since the report of the first diketopyrrolopyrrole (DPP)-based polymer semiconductor, such polymers have received considerable attention as a promising candidate for high-performance polymer semiconductors in organic thin-film transistors (OTFTs). This Progress Report summarizes the advances in the molecular design of high-mobility DPP-based polymers reported in the last few years, especially focusing on the molecular design of these polymers in respect of tuning the backbone and side chains, and discussing the influences of structural modification of the backbone and side chains on the properties and device performance of corresponding DPP-based polymers. This provides insights for the development of new and high-mobility polymer semiconductors.