Xike Gao

Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm2 V–1 s–1

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ~0.001 to >3.0 cm(2) V(-1) s(-1). Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C(11,6) results in a dense in-plane molecular packing with an unit cell area of 127 Å(2), larger domain sizes of up to 1000 × 3000 nm(2), and an electron mobility of up to 3.50 cm(2) V(-1) s(-1), which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities.

Core-Expanded Naphthalene Diimides Fused with 2-(1,3-Dithiol-2-Ylidene)Malonitrile Groups for High-Performance, Ambient-Stable, Solution-Processed n-Channel Organic Thin Film Transistors

A new class of n-type semiconductors for organic thin film transistors (OTFTs), based on core-expanded naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malonitrile groups, is reported. The first two representatives of these species, derived from long branched N-alkyl chains, have been successfully used as active layers for high-performance, ambient-stable, solution-processed n-channel OTFTs. Their bottom-gate top-contact devices fabricated by spin-coating methods exhibit high electron mobilities of up to 0.51 cm(2) V(-1) s(-1) with current on/off ratios of 10(5)-10(7), and small threshold voltages below 10 V under ambient conditions. As this class of n-type organic semiconductors has relatively low-lying LUMO levels and good film-formation ability, they also displayed good environmental stability even with prolonged exposure to ambient air. Both the device performance and the ambient stability are among the best for n-channel OTFTs reported to date.

Flexible suspended gate organic thin-film transistors for ultra-sensitive pressure detection

The utilization of organic devices as pressure-sensing elements in artificial intelligence and healthcare applications represents a fascinating opportunity for the next-generation electronic products. To satisfy the critical requirements of these promising applications, the low-cost construction of large-area ultra-sensitive organic pressure devices with outstanding flexibility is highly desired. Here we present flexible suspended gate organic thin-film transistors (SGOTFTs) as a model platform that enables ultra-sensitive pressure detection. More importantly, the unique device geometry of SGOTFTs allows the fine-tuning of their sensitivity by the suspended gate. An unprecedented sensitivity of 192 kPa−1, a low limit-of-detection pressure of <0.5 Pa and a short response time of 10 ms were successfully realized, allowing the real-time detection of acoustic waves. These excellent sensing properties of SGOTFTs, together with their advantages of facile large-area fabrication and versatility in detecting various pressure signals, make SGOTFTs a powerful strategy for spatial pressure mapping in practical applications.

All‐Solution‐Processed, High‐Performance n‐Channel Organic Transistors and Circuits: Toward Low‐Cost Ambient Electronics

Exploration of high-performance solution-processed n-channel organic transistors with excellent stability is a critical issue for the development of powerful printed circuits. Solution-processed, bottom-gate transistors exhibiting a record electron mobility of up to 1.2 cm(2) V(-1) s(-1) are reported. The devices show excellent stability, which enables the construction of all-solution-processed flexible circuits with all fabrication procedures performed in air.

Ultrathin Film Organic Transistors: Precise Control of Semiconductor Thickness via Spin-Coating

Construction of ultrathin film organic transistors is an important challenge towards deeper understanding of the charge transport mechanism and multifunctional applications. We report on precise thickness control of ultrathin films of several organic semiconductors by using a simple spin-coating approach. Ultrathin film, n-channel organic transistors with mobilities well over 1.0 cm(2) V(-1) s(-1) have been realized and their potential in high-sensitivity gas sensing and other applications is demonstrated.

High mobility organic semiconductors for field-effect transistors

Organic field-effect transistors (OFETs) are attracting more and more attention due to their potential applications in low-cost, large-area and flexible electronic products. Organic semiconductors (OSCs) are the key components of OFETs and basically determine the device performance. The past five years have witnessed great progress of OSCs. OSCs used for OFETs have made rapid progress, with field-effect mobility much larger than that of amorphous silicon (0.5–1.0 cm2/(V s)) and of up to 10 cm2/(V s) or even higher. In this review, we demonstrate the latest progress of OSCs for OFETs, where more than 50 representative OSCs are highlighted and analyzed to give some valuable insights for this important but challenging field.

High‐Performance Organic Field‐Effect Transistors Based on Single and Large‐Area Aligned Crystalline Microribbons of 6,13‐Dichloropentacene

Single and large-area aligned crystalline microribbons of 6,13-dichloropentacene (DCP) are prepared. The ribbons display excellent charge transport properties with a mobility of up to 9.0 cm(2) V(-1) s(-1) in ambient conditions, one of the highest values reported for organic semiconductors. The slipped π-π stacking with large intermolecular overlap is responsible for the high mobility of DCP ribbons.

One-pot synthesis of core-expanded naphthalene diimides: enabling N-substituent modulation for diverse n-type organic materials.

A mild and versatile one-pot synthesis of core-expanded naphthalene diimides has been developed, which undergoes a nucleophilic aromatic substitution reaction and then an imidization reaction, allowing an easy and low-cost access to diverse n-type organic materials. Some newly synthesized compounds by this one-pot operation exhibited high electron mobility of up to 0.70 cm(2) V(-1) s(-1) in ambient conditions.

Linear benzene-fused bis(tetrathiafulvalene) compounds for solution processed organic field-effect transistors

Three new linear benzene-fused bis(tetrathiafulvalene) compounds (4–6) were synthesized and characterized. Compounds 4 and 5, comprising branched alkyl chains (2-ethylbutyl and 2-ethylhexyl, respectively), had very good solubility in common organic solvents and were successfully used as p-channel semiconductors in solution-processed organic field-effect transistors (FET). The FET device based on 5 showed a mobility of 5.6 × 10–4 cm2 V–1 s–1 with an on/off ratio of 1.6 × 104, and the devices based on 4 and 5 displayed good reproducibility in air. In addition, 6, bearing the long n-dodecyl chains, exhibited self-assembly behavior; one-dimensional self-assembly micron-scale ribbons formed in solution and were revealed by scanning electron microscopy on SiO2 substrates.

Specific and Reproducible Gas Sensors Utilizing Gas‐Phase Chemical Reaction on Organic Transistors

Utilizing a textbook reaction on the surface of an organic active channel, achieves sensitive detection of HCl, NH3 and NO2, with good selectivity, excellent reproducibility, and satisfactory stability. These results reveal gas-phase reaction assisted detection as a unique and promising approach to construct practical applicable gas sensors with typical organic transistors.