Progresses in organic field-effect transistors and molecular electronics

In the past years, organic semiconductors have been extensively investigated as electronic materials for organic field-effect transistors (OFETs). In this review, we briefly summarize the current status of organic field-effect transistors including materials design, device physics, molecular electronics and the applications of carbon nanotubes in molecular electronics. Future prospects and investigations required to improve the OFET performance are also involved. __________ Translated from Huaxue Tongbao (Chemistry), 2006, 69(6) (in Chinese)     

High-performance and multifunctional organic field-effect transistors

Organic field-effect transistors(OFETs) refer to field-effect transistors that use organic semiconductors as channel materials. Owing to the advantages of organic materials such as solution processability and intrinsic flexibility, OFETs are expected to be applicable in emergent technologies including wearable electronics and sensors, flexible displays, internet-of-things, neuromorphic computing, etc. Improving the electrical performance and developing multifunctionalities of OFETs are two major...     

High-performance organic field-effect transistors: molecular design, device fabrication, and physical properties

In the past decade, tremendous progress has been made in organic field-effect transistors (OFETs). Their real applications require further development of device performance. OFETs consist of organic semiconductors, dielectric layers, and electrodes. Organic semiconductors play a key role in determining the device characteristics. The properties of the organic semiconductors, such as molecular structure and packing, as well as molecular energy levels, can be properly controlled by molecular design. Therefore, we designed and synthesized a series of organic molecules. The synthesized organic semiconductors exhibit excellent field-effect properties due to strong intermolecular interactions and proper molecular energy levels. Meanwhile, the influence of the device fabrication process, organic semiconductor/dielectric layer interface, and organic layer/electrode contact on the device performance was investigated. A deep understanding of these factors is helpful to improve field-effect properties. Furthermore, single-crystal field-effect transistors are highlighted because the single-crystal-based FETs can provide an accurate conducting mechanism of organic semiconductors and higher device performance as compared with thin film FETs.     

高性能双极性聚合物半导体材料与晶体管器件

有机聚合物半导体材料与晶体管器件是融合了化学、材料、半导体以及微电子等学科的前沿交叉研究方向.聚合物半导体材料分子是该领域研究的重要内容,其中双极性聚合物分子半导体材料,兼具了电子和空穴的双重载流子输运能力而受到学术界的广泛关注.本文总结了双极性聚合物半导体材料与器件的研究进展,重点介绍了我们在D-A型双极性聚合物分子半导体材料设计、加工技术与器件制备以及功能应用方面的研究工作,并论述了双极性聚合物分子半导体材料与器件研究过程中存在的科学问题及发展方向.     

Noncovalent conformational locks in organic semiconductors

Highly planar conformation is considered to be one of the most important properties for high performance organic semiconductors. Among all kinds strategies for designing highly performing materials, noncovalent conformational locks(NCLs)have been widely used to increase the planarity and rigidity for π-conjugated systems. This review summarizes π-conjugated small molecules and polymers by employing various NCLs for controlling molecular conformation in the past two years. The optoelectronic properties of the conjugated materials, together with their applications on organic field-effect transistors(OFETs)and organic photovoltaics(OPVs) are discussed. Besides, the outlook and challenges in this field are also presented. It is obvious that NCLs play an important role in the design and synthesis of high-performance organic semiconductors.     

Enhanced mobility and environmental stability in all organic field‐effect transistors: The role of high dipole moment solvent

Low‐operating voltage, high mobility, and stable organic field‐effect transistors (OFETs) using polymeric dielectrics such as pristine poly(4‐vinyl phenol) (PVP) and poly(methyl methacrylate) (PMMA), dissolved in solvents of high dipole moment, have been achieved. High dipole moment solvents such as propylene carbonate and dimethyl sulfoxide used for dissolving the polymer dielectric enhance the charge carrier mobilities by three orders of magnitude in pentacene OFETs compared with low dipole moment solvents. Fast switching circuits with patterned gate PVP‐based pentacene OFETs demonstrated a switching frequency of 75 kHz at input voltages of |5 V|. The frequency response of the OFETs is attributed to a high degree of dipolar‐order in dielectric films obtained from high‐polarity solvents and the resulting energetically ordered landscape for transport. Remarkably, these pentacene‐based OFETs exhibited high stability under bias stress and in air with negligible shifts in the threshold voltage. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1533–1542     

Angular-shaped naphthalene tetracarboxylic diimides for n-channel organic transistor semiconductors

A series of compounds based on the angular-shaped naphthalene tetracarboxylic diimide core have been synthesized, characterized and used as active layers of organic field-effect transistors (OFETs). The fabricated OFET devices exhibit n-type semiconducting characteristics, demonstrating the first examples of semiconductors based on angular-shaped naphthalene tetracarboxylic diimides.     

Gate-planarized low-operating voltage organic field-effect transistors enabled by hot polymer pressing/embedding of conducting metal lines

A new process for fabricating patterned, gate-planarized organic field-effect transistors (OFETs) based on hot polymer pressing/embedding of conducting metal features is demonstrated. This methodology is applicable to a variety of gate conductors and polymer matrices. Vapor-deposited Al and Au and printed Ag lines as narrow as 15 mum are transferred from a substrate donor to the hot-pressed polymer, resulting in a new smooth, flat, self-planarized gate-plastic substrate composite on which thin polymer insulators can be spin-coated with great uniformity. OFETs fabricated on these structures with both p- and n-type semiconductors function at low voltages, opening new routes to printed electronic circuits and products.     

Organic transistors in the new decade: Toward n-channel,printed, and stabilized devices

Significant progress has been made in designing organic semiconducting materials (OSCs) for the past few decades for organic field-effect transistors (OFETs). Much attention has been paid to the development of p-channel OSCs, with less but highly significant progress on n-channel OSCs. In this review, we focus on the advances made with OFETs in the last few years to achieve high performance in n-channel modes, air stability, and solution processability, leading to printable active electronics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Air-stable n-type semiconductor: core-perfluoroalkylated perylene bisimides

A series of core-perfluoroalkylated perylene bisimides (PBIs) have been efficiently synthesized by copper-mediated perfluoroalkylation of dibrominated PBIs. Their aromatic cores are highly twisted due to the steric encumbrance in the bay regions as revealed by single-crystal X-ray analysis. The organic field-effect transistors (OFETs) incorporating these new n-type semiconductors show remarkable air-stability and good field effect mobility.     

Fully printed flexible audio system on the basis of low‐voltage polymeric organic field effect transistors with three layer dielectric

Fully mass printed, flexible and truly polymeric organic field effect transistors consisting of a three layer dielectric made of CYTOP (low‐k), PVA (intermediate) and P(VDF‐TrFE‐CTFE)(high‐k) are introduced. Gravure‐, flexo‐and screen printing were selected as highly productive manufacturing technologies. These OFETs work at strongly reduced voltages and show high field effect mobility (µ = 0.2 cm²/Vs) and remarkable good bias stress stability at very high current density (50 µA/mm). Fully printed OFETs are used for the realization of ring oscillators working in the kHz regime at reduced supply voltage (10 V). In combination with printed fully polymeric piezoelectric loudspeakers, this work shows for the first time fully printed flexible audio systems. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1409–1415     

Organic semiconductors for solution-processable field-effect transistors (OFETs)

The cost-effective production of flexible electronic components will profit considerably from the development of solution-processable, organic semiconductor materials. Particular attention is focused on soluble semiconductors for organic field-effect transistors (OFETs). The hitherto differentiation between "small molecules" and polymeric materials no longer plays a role, rather more the ability to process materials from solution to homogeneous semiconducting films with optimal electronic properties (high charge-carrier mobility, low threshold voltage, high on/off ratio) is pivotal. Key classes of materials for this purpose are soluble oligoacenes, soluble oligo- and polythiophenes and their respective copolymers, and oligo- and polytriarylamines. In this context, micro- or nanocrystalline materials have the general advantage of somewhat higher charge-carrier mobilities, which, however, could be offset in the case of amorphous, glassy materials by simpler and more reproducible processing.     

Photoresponsive n-channel organic field-effect transistors based on a tri-component active layer

Photoresponsive OFETs were fabricated based on a tri-component active layer (NDI2OD-DTYM2, spiropyran and polystyrene). The results demonstrated that these OFETs displayed photoresponsive feature to alternate UV and vis light due to the photoisomerization of spiropyran between the closed-ring state and ionic open-ring state.     

Influence of molecular weight on the short‐channel effect in polymer‐based field‐effect transistors

In this study, we demonstrate how the intrinsic properties of a polymer can influence the electrical characteristics of organic field‐effect transistors (OFETs). OFETs fabricated with three batches of poly[2‐methoxy,5‐(3′,7′‐dimethyl‐octyloxy)]‐p‐phenylene vinylene (MDMO‐PPV) were investigated. The properties of the polymers were initially investigated using Fourier transform infrared spectroscopy (FTIR), impedance spectroscopy (IS), gel permeation chromotography (GPC), and cyclic voltammetry (CV), respectively. The structure and purity of the polymer batches were found to be very comparable, but the molecular weight (M_n and M_w) and polydispersity (PDI = M_w/M_n), varied between the samples and the HOMO and LUMO levels of the polymers were found to depend on the molecular weight properties. OFETs were then fabricated with the polymers and electrically characterized. It was observed that the channel current and the field‐effect mobility increase with increasing polymer molecular weight. The output characteristics of the transistors, on the other hand, were found to depend on the PDI of the polymer. Saturation of the channel current occurs at higher source–drain voltages and short‐channel behavior was observed to start at longer channel lengths for polymers with a higher PDI. This behavior is observed to be thickness dependent, and the short‐channel behavior was more pronounced for thicker MDMO‐PPV films. These results are explained in terms of influences of chain packing and ordering and high bulk currents on the FET output and transistor parameters. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: 117–124, 2012     

Recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells

Organic semiconductor materials, especially donor–acceptor (D–A) polymers, have been increasingly applied in organic optoelectronic devices, such as organic field-effect transistors (OFETs) and organic solar cells (OSCs). Plenty of high-performance OFETs and OSCs have been achieved based on varieties of structurally modified D–A polymers. As the basic building block of D–A polymers, acceptor moieties have drawn much attention. Among the numerous types, lactam- and imide-functionalized electron-deficient building blocks have been widely investigated. In this review, the structural evolution of lactam- or imide-containing acceptors (for instance, diketopyrrolopyrrole, isoindigo, naphthalene diimide, and perylene diimide) is covered and their representative polymers applied in OFETs and OSCs are also discussed, with a focus on the effect of varied structurally modified acceptor moieties on the physicochemical and photoelectrical properties of polymers. Additionally, this review discusses the current issues that need to be settled down and the further development of new types of acceptors. It is hoped that this review could help design new electron-deficient building blocks, find a more valid method to modify already reported acceptor units, and achieve high-performance semiconductor materials eventually.

This review highlights the recent structural evolution of lactam- and imide-functionalized polymers applied in organic field-effect transistors and organic solar cells.     

A highly pi-stacked organic semiconductor for field-effect transistors based on linearly condensed pentathienoacene

We present the synthesis and characterization of a fused-ring compound, dithieno[2,3-d:2',3'-d']thieno[3,2-b:4,5-b']dithiophene (pentathienoacene, PTA). In contrast to pentacene, PTA has a larger band gap than most semiconductors used in organic field-effect transistors (OFETs) and therefore is expected to be stable in air. The large pi-conjugated and planar molecular structure of PTA would also form higher molecular orders that are conductive for carrier transport. X-ray diffraction and atomic force microscopy experiments on its films show that the molecules stack in layers with their long axis upright from the surface. X-ray photoelectron spectroscopy suggests that there are no chemical bonds at the PTA/Au interface. OFETs based on the PTA have been constructed, and their performances as p-type semiconductors are also presented. A high mobility of 0.045 cm(2)/V s and an on/off ratio of 10(3) for a PTA OFET have been achieved, demonstrating the potential of PTA for application in future organic electronics.     

Electron transporting semiconducting polymers in organic electronics

Significant progress has been achieved in the preparation of semiconducting polymers over the past two decades, and successful commercial devices based on them are slowly beginning to enter the market. However, most of the conjugated polymers are hole transporting, or p-type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or n-type, materials that have lagged behind their p-type counterparts. Organic electron transporting materials are essential for the fabrication of organic p-n junctions, organic photovoltaic cells (OPVs), n-channel organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs) and complementary logic circuits. In this critical review we focus upon recent developments in several classes of electron transporting semiconducting polymers used in OLEDs, OFETs and OPVs, and survey and analyze what is currently known concerning electron transporting semiconductor architecture, electronic structure, and device performance relationships (87 references).     

High speeds complementary integrated circuits fabricated with all‐printed polymeric semiconductors

Inkjet‐printed high speed polymeric complementary circuits are fabricated using an n‐type ([poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐dithiophene)} [P(NDI2OD‐T2), Polyera ActivInk N2200] and two p‐type polymers [poly(3‐hexylthiophene) (P3HT) and a dithiophene‐based polymer (Polyera ActivInk P2100)]. The top‐gate/bottom‐contact (TG/BC) organic field‐effect transistors (OFETs) exhibit well‐balanced and very‐high hole and electron mobilities (μ_FET) of 0.2–0.5 cm²/Vs, which were enabled by optimization of the inkjet‐printed active features, small contact resistance both of electron and hole injections, and effective control over gate dielectrics and its orthogonal solvent effect (selection of poly(methyl methacrylate) and 2‐ethoxyethanol). Our first demonstrated inkjet‐printed polymeric complementary devices have been integrated to high‐performance complementary inverters (gain >30) and ring oscillators (oscillation frequency ～50 kHz). We believe that the operating frequency of printable organic circuits can be further improved more than 10 MHz by fine‐tuning of the device architecture and optimization of the p‐ and n‐channel semiconductor processing. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Photoresponsive organic field-effect transistors involving photochromic molecules

In recent years, organic field-effect transistors (OFETs) with high performance and novel multifunctionalities have attracted considerable attention. Meanwhile, featured with reversible photoisomerization and the corresponding variation in color, chemical/physical properties, photochromic molecules have been applied in sensors, photo-switches and memories. Incorporation of photochromic molecules to blend in the device functional layers or to modify the interfaces of OFETs is common way to build photo-transistors. In this review, we focus on the recent advantages on the study of photoresponsive transistors involving one of three typical photochromic compounds spiropyran, diarylethene and azobenzene. Three main strategies are demonstrated in detail. Firstly, photochromic molecules are doped in active layers or combined with semiconductor structure thus forming photoreversible active layers. Secondly, the modification of dielectric layer/active layer interface is mainly carried out by bilayer dielectric. Thirdly, the photo-isomerization of self-assembled monolayer (SAM) on the electrode/active layer interface can reversibly modulate the work functions and charge injection barrier, result in bifunctional OFETs. All in all, the combination of photochromic molecules and OFETs is an efficient way for the fabrication of organic photoelectric devices. Photoresponsive transistors consisted of photochromic molecules are potential candidate for real applications in the future.