High-performance Organic Field-effect Transistors Research Articles

Controlled 2D growth of organic semiconductor crystals by suppressing “coffee-ring” effect

Owing to enhanced charge transport efficiency arising from the ultrathin nature, two-dimensional (2D) organic semiconductor single crystals (OSSCs) are emerging as a fascinating platform for high-performance organic field-effect transistors (OFETs). However, “coffee-ring” effect induced by an evaporation-induced convective flow near the contact line hinders the large-area growth of 2D OSSCs through a solution process. Here, we develop a new strategy of suppressing the “coffee-ring” effect by using an organic semiconductor: polymer blend solution. With the high-viscosity polymer in the organic solution, the evaporation-induced flow is remarkably weakened, ensuring the uniform molecule spreading for the 2D growth of the OSSCs. As an example, wafer-scale growth of crystalline film consisting of few-layered 2,7-didecylbenzothienobenzothiophene (C10-BTBT) crystals was successfully accomplished via blade coating. OFETs based on the crystalline film exhibited a maximum hole mobility up to 12.6 cm2·V−1·s−1, along with an average hole mobility as high as 8.2 cm2·V−1·s−1. Our work provides a promising strategy for the large-area growth of 2D OSSCs toward high-performance organic electronics.

Dinaphthothiepine Bisimide and Its Sulfoxide: Soluble Precursors for Perylene Bisimide.

The synthesis and properties of dinaphtho[1,8-bc:1',8'-ef]thiepine bisimide (DNTBI) and its oxides are described. Their molecular design is conceptually based on the insertion of a sulfur atom into the perylene bisimide (PBI) core. These sulfur-inserted PBI derivatives adopt nonplanar structures, which significantly increases their solubility in common organic solvents. Upon electron injection, light irradiation, or heating, DNTBI and its sulfoxides undergo sulfur extrusion reactions to furnish PBI. The photoinduced and thermal sulfur extrusion reactions proceed almost quantitatively. This unique reactivity enabled the fabrication of a high-performance solution-processed n-type organic field-effect transistor with an electron mobility of up to 0.41 cm2 V-1 s-1.

π-Conjugated oligomers based on aminobenzodifuranone and diketopyrrolopyrrole

A new donor-acceptor type oligomer, named as O1 with aminobenzodifuranone (ABDF) capped at both ends of thiophene DPP, was synthesized and characterized. The amino groups (NH) in this oligomer react with the carbonyl units (CO) from the neighboring molecules forming hydrogen bonding (NH…OC), which results in hydrogen bonded oligomer O1. The thermal, optical, electrochemical, thin film properties as well as the density functional theory computational simulation of O1 were investigated. O1 thin film presented not only strong aggregation, but also hydrogen bonding association. This is beneficial for the charges to be carried across the individual molecules. The high HOMO levels of O1 (−5.31 eV) facilitate charge injection and ohmic contacts with electrodes. Apart from that, the good planarity backbone combined with strong D-A system for the π-conjugation of O1 is favorable for the intracharge transport. These properties provided the O1 thin film based organic field-effect transistors (OFET) with hole mobility (μh) of 0.28 cm2/V·s, and after annealing, the μh increased up to 0.39 cm2/V·s. Such hydrogen-bonding-mediated enhancement of π – π stacking exhibits potential for improving the performance of solution processable organic electronic materials. This work shows that the hydrogen bonding association is a simple and useful strategy to design π-conjugation oligomers for high performance OFETs.

Facile and cost-effective liver cancer diagnosis by water-gated organic field-effect transistors

The rise of mortality rate caused by hepatocellular carcinoma accelerates requirements of biosensors for early liver cancer diagnosis and treatment to improve the clinical prognosis and prolong the survival of patients. However, how to realize label-free, low-cost, easy and fast-detection is the major challenge in the design of biosensors. Water-gated organic field-effect transistors efficiently bridge the gap between semiconductor devices and biological systems, leading to an organic device suitable for health or body signal monitoring. Herein, a kind of high-performance water-gated organic field-effect transistor is developed through the optimization process. This method provides a label-free general sensing platform for the determination of liver cancer biomarker alpha-fetoprotein in 45 minutes, much quicker than traditional methods such as the enzyme-linked immunosorbent assay for several hours. In addition, with the detection limit lower than the cut-off value as well as the ability to achieve quantitative detection, this novel water-gated organic field-effect transistor enables a much broader analysis of other biomarkers in cancer patient samples.

Work Function Engineering of Electrohydrodynamic-Jet-Printed PEDOT:PSS Electrodes for High-Performance Printed Electronics.

Poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) has demonstrated outstanding performance as a charge transport layer or an electrode in various electronic devices, including organic solar cells, organic light-emitting diodes, and organic field-effect transistors (OFETs). The electrical properties of these devices are affected by the contact properties at the PEDOT:PSS-semiconductor junction. In this research, we performed work function (WF) engineering of electrohydrodynamic (EHD)-jet-printed PEDOT:PSS and successfully used it as an electrode to fabricate high-performance OFETs and complementary logic circuits. Two types of PEDOT:PSS materials-one with a high WF (HWF, 5.28 eV) and the other with a low WF (LWF, 4.53 eV)-were synthesized and EHD-jet-printed. The WF of PEDOT:PSS was deterministically modulated by approximately 0.75 eV through simple mixing of the two synthesized PEDOT:PSS materials in various ratios. OFETs fabricated with HWF and LWF PEDOT:PSS electrodes showed excellent electrical properties, including the ON/OFF switching ratio higher than 107 and the highest carrier mobility greater than 1 cm2·V-1·s-1. Furthermore, the HWF and LWF PEDOT:PSS electrodes were integrated to fabricate complementary metal-oxide-semiconductor (CMOS) NOT, NOR, and NAND circuits.

Organic thin-film transistors with flame-annealed contacts

Reducing contact resistance is critical to developing high-performance organic field-effect transistors (OFETs) since it impacts both the device mobility and switching speed. Charge injection and collection has been optimized by applying chemical treatments to the contacts, such as self-assembled monolayers (SAMs), oxide interlayers, or dopants. Here, we tested how flame annealing the surface of the electrodes impacts the interface and bulk components of the contact resistance, as well as the overall device performance. A butane micro torch was used to flash-anneal the gold electrodes, which allowed gold grains to crystallize into larger domains. We found that, along with the grain size, the surface roughness of the contacts was also increased. SAM treatment created a lower work function shift on a flame annealed electrode than when deposited on an untreated surface, due to the greater surface roughness. This resulted in a larger interface contact resistance. However, flame annealing also produced an order of magnitude reduction in the density of trap states in the semiconductor layer, which reduced the bulk contact resistance and channel resistance. These competing effects yielded OFETs with similar performance as untreated devices.

Methoxylation of quinoidal bithiophene as a single regioisomer building block for narrow-bandgap conjugated polymers and high-performance organic field-effect transistors

Methoxyl group was introduced to obtain isomer-free methoxylation quinoidal bithiophene building block. Four polymers displayed narrow bandgap (<1.20 eV) and hole mobility of up to 2.70 cm2 V−1 s−1.

High-performance amorphous organic semiconductor-based vertical field-effect transistors and light-emitting transistors.

Herein, two kinds of vertical organic optoelectronic devices, vertical organic field-effect transistors (VOFETs) and light-emitting transistors (VOLETs), were constructed based on amorphous organic semiconductors of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB) as hole injecting and transport layers and tris(8-hydroxy-quinolinato) aluminum (Alq3) as the emitting layer. High device performances with a large on/off ratio of ∼6 × 103, current density of ∼40 mA cm-2, and fast response of ∼5 ms at a frequency of 20 Hz and a brightness of 126 cd m-2 were demonstrated for these two vertical devices with good device stability and repeatability. These results suggest the potential applications of amorphous organic semiconductors with good film-forming characteristics and easy device fabrication ability in vertical optoelectronic circuits.

One-step synthesis of an acceptor–donor–acceptor small molecule based on indacenodithieno[3,2-b]thiophene and benzothiadiazole units for high-performance solution-processed organic field-effect transistors

We reported a one-step synthesis of a high-performance acceptor–donor–acceptor small molecule based on indacenodithieno[3,2-b]thiophene (IDTT) as the donor unit and benzothiadiazole (BT) as the acceptor unit.

Meniscus-guided coating of organic crystalline thin films for high-performance organic field-effect transistors

In this review, recently reported strategies applied to control the morphology, molecule packing, and orientation of organic semiconductors thin films in the context of meniscus-guided coating techniques are summarized and discussed.

Theoretical Studies of Bipolar Transport in CnBTBT-FmTCNQ Donor-Acceptor Cocrystals.

The development of crystals with bipolar transport characteristics is essential for high-performance organic field effect transistor (OFET) devices. In this work, we theoretically investigated the bipolar transport behaviors in CnBTBT-FmTCNQ cocrystals. It is found that bipolar transport can be realized in C8BTBT-TCNQ and C12BTBT-TCNQ cocrystals with room-temperature electron/hole mobility up to 1.8/0.75 and 2.5/1.8 cm2 V-1 s-1, respectively. The comparable electron- and hole-transfer integrals between the nearest-neighbor molecule pairs as well as the small hole reorganization energy of the TCNQ molecule are responsible for the balanced electron and hole mobilities. Moreover, because of the π-π stacking between neighboring molecules, all cocrystals show strong anisotropic transport characteristic for both electron and hole transport with the mobility along the π-π stacking direction much larger than those along the other two directions. This work provides the possibility of high-performance OFET engineering and also enriches the OFET families with bipolar transport characteristics.

Precisely Controlling the Structure of Ultrathin Semiconducting Films by a Laminating Method for High-Performance Organic Field-Effect Transistors.

High-performance organic field-effect transistors (OFETs) based on conjugated polymers have received extensive attention in recent years. However, the relationships between the multiscale structures of conjugated polymers and electrical properties are not well established. Here, laminated, ultrathin poly(3-hexylthiophene) (P3HT) films have been prepared using a sequential, repeated transfer-etching process from the precursor conjugated/insulating polymer blend films and used to study the structure-property relationship at the transition of the film structure from 2D to 3D. The molecular packing of the films is improved by lamination as certified by grazing incidence X-ray diffraction, UV-visible spectroscopy, and Raman spectroscopy. The laminated ultrathin P3HT films exhibit excellent electrical properties with a maximum mobility of 0.23 cm2 V-1 s-1 at three layers, which is close to the highest value reported for undoped P3HT OFETs. Temperature-dependent FET characteristics reveal that the laminated films have low activation energy and a 2D charge transport profile regardless of the number of layers. These charge transport properties are attributed to the well-ordered molecular packing and low trap density in the films, which are enabled by the phase separation of the precursor blend films and the lamination process. In addition, OFETs based on these films have good photostability under different wavelengths of light, indicating that this approach has promising practical application prospects.

Highly aligned and crystalline poly(3-hexylthiophene) thin films by off-center spin coating for high performance organic field-effect transistors

Controlling the morphology, molecular packing and alignment of conjugated polymer remain a primary challenge that hinders their widespread applications in electronic and optoelectronic devices. Here, we have demonstrated an extremely simple and effective strategy for nucleation and growth of highly crystalline and aligned nanowires of regioregular poly (3-hexylthiophene) (P3HT) in the solvent that exhibits a remarkable enhancement in charge transport properties. We have used the combination of marginal solvent (dichloromethane) and ultrasonication to obtain highly crystalline, aligned and ordered nanowires of P3HT by means of off-center spin coating and compared with their on-center counterparts also. Macroscopically ordered and aligned nanowires of P3HT are confirmed by atomic force microscopy (AFM) and polarized optical microscopy (POM). Grazing incidence X-ray diffraction (GIXD) and high-resolution transmission electron microscopy (HR-TEM) demonstrated highly crystalline π–π stacked lamellar structure. The organic field-effect transistors (OFETs) based on highly oriented and crystalline P3HT nanowires exhibited a 50-fold enhancement in charge carrier mobility (average mobility 0.053 cm2V−1s−1) compared to OFETs based on P3HT film prepared in a good solvent (chloroform), measured under the ambient condition. The results are further supported by UV–vis spectroscopy, cyclic voltammetry, and theoretical calculation. Our study is a promising strategy for achieving large-scale aligned nanostructures of solution processable conjugated polymer film to enable high-performance organic electronic devices.

Reduction Treatment of Molecular-Doped Polymer Semiconductors for High-Performance N-Channel Organic Field-Effect Transistors

Here, we study an efficient reduction treatment for improving the electron transport properties in n-type organic field-effect transistors (OFETs) based on solution-processed polymer semiconductors. If complementary-type printed electronic circuits are to be enabled, equivalently high-performance p-type and n-type organic semiconductors have to develop. Most organic semiconductors have p-type performance superior to the n-type performance. Therefore, high-performance n-type organic semiconductors and their doping process are essential. Reduced-state viologen is known to be a powerful molecular dopant via donating electrons to the conjugated polymer semiconductors. However, the reduced-state viologen readily converts to its initial ionic species under ambient conditions, which leads to insufficient n-doping performance, especially for low electron affinity organic semiconductors. N-doped polymer semiconductors using pristine viologen molecular dopants show inadequate n-channel OFET behavior. For enhancing the electron transfer doping process, solution-based reduction treatment is used to improve remarkably the performance of n-channel OFETs based on initially p-type dominant donor-acceptor-type ambipolar polymer semiconductors. This solution-processed organic active channel layer can be produced at low cost and can be applied to flexible electronic and optoelectronic devices by using various graphic art printing techniques.

Programmed Design of Highly Crystalline Organic Semiconductor Patterns with Uniaxial Alignment via Blade Coating for High-Performance Organic Field-Effect Transistors.

A solution-printing technique that enables the patterning and aligning of organic semiconducting crystals is necessary for their practical application. Here, we report the facile growth of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) semiconducting crystal patterns via a novel blade-coating technique. Defining low/high shearing-speed regions alternatively in a programmed manner enables the growth of TIPS-PEN crystals in low-speed regions and their patterning in high-speed regions. Various crystal-analysis tools, including polarized UV-vis absorption spectroscopy, grazing-incidence wide-angle X-ray scattering, and near-edge X-ray absorption fine structure, reveal that a crystal grown at an optimum shearing speed is highly oriented along the shearing direction with high crystallinity, and its molecules have a more edge-on orientation for efficient lateral-charge transport. As a result, organic field-effect transistors comprised of these crystals show a high field-effect mobility of up to 1.74 cm2/(V s). In addition, various crystal patterns can be created by simply changing the programming parameters, suggesting the broad utility of the crystal patterns and printing technique.

Large-Area MXene Electrode Array for Flexible Electronics.

MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have potential for application as high-performance, low-cost electrodes in organic field-effect transistors (OFETs) because of their water dispersibility, high conductivity, and work-function tunability. In this study, we successfully fabricated a large-scale, uniform Ti3C2Tx MXene electrode array on a flexible plastic substrate for application in high-performance OFETs. The work function of the Ti3C2Tx MXene electrodes was also effectively modulated via chemical doping with NH3. The fabricated OFETs with Ti3C2Tx MXene electrodes exhibited excellent device performance, such as a maximum carrier mobility of ∼1 cm2·V-1·s-1 and an on-off current ratio of ∼107 for both p-type and n-type OFETs, even though all the electrode and dielectric layers were fabricated on the plastic substrate by solution processing. Furthermore, MXene-electrode-based complementary logic circuits, such as NOT, NAND, and NOR, were fabricated via integration of p-type and n-type OFETs. The proposed approach is expected to expand the application range of MXenes to other OFET-based electronic devices, such as organic light-emitting displays and electronic skins.

High-Performance Organic Field-Effect Transistors Fabricated with High-k Composite Polymer Gel Dielectrics

This study presents a composite gel-like dielectric material for organic field-effect transistors (OFET) applications. Poly(methyl methacrylate) (PMMA) gelled with propylene carbonate was used as gel dielectric material. Copper phthalocyanine was used as active layer in the OFET structures. In order to enhance the performance of the PMMA-gel dielectric material, silicium dioxide (SiO2) was used as an additive material. Various ratios of SiO2 were added to the gel dielectric and the effect of SiO2 on the OFET performance was investigated. It was clearly observed that SiO2 enhanced the performance and source-drain current of the fabricated OFETs. SiO2 was added to the PMMA-gel with different doping ratios of 0%, 10%, 30%, 50% and 100% by using a solution-processing method. The dielectric properties of the PMMA-gel:SiO2 composite materials were analyzed with impedance spectroscopy in terms of their effective capacitance ©I), tangent factor (tan(δ)), phase angle and complex dielectric constant (e′ and e″). The hole mobility of the OFETs was enhanced by 50% SiO2 nanoparticles in PMMA-gel dielectric materials from 6.83 × 10−1 cm2 V−1 s−1 to 4.66 × 100 cm2 V−1 s−1 (at VDS = − 0.5 V). The time-dependent IDS curves were analyzed for OFETs fabricated with PMMA-gel:SiO2 composite dielectric layers. It was found that all the devices worked stably under bias stress and gave fast responses for all gate voltages.

Controllable microstructure of polymer-small molecule blend thin films for high-performance organic field-effect transistors

The carrier mobility is an important figure of merit and is sensitively influenced by the microstructure of semiconducting films such as crystallization and morphology. Here, a method of blending semiconducting small molecule 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and insulating polymer binder is reported to well control the microstructures of blend semiconducting films. It is found that the insulating polymer in the blends prevents the dewetting of C8-BTBT from the SiO2 substrate and provides a platform for C8-BTBT molecules to segregate and crystallize at the air-film interface. By controlling the spinning speed, polymer type and ratio of the blend solution, it is possible to regulate the domain size and the surface roughness of C8-BTBT films, which in turn determines the performances of OTFTs. The bottom gate transistors based on C8-BTBT/polystyrene(PS) blend demonstrate excellent stability and a mobility up to 6.80 cm2/Vs due to the large domain size, smooth grain boundary and the phase-separated interface, which shows the potential applications in high-performance flexible and printed electronics.

An A−D−Aʹ−Dʹ strategy enables perylenediimide-based polymer dyes exhibiting enhanced electron transport characteristics

In this work, we develop two new A−D−Aʹ−Dʹ type polymer dyes, namely PPDIDTBT and PPDIDTBT-2F, containing perylenediimide (PDI), thiophene, and benzothiadiazole units. Both polymer dyes have high thermal stability and broad absorption throughout the UV–vis region. More importantly, the introduction of the second electron-withdrawing units, benzothiadiazoles, endows both polymer dyes with low-lying LUMO energy levels of −3.85 eV for PPDIDTBT and −3.88 eV for PPDIDTBT-2F. The charge transport properties of the two polymer dyes were examined by fabricating organic field-effect transistors (OFETs) with top-gate/bottom-contact configuration. Both polymer dyes exhibit enhanced electron transport characteristics with the highest electron mobility of 0.070 cm2 V−1 s−1, which is among the highest values for PDI-based polymer semiconductors reported to date. Our results highlight that the A−D−Aʹ−Dʹ strategy is a useful approach in tuning orbital energetic and charge transport properties of polymer dyes for developing high-performance OFETs.

Side-Chain Engineering To Optimize the Charge Transport Properties of Isoindigo-Based Random Terpolymers for High-Performance Organic Field-Effect Transistors

Random copolymerization is a simple and effective method to regulate and improve the performance of conjugated polymers. Herein, random copolymerization is employed to finely tune the transport beh...