High-performance Organic Field-effect Transistors Research Articles

P-n heterojunctions composed of two-dimensional molecular crystals for high-performance ambipolar organic field-effect transistors

Bilayer p-n heterojunctions are promising structures to construct ambipolar organic field-effect transistors (aOFETs) for organic integrated circuits. However, due to the lack of effective strategies for high-quality p-n heterojunctions with clear interfaces, the performance of aOFETs is commonly and substantially lower than that of their unipolar counterparts, which hinders the development of aOFETs toward practical applications. Herein, a one-step solution crystallization strategy was proposed for the preparation of high-quality bilayer p-n heterojunctions. A mixed solution of a p- and an n-type organic semiconductor was dropped on a liquid substrate, and vertical phase separation occurred spontaneously during crystallization to produce bilayer p-n heterojunctions composed of molecularly thin two-dimensional molecular crystals. Due to the clear interface of the bilayer p-n heterojunctions, the maximum mobility (average mobility) reached 1.96 cm2 V−1 s−1 (1.12 cm2 V−1 s−1) for holes and 1.27 cm2 V−1 s−1 (0.61 cm2 V−1 s−1) for electrons in ambient air. So far as we know, these values were the highest among double-channel aOFETs measured in ambient air. This work provides a simple yet efficient strategy to construct high-quality bilayer p-n heterojunctions, which lays a foundation for their application in high-performance optoelectronic devices.

Diketopyrrolopyrrole Based Organic Semiconductor Materials for Field-Effect Transistors.

Over the past several decades, organic conjugated materials as semiconductors in organic field effect transistors (OFETs) have attracted more and more attention from the scientific community due to their intriguing properties of mechanical flexibility and solution processability. However, the device fabrication technique, design, and synthesis of novel organic semiconductor materials with high charge carrier mobility is crucial for the development of high-performance OFETs. In the past few years, more and more novel materials were designed and tested in the OFETs. Among which, diketopyrrolopyrrole (DPP) and its derivatives, as the electron acceptors to build donor-acceptor (D-A) typed materials, are the perspective. In this article, recently reported molecules regarding the DPP and its derivatives for OFETs application are reviewed. In addition, the relationship between the chemical structures and the performance of the device are discussed. Furthermore, an outlook of DPP-based materials in OFETs with a future design concept and the development trend are provided.

Dual-function surfactant strategy for two-dimensional organic semiconductor crystals towards high-performance organic field-effect transistors

Dual-function surfactant strategy for two-dimensional organic semiconductor crystals towards high-performance organic field-effect transistors

Recent advances on π-conjugated polymers as active elements in high performance organic field-effect transistors

As high-performance organic semiconductors, π-conjugated polymers have attracted much attention due to their charming advantages including low-cost, solution processability, mechanical flexibility, and tunable optoelectronic properties. During the past several decades, the great advances have been made in polymers-based OFETs with p-type, n-type or even ambipolar characterics. Through chemical modification and alignment optimization, lots of conjugated polymers exhibited superior mobilities, and some mobilities are even larger than 10 cm2·V−1·s−1 in OFETs, which makes them very promising for the applications in organic electronic devices. This review describes the recent progress of the high performance polymers used in OFETs from the aspects of molecular design and assembly strategy. Furthermore, the current challenges and outlook in the design and development of conjugated polymers are also mentioned.

High-Performance Organic Field-Effect Transistors Gated by Imidazolium-Based Ionic Liquids

Inducement of high charge carrier density in organic field-effect transistors (OFETs) is a subject of immense interest due to its fundamental and applied impact on device operation. Here, the high ...

2,2′-(Arylenedivinylene)bis-8-hydroxyquinolines exhibiting aromatic π–π stacking interactions as solution-processable p-type organic semiconductors for high-performance organic field effect transistors

A new class of solution-processable small molecule organic semiconductors that are capable of functioning at low operating voltages (∼6 V) have been developed and their structure–activity correlation has been studied using crystallographic analyses.

Interface engineering of moisture-induced ionic albumen dielectric layers through self-crosslinking of cysteine amino acids for low voltage, high-performance organic field-effect transistors.

The interface roughness between the semiconducting and dielectric layers of organic field-effect transistors (OFETs) plays a crucial role in the charge transport mechanism through the device. Here we report the interface engineering of a moisture induced ionic albumen material through systematic control of the temperature-dependent self-crosslinking of cysteine amino acids in the dielectric layer. The evolution of the surface morphologies of albumen and pentacene semiconducting films has been studied to achieve a smooth interface for enhanced charge transport. A structural transition of pentacene films from crystalline dendrite to amorphous was induced by the higher surface roughness of the albumen film. The devices showed a high transconductance of 11.68 μS at a lower threshold voltage of -0.9 V.

2D molecular crystal templated organic p–n heterojunctions for high-performance ambipolar organic field-effect transistors

2D molecular crystals were used as the templates to produce bilayer p–n heterojunctions for high-performance ambipolar organic field-effect transistors.

Stencil mask defined doctor blade printing of organic single crystal arrays for high-performance organic field-effect transistors

A novel printing method to realize a solution processed large-area, high-resolution organic single crystal array for OFET applications.

Role of new synthesized diethylenetriamine-based protic ionic liquids in organic semiconductor devices

The charge carrier transport and related transport layers are extremely essential issue for obtaining high-efficient Organic Light Emitting Devices (OLEDs) and high-performance Organic Field Effect Transistor (OFETs). In an attempt to light on the effects of diethylenetriamine (DETA) based protic ionic liquids (PILs) on OLEDs and OFET, PILs were used as a hole transport layer and dielectric layer in OLEDs and OFETs, respectively. Charge transfer properties of synthesized two novel DETA based PILs (PIL1 and PIL2) on OLED device performance were investigated by spinning on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) due to their ionic characteristics. The PIL2 based OLED device showed threefold luminance (1038 cd/m2) compared to reference device (360 cd/m2) without effecting its light output emission wavelength. When analyzed OFET performances, device with PIL1 possessed the highest performance in terms of low threshold voltage (VTh∼1.5 V), operating voltage (V∼5 V) and reproducibility in the ambient atmosphere in comparison to device without PILs. Moreover, Density Functional Theory (DFT) calculations and impedance spectroscopic analysis of ionic liquids (ILs) were investigated. The carrier transport properties and mobility measurement has been identified with impedance spectroscopy of the devices. PIL1 had p-type transport property with mobility up to 0.198 cm2/Vs while the device without PILs had mobility 0.0237 cm2/Vs.

Effect of Alkyl Chain Length on Charge Transport Property of Anthracene-Based Organic Semiconductors.

Anthracene, a simple planar building block for organic semiconductors, shows strong intermolecular interactions and exhibits strong blue fluorescence. Thus, its derivatives have a great potential to integrate considerable charge carrier mobility and strong emission within a molecule. Here, we systematically studied the influence of alkyl chain length on the crystal structures, thermal properties, photophysical characteristics, electrochemical behaviors, and mobilities for a series of 2,6-di(4-alkyl-phenyl)anthracenes (Cn-Ph-Ants, where n represents the alkyl chain length). Among them, Cn-Ph-Ants (n = 0, 1, 2, and 3) display similar layered herringbone (LHB) packing motifs, which facilitate two-dimensional charge transport and thereby enables high-performance organic field-effect transistors (OFETs). All Cn-Ph-Ants exhibit similar work functions and show strong blue fluorescence with photoluminescence quantum yields (PLQY) of approximately 40% in toluene. In addition, the absolute powder PLQYs of C0-, C2-, C3-, C4-, and C6-Ph-Ants are 24.6, 8.2, 5.7, 10.9, and 8.6%, respectively. Note that the alkyl chain length shows a significant effect on the charge mobilities of Cn-Ph-Ants. Our newly synthesized C1-, C3-, and C4-Ph-Ants show hole mobilities of up to 2.40, 1.34, and 1.00 cm2 V-1 s-1, respectively, with mobilities of 3.40, 1.57, and 0.82 cm2 V-1 s-1 for C0-, C2-, and C6-Ph-Ants, indicating an increasing tendency of mobility with shorter alkyl chain length. This feature is related to the microstructures of the thin films, which reveal the enhanced film order, crystallinity, and grain size with a decrease in the alkyl chain length. Moreover, we theoretically analyze the intermolecular transfer integrals of HOMOs, which increase at T-shaped contacts as the alkyl chain length decreases, which improves the intermolecular charge transport properties, leading to the increases in mobility. Interestingly, the anisotropy of the transfer integral tends to decrease upon substitution with longer alkyl chains, suggesting that alkyl chain adjustments may facilitate isotropic charge transport property in 2,6-alkylated anthracenes.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity.

SummaryPolyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

Polymer Electret Improves the Performance of the Oxygen-Doped Organic Field-Effect Transistors

Chemical doping is widely used in the electronic devices. In p-type semiconductor thin films, oxygen doping fills the hole traps and increases hole concentrations, improving the performance of the organic field-effect transistors (OFETs). Due to the low ionization potential for p-type semiconductors, the superfluous holes induced by the oxygen doping degrades the OFETs off-state leakage performance. On the other hand, for p-type semiconductors with high ionization potential (up to 5.5-6.0 eV), the limited oxidation of oxygen is hard to achieve satisfactory doping concentrations to fill the trap states. This refers to the well-known intrinsic incompatibility between the oxygen doping and high-performance OFETs. Herein, a novel strategy is introduced to overcome the incompatibility and achieve high-performance OFETs by using the structural polymer electret. That is, moderate hole concentrations induced by low-pressure (30 Pa) oxygen plasma fill the hole traps within semiconductor. And the built-in field resulted from polymer electret accumulates the holes inside semiconductor near the semiconductor/electret interface, thus improving the OFETs performance. Using a model organic semiconductor with high ionization potential-2,7-didodecyl[1]benzothieno [3,2-b][1]benzothiophene (C12-BTBT) as an example, the high-performance OFETs with field-effect mobility ( $\mu _{FET}$ ) of 3.5 cm2V−1 s−1, subthreshold-swing ( SS ) of 110 mV decade−1, on-off ratio of 104, and widely-tunable threshold voltage ( ${V} _{t}$ ) are realized at a low voltage below 2 V in the open air.

Direct Printing of Asymmetric Electrodes for Improving Charge Injection/Extraction in Organic Electronics.

Engineering the energy levels of organic conducting materials can be useful for developing high-performance organic field-effect transistors (OFETs), whose electrodes must be well controlled to facilitate easy charge carrier transport from the source to drain through an active channel. However, symmetric source and drain electrodes that have the same energy levels are inevitably unfavorable for either charge injection or charge extraction. In this study, asymmetric source and drain electrodes are simply prepared using the electrohydrodynamic (EHD)-jet printing technique after the careful work function engineering of organic conducting material composites. Two types of additives effectively tune the energy levels of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate-based composites. These solutions are alternately patterned using the EHD-jet printing process, where the use of an electric field makes fine jet control that enables to directly print asymmetric electrodes. The asymmetric combination of EHD-printed electrodes helps in obtaining advanced charge transport properties in p-type and n-type OFETs, as well as their organic complementary inverters. This strategy is believed to provide useful guidelines for the facile patterning of asymmetric electrodes, enabling the desirable properties of charge injection and extraction to be achieved in organic electronic devices.

Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{--}\text{decyl}\text{--}7\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b]\text{naphtho}[2,3\text{--}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

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.