Performance Of Organic Transistors Research Articles

High‐Resolution Lithography for High‐Frequency Organic Thin‐Film Transistors

AbstractOrganic thin‐film transistors are field‐effect transistors comprising a semiconductor in the form of a thin, typically polycrystalline layer of conjugated organic molecules. Since organic transistors can often be fabricated at temperatures no higher than about 100 °C, they are potentially useful for flexible, large‐area electronics applications. An important performance parameter of organic transistors is the transit frequency, which is the highest frequency at which the transistors can be operated. The transit frequency of organic transistors is determined in large part by the channel length and the parasitic gate‐to‐source and gate‐to‐drain overlap lengths. How small these dimensions can be made depends (at least in the case of transistors fabricated in the lateral device architecture) greatly on the resolution of the lithography method that is utilized for the patterning of the gate electrodes and the source and drain contacts. Patterning methods that have yielded organic transistors with lateral dimensions sufficiently small to provide transit frequencies above 10 MHz include photolithography, laser lithography, stencil lithography, and electron‐beam lithography. In this review, these four lithography methods and their roles in the fabrication of high‐frequency organic transistors, as well as their prospects for future improvements in the dynamic performance of organic transistors, will be illuminated.

Direct Writing of Aligned Conjugated Polymer Micro-Ribbons for High-Performance Organic Electronics

Organic semiconductors (OSCs) and relevant devices have presented their great potential in flexible and wearable electronics. Controlled solution deposition of organic semiconductors at a limited local region is highly expected because it can realize feasible and low-cost fabrication of highly integrated devices with low current leakage and free crosstalk between adjacent devices. Here, we reported a feasible microscale meniscus-guided direct writing method to purposely deposit conjugated polymer poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno[3,2-b]thiophene)] micro-ribbons at preset local regions. The direct writing of OSCs was carried out on a microscale with the width of written micro-ribbons down to 5 μm, which was seldomly reported before. Direct writing operation induced anisotropic crystallization of organic semiconducting micro-ribbons, which further enhanced the electrical performance of organic transistors with a device on/off ratio of 5.1 × 104 and carrier mobility of 1.05 cm2 V–1 s–1. Small-angle X-ray diffraction analysis further proved the existence of anisotropic crystallization behavior. This technique shed light on the feasible and convenient fabrication of future high-performance organic electronics.

Organic bipolar transistors

Devices made using thin-film semiconductors have attracted much interest recently owing to new application possibilities. Among materials systems suitable for thin-film electronics, organic semiconductors are of particular interest; their low cost, biocompatible carbon-based materials and deposition by simple techniques such as evaporation or printing enable organic semiconductor devices to be used for ubiquitous electronics, such as those used on or in the human body or on clothing and packages1–3. The potential of organic electronics can be leveraged only if the performance of organic transistors is improved markedly. Here we present organic bipolar transistors with outstanding device performance: a previously undescribed vertical architecture and highly crystalline organic rubrene thin films yield devices with high differential amplification (more than 100) and superior high-frequency performance over conventional devices. These bipolar transistors also give insight into the minority carrier diffusion length—a key parameter in organic semiconductors. Our results open the door to new device concepts of high-performance organic electronics with ever faster switching speeds.

Immobilizing a π-Conjugated Catecholato Framework on Surfaces of SiO2 Insulator Films via a One-Atom Anchor of a Platinum Metal Center to Modulate Organic Transistor Performance.

Chemical modification of insulating material surfaces is an important methodology to improve the performance of organic field-effect transistors (OFETs). However, few redox-active self-assembled monolayers (SAMs) have been constructed on gate insulator film surfaces, in contrast to the numerous SAMs formed on many types of conducting electrodes. In this study, we report a new approach to introduce a π-conjugated organic fragment in close proximity to an insulating material surface via a transition metal center acting as a one-atom anchor. On the basis of the reported coordination chemistry of a catecholato complex of Pt(II) in solution, we demonstrate that ligand exchange can occur on an insulating material surface, affording SAMs on the SiO2 surface derived from a newly synthesized Pt(II) complex containing a benzothienobenzothiophene (BTBT) framework in the catecholato ligand. The resultant SAMs were characterized in detail by water contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy, and cyclic voltammetry. The SAMs served as good scaffolds of π-conjugated pillars for forming thin films of a well-known organic semiconductor C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), accompanied by the engagements of the C8-BTBT molecules with the SAMs containing the common BTBT framework at the first layer on SiO2. OFETs containing the SAMs displayed improved performance in terms of hole mobility and onset voltage, presumably because of the unique interfacial structure between the organic semiconducting and inorganic insulating layers. These findings provide important insight into creating new elaborate interfaces through installing coordination chemistry in solution to solid surfaces, as well as OFET design by considering the compatibility between SAMs and organic semiconductors.

Engineering Asymmetric Charge Injection/Extraction to Optimize Organic Transistor Performances

The introduction of an appropriate functionality on the electrode/active layer interface has been found to be an efficient methodology to enhance the electrical performances of organic field-effect transistors (OFETs). Herein, we efficiently optimized the charge injection/extraction characteristics of source/drain (S/D) electrodes by applying an asymmetric functionalization at each individual electrode/organic semiconductor (OSC) interface. To further clarify the functionalizing effects of the electrode/OSC interface, we systematically designed five different OFETs: one with pristine S/D electrodes (denoted as pristine S/D) and the remaining ones made by symmetrically or asymmetrically functionalizing the S/D electrodes with up to two different self-assembled monolayers (SAMs) based on thiolated molecules, the strongly electron-donating thiophenol (TP) and electron-withdrawing 2,3,4,5-pentafluorobenzenethiol (PFBT). Both the S and D electrodes were functionalized with TP (denoted as TP-S/D) in one of the two symmetric cases and with PFBT in the other (PFBT-S/D). In each of the two asymmetric cases, one of the S/D electrodes was functionalized with TP and the other with PFBT (to produce PFBT-S/TP-D and TP-S/PFBT-D OFETs). The vapor-deposited p-type dinaphtho[2,3- b:2',3'- f]thieno[3,2- b]thiophene was used as the OSC active layer. The PFBT-S/TP-D case exhibited a field-effect mobility (μFET) of 0.86 ± 0.23 cm2 V-1 s-1, about three times better than that of the pristine S/D case (0.31 ± 0.12 cm2 V-1 s-1). On the other hand, the μFET of the TP-S/PFBT-D case (0.18 ± 0.10 cm2 V-1 s-1) was significantly lower than that of the pristine case and even lower than those of the TP-S/D (0.23 ± 0.07 cm2 V-1 s-1) and PFBT-S/D (0.58 ± 0.19 cm2 V-1 s-1) cases. These results were clearly correlated with the additional hole density, surface potential, and effective work function. In addition, the contact resistance ( RC) for the asymmetric PFBT-S/TP-D case was 10-fold less than that for the TP-S/PFBT-D case and more than five times lower than that for the pristine case. The results contribute a meaningful step forward in improving the electrical performances of various organic electronics such as OFETs, inverters, solar cells, and sensors.

Enhancing the performance of solution-processed organic thin-film transistors by blending binary compatible small molecule semiconductors

Abstract Physical blending is a facile and effective way to improve the performance of solution processed organic thin-film transistors (OTFTs). Blending small molecule semiconductors with soluble polymers has been extensively studied in recent years. However, blending between binary small molecule semiconductors is rare due to the difficulty to obtain ideal thin films. Herein, we systematically investigate the blending effects on the morphologies of thin films and their field-effect performance by using two small molecule semiconductors, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, (C12-Ph-BTBT), which have the same aromatic skeleton. Molecular ordering and better crystallinity are observed in most of spin-coated blend thin films, thanks to the enhanced molecular interaction after blending. As a result, OTFTs based on blend thin films exhibit improved performance in most cases, with the highest average hole mobility about 1.5 cm2 V−1 s−1 demonstrated. Further device performance improvements are demonstrated by blending polystyrene with Ph-BTBT and C12-Ph-BTBT blends. The results here indicate that blending between small molecule semiconductors with compatible fused ring structures may be a promising strategy to enhance the performance of organic transistors.

Cu-Thienoquinone Charge-Transfer Complex: Synthesis, Characterization, and Application in Organic Transistors.

A facile and unusual reaction between thienoquinone compound QDTBDT2C and copper is reported. The formation of Cu-QDTBDT2C complex is proved by absorption spectra, IR spectra, Raman spectra, and X-ray photoelectron spectroscopy data. This complex can serve as a doping layer at the interface of Cu/QDTBDT2C and greatly improve the performance of organic transistors in which the copper electrode is source/drain electrodes and QDTBDT2C is an active layer. The transistors display an electron mobility of 0.95 cm2 V-1 s-1, to our knowledge, the highest electron mobility reported for copper electrode-based n-type transistors and nearly two times higher than that of the Au electrode-based devices. These results demonstrate the potential applications of Cu-QDTBDT2C complex in organic electronics, and the unique properties of QDTBDT2C (spontaneously reacting with copper) provide a new insight into the design of n-type organic semiconductors for copper electrode-based organic transistors.

Will We See Gigahertz Organic Transistors?

AbstractDespite the many advances in the materials and technology of organic transistors, the highest transit frequency reported for organic transistors has not been improved for more than half a decade and remains far below 1 GHz. One reason is that most contributions to the field have traditionally focused on parameters that have little or no impact on the high‐frequency performance of organic transistors. By analyzing the fundamental equations for the transit frequency and the effective (or apparent) carrier mobility, the requirements under which organic transistors can be expected to show a transit frequency of 1 GHz are reiterated. Not surprisingly, it is found that the critical parameter in this quest is not the charge‐carrier mobility in the organic semiconductor layer, but the contact resistance, along with the channel length and the parasitic gate‐to‐contact overlaps.

Systematic optimization of MWCNT-PEDOT:PSS composite electrodes for organic transistors and dye-sensitized solar cells: Effects of MWCNT diameter and purity

Abstract Multi-walled carbon nanotubes (MWCNTs) and polymer composites have attracted significant attention as metal-free electrode materials for various electronic devices. Many studies have presented strategies to enhance the electrical conductivity of MWCNT/polymer composites. However, systematic studies on the effects of MWCNT features on the composite properties are still missing. Such studies can further the development of MWCNT/polymer-based conducting materials. Herein, we characterize composite films of MWCNTs and poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS), with variations in the MWCNT diameter and purity. MWCNT features greatly affect the physical/chemical/electrical properties of the MWCNT/PEDOT:PSS composite films. In addition, the diameter and purity of MWCNTs also influenced the transition behavior of the composite films during the hydrochloric acid-methanol treatment, which was performed for increasing electrical conductivity. As a result, we could optimize the performance of organic transistors and dye-sensitized solar cells (DSSCs) using MWCNT/PEDOT:PSS composite films as source/drain electrodes and catalytic counter electrodes, respectively. Furthermore, using the optimized device parameters, we successfully fabricated high performance fluorine doped tin oxide-free DSSCs and high-gain organic complementary inverters using MWCNT/PEDOT:PSS composite electrodes.

Effect of alkyl side chains on properties and organic transistor performance of 2,6-bis(2,2′-bithiophen-5-yl)naphthalene

Herein we report the synthesis and characterization of C2-symmetric 2,6-bis(5′-alkyl-2,2′-bithiophen-5-yl)naphthalene derivatives with alkyl side chains of various length and shape, including even the less commonly examined cyclohexyl and cyclohexylalkyl groups. By investigation of how these side chains influence solubility, self-assembly and charge mobility, we aimed to extend the knowledge of structure-function property relationship of the oligothiophene compounds. Charge mobilities observed in this study ranged from 10−3 to 10−1cm2 V−1s−1. The highest values referred to thin layers of derivatives end-capped with linear alkyl chains. XRD methods uncovered OFET-favorable layering of tilted molecular arrangement with the molecular long axes in the out of plane direction. Although the charge mobilities showed variation of as much as two orders of magnitude, XRD measurements of these derivatives revealed similar packing and self-assembly behavior. Hence, the charge mobility differences are more a result of the naturally diverse spatial demands of the tested alkyl groups, with the linear alkyl chains offering the tightest close packing of the π-conjugated backbone. On the other hand, the bulky 2-hexyldecyl group turned out to be the only group that made it possible to avoid insolubility of these derivatives and provided excellent solubility (23mg/mL of chloroform), while the charge mobility remained acceptable even of the spin-coated thin layers.

Mapping charge carrier density in organic thin-film transistors by time-resolved photoluminescence lifetime studies

The device performance of organic transistors is strongly influenced by the charge carrier distribution. A range of factors effect this distribution, including injection barriers at the metal-semiconductor interface, the morphology of the organic film, and charge traps at the dielectric/organic interface or at grain boundaries. In our comprehensive experimental and analytical work we demonstrate a method to characterize the charge carrier density in organic thin-film transistors using time-resolved photoluminescence spectroscopy. We developed a numerical model that describes the electrical and optical responses consistently. We determined the densities of free and trapped holes at the interface between the organic layer and the SiO2 gate dielectric by comparison to electrical measurements. Furthermore by applying fluorescence lifetime imaging microscopy we determine the local charge carrier distribution between source and drain electrodes of the transistor for different biasing conditions. We observe the expected hole density gradient from source to drain electrode.

Blending crystalline/liquid crystalline small molecule semiconductors: A strategy towards high performance organic thin film transistors

Solution processed small molecule polycrystalline thin films often suffer from the problems of inhomogeneity and discontinuity. Here, we describe a strategy to solve these problems through deposition of the active layer from a blended solution of crystalline (2-phenyl[1]benzothieno[3,2-b][1]benzothiophene, Ph-BTBT) and liquid crystalline (2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, C12-Ph-BTBT) small molecule semiconductors with the hot spin-coating method. Organic thin film transistors with average hole mobility approaching 1 cm2/V s, much higher than that of single component devices, have been demonstrated, mainly due to the improved uniformity, continuity, crystallinity, and stronger intermolecular π-π stacking in blend thin films. Our results indicate that the crystalline/liquid crystalline semiconductor blend method is an effective way to enhance the performance of organic transistors.

Interplay between Vacuum-Grown Monolayers of Alkylphosphonic Acids and the Performance of Organic Transistors Based on Dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene.

Monolayers of six alkylphosphonic acids ranging from C8 to C18 were prepared by vacuum evaporation and incorporated into low-voltage organic field-effect transistors based on dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). Similar to solution-assembled monolayers, the molecular order for vacuum-deposited monolayers improved with increasing length of the aliphatic tail. At the same time, Fourier transform infrared (FTIR) measurements suggested lower molecular coverage for longer phosphonic acids. The comparison of FTIR and vibration frequencies calculated by density functional theory indicated that monodentate bonding does not occur for any phosphonic acid. All monolayers exhibited low surface energy of ∼17.5 mJ/m(2) with a dominating Lifshitz-van der Waals component. Their surface roughness was comparable, while the nanomechanical properties were varied but not correlated to the length of the molecule. However, large improvement in transistor performance was observed with increasing length of the aliphatic tail. Upon going from C8 to C18, the mean threshold voltage decreased from -1.37 to -1.24 V, the field-effect mobility increased from 0.03 to 0.33 cm(2)/(V·s), the off-current decreased from ∼8 × 10(-13) to ∼3 × 10(-13) A, and for transistors with L = 30 μm the on-current increased from ∼3 × 10(-8) to ∼2 × 10(-6) A, and the on/off-current ratio increased from ∼3 × 10(4) to ∼4 × 10(6). Similarly, transistors with longer phosphonic acids exhibited much better air and bias-stress stability. The achieved transistor performance opens up a completely "dry" fabrication route for ultrathin dielectrics and low-voltage organic transistors.

Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Effect of the initial stage of film growth on device performance of organic transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

The initial stage of organic film growth is considered to be vital for the carrier transport in organic thin-film transistors with bottom gate configuration. The same topographies of 40nm dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) films on para-sexiphenyl (p-6P) monolayer and bare SiO2 exhibited quite different field-effect mobilities, 1.9 and 0.1cm2/Vs, respectively. The further investigation indicated there were different growth behaviors at their initial stages of film growth. Column islands with high density were observed on SiO2, while lamina islands on p-6P monolayer due to the good diffusion ability and their good epitaxial relationship. The latter is beneficial to obtain high quality film with less boundaries and defects. The work demonstrated that the initial stage of film growth is an important factor to determine the device performance of organic transistors, which is significant to improve the device fabrication and optimize the device performance.

High-speed organic transistors with three-dimensional organic channels and organic rectifiers based on them operating above 20 MHz

Three-dimensional organic transistors (3D-OFETs) comprising vertical short channels are developed to raise the operational speed of organic transistors. The devices with a short-channel length of 0.8μm and reduced parasitic capacitance operate at up to 20MHz with an applied drain voltage of −15V. Organic rectifiers based on the diode-connected 3D-OFETs are also demonstrated to operate at above 20MHz, even with an applied effective voltage of about 4V, which is higher than the speed of radio frequency identification tags of 13.56MHz required in near field communication. These techniques boost the performance of organic transistors and can help to realize the breakthrough for practical applications of organic logic circuits used as key components in various flexible or plastic devices.

Mesoscale control of organic crystalline thin films: Effects of film morphology on the performance of organic transistors

We report mesoscale control of small molecular 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) crystalline thin films by varying the solute concentration in the fluidic channel method. A stepwise increase in the TIPS-pentacene concentration in the solution enabled us to prepare highly-crystallized ribbons, thin films, and thick films in a mesoscale range, respectively. All three types of deposited films exhibited an in-plane crystalline nature of (001) direction being normal to the substrate as well as crystalline domain growth parallel to the direction of the receding meniscus inside the fluidic channel. In addition, the film’s morphology and thickness were found to have a great influence on the field-effect mobility of the transistors, and the highest average and maximum mobilities were achieved from transistors with thin-film semiconductor channels.

Poly(3-hexylthiophene) (P3HT)/Graphene Nanocomposite Material Based Organic Field Effect Transistor with Enhanced Mobility

The major drawback of organic field effect transistors (OFETs) is its lower mobility, which restricts their applications for high performance devices. Recently, graphene exhibits excellent carrier mobility, therefore, is used as a novel electronic substance for the fundamental research and several potential applications. Pristine graphene is not applicable in field effect transistors (FETs) for satisfactory on/off current ratio as it has no forbidden energy gap. Here, we report the fabrication as well as characterizations of poly-3-hexylthiophene (P3HT)/graphene nanocomposite (with two distinct concentrations i.e., 0.05 and 0.1 mg/ml of graphene in P3HT solution) based FETs to over come the limitations. The current-voltage (I-V) characteristics of P3HT/graphene based FETs are measured and key performance parameters of device are compared against only polymer P3HT based FETs. The analysis demonstrates that, in P3HT/graphene transistors some crucial parameters such as drain saturation current and mobility enhanced drastically although the on/off ratio reduced significantly. Our study demonstrates that presence of graphene in organic semiconductor and a synergic effect due to uniform distribution in the nanospace is an ordinary route to achieve high mobility OFETs which impart an affordable way for raising the performance of organic transistors.

A New Route to Low Resistance Contacts for Performance‐Enhanced Organic Electronic Devices

The barrier to charge carrier injection across the semiconductor/electrode interface is a key parameter in the performance of organic transistors and optoelectronic devices, and the work function of the electrode material plays an important role in determining the size of this barrier. We present a new, chemical route for making metal surfaces with low work functions, by functionalizing gold surfaces with self‐assembled monolayers of n,n‐dialkyl dithiocarbamates. Ultraviolet photoemission spectroscopy measurements show that work functions of 3.2 eV ± 0.1 eV can be achieved using this surface modification. Electronic structure calculations reveal that this low work function is a result of the packing‐density, polarization along the N‐C bond, and charge rearrangement associated with chemisorption. We demonstrate that electrodes functionalized with these monolayers significantly improve the performance of organic thin‐film transistors and can potentially be employed in charge selective contacts for organic photovoltaics.

有機半導体薄膜表面の自己凝集構造を用いた有機トランジスタの高性能化

有機半導体薄膜表面の自己凝集構造を用いた有機トランジスタの高性能化