High-performance Organic Thin-film Transistors Research Articles

Polymer blended small molecule organic field effect transistors with improved device-to-device uniformity and operational stability

We report high performance organic thin-film transistors (OTFTs) with improved device-to-device uniformity and operational stability using polymer-blended small molecule organic semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). The diF-TESADT blended with poly(α-methylstyrene) was spin-cast to form bottom-contact OTFTs, and an average carrier mobility of more than 0.16cm2/Vs with more uniform surface morphology and device-to-device uniformity compared to neat diF-TESADT devices were achieved. Additionally, the polymer-blended OTFTs have shown improved operational stability under gate bias-stress possibly due to blocking of ambient oxygen and moisture by vertically separated insulating matrix polymer.

A high-performance planar organic thin-film transistor

A high-performance planar organic thin-film transistor

Deposition-Pressure-Induced Optimization of Molecular Packing for High-Performance Organic Thin-Film Transistors Based on Copper Phthalocyanine

Copper phthalocyanine (CuPc) films have been prepared on SiO2 and octadecyltrichlorosilane (OTS)-treated SiO2 (OTS/SiO2) substrates by vacuum deposition under different deposition pressures (Pdep) ranging from 10–4 to 10–1 Pa. Experimental results indicate that Pdep has an obvious influence on the morphologies and molecular packing structures of the CuPc films and, therefore, the performance of the resulting thin-film transistors (TFTs). Specifically, the grain sizes of the CuPc films keep almost unchanged for Pdep below 10–2 Pa and significantly increase for further increasing Pdep to 10–1 Pa on both SiO2 and OTS/SiO2. The interplanar spacings (D values) of the films increase on SiO2 and keep unchanged on OTS/SiO2 with increasing Pdep, which has also been rationalized by the molecular dynamics simulation. Field-effect measurements indicate that the electronic properties of the CuPc thin films are closely correlated with their microstructures, and the film prepared at higher Pdep gives the higher mobility. In addition to the general results that larger grain size favors the higher mobility similar to literature reports, both our experimental and theoretical results reveal that the interplanar spacing of the film also has a sensitive influence on the mobility of the CuPc TFTs, and the larger D value leads to the higher mobility. This study provides a convenient way to tune the morphology and molecular packing of the CuPc films to optimize the performance simply by regulating Pdep and discloses an important parameter, that is, the D value, associated with the device performance, which is significant for basic understanding and potential applications.

High-performance organic thin-film transistors with polymer-blended small-molecular semiconductor films, fabricated using a pre-metered coating process

We herein present results obtained from our study of flat and uniform polymer-blended small-molecular semiconductor thin films. These films were produced for organic thin film transistors (OTFTs), using a simple pre-metered horizontal dipping process. The organic semiconductor thin films were composed of a small molecular 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) composite blended with a polymer binder of poly(α-methylstyrene) (PaMS). We show that the pre-metered solution-coating process allowed the critical control of the thickness of the TIPS-PEN:PaMS film; extremely thin films could be produced using the downstream meniscus of the solution at high speeds (of the order of a few metres per minute). The fabricated TIPS-PEN:PaMS OTFTs exhibited a maximum field-effect mobility of 0.22 cm2 V−1 s−1, and on/off ratios of over 105, that were consistently superior to those of conventional spin-cast devices. These results demonstrate that horizontal dip-coated TIPS-PEN:PaMS films show considerable promise for the production of reliable, reproducible, high-performance OTFTs.

Polymer blends with semiconducting nanowires for organic electronics

This article reviews the recent advances in organic electronics based on polymer blends with one-dimensional (1D) nanowires (NWs) of π-conjugated polymers. Self-assembled polymer semiconducting NWs are fascinating building blocks for their directional extension of inter- and intramolecular π-conjugation. This extensive conjugation provides unique electrical and optical properties that benefit applications in organic electronic devices. Outstanding performances are particularly expected from blending these NWs with insulating polymers or semiconducting molecules. Several representative reports describing NWs prepared from semiconducting/insulating polymer blends, conjugated block copolymers, or electrospinning for use as high-performance organic thin-film transistors (OTFTs) are discussed. The concepts of phase-separation behavior and the growth of crystalline NWs from multi-phase blend solutions are also illustrated. Research into the solubility-induced formation of NWs from semiconducting polymer/n-type small molecule blends for high-efficiency organic photovoltaic solar cells (OPVs) is introduced. In addition, the effects of the chemical properties of conjugated polymers on the optical and electrical properties of NWs and the use of ordered NW structures in OPVs are summarized. This critical review provides insights and a new perspective on the optimization of blend morphologies, which consequently enhances the performance of organic electronic devices.

High Performance Organic Thin Film Transistors with Solution Processed TTF-TCNQ Charge Transfer Salt as Electrodes

Fabrication of high-performance organic thin film transistors (OTFTs) with solution processed organic charge transfer complex (TTF-TCNQ) film as bottom contact source-drain electrodes is reported. A novel capillary based method was used to deposit the source-drain electrodes from solution and to create the channel between the electrodes. Both p- and n-type OTFTs have been fabricated with solution deposited organic charge transfer film as contact electrodes. Comparison of the device performances between OTFTs with TTF-TCNQ as source-drain electrodes and those with Au electrodes (both top and bottom contact) indicate that better results have been obtained in organic complex film contacted OTFT. The high mobility, low threshold voltage, and efficient carrier injection in both types of OTFTs implies the potential use of the TTF-TCNQ based complex material as low-cost contact electrodes. The lower work function of the TTF-TCNQ electrode and better contact of the complex film with the organic thin film owing to the organic-organic interface results in efficient charge transfer into the semiconductor yielding high device performance. The present method having organic metal as contact materials promises great potential for the fabrication of all-organics and plastic electronics devices with high throughput and low-cost processing.

Bilayer Source/Drain Electrodes Self-Aligned With Fluoropolymer Dielectrics for Stable High-Performance Organic TFTs

We propose a systematic fabrication method for stable high-performance organic thin-film transistors (TFTs) that are potentially compatible with high-density integrated circuits. Ag/PEDOT:PSS bilayers provide source/drain (S/D) electrodes with low sheet resistance and efficient hole injection capabilities, leading to high-performance bottom-contact pentacene TFTs with a saturation mobility of 0.19 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> / V·s. Patterned dielectric layers based on a fluoropolymer Cytop function as a hydrophobic bank structure to define S/D electrodes in a self-aligned manner from Ag ink and a PEDOT:PSS solution while simultaneously improving the electrical stability of pentacene TFTs.

Study of Surface-Modified PVP Gate Dielectric in Organic Thin Film Transistors with the Nano-Particle Silver Ink Source/Drain Electrode

We have fabricated the flexible pentacene based organic thin film transistors (OTFTs) with formulated poly[4-vinylphenol] (PVP) gate dielectrics treated by CF4/O2 plasma on poly[ethersulfones] (PES) substrate. The solution of gate dielectrics is made by adding methylated poly[melamine-co-formaldehyde] (MMF) to PVP. The PVP gate dielectric layer was cross linked at 90 degrees under UV ozone exposure. Source/drain electrodes are formed by micro contact printing (MCP) method using nano particle silver ink for the purposes of low cost and high throughput. The optimized OTFT shows the device performance with field effect mobility of the 0.88 cm2/V s, subthreshold slope of 2.2 V/decade, and on/off current ratios of 1.8 x 10(-6) at -40 V gate bias. We found that hydrophobic PVP gate dielectric surface can influence on the initial film morphologies of pentacene making dense, which is more important for high performance OTFTs than large grain size. Moreover, hydrophobic gate dielelctric surface reduces voids and -OH groups that interrupt the carrier transport in OTFTs.

The ultraviolet-ozone effects on organic thin-film transistors with double polymeric dielectric layers

Organic thin-film transistors (OTFTs) were fabricated based on the double polymeric insulators consisting of polyvinyl alcohol (PVA) and polystyrene (PS). The using of double dielectric layers exhibited an improved device performance comparing with the conventional devices with single dielectric layer. By exposing the polymer insulating layer (PS) under ultraviolet (UV) light for different times (0, 5, 10 and 15 min), the surface properties of the PS film has been further optimized for the growth of pentacene. The grain size and crystallization of pentacene film grown on PS are apparently improved as the increase of surface energy of PS. As a result, a field-effect mobility of 1.38 cm 2 V −1 S −1 and an on/off current ratio of 10 4 had been obtained by UV treatment for 10 min. All results indicate the surface energy of dielectric layer is a crucial factor to influence the devices performance that can be well tuned by UV treatment. The polymeric materials pair consisting of PS and PVA is a promised candidate as the dielectric layers for high-performance OTFTs.

Abrupt heating-induced high-quality crystalline rubrene thin films for organic thin-film transistors

We present a simple but effective approach to fabricate high-quality crystalline rubrene thin-film active layers for organic thin-film transistors (OTFTs) based on an abrupt heating process. Through this method, continuous, highly ordered, and highly oriented crystalline rubrene thin films comprising large single-crystalline grains (average size: ∼80 μm) can be remarkably rapidly produced in just 1 min without any dielectric surface modification process. OTFTs with carrier mobility as high as 1.21 cm 2 V −1 s −1 and on/off current ratios greater than 10 6 are demonstrated under air-ambient condition using the approach. These results suggest that our approach is very promising to fabricate high-performance OTFTs for practical applications in organic electronics.

Reduced Graphene Oxide Electrodes for Large Area Organic Electronics

Interlayer lithography is used to pattern highly conductive, solution-processed, reduced graphene oxide source and drain electrodes down to 10 μm gaps. These patterned electrodes allow for the fabrication of high-performance organic thin-film transistors and complementary circuits. The method offers a viable route towards organic electronics fabricated entirely by solution processing.

Diketopyrrolopyrrole as a p-channel organic semiconductor for high performance OTFTs

A new diketopyrrolopyrrole derivative 1 exhibits excellent hole mobilities of 0.7 cm(2) V(-1) s(-1) and a current on/off ratio of 10(6) under ambient conditions in bottom-gate, top-contact organic thin film transistors (OTFTs) fabricated by vacuum deposition.

Making Contacts to n-Type Organic Transistors Using Carbon Nanotube Arrays

We investigated the performance of carbon nanotube (CNT) array electrodes applied to n-type and ambipolar phenyl-C61-butyric acid methyl ester (PCBM) thin film transistors on a SiO(2) dielectric substrate. Compared to conventional Au electrodes, CNT arrays provide better injection efficiency, improved switching behavior, higher electron mobility, and lower contact resistance. Experiments on ambipolar PCBM transistors indicate that the injection performance is enhanced by the electrostatics of the CNT contacts, which promotes electron and hole tunneling across Schottky barriers at the PCBM/nanotube interface. The use of CNT arrays is a valid replacement to low workfunction metals, which are often reactive in air and difficult to process. Our work paves the way for a widespread use of carbon nanotube array electrodes in high-performance n-type and p-type organic thin film transistors.

Fabrication of organic thin-film transistors by spray-deposition for low-cost, large-area electronics

We report on high performance organic thin-film transistors fabricated by spray-deposition, an innovative coating technique for organic electronic devices. This method is simple and inexpensive, compatible with room-temperature processing and easily scalable from laboratory-based devices to large-area electronics. Our spray-deposited 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene transistors show very good homogeneity, with mobilities of 0.2 cm 2 / V s , and on/off ratios of 10 7 . This performance is comparable to that of our best spin-coated devices fabricated on the same substrate type, while offering additional advantages such as using 20 times less organic semiconductor and facilitating fast coating of large-areas.

Dual-gate field-effect transistors of octathio[8]circulene thin-films with ionic liquid and SiO2 gate dielectrics

Dual-gate organic thin-film transistors (OTFTs) of octathio[8]circulene (1) with ionic liquid and SiO2 gate dielectrics lead to good transistor performance with a high on/off ratio (∼105), a low threshold voltage (∼−5 V), a low subthreshold slope (∼150 mV/decade), and low power operation, thus surpassing the performance of the single-gate OTFTs of 1. This operation is a promising method for improving the carrier concentrations in OTFTs and for realizing high-performance OTFTs.

Photoactive Gate Dielectrics

Ageneral methodology towards stable high-performance photocontrollable organic thin-film transistors (OTFTs) by using photochromic spiropyrans (SP) combined with polymethyl methacrylate (PMMA) as a photoactive gate dielectric is reported. Capacitance and mobility transitions are demonstrated theoretically and experimentally when the SP molecules undergo their documented reversible photoisomerization. This universal strategy of incorporating molecular functionalities into OTFTs offers attractive new prospects for the development of molecular devices with tailored electronic and other properties. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

High Performance Organic Thin-Film Transistor based on Amorphous A,B-Alternating Poly(arylenevinylene) Copolymers

A series of amorphous polymers of poly(arylenevinylene) copolymers, in which heterocycles (furan, thiophene, selenophene) and dialkoxy phenylenes were alternatingly linked by vinylene unit, was prepared by the Horner−Emmons reaction. Because of high regularity of the polymer microstructure by selective formation of E olefin, the resulting polymers showed good interchain π−π stacking in thin film state despite being amorphous polymers. When the A,B-alternating poly(phenylene thiophene vinylene), in particular with the bis(heptoxy) group, was used as a semiconductor material in an organic thin-film transistor, the best hole mobility up to 0.03 cm2/(V s) was observed, which is one of the highest values recorded from amorphous polymer film. The mobility was even improved to 0.06 cm2/(V s) when the polymer was blended with well-dispersed single-wall carbon nanotubes (SWCNT). Although this mobility is lower than that from the best crystalline polymers, these amorphous polymers showed advantages such as the devi...

An Amorphous Polythiophene as a Binder Material for Organic Thin-Film Transistor Channel Applications

Blending of crystalline low molecular-weight organic semiconductors, e.g., 6,13-bis (triisopropylsilylethynyl)pentacene (TIPS-Pentacene), and binder polymers has recently been known to be an effective way to achieve high performance organic thin-film transistors (OTFTs). However, only limited number of examples has been reported as binder materials for OTFT applications, e.g., poly(triarylamine), poly(α-methylstyrene), and etc. In this work, an amorphous polythiophene derivative, poly(3-(2-ethylhexyl)thiophene) (P3EHT), was prepared by introducing 2-ethylhexyl group as a side-chain to the polythiophene. The P3EHT was synthesized by Grignard metathesis (GRIM) polymerization to give a molecular weight (Mn) of 4,100 (PDI = 3.4), and it was readily soluble in common organic solvents, such as chloroform, tetrahydrofuran, toluene, and etc. An amorphous nature of the P3EHT was confirmed from differential scanning calorimetry (DSC) and ultraviolet-visible spectroscopy results. A mobility of 2.5 × 10−4 cm2/Vs, an on/off ratio of 0.57 × 103 and a threshold voltage of 1.6 V have been obtained with OTFTs using a P3EHT/TIPS-Pentacene blend.

A New Amorphous Semiconducting Polythiophene for High-Performance Organic Thin-Film Transistors

A new amorphous semiconducting polymer containing dodecylthiophene rings and a rigid thieno[3,2-b]thiophene ring, poly(2,5-bis(3'-dodecyl-2,2'-bithiophen-5-yl)thieno[3,2-b]thiophene) (NAP), was synthesized via a microwave-assisted Stille coupling reaction. The presence of the flexible unsubstituted thiophene ring units next to the rigid fused thiophene ring caused NAP to have an amorphous structure. This structure was confirmed by XRD, AFM, and computational calculations. In particular, the out-of-plane XRD patterns of NAP thin films exhibited no reflection peaks before or after the annealing process, indicating that the films had amorphous microstructures. In addition, AFM images of the NAP thin films showed amorphous surface morphologies with very small root-mean-square (rms) surface roughnesses of 0.3-0.5 nm, independent of surface treatment or heat treatment. Computational calculations performed to investigate the preferred conformation of the polymer confirmed the amorphous characteristics of the NAP structure. On the basis of these findings, we propose how an amorphous NAP semiconductor can maintain high carrier mobility. A NAP-based TFT device exhibited a very high carrier mobility of 0.02 cm(2) V(-1) s(-1) with an on/off ratio of 1 x 10(5) and a very small threshold voltage of -2.0 V. This carrier mobility is the highest yet reported for TFTs based on amorphous semiconductors. Thus, the present findings suggest that an amorphous semiconductor layer comprised of NAP would be suitable for use in high-performance organic TFTs fabricated via simple processes in which neither surface treatment nor heat treatment is necessary.

High-performance n-channel organic thin-film transistors based on the dual effects of heterojunction and surface modification

This paper presents two n-channel organic heterojunction transistors with modified insulator by using hexadecafluorophthalocyaninatocopper (F16CuPc)/copper phthalocyanine (CuPc) and F16CuPc/pentacene as the active layers. Compared with a single-layer device, it reports that an improved field-effect mobility and a 6-fold higher drain current are observed. The highest mobility of 0.081 cm2/(V · s) was obtained from F16CuPc/CuPc heterojunction devices. This result is attributed to the dual effects of the organic heterojunction and interface modification. Furthermore, for two heterojunction devices, the performance of the F16CuPc/CuPc-based transistor is better than that of F16CuPc/pentacene. This is attributed to the morphologic match of two organic components.