Transparent Organic Thin-film Transistors Research Articles

Modification of Indium Tin Oxide Electrodes by Fluorinated Silanes for Transparent Organic Thin-Film Transistors

Modification of Indium Tin Oxide Electrodes by Fluorinated Silanes for Transparent Organic Thin-Film Transistors

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Summary Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1.

Solution-processable liquid crystalline chrysene semiconductors with wide band gap: Self-organization and carrier transport properties

Abstract High-performance solution-processable organic semiconductors (OSCs) with wide bang gap (≥3.3eV) are highly required for fabricating transparent organic thin-film transistors (OTFTs) with the inexpensive and low-temperature printed technique. Herein, in this study, we reported the newly synthesized chrysene-based OSCs (di-CR-C12, mono-CR-C12) with considerably large band gap of 3.70 eV, superior liquid crystallinity and solution-processed OTFTs mobility as high as 0.5 cm2/V. Such large band gap resulted in complete optical transparency of thin film in the visible-light region and good photostability. Highly ordered smectic E (SmE) phase of mono-CR-C12 was the first reported case of the monoalkylated single core structure demonstrating advantageous capacity of CR backbone in stablizing Sm mesophase. Remarkably, high hole mobility exceeding 0.1 cm2/V in the bulk phase was determined by time-of-flight (TOF) technique intrinsically revealing their satisfactory carrier transport characteristics. Good solubility of mono-CR-C12 enables solution-processed OTFTs with Au nanoparticles deposited at room-temperature and under ambient conditions, exhibiting good device performance with mobility up to 0.5 cm2/V. All results demonstrate that they are very promising candidates as active component for transparent electronics based on OTFTs.

Air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc-F16CuPc bi-channel structure

We report the fabrication of air-stable, transparent flexible ambipolar organic thin film transistors based on CuPc and F16CuPc bi-channel films deposited under 10−1 Pa, by using graphene gate electrode, fluoropolymer Cytop dielectric layer, and Au source/drain electrodes on polyethylene terephthalate substrates. The devices show outstanding air-stability and high optical transparency of above 75% with electron and hole mobilities up to 0.031 and 0.029 cm2/V·s, respectively. The excellent ambipolar performance, along with the outstanding air-stability and high optical transparency, make CuPc-F16CuPc bi-channel transistors as a promising candidate for organic integrated circuits.

One-Step Interface Engineering for All-Inkjet-Printed, All-Organic Components in Transparent, Flexible Transistors and Inverters: Polymer Binding.

We report a one-step interface engineering methodology which can be used on both polymer electrodes and gate dielectric for all-inkjet-printed, flexible, transparent organic thin-film transistors (OTFTs) and inverters. Dimethylchlorosilane-terminated polystyrene (PS) was introduced as a surface modifier to cured poly(4-vinylphenol) dielectric and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) electrodes without any pretreatment. On the untreated and PS interlayer-treated dielectric and electrode surfaces, 6,13-bis(triisopropylsilylethynyl)pentacene was printed to fabricate OTFTs and inverters. With the benefit of the PS interlayer, the electrical properties of the OTFTs on a flexible plastic substrate were significantly improved, as shown by a field-effect mobility (μFET) of 0.27 cm2 V-1 s-1 and an on/off current ratio (Ion/Ioff) of greater than 106. In contrast, the untreated systems showed a low μFET of less than 0.02 cm2 V-1 s-1 and Ion/Ioff ∼ 104. Additionally, the all-inkjet-printed inverters based on the PS-modified surfaces exhibited a voltage gain of 7.17 V V-1. The all-organic-based TFTs and inverters, including deformable and transparent PEDOT:PSS electrodes with a sheet resistance of 160-250 Ω sq-1, exhibited a light transmittance of higher than 70% (at wavelength of 550 nm). Specifically, there was no significant degradation in the electrical performance of the interface engineering-assisted system after 1000 bending cycles at a radius of 5 mm.

Transparent ambipolar organic thin film transistors based on multilayer transparent source-drain electrodes

A fabrication method for transparent ambipolar organic thin film transistors with transparent Sb2O3/Ag/Sb2O3 (SAS) source and drain electrodes has been developed. A pentacene/N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic di-imide (PTCDI-C13) bilayer heterojunction is used as the active semiconductor. The electrodes are deposited by room temperature electron beam evaporation. The devices are fabricated without damaging the active layers. The SAS electrodes have high transmittance (82.5%) and low sheet resistance (8 Ω/sq). High performance devices with hole and electron mobilities of 0.3 cm2/V s and 0.027 cm2/V s, respectively, and average visible range transmittance of 72% were obtained. These transistors have potential for transparent logic integrated circuit applications.

Strategy for preparation of transparent organic thin film transistors with PEDOT:PSS electrodes and a polymeric gate dielectric

Fully transparent organic field effect transistors (OFET) in top- and bottom-gate layouts were prepared. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was structured photolithographically as either gate- or source- and drain-contact on top of poly(4-vinylphenol) (PVP). The dielectric material was fitted to the sequence of materials by combining water soluble polyvinyl alcohol (PVA) and PVP. N-type conducting hexadecafluorophthalocyaninato-copper (F16PcCu) was used as organic semiconductor sensitive to interface traps. Critical boundary surfaces for film growth were examined by microscopic methods, which revealed smooth polymer surfaces as a result of our preparation and lithography steps characterizing the presented concept as a viable alternative to existing approaches.

Highly transparent thin-film transistors using wide-bandgap organic semiconductors and multilayer transparent electrodes

Highly transparent organic thin-film transistors operable at low voltages were demonstrated using a wide-bandgap organic semiconductor for channel layers, metal-based multilayers for transparent source/drain (S/D) electrodes, and an ultrathin fluoropolymer for gate dielectric layers. The devices optimized for high transparency based on characteristic-matrix formalism exhibited transmittances over 85% for the channel region and over 70% for the S/D region in almost the entire visible range. The validity of the multilayer S/D contact and the ultrathin fluoropolymer dielectrics on indium tin oxide (ITO) gate electrodes for favourable electrical characteristics were also verified. The resultant transparent thin-film transistors showed a typical p-channel operation switchable under 2 V with ca. 0.72 cm2/V s mobility.

Towards Colorless Transparent Organic Transistors: Potential of Benzothieno[3,2‐b]benzothiophene‐Based Wide‐Gap Semiconductors

Colorless, highly transparent organic thin-film transistors (TOTFTs) with high performance are realized based on benzothieno[3,2-b]benzothiophene (BTBT) derivatives that simultaneously exhibit a wide energy gap and high transport properties. Multilayer transparent source/drain electrodes maintain the transparency, and ultrathin fluoropolymer dielectric layers enable stable, low-voltage operation of the proposed TOTFTs.

N-channel transparent organic thin-film transistors with Ag/LiF bilayer transparent source–drain electrodes fabricated by thermal evaporation

In this paper, we report the use of Ag/LiF (AL) bilayer transparent source–drain (S/D) electrodes for n-channel F16CuPc-based transparent organic thin-film transistors (OTFTs). The transparent electrodes are deposited at room temperature by thermal evaporation, with devices subsequently fabricated without damaging the active layer or breaking the vacuum. Consequently, we obtain transparent OTFTs based on AL bilayer transparent S/D electrodes with good electron mobility of 1.31 × 10−2 cm−2·V−1·s−1, a high on/off ratio of 4.2 × 106, and an average visible range transmittance of 55.6%.

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

Organic semiconductors with higher carrier mobility and better transparency have been actively pursued for numerous applications, such as flat-panel display backplane and sensor arrays. The carrier mobility is an important figure of merit and is sensitively influenced by the crystallinity and the molecular arrangement in a crystal lattice. Here we describe the growth of a highly aligned meta-stable structure of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from a blended solution of C8-BTBT and polystyrene by using a novel off-centre spin-coating method. Combined with a vertical phase separation of the blend, the highly aligned, meta-stable C8-BTBT films provide a significantly increased thin film transistor hole mobility up to 43 cm(2) Vs(-1) (25 cm(2) Vs(-1) on average), which is the highest value reported to date for all organic molecules. The resulting transistors show high transparency of >90% over the visible spectrum, indicating their potential for transparent, high-performance organic electronics.

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

Transparent organic thin film transistors with WO3/Ag/WO3 source-drain electrodes fabricated by thermal evaporation

High-performance transparent organic thin film transistors using a WO3/Ag/WO3 (WAW) multilayer as the source and drain electrodes have been developed without breaking the vacuum. The WAW electrodes were deposited by thermal evaporation at room temperature, leading to little damage to organic film. The optimized WAW electrode shows high transmittance (86.57%), low sheet resistance (11 Ω/sq), and a high work function (5.0 eV). Consequently, we obtained high performance devices with mobility of 8.44 × 10−2 cm2/V · s, an on/off ratio of approximately 1.2 × 106, and an average visible range transmittance of 81.5%.

Transparent Organic Thin Film Transistors Using an Oxide/Metal/Oxide Trilayer as Low-Resistance Transparent Source/Drain Electrodes

This work presents transparent pentacene-based organic thin film transistors (OTFTs) in which an oxide/metal/oxide trilayer provides low-resistance transparent source/drain electrodes. The device structure is glass/indium–tin oxide (ITO)/poly(methyl methacrylate) (PMMA)/pentacene/transparent source–drain electrode. The transparent source/drain electrode consists of a low-resistance metal, silver, that is sandwiched by a high-transmittance oxide, WO3. The structure of the transparent source–drain electrode is optimized by tuning the thickness of each layer. The optimized structure has a sheet resistance of 6.2 Ω/sq. and an optimum transmittance of 70% in the visible wavelength range of 380–780 nm. The pentacene-based OTFT employing optimized transparent source–drain electrode yields an output current (IDS) of -7.08 µA, a field-effect mobility (µ) of 0.22 cm2 V-1 s-1, an on–off drain current ratio of 1.8×105, and a threshold voltage of -15.1 V. OTFTs in which WO3 (5 nm)/Ag (10 nm)/WO3 (30 nm) is used as the source/drain electrode greatly outperform OTFTs in which silver is utilized, because the work function of WO3/Ag/WO3 substantially exceeds that of silver.

Transparent organic thin-film transistors based on high quality polycrystalline rubrene film as active layers

Transparent organic thin-film transistors (OTFTs) with high performance are demonstrated by using high quality polycrystalline 5,6,11,12-Tetraphenylnaphthacene (rubrene) as an active layer, which is prepared by weak epitaxy growth (WEG) method. Benefiting from epitaxial relationship is formed between the inducing layer and the rubrene films, highly oriented and continuous organic polycrystalline thin films with large grains were obtained, which enhances the carrier transport in the film plane. The mobility of devices reaches 1.3cm2/Vs, the threshold voltage is lower than −0.9V and the on–off current ratio (Ion/Ioff) is higher than 106 after the photolithography process. Moreover, the array consisting of the transparent thin-film transistors displays a high optical transparency more than 65% in visible light regions. The high-performance transparent OTFTs promote the practical applications for large-area and flexible active-matrix organic light-emitting diodes (AMOLEDs) display.

Enhanced conductivity of Y-doped ZnO thin films by incorporation of multiple walled carbon nanotubes

Multiple walled carbon nanotubes have been incorporated into yttrium-doped zinc oxide (YZO) films through sol–gel method. Sol–gel process is used since it is an inexpensive means of thin film deposition when compared to the more expensive and time consuming techniques like RF sputtering. The structural, electrical and optical properties of the YZO films with and without multiple walled carbon nanotubes (MWCNT) are investigated. The carrier concentration of YZO/MWCNT composite films (2.95×1019cm−3) is found to be twice that of the YZO films (1.49×1019cm−3) achieving lower electrical resistivity of 4.36×10−2Ω-cm and an optical transparency over 85% making it suitable for application as channel injection layer in transparent organic thin film transistors. The spin coating or printable procedure of the solution processed YZO/MWCNT for incorporating a channel injection layer can replace expensive vacuum processes techniques.

Enhanced Stability of All Solution-Processed Organic Thin-Film Transistors Using Highly Conductive Modified Polymer Electrodes

Enhanced stability of all solution-processed organic thin-film transistors (OTFTs) has been achieved by replacing metallic electrodes with glycerol-modified poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) polymer electrodes. The modified PEDOT:PSS showed a substantially low electrical resistivity of 6.4×10-3 Ω cm with improved environmental stability and water-resistant characteristics, which are crucial for highly reliable applications. Additionally, the modified PEDOT:PSS electrodes were highly stable under intense mechanical stress, allowing their application to flexible electronics. Particularly, all solution-processed flexible and transparent OTFTs with the modified PEDOT:PSS electrodes showed a field-effect mobility decrease of only 2.7% after a tensile mode mechanical fatigue test, while OTFTs with metallic electrodes showed a mobility decrease of 56.6% under identical test conditions.

Transparent organic thin-film transistors and nonvolatile memory devices fabricated on flexible plastic substrates

Transparent and flexible organic thin-film transistors (OTFTs) and nonvolatile memory devices were developed. The memory device is based on OTFTs by incorporating self-assembled gold nanoparticles in the organic dielectric layers as the charge storage elements. The organic memory devices exhibited good programmable memory properties with respect to the program/erase operations, resulting in controllable and reliable threshold voltage shifts by hole trapping/detrapping in the gold nanoparticles. The organic memory devices could be electrically programmed and erased, leading to a large memory window of around 15 V. Additionally, the long term data retention (>105 s) and the stable cyclic bending tests (>2000 cycles) confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. The organic memory devices were composed of a solution-processed organic dielectric layer/metallic nanoparticles and a low-temperature processed organic semiconductor layer. Additionally, transparent indium-tin oxide (ITO) deposited at room temperature was used for the gate and source–drain electrodes to fabricate transparent memory devices. Therefore, this approach could potentially be applied to advanced transparent and flexible electronic devices as well as integrated organic device circuits.

Performance improvement in transparent organic thin-film transistors with indium tin oxide/fullerene source/drain contact

With the use of fullerene (C60)/indium tin oxide (ITO) source/drain electrode, the performance of a transparent thin-film transistor could be enhanced dramatically. The drain current can be increased by a factor of more than 5. The improvements are attributed to the reduction of the injection barrier at the ITO/pentacene interface, which can be confirmed by the work function measured at the ITO/C60 and the contact resistance obtained by transmission line method. Meanwhile, the average transmittance in the visible region with a 3.5 nm C60 buffer layer for 65-nm-thick pentacene organic thin film transistors remains at 62.98%.

Sol–gel ZnO in organic transistor-based non-volatile memory

There are many types of non-volatile memory devices but they are generally constructed from silicon. With the development of transparent organic thin film transistors, there is a need to also develop memory devices to allow the complete integration of digital circuitries. The aim of this research is to develop a fabrication route of an all-solution processing of optically transparent organic field effect transistor-based non-volatile memory (OFET-NVM). The OFET-NVMs can be programmed and erased at a relatively low voltage (±15 V). The OFET-NVM has a charge mobility of 0.125 cm2/V-s, threshold voltage shift of approximately 3 V between programmed and erased transistor and a sub-threshold slope of 1.5 V/decade. Although these figure-of-merits are not comparable to its silicon counterpart, the creation of an all solution processed OFET-NVM that is optically transparent (~70–85%) has been demonstrated.