Fabrication of Carbon Nanotube Thin Films for Flexible Transistors by Using a Cross-Linked Amine Polymer

Owing to their remarkable properties, single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) are expected to be used in various flexible electronics applications. To fabricate SWCNT channel layers for TFTs, solution-based film formation on a self-assembled monolayer (SAM) covered with amino groups is commonly used. However, this method uses highly oxidized surfaces, which is not suitable for flexible polymeric substrates. In this work, a solution-based SWCNT film fabrication using methoxycarbonyl polyallylamine (Moc-PAA) is reported. The NH₂-terminated surface of the cross-linked Moc-PAA layer enables the formation of highly dense and uniform SWCNT networks on both rigid and flexible substrates. TFTs that use the fabricated SWCNT thin film exhibited excellent performance with small variations. The presented simple method to access SWCNT thin film accelerates the realization of flexible nanoelectronics.     

Low Temperature Preparation of Titania Coated PAN Fiber and Its Photocatalytical Property

Titania thin film was deposited successfully on polyacrylonitrile (PAN) fiber by the sol‐gel process with the assistance of tetraethyl silicate (TEOS) at low temperature. It was found that the densification and crystallization of the film was a result of the post‐treatment in boiling water because of the hydrolysis of the Si‐O‐Ti bonds and dissolution of the silica component formed in the film. XRD patterns revealed the existence of anatase phase in the continuous titania layer. The product, titania coated polyacrylonitrile fiber (TiO₂/PAN), showed a high photocatalytic property and good repetition on the photodegradation of methylene blue (MB). The proposed method is expected to be used for the preparation of novel photo‐catalysts based on thermally sensitive substrates.     

The highly conducting carbon electrodes derived from spin-coated polyacrylonitrile films

Carbon films prepared from pyrolyzation of spin-casted polyacrylonitrile (PAN) thin films display high electrical conductivity (600 S/cm, at 1000 °C carbonization), low sheet resistance (about 100 Ω/square at the PAN film thickness of 70 nm) and partial transmittance. These pyrolyzed PAN (PPAN) films were patterned as bottom electrodes by photolithography, and utilized as drain and source electrodes to fabricate organic field-effect transistor (OFET) devices with a p-type semiconductor (P3HT) and an n-type semiconductor (DPP-containing quinoidal small molecule) through a spin-coating procedure. The results showed that the devices with the PAN electrodes exhibited almost the same excellent performance without any further modification compared to those devices with traditional Au electrodes. Since these PPAN films had the advantages of low-cost, high performance, easier for large-area fabrication, thermal and chemical stability, it should be a promising electrode material for organic electrodes.     

Superamphiphilic Janus fabric

Janus fabrics with superamphiphilicity were fabricated via electrospinning of polyacrylonitrile (PAN). PAN nanofibrous mats were formed on an aluminum foil substrate and then thermally treated to cause hydrolysis. An identical PAN solution was subsequently electrospun onto the hydrolyzed PAN layer, followed by peeling off of the bicomposite film from the collector substrate to produce a free-standing Janus fabric. On one side, the electrospun PAN mat exhibited superhydrophobic properties, with a water contact angle of 151.2°, whereas the initially superhydrophobic PAN sheet on the opposite side of the fabric was converted to a superhydrophilic surface (water contact angle of 0°) through hydrolysis of the surface functional groups induced by the thermal treatment. The resulting Janus fabrics exhibited both superhydrophobicity, repelling water on the one side, and superhydrophilicity, absorbing water on the other side. The organic solvent resistance of the PAN nanofibrous sheets was remarkably improved by incorporation of a tetraethyl orthosilicate. This facile and simple technique introduces a new route for the design and development of functional smart, robust fabrics from an inexpensive, commercially available polymer.     

2,6-Bis[2-(4-pentylphenyl)vinyl]anthracene: a stable and high charge mobility organic semiconductor with densely packed crystal structure

The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This paper presents the design, synthesis, optical and electrochemical characterization, crystal packing, modeling and thin film morphology, and organic thin film field effect transistor (OTFT) device data analysis for a novel 2,6-bis[2-(4-pentylphenyl)vinyl]anthracene (DPPVAnt) organic semiconductor. We observed a hole mobility of up to 1.28 cm2/V.s and on/off current ratios greater than 107 for OTFTs fabricated using DPPVAnt as an active semiconductor layer. The mobility value is comparable to that of the current best p-type semiconductor pentacene-based device performance. In addition, we found a very interesting relationship between the charge mobility and molecule crystal packing in addition to the thin film orientation and morphology of the semiconductor as determined from single-crystal molecule packing study, thin film X-ray diffraction, and AFM measurements. The high performance of the semiconductor ranks among the best performing p-type organic semiconductors reported so far and will be a very good candidate for applications in organic electronic devices.     

One-Step Fabrication of Bio-Compatible Coordination Complex Film on Diverse Substrates for Ternary Flexible Memory

Recently, resistance random access memories (RRAMs) have been studied extensively, because the demand for information storage is increasing. However, it remains challenging to obtain a flexible device because the active materials involved need to be nontoxic, nonpolluting, distortion-tolerable, and biodegradable as well adhesive to diverse flexible substrates. In this paper, tannic acid (TA) and an iron ion (Fe^III) coordination complex were employed as the active layer in a sandwich-like (Al/active layer/substrate) device to achieve memory performance. A nontoxic, biocompatible TA-Fe^III coordination complex was synthesized by a one-step self-assembly solution method. The retention time of the TA-Fe^III memory performance was up to 15 000 s, the yield up to 53 %. Furthermore, the TA-Fe^III coordination complex can form a high-quality film and shows stable ternary memory behavior on various flexible substrates, such as polyethylene terephthalate (PET), polyimide (PI), printer paper, and leaf. The device can be degraded by immersing it in vinegar solution. Our work will broaden the application of organic coordination complexes in flexible memory devices with diverse substrates.     

Solution-processable precursor route for fabricating ultrathin silica film for high performance and low voltage organic transistors

In this paper, we introduce a simple solution spin-coating method to fabricate silica thin film from precursor route in the condition of low temperature and atmospheric environment, which possesses a low leakage current, high capacitance, and low surface roughness. With silica film (~ 50 nm), high performance and low voltage (< 4 V) p-/n-type organic transistors are fabricated. This method shows great potential for industrialization owing to its characteristic of low consumption and energy saving, time-saving and easy to operate.     

All-solution-processed PIN architecture for ultra-sensitive and ultra-flexible organic thin film photodetectors

An ideal organic thin film photodetectors (OTFPs) should adopt a hierarchical, multilayer p-type/blend-type/n-type (PIN) structure, with each layer having a specific purpose which could greatly improve the exciton dissociation while guarantee efficient charge transport. However, for the traditional layer-by-layer solution fabrication procedure, the solvent used can induce organic material mixing and molecular disordering between each layer. Hence, such architecture for OTFPs can now only be formed via thermal evaporation. In this paper, a contact-film-transfer method is demonstrated to all-solution processing organic PIN OTFPs on flexible substrates. The fabricated PIN OTFPs exhibit high photoresponse and high stability under continuous mechanical bending. Hence, the method we described here should represent an important step in the development of OTFPs in the future.     

Inorganic oxide core, polymer shell nanocomposite as a high K gate dielectric for flexible electronics applications

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.     

Preparation and Properties of Sol–Gel Thin Film Containing Rhodamine B Derivative with Ethoxy Silano Group on Organic Substrate and its Application to Surface-Treatment Thin Film for Display

A thin film containing rhodamine B derivative with ethoxy silano group was formed on organic film substrate using the sol–gel method. Rhodamine B derivative with a triethoxysilano group, SiO₂ sol and acrylic polymer having a triethoxy group were reacted in alcohol to give a coating solution for film formation, followed to be roll-coated on polyethylene terephthalate (PET) film and heat-treated at 130°C. This thin film consists of inorganic polymer (SiO₂), organic polymer (acrylic resin) and organic dye. These component parts become interconnected through mutual chemical bonding. This thin film has an absorption peak at 578 nm and superb water resistant characteristics (almost no dye elution in 50°C water for 150 min) as a result of chemical bonding between the organic dye and the matrix skeleton. It also has good flexibility. The film can be used as a wavelength-selective absorption film for displays to improve contrast.     

Redox-active ionic-liquid-assisted one-step general method for preparing gold nanoparticle thin films: applications in refractive index sensing and catalysis

We describe a general one-step facile method for depositing gold nanoparticle (GNP) thin films onto any type of substrates by the in situ reduction of AuCl(3) using a newly designed redox-active ionic liquid (IL), tetrabutylphosphonium citrate ([TBP][Ci]). Various substrates such as positively charged glass, negatively charged glass/quartz, neutral hydrophobic glass, polypropylene, polystyrene, plain paper, and cellophane paper are successfully coated with a thin film of GNPs. This IL ([TBP][Ci]) is prepared by the simple neutralization of tetrabutylphosphonium hydroxide with citric acid. We also demonstrate that the [TBP][Ci] ionic liquid can be successfully used to generate GNPs in an aqueous colloidal suspension in situ. The deposited GNP thin films on various surfaces are made up of mostly discrete spherical GNPs that are well distributed throughout the film, as confirmed by field-emission scanning electron microscopy. However, it seems that some GNPs are arranged to form arrays depending on the nature of surface. We also characterize these GNP thin films via UV-vis spectroscopy and X-ray diffractometry. The as-formed GNP thin films show excellent stability toward solvent washing. We demonstrate that the thin film of GNPs on a glass/quartz surface can be successfully used as a refractive index (RI) sensor for different polar and nonpolar organic solvents. The as-formed GNP thin films on different surfaces show excellent catalytic activity in the borohydride reduction of p-nitrophenol.     

Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Polymorphism of organic semiconductor films is of key importance for the performance of organic thin film transistors(OTFTs).Herein, we demonstrate that the polymorphism of solution-processed organic semiconductors in thin film transistors can be controlled by finely tuning the surface nanostructures of substrates with self-assembled monolayers(SAMs). It is found that the SAMs of 12-cyclohexyldodecylphosphonic acid(CDPA) and 12-phenyldodecylphosphonic acid(Ph DPA) induce different polymorphs in the dip-coated films of 2-dodecyl[1]benzothieno[3,2-b][1]benzothiophene(BTBT-C12). The film of BTBT-C12 on CDPA exhibits field effect mobility as high as 28.1 cm~2 V~(-1) s~(-1) for holes, which is higher than that of BTBT-C12 on Ph DPA by three times. The high mobility of BTBT-C12 on CDPA is attributable to the highly oriented films of BTBT-C12 with a reduced in-plane lattice and high molecular alignment.     

Changes in silicon elastomeric surface properties under stretching induced by three surface treatments

Poly(dimethylsiloxane) (PDMS) substrates are used in many applications where the substrates need to be elongated and various treatments are used to regulate their surface properties. In this article, we compare the effect of three of such treatments, namely, UV irradiation, water plasma, and plasma polymerization, both from a molecular and from a macroscopic point of view. We focus our attention in particular on the behavior of the treated surfaces under mechanical stretching. UV irradiation induces the substitution of methyl groups by hydroxyl and acid groups, water plasma leads to a silicate-like layer, and plasma polymerization causes the formation of an organic thin film with a major content of anhydride and acid groups. Stretching induces cracks on the surface both for silicate-like layers and for plasma polymer thin coatings. This is not the case for the UV irradiated PDMS substrates. We then analyzed the chemical composition of these cracks. In the case of water plasma, the cracks reveal native PDMS. In the case of plasma polymerization, the cracks reveal modified PDMS. The contact angles of plasma polymer and UV treated surfaces vary only very slightly under stretching, whereas large variations are observed for water plasma treatments. The small variation in the contact angle values observed on the plasma polymer thin film under stretching even when cracks appear on the surface are explained by the specific chemistry of the PDMS in the cracks. We find that it is very different from native PDMS and that its structure is somewhere between Si(O2) and Si(O3). This is, to our knowledge, the first study where different surface treatments of PDMS are compared for films under stretching.     

Syntheses and characterization of functionalized polynorbornene dicarboximides for flexible substrates

In this study, we synthesized polynorbornene (PNB) dicarboximides substituted by monochlorophenyl group (PMCPhNDI) and dichlorophenyl group (PDCPhNDI) via ring‐opening polymerization using a ruthenium catalyst and investigated their thermal, mechanical, and optical properties. We also discussed the performance and application of the functionalized PNB dicarboximide films as flexible substrates for organic light‐emitting devices (OLEDs). The polymer films exhibited good optical transparency with an average transmittance of around 97% in the visible light region and good thermal stability with a 5% degradation temperature of >440°C. The polymers were applied for flexible displays, which were coated on indium tin oxide (ITO) thin films using a radio‐frequency planar magnetron sputtering system with changing the deposition substrate temperatures. A flexible OLED that was fabricated on the ITO‐grown polymer substrates exhibited a performance as comparable to the corresponding ITO‐grown glass substrates. Copyright © 2012 John Wiley & Sons, Ltd.     

Flexible Polymer–Organic Solar Cells Based on P3HT:PCBM Bulk Heterojunction Active Layer Constructed under Environmental Conditions

In this study, some crucial parameters were determined of flexible polymer–organic solar cells prepared from an active layer blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) mixed in 1:1 mass ratio and deposited from chlorobenzene solution by spin-coating on poly(ethylene terephthalate) (PET)/ITO substrates. Additionally, the positive effect of an electron transport layer (ETL) prepared from zinc oxide nanoparticles (ZnO np) on flexible photovoltaic elements’ performance and stability was investigated. Test devices with above normal architecture and silver back electrodes deposed by magnetron sputtering were constructed under environmental conditions. They were characterized by current-voltage (I–V) measurements, quantum efficiency, impedance spectroscopy, surface morphology, and time–degradation experiments. The control over morphology of active layer thin film was achieved by post-deposition thermal treatment at temperatures of 110–120 °C, which led to optimization of device morphology and electrical parameters. The impedance spectroscopy results of flexible photovoltaic elements were fitted using two R||CPE circuits in series. Polymer–organic solar cells prepared on plastic substrates showed comparable current–voltage characteristics and structural properties but need further device stability improvement according to traditionally constructed cells on glass substrates.     

An inkjet printing soft photomask and its application on organic polymer substrates

This article presents a simple, fast and low-cost method to fabricate a flexible UV light photomask. The designed micropatterns were directly printed onto transparent hybrid composite film of biaxially oriented polypropylene coated with silica oxide (BOPP-SiO _x ) by an inkjet printer. Compared to the conventional chrome-mask, it is of advantages such as suitable for non-planar substrates, scalable for large area production, and extreme low cost. Combined with the confined photo-catalytic oxidation (CPO) reaction, the printed flexible BOPP-SiO _x photomask was successfully used to pattern the shape of wettability of organic polymer surfaces, and then polyaniline patterns were deposited on the modified substrates with strong adhesion. With the above photomasks, the polyacrylic acid graft chains were duplicated on the poly (ethylene terephthalate) (PET) and BOPP substrates by photografting polymerization. We grafted polyacrylic acid (PAA) on a non-planar plastic substrate with this soft and thin plastic photomask. Scanning electron microscopy (SEM) and optical microscopy were used to characterize the surface morphology and thickness of ink layers of the printed photomask. Optical microscopy was used to characterize the deposition polyaniline micropatterns. It was found that the desired patterns were precisely printed on the modified polymer films and were applied in modifying organic polymer substrates. The printed photomask could be exploited in the fields such as prototype microfluidics, micro-sensors, optical structures and any other kind of microstructures which does not require high durability and dimensional stability.     

Ternary single‐source precursors for polycrystalline thin‐film solar cells

The development of thin‐film solar cells on flexible, lightweight, space‐qualified substrates provides an attractive cost solution for fabricating solar arrays with high specific power (W kg^?1). The use of a polycrystalline chalcopyrite absorber layer for thin‐film solar cells is considered as the next generation in photovoltaic devices. At NASA GRC we have focused on the development of new single‐source precursors (SSPs) and their utility to deposit the chalcopyrite semiconducting layer (CIS) onto flexible substrates for solar‐cell fabrication. The syntheses and thermal modulation of SSPs via molecular engineering are described. Thin‐film fabrication studies demonstrate that the SSPs can be used in a spray chemical vapor deposition process for depositing CIS at reduced temperatures, and result in electrical properties that are suitable for photovoltaic devices. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Cross‐linked Polymer‐Blend Gate Dielectrics through Thermal Click Chemistry

New cross‐linking reagents were synthesized and mixed with polystyrene (PS) in solution to form a blend. Thin‐films were spin‐coated from the blend and then cross‐linked by thermal activation at relatively low temperature (100 °C) to form cross‐linked gate dielectrics. This new method is compatible with plastic substrates in flexible electronics. The azide and alkyne cross‐linking reagents are kinetically stable at room temperature, so any premature cross‐linking is avoided during processing. This method also significantly improved the dielectric performances of PS thin films. Solution‐processed top‐gate organic field‐effect transistor devices with indacenodithiophene–benzothiadiazole copolymer as semiconductor layer and the cross‐linked PS blend as dielectric layer showed improved performances with lower gate leakages and higher operation stabilities than devices with neat PS film as dielectric layer.     

Characterization of a passivation layer comprising MgO?SiO2 and ZrO2

Thin films with magnesium oxide (MgO) and silicon oxide (SiO₂) compounds mixed at various mixture ratios were deposited on flexible polyether sulfone (PES) substrates by an e‐beam evaporator to investigate their potential for transparent barrier applications. In this study, as the MgO fraction increased, thin films comprising MgO and SiO₂ compounds became more amorphous, and their surface morphologies became smoother and denser. In addition, zirconium oxide (ZrO₂) was added to the above‐mentioned compound mixtures, and the properties of the compound mixture comprising Mg? Si? Zr? O were then measured. ZrO₂ made the thin mixture films more amorphous, and made the surface morphology denser and more uniform. Whole thin films of 250 ± 30 nm in thickness were formed, and their water vapor transmission rates (WVTRs) decreased rapidly. The best WVTR was obtained by depositing thin films of Mg? Si? Zr? O compound among the whole thin films. The WVTRs of the PES substrate in the bare state decreased from 47 to 0.8 g m^?2 day^?1. This Mg? Si? Zr? O compound was deposited on polyethylene terephtalate (PET) substrates again to confirm the availability of the compound mixture. Thin films on the PET substrates decreased the WVTRs remarkably from 2.96 to 0.01 g m^?2 day^?1. These results were similar to those of thin films on PES substrates. As the thin mixture films became more amorphous and surface morphology denser and more uniform, the WVTRs decreased. Therefore, the thin mixture films became more suitable for flexible organic light emitting displays (OLEDs) as transparent passivation layers against moisture in air. Copyright © 2006 John Wiley & Sons, Ltd.