Vacuum production of OTFTs by vapour jet deposition of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) on a lauryl acrylate functionalised dielectric surface

Roll-to-roll (R2R) production of organic transistors and circuits require patterned deposition of organic layers at high deposition rate. Here we demonstrate a vapour-jet process for the rapid deposition of the organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT). The deposition rate achieved, equivalent to ∼200 nm/s onto a stationary substrate, was several orders of magnitude faster than ordinary thermal evaporation. Nevertheless, transistor yield was 100% with an average mobility of 0.4 cm²/V in a single pass deposition onto a substrate moving at 0.15 m/min. We also demonstrate a vacuum, high rate R2R-compatible process for surface-functionalising a gate dielectric layer with lauryl acrylate which enabled an all-vacuum route to the fabrication of a five-stage ring oscillator.     

Organic field-effect transistors and memory elements using deoxyribonucleic acid (DNA) gate dielectric

Deoxyribonucleic acid (DNA) bio-polymers derived from fish waste products are employed as gate dielectric in n-type methanofullerene as well as p-type pentacene based organic field-effect transistors working at low voltage levels and low gate leakage currents. Based on the large hysteresis in the transfer characteristics, operation of the transistor as a non-volatile memory element is shown. Practically hysteresis free operation of DNA based transistors is obtained at low voltage levels by adding an additional aluminium oxide blocking layer between the organic semiconductor and the DNA gate dielectric.     

All-Inkjet-Printed Bottom-Gate Thin-Film Transistors Using UV Curable Dielectric for Well-Defined Source-Drain Electrodes

Technological restrictions of the inkjet printing technology for printed electronics can hinder its application potential, mainly due to the limited resolution and layer homogeneity in comparison to conventional manufacturing techniques for electronics. The manufacturing of active devices such as thin-film transistors with appropriate performance using printing technologies is still one of the current challenges towards industrial applications. This work demonstrates the application of an ultraviolet (UV) curable ink as insulating material for the gate dielectric. The advantage of the UV curable ink is its fast curing and the smooth surface enabling high resolution patterns on top of it. In this way, all-inkjet-printed organic thin-film transistors (OTFTs) were fabricated with silver electrodes, UV curable gate dielectric, and 6,13-bis(triisopropylsilylethynyl)pentacene for the active semiconductor layer. By fine tuning of processing parameters and pattern geometries, a stable channel length of about 10 μm was obtained in the bottom-gate configuration without the need of additional steps, suggesting a way to build low-cost all-inkjet-printed OTFTs with well-defined source-drain electrodes and fast UV curable dielectric without any additional steps. The inkjet-printed device is characterized by an electron mobility of 0.012 cm² V^?1 s^?1 and on/off ratio of 10³.     

Humidity sensitive organic field effect transistor

This paper reports the experimental results for the humidity dependent properties of an organic field effect transistor.The organic field effect transistor was fabricated on thoroughly cleaned glass substrate,in which the junction between the metal gate and the organic channel plays the role of gate dielectric.Thin films of organic semiconductor copper phthalocynanine (CuPc) and semitransparent Al were deposited in sequence by vacuum thermal evaporation on the glass substrate with preliminarily deposited Ag source and drain electrodes.The output and transfer characteristics of the fabricated device were performed.The effect of humidity on the drain current,drain current-drain voltage relationship,and threshold voltage was investigated.It was observed that humidity has a strong effect on the characteristics of the organic field effect transistor.     

Humidity sensitive organic field effect transistor

This paper reports the experimental results for humidity dependent properties of organic field effect transistor. The organic field effect transistor was fabricated on thoroughly cleaned glass substrate, in which the junction between metal gate and organic channel plays the role of gate dielectric. The thin films of organic semiconductor copper phthalocynanine (CuPc) and semitransparent Al were deposited in sequence by vacuum thermal evaporation on the glass substrate with preliminary deposited Ag source and drain electrodes. Output and transfer characteristics of the fabricated device were performed. The effect of humidity on the drain current, drain current-drain voltage relationship, and threshold voltage have been investigated. It was observed that humidity has strong effect on the characteristics of organic field effect transistor (OFET).     

Hybrid Nanodielectrics for Low‐Voltage Organic Electronics

Nanoscale hybrid dielectrics composed of an ultra‐thin polymeric low‐κ bottom layer and an ultra‐thin high‐κ oxide top layer, with high dielectric strength and capacitances up to 0.25 μFcm^?2, compatible with low‐voltage, low‐power, organic electronic circuits are demonstrated. An efficient and reliable fabrication process, with 100% yield achieved on lab‐scale arrays, is demonstrated by means of pulsed laser deposition (PLD) for the fast growth of the oxide layer. With this strategy, high capacitance top gate (TG), n‐type and p‐type organic field effect transistors (OFETs) with high mobility, low leakage currents, and low subthreshold slopes are realized and employed in complementary‐like inverters, exhibiting ideal switching for supply voltages as low as 2 V. Importantly, the hybrid double‐layer allows for a neat decoupling between the need for a high capacitance, guaranteed by the nanoscale thickness of the double layer, and for an optimized semiconductor–dielectric interface, a crucial point in enabling high mobility OFETs, thanks to the low‐κ polymeric dielectric layer in direct contact with the polymer semiconductor. It is shown that such decoupling can be achieved already with a polymer dielectric as thin as 10 nm when the top oxide is deposited by PLD. This paves the way for a very versatile implementation of the proposed approach for the scaling of the operating voltages of TG OFETs with very low level of dielectric leakage currents to the fabrication of low‐voltage organic electronics with drastically reduced power consumption.     

Monitoring the Channel Formation in Organic Field‐Effect Transistors via Photoinduced Charge Transfer

Conducting channel formation in organic field‐effect transistors (OFETs) is considered to happen in the organic semiconductor layer very close to the interface with the gate dielectric. In the gradual channel approximation, the local density of accumulated charge carriers varies as a result of applied gate bias, with the majority of the charge carriers being localized in the first few semiconductor monolayers close to the dielectric interface. In this report, a new concept is employed which enables the accumulation of charge carriers in the channel by photoinduced charge transfer. An OFET employing C₆₀ as a semiconductor and divinyltetramethyldisiloxane‐bis(benzocyclobutene) as the gate dielectric is modified by a very thin noncontinuous layer of zinc‐phthalocyanine (ZnPc) at the semiconductor/dielectric interface. With this device geometry, it is possible to excite the phthalocyanine selectively and photogenerate charges directly at the semiconductor/dielectric interface via photoinduced electron transfer from ZnPc onto C₆₀. Thus the formation of a gate induced and a photoinduced channel in the same device can be correlated.     

A process for the formation of submicron V-gate by micromachinedV-grooves using GaInP/GaAs selective etching technique

A process for the formation of submicron V-gates by V-grooves was demonstrated for the first time. The fabrication steps of V-gates consist of anisotropic wet etching of the undoped GaAs layer grown on top of a GaInP layer and subsequent metal evaporation and lift-off process. Owing to the outward slope of the sidewalls of the micromachined V-groove, submicron gate length could be easily obtained by normal 1 μm UV photolithography. The submicron V-gate process was also applied successfully to the fabrication of V-gate GaInP/GaAs/InGaAs metal semiconductor field effect transistors with a gate length of 0.6 μm     

Organic Field Effect Transistors Based on Graphene and Hexagonal Boron Nitride Heterostructures

Enhancing the device performance of single crystal organic field effect transistors (OFETs) requires both optimized engineering of efficient injection of the carriers through the contact and improvement of the dielectric interface for reduction of traps and scattering centers. Since the accumulation and flow of charge carriers in operating organic FETs takes place in the first few layers of the semiconductor next to the dielectric, the mobility can be easily degraded by surface roughness, charge traps, and foreign molecules at the interface. Here, a novel structure for high‐performance rubrene OFETs is demonstrated that uses graphene and hexagonal boron nitride (hBN) as the contacting electrodes and gate dielectric layer, respectively. These hetero‐stacked OFETs are fabricated by lithography‐free dry‐transfer method that allows the transfer of graphene and hBN on top of an organic single crystal, forming atomically sharp interfaces and efficient charge carrier‐injection electrodes without damage or contamination. The resulting heterostructured OFETs exhibit both high mobility and low operating gate voltage, opening up new strategy to make high‐performance OFETs and great potential for flexible electronics.     

有机半导体薄膜三极管的研制

采用真空蒸镀法和有机半导体材料酞菁铜,制作Au/CuPc/Al/ CuPc/Au三明治结构的肖特基型栅极有机静电感应三极管.该三极管导电沟道垂直于CuPc薄膜,与采用MOSFET结构的有机薄膜三极管相比导电沟道大幅缩短,有利于克服有机半导体电学性能的缺点.实验结果表明,该三极管驱动电压低,呈不饱和电流-电压特性.其工作特性依赖于栅极电压和梳状铝电极的结构.通过合理设计、制作梳状铝电极,获得了良好的三极管静态、动态特性.     

Super‐ and Ultrathin Organic Field‐Effect Transistors: from Flexibility to Super‐ and Ultraflexibility

Physically flexible electronics offer a wide range of benefits, including the development of next‐generation consumer electronics and healthcare products. The advancement of physical flexibility, typically achieved by the reduction of the total device thickness, including substrates and encapsulation layers, shows great promise for skin‐laminated electronics. Organic electronics—devices relying on carbon‐based materials—offer many advantages over their inorganic counterparts, including the following: significantly lower fabrication temperatures resulting in alternative fabrication techniques, including inkjet and roll‐to‐roll printing, enabling low‐cost and large‐area fabrication; biocompatibility; and spectacular physical flexibility. This article presents a review, spanning the last two decades, of organic field‐effect transistors with the total thickness of just a few microns as well as devices demonstrated in this decade with a total thickness of few hundred of nanometers. A handful of demonstrations of other organic electronic thin film devices are also presented.     

Self‐Organization of Inkjet‐Printed Organic Semiconductor Films Prepared in Inkjet‐Etched Microwells

The high‐precision deposition of highly crystalline organic semiconductors by inkjet printing is important for the production of printed organic transistors. Herein, a facile nonconventional lithographic patterning technique is developed for fabricating banks with microwell structures by inkjet printing solvent droplets onto a polymer layer, thereby locally dissolving the polymer to form microwells. The semiconductor ink is then inkjet‐printed into the microwells. In addition to confining the inkjet‐printed organic semiconductor droplets, the microwells provide a platform onto which organic semiconductor molecules crystallize during solvent evaporation. When printed onto the hydrophilic microwells, the inkjet‐printed 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) molecules undergo self‐organization to form highly ordered crystalline structures as a result of contact line pinning at the top corner of the bank and the outward hydrodynamic flow within the drying droplet. By contrast, small crystallites form with relatively poor molecular ordering in the hydrophobic microwells as a result of depinning of the contact line along the walls of the microwells. Because pinning in the hydrophilic microwells occurred at the top corner of the bank, treating the surfaces of the dielectric layer with a hydrophobic organic layer does not disturb the formation of the highly ordered TIPS_PEN crystals. Transistors fabricated on the hydrophilic microwells and the hydrophobic dielectric layer exhibit the best electrical properties, which is explained by the solvent evaporation and crystallization characteristics of the organic semiconductor droplets in the microwell. These results indicate that this technique is suitable for patterning organic semiconductor deposits on large‐area flexible substrates for the direct‐write fabrication of high‐performance organic transistors.     

High‐Mobility and Low Turn‐On Voltage n‐Channel OTFTs Based on a Solution‐Processable Derivative of Naphthalene Bisimide

In organic electronics solution‐processable n‐channel field‐effect transistors (FETs) matching the parameters of the best p‐channel FETs are needed. Progress toward the fabrication of such devices is strongly impeded by a limited number of suitable organic semiconductors as well as by the lack of processing techniques that enable strict control of the supramolecular organization in the deposited layer. Here, the use of N,N′‐bis(4‐n‐butylphenyl)‐1,4,5,8‐naphthalenetetracarboxylic‐1,4:5,8‐bisimide (NBI‐4‐n‐BuPh) for fabrication of n‐channel FETs is described. The unidirectionally oriented crystalline layers of NBI‐4‐n‐BuPh are obtained by the zone‐casting method under ambient conditions. Due to the bottom‐contact, top‐gate configuration used, the gate dielectric, Parylene C, also acts as a protective layer. This, together with a sufficiently low LUMO level of NBI‐4‐n‐BuPh allows the fabrication and operation of these novel n‐channel transistors under ambient conditions. The high order of the NBI‐4‐n‐BuPh molecules in the zone‐cast layer and high purity of the gate dielectric yield good performance of the transistors.     

The mutual influence of surface energy and substrate temperature on the saturation mobility in organic semiconductors

We report on a mutual correlation between the substrate temperature during semiconductor deposition and the surface energy of the gate dielectric on the charge carrier mobility in bottom gate top contact organic field effect transistors (OFETs) with N,N′-diphenyl-3,4,9,10-perylene tetracarboxylic diimide (DP-PDI) as organic semiconductor.     

Flexible organic complementary circuits

We report the first organic complementary circuits on flexible substrates. Organic thin-film transistors were fabricated using pentacene as the semiconductor for the p-channel devices and hexadecafluorocopperphthalocyanine (F/sub 16/CuPc) as the semiconductor for the n-channel devices. Both semiconductors were purchased from commercial sources and deposited by evaporation in vacuum. The pentacene layer was photolithographically patterned to simplify the circuit layout and reduce the circuit area. The transistors and circuits were manufactured on thin, transparent sheets of polyethylene naphthalate. Evaporated metals were used to define all contacts and interconnects, and a 50-nm-thick layer of solution-processed polyvinylphenol was used as the gate dielectric. Transistors and circuits operate at supply voltages as low as 8 V, and ring oscillators have a signal propagation delay as low as 8 /spl mu/s per stage. To our knowledge, these are the fastest organic complementary circuits reported to date.     

Fabrication of vertical channel top contact organic thin film transistors

Vertical channel top contact (TC) organic thin film transistors (OTFTs) have been successfully realized on Si substrates with SiO₂ as gate insulators and P3HT(poly ∼3-hexylthiophene) as organic semiconductors. The active channel region was defined by a steep step through a Si etching method. Source and drain metal contacts were deposited by vacuum evaporation through a shadow mask at a high tilting angle. Top contact transistors with channel lengths 5 μm can be fabricated with a relatively simple and efficient (yield >85%) fabrication process with only two photolithography steps (two photo masks) while no need for high-resolution and precision alignment for channel definition. Measurement results showed that the vertical channel BC (bottom contact) devices have compatible performance with planar BC devices. However, vertical channel TC transistors showed improved performance with double field effect mobilities and three times larger current ON/OFF ratios than vertical channel BC devices.     

Polylactic acid as a biodegradable material for all-solution-processed organic electronic devices

In this paper we report on the fabrication of spin-coated biodegradable polylactic acid (PLA) thin films to be used as substrates for the realisation of all-solution-processed organic electronic devices. The full mechanical and electrical characterisation of these substrates shows that they exhibit good mechanical and dielectric properties and are therefore suitable for the fabrication of disposable electronics. To demonstrate practically the functionality of such PLA thin films, organic electronic devices were realised on the top of them, exclusively by means of solution-process fabrication techniques and in particular inkjet-printing. Also, a photonic curing procedure is here presented as a means for sintering the conductive inks without heating up the PLA substrates. Two types of organic transistors were fabricated on the top of PLA: organic field-effect transistors (OFETs), where the PLA film was used not only as a substrate but also as the gate dielectric, and all-inkjet-printed organic electrochemical transistors (OECTs). The second typology of transistors exhibited one of the highest transconductance reported so far in the literature (up to 2.75 mS). This study opens an avenue for the fabrication of disposable, low-cost organic electronic devices.     

A novel gate insulator for flexible electronics

Field effect transistors using a poly(triaryl amine) p-channel organic semiconductor in conjunction with anodised aluminium oxide as the gate insulator (Al₂O₃ on Al) are demonstrated. Anodised films are pinhole-free, homogenous oxide layers of precisely controlled thickness. The anodisation process requires no vacuum steps; anodised Al₂O₃ is insoluble in organic solvents, and Al films are cheaply available as laminates on flexible substrates. Anodised Al₂O₃ is confirmed to have high gate capacitance (≈60 nF/cm²) and electric breakdown strength (>3 MV/cm in the working device). This property profile answers to the demands on gate insulators for flexible electronics applications.     

Cover Picture: Self‐Organization of Ink‐jet‐Printed Triisopropylsilylethynyl Pentacene via Evaporation‐Induced Flows in a Drying Droplet (Adv. Funct. Mater. 2/2008)

Charge carrier transport in organic electronic devices is influenced by the crystalline microstructure and morphology of the organic semiconductor film. Evaporation behavior during drying plays a vital role in controlling the film morphology and the distribution of solute in inkjet‐printed films. On p. 229, Kilwon Cho and co‐workers demonstrate the influence of the evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of inkjet‐printed 6,13‐bis((triisopropylsilylethynyl) pentacene. The results provide an excellent method for direct‐write fabrication of high‐performance organic electronics. We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm² V^–1 s^–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.     

Fabrication of pentacene organic field-effect transistors With polyimide gate dielectric layer

The organic field effect transistors had been fabricated using the pentacene by vacuum evaporation as the active layer, the polyimide by spin coating as insulator layer, and aluminum by vacuum evaporation as gate, source and drain electrodes respectively. The field-effect mobility of 0.079 cm2/V.s was tested at Vds=70 V, and on/off radio up to 1.7×104.