Source-drain Electrodes Research Articles

High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates

The electronic properties of organic single crystals have been intensely studied for well over 40 years. Until recently, organic single-crystal field-effect transistors have generated results that are comparable to and sometimes better in performance than hydrogenated amorphous silicon. Organic thin-film transistors are being actively pursued for a broad area of electronic applications, but their charge-carrier mobilities are limited by structural imperfections (i.e., grain boundaries) and impurities. Organic single crystals, on the other hand, have been limited to charge-transport studies mainly because the fabrication of single-crystal transistors poses a technological challenge. Novel methods for fabricating single-crystal devices include the flip-crystal technique, elastomeric stamp platforms, and freestanding devices, where the source–drain electrodes, dielectric, and gate are all fabricated onto the crystal surface. For the most part, a relatively thick and rigid single crystal is employed (5–500 lm thick). Because the fragility makes them difficult to handle, their use has been restricted to simple and basic devices and wide-ranging applications in sensors or plastic transistors for flexible electronics have not yet been possible. Thus, there is a strong need for the development of mechanically flexible, nondestructive, single-crystal devices with prospective applications in organic electronics while maintaining the intrinsic properties and characteristics of organic single crystals. We demonstrate field-effect transistors fabricated from thin and conformable organic single crystals. We report on proofof-concept “flexible” organic single-crystal field-effect transistors with performance exceeding those of previously reported organic thin-film flexible devices. Rubrene single-crystal devices constructed on low-cost flexible substrates (Fig. 1b) yielded mobilities as high as 4.6 cm V s and on/off ratios of approximately 10.

Organic inverter circuits employing ambipolar pentacene field-effect transistors

Ambipolar transport has been observed in pentacene films grown on polyvinyl alcohol gate dielectric with hole and electron mobilities of 0.3 and 0.04cm2∕Vs, respectively. A simple device structure with Au as source-drain electrode can be used to operate a transistor in both p-channel and n-channel modes without employing low work function metal electrodes for ambipolar charge injection. Using ambipolar pentacene field-effect transistors, we construct a complementarylike inverter with voltage inversion gain of ∼10. These inverters are able to operate both in first and third quadrants of the voltage output to voltage input characteristics which is a unique feature of employing ambipolar transistors.

Low-cost fabrication of submicron all polymer field effect transistors

All polymer field effect transistors have been fabricated combining nanoimprint lithography and inkjet printing. Trenches with hydrophilic bottoms confined by hydrophobic walls with considerable height are patterned by nanoimprint lithography. Conducting polymer solutions were then delivered into these trench liquid containers by inkjet printing. Dried conducting polymer in nearby trenches forms source-drain electrodes with the channel length accurately defined by the gap in between the designed two trenches. Top-gate all polymer field effect transistors with submicron channel lengths were successfully realized by such low-cost process.

Patterning Organic Semiconductors Using “Dry” Poly(dimethylsiloxane) Elastomeric Stamps for Thin Film Transistors

This communication demonstrates a method of transferring unreacted low molecular weight (LMW) siloxane oligomers from freshly prepared "dry" PDMS stamps for patterning organic semiconductors and conducting polymers into functional devices via selective wetting. The semiconductors were patterned onto the modified surfaces via dip-coating with well-resolved feature sizes as small as 1 mum. Functional transistor arrays exhibited field-effect mobilities as high as 0.07 cm2/Vs. The proposed printing method eliminates the need to ink SAMs for fabricating patterns and results in a simple, fast, and highly reproducible method of patterning organic semiconductors from solution. The method herein also produced a flexible transistor composed of patterned PEDOT source-drain electrodes.

Estimation of carrier recombination and electroluminescence emission regions in organic light-emitting field-effect transistors using local doping method

To elucidate the electroluminescence (EL) mechanism of organic light-emitting field-effect transistors (OLEFETs), we determined the carrier recombination and EL emission regions using the local doping method. We demonstrated that the local doping method is a useful technique for estimating the width of these regions in OLEFETs. We inserted an ultrathin rubrene doped 1,3,6,8-tetraphenylpyrene (TPPy) layer (d=10nm) as a sensing layer in a TPPy layer (80nm) and measured the luminance-drain current-drain voltage characteristics and the EL spectra depending on the position of the sensing layer. We confirmed that the EL emission region expanded almost to the height (h≃40nm) of the source-drain electrodes and was independent of the gate bias voltage (Vg). Further, we observed that the EL external quantum efficiency (ηext) significantly decreased as Vg increased, suggesting that excitons generated in a TPPy host layer by carrier recombination are quenched by the application of Vg.

Ambipolar Carrier Transport in Polycrystalline Pentacene Thin-Film Transistors

We have investigated the dependence of polycrystalline pentacene thin-film transistors (TFTs) characteristics on work function of source-drain contact metals. The pentacene TFTs using Mg, Al, Ag, and Au source-drain electrodes showed only p-type characteristics and the field-effect hole mobilities were strongly dependent on their work function. On the other hand, the pentacene TFTs using Ca source-drain electrodes showed typical ambipolar characteristics. The field-effect hole mobility of 4.5 × 10−4 cm2/Vs and field-effect electron mobility of 2.7 × 10−5 cm2/Vs were estimated from saturation currents. Appearance of an electron enhancement mode in pentacene TFTs was ascribed to the lowering of barrier for electron injection at source electrodes.

Preparation of Conductive Organic–Inorganic Cubic Perovskite Thin Films by Dual-Source Vacuum Vapor Deposition

We successfully obtained conductive organic–inorganic cubic perovskite CH3NH3SnI3 thin films by vacuum vapor deposition. The vacuum-deposited thin films showed a low electrical resistivity of 8.7×10-4 Ω cm. Furthermore, the cubic perovskite thin films could be patterned in the shape of a shadow mask by vacuum vapor deposition. A field-effect transistor (FET) comprising a pentacene thin film as a semiconductor and the cubic perovskite thin films as source–drain electrodes was fabricated. The FET with the cubic perovskite top-contact source–drain electrodes revealed typical field-effect characteristics. The saturation-regime hole mobility, drain current on/off ratio, and threshold voltage of the FET were 0.004 cm2/(V·s), 1.2×103, and -7.2 V, respectively.

Bipolar Characteristics of an Organic Light-Emitting Filed Effect Transistor Using a TPTPT and NTCDA Co-Deposited Layer

We fabricated an organic light-emitting field effect transistors (OLEFET) using a co-deposited layer of TPTPT and NTCDA which has p-type and n-type semiconducting behaviors, respectively. The organic layer was formed on a comb type electrode (source-drain electrodes) with a channel length of 25μm on a p-Si/SiO2 substrate. Bipolar and EL characteristics were investigated by changing co-deposition ratio of TPTPT and NTCDA. In the optimum deposition ratio, we observed typical ambipolar characteristics, demonstrating saturation current characteristics at low Vd (source-drain voltage), followed by divergent current characteristics at high Vd.

P- and n-Type Charge Transport in Field-Effect Transistors of Pristine Poly(p-Phenylenevinylene)

ABSTRACTPoly(p-phenylenevinylene) (PPV) thin films were prepared by using drop casting under high gravity condition and p- and n-type PPV based field effect transistors (FET) have been fabricated. PPV FETs with gold source-drain electrodes showed the p-channel FET conduction. The FET characteristics of PPV were improved by annealing and the field-effect hole mobility was 8.8×10−4cm2V−1s−1. On the other hand, PPV FET with calcium source-drain electrodes showed the n-channel FET conduction and the field-effect electron mobility was 1.0×10−6cm2V−1s−1.

Organic Thin-Film Transistors with Ink-jet Printed Metal Electrodes

Ink-jet printing technology is widely used in numerous applications. Various inks are developed for the demand of electronic devices. Ink-jet printing conductive metal nanopastes have been used in printed circuit boards and passive components. This technique provides a manufacturing method that can replace more expensive process such as lithography or metal evaporation. In this paper, we demonstrate a new method for manufacturing conductive electrodes of organic thin-film transistors (OTFTs). Adjusting ink properties and jetting performance can control the qualities of the printed nanopaste film. Curing temperature and time are also the key issues which are required for the better morphology of the film. Soluble conjugated polymers such as poly 3-hexylthiophene (P3HT) are very promising candidates for a cheap electronic device on various substrates and suitable for the applications of ink-jet printing technology. We compare the characteristics of OTFTs for the evaporated and the ink-jet printed source-drain electrodes. This approach presents a simple method for fabricating OTFT device and is suitable for a flexible display displays and circuits.

Patterned Carbon Nanotube Thin-Film Transistors with Transfer-Print Assembly

ABSTRACTConditions for the transfer printing of patterned carbon nanotube (CNT) films, along with a Au-gate, a poly methylmethacrylate (PMMA) dielectric layer and Au source-drain electrodes have been developed for the fabrication of thin-film transistors on a polyethylene terephthalate (PET) substrate. Chemical vapor deposition (CVD) grown CNTs were patterned using a photolithographic method.Transfer printing was used to fabricate devices having both top gate and bottom gate configurations. Replacement of the SiO2 dielectric with PMMA correlates with a decreased hysteresis in the transconductance behavior. Encapsulation of the CNTs between the polymeric substrate and dielectric layer yields ambipolar behavior. Variations in device performance are also observed as a function of CNT film density and channel length, suggesting changing contributions of the metallic and semiconducting CNTs to the transport mechanism.

Short-channel polymer field-effect-transistor fabrication using spin-coating-induced edge template and ink-jet printing

A method to fabricate polymer field-effect transistors with submicron channel lengths is described. A thin polymer film is spin coated on a prepatterned resist with a low resolution to create a thickness contrast in the overcoated polymer layer. After plasma and solvent etching, a submicron-sized line structure, which templates the contour of the prepattern, is obtained. A further lift-off process is applied to define source-drain electrodes of transistors. With a combination of ink-jet printing, transistors with channel length down to 400 nm have been fabricated by this method. We show that drive current density increases as expected, while the on/off current ratio 106 is achieved.

High-performance organic thin-film transistors with metal oxide/metal bilayer electrode

We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between the organic layer and the metal electrode.

Utilizing roll-to-roll techniques for manufacturing source-drain electrodes for all-polymer transistors

The first all-polymer field effect transistor was reported already ten years ago. In the last few years rapid progress has been made in the field of plastic electronics. This progress has been based on developments in materials and in fabrication technology. In this work we report an all-polymer transistor in which source and drain electrodes are printed by using flexographic printing. This method is widely used in the printing industry. In the all-polymer transistor device, polyaniline (PANI), regio-regular poly(3-hexylthiophene) (rr-PHT), poly(vinylphenol) (PVP) and poly(3,4)ethylenedioxythiophene/potystyrenesulfonate (PEDOT/PSS) are used as source and drain electrodes, active channel semiconductor material, insulator and gate electrode, respectively. Flexible polyethylene terephthalate (PET) and poly(ethylene-2,6-naphthalate) (PEN) sheets are used as substrates. First, source and drain electrodes are printed by using a laboratory scale FLEXO printing machine, rr-PHT and PVP are spin coated and finally PEDOT/PSS is drop cast. As a result, conductivity, spreading, optical quality and thickness of the produced source-drain electrode pairs are measured and the transistor devices are characterized. Manufacturing of organic transistors entirely by a roll-to-roll process is evaluated.

Control of the nano electrostatic phenomena at a pentacene/metal interface for improvement of the organic FET devices

The effect of the UV/ozone treatment of substrate on transistor properties of the bottom contact pentacene FET is discussed in the context of the carrier accumulation around the source-drain electrode. Significant change of the threshold voltage, i.e., the decrease of the V th, by a UV/ozone treatment of the substrate was observed, and can be reasonably assigned to accumulated charges around source-drain electrode. In addition, to investigate the channel formation process on this FET, capacitance–voltage ( C– V) characteristics were measured, and significant difference between UV/ozone-treated and untreated FET was observed. Only a small change of a capacitance was observed for untreated sample with applied voltage, whereas drastic increase of the capacitance was observed for UV/ozone-treated FET. The capacitance increase corresponds to the accumulation of carriers at the interfacial region of pentacene, and this behavior was in good agreement with the result of the FET measurement. Finally, these C– V characteristics were successfully reproduced by Maxwell–Wagner model.

Fabrication of nanoscale C60 field-effect transistors with carbon nanotubes

A nanoscale C60 field-effect transistor has been fabricated with carbon nanotubes (C60CNT-FET). A wire of multiwalled carbon nanotube has been anchored by metal pads on a Si wafer, and cut by bombardment with focused Ga2+ ion beam. The cut ends of the wire have been integrated as source-drain electrodes into the C60CNT-FET, with a vacuum evaporated C60 thin film. The C60CNT-FET has exhibited an excellent performance of a low-voltage drive operation, without any short-channel effect even at as small as 100 nm of channel length.

Transparent thin-film transistors with pentacene channel, AlOx gate, and NiOx electrodes

We report on the fabrication of pentacene-based transparent thin-film transistors (TTFT) that consist of NiOx, AlOx, and indium-tin-oxide (ITO) for the source-drain (S/D) electrode, gate dielectric, and gate electrode, respectively. The NiOx S/D electrodes of which the work function is well matched to that of pentacene were deposited on a 50-nm-thick pentacene channel by thermal evaporation of NiO powder and showed a moderately low but still effective transmittance of ∼25% in the visible range along with a good sheet resistance of ∼60Ω∕◻. The maximum saturation current of our pentacene-based TTFT was about 15μA at a gate bias of −40V showing a high field effect mobility of 0.9cm2∕Vs in the dark, and the on/off current ratio of our TTFT was about 5×105. It is concluded that jointly adopting NiOx for the S/D electrode and AlOx for gate dielectric realizes a high-quality pentacene-based TTFT.

3.5: Invited Paper: Recent Progress in Large Sized & High Performance Organic TFT Array

Recently, we have developed 15.0 inch XGA organic TFT array and its LCD module, which might be world-largest array size and highest resolution in organic based device. This achievement is to be combined with several technical breakthroughs include new organic gate dielectric material, novel ohmic contact technique, and well-optimized process-architectures. Base on the newly developed organic gate dielectric with pentacene, we have obtained world-record field effect TFT mobilities in excess of 7cm2/Vsec and excellent on/off ratios as ∼106. For jumping up the OTFT research activities from laboratory scale to real production line, we have developed two key technologies. One is the new ohmic contact process with very common materials in the current LCD manufacturing such as ITO; the bottom contact structured OTFT with ITO source-drain electrode shows much the same performances as that with gold source-drain electrode. The other key technology to realize the large size organic device is that of optimizing the array structure to minimize the OTFT performance degradations. By combining these novel technologies, a 15.0″ XGA prototype LCD panel is fabricated and shows good display performance.

Organic field-effect transistors based on high electron and ambipolar carrier transport properties of copper–phthalocyanine

An electron accumulation-mode operation in organic field-effect transistor (OFET) using a thin film of copper–phthalocyanine (CuPc) and Ca source-drain electrodes was demonstrated. Appearance of the electron accumulation-mode in OFET was ascribed to the lowering of barrier for electron injection at a source electrode. The OFET also showed a p-channel and typical ambipolar transport property. The field-effect electron mobility of 1.0 × 10 −3 cm 2/V s and field-effect hole mobility of 2.5 × 10 −4 cm 2/V s were estimated from the saturation currents above the threshold voltage.

Emergence of n-Type Characteristic of Conjugated Polymer Field-Effect Transistors with Calcium Source-Drain Electrodes

We fabricated field-effect transistors (FETs) using a highly soluble fluorene-based polymer as an active layer utilizing a low-work-function metal, calcium as a source-drain electrode material. The n-type characteristic of polymer FETs was successfully realized by lowering the electron injection barrier, although polymer FETs with gold source-drain electrodes did not exhibit n-type or p-type characteristic. The well-defined n-type FET provides a useful means of measuring the electron transport property of new organic semiconductors.