Electrodes In Organic Field-effect Transistors Research Articles

Recovery of gold from e-waste via food waste byproducts

Global materials’ and energy constraints and environmental issues call for a holistic approach to waste upcycling. We propose a chemically rational, cost-effective and environmentally friendly recovery of non-leaching gold from e-waste using aqueous chemistry with hydrogen peroxide, an environmentally benign oxidant, and lactic acid, a food chain byproduct. The oxidation of the base metals enables the release of gold in its metallic state in the form of flakes subsequently separated via filtration. Our main byproduct is a precursor of Cu2O, a relevant metal oxide for solar energy conversion applications. The recovered gold was characterized by scanning electron microscopy, energy dispersive spectroscopy and x-ray photoelectron spectroscopy to gain insight into the morphology of the flakes and their chemical composition. Furthermore, recovered gold was used to successfully fabricate the source and drain electrodes in organic field-effect transistors.

Improvement of hole injection characteristics in wet-processed organic field-effect transistor based on oxidation of silver electrode surface

Source–drain electrodes for organic field-effect transistors (OFETs) were fabricated with Ag nanoink on a Si wafer with a 300 nm thick oxide layer using the repellent patterning method, and a depth-profile analysis of the composition and physical properties of the electrode surface was performed once the electrode was oxidized by UV–ozone irradiation. Additionally, OFETs with a wet-processed electrode and 9,10-diphenylanthracene layer were fabricated, and their electrical characteristics were measured. The chemical composition of the Ag electrode surface changed to silver oxide (Ag2O or AgO) due to the longer oxidation treatment time, and the work function value increased. In the OFET with the electrode oxidized for 600 s, increased drain current ∣I D∣ was observed around a gate voltage of 0 V. Furthermore, good OFET characteristics were obtained [maximum ∣I D∣ = 326.2 μA, mobility μ = 0.91 cm2V −1·s−1], which were similar to those of the OFETs manufactured using a dry process.

Geometry Control of Source/Drain Electrodes in Organic Field-Effect Transistors by Electrohydrodynamic Inkjet Printing.

In this work we study the influence of dielectric surface and process parameters on the geometry and electrical properties of silver electrodes obtained by electrohydrodynamic inkjet printing. The cross-section and thickness of printed silver tracks are optimized to achieve a high conductivity. Silver overprints with cross-section larger than 4 μm2 and thickness larger than 90 nm exhibit the lowest resistivity. To fabricate electrodes in the desired geometry, a sufficient volume of ink is distributed on the surface by applying appropriate voltage amplitude. Single and multilayer overprints are incorporated as bottom contacts in bottom gate organic field-effect transistors (OFETs) with a semiconducting polymer as active layer. The multilayer electrodes result in significantly higher electrical parameters than single layer contacts, confirming the importance of a careful design of the printed tracks for reliable device performance. The results provide important design guidelines for precise fabrication of electrodes in electronic devices by electrohydrodynamic inkjet printing.

Non-lithographic direct patterning of carbon nanomaterial electrodes via electrohydrodynamic-printed wettability patterns by polymer brush for fabrication of organic field-effect transistor

In this study, we report a new approach for patterning of carbon nanomaterials via the solution route through wettability patterning using electrohydrodynamic (EHD) printing. Differences in wettability were successfully obtained by treating dimethylchlorosilane-terminated polystyrene (PS-brush) with the micro-dripping mode of EHD printing up to 6-μm line width and scaled fine patterns with various shapes at 6-μm printing conditions. We obtained sub-micrometer scaled carbon nanomaterial patterns, which were utilized as source and drain (S/D) electrodes for organic field-effect transistors (OFETs), without using any lithographic process. In addition, the PS-brush treatment modified the surface hydrophobicity of the substrates that served as the PS-brush and SiO2 bilayer dielectrics to guarantee high stability during the OFET operation. As a result, the OFETs employing S/D electrodes based on carbon nanomaterial patterns exhibited the electrical performance of typical p-type OFETs with negligible hysteresis. Additionally, we confirmed that a solution processing organic semiconductor forms a more suitable crystal structure than a vacuum process based organic semiconductor by several tools like CPOM, AFM, and 2D-GIXD.

Large-Area MXene Electrode Array for Flexible Electronics.

MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have potential for application as high-performance, low-cost electrodes in organic field-effect transistors (OFETs) because of their water dispersibility, high conductivity, and work-function tunability. In this study, we successfully fabricated a large-scale, uniform Ti3C2Tx MXene electrode array on a flexible plastic substrate for application in high-performance OFETs. The work function of the Ti3C2Tx MXene electrodes was also effectively modulated via chemical doping with NH3. The fabricated OFETs with Ti3C2Tx MXene electrodes exhibited excellent device performance, such as a maximum carrier mobility of ∼1 cm2·V-1·s-1 and an on-off current ratio of ∼107 for both p-type and n-type OFETs, even though all the electrode and dielectric layers were fabricated on the plastic substrate by solution processing. Furthermore, MXene-electrode-based complementary logic circuits, such as NOT, NAND, and NOR, were fabricated via integration of p-type and n-type OFETs. The proposed approach is expected to expand the application range of MXenes to other OFET-based electronic devices, such as organic light-emitting displays and electronic skins.

Electrohydrodynamic (EHD) jet printing of carbon-black composites for solution-processed organic field-effect transistors

Abstract In this study, carbon-black (CB) conductive electrodes were successfully printed using the high-resolution electrohydrodynamic (EHD) jet printing technique. The wrapping of CB bundles with a polymeric surfactant, Triton X-100 (TX-100), enabled the CB/TX-100 composites to well disperse in ethanol/deionized water for use in the preparation of conductive inks for EHD jet printing. By adjusting the voltage and operation distance, the applied electrostatic and gravity forces to the loaded CB/TX-100 inks overcame the fluid forces (viscosity and surface tension) to elongate the droplet and provide continuous jet lines, where the ink widths were smaller than the diameter of the nozzle. The EHD-printed CB/TX-100 in the stable cone-jet mode formed conductive lines and various pattern shapes. These conductive lines were utilized as source and drain electrodes of organic field-effect transistors (OFETs) with solution-processed organic semiconductors. The OFET with printed CB/TX-100 electrodes exhibited better electrical performances, including a higher saturation mobility and smaller hysteresis, than those of the reference OFET with Au electrodes.

A facile way to pattern PEDOT:PSS film as an electrode for organic devices

We report a facile and high resolution patterning method for conductive polymer films as transparent electrodes for organic devices on various substrates. The reliable pattern of PEDOT:PSS film is demonstrated by selective oxygen plasma etching via a defined copper (Cu) protection layer deposited by thermal evaporation. The overcoated Cu layer is removed by a wet etching process without damage to the underlying PEDOT:PSS layer. The minimum patterning feature size is determined by the resolution of the mask for Cu deposition and has shown to reach twenties of micro-meters. The patterned PEDOT:PSS film is successfully applied as source-drain electrodes for the organic field effect transistors (OFETs), and as a transparent anode in organic photodiodes (OPDs). Performance of flexible PEDOT:PSS-based OPDs exceeds a conventional flexible IZO-anode OPDs (6.6 mA/W vs. 1.4 mA/W, respectively). The contact resistance of PEDOT:PSS electrodes in OFETs is found to be 10 times lower (9.8 × 102 Ω cm), than that for Au counterparts. The hole mobility was 0.29 cm2/V s for both electrode materials and used DPPT-TT semiconductor polymer.

Holey Contacts: A New Approach to Enhance Charge Injection through Low‐Cost Nanopore‐Structured Silver Electrodes in Bottom‐Gate Bottom‐Contact (BGBC) Organic Field‐Effect Transistors

Nanosphere lithography is utilized to construct nanopore structure of the low-cost silver source/drain electrodes in organic field-effect transistors. The mechanism of charge transport with these nanopore-structured electrodes is systematically investigated. It is found that nanopore-structured silver electrodes enhance charge injection for high performance and have the potential to replace the gold electrode in the application of organic electronics. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to 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.

Direct patterning of conductive carbon nanotube/polystyrene sulfonate composites via electrohydrodynamic jet printing for use in organic field-effect transistors

Conductive MWCNT/PSS composites have been directly patterned via electrohydrodynamic printing for application as source/drain electrodes in organic field-effect transistors.

Combining Bottom-Up and Top-Down Segmentation: A Way to Realize High-Performance Organic Circuit

In this letter, the top-down and bottom-up segmentations were integrated together to realize high-performance organic devices and circuits. By combining photolithography with in situ polymerization, high conductive polypyrrole was patterned and used as electrodes for organic field-effect transistors (OFETs). With pentacene as P-type organic semiconductor and N,N’-bis (n-octyl)-Dicyanop-erylene-3,4:9,10-is (dicarboximide) (PDI-8CN2)/copper hexadecafluorophthalocyanine (CuPcF16) as N-type ones, both P-type and N-type OFETs exhibit excellent stability. Air-stable complementary inverters with the highest gain of 16.1 were realized. The results showed that the method was suitable for the fabrication of organic circuits.

3D Hollow Framework Silver Nanowire Electrodes for High-Performance Bottom-Contact Organic Transistors

We successfully fabricated high performance bottom-contact organic field-effect transistors (OFETs) using silver nanowire (AgNW) network electrodes by spray deposition. The synthesized AgNWs have the dimensions of 40-80 nm in diameter and 30-80 μm in length and are randomly distributed and interconnected to form a 3D hollow framework. The AgNWs networks, deposited by spray coating, yield an average optical transmittance of up to 88% and a sheet resistance as low as 10 ohm/sq. For using AgNWs as source/drain electrodes of OFETs with a bottom-contact configuration, the large contact resistance at the AgNWs/organic channel remains a critical issue for charge injection. To enhance charge injection, we fabricate semiconductor crystals on the AgNW using an adsorbed residual poly(N-vinylpyrrolidone) layer. The resulting bottom-contact OFETs exhibit high mobility up to 1.02 cm(2)/(V s) and are similar to that of the top-contact Au electrodes OFETs with low contact resistance. A morphological study shows that the pentacene crystals coalesced to form continuous morphology on the nanowires and are highly interconnected with those on the channel. These features contribute to efficient charge injection and encourage the improvement of the bottom-contact device performance. Furthermore, the large contact area of individual AgNWs spreading out to the channel at the edge of the electrode also improves device performance.

A simple PAN-based fabrication method for microstructured carbon electrodes for organic field-effect transistors

Facile and efficient fabrication of polyacrylonitrile (PAN)-based conductive graphitic carbon microstructures (GCMs) and their application to the electrodes of organic field-effect transistors (OFETs) is described. The PAN thin films spin-coated on a SiO2-deposited Si wafer was irradiated through a pattern mask with 150keV H+ ions at various fluences, and subsequently developed to form PAN microstructures. The resulting PAN microstructures were carbonized at various temperatures to create the GCMs. The analytical results revealed that the optimized fluence and carbonization temperature for well-defined GCMs was 3×1015ionscm−2 and 100°C, respectively, and that the resulting GCMs created at the optimized condition exhibited a greatly low surface roughness of 0.36nm, a good electrical conductivity of about 600Scm−1, and a high work function of 5.11eV. Noticeably, the GCM electrodes-based p-type OFET showed a comparable performance to that of the gold electrode-based one, demonstrating that the practical use of GCMs as cheap electrodes to replace expensive metallic ones for organic electronic devices.

Improving charge injection in high-mobility rubrene crystals: From contact-limited to channel-dominated transistors

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances Rc to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that Rc decreases from 105–106 Ω cm for 15 min air exposure to 3 × 103 Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of Rc to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V0. In channel-dominated (low Rc) FETs, the mobility is in the range of 10–17 cm2/(Vs); in contrast, in contact-limited (high Rc) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ – Rc dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts beyond traditional metal electrodes.

Self-Assembly of Mono- And Bidentate Oligoarylene Thiols onto Polycrystalline Au

Four thiolated oligoarylene molecules (i) 4-methoxy-terphenyl-4″-methanethiol (MTM), (ii) 4-methoxy-terphenyl-3″,5″-dimethanethiol (MTD), (iii) 4-nitro-terphenyl-4″-methanethiol (NTM), and (iv) 4-nitro-terphenyl-3″,5″-dimethanethiol (NTD) were synthesized and self-assembled as monolayers (SAMs) on polycrystalline Au electrodes of organic field-effect transistors (OFETs). SAMs were characterized by contact angle and AC/DC electrochemical measurements, whereas atomic force microscopy was used for imaging the pentacene films grown on the coated electrodes. The electrical properties of functionalized OFETs, the electrochemical SAMs features and the morphology of pentacene films were correlated to the molecular organization of the thiolated oligoarylenes on Au, as calculated by means of the density functional theory. This multi-methodological approach allows us to associate the systematic replacement of the SAM anchoring head group (viz. methanethiol and dimethanethiol) and/or terminal tail group (viz. nitro-, -NO2, and methoxy, -OCH3) with the change of the electrical features. The dimethanethiol head group endows SAMs with higher resistive features along with higher surface tensions compared with methanethiol. Furthermore, the different number of thiolated heads affects the kinetics of Au passivation as well as the pentacene morphology. On the other hand, the nitro group confers further distinctive properties, such as the positive shift of both threshold and critical voltages of OFETs with respect to the methoxy one. The latter experimental evidence arise from its electron-withdrawing capability, which has been verified by both DFT calculations and DC electrochemical measurements.

Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors.

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.

Patterned conductive nanostructures from reversible self-assembly of 1D coordination polymer

In this study, the outstanding ability of the coordination polymer [Pt2(nBuCS2)4I]n (nBu = n-butyl) (1) to reversibly self-organize from solution was demonstrated. This feature allowed us to generate highly electrical conductive structures located upon demand on technologically relevant surfaces, by easy-to-handle and low cost micromolding in capillaries (MIMIC) and lithographically controlled wetting (LCW). Electrical characterization reveals a near Ohmic behaviour and a high stability of the stripes (in air). Electrodes produced by the MIMIC technique from a solution of compound 1 demonstrated that this material can be efficiently used as electrodes for organic field-effect transistors (OFETs).

Self-assembly of reduced graphene oxide at liquid–air interface for organic field-effect transistors

A new method to prepare reduced graphene oxide (RGO) films was reported. In this method, RGO thin films can be prepared for large areas with controllable thickness and high temperature (>500 °C) is not required. The applications of RGO thin films as electrodes of organic field-effect transistors (OFETs) were investigated by bottom-contact p- and n-channel OFETs using copper phthalocyanine and copper hexadecafluorophthalocyanine as semiconductors. Transistor characterization showed the RGO electrode devices displayed higher mobilities than similar OFETs with Au electrodes, suggesting RGO is a good candidate for OFET electrodes.

Enhanced characteristics of pentacene field-effect transistors with graphene electrodes and substrate treatments

Pentacene organic field-effect transistors (OFETs) were fabricated with multilayer graphene films as the source and drain electrodes. The electrical properties of graphene electrode OFETs were monitored as the fabrication conditions were varied with surface treatments on the dielectric layer and substrate. With surface treatments, the performance of the graphene-electrode pentacene OFETs were significantly enhanced; the output currents increased by more than tenfold, and the mobility increased to levels as high as 1.2 cm2/V s. Our results may foster the wide application of graphene electrodes in OFETs and other types of organic electronic devices.

Charge injection from organic charge-transfer salts to organic semiconductors

Highly conducting films of organic charge-transfer (CT) salts are fabricated by a solution process from the dispersions stabilized by poly(vinylpyrrolidone). This method provides a general way to obtain conducting films of nonvolatile organic cation- and anion-radical salts with inorganic counter ions. Carrier injection from organic CT salts to organic semiconductors is investigated by using these films as electrodes in organic field-effect transistors. Efficient hole injection is observed not only from organic cation-radical salts but also from anion-radical salts to pentacene and sexithiophene. Electron injection is dominant from both types of CT salts to C60, but hole injection and ambipolar characteristics are observed for cation-radical salts. The Fermi levels of the CT salts are discussed on the basis of these observations.