Source-drain Electrodes Research Articles

Controlling the crack formation in inkjet-printed silver nanoparticle thin-films for high resolution patterning using intense pulsed light treatment

During the last years, intense pulsed light (IPL) processing has been employed and studied intensively for the drying and sintering of metal nanoparticle layers deposited by means of printing methods on flexible polymer substrates. IPL was found to be a very fast and substrate-gentle approach qualified for the field of flexible and large-area printed electronics, i.e. manufactured via roll-to-roll processing. In this contribution, IPL is used for the fine-patterning of printed silver nanoparticle layers. The patterning is obtained by induced and controlled crack formation in the thin silver layer due to the intense exposure of IPL. The crack formation is controlled by selection of the substrate material, the fine-tuning of the morphology of the silver layer and an application of a dielectric layer on top of the silver layer that acts as a stress concentrator. Careful optimization of the IPL parameters allowed to adjust the lateral width of the crack. This novel approach turned out to be a fast and reproducible high-resolution patterning process for multiple applications, e.g. to pattern the source-drain electrodes for all-inkjet-printed thin-film transistors.

Ultrasensitive and Selective Organic FET-type Nonenzymatic Dopamine Sensor Based on Platinum Nanoparticles-Decorated Reduced Graphene Oxide.

Dopamine (DA), a catecholamine hormone, is an important neurotransmitter that controls renal and cardiovascular organizations and regulates physiological activities. Abnormal concentrations of DA cause unfavorable neuronal illnesses such as Parkinson's disease, schizophrenia, and attention deficit hyperactivity disorder/attention deficit disorder. However, the DA concentration is exceedingly low in patients and difficult to detect with existing biosensors. In this study, we developed an organic field-effect-transistor-type (OFET) nonenzyme biosensor using platinum nanoparticle-decorated reduced graphene oxide (Pt_rGO) for ultrasensitive and selective DA detection. The Pt_rGOs were fabricated by reducing GO aqueous solution-containing Pt precursors (PtCl4) with a chemical reducing agent. The Pt_rGOs were immobilized on a graphene substrate by π-π interactions and a conducting-polymer source-drain electrode was patterned on the substrate to form the DA sensor. The resulting OFET sensor showed a high sensitivity to remarkably low DA concentrations (100 × 10-18 M) and selectivity among interfering molecules. Good stability was expected for the OFET sensor because it was fabricated without an enzymatic receptor, and π-π conjugation is a part of the immobilization process. Furthermore, the OFET sensors are flexible and offer the possibility of wide application as wearable and portable sensors.

Solution-processed organic field-effect transistors based on dinaphthothienothiophene precursor with chemically modified electrodes

Bottom-gate organic field-effect transistors (OFETs) based on a soluble precursor of dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) with bottom-contact source-drain electrodes chemically modified with pentafluorobenzenethiol (PFBT) have been fabricated. The preparation of DNTT films using CYTOP overcoat layers allows the solution processing of bottom-gate/bottom-contact DNTT FETs with good electrical contacts between the PFBT-treated Au electrodes and the DNTT molecules. The DNTT FETs processed using CYTOP overcoat layers exhibit the field-effect mobilities of up to 0.37 cm2 V−1 s−1. High maximum mobility of 0.29 cm2 V−1 s−1 has been achieved in solution-processed DNTT FETs with channel length of 5 μm.

Gate-controlled reversible rectifying behaviour in tunnel contacted atomically-thin MoS2 transistor

Atomically thin two-dimensional semiconducting materials integrated into van der Waals heterostructures have enabled architectures that hold great promise for next generation nanoelectronics. However, challenges still remain to enable their applications as compliant materials for integration in logic devices. Here, we devise a reverted stacking technique to intercalate a wrinkle-free boron nitride tunnel layer between MoS2 channel and source drain electrodes. Vertical tunnelling of electrons therefore makes it possible to suppress the Schottky barriers and Fermi level pinning, leading to homogeneous gate-control of the channel chemical potential across the bandgap edges. The observed features of ambipolar pn to np diode, which can be reversibly gate tuned, paves the way for future logic applications and high performance switches based on atomically thin semiconducting channel.

Top-Contact Pentacene-Based Organic Thin Film Transistor (OTFT) with N,N′-Bis(3-Methyl Phenyl)-N,N′-Diphenyl Benzidine (TPD)/Au Bilayer Source-Drain Electrode

A top-contact Pentacene-based organic thin film transistor (OTFT) with N,N′-Bis (3-methyl phenyl)-N,N′-diphenyl benzidine (TPD)/Au bilayer source-drain electrode is reported. The devices with TPD/Au bilayer source-drain (S-D) electrodes show better performance than the single layer S-D electrode OTFT devices. The field-effect mobility of 4.13 cm2 v−1 s−1, the on–off ratio of 1.86 × 107, the threshold voltage of −4 v and the subthreshold slope of .27 v/decade, respectively, are obtained from the device with a TPD/Au bilayer source-drain electrode.

Modelling and Realization of a Water-Gated Field Effect Transistor (WG-FET) Using 16-nm-Thick Mono-Si Film

We introduced a novel water-gated field effect transistor (WG-FET) which uses 16-nm-thick mono-Si film as active layer. WG-FET devices use electrical double layer (EDL) as gate insulator and operate under 1 V without causing any electrochemical reactions. Performance parameters based on voltage distribution on EDL are extracted and current-voltage relations are modelled. Both probe- and planar-gate WG-FETs with insulated and uninsulated source-drain electrodes are simulated, fabricated and tested. Best on/off ratios are measured for probe-gate devices as 23,000 A/A and 85,000 A/A with insulated and uninsulated source-drain electrodes, respectively. Planar-gate devices with source-drain insulation had inferior on/off ratio of 1,100 A/A with 600 μm gate distance and it decreased to 45 A/A when gate distance is increased to 3000 μm. Without source-drain electrode insulation, proper transistor operation is not obtained with planar-gate devices. All measurement results were in agreement with theoretical models. WG-FET is a promising device platform for microfluidic applications where sensors and read-out circuits can be integrated at transistor level.

Proving Scalability of an Organic Semiconductor To Print a TFT-Active Matrix Using a Roll-to-Roll Gravure.

Organic semiconductor-based thin-film transistors’ (TFTs) charge-carrier mobility has been enhanced up to 25 cm2/V s through the improvement of fabrication methods and greater understanding of the microstructure charge-transport mechanism. To expand the practical feasibility of organic semiconductor-based TFTs, their electrical properties should be easily accessed from the fully printed devices through a scalable printing method, such as a roll-to-roll (R2R) gravure. In this study, four commercially available organic semiconductors were separately formulated into gravure inks. They were then employed in the R2R gravure system (silver ink for printing gate and drain–source electrodes and BaTiO3 ink for printing dielectric layers) for printing 20 × 20 TFT-active matrix with the resolution of 10 pixels per inch on poly(ethylene terephthalate) (PET) foils to attain electrical properties of organic semiconductors a practical printing method. Electrical characteristics (mobility, on–off current ratio, threshold voltage, and transconductance) of the R2R gravure-printed 20 × 20 TFT-active matrices fabricated with organic semiconducting ink were analyzed statistically, and the results showed more than 98% device yield and 50 % electrical variations in the R2R gravure TFT-active matrices along the PET web.

Reducing the contact resistance in bottom-contact-type organic field-effect transitors using an AgOx interface layer

Bottom-contact organic field-effect transistors (OFETs) are prepared by inserting an AgOx layer between a pentacene layer and the source–drain electrodes. The contact resistance in the device is ∼8.1 kΩ·cm with an AgOx layer oxidized for 60 s but reaches 116.9 kΩ·cm with a non-oxidized Ag electrode. The drain current and mobility in the OFETs with the AgOx layer increase with the oxidization time and then gradually plateau, and this trend strongly depends on the work function of the Ag surface. Further, the hole injection is enhanced by the presence of Ag2O but inhibited by the presence of AgO.

Halide Welding for Silver Nanowire Network Electrode.

We developed a method of chemically welding silver nanowires (AgNWs) using an aqueous solution containing sodium halide salts (NaF, NaCl, NaBr, or NaI). The halide welding was performed simply by immersing the as-coated AgNW film into the sodium halide solution, and the resulting material was compared with those obtained using two typical thermal and plasmonic welding techniques. The halide welding dramatically reduced the sheet resistance of the AgNW electrode because of the strong fusion among nanowires at each junction while preserving the optical transmittance. The dramatic decrease in the sheet resistance was attributed to the autocatalytic addition of dissolved silver ions to the nanowire junction. Unlike thermal and plasmonic welding methods, the halide welding could be applied to AgNW films with a variety of deposition densities because the halide ions uniformly contacted the surface or junction regions. The optimized AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding significantly enhanced the mechanical flexibility of the electrode compared with the as-coated AgNWs. The halide-welded AgNWs were successfully used as source-drain electrodes in a transparent and flexible organic field-effect transistor (OFET). This simple, low-cost, and low-power consumption halide welding technique provides an innovative approach to preparing transparent electrodes for use in next-generation flexible optoelectronic devices.

One-Step Synthesis of Antioxidative Graphene-Wrapped Copper Nanoparticles on Flexible Substrates for Electronic and Electrocatalytic Applications

In this study, we report a novel, one-step synthesis method to fabricate multilayer graphene (MLG)-wrapped copper nanoparticles (CuNPs) directly on various substrates (e.g., polyimide film (PI), carbon cloth (CC), or Si wafer (Si)). The electrical resistivities of the pristine MLG-CuNPs/PI and MLG-CuNPs/Si were measured 1.7 × 10-6 and 1.4 × 10-6 Ω·m, respectively, of which both values are ∼100-fold lower than earlier reports. The MLG shell could remarkably prevent the Cu nanocore from serious damages after MLG-CuNPs being exposed to various harsh conditions. Both MLG-CuNPs/PI and MLG-CuNPs/Si retained almost their conductivities after ambient annealing at 150 °C. Furthermore, the flexible MLG-CuNPs/PI exhibits excellent mechanical durability after 1000 bending cycles. We also demonstrate that the MLG-CuNPs/PI can be used as promising source-drain electrodes in fabricating flexible graphene-based field-effect transistor (G-FET) devices. Finally, the MLG-CuNPs/CC was shown to possess high performance and durability toward hydrogen evolution reaction (HER).

Nozzle-jet printed flexible field-effect transistor biosensor for high performance glucose detection

Printable electronics is a subject of great interest for low-cost, facile and environmentally-friendly large scale device production. But, it still remains challenging for printable biosensor development. Herein, we present the fabrication of nozzle-jet printed flexible field-effect transistor (FET) glucose biosensor. The silver source-drain electrodes and ZnO seed layers were printed on flexible substrate by nozzle-jet printer followed by ZnO nanorods (ZnO NRs) synthesis and glucose oxidase (GOx) immobilization. Utilization of nozzle-jet printing methods resulted in highly reproducible electrodes with well-defined vertical grown ZnO NRs for high GOx loading and enhanced glucose sensing performance in a wide glucose detection range. The stability, anti-interference ability, reproducibility, reusability, and applicability in human serum samples were also assessed. Overall, biosensor fabrication using nozzle-jet printer will not only provide large scale production of highly reproducible electrodes but also reduce the fabrication cost. Additionally, printed electrodes can be modified accordingly for different analyte detection.

Simultaneous improvement of on- and off-current features in organic transistors via dual-targeted hydrophobic adhesion

Abstract We developed a facile technique, termed as dual-targeted hydrophobic adhesion (DHA) to improve on- and off-current features simultaneously in solution-processed organic thin-film transistors (OTFTs). Simple DHA process not only modifies the interface between source-drain electrodes and organic semiconductor (OSC), but also acts as passivation in OSC fabrication process. The modified interface induces shift of the Fermi energy level and decreases the contact resistance. Moreover, the passivation function of DHA suppresses water permeation and prevents the increase of trap-induced conductivity at off-state. As a result, DHA-treated OTFTs exhibit a field-effect mobility with a threefold increase and on/off current ratio (by two orders of magnitude), as compared to non-treated transistors.

Fabrication and investigation of P3HT:PCBM bulk heterojunction based organic field effect transistors using dielectric layers of PMMA:Ta2O5 nanocomposites

In this work, we have fabricated top-gate/bottom-contact type Organic Field Effect Transistors (OFETs). A poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend with 1:1 wt/wt ratio was used as the semiconducting layer, and poly(methyl methacrylate)(PMMA):Ta2O5 blends with varying concentration of Ta2O5 were used as dielectric layers of the devices. Interdigitated type source-drain electrodes on a glass substrate were coated with a semiconducting layer, which was followed by the deposition of dielectric layer and a top aluminum gate contact in sequence. Dielectric layers were prepared as the blends of PMMA:Ta2O5 with Ta2O5 wt/wt concentrations of 0%, 3%, 7%, 10%, 20% and 50%. Structural and electrical characterization of PMMA:Ta2O5 nanocomposite films were carried out using scanning electron microscope (SEM) and dielectric analyzer respectively. Dielectric constant of PMMA:Ta2O5 nanocomposites was found to increase with the increasing concentration of Ta2O5. Devices that were processed using 0wt% and 3wt% concentration of Ta2O5 exhibited a typical p-type OFET behavior with hole mobility of 2.68×10−3 and 3.42×10−3cm2/Vs respectively. Whereas, the devices with 7wt% and higher concentration of Ta2O5 showed electron mobility in addition to hole mobility, indicating ambipolar behavior of the devices. Furthermore, electron mobility was found to increase with increasing concentration of Ta2O5. The hole and electron mobility values were found to be 1.08×10−3 and 4.42×10−3cm2/Vs at 50% wt concentration of Ta2O5 respectively. Based on excellent saturation behavior and promising mobility values of the investigated solution processed devices, it is expected that the investigated devices can possibly be utilized in some practical applications, such as low-cost large area sensing arrays in future.

Influence of Asymmetric Contact Form on Contact Resistance and Schottky Barrier, and Corresponding Applications of Diode.

We have fabricated carbon nanotube and MoS2 field-effect transistors with asymmetric contact forms of source-drain electrodes, from which we found the current directionality of the devices and different contact resistances under the two current directions. By designing various structures, we can conclude that the asymmetric electrical performance was caused by the difference in the effective Schottky barrier height (ΦSB) caused by the different contact forms. A detailed temperature-dependent study was used to extract and compare the ΦSB for both contact forms of CNT and MoS2 devices; we found that the ΦSB for the metal-on-semiconductor form was much lower than that of the semiconductor-on-metal form and is suitable for all p-type, n-type, or ambipolar semiconductors. This conclusion is meaningful with respect to the design and application of nanomaterial electronic devices. Additionally, using the difference in barrier height caused by the contact forms, we have also proposed and fabricated Schottky barrier diodes with a current ratio up to 104; rectifying circuits consisting of these diodes were able to work in a wide frequency range. This design avoided the use of complex chemical doping or heterojunction methods to achieve fundamental diodes that are relatively simple and use only a single material; these may be suitable for future application in nanoelectronic radio frequency or integrated circuits.

Roles of 3-Ethylrhodanine in Attaining Highly Ordered Crystal Arrays of Ambipolar Diketopyrrolopyrrole Oligomers.

Until now, only limited DPP oligomers delivered ambipolar semiconductor characteristics. To develop a facile strategy of preparing ambipolar mono-DPP oligomers, two dithienyl diketopyrrolopyrrole (DPPT) based-conjugated molecules, DPPT-RD and DPPT-DCV, which contain 3-ethylrhodanine (RD) and dicyano-2-vinyl (DCV) end substituents were synthesized. The influences of the -RD end substituents on the molecular properties, solid-state morphology, and OFET performances of the DPPT oligomer were investigated. The UV-vis absorption and CV results showed that the RD end substituents provide the DPPT oligomer suitable EHOMO and ELUMO for hole and electron injection from the Au source-drain electrodes. Moreover, the RD end substituents also improve the crystalline nature of the DPPT oligomer. That is, DPPT-RD can form crystal arrays with good lattice orientation, larger crystalline size, and without polymorphism. With those properties, DPPT-RD thus display ambipolar characteristic with μh and μe reaching 2.16 × 10-2 and 7.27 × 10-2 cm2 V-1 s-1, respectively.

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm2V−1s−1, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.

A novel synthesis method for large-area MoS2 film with improved electrical contact

Two-dimensional layered materials (2DLMs) are radically different from conventional bulk semiconductors, and major challenges in achieving practical 2DLM devices with good performance lie in the areas of improving the electrical contacts that connect 2DLMS with electrodes rather than in improving the quality of the channel material itself. To meet this challenge, we have devised a promising large-scale synthesis method for few-layer MoS2 film integrated with unique fabrication process that provides good contact. A thin layer of Mo was first deposited using sputtering evaporation and defined by a shadow mask, instead of immediate sulfurization in a chemical vapor deposition (CVD) furnace. Au contact electrodes were then deposited on Mo, followed by further sulfurization, which readily provides source-drain electrodes for synthesized MoS2. Top-gated transistors based on such MoS2 film show improved device performance compared to transistors fabricated using the traditional method. The transfer line method was further applied to verify that the proposed method could effectively decrease the contact resistance by more than 10 times, which can be attributed to the incomplete sulfurization of Mo atoms beneath the Au electrodes. It is envisioned that the proposed method could eventually be used to provide uniform and low contact resistance for CVD-grown 2DLM devices.

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.

Effects of V2O5/Au bi-layer electrodes on the top contact Pentacene-based organic thin film transistors

Top-contact Pentacene-based organic thin film transistors (OTFTs) with a thin layer of Vanadium Pent-oxide between Pentacene and Au layer are fabricated. Here we have found that the devices with V2O5/Au bi-layer source–drain electrode exhibit better field-effect mobility, high on–off ratio, low threshold voltage and low sub-threshold slope than the devices with Au only. The field-effect mobility, current on–off ratio, threshold voltage and sub-threshold slope of V2O5/Au bi-layer OTFT estimated from the device with 15 nm thick V2O5 layer is .77 cm2 v−1 s−1, 7.5×105, −2.9 V and .36 V/decade respectively.

Solution-Processed Organic Thin-Film Transistor Arrays with the Assistance of Laser Ablation

A key step toward commercialization of organic thin-film transistors (OTFTs) is to manufacture large-area OTFT arrays with desired uniform device performance. In this work, for the first time, solution-processed OTFT arrays were fabricated with the assistance of laser ablation. The source-drain electrodes and the whole devices were patterned by precise control of laser intensity and process path. Compared with traditional methods, this approach significantly simplifies the fabrication process of OTFT arrays with high quality and high yield. A careful selection of laser processing parameters is key to obtaining high quality and high performance OTFT arrays. The grazing incidence X-ray diffraction experiments and device performance tests ensured the selection of proper laser ablation intensity. Eventually, the OTFT arrays on silicon wafer and ITO glass exhibited uniform electrical characteristics with the mean mobility of 0.16 and 0.10 cm2 V-1 s-1, respectively. These results demonstrated that the laser ablation process provides a promising tool to simplify the fabrication of solution-processed OTFT arrays with low cost and high yield, which has great potential in upscaling of high performance OTFT arrays for display and circuits.