Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm²/Vs similar to that obtained from a SiO₂ gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.     

Controlled growth of a graphene charge-floating gate for organic non-volatile memory transistors

We report memory application for graphene as a floating gate in organic thin-film transistor (OTFT) structure. For graphene floating gate, we demonstrate a simpler synthesis method to form a discrete graphene layer by controlling the growth time during a conventional CVD process. The resulting organic memory transistor with the discrete graphene charge-storage layer is evaluated. The device was demonstrated based on solution-processed tunneling dielectric layers and evaporated pentacene organic semiconductor. The resulting devices exhibited programmable memory characteristics, including threshold voltage shifts (∼28 V) in the programmed/erased states when an appropriate gate voltage was applied. They also showed an estimated long data retention ability and program/erase cycles endurance more than 100 times with reliable non-volatile memory properties although operated without encapsulation and in an ambient condition.     

Low-voltage operation of organic thin-film transistors based on ultrafine printed silver electrodes

We report the low-voltage operation of organic thin-film transistors (OTFTs) based on high-resolution printed source/drain electrodes that are produced by a surface photoreactive nanometal printing (SuPR-NaP) technique. We utilized an ultrathin layer of perfluoropolymer, Cytop, that functions not only as a gate dielectric layer in the OTFTs but also as a base layer for producing a patterned reactive surface for silver nanoparticle chemisorption in the SuPR-NaP technique. We successfully demonstrate 2 V operation with negligible hysteresis in the polycrystalline pentacene OTFT with a gate dielectric thickness of 22 nm, and we achieved current amplification by the printed electrodes modified with pentafluorobenzenethiol. The SuPR-NaP technique enables the production of high-resolution printed silver electrodes required for high-performance OTFTs, which have potential practical electronic device applications.     

Printing-induced improvements of organic thin-film transistors

To understand the observation of improved pentacene (Pn) thin-film transistor mobility in flexible printed devices, a method for performing electrical measurements of organic thin-film transistors (OTFT) during the process of transfer printing has been developed. Different sample configurations were designed to test two aspects of the printing process: (1) the formation of the source/drain contacts a Pn thin-film, and (2) the formation of the transfer printed Pn/dielectric interface. In situ measurements show that pressure-induced contacts of gold (Au) electrodes result in a factor of seven mobility improvement compared with evaporation of top Au electrodes on an otherwise identical device configuration. Annealing the laminated device up to 90 °C caused no further improvement, and heating above 90 °C degraded performance. The mobility of a transfer printed device with the rough, as-grown top surface of the Pn in contact with the dielectric was found to increase dramatically with subsequent annealing for a sample temperature up to 120 °C. This is attributed to annealing-induced structural changes in the Pn film at elevated temperatures, consistent with X-ray bulk measurements showing enhanced crystal morphology in transfer printed Pn thin-films.     

Surface modification of printed silver electrodes for efficient carrier injection in organic thin-film transistors

The modification of printed silver electrode surfaces for use as the bottom-contact electrodes of organic thin-film transistors (OTFTs) is reported. Printed silver electrodes fabricated using the surface photoreactive nanometal printing (SuPR-NaP) technique are inevitably covered with an inert surface layer of alkylamines that is originally used for encapsulation of the silver nanoparticles (AgNPs). However, it may act as a built-in protective layer against carrier injections. We demonstrate that a simple vapor exposure method is sufficient for converting the protective layer into a layer that assists carrier injection. As modifiers, we used various types of fluorinated benzenethiols that exhibit a stronger coordination with the silver surfaces than the alkylamimes. We detected the chemical conversion from alkylamine encapsulation to thiol coordination by surface enhanced Raman spectroscopy (SERS) and evaluated the improvement in the carrier injection using a transfer length method (TLM) for the OTFTs. Among the modifiers, the pentafluorobenzenethiol (PFBT) treatment significantly improves the device performance and stability of the OTFTs.     

Fluoropolymer-assisted graphene electrode for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) were fabricated on a graphene electrode, with synthesized graphene being transferred and simultaneously doped with supporting polymers. Poly[methyl methacrylate] (PMMA) and fluoropolymer (CYTOP) layers were used as the supporting polymers. The sheet resistance of CYTOP-assisted graphene (CYTOP-G) with 4 layers of graphene is 200 Ω/sq., which is lower than that of PMMA-assisted graphene (PMMA-G, 330 Ω/sq.) The transmittance value of PMMA-G and CYTOP-G with 4 graphene layers is higher than 85%. CYTOP-G is shown to exhibit a higher tolerance to UV–O₃ treatment and thermal annealing than PMMA-G. Work function of CYTOP-G is 4.7 eV, which is higher than that of PMMA-G (4.3 eV). X-ray photoemission and Raman spectroscopy data indicate that CYTOP-G has numerous C-F bonds on the surface exhibiting p-type semiconductor properties, owing to the high electronegativity of fluorine. The turn-on voltage of an OLED based on CYTOP-G with 4 graphene layers is 4.2 V, which is lower than that of indium tin oxide (ITO)-based one (4.5 eV). Furthermore, the luminance ratio of graphene-based OLEDs to ITO-based OLEDs was calculated to be 104% for CYTOP-G, and 97% for PMMA-G. According to the ultraviolet photoemission spectra, the hole injection barrier in CYTOP-G is lower by about 0.5 eV than the hole injection barrier in PMMA-G. These results are very encouraging to the prospect of replacing ITO electrodes with graphene ones in OLED applications.     

Mechanical bending of flexible complementary inverters based on organic and oxide thin film transistors

Flexible complementary inverters composed of p-channel pentacene thin-film transistors (TFTs) and n-channel amorphous indium gallium zinc oxide TFTs were fabricated on polymer substrates. The characteristics of the TFTs and inverters were evaluated at different bending radii. Throughout the bending experiments, the relationship between the performances of the inverters and the characteristics of the TFTs under mechanical deformation was analyzed. The mechanically applied strain led to a change in the voltage transfer characteristics of the complementary inverters, as well as the source–drain saturation current, field-effect mobility and threshold voltage of the TFTs. The switching threshold voltage of the fabricated inverters decreased with decreasing bending radius, which was related to changes in the field-effect mobility and the threshold voltage of the TFTs.     

Significant effect of the molecular formula in silver nanoparticle ink to the characteristics of fully gravure-printed carbon nanotube thin-film transistors on flexible polymer substrate

Single-walled carbon nanotubes (SWNTs) are a valuable material for use in not only nanoelectronics but also printed electronics because of their stability, tunable operation speed, and scalability. However, the device characteristics of fully printed, SWNT-based thin film transistors (SWNT-TFTs) often have large variations, and the fundamental cause of these inconsistencies are not yet well understood. Therefore, fully printed SWNT-TFT-based electronic devices have not been practically realized in the market. In this study, the significant variation in the electrical parameters of printed SWNT-TFTs that is caused by minor molecular variations in the formulation of silver nanoparticle-based ink is reported. Strikingly, a very small difference in the chemical structure between ethylene glycol and diethylene glycol in the silver nanoparticle-based ink, which is used to print drain-source electrodes in the SWNT-TFTs, with everything else identical, induced a difference of approximately 70 meV in the barrier height between the drain-source electrodes and the SWNT layer at 300 K. The modification of the absorbed polymer binder in the silver nanoparticle ink due to the additive is the major cause of the observed barrier height difference. These results allow for a better understanding of the relationship between the ink rheology at the molecular level and the printed device properties, and enable a more precise design and control of device properties which will have profound impacts on printed electronic devices.     

Crosslinked polymer-mixture gate insulator for high-performance organic thin-film transistors

In this paper, we report on the fabrication of a crosslinked polymer-mixture gate insulator for high-performance organic thin-film transistors (TFTs). We used cyanoethylated pullulan (CEP) as a crosslinkable high-k polymer matrix and poly(ethylene-alt-maleic anhydride) (PEMA) as a polymeric crosslinking agent. Because PEMA has a high number of functional groups reactive to the hydroxyl groups of CEP, the use of PEMA is effective for minimizing the amount of remaining hydroxyl groups strongly related to the large current hysteresis and high off current of the organic TFTs. To investigate the potential of the CEP-PEMA mixture as a gate insulator, we fabricated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) TFTs. The C₈-BTBT TFT with the 60 nm-thick CEP-PEMA gate insulator showed excellent TFT performance with a field-effect mobility of 1.4 cm²/V s and an on/off ratio of 2.4 × 10⁶.     

Megahertz operation of flexible low-voltage organic thin-film transistors

Bottom-gate, top-contact (inverted staggered) organic thin-film transistors with a channel length of 1 μm have been fabricated on flexible plastic substrates using the vacuum-deposited small-molecule semiconductor 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT). The transistors have an effective field-effect mobility of 1.2 cm²/V s, an on/off ratio of 10⁷, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 ns per stage at a supply voltage of 3 V. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V.     

Ta2O5 as gate dielectric material for low-voltage organic thin-film transistors

In this paper we report the use of Ta₂O₅ as gate dielectric material for organic thin-film transistors. Ta₂O₅ has already attracted a lot of attention as insulating material for VLSI applications. We have deposited Ta₂O₅ thin-films with different thickness by means of electron-beam evaporation. Being a relatively low-temperature process, this method is particularly suitable for organic thin-film transistor fabrication on plastic substrates. Deposition and patterning are achieved in one step by the use of shadow masks. The dielectric can be evaporated on top of the semiconducting layer. In this way a large variety of structures can be realized. Poly(3-hexylthiophene) was used as semiconducting material in the transistor structure. Such transistors are operating at voltages smaller than −3 V. Having a high dielectric constant (ε_r=21), Ta₂O₅ facilitates the charge carrier accumulation in the transistor channel at much lower electrical fields. The properties of the dielectric material as well as the operation of the organic transistors with a Ta₂O₅ gate dielectric are discussed.     

Micro-scale twistable organic field effect transistors and complementary inverters fabricated by orthogonal photolithography on flexible polyimide substrate

We fabricated micro-scale organic field effect transistors (OFETs) and complementary inverters on a twistable polyimide (PI) substrate by applying orthogonal photolithography. By applying a highly fluorinated photoresist and development solvent, it becomes possible to create organic electronic devices with a micro-scale channel length without damaging the underlying polymer films. The 3 μm-channel twistable pentacene OFET devices and complementary inverters created using p-type pentacene and n-type copper hexadecafluorophthalocyanine exhibited stable electrical characteristics from flat to twist configurations (angle of up to ∼50°). The realization of twistable micro-scale OFETs and inverter devices on a PI substrate may enable the production of functioning organic devices in practical, flexible configurations.     

Enhanced storage/operation stability of small molecule organic photovoltaics using graphene oxide interfacial layer

Graphene and graphene oxide (GO) have been applied in flexible organic electronic devices with enhanced efficiency of polymeric photovoltaic (OPV) devices. In this work, we demonstrate that storage/operation stability of OPV can be substantially enhanced by spin-coating a GO buffer layer on ITO without any further treatment. With a 2 nm GO buffer layer, the power conversion efficiency (PCE) of a standard copper phthalocyanine (CuPc)/fullerene (C₆₀) based OPV device shows about 30% enhancement from 1.5% to 1.9%. More importantly, while the PCE of the standard device drop to 1/1000 of its original value after 60-days of operation-storage cycles; those of GO-buffered device maintained 84% of initial PCE even after 132-days. Atomic force microscopy studies show that CuPc forms larger crystallites on the GO-buffered ITO substrate leading to better optical absorption and thus photon utilization. Stability enhancement is attributed to the diffusion barrier of the GO layer which slow down diffusion of oxygen species from ITO to the active layers.     

Variation-based design of an AM demodulator in a printed complementary organic technology

In this work, the design of a high-frequency AM demodulator in a printed complementary organic technology is presented. The behaviour and the variability of printed circuits are predicted by means of accurate transistor modelling, statistical characterization, and Monte Carlo simulations. The effectiveness of the design approach is readily verified by comparing measurements and simulations of simple digital blocks as well as two differential amplifiers. These amplifiers can be used as continuous-time comparators in the demodulator. In addition, the possibility of high-frequency rectification using the printed organic TFTs is shown by providing the experimental results of an envelope detector measured under different load and input conditions. All the measurements are performed in air. Finally, the simulation of a complete AM demodulator system including the measured blocks is demonstrated.     

Polyimide/polyvinyl alcohol bilayer gate insulator for low-voltage organic thin-film transistors

In this paper, we report the fabrication of a polyimide/polyvinyl alcohol (PVA) bilayer gate insulator for low-voltage organic thin-film transistors (TFTs). The introduction of a PVA layer to form a bilayer structure improves the dielectric and insulating properties of the gate insulator. Organic TFTs with 150 nm-thick polyimide and PVA gate insulators were inactive at low operation voltages below 5 V. Conversely, organic TFTs with 150 nm-thick polyimide/PVA bilayer gate insulators exhibited excellent device performances. Our results suggest that the introduction of a PVA layer with a high dielectric constant could be a simple and efficient way to improve the device performance of low-voltage organic TFTs.     

Effect of the work function of gate electrode on hysteresis characteristics of organic thin-film transistors with Ta2O5/polymer as gate insulator

Organic thin-film transistors (OTFTs) using high dielectric constant material tantalum pentoxide (Ta₂O₅) and benzocyclobutenone (BCBO) derivatives as double-layer insulator were fabricated. Three metals with different work function, including Al (4.3 eV), Cr (4.5 eV) and Au (5.1 eV), were employed as gate electrodes to study the correlation between work function of gate metals and hysteresis characteristics of OTFTs. The devices with low work function metal Al or Cr as gate electrode exhibited high hysteresis (about 2.5 V threshold voltage shift). However, low hysteresis (about 0.7 V threshold voltage shift) OTFTs were attained based on high work function metal Au as gate electrode. The hysteresis characteristics were studied by the repetitive gate voltage sweep of OTFTs, and capacitance–voltage (C–V) and trap loss-voltage (G_p/ω?V) measurements of metal–insulator–semiconductor (MIS) devices. It is proved that the hysteresis characteristics of OTFTs are relative to the electron injection from gate metal to Ta₂O₅ insulator. The electron barrier height between gate metal and Ta₂O₅ is enhanced by using Au as gate electrode, and then the electron injection from gate metal to Ta₂O₅ is reduced. Finally, low hysteresis OTFTs were fabricated using Au as gate electrode.     

Facile polymer gate dielectric surface-modification for organic thin-film transistors using self-assembled surfactant layer

We demonstrate facile polymer gate dielectric surface-modification method for organic thin-film transistors (OTFTs). We simply introduce self-assembled surfactant layer onto the top surface of poly(4-vinylphenol) (PVP) dielectric by spin coating PVP solution mixed with sodium dodecyl sulfate and tridecafluorohexane-1-sulfonic acid potassium salt as additive agents. The surfactant-modified PVP layer acquires various merits compared to pristine PVP layer in terms of surface smoothness and hydrophobicity, as confirmed by contact angle measurement, atomic force microscopy analyses, grazing incident X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy. The resulting OTFTs with the conventional semiconducting poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) as the active layer and surfactant-modified PVP as the dielectric layer reveal overall ascendency over the OTFT with pristine PVP, especially in terms of operating hysteresis and reliability. The effects of hydrophobicity of surfactants on the surface properties of PVP as well as the OTFT performances are fully discussed in conjunction with various characterization tools.     

Degradation of all-inkjet-printed organic thin-film transistors with TIPS-pentacene under processes applied in textile manufacturing

Printed electronics represent an alternative solution for the manufacturing of low-temperature and large area flexible electronics. The use of inkjet printing is showing major advantages when compared to other established printing technologies such as gravure, screen or offset printing, allowing the reduction of manufacturing costs due to its efficient material usage and the direct-writing approach without requirement of any masks. However, several technological restrictions for printed electronics can hinder its application potential, e.g. the device stability under atmospheric or even more stringent conditions. Here, we study the influence of specific mechanical, chemical, and temperature treatments usually appearing in manufacturing processes for textiles on the electrical performance of all-inkjet-printed organic thin-film transistors (OTFTs). Therefore, OTFTs where manufactured with silver electrodes, a UV curable dielectric, and 6,13-bis(triisopropylsilylethynyl) pentance (TIPS-pentacene) as the active semiconductor layer. All the layers were deposited using inkjet printing. After electrical characterization of the printed OTFTs, a simple encapsulation method was applied followed by the degradation study allowing a comparison of the electrical performance of treated and not treated OTFTs. Industrial calendering, dyeing, washing and stentering were selected as typical textile processes and treatment methods for the printed OTFTs. It is shown that the all-inkjet-printed OTFTs fabricated in this work are functional after their submission to the textiles processes but with degradation in the electrical performance, exhibiting higher degradation in the OTFTs with shorter channel lengths (L = 10 μm).     

高性能PTCR电极浆料生产工艺的改进

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