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We demonstrate printed organic Schottky diodes for the rectification of the 13.56 MHz near-field communication carrier wave. Printed silver electrodes covered with a self-assembled monolayer of 4-methylbenzenethiol were employed for the Schottky barrier. Optimizing the conditions of the self-assembled monolayer treatment was performed to improve the rectification ratio to 106 and the forward current density to 1 A cm−2. Finally, organic half-wave and full-wave rectifiers were demonstrated with a high-frequency response of output rectifying voltage of more than 5 V at 13.56 MHz.

In recent years, softening a ferroelectric polymer for wearable tactile electronics has been challenging because of its high robustness and crystallinity. In this study, a functional piezoelectric material system was fabricated by optimizing the blend ratio of a copolymer of PMMA-PBA, which is an amorphous polymer, and a ferroelectric polymer P(VDF-TrFE). The stretchable film was able to extend it up to maximum 400%. By fabricating a capacitor-type device combined with a conductive polymer, we demonstrated the possibility of applying it to a stretchable sensor.

Herein, two donor–acceptor conjugated polymers based on dithieno[3,2-b:2′,3′-d]thiophene (DTT) and thiophene-flanked diketopyrrolopyrrole (DPP) or furan-flanked DPP, namely PDPPT-DTT and PDPPF-DTT were synthesised, characterised and evaluated for their electrical performance in transistor devices for comparison. The influence of the heteroatoms (thiophene and furan) flanked DPP on their thermal, optical, electronic structure and charge carrier transport properties were investigated. The absorption spectra of polymer PDPPT-DTT are broader and red-shifted (26–30 nm) compared with that of polymer PDPPF-DTT, indicating the resonance energy of thiophene is greater than furan, which may allow for different electron localization and result in the difference of optical properties. In addition, energy levels of polymers were slightly affected by the aromatic remote end-groups (thiophene to furan) in DPP-based molecule. Hole transport properties of copolymers were investigated by fabricating the field-effect transistors in the bottom gate top contact (BGTC) configurations for three different self-assembled monolayers (SAMs)/gate dielectric interfaces and different annealing temperatures of polymeric active layer. The BGTC organic thin film transistor (OFET) devices having PDPPT-DTT and PDPPF-DTT thin film annealed at 200 °C exhibit the hole mobility of 0.18 and 0.20 cm2 V−1 s−1, respectively. The OFET devices with trichloro(octyl)silane SAM, fabricated and characterized in ambient environment (temperature ∼25 °C, humidity ∼50%), were found to retain 90% of their performance up to 1000 h.

In recent years, the use of printing methods to fabricate electronic devices (printed electronics) has attracted attention because of their low cost and low environmental impact. Printing technology enables the high-throughput fabrication of electrical circuits on film substrates, thereby providing inexpensive personal healthcare devices to monitor health status in real-time, for example. Temperature detection is one of the central concerns as a fundamental physical quantity in various fields. In 2013, a highly sensitive flexible thermistor was reported by formulating aqueous inks of nickel oxide (NiO) nanoparticles for inkjet printing. However, it required a high-temperature calcination process of more than 200 °C, which led to the use of expensive polyimide films with high heat resistance. It is necessary to promote further the development of low-temperature processes for printed thermistors to realize flexible negative temperature coefficient (NTC) thermistors at low cost using printed electronics technology. Moreover, in screen printing and inkjet printing, the definition of the ink pattern applied on the substrate changes due to spreading and coffee distortion phenomena, and the thickness between sensors becomes non-uniform, which is a structural consistency problem that can lead to variations in sensing performance. Therefore, in this study we developed low-temperature processable printed NTC thermistors with a temperature-sensitive layer of NiO by using reverse offset printing. The NTC thermistors were fabricated by printing a comb-like pattern of silver nanoparticles and a thin NiO film. In addition, the low-temperature formation of a NiO layer by oxygen plasma treatment was investigated, and x-ray photoelectron spectroscopy was used to carry out compositional analysis of the surface. Together with the plasma-assisted calcination, a flexible NTC thermistor formed on polyethylene terephthalate film is demonstrated.

We report printed single and dual-gate organic thin film transistors (OTFTs) and p-channel metal-oxide-semiconductor (PMOS) inverters fabricated on 125 µm thick flexible polyethylene naphthalate substrate. All the electrodes (gate, source, and drain) are inkjet-printed, while the parylene dielectric is formed by chemical vapor deposition. A dispenser system is used to print the active channel material using a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo [1,2-b;4,5-b′]dithiophene and polystyrene in tetralin solvent, which gives highest mobility of 0.43 cm2 V−1s−1. Dual-gate OTFTs are characterized by keeping the other gate electrode either in grounded or floating state. Floating gate electrode devices shows higher apparent mobility and current ratio due to additional capacitance of the parylene dielectric. PMOS inverter circuits are characterized in terms of gain, trip point and noise margin values calculated from the voltage transfer characteristics (VTC). Applied top gate voltage on the load OTFT control the conductivity or threshold voltage (V Th) of the bottom TFT and shift the trip point towards the middle of the VTC curve, and hence increase the noise margin.

Low-power, flexible, and properly encapsulated integrated circuits are the basic requirements of the solution-processed printed and wearable electronic prototypes for various emerging applications including display circuits, sensors, and radio-frequency identification tags. The organic field-effect transistor is one of the important types of devices used in such prototypes and its industrial applicability is essential for the printed electronics technology. The performance deterioration upon encapsulated through the thick layer of air-stable dielectric material such as amorphous fluoropolymer [CYTOP] - must be compensated by device engineering. In this work, we used furan and thiophene flanked diketopyrrolopyrrole donor-acceptor conjugated polymers namely PDPPF-DTT and PDPPT-DTT, and its comparative study was performed using Cytop as a dielectric material. The work advances interface engineering towards the single-gate and dual-gate organic transistors. Dual-gate transistors performance modulation using Cytop dielectric opens new research avenues towards stability enhancement of such transistors for real-world applications.

Low-operating-voltage (µm-thick flexible polyethylene naphthalate substrate using a blend of 2,7-dihexyl-dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene and polystyrene as an active p-type organic semiconducting material. All three electrodes (gate, source, and drain) are inkjet-printed, while the active semiconducting material is deposited by a dispenser system to achieve a saturation mobility of 0.32 cm2 V−1 s−1 at V GS = −2 V. Two different PMOS inverters are fabricated, for which the signal gain peak values are resolved for an ultra-low supply voltage, V DD = −0.5 V. We achieve a signal gain of 2.73 at V DD = −0.5 V. The effect of channel width is demonstrated for both OTFTs and PMOS devices. The ‘on’ current increases with channel width, and the switching point of the PMOS inverters shifts toward the middle of the voltage transfer characteristics, and hence improves the noise margin.

We have been employing novel printing technologies in three-dimensional printed electronics applications, to form electronic devices on curved or three-dimensional object surfaces. In this paper, we report on a newly developed printing technology named ‘soft blanket reverse-offset’ (SBR) printing, which was developed to create wide and flat patterned layers with uniform thicknesses on curved surfaces. SBR printing is an extension of conventional reverse-offset printing and uses an extremely thick and soft blanket (referred to as a “soft blanket”) as an offset roller. We also created a new thick stencil printing plate and a two-layered soft blanket construction that avoids the formation of contact-defects. In addition, we have also succeeded in printing wide and flat patterned layers onto curved surfaces using commercially available silver nanoparticle inks. The printed layers are also characterized by uniform thicknesses and low resistivities, equivalent to those of layers printed on planar substrates.

We report an in-depth study of three-dimensional (3D) inkjet-printed flexible organic field-effect transistors (FETs) and integrated circuits (ICs), and demonstrate the necessity, the feasibility and the key aspects of modelling-driven design and analysis. In particular, we performed compact modelling of the flexible printed organic FETs to determine transistor parameters from measured electrical characteristics, and to optimize the design of transistors for a balanced inverter. We also conducted a SPICE simulation of both static and dynamic behaviours of flexible printed organic complementary inverters and ring oscillators. This study revealed design rules to guide fabrication of the 3D structure of the inverter. The study also showed that the parasitic capacitances are dominant factors that determine the transient behaviour of the ring oscillator. Our work provides insights that can guide further improvement of flexible printed organic FETs and ICs towards the realisation of increasingly complex organic ICs and unconventional electronic applications.

Organic thin-film transistors (OTFTs) using high-k polymeric materials have attracted considerable attention. Their fabrication processes based on printing can dramatically reduce material waste and manufacturing costs compared with conventional photolithography. However, the high operating voltage of printed OTFTs remains a significant problem. In this study, we fabricated OTFTs that can operate in low voltages by using a high-k dielectric bilayer and a paraelectric polymers. The bilayer structure provided a much higher dielectric constant (>18). The bilayer dielectric exhibited a residual polarization value of 0.3 μC cm−2 and the measured field-effect mobility of the printed OTFT was 0.91 cm2 V s−1.

This paper reports on results demonstrating printed interconnect layers fabricated on the surface of three-dimensional (3D) plastic objects with low heat-resistance using omnidirectional inkjet (OIJ) 3D printing method in conjunction with continuous-wave xenon light sintering. The xenon light sintering was carried out by attaching a condenser lens to the OIJ printing apparatus. The combination of xenon light focused by a condenser lens and a long time exposure helped realize large energy densities of 100 J cm−2, which are comparable to pulsed xenon light energy. The combination of OIJ and continuous-wave xenon light technologies allowed us to fabricate 3D interconnect layers having resistivities of 13.3 ± 0.8 Ω along the 3D shape of plastic substrates. Furthermore, we found that the low thermal conductivity of plastic substrates makes it possible to carry out continuous-wave xenon light sintering without requiring assistive heating of the substrate.

We have developed a "Soft Blanket Gravure" (SBG) printing technology that can form patterns onto curved or three-dimensional solid surfaces using functional ink. In SBG printing, a soft and thick blanket (called a "Soft Blanket") is employed in the similar process to conventional gravure offset printing processes. We found and reported that SBG printing could completely receive silver ink from printing plates and transfer it to target substrates more effectively than conventional gravure offset printing under various conditions. In this research work, we measured the swelling ratio of the Soft Blanket using two different organic solvents and performed SBG printings using printing plates with groove depths of 10–100 µm and two silver inks that contained the two different solvents. SBG printing could form patterns with thickness of more than 11 µm and the same widths as the grooves on the printing plate (50 µm) using the optimal ink solvent. We also observed that the Soft Blanket could receive silver ink from grooves without cobwebbing, in contrast to conventional blanket. These results indicate that the SBG printing technology significantly improves the printing process because the Soft Blanket can absorb more solvent from silver ink.

We performed the spectral and cyclic voltammetry measurements of the new blue-light-emitting polymer poly(9,9-dioctylfluorenyl-2,7-diyl) (PDOFO) end capped with polyhedral oligomeric silsesquioxanes (POSS). From the oxidation and reduction potentials are determined, the estimated ionization potential Ip=5.73 eV and electron affinity Ea=2.2 eV were obtained. The luminescent properties of thin films and electroluminescent devices on their base were investigated. The devices on a PDOFO–POSS base showed a pure blue emission with a maximum of 423 nm. The green emission characteristic of PFO-based films and organic light-emitting devices (OLEDs) was suppressed by end capping the PFO backbone with POSS.

We report on the direct measurement of the external quantum efficiency of electroluminescence of organic light-emitting devices with different emission colors and patterns using an integrating sphere and a highly sensitive charge-coupled device spectral analyzer. Using this direct-measurement method, precise external quantum efficiency can be obtained more easily and more quickly than using the conventional luminance-conversion method. This method is shown to be very useful for determining the external quantum efficiency of organic light-emitting devices.

The characteristics of pentacene-based organic thin-film transistors (OTFTs) on plastic substrates with anodized gate insulators using different electrolyte solutions were investigated. The characteristics were significantly improved by using ammonium borate as the solution. The results show a good current-saturation characteristic, field-effect mobility of 0.51 cm2/Vs and a current on/off ratio of 105 at a low drain voltage of 3 V. To investigate the reason for the improvements, the structural and electrical characteristics of OTFTs and insulators were examined using various methods such as X-ray diffraction, atomic force microscope, capacitance-voltage measurement and secondary ion mass spectroscopy. From the results of these experiments, it was found that the orientation of pentacene was improved and the concentration of impurities in the insulator decreased in the case of ammonium borate. We also examined low-voltage driving of liquid crystal devices by the OTFT, targeting its application for flexible displays. Improvement of characteristics by the solution was also confirmed in the driving.

We have fabricated simple polymer electrophosphorescent light-emitting devices based on a single emitting layer containing a hole-transporting host polymer, poly(N-vinylcarbazole), and an electron-transporting 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene, doped with iridium complexes. We obtained a white light emission, whose Commission Internationale de l'Eclairage color coordinates are x=0.33 and y=0.36, by double-doping with phosphorescent materials: 11.8 wt% blue-light-emitting bis[(4,6-difluorophenyl)-pyridinato-N,C2'](picolinate) iridium(III) and 0.2 wt% red-light-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3'](acetylacetonate) iridium(III). At a practical luminance of 100 cd/m2, the external quantum efficiency of electroluminescence and the power luminous efficiency were 4.9% and 2.6 lm/W, respectively. This work demonstrates that high-efficiency white-light-emitting devices can be constructed from polymers and phosphorescent materials by using simple wet fabrication processes.

Organic thin-film transistors (OTFTs) which use Ta2O5 as the gate insulators were fabricated on plastic substrates. The gate insulators were synthesized by anodizing the gate electrodes fabricated as stacked structures of aluminum (Al) and tantalum (Ta) at room temperature. The stacked structure suppressed the stress at the interface between the substrate and metal electrodes so that cracks on Ta could be avoided. The organic semiconductor pentacene was used as the active layer on the gate insulator. The OTFTs showed the best performance at an operating voltage of 5 V and a good field-effect mobility of 0.36 cm2/Vs on plastic substrates. The structural order of the pentacene in the film and the temperature dependence of the leakage current in the insulator were also studied to improve the device characteristics.

A newly developed patterning technology called 'Soft Blanket Gravure' (SBG) printing is introduced, which uses a thick offset blanket made of a soft material (soft blanket). The SBG printing technology is based on conventional gravure offset printing, but is able to receive ink from a printing plate and transfers it to target substrates more completely under a variety conditions. In this research work, the printability of conductive lines using silver inks was studied for this patterning technology under atypical printing conditions. The SBG printing technology can successfully print silver conductive lines that are uniform and thicker under various conditions, as well as relatively thin lines, by using the same printer configuration and printing parameters. In addition, we validated the gravure offset printing mechanisms using the soft blanket.

A newly developed silver nanoparticle ink for gravure offset printing has been prepared through the thermal decomposition reaction of silver oxalate. The resistivity of silver thin films made of the ink was estimated to be 3.1 µΩ cm by sintering at 250 °C, which is comparable to that of bulk silver. In addition, we achieved the patterning of fine lines with a width of 20 µm using the combination of the ink and gravure offset printing. The prepared ink could be used to fabricate various printed electronic circuits in the near future.

In this paper, we report on a newly developed printing method called the "soft blanket gravure" (SBG) printing method, which is based on a conventional gravure offset printing method but unlike it, SBG printing uses a very soft and thick offset blanket. SBG printing onto various curved, nonplanar surfaces as well as on planar surfaces using silver ink was successfully demonstrated by optimizing printing conditions such as printing pressure and printing speed. Finely printed conductive silver lines with a 30 µm line width were formed on the curved surfaces. This printing method will have new applications in three-dimensional printed electronics.

Silver nanowires (AgNWs) are attracting much attention for their potential use in flexible or stretchable conducting film applications and in printed and wearable electronics devices. In this study, we fabricated flexible hybrid conducting films using different concentrations of composite silver ink prepared by mixing AgNW and silver nanoparticle (AgNP) materials, which we synthesized, for flexible interconnects and electrodes. These films had low resistivities at low sintering temperatures. Furthermore, they were also stable to tensile bending in comparison with a pure AgNP film. We found that there is an optimum concentration of AgNWs in the composite silver ink for the hybrid conducting film to have a lower resistivity even with sintering below 100 °C. We also found that the reason why hybrid conducting films are stable to repeated tensile bending is that AgNWs maintain functional conducting paths by making bridges that compensate for cracks between AgNPs during film bending.

In this work, we demonstrate that highly conductive metal interconnects can be fabricated on the surface of three-dimensional objects using "omnidirectional ink jet" (OIJ) printing technology. OIJ printing technology makes it possible to perform ink jet printing in all directions by combining the motion of a 6-axis vertically articulated robot with precise positioning and a thermal drying process, which allows for the printing of stacked layers. By using OIJ technology, we were the first to successfully fabricate printed interconnect layers having a very low electrical resistance of 12 mΩ over a 10 mm length. Moreover, the results of the high-current test demonstrated that the printed interconnects can withstand high-current-flow of 5 A for 30 min or more.

We demonstrate that by using thin films of metal oxides, such as vanadium oxide (VOx), molybdenum oxide (MoOx) and ruthenium oxide (RuOx), as a hole-injecting layer for an organic electroluminescent (EL) device consisting of N,N'-diphenyl-N,N-bis(3-methylphenyl1)1,1'-biphenyl-4,4' diamine (TPD) and tris-(8-quinolinolato) aluminium (Alq), the EL device performance can be significantly improved. The `operating voltage' of the device is reduced with respect to a device with a well known indium - tin-oxide (ITO) electrode for hole injection. We consider that the improvement of the operating voltage is attributable to the lower energy barrier for hole injection at the metal oxide/TPD interface.

We report on the phosphorescence properties of green-light-emitting fac tris(2-phenylpyridine) iridium [Ir(ppy)3] and red-light-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3'](acetylacetonate) iridium [Btp2Ir(acac)] doped into polycarbonate or 4,4'-N,N'-dicarbazol-biphenyl host matrices. We observed that the phosphorescence intensity of Ir(ppy)3 was almost independent of temperature, indicating a high phosphorescence quantum yield of nearly 100%. On the other hand, the phosphorescence intensity of Btp2Ir(acac) gradually decreased with increasing temperature above 200 K. This suggests that the nonradiative processes from triplet states are intrinsic properties, and that the phosphorescence quantum yield is lower than that of Ir(ppy)3. We discuss the temperature dependence of the transient phosphorescence characteristics.

We report on the fabrication and demonstration of fully printed ferroelectric capacitors using poly(vinylidene fluoridetrifluoroethylene) [P(VDF–TrFE)]. The printed ferroelectric capacitors were primarily fabricated by ink-jet printing on a thin plastic film substrate. The annealing process for the P(VDF–TrFE) layer was optimized from the viewpoints of surface morphology and crystallinity. A good ferroelectric polarization–electric field loop and piezoelectricity in the P(VDF–TrFE) were achieved for the printed ferroelectric capacitors. We have succeeded in the detection of a weak pressure of 150 mbar using the printed ferroelectric capacitor, which is an indication of a potential application to health-care biosensors. These results were realized by the optimization of the annealing temperature for the P(VDF–TrFE) layer.

Here, we demonstrate ultrathin, flexible nonvolatile memory devices with excellent durability under compressive strain. Ferroelectric-gate field-effect transistors (FeFETs) employing organic semiconductor and polymer ferroelectric layers are fabricated on a 1-µm-thick plastic film substrate. The FeFETs are characterized by measuring their transfer characteristics, programming time, and data retention time. The data retention time is almost unchanged even when a 50% compressive strain is applied to the devices. To clarify the origin of the excellent durability of the devices against compressive strain, an intermediate plane is calculated. From the calculation result, the intermediate plane is placed close to the channel region of the FeFETs. The high flexibility of the ferroelectric polymer and ultrathin device structure contributes to achieving a bending radius of 0.8 µm without the degradation of memory characteristics.

Flexible organic thin-film transistors (OTFTs) are studied for novel devices such as wearable and low-cost electronics. However, a change of characteristics of their devices under tensile strain was recognized. Here, we investigated the effects that applied strain has on flexible printed OTFT devices by concentrating on the contact between printed source/drain electrodes and semiconducting layers. The semiconducting layer contributed only to increases in the contact resistance R c, which depended on the physical stability of an interface between the printed electrodes and semiconducting layers in the OTFT devices under tensile strain. It was found that the small-molecule semiconductor OTFT devices were more sensitive to strain, exhibiting increases in R c.

In this note, we report on the electrical detection of a mercury(II) ion (Hg2+) in water using an organic field-effect transistor (OFET). The fabricated OFET device possesses an extended-gate gold electrode functionalized with l-cysteine (known as a Hg2+ ligand). We have investigated the detection ability of the OFET by immersing the extended-gate electrode in an aqueous solution with various Hg2+ concentrations. As a result, we have successfully observed the decrease in the drain current of the OFET upon the addition of incremental amounts of Hg2+ in water. The limit of detection for Hg2+ in this device is estimated to be 31 ppb.

We have characterized mechanisms to improve the adhesion between printed electrodes prepared from silver nanoparticle inks and underlying polymer layers. Adhesion strength was significantly improved by sintering the inks above the glass transition temperature of the polymer underlayers, whereby enhanced adhesion was realized through interfacial fusion between the silver electrode layer and the underlying polymer layer. The surface energy of the underlayer was found to be an important factor in the improvement of adhesive strength, in that larger and thicker interfused regions between layers were observed for underlayers that had higher surface energies.

Organic reset–set (RS) flip-flop logic circuits based on pseudo-CMOS inverters have been fabricated using full solution processing at a relatively low process temperatures of 150 °C or less. The work function for printed silver electrodes was increased from 4.7 to 5.4 eV through surface modification with a self-assembled monolayer (SAM) material. A bottom-gate, bottom-contact organic thin-film transistor (OTFT) device using a solution-processable small-molecular semiconductor material exhibited field-effect mobility of 0.40 cm2 V−1 s−1 in the saturation region and a threshold voltage (VTH) of −2.4 V in ambient air operation conditions. In order to reduce the variations in mobility and VTH, we designed a circuit with six transistors arranged in parallel, in order to average out their electrical characteristics. As a result, we have succeeded in reducing these variations without changing the absolute values of the mobility and VTH. The fabricated RS flip-flop circuits were functioned well and exhibited short delay times of 3.5 ms at a supply voltage of 20 V.

We report on the detection of cysteine in water using an extended-gate-type organic field-effect transistor (OFET). The fabricated OFET device can be operated below 3 V. The portion of the designed device used for cysteine detection is the extended-gate electrode prepared by vacuum deposition of a gold (Au) layer. Cysteine can bind to the Au electrode surface through chemisorption. By this technique, we have successfully observed a shift in the threshold voltage of the OFET upon the addition of cysteine in an aqueous solution, whereby the detection limit for cysteine was found to be 39 ppb.

As part of our ongoing research program to develop health care sensors based on organic field-effect transistor (OFET) devices, we have attempted to detect histamine using an extended-gate OFET. Histamine is found in spoiled or decayed fish, and causes foodborne illness known as scombroid food poisoning. The new OFET device possesses an extended gate functionalized by carboxyalkanethiol that can interact with histamine. As a result, we have succeeded in detecting histamine in water through a shift in OFET threshold voltage. This result indicates the potential utility of the designed OFET devices in food freshness sensing.

A photolithographic patterning technique of perfluoropolymers was developed for the micropatterning of solution-processed organic semiconductors. We found that a small amount of fluorosurfactant additives in photoresist solutions enabled preparation of the spin-coated films even on the dewetting surfaces of perfluoropolymers. Subsequent photolithography and dry/wet etching achieved excellent patterning of perfluoropolymers, which provided a high contrast of wettability in microscale on the substrate. The dewetting patterns were applied for isolating solution-processed organic thin-film transistor arrays for a 100 ppi active-matrix display by selective dewetting. The isolation of the semiconducting layers achieved significant improvement in the current on/off ratio with a high mobility of >1 cm2 V−1 s−1.

Using a simple spin-coating process, we have demonstrated a fine patterning method for silver nanoparticle ink by combining hydrophobic and hydrophilic substrate surface properties. The hydrophobic/hydrophilic treated areas were precisely formed on a glass substrate by employing a hydrophobic photoresist and a hydrophilic polymer with a conventional photolithographic patterning process. Using this technique, fine silver electrodes with spaces of 5 µm were successfully formed. We applied this method to the short-channel source/drain electrodes in organic thin-film transistor (TFT) devices and succeeded in fabricating a TFT electrode array with a resolution of 100 ppi. The short-channel pentacene TFT with silver nanoparticle electrodes showed good electrical characteristics with high yields and no hysteresis.

Fully solution-processed organic thin-film transistor (OTFT) devices have been fabricated with simple patterning process at a relatively low process temperature of 100 °C. In the patterning process, a hydrophobic amorphous fluoropolymer material, which was used as the gate dielectric layer and the underlying base layer, was treated with an oxygen plasma to selectively change its surface wetting properties from hydrophobic to hydrophilic. Silver source and drain electrodes were successfully formed in the treated areas with highly uniform line widths and without residues between the electrodes. Nonuniformities in the thickness of the silver electrodes originating from the “coffee-ring” effect were suppressed by optimizing the blend of solvents used with the silver nanoparticles, such that the printed electrodes are appropriate for bottom-gate OTFT devices. A fully solution-processed OTFT device using a polymer semiconductor material (PB16TTT) exhibited good electrical performance with no hysteresis in its transfer characteristics and with good linearity in its output characteristics. A relatively high carrier mobility of 0.14 cm2 V-1 s-1 and an on/off ratio of 1×105 were obtained with the fabricated TFT device.

A new driving scheme for an active-matrix organic light emitting diode (AMOLED) display was developed to prevent the picture quality degradation caused by the hysteresis characteristics of organic thin film transistors (OTFTs). In this driving scheme, the gate electrode voltage of a driving-OTFT is directly controlled through the storage capacitor so that the operating point for the driving-OTFT is on the same hysteresis curve for every pixel after signal data are stored in the storage capacitor. Although the number of OTFTs in each pixel for the AMOLED display is restricted because OTFT size should be large enough to drive organic light emitting diodes (OLEDs) due to their small carrier mobility, it can improve the picture quality for an OTFT-driven flexible OLED display with the basic two transistor-one capacitor circuitry.

We fabricated highly efficient organic light-emitting diodes (OLEDs) using the solution processible iridium complexes which have long alkyl or cycloalkyl substituents in an ancillary ligand as a host material and a red phosphorescent iridium complex as an emitting guest by spin coating method. The OLEDs emitted red phosphorescence from the guest and the luminance reached around 10,000 cd/m2. The efficiency of the OLED improved after we added an electron transport material (ETM) to the emitting layer. The OLED using a mixture of the alkyl substituted iridium complex and ETM as the host showed an external quantum efficiency of 10%.

We report on n-channel organic thin-film transistors (OTFTs) based on the novel n-type organic semiconductor, perfluoropentacene. The transistor exhibits excellent electrical characteristics, with a high electron mobility of 0.22 cm2/(V s) and a good current on/off ratio of 105. The electron mobility is comparable to the hole mobility of a pentacene OTFT. By combining the n-type perfluoropentacene and the p-type pentacene, we have fabricated ambipolar OTFTs and complementary inverter circuits. The OTFTs with heterostructures of the p- and n-type organic semiconductors can operate as an ambipolar device with high electron and hole mobilities of 0.042 and 0.041 cm2/(V s). The complementary inverter using an n-channel perfluoropentacene OTFT and a p-channel pentacene OTFT exhibits excellent transfer characteristics with a voltage gain of 45. A complementary inverter using the ambipolar OTFTs is also demonstrated.

We report a novel class of light-emitting materials for use in organic light-emitting diodes (OLEDs): multifunctional phosphorescent dendrimers that have a phosphorescent core and dendrons based on charge-transporting building blocks. We synthesized first-generation and second-generation dendrimers consisting of a fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] core and hole-transporting phenylcarbazole-based dendrons. Smooth amorphous films of these dendrimers were formed by spin-coating them from solutions. The OLEDs using the dendrimer exhibited bright green or yellowish-green emission from the Ir(ppy)3 core. The OLEDs using the film containing a mixture of the dendrimer and an electron-transporting material exhibited higher efficiency than those using the neat dendrimer film. The external quantum efficiency of OLEDs using the film containing a mixture of the first-generation dendrimer and an electron-transporting material was as high as 7.6%.

Organic thin-film transistor (OTFT)-driven 5×5 polymer-dispersed liquid crystal (PDLC) display cells on a flexible plastic substrate have been developed. It is necessary to increase the maximum usable gate voltage of OTFT to obtain a high contrast ratio. We propose a stacked gate insulator that consists of polyvinylphenol and anodized Ta2O5 for decreasing the gate leakage current and increase the maximum voltage. The OTFT with the insulator showed a field-effect mobility of 0.4 cm2/(V s), a current on/off ratio of 105, a low threshold voltage of 1.1 V, and a subthreshold slope of 0.2 V/decade. Leakage current was successively decreased up to a gate voltage of 15 V, maintaining a low-voltage operation of OTFT. Double passivation layers using polyvinylalcohol and photosensitive acrylic material are also proposed to prevent the degradation of OTFT by liquid crystal. The bending characteristics of OTFT on plastic substrates were also measured for various radiuses of curvature. The OTFT can operate at a radius of curvature exceeding 20 mm. On the fabricated display cells, we confirmed a good display operation with a contrast ratio of 10:1 with a low driving voltage of 12–13 V.

The optical absorption and reflectance of poly(2,5-thienylenevinylene) and its precursor polymer thin films were measured. It was found that the optical properties drastically changed and metallic reflectance appeared during conversion from the precursor polymer to poly(2,5-thienylenevinylene).

We have succeeded in the fabrication of a stable organic electroluminescent (EL) device with a three-layer structure; hole transport layer/emitting layer/electron transport layer. The EL device was prepared by vacuum evaporation. Efficient carrier double injection into the emitting layer was realized by the use of separate hole and electron transport layers. Bright EL emission was observed in a darkened room at the dc bias voltage of 50 V. Stable emission lasted for more than five hours at this condition. The emission spectrum could be changed with variation of the organic material for the emitting layer.

An electroluminescent device with a three-layer structure was constructed using an intensely fluorescent material, 12-phthaloperinone derivative, for the emitter material. The cell structure of Mg-electrode/amorphous electron-transport layer/polycrystalline emitting layer/amorphous hole-transport layer/Au-electrode was constructed on a glass substrate. Yellow EL emission was clearly observed in normal room lighting at the dc bias voltage of 60 V. The emission intensity was proportional to the injection current over a wide current range of 10-7 to 10-3 A/cm2. The EL intensity exceeded 1 µW/cm2 at the injection current of 2 mA/cm2.