酒井 正俊

Electrophotography is a digital, on-demand, dry, and page printing technique that operates based on toner particles of electronic materials using an electrostatic force and generates an electrical circuit via distribution of the toner particles. We developed a 10 μm linewidth resolution with various electronic materials, including conductors, semiconductors, and insulators, without any chemical pretreatments on the substrate films, while a 5 μm resolution was also possible for limited materials. The electrical resistivity of the printed Ag–Ni after an intense pulse light sintering was comparable to that of commercial indium tin oxide transparent films.

The metal–insulator transition induced by the gate electric field in the charge order phase of the α-(BEDT-TTF)2I3 single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region.

Printed electronics are a set of additive manufacturing methods for creating future flexible electronics on thin polymeric sheets. We proposed the toner-type, dry, page-printing of Ag–Ni composite conductive particles on flexible plastic sheets without pre-treatment. No chemical solvents are necessary to compose the inks of the electronic materials used for the toner-type printing, and no chemical treatment is required for the plastic film substrate surface. In addition, multilayer printing is simple when using toner printing because previously printed materials do not need to be resolved; furthermore, composing the thick films of the electronic materials is relatively simple. In this study, we fabricated an Ag–Ni composite toner to improve the fluidity of the toner particles compared to bare Ag particles. We successfully printed IC peripheral circuits at a resolution of 0.20 mm and demonstrated that the actual electrical circuit pattern can be formed using our method.

Organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in large area and flexible sensor devices. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on any substrate. Moreover, their electronic properties can be easily tuned to optimize, charges transport depending on the targeted application in the field of imaging, tactile or biomedical sensing. We made curved surface sensor array by thermal molding of planer device array on thin plastic film.

A solvent-free printing process for printed electronics is successfully developed using toner-type patterning of organic semiconductor toner particles and the subsequent thin-film formation. These processes use the same principle as that used for laser printing. The organic thin-film transistors are prepared by electrically distributing the charged toner onto a Au electrode on a substrate film, followed by thermal lamination. The thermal lamination is effective for obtaining an oriented and crystalline thin film. Toner printing is environmentally friendly compared with other printing technologies because it is solvent free, saves materials, and enables easy recycling. In addition, this technology simultaneously enables both wide-area and high-resolution printing.

In this work, a thermal molding technique is proposed for the fabrication of plastic electronics on curved surfaces, enabling the preparation of plastic films with freely designed shapes. The induced strain distribution observed in poly(ethylene naphthalate) films when planar sheets were deformed into hemispherical surfaces clearly indicated that natural thermal contraction played an important role in the formation of the curved surface. A fingertip-shaped organic thin-film transistor array molded from a real human finger was fabricated, and slight deformation induced by touching an object was detected from the drain current response. This type of device will lead to the development of robot fingers equipped with a sensitive tactile sense for precision work such as palpation or surgery.

The usual silicon-based display back planes require fairly high process temperature and thus the development of a low temperature process is needed on flexible plastic substrates. A new type of flexible organic light emitting transistor (OLET) had been proposed and investigated in the previous work. By using ultraviolet/ozone (UV/O-3) assisted thermal treatments on wet processed zinc oxide field effect transistor (ZnO-FET), through low-process temperature, ZnO-FETs were fabricated which succeeded to achieve target drain current value and mobility. In this study, physical property evaluation of ZnO was conducted in term of their crystallinity, the increase composition of ZnO formed inside the thin film and the decrease of the carbon impurities originated from aqueous solution of the ZnO itself. The X-ray diffraction (XRD) evaluation showed UV/O-3 assisted thermal treatment has no obvious effect towards crystallinity of ZnO in the range of low process temperature. Moreover, through X-ray photoelectron spectroscopy (XPS) evaluation and Fourier transform infrared (FT-IR) spectroscopy evaluation, more carbon impurities disappeared from the ZnO thin film and the increase of composition amount of ZnO, when the thin film was subjected to UV/O-3 assisted thermal treatment. Therefore, UV/O-3 assisted thermal treatment contributed in carbon impurities elimination and accelerate ZnO formation in ZnO thin film, which led to the improvement in the electrical property of ZnO-FET in the low-process temperature.

Zinc oxide (ZnO) nanowires (NWs) are grown on fluorine-doped tin oxide (FTO) glass substrates via a simple reactive evaporation method without the presence of any catalysts or additives. The ZnO NWs show high crystallinity and preferential elongation along the c-axis of the hexagonal wurtzite crystal structure. The highly crystalline NWs as electron transporting layer have been used to fabricate the CH3NH3PbI3 perovskite solar cells and their photovoltaic performance were investigated. In this report, we studied the effect of filtration of PbI2-solution on surface morphology of CH3NH3PbI3 layer. Spin-coating of the filtered PbI2-solution leads to a better crystallization and relatively homogenous coverage of the CH3NH3PbI3 film, resulting in an enhancement of the solar cell efficiency compared to the cell fabricated using non-filtrated PbI2-solution. By synthesizing the CH3NH3PbI3 film using filtrated PbI2-solution, we achieved the best power conversion efficiency of 4.8% with a current density of 7.6 mA cm(-2), the open circuit voltage of 0.79V and fill factor of 0.63. (C) 2016 The Japan Society of Applied Physics

For the better performance of organic thin-film transistors (TFTs), gate-insulator surface treatments are often applied. However, the origin of mobility increase has not been well understood because mobility-limiting factors have not been compared quantitatively. In this work, we clarify the influence of gate-insulator surface treatments in pentacene thin-film transistors on the limiting factors of mobility, i.e., size of crystal-growth domain, crystallite size, HOMO-band-edge fluctuation, and carrier transport barrier at domain boundary. We quantitatively investigated these factors for pentacene TFTs with bare, hexamethyldisilazane-treated, and polyimide-coated SiO2 layers as gate dielectrics. By applying these surface treatments, size of crystal-growth domain increases but both crystallite size and HOMO-band-edge fluctuation remain unchanged. Analyzing the experimental results, we also show that the barrier height at the boundary between crystal-growth domains is not sensitive to the treatments. The results imply that the essential increase in mobility by these surface treatments is only due to the increase in size of crystal-growth domain or the decrease in the number of energy barriers at domain boundaries in the TFT channel. (C) 2015 AIP Publishing LLC.

Step-edge vertical channel organic field-effect transistors (SVC-OFETs) with a very short channel have been fabricated by a novel selective electrospray deposition (SESD) method. We propose the SESD method for the fabrication of SVC-OFETs based on a 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) semiconductor layer formed by SESD. In the SESD method, an electric field is applied between the nozzle and selective patterned electrodes on a substrate. We demonstrated that the solution accumulates on the selected electrode pattern by controlling the voltage applied to the electrode.

Self-assembly silver nanoparticles were successfully prepared on indium tin oxide coated glass substrates by means of a vacuum vapor evaporation method followed by thermal annealing. The nanoparticles were characterized using a field-emission scanning election microscopy with an energy dispersive X-ray analyzer and a UV-vis spectrometer. It was found that the particle morphology was drastically changed depending both on the evaporated silver mass thickness and the length of time of the thermal annealing. Some absorption peaks were observed in the UV-vis spectra and the peaks showed red-shift with the average particle size increase. These facts suggest that the absorption peaks may arise from the localized surface plasmon resonance. These controllable absorptions may be suitable for photon management in dye sensitized solar cells. (C) 2014 The Japan Society of Applied Physics

We have demonstrated NAND and NOR logic circuit operations of stacked-structure complementary thin-film transistors (TFTs) using 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) and soluble ZnO as active layers. Bottom-gate-type TIPS-pentacene TFTs, as p-channel transistors, were formed on n-channel ZnO TFTs with common gate electrodes. Solution-processed silicone-resin layers were used as gate dielectric and electrical interconnection layers between lower and upper TFTs. The stacked-structure integrated circuits have several advantages such as ease of active layer formation, compact device area per stage, and the short length of the interconnection compared with the planar configuration in a conventional logic circuit. (C) 2014 The Japan Society of Applied Physics

Wet processed ZnO films for semiconductor layers bring positive impact on fabrication of field-effect transistors (FETs) in terms of their low process temperature and manufacturing cost reduction. In our work, with the aim of fabricating organic light-emitting transistors (OLETs) using transparent ZnO FETs as the driver transistors, we have fabricated a wet processed ZnO film as a semiconductor layer, and investigated the effects of ultraviolet/ozone (UV/O-3)-assisted thermal treatments on ZnO FET characteristics. The results on the carrier mobility and thermal treatment temperature of ZnO films showed that the carrier mobility is almost proportional to treatment temperature, and the UV/O-3-assisted thermal treatment lowers the thermal treatment temperature. Moreover, devices subjected to UV/O-3-assisted thermal treatments showed excellent electrical characteristics compare with devices without the treatments. These results show that the UV/O-3-assisted thermal treatment is expected to facilitate OLET fabrication on flexible plastic films. (C) 2014 The Japan Society of Applied Physics

In recent years, organic flexible devices have been investigated extensively and sub-millimeter bending stability of organic thin film devices has been achieved by placing the active layer of the transistor on a neutral strain surface. Around the neutral strain surface, an organic thin film has a high bending durability because the in-plane tensile and compressive strain cancel each other. However, this type of highly flexible device is also destroyed or undergoes irreversible degradation when subjected to hard bending, and the breaking point is very difficult to detect experimentally. Therefore, we performed a finite element analysis of a flexible device and found a possible breaking point at the boundary between the contact electrode and the organic layer. This was the result of a strain concentration at the boundary due to the difference in Young's modulus of the organic semiconductor and Au. In addition, the shear stress is concentrated around the edge of the Au electrode. These results indicate that the most likely breaking point for this type of flexible transistor is a rupture of the organic layer at the interface with the Au electrode.

Step-edge vertical-channel organic field-effect transistors (SVC-OFETs) with a very short channel have been fabricated by a novel electrospray deposition (ESD) method. ESD is the direct patterning process in which a solution is sprayed by using an electric field between the nozzle and electrodes formed on samples. The electrosprayed solution accumulates on the electrode pattern, and SVC-FETs based on 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and a ZnO layer formed by ESD showed typical p- and n-channel FET characteristics, respectively. The results demonstrate that this ESD direct wet patterning is a useful method for complementary inverters and integrated circuit applications. (C) 2014 The Japan Society of Applied Physics

We propose a novel and complementary method for fabrication of flexible electronics. This method is not based on conventional printing using inks, but is based on the application of a toner-based method such as Xerox or laser printing, followed by a lamination process. The lamination method is a solvent-free and material-saving process that simultaneously seals the devices, and the fabricated flexible devices have structural durability against bending. We have also shown that thermal lamination has an oriented growth effect, and the electrical characteristics of flexible organic field-effect transistors did not degrade under a bending radius of 1 mm. [GRAPHICS] In our thermal lamination method, organic thin film transistors are fabricated by a desktop laminator, like making plastic laminated cards. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High throughput printing processes have been developed for the fabrication of organic flexible sheet devices. However, printing processes inevitably use toxic solvents to prepare inks of organic semiconductors. Here we propose a novel and complementary method for the preparation of flexible sheet electronics. A thermal pressing method has several advantages, such as (i) being solvent-free, (ii) resulting in flexible device with structural durability against bending, (iii) making simultaneous sealing available to prevent physical or atmospheric damage, and (iv) providing seamless application to roll-to-roll processes by combination with toner technology. We have demonstrated that the characteristics of a flexible organic field-effect transistor (OFET) fabricated by the thermal press process were not degraded during or after the bending test until a bending radius of 2mm was achieved, which is sufficient for the concept of a rolled-up type flexible sheet display. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Innovative sensing systems based on THz electromagnetic waves have been attracting a great deal of attention. Although many THz detectors have been developed over the years, it is currently difficult to manufacture low-cost THz sensing/imaging devices. In the present study, we propose to use organic field-effect transistors (OFETs) and small potential fluctuation against the carriers within them (N. Ohashi, H. Tomii, R. Matsubara, M. Sakai, K. Kudo, M. Nakamura, Appl. Phys. Lett. 91 (2007) 162105). We use THz time-domain spectroscopy for OFETs in which the carrier density in the pentacene active layer is modulated by the gate bias. We found evidence that the accumulated free holes in pentacene films can be excited by THz photons to overcome the surrounding barriers in the fluctuating potential. The Drude-Lorentz model could not account for the shape of the absorption spectra, which suggests that the holes are weakly restricted by the potential fluctuation. The integrated absorption intensity was proportional to the transfer characteristics of the OFETs. The present findings represent an important step toward developing a new class of THz-wave sensors. (C) 2013 Elsevier B.V. All rights reserved.

In-situ observations of vapor-phase growth of tetrathiafulvalene (TTF)-tetracyanoquinodimethane (TCNQ) crystals under an electric field were conducted without influencing the actual crystal growth process. The shortest incubation time of TTF-TCNQ nuclei and the highest initial growth rate of the crystals are obtained on the anode side and in high electric field regions. It is demonstrated that the distribution of molecules thermally diffusing on the substrate surface is controlled by an external electric field. These results indicate the potential for selective growth of highly conductive organic wires for micro- and nanoscale wiring in organic nanodevices. © 2013 AIP Publishing LLC.

We have fabricated printed active antenna for flexible information tag which have a loop antenna combined with step-edge vertical channel organic field-effect transistor (SVC-OFET). Fabrication using printing process, characterization of SVC-OFETs, and performances of active antenna elements are discussed in detail.

Flexible and low-cost organic field-effect transistors (OFETs) are desired for a variety of organic electronics. In this paper, we describe step-edge vertical-channel OFETs (SVC-OFETs) having excellent device performance fabricated by nanoimprint lithography and a self-aligned process. SVC-OFETs can be used to fabricate a submicron channel by forming the channel region around the step edge. The carriers flow in the vertical direction in the short channel along the step-edge structure. Both n- and p-channel FETs are also realized by a solution process. (C) 2012 The Japan Society of Applied Physics

A gate-induced thermally stimulated current (TSC) on beta'-(BEDT-TTF)(TCNQ) crystalline FET were conducted to elucidate the previously observed ferroelectric-like behaviors. TSC which is symmetric for the polarization of an applied V-G(p) and has a peak at around 285 K was assigned as a pyroelectric current. By integrating the pyroelectric current, temperature dependence of the remnant polarization charge was obtained and the existence of the ferroelectric phase transition at 285 K was clearly demonstrated. We have tentatively concluded that the phase transition between dimer Mott insulator and charge ordered phase occurred at around the interface of organic crystal and substrate.

We controlled the in-plane orientation of pentacene grains by graphoepitaxy using patterned amorphous-SiO2/Si substrates on which periodic grooves with slope edges had been formed. Pentacene crystals exhibited a clear tendency to align their b-axes perpendicular to the groove edges. Organic thin-film transistors were fabricated on the patterned and flat substrates and their transistor characteristics were compared. Although the patterned substrates increased the apparent mobility by only 10-20%, the number of grain boundaries with high potential barriers to carrier transport was reduced to half of that on flat substrates. This improvement is expected to enhance the response speed of pentacene organic thin-film transistors and suppress the sensitivity of their characteristics to the operating temperature. (C) 2012 Elsevier B. V. All rights reserved.

Break junction have been proposed as one of methods for making a molecular-scale nano-gaps between the two metallic electrodes. Here we had made organic metallic wire bridges being consist of tetrathiafulvalene-tetracyanouinodimethane (TTF-TCNQ) organic charge transfer complex, and then gradually disconnected the bridged molecular wires by a Joule heating to make a molecular-size gap or single-molecular junction. The observed conductance of the initially high conductive wires decreased by a direct current (DC) Joule heating. The conductance exhibited several different behaviors with the stage of the thermal desorption of molecules, and finally disconnected. Significant increase and decrease of the conductance were observed just before the disconnection. After the DC treatment and before the disconnection of the wires, we switched to alternation current (AC) Joule heating to make a single-molecular junction which would work as a single-molecular transistor. During the AC Joule heating, many discrete steps were found in the time evolution of the conductance. The steps were approximately 1 pS or it's multiple, and the duration of the step were nearly the same (approx. 300 - 400 s) in the final stage of the AC Joule heating. We conclude that the step height corresponds to the single-molecular conductance and the duration reflects the probability of single TCNQ molecular desorption.

To realize a single or several molecule device, the following are necessary: (1) an electrical wiring method that is not destructive to the molecular aggregates and does not affect the electronic state of the molecules, (2) noncontact and controllable molecular manipulation technology, (3) oriented growth techniques especially to prepare a nanodevice employing an anisotropic molecular system. In this paper, recently developed electric-field assisted growth and its application to molecular device fabrication are presented.

To elucidate the effects of a static electric field on the crystal growth of a molecule with both a molecular dipole and a quadrupole moment, we performed physical vapor growth of monoclinic lead phthalocyanine (PbPc) crystals under quasithermal equilibrium conditions and observed distinct effects of the applied electric field on the drift motion of the molecules that diffused over the substrate surface. The density of crystals grown on the Au electrode with a high electric potential exceeds the crystal density on both an electrically grounded electrode and a glass substrate surface. In addition, this difference in crystal density increases with applied voltage. This biased distribution of crystal growth is explained by the drift motion of diffusing molecules, which is induced by the interaction of the electric field with molecular dipole and quadrupole moments. However, the long-range oriented growth that is clearly seen in the physical vapor growth with an electric field for copper phthalocyanine is not observed in the crystal growth of PbPc; only a locally oriented growth is observed in the vicinity of the Au electrode. For electric-field-sensitive PbPc, it is difficult to maintain quasithermal equilibrium conditions over a wide area because the distribution of the diffusing molecules varies as a function of the spatially nonuniform electric field, which disturbs the long-range oriented growth. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553859]

To clarify relationship between crystallographic and electronic structures in pentacene polycrystalline films grown on SiO2, in-plane crystallite size and random strain of the films were analyzed by grazing incidence X-ray diffraction (GIXD) using synchrotron radiation source. The results indicate that the diffraction peak width is not determined by random strain but by crystallite size. The crystallite size remains constant within the range of 25-50 nm even when the size of polycrystalline domain, or crystal grain, increases more than tenfold by elevating the growth temperature. The crystallite size agrees well with characteristic periods of both HOMO-band-edge fluctuations in pentacene films, which was reported in our previous paper, and surface corrugation of the substrate. These facts strongly suggest that roughness of the SiO2 surface limits the crystallite size and the interruption of long-range order in pentacene lattice introduces the HOMO-band-edge fluctuation. (C) 2010 Elsevier B.V. All rights reserved.

Conductivity anisotropy in a crystal grain of thin-film-phase pentacene has been estimated by a combination of atomic-force-microscope potentiometry (AFMP) and electrostatic simulation. The surface potential distribution and topography of a grain in a working pentacene thin-film transistor are simultaneously measured by AFMP. Then, the nonlinear potential profile due to the thickness variation is simulated by changing the anisotropic ratio of conductivity. The anisotropic ratio (corresponding to the anisotropy of carrier drift mobility) is estimated to be sigma(x):sigma(z)= 45:1, where x is the horizontal (harmonic mean of those in a- and b-axes) direction and z is the vertical (c-axis) direction. (C) 2010 American Institute of Physics. [doi:10.1063/1.3430041]

A ferroelectriclike dielectric response was observed in a field-effect transistor using oriented bis(ethylenedithio)tetrathiafulvalene-tetracyanoquinodimethane crystals. Phase transitions at 285 and 320 K were clearly observed in the temperature dependence of field-effect electron mobility. The phase transition at 320 K corresponds to the metal-insulator transition previously reported in a bulk crystal. On the other hand, the field-effect electron and hole mobilities exhibited an abrupt increase at 285 K, which had not been discovered by other physical measurements in the bulk crystal and is nonetheless sufficiently stable and reproducible. In addition, the abrupt increase in carrier mobilities was clearly correlated with the decrease in the dielectric response. The temperature variation in difference hysteresis curves demonstrated the feature of ferroelectric transition.

A spontaneous activation method for self-aligned organic nanochannel transistors with organic conductive wires was developed. With the continuous application of an alternating electrical current through connected organic wires that did not exhibit field-effect transistor characteristics, n-channel field-effect transistor characteristics appeared with a decrease in the electrical conductance of the wires. In principle, this process spontaneously ceased without breaking the organic wires. The yield rate of self-aligned nanotransistors should be effectively improved by using this technique. (C) 2008 The Japan Society of Applied Physics.

Crystalline domain size and temperature dependences of the carrier mobility of commonly used pentacene polycrystalline films on SiO(2) have been studied by four-point-probe field-effect transistor measurements. The mobility is found to be proportional to the crystalline domain size and thermally activated. This behavior is well explained by a polycrystalline model with the diffusion theory, and thereby the barrier height at boundary and the mobility in domain are calculated to be 150 meV and 1.0 cm(2)/V s, respectively. The in-domain mobility is lower than those expected in single crystals, which suggests that there exist some other limiting factors of carrier transport than the domain boundaries. (C) 2008 American Institute of Physics.

Surface topography and high-resolution potential images in a thin-film transistor with a polycrystalline pentacene active layer have been measured by atomic-force-microscope potentiometry. A potential fluctuation independent of topographic features was found in large flat molecular terraces. The origin of the potential fluctuation was concluded to be the fluctuation of the top level of the highest-occupied-molecular-orbital band, which results in the variation of local carrier concentration. The full width at half maximum of the band fluctuation was estimated to be 12 meV, which might reduce the mean carrier velocity in crystalline domains. (C) 2007 American Institute of Physics.

Ambipolar carrier conduction has been observed in a metal-insulator-semiconductor field-effect transistor made using (BEDT-TTF)(TCNQ) crystals. The temperature dependence of the source current with the applied positive gate voltage exhibits metal-like behavior at around room temperature. The metal-like conduction transforms into thermal-activation-type behavior below 240 K. The I-S-V-DS curve for an applied gate voltage of 80 V exhibited a corresponding change in the curvature below 240 K.