We have developed the pressure annealing technique for fabricating low work function metal pattern on plastic substrate. In general, the difficulty to print conductive low work function metal patterns is caused by the insulating metal oxide layer covering on metal particles included in metal paste. The pressure annealing technique can destruct the metal oxide layer and can form conductive layer on printed metal pattern. Further, we have confirmed that a binary solid solution is easily formed on metal patterns including two kinds of metal particles by using the pressure annealing technique. Changing the composition ratio of the binary metal paste led to the work function control of the pressure-annealed metal patterns. Formation of the binary solid solution was confirmed by using XRD spectra, and work function values were measured by using photoelectron emission spectra. In the case of the binary metal paste of Cu and Zn, we have succeeded in controlling work function from 3.8 eV to 5.0 eV. Since the Cu-Zn paste is composed of relatively low price metals, this would be applicable to large-scale flexible electronic devices.

We have already reported that low-temperature (about 170 °C) preparation technique of silicon dioxide (SiO2) dielectric thin film that has high resistivity and extremely smooth surface by the photo oxidation process. In this paper, we have developed a low-damage preparation technique to fabricate a SiO2 thin film by the photo-assisted low temperature oxidation process in order to apply this process to the flexible electronics using for convenient plastic films. We have reported that the SiO2 dielectric thin film with a high insulation performance can be prepared by the low temperature processing below 100°C by improving the pre-processing of the photo oxidation of thin film.

It is mostly important to develop the fabrication technology of the dielectric thin film with high insulation performance and surface flatness by large-area printing process. We have developed a technique to fabricate a silicon dioxide (SiO2) dielectric thin film by the low temperature solution process. The thin film prepared by about 170°C showed excellent dielectric performance with high resistivity in the order of 10 to the 16 Ωcm and surface flatness with the same degree of thermal oxidized SiO2 thin film on the silicon wafer (RMS=0.15nm). In addition, it is showed that the production of thick film of SiO2 with high dielectric performance and surface flatness is possible by applying over-coating technique. These indicate that this SiO2 production technique is greatly useful for the large-area printed electronics technology.

A print technique of an electrode on a plastic substrate is one of the most important techniques for developing a printed large area device. Especially, preparation of a metal electrode with low work function by printing is very important to develop printed active devices such as diode and transistors. Work function values of electrodes strongly affect device properties (1,2), because the work function difference between a semiconductor and an electrode is related to charge injection from the electrode to the semiconductor. Recently, many solution-processable n-type semiconductors have been developed (3), because these are required to fabricate printable EL devices, printable CMOS devices, and so on. Low-work function metals are necessary to inject electrons to n-type semiconductor. In spite of this fact, very few printable low-work function metal inks have been developed, because low-work function metals are easily oxidized during print process owing to high temperature annealing treatment and their conductivities are lost.

In this study, we have examined to develop a new annealing technique on a printed electrode to reduce the process temperature during metal printing. We have newly developed a mechanical sintering technique in which mechanical forces is applied on a printed metal pattern. Control of the direction balance of applied mechanical force was effective to reduce resistivity of the printed metal without any destruction of plastic substrate. Furthermore, distribution control of metal particle in the metal ink was also effective to reduce resistivity. By using this technique, we have succeeded in the preparation of an aluminum, zinc, copper and tin electrode on a plastic substrate. On the other hand, we have tried to prepare a metal alloy ink to control the work function of printed electrode. Metal alloy ink was composed of two kinds of metal particles. Work function of the electrode was controlled by changing composition of these metal contents in an alloy ink. By applying our developed mechanical sintering technique on the printed alloy pattern, printed electrode with various work functions from 3.5eV to 5eV could be prepared on a plastic substrate. These printed alloys were effective to improve the performance of printed diode and transistors.

Various flexible electronic devices have been intensively studied and developed in this decade. Most of consumers expect to obtain these flexible electronic devices in the next decade. The spread of flexible electronic devices depends on the process costs and material costs. Today prices of materials and energy sources are continuously increasing. Therefore, using of expensive materials and wasteful processes for fabricating electronic devices should be reconsidered. Printing techniques, a representative solution process, are inexpensive and very effective for mass production of electronic devices. We have developed a low-temperature process for fabricating flexible printed patterns of metals and semiconductors by using relatively inexpensive commercial pastes. A characteristic of our process is to utilize mechanical energies for sintering particles contained in pastes. In our process, the precise three-dimensional pressure control brings about the improvement of electrical properties in the printed patterns, the preservation of pattern accuracy and the mechanical durability of the printed patterns. In our experiment, we have already fabricated very low resistivity metal patterns (about 6×10−6Ω·cm) at ca.120°C without using any kinds of nano-particle paste, and also fabricated metal oxide semiconductor patterns.

Printable devices have been attracting considerable interest because of their application to flexible large-area devices in low cost printable electronics. In order to fabricate such devices, it is necessary to discover a passivation film and develop an efficient process for its preparation. We have previously reported that silicone oxide film is obtained with high density by UV irradiation to silazane compound film at low temperature [1]. The silicon oxide film has electric resistivity more than 10−15 Ωcm and electric strength more than 7 MV/cm. In order to apply the film to passivation layer in printable device, in this paper, preparation of nano-composite film with silicon oxide and clay mineral was investigated.

The photo FET having a photosensitive insulator layer showed excellent photo-switching properties upon illuminating the gate dielectric layer, because the gate capacitor was rapidly charged up by providing a large amount of photo-generated charges in the dielectric layer to the gate capacitor. In this study, we have tried to give a memory function to the photo FET. Further, we have tried to fabricate flexible photo FETs by using only solution processes in order to utilize the photo FET as flexible photo sensor and photo memory devices. As a result, we have succeeded in observing the memory effect of the photo FET and the photo-switching behavior of the flexible photo FETs

A polymer field-effect transistor (FET) was fabricated with poly(3-hexylthophene) as an active layer and poly(methyl-methacrylate) gate dielectric modified with poly(vinyl alcohol). The influences of air atmosphere on the FET property were investigated. Under the air atmosphere, FET properties such as modulation of drain current by gate bias, leakage current and threshold voltage shift were strongly influenced by humidity. An influence was investigated by temperature dependence of FET properties. It cleared the relationship between the FET property and intrinsic mobile ion in the gate dielectric. The influence of humidity on the FET property can be prevented by insertion of a clay mineral layer, which clogs motion of ion transport in gate dielectric layer.

The relationship between impurity species included in regioregular poly(3-hexylthiophene) (P3HT) and the field effect transistors (FETs) property was investigated. P3HT synthesized by the Rieke method contained only Zn, Ni and Br (free halogen) as impurities. Several kinds of P3HT with different purities by using purification techniques were prepared, and those P3HT-FETs properties were estimated. As a result, it is revealed that the free halogen is effective dopant to increase the FET mobility, and the removal of the catalyst metal (Zn and Ni) is also effective to decrease off-current.

The spectral imaging for electroluminescence (EL) characterization in the light emitting diode (LED) based on blends of poly[2,7-(9,9-di-n-octylfluorene)] (PFO) and poly[2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)amino)-1,4-phenylene) ] (TFB) was performed using the two dimensional imaging micro-spectroscopy system. We found that EL spectral images varied with increasing applied voltages. The origin for such variation of EL spectral images will be discussed in this report.

A novel process for fabricating alignment-free, printable, organic thin-film transistors is presented. This process exploits a self-assembly phenomenon in which soluble nanomaterials such as metal nanoparticles and organic molecules are self-assembled into a device structure. To demonstrate this process, solution-processed source and drain electrodes were self-aligned to a gate electrode by using a hydrophobic self-assembled monolayer (SAM) optically patterned onto the gate electrode with a back-substrate exposure technique. An organic semiconductor film deposited on the patterned SAM was selectively ordered and substantially self-aligned to the gate electrode. A field-effect mobility of 0.15 cm2/Vs and on/off current ratio of 105 were experimentally demonstrated when pentacene molecules were used as the semiconductor and silver nanoparticles were used as electrode materials.

We studied the features of the newly developed Top and Bottom Contact Field Effect Transistor (TBCFET) with organic semiconductor layers. TBCFETs with ca.0.5μm channel length (L) were fabricated and their transistor properties were measured. The output drain-source currents (IDS) of TBCFET were 1 to 2 orders of magnitude higher than those of ordinary planar type FETs with 100μm channel length. On the other hand, because of the TBC structure, off current tends to become larger in the TBCFET. Therefore, in order to solve this off-current problem, we intentionally formed the Schottky junction at the top electrode/semiconductor interface. As a result, the off current became about 2 orders of magnitude smaller than before the formation of the Schottky junction.

We have developed a current flow analysis system for Field Effect Transistors (FET) using a conductive cantilever of a probe microscope. Using this analysis system, nanoscale current images were observed on a pentacene active layer grown on a bottom electrode in ordinary FETs. When gold (Au) was employed as a bottom electrode, we found the special region around the electrode edge. The current flow hardly occurred in this region. On the other hand, using a conductive polymer (PEDOT) bottom electrode, such a region was not observed. From the detailed observation around the electrode edge, we concluded that the special region would be caused by diffusion of Au particles that evoked heterogeneous pentacene growth, and then the heterogeneity worked as a contact resistance.