Kouji Suemori

Threshold voltage stability of organic field-effect transistors for various chemical species in the insulator surface

The relationship between the threshold voltage (Vt) stability and the chemical species of the insulator surface was investigated by using organic field-effect transistors with different types of self-assembled monolayers on a SiO2 insulator. The Vt shift induced by gate bias stressing was considerably increased by the introduction of long-chain chemical species to the SiO2 surface. In order to obtain high-performance and high-stability organic transistors, insulator surfaces with short-chain chemical species that can improve transistor performance without degrading stability are required.

Influence of fine roughness of insulator surface on threshold voltage stability of organic field-effect transistors

We have investigated the influence of the surface roughness of an insulator on the threshold voltage shift caused by gate bias stressing in organic field-effect transistors (OFETs). Our investigation was conducted for OFETs with SiO2 insulators. We observed that the threshold voltage shift is extremely sensitive to changes in the fine roughness of the SiO2 surface; the shift increased with the roughness. The large shift in OFETs with rough SiO2 insulators can be attributed to lattice distortion in pentacene layers deposited on rough SiO2 surfaces.

Carbon nanotube bundles/polystyrene composites as high-performance flexible thermoelectric materials

Lightweight and flexible thermoelectric devices consisting of carbon nanotube (CNT)-based materials have the potential to be used for the various applications, such as energy harvesting from the low-temperature waste heat that exists ubiquitously in living areas. Because high-performance CNT-based materials are crucial for the broad-ranging employment of CNT-based thermoelectric devices, considerable efforts are being made to improve the power-generation capability of CNT-based thermoelectric materials. Here, we report high-performance thermoelectric composites consisting of CNT bundles and polystyrene fabricated by a planetary ball milling-based dispersion technique, which allows for the direct dispersion of the CNT bundles within the polystyrene matrix without causing the disaggregation of the bundled CNTs into individual ones. The CNT-bundles/polystyrene composites reported here exhibit a power factor of 413 μW/K2·m.

Temporal Changes in Source--Drain Current for Organic Field-Effect Transistors Caused by Dipole on Insulator Surface

The influence of the dipole of an insulator surface on temporal changes in the source–drain current was investigated by using organic field-effect transistors with a surface-modified SiO2 insulator. The source–drain current decreased drastically with respect to time when the dipoles of the insulator surface displaced slightly. In order to obtain highly stable organic transistors, it is thus necessary to remove the mobile dipoles from the insulator surface.

Highly Sensitive Organic Photo-FET Using Photosensitive Polymer Insulator

We fabricated a photo field effect transistor (photo FET) having a poly (N-vinyl carbazole) (PVK) insulator layer. PVK was employed as a photosensitive insulator material for the Photo FET. In this photo FET, illumination of a blue light to the PVK insulator drastically improved the field effect mobility and the gate-switching ratio. This result would be due to that the photo-generated carriers in the PVK layer were effectively accumulated in the gate capacitor of the photo FET.

Effect of positively charged particles on sputtering damage of organic electro-luminescent diodes with Mg:Ag alloy electrodes fabricated by facing target sputtering

We investigated the influence of the positively charged particles generated during sputtering on the performances of organic light-emitting diodes (OLEDs) with Mg:Ag alloy electrodes fabricated by sputtering. The number of positively charged particles increased by several orders of magnitude when the target current was increased from 0.1 A to 2.5 A. When a high target current was used, many positively charged particles with energies higher than the bond energy of single C–C bonds, which are typically found in organic molecules, were generated. In this situation, we observed serious OLED performance degradation. On the other hand, when a low target current was used, OLED performance degradation was not observed when the number of positively charged particles colliding with the organic underlayer increased. We concluded that sputtering damage caused by positively charged particles can be avoided by using a low target current.

Printed Electrode for All-Printed Polymer Diode

Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5,1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, JapanReceived September 16, 2010; revised November 4, 2010; accepted November 8, 2010; published online April 20, 2011We developed a novel pressure-annealing method for fabricating printed low-work-function metal patterns and printed metal alloy patterns. Thepressure-annealed metal electrodes were used as bottom electrodes of printed polymer diodes. As a result, all-printed polymer diodes havingvarious rectification properties were successfully produced. # 2011 The Japan Society of Applied Physics

Mechanical Sintering Techniques for Printed Electrodes with Various Work-function on a Plastic Substrate

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.