Noriyuki Takada

Influence of moisture on device characteristics of polythiophene-based field-effect transistors

We investigated a field-effect transistor (FET) based on a poly(3-n-hexylthiophene) (P3HT) to determine the influence of moisture on device characteristics and thus gain a deep understanding of the mechanism underlying the susceptibility to air of the operation of FETs of this kind. The fundamental output characteristics, which include effective field-effect modulation and saturation behavior in the output current, remained almost the same for every current–voltage profile in a vacuum, N2 and O2. By contrast, operation in N2 humidified with water resulted in enlarged off-state conduction and deterioration in the saturation behavior, in the same manner as that experienced with exposure to room air. We concluded that atmospheric water had a greater effect on the susceptibility of the device operation to air than O2, whose p-type doping activity as regards P3HT caused only a small increase in the conductivity of the active layer and a slight decrease in the field-effect mobility with exposure at ambient pres...

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.

Polariton emission from polysilane-based organic microcavities

We report the observation of strong coupling between exciton and photon modes in a conjugated polymer-based semiconductor microcavity. Thin films of the σ-conjugated poly[bis(p-butylphenyl)silane] (PBPS) were inserted between metal and dielectric mirrors to form the microcavity structures. Variation of the PBPS film thickness between 80 and 140 nm allowed the cavity photon resonance to be tuned in the vicinity of the free exciton energy. The expected anticrossing behavior, with intensity and linewidth averaging, was observed at room temperature in the cavity reflection spectra and the vacuum Rabi splitting was found to be ⩽430u2009meV. This large value is consistent with the expectations of transfer matrix reflectivity calculations performed with optical constants data derived from a Kramers–Kronig analysis of the PBPS absorption spectrum. Angle-dependent photoluminescence measurements were performed for the microcavity with a 120 nm thickness PBPS layer. Unlike the emission from a standard, weakly coupled, c...

Light up-conversion from near-infrared to blue using a photoresponsive organic light-emitting device

A photoresponsive organic light-emitting device combining blue-emitting organic electroluminescent (EL) diode with titanyl phthalocyanine as a near-infrared (IR) sensitive layer was fabricated. By irradiating near-IR light to the device, blue emission occurred in the lower drive voltage (between 5 and 12 V). The result indicates that the device acts as a light switch and/or an up-converter from near-IR light (1.6 eV) to blue (2.6 eV). The EL response times of rise and decay using a near-IR light trigger were 260 and 330 μs, respectively. At a higher voltage (above 12 V), enhancement of blue emission was observed with near-IR light irradiation. The ON/OFF ratio reached a maximum of 103.

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.

Mechanoluminescent properties of europium complexes

Mechanoluminescence (ML) from polymer-dispersed films of europium complexes was investigated for the first time. We synthesized four new kinds of europium complexes and examined their mechanoluminescent properties in different sample forms: powder and polymer-dispersed film. While only two complexes exhibited ML in powder form, all the four became mechanoluminescent when dispersed in polymer matrix. The excitation mechanism of ML of the complexes is studied by the measurement of the spectral and transient behavior.

Probing buried organic layers in organic light-emitting diodes under operation by electric-field-induced doubly resonant sum-frequency generation spectroscopy

Electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) spectroscopy was used to study the electric field distribution in multilayer organic light-emitting diodes (OLEDs). Remarkable correlations between the DR-SFG signal enhancement and the applied bias voltage were observed. The SFG signals attributed to 4,4′-bis[N-(1-naphthyl-N-phenylamino)-biphenyl] were significantly enhanced by applying a forward voltage, whereas those from Alq3 were increased by applying a reverse voltage. The large enhancement in EFI-DR-SFG intensity enables us to nondestructively probe the local electric field distribution at the buried organic layer within the OLED.

Near-infrared electroluminescent devices using single-wall carbon nanotubes thin flms

We have fabricated near-infrared electroluminescent (EL) devices utilizing single-wall carbon nanotubes (SWNTs) finely dispersed in a polymer, such as poly[2-methoxy-5-(2′-ethylhexyloxy]-1,4-phenylenevinylene (MEHPPV). Al/SWNT-MEHPPV/indium tin oxide thin-film devices exhibit a very promising EL response over a broad spectrum, including the range of 900–1600nm. From the analysis of the optical absorption, photoluminescence and EL spectra, as well as the current-voltage characteristics, we demonstrate that those devices exploit the intrinsic near-infrared light-emitting properties of semiconducting SWNTs and the electronic transport properties of SWNT-doped MEHPPV. Those achievements are essential for the future development of thin-film SWNT optoelectronic devices.

Progress in Emission Efficiency of Organic Light-Emitting Diodes: Basic Understanding and Its Technical Application

The technical history of when and how the basic understanding of the emission efficiency of organic light-emitting diodes (OLEDs) was established over the last 50 years is described. At first, our understanding of emission efficiency in single-crystal and thin-film electroluminescence (EL) devices in the early stages before the Eastman-Kodak breakthrough, that is, the introduction of the concept of multilayer structures, is examined. Then our contemplation travels from the Eastman-Kodak breakthrough towards the presently widely accepted concept of emission efficiency. The essential issues concerning the emission efficiency of OLEDs are summarized to help readers to obtain a common understanding of OLED efficiency problems, and detailed discussions on the primary factors that determine emission efficiency are given. Finally, some comments on remaining issues are presented.

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.