To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO(2-x)N(x) have revealed an improvement over titanium dioxide (TiO2) under visible light (wavelength < 500 nanometers) in optical absorption and photocatalytic activity such as photodegradations of methylene blue and gaseous acetaldehyde and hydrophilicity of the film surface. Nitrogen doped into substitutional sites of TiO2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.

One of the keys to highly efficient phosphorescent emission in organic light-emitting devices is to confine triplet excitons generated within the emitting layer. We employ “starburst” perfluorinated phenylenes (C60F42) as a both hole- and exciton-block layer, and a hole-transport material 4,4′,4″-tri(N-carbazolyl) triphenylamine as a host for the phosphorescent dopant dye in the emitting layer. A maximum external quantum efficiency reaches to 19.2%, and keeps over 15% even at high current densities of 10–20 mA/cm2, providing several times the brightness of fluorescent tubes for lighting. The onset voltage of the electroluminescence is as low as 2.4 V and the peak power efficiency is 70–72 lm/W, promising for low-power display devices.

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TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Raman spectra of n-type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high-frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line-shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation-potential and electro-optic mechanisms.

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.

Thermal stability of the electroluminescent (EL) devices using various hole-transporting materials based on triphenylamine, and a typical emitting material, tris(8-quinolinolato) aluminum has been systematically studied. The thermal stability of the EL devices is clearly seen to depend on the glass transition temperature (Tg) of the hole-transporting material. The highest thermal stability up to 155 °C is obtained in the device using the pentamer of triphenylamine. It has been found that the linear linkage of triphenylamine is useful to attain high Tg rather than the branch linkage.

A dependency between the molecular weight distribution of polyfluorene (PFO) and its aggregation tendency on operation in organic light-emitting devices (OLED) is described. As a result of these findings, low molecular parts of the polymer could be made responsible for the aggregation in liquid crystalline PFO. Consequently, a procedure was developed which leads to PFO-based OLEDs which do not show aggregation on operation anymore but exhibit highly efficient and stable blue electroluminescence.