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Electronic devices
Organic materials were believed to be insulators. However, the first organic semiconductor was discovered in 1950s. This discovery opened a new research field of molecular conductors and superconductors. After 20 years of the discovery Hideki Shirakawa succeeded to synthesize polyacetylene films. Later he found that conductivity of polyacetylene extremely increases by doping in collaboration with Prof. Heeger. Following the finding, many researchers stated studies of conjugated polymers including polythiophene, poly-p-phenylene, polydiacetylene, and others. Based on basic researches of these substances, practical organic electronic devices such as organic light-emitting devices(OLED) and organic field effect transistors (OFET) have been extensively studied and some commercial devices have appeared.
Electronic properties of organic materials are governed by pi-electrons exist on double and triple bonds. Most of organic materials having multiple bonds are linear or planar in shape. Accordingly, overlaps of pi-electrons among molecules in organic molecular crystals are anisotropic and conductivities too. Aligning molecular orientation so as to use the most conductive direction is very important to realize high performance organic electronic devices. We have developed alignment techniques of organic thin films and study opt-electronic properties of organic materials. Research issues are following;
- Control of molecular alignment and opt-electronic properties of organic thin films by molecular beam epitaxy(MBE)
MBE technique enables us to fabricate organic thin films with definite structures controlled in a layer level. We have fabricated transparent transistors using epitaxial phthalocyanine films grown on single-crystalline transparent electrodes. We have also studied the growth of phthalocyanines on alkali-halide substrates and heteroepitaxial structure of phthalocyanines to fabricate organic superlattice devices.
- Fabrication of in-plane oriented organic thin films and thin film transistors
We have fabricated in-plane oriented vacuum-deposited organic thin films by rubbing technique commonly used in the liquid crystal industry. Orientation of films determined by X-ray diffraction and polarized optical spectra were quite high. Using these films, we have fabricated OFETs and evaluate the anisotropy of mobility parallel and perpendicular to the molecular stack axis.
- Electronic conductivity of liquid crystals
In liquid crystal phases, molecules orient themselves in one direction. This orientation makes charge carrier mobility of liquid crystals higher than non-oriented amorphous organic films. Although the mobility is not so high as organic single crystals, liquid crystalline materials have advantage in fabrication of large-size defect-free films. We study electronic conduction in liquid crystal phases. Experimental results are analysed in comparison with computer simulation.