Organic Light-Emitting Diode (OLED) Materials

　Organic light-emitting diode (OLED) devices have received much attention, because they are expected to be a next generation display and light source, thanks to lightweight and flexible organic materials. The OLED was focused on practical use, after Tang et al. first observed the OLED device by use of a two layered organic thin film.¹⁾ Adachi et al. further reported a three layered device, in which a host layer is sandwiched by hole transport and electron transport layers. In addition, they reported a two layered system, in which one layer has roles of host and electron transport properties.^2,3) A five layered system including electron injection and hole injection layers has been also studied in order to improve the efficiency of carrier injection. One can control RGB colors of emission by selection of a dopant into a host layer. A suitable combination of the dopant can give a white colored device.^4,5) An application using the white organic light-emitting device (WOLED) is an OLED light panel.⁶⁾ An amorphous material is useful for an OLED device, because it is transparent, homogeneous, isotropic and easily processible. A practical OLED device further requires excellent heat-resistance and durability. Many hole transport materials based on triphenylamine derivatives (TPD) are widely usable, because they are heat-resistant and amorphous.^7,8) In addition to the TPDs, oxadiazole derivatives (PBD) having an electron transport property,⁹⁾ Alq₃ as a host material,¹⁾ and blue emissive distylyl derivatives¹⁰⁾ are fundamental materials for amorphous OLED devices.