Tomoyuki Ogata

Characteristics of organic electroluminescent devices with new dopants

Abstract We conducted an investigation of electroluminescent (EL) characteristics of the organic EL devices with new dopants: 5,12-diphenyl-tetracene (DPT) and benzothioxanthene (BTX) derivatives. The organic EL cell doped with DPT showed a green emission with high color purity and also good operational stability. Concentration quenching was found out to be more severe for DPT, compared with rubrene. BTX derivatives exhibited an orange emission as a dopant both in the electron-transporting layer and in the hole-transporting layer. We demonstrated a white emission by the combination of an orange-emitting hole transport layer and a blue-emitting layer.

Organic electroluminescent devices with polymer buffer layer

We have developed a new type of polymers having TPD unit combined with non-conjugated spacer group, poly(arylene ether sulfone)-containing and poly(arylene ether ketone)- containing tetraphenylbenzidine (PTPDES an PTPDEK) and also polymers with directly coupled triphenyl amine units (PPBA). When these polymers are mixed with strong acceptor such as TBPAH or DDQ, they indicated higher conductivity and facilitated hole injection from ITO to the hole transport layer. Spin-coating of such polymer from an organic solution on ITO was found to improve the surface roughness of ITO, resulting in reduced defects that cause electric short circuit between ITO and cathode. These buffer materials lowered the operation voltage and improved the thermal stability of the device. After storage of 1,000 hours at 85 degrees Celsius, the device with polymer buffer showed no degradation in luminance and small increase of operation voltage. In comparison with CuPc buffer, it is clear that the doped polymer is superior in terms of both efficiency and thermal stability.

Interface and material considerations of OLEDs

Three interfaces, anode interface, hole blocking layer and cathode interface were considered mainly from the viewpoint of materials. Vinyl polymers containing triphenylamine as a side group were investigated as an ITO buffer layer. When these polymers were doped with strong acceptor, they lowered operation voltage of OLED and also improved the thermal stability. Employment of high Tg hole transport material was also found effective for the thermally stable EL characteristics. Hole blocking material with a wider optical gap improved color purity of blue-emitting device. Various inorganic compounds were examined as a cathode interface layer to demonstrate that MgF2 was effective to improve operation lifetime of OLED.