El Behavior of Stylyl Compounds with Benzoxazole and Benzothiazole for Organic Light-Emitting-Diode

ABSTRACTWe studied the EL behaviors of benzoheterocyclic derivatives as emitting and hole transport layers for organic light-emitting-diodes (OLEDs). First, we studied the benzoheterocyclic derivatives having stylyl group, triphenylamine group and benzoxazole or benzothiazole group as an emission layer. These devices emitted a blue-green light. The current densities of the OLED having these benzoheterocyclic derivatives as an emission layer were higher than that of the Alq3 OLED at same applied voltage. However, these devices did not have a high EL efficiency (maximum 0.1 lm/W). From these results, in these benzoheterocyclic derivatives having a triphenylamine group, we thought that holes could flow out from hole transport to cathode. We newly synthesized dimer-type benzoheterocyclic derivatives without a triphenylamine group, which have benzoheterocycle at both sides. The current densities of the OLEDs having dimer-type benzoheterocyclic derivatives was more strongly suppressed than that of the OLEDs having benzoheterocyclic derivatives with a triphenylamine group at same applied voltage, but the EL efficiency could not be improved by dimerization and eliminating of a triphenyldiamine group obtained. Next, we studied the TPD derivatives having benzoxazole, benzothiazole and stylyl groups as hole transport layer. In new TPD derivatives, the EL efficiency the OLEDs having the TPD derivatives with stylyl groups was the best efficiency of all. The EL efficiency of ITO/a TPD derivative with stylyl groups/Alq3/AlLi is 1. 1 lm/W (max. luminance 12000cd/m2).