AbstractInfluence of emission sites on emission efficiency and running durability of molecular organic electroluminescent devices was investigated. When fluorescent dye rubrene was doped into TPD diamine hole transport layer, rubrene molecule emitted bright yellow light with a max. luminance of 61 000 cd/m2. The device had the highest efficiency of 9.8 Im/W at 100 cd/m2. Half‐decay time of the device from initial luminance of 530 cd/m2under constant direct current was 3 600 h. On the other hand, when rubrene was doped into BeBq2complex electron transport layer, the efficiency was 4.4 Im/W and the half‐decay time was 110 h. Thus doping site is found to exert significant influence both on the emission efficiency and the running durabil