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The influence of the emission site on the running durability of organic electroluminescent devices was examined. The fundamental device structure of MgIn/BeBq 2 /TPD/MTDATA/ITO ( BeBq 2 =bis(10-hydroxybenzohquinolinato)beryllium, TPD=N,N ′ -diphenyl-N,N ′ -(3-methylphenyl)-1,1 ′ -biphenyl-4,4 ′ -diamine, MTDATA=4,4 ′ ,4 ′′ -tris(3-methylphenylphenylamino)triphenylamine) was employed. BeBq 2 has the electron transport property, and TPD and MTDATA have the hole transport property. The emitting material rubrene was doped in the TPD layer of device A and in the BeBq 2 layer of device B. Both devices A and B showed high luminance of more than 10000 cd/m 2 . However, the running lifetimes of devices A and B, in which the initial luminance of 500 cd/m 2 was reduced to one-half under a constant driving current, were 3554 h and 110 h, respectively. It was thus found that the emission site exerts an influence on the running durability.
Hamada et al. (1995) studied this question.