Temperature dependence of operational stability of organic light emitting diodes based on mixed emitter layers

Temperature dependence of electroluminescence degradation is studied in organic light emitting diodes (OLEDs) containing an emitting layer composed of a mixture of tris(8-hydroxyquinoline)aluminum (AlQ3) electron transport and emitter material and four different hole transport materials (HTMs) (N,N′–di(naphthalene-1-yl)-N,N′-diphenyl-benzidine) (NPB), N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD), 1,1′-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) or 4,4′,4″-tris[3-methylphenyl(phenyl)amino]triphenylamine (MTDATA). The emitting layer is sandwiched between hole and electron transport layers. Activation energy of device degradation was determined from operational stability measurements for devices with different HTMs operated at various temperatures. It was found that the device lifetime correlates with the difference in HOMO energy level between AlQ3 and the HTM, where greater energy differences lead to longer device lifetimes.