Study of organic light emitting devices with a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer

We investigated the stability of an organic light emitting device (OLED) with structure of indium–tin–oxide (ITO) anode/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole transport layer (HTL)/tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer/Mg:Ag cathode, in which different portions of the HTL were doped with 5,6,11,12-tetraphenylnaphthacene (rubrene). Compared to undoped devices, the stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas the stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices in which the whole HTL was doped. The color of the emission depended only on the material in immediate contact with AlQ3. The results demonstrate that increasing the OLED’s stability by means of doping the HTL is associated with changes in bulk HTL properties rather than interfacial properties, and is consistent with the OLED degradation mechanism based on the instability of AlQ3 cationic species.