Improved positive polaron stability of the p-type host for long lifetime in phosphorescent organic light-emitting diodes

A long lifetime is one of requirements for the commercialization of organic light-emitting diodes (OLEDs). In this paper, we improved the device lifetime of phosphorescent OLEDs by enhancing the oxidation stability of the hole-transport-type host because the host material with steadiness against oxidation has a good positive polaron stability for improved device lifetime. Two p-type host materials, 9-(dibenzo[b,d]thiophen-2-yl)-3′,6,6′-triphenyl-9H-3,9′-bicarbazole (2-TPhDCz) and 9-(dibenzo[b,d]thiophen-4-yl)-3′,6,6′-triphenyl-9H-3,9′-bicarbazole (4-TPhDCz), were synthesized for improved positive polaron stability to obtain a long device lifetime in phosphorescent OLEDs by substituting the active site of a carbazole unit with a phenyl unit. The oxidation stability and positive polaron stability of the host were enhanced by the phenyl protection of the active site, which extended the device lifetime of the phosphorescent OLEDs compared with conventional hole-transport-type hosts. In particular, the 2-TPhDCz host doubled the device lifetime of the phosphorescent OLEDs.