Improvement in the stability of phosphorescent OLED with solution-coated hole-transport layer via exciplex–triplet energy transfer

Abstract

An approach to enhance the lifetime of a phosphorescent organic light-emitting diode (PHOLED) with a solutionprocessed hole-transport layer (HTL) by employing energy transfer from an exciplex to a phosphorescent emitter is presented. Using the structure of solution-coated Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-secbutylphenyl) diphenylamine)] (TFB) as a HTL and vacuum-deposited 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) as a host provides a suitable situation for the formation of exciplex between CBP and TFB because energy gap between the LUMO level of CBP and the HOMO level of TFB becomes much smaller than the HOMO-LUMO gap of CBP or TFB. As the PHOLED emission is solely from the phosphorescence, the intermediate exciplex state rapidly transfers its energy to the dopant triplet. Since singlet-excited state of exciplex requires much lower energy than singlet-excited state of the host, driving voltage for PHOLEDs using exciplex–triplet energy transfer (ExTET) is lower than conventional, leading to longer device lifetime. The results show that the electroluminescence half-life (LT50) of fabricated device with the structure of HTL TFB and host CBP in which exciplex can form between CBP and TFB is about 5785 h (for an initial luminance of 1000 cd m−2). On the other hand, in the other fabricated devices with the same structure and just mixing TFB with other materials with deeper HOMO level, where the ability to form exciplex between TFB and CBP is suppressed, the device lifetime is significantly shorter. To the best of our knowledge, it is the first time that ExTET is obtained in a hybrid structure involving solution-coated and vacuum-deposited layers.