Phosphorescent organic light-emitting diodes with simplified device architecture

Abstract

We demonstrated that single-layered red phosphorescent organic light-emitting diodes (OLEDs) can have high a efficiency without carrier transport and injection layers. This high efficiency is caused by the direct injection of carriers from electrodes into a dopant, bis(2-phenylquinoline) iridium(III) (acetylacetonate) [Ir(ppy)2(acac)]. This mechanism is proved by analyzing the single-layered devices with various hosts, 4,4'-N,N'- dicarbazole-biphenyl (CBP), 9-phenyl-3-[4-(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl]-9H-carbazole (LPGH 114), 9-(naphthalen-2-yl)-3-[4-(1- phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-9H-carbazole (LPGH 124), and 9-phenyl-3,6-bis[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-9H-carbazole (LPGH 153). Among the devices, the single-layered device with LPGH 153 shows a luminous efficiency, a power efficiency, and a quantum efficiency of 9.3 cd/A, 5.2 lm/W, and 6.2%, respectively. The single-layered device with CBP was compared with a multilayered device with CBP. As a result, the single-layered device shows a reduced operating voltage, an enhanced roll-off efficiency, and a pure emitting color in comparison with the multilayered device owing to the direct injection of carriers into a dopant and the suppression of exciplex formation. © 2010 The Japan Society of Applied Physics.