The improved performance of phosphorescent p-i-n organic light emitting diodes via enhancing hole injection and reducing triplet-polaron annihilation

Red phosphorescent p-i-n organic light emitting diodes (OLEDs) have been fabricated incorporating bis(1-(phenyl) isoquinoline) iridium(III) acetylacetonate doped host of N,N'-bis-(1-naphthyl)-diphenyl-1,1'-biphenyl-4,4'-diamine[NPB:Ir(piq)(2)(acac)] and cohost of NPB and 4,4'-N,N'-dicarbazole-biphenyl [NPB:CBP:Ir(piq)(2)(acac)]. The current efficiency based on NPB:CBP:Ir(piq)(2)(acac) (x:100-x:25 by mass) is increased than that based on NPB:Ir(piq)(2)(acac) (100:25), where x = 40, 50, 60, and 70, mostly because the CBP contributes to electron transport, decreasing the number of electrons conducted by Ir(piq)(2)(acac) and thereby relieving triplet-polaron annihilation. Compared to MoO3 p-doped NPB (NPB:MoO3), the combined MoO3 p-doped CBP (CBP:MoO3) and NPB:MoO3 enhances hole current into emissive layer, mainly due to the fact that the hole injection from indium tin oxide (ITO) to CBP:MoO3 is more efficient than that from ITO to NPB:MoO3. The device applying NPB:CBP:Ir(piq)(2)(acac) (60:40:25) and CBP:MoO3/NPB:MoO3 shows comparable current density versus voltage curve while increased current efficiency than the one employing NPB:CBP:Ir(piq)(2)(acac) (50:50:25) and NPB:MoO3. The current research provides some insight to optimize the architectures of phosphorescent p-i-n OLEDs.