High efficiency blue phosphorescent organic light emitting diodes

Abstract

Organic light emitting devices (OLEDs) have demonstrated the potential for solid state lighting as well as full color display applications. Use of triplet harvesting phosphorescent materials has led to very high efficiency OLEDs especially in green and red phosphorescent OLEDs. However in case of blue OLEDs the efficiency achieved is still room for improvement. Charge balance is a very important factor for achieving high efficiency organic light emitting diodes. In most OLED devices, hole mobility of hole transport layer is orders of magnitude higher than the electron mobility of electron transport layer. We study how this affects the charge balance and hence the device performance in the blue phosphorescent OLEDs with Iridium (III)bis [(4,6-di-fluorophenyl)- pyridinato-N,C2´] picolinate (FIrpic) emitter. Charge balance is studied in these devices and the devices are found to be hole dominant. Additionally, effect of charge balance on device performance is demonstrated with different electron transport layers. Using this approach, a very high efficiency of 60 Cd/A (50 lm/W) is achieved with 3,5´-N,N´-dicarbazole-benzene (mCP) host.