Improvement of performance of tetraphenylporphyrin-based red organic light emitting diodes using WO3 and C60 buffer layers

One of the porphyrin derivatives, meso-tetraphenylporphyrin (TPP), has been synthesized and examined as an emitter material (EM) for efficient fluorescent red organic light-emitting diodes (OLEDs). By inserting a tungsten oxide (WO3) layer into the interface of anode (ITO) and hole transport layer N,N'-Di-[(1-napthyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (NPB) and by using fullerene (C-60) in contact with a LiF/Al cathode, the performance of devices was markedly improved. The current density-voltage-luminance (J-V-L) characterizations of the samples show that red OLEDs with both WO3 and C60 as buffer layers have a lower driving voltage and higher luminance compared with the devices without buffer layers. The red OLED with the configuration ITO/WO3 (3 nm)/NPB (50 nm)/TPP (60 nm)/BPhen (30 nm)/C-60 (5 nm)/LiF (0.8 nm)/Al (100 nm) achieved the high luminance of 6359 cd/m(2) at the low driving voltage of 8 V. At a current density of 20 mA/cm(2), a pure red emission with CIE coordinates of (0.65; 0.35) is observed for this device. Moreover, a power efficiency of 2.07 lm/W and a current efficiency of 5.17 cd/A at 20 mA/cm(2) were obtained for the fabricated devices. The study of the energy level diagram of the devices revealed that the improvement in performance of the devices with buffer layers could be attributed to lowering of carrier-injecting barrier and more balanced charge injection and transport properties. (C) 2012 Elsevier B.V. All rights reserved.