New materials development in organic light-emitting diodes

Abstract

A new pyrazine derivative, 5,6-Bis-[4-(naphthalene-1-yl-phenyl-amino)-phenyl]-pyrazine-2,3-dicarbo-nitrile (BNPPDC) with naphthalene-1-yl-phenyl-amino as an electron donor and pyrazine-2,3-dicarbonitrile as an acceptor, were designed, synthesized, and studied. The quantum chemical properties, absorption and photoluminescence properties, energy levels, and electroluminescence (EL) performance of the compounds were investigated and discussed. A double-layer EL device (ITO/BNPPDC/TPBI/Mg:Ag) was fabricated and shown to have good EL performance with a brightness of 100 cd/m2 at 4.5 V, a maximum brightness of 17400 cd/m2 at 11 V, a low turn-on voltage of 2.9 eV, and a maximum current efficiency of 3.61 cd/A as well. The results indicate that BNPPDC can be used as a hole-transporting emitter. Another series of EL devices using BNPPDC as a host and a newly developed red emitter, 2,3-bis[[(2-hydroxy-4-diethylamino)phenyl][(methylene)]amino]-2-butanedinitrile (BDPMB) as dopant have been fabricated. An important feature of this dopant-host system is the hypsochromic shift of the emission wavelength of the dopant BDPMB relative to the same dopant doped in the prototypical host, Alq3. The resulting hypsochromic shift leads to substantially improved performance of the red OLED in terms of turn-on voltage, current and power efficiencies. The OLED fabricated from this dopant-host system achieved a high brightness of 9730 cd/m2 at a low voltage of 11 V, a high power efficiency of 2.35 lm/W (current efficiency of 3.36 cd/A) at 4.5 V (at a brightness of 190 cd/m2), a very low turn-on voltage of 3.0 V, and CIE coordinates of (x=0.64, y=0.35). To our best knowledge, the EL performance of the present device is superior or equal to the best fluorescent red-emitting OLEDs, including the state-of-the-art red-emitting OLED based on the fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulo-lidyl-9-enyl)-4H-pyran (DCJTB) doped in a cohost-emitter system of 5,6,11,12-tetraphenylnathacene (rubrene)/Alq3 with a device structure of ITO/CFx/NPB/CHE (rubrene:Alq3:DCJTB; 60:40:2)/Alq3/LiF/Al. A new series of phosphorescent iridium complexes with the pinene group spacer in the framework of 2-phenylpyridine to minimize the aggregation quenching were designed, synthesized, and studied. The quantum chemical properties, absorption and photoluminescence properties, energy level, and electroluminescence performance of these complexes were investigated and discussed. The EL performance of the device configuration (ITO/NPB/blue iridium (III) tris(5-(2,4-difluoro-phenyl)-0,10-dimethyl-4-aza-tricycloundeca-2,4,6-triene) (Ir(F2-mppy)3) complex (emission peak at 475 nm)doped in CBP/BCP/Alq3/Mg:Ag) achieved a maximum luminance of 6000 cd/m2 (at 11.5 V), 9001 cd/m2 (12.5 V), 6986 cd/m2 (at 13.5 V), 4671 cd/m2 (at 15.5 V), 2701 cd/m2 (at 10.0 V) and 1251 cd/m2 (at 11.5 V) for doping concentrations of 10, 15, 19, 50, 100 % of Ir(F2-mppy)3 and 100% of Ir(F2-ppy)3, respectively. It also achieved a maximum power efficiencies (η p ) of 7.68 lm/W, 9.58 lm/W, 8.04 lm/W, 4.76 lm/W, 2.64 lm/W and 0.93 lm/W with CIE coordinates of (x=0.17, y=0.29), (x=0.18, y=0.36), (x=0.20, y=0.37), (x=0.21, y=0.40), (x=0.24,y=0.43) and (x=0.26, y=0.45) for a doping concentration of 10, 15, 19, 50, 100 % of Ir(F2-mppy)3 and 100 % of Ir(F2-ppy)3, respectively. The superior performance of the present device can be appreciated by benchmarking against the state-of-art devices. For further exploration of iridium complexes, another new series of iridium complexes were designed, synthesized, and studied. Their quantum chemical properties, absorption and photoluminescence properties, energy level, and electroluminescence performances were investigated and discussed. The EL performance of the device configuration (ITO/NPB/green iridium(III)bis(2,5-bis(2,4-difluoro-phenyl)-pyridazinato)picolinate ((bdpp)2Ir(pic)) complex (emission peak at 516 nm) doped in CBP/BAlq/Mg:Ag) achieved a maximum brightness of 48945 cd/m2 and a maximum current efficiency of 36.45cd/A. Besides that, the device with the yellow iridium(III)bis(2,5-diphenyl)-pyridazinato)picolinate ((dpp)2Ir(pic)) complex (emission peak at 556 nm)as dopant achieved a maximum brightness of 47525 cd/m2 and a maximum current efficiency and power efficiency of 31.67 cd/A and 25.30 lm/W, respectively. Finally, a new series of white OLED devices were fabricated. First, a device with a configuration of ITO/NPB/CBP:8 wt% Ir(F2-mppy)3:x wt% DCM2/BAlq/Mg:Ag was fabricated by using the new blue emission iridium complex Ir(F2-mppy)3 sensitized red fluorescent dye DCM2. White emission with a color rendering index of 70 was obtained in a device with 8 wt% Ir(F2-mppy)3 and 0.5 wt% DCM2. The maximum luminance and current efficiency of the device are respectively 16218 cd/m2 and 9.28 cd/A. Secondly, a device with a configuration of ITO/NPB (50 nm)/DPVBi (15 nm)/CBP:6wt% (dpp)2Ir(pic) (15 nm)/BAlq (20 nm)/LiF (0.5 nm)/Mg:Ag (100 nm)(10:1) was fabricated, where (dpp)2Ir(pic) was a newly synthesized highly efficient yellow iridium complex and DPVBi was a fluorescent blue emitter. The device achieved a maximum luminance of 26340 cd/m2 at 17 V, and a maximum current and power efficiencies of 12.85 cd/A (at 5.60 mA/cm2) and 5.33 lm/W (at 0.05 mA/cm2), respectively. The CIE coordinates are (0.35, 0.38) at 7 V and change from (0.35, 0.39) at 9 V to (0.36, 0.41) at 15 V that are in the white region.