Oxadiazole host for a phosphorescent organic light-emitting device

In this paper, we demonstrate a phosphorescent organic light-emitting device (OLED) with enhanced current efficiency (in terms of cd/A) based on an oxadiazole (OXD) derivative as the electron-transporting host of the emitting layer (EML) doped with a phosphorescent dopant, iridium(III) bis[4,6-(di-fluorophenyl)-pyridinato-N, C2'] picolinate (FIrpic). The maximum current efficiency of OXD-based OLEDs was 13.0 cd/A. Compared to the phosphorescent OLED with a conventional hole-transporting host, 1,3-bis(carbazol-9-yl)benzene (mCP) with 11.1 cd/A in maximum current efficiency, 17.2% improvement was achieved. However, in terms of external quantum efficiency (EQE), the OXD- and mCP-based OLEDs were 4.01 and 4.66%, respectively, corresponding to a 13.9% decrease. Such a discrepancy can be understood from the electroluminescence (EL) variation. Contrary to the hole-transporting mCP, OXD exhibited electron transporting characteristics which shifted the recombination zone toward the anode. The optical interference effect result was that the relative intensity at long wavelengths (500-600 nm) was higher in the OXD-based OLED, which was more sensitive to the human eye and increased the current efficiency, even though the EQE was lower. Besides, in OXD-OLED, the recombination zone shifted toward the anode side with a high driving voltage, which was also deduced from the EL spectral variations. Under a high driving voltage, we observed the relative intensity of FIrpic emission ata longer wavelength increased which resulted from the optical interference effect, and emission from the hole-transporting layer increased. By using hole-transporting mCP and electron-transporting OXD as the hosts of double EML (DEML), the maximum current-efficiency and EQE of the optimized DEML-OLED further increases to 17.6 cd/A and 7.06%, which corresponds to improvements of 58.6% and 51.5%, compared to the single mCP-OLED, and by 35.4% and 76.1%, compared to the single OXD-OLED, respectively. This was a result of the better charge balance in DEML, and less quenching effects from transporting materials. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3581071]