Oxadiazole host for a phosphorescent organic light-emitting device

Abstract

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.