Abstract
In this paper, we investigate the carrier injection and transport characteristics in iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) doped phosphorescent organic light-emitting devices (OLEDs) with oxadiazole (OXD) as the bipolar host material of the emitting layer (EML). When doping Firpic inside the OXD, the driving voltage of OLEDs greatly decreases because FIrpic dopants facilitate electron injection and electron transport from the electron-transporting layer (ETL) into the EML. With increasing dopant concentration, the recombination zone shifts toward the anode side, analyzed with electroluminescence (EL) spectra. Besides, EL redshifts were also observed with increasing driving voltage, which means the electron mobility is more sensitive to the electric field than the hole mobility. To further investigate carrier injection and transport characteristics, FIrpic was intentionally undoped at different positions inside the EML. When FIrpic was undoped close to the ETL, driving voltage increased significantly which proves the dopant-assisted-electron-injection characteristic in this OLED. When the undoped layer is near the electron blocking layer, the driving voltage is only slightly increased, but the current efficiency is greatly reduced because the main recombination zone was undoped. However, non-negligible FIrpic emission is still observed which means the recombination zone penetrates inside the EML due to certain hole-transporting characteristics of the OXD.
Highlights
Organic light-emitting devices have attracted lots of attention in display and lighting applications due to the various advantages such as self-emission, flexible-substrate compatibility, and large-sized fabrication [1–5]
For efficient use of the triplet exciton for electroluminescence, phosphorescent dopant is employed in the matrix as the emitting layer (EML) of the organic light-emitting devices (OLEDs) [6–15]
We demonstrated an efficient blue phosphorescent OLED consisting of iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) doped into 2-phenyl-5-(2',4',6'-trimethyl-[1,1'-biphenyl]-4-yl)-1,3,4-oxadiazole (OXD)
Summary
Organic light-emitting devices have attracted lots of attention in display and lighting applications due to the various advantages such as self-emission, flexible-substrate compatibility, and large-sized fabrication [1–5]. With doping FIrpic inside the OXD, the driving voltage is decreased which means the dopants help carrier injection and transport. From EL spectra analysis, it can be found that the recombination zone shifts toward the anode side This means the dopant material improves the electron injection and transport capability. When increasing the driving voltage, the relative intensity at longer wavelength of the EL spectra increases and the recombination zone shifts from inside the EML toward the anode side [33,34]. The J-V characteristics are only slightly shifted for the case with the undoped region close to the anode, the current efficiency decreases a lot because there are no dopants inside the main recombination zone. There is still observable light emission, which means the hole is transported over the undoped region (pure OXD) and recombines with an electron
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