The electrical conductivity behavior of a fluorinated self-assembled monolayer (FSAM) of a molybdenum oxide (MoOx)-doped α-naphthyl diamine derivative (α-NPD) in organic light-emitting diodes (OLEDs) was investigated. The current density of the MoOx-doped α-NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature-dependent characteristics of this device were investigated. The current density-voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 10(-2) [eV] and was very low due to the inelastic electron tunneling of FSAM molecules.
We investigated the effects of using a Ag anode with the co-evaporation of molybdenum oxide (MoO
x
) and N,N '-bis(1-naphthyl)-N,N '-diphenyl-1,1'-diphenyl-1,4'-diamine (α-NPD). The current density of the device with a Ag anode and a co-evaporation thin film was the highest among the various devices used owing to a low sheet resistance and charge transport. We discussed the mobility and carrier density of the co-evaporation thin film as compared with the electrical properties of the α-NPD thin film. The device with a Ag anode achieved high electroluminescence (EL) efficiency due to the optical design. In particular, the device with a Ag anode and a co-evaporation thin film showed an approximately 55% higher current efficiency than that with indium–tin oxide (ITO) and a MoO
x
layer.
We investigate the effect of the interface phenomena of a self-assembled monolayer (SAM) molecules on hole injection. A SAM represents an easy and accurate approach to the modification of surface properties. The dipole moment of a fluorinated SAM (FSAM) improves hole injection from Indium Tin Oxide (ITO). We prepare several kinds of FSAM with different number of fluorine atoms FnSAM. As the number of fluorine atom increases, the dipole moment also increases. Increasing the chain length increases the van der Waals interaction between alkyl chains. We examine the interface characteristics of SAM molecules on the hole injection.
In this paper, the high-performance characteristics of a device that uses dipyrazino[2,3-f:2′,3′-h] quinoxaline-2,3 (HAT_CN) as the hole-injection layer are reported. Devices with HAT_CN showed a higher current density (of about 3.97 A/cm2 at 3 V) than those without a hole-injection layer. The higher current density of the devices with HAT_CN can be attributed to the nonexistence of a hole-injection barrier because of the HAT_CN, and the carrier transfer effect that is facilitated by the small difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital of the N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine in the hole-injection layer. The device with HAT_CN achieved a high maximum current efficiency of 11.45 cd/A, despite the collapse of the carrier balance. The external quantum efficiency of the device with HAT_CN was increased by approximately 0.4% compared to that of a device without the hole-injection layer.
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