Sang-Geon Park

Electrical conduction behavior of organic light-emitting diodes using fluorinated self-assembled monolayer with molybdenum oxide-doped hole transporting layer

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.

Interface Phenomena of Self-Assembled Monolayer with Various Alkyl Chain as a Hole-Injection Layer for Organic Light-Emitting Diodes

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.

Effects of Self-Assembled Monolayers with Fluorinated Alkyl Chain

The electrical characteristics of fluorinated self-assembled monolayers (FSAMs) in organic light-emitting diodes (OLEDs) were investigated according to the change in the lengths of their alkyl chains. As the number of fluorine atom increases, the device with long alkyl chain length allows the flow of a large amount of current because of the improved barrier height of the hole injection. However, the current efficiency under long alkyl chain length was decreased by the switched carrier balance. The device with the shortest FSAM alkyl chain length demonstrated an approximately 68% higher current efficiency than that with the longest alkyl chain length.

High Efficiency Organic Light-Emitting Diode by Ag Anode Technique

The characteristics of a Ag anode with molybdenum oxide (MoOx)-doped N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-diphenyl-1,4′-diamine (α-NPD) are reported. The current efficiency of devices with a Ag anode was achieved over 8.2 cd/A due to the microcavity effect. The current density of the device with a 25% concentration of MoOx was higher than those of a 10% concentration due to the improved charge transfer effect. The device of a Ag anode with a 25% concentration of MoOx indicated an approximate 227% higher current efficiency than that of an ITO anode with a 25% concentration of MoOx. The driving voltage of a Ag anode with a 25% concentration of MoOx was achieved at 4.4V with 10−2 A/cm2.

Performance enhancement of organic light-emitting diodes using fluorinated self-assembled monolayer

ABSTRACT In order to enhance the performance of organic light-emitting diodes, fluorinated self-assembled monolayer (F-SAM) with long alkyl chain was introduced to the device between the anode and hole-transport layer. CF3-terminated F-SAM has a relatively high electric dipole moment, which may affect hole injection at the indium-tin-oxide (ITO) anode. Two kinds of F-SAM with different alkyl chain lengths (F3-SAM and F10-SAM) were formed on the ITO substrate through hydrolysis and dehydration process. Electric dipole moments of these F-SAMs were obtained from a simulation using Gauss View program. A device was made in a structure of ITO (170 nm) / F-SAM / TPD (40 nm) / Alq3 (60 nm) / Al (100 nm). Formation of the SAM was confirmed by contact-angle measurements. It was found that the contact angles for the F3-SAM and F10-SAM are about 84° and 112°, respectively. A device with properly treated self-assembled monolayer at the anode gives an improvement in turn-on voltage, luminance, and efficiency compared to those of the device without SAM. Turn-on voltage was lowered from 4.25 V to 3.50 V in the device with SAM compared to the one without SAM, which indicates a reduction of hole-injection barrier height due to the electric dipole moment of the F-SAM. An external quantum efficiency of the device with F3-SAM was increased by a factor of about 3.5 compared to the one without SAM.

Effect of self-assembled monolayer on polymeric and organic light-emitting diodes

In this report, we discuss the effect of FSAM on both hole-injection and hole-transport functions. The thiophene derivative PEDOT:PSS is widely used as a hole-injection layer of conducting-polymer based LEDs (PLEDs). However, since PEDOT:PSS has some problems of strong acidity and metal cation impurities, a substitute material has been developed as a substitute. We used FSAM as a directly hole-injection layer of PLEDs and a single Alq3 layer. The introduction of FSAM lowered the driving voltage of devices and improved EL efficiency per current. In the former case, the EL properties of PLED with PEDOT:PSS is superior to those of PLED with FSAM. In the latter case, it was found the increase in hole injection contributed to high conduction of OLEDs. We reviewed the basic characteristics of polymer and OLED using FSAM.