Kwangho Jeong

The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.

Enhancing the electroluminescent properties of organic light-emitting devices using a thin NaCl layer

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.

The interface state assisted charge transport at the MoO 3 /metal interface

The interface formation between a metal and MoO(3) was examined. We carried out in situ ultraviolet and x-ray photoemission spectroscopy with step-by-step deposition of MoO(3) on clean Au and Al substrates. The MoO(3) induces huge interface dipoles, which significantly increase the work functions of Au and Al surfaces. This is the main origin of the carrier injection improvement in organic devices. In addition, interface states are observed at the initial stages of MoO(3) deposition on both Au and Al. The interface states are very close to the Fermi level, assisting the charge transport from the metal electrode. This explains that thick MoO(3) layers provide good charge transport when adopted in organic devices.

Origin of the improved luminance-voltage characteristics and stability in organic light-emitting device using CsCl electron injection layer

The luminance-voltage characteristics and stability were highly improved by replacing LiF with CsCl in organic light-emitting devices. To investigate the origin of these improvements, ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy were used. The additional shifts of the vacuum, highest occupied molecular orbital, and lowest unoccupied molecular orbital levels due to the CsCl layer reduce the width and height of the electron injection barrier, resulting in the improved luminance-voltage characteristics of the devices. Additionally, the intervening CsCl layer between Al and Alq3 prevents N–Al reactions among Alq3 molecules and Al, which reduces distortion or breakdown of Alq3 molecules and slows the degradation of the device.

Controlling hole injection in organic electroluminescent device by sputter-grown Cu-phthalocyanine thin films

A sputter-grown Cu-phthalocyanine (SG-CuPc) thin films have been employed to control the anode interface of organic electroluminescent device (OELD). Insertion of a thin SG-CuPc between the indium tin oxide and hole-transport layers enhances the hole injection in a controllable manner, which can increase the device efficiency and decrease the operation voltage without increasing the interface roughness. Time–voltage–luminescence measurements show the improved operational durability of the thin SG-CuPc inserted OELD.

Light-emitting diode based on oligo-phenylene vinylene and butyl-PBD blends

Abstract We have fabricated light-emitting diodes (LEDs) using organic materials; a polymer blend dispersing oligo-phenylene vinylene (oligo-PV), 1,4-dis-tyrylbenzene and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl-PBD) as emissive materials into a soluble polyimide mixed with polyaniline (PANI) of emeraldine salt used as a hole transport material. These polymer dispersed materials were sandwiched between In and indium-tin-oxide (ITO) electrodes. In order to increase the electron injection into the emissive material, we have inserted a thin Mg layer between In and polymer blends. The electroluminescence (EL) spectra of LEDs showed noticeable enhancement of the oscillator strength of oligo-PV peak at 2.76 eV. This implies improved quantum efficiency of this blue light-emitting diode, resulting from the excitonic migration from butyl-PBD to oligo-PV. We have found that the EL device with host polymers, polyimide and PANI, displayed increasing device performance, lowering the turning point in I - V characteristics, compared to that of LED without PANI. Under normal illumination conditions, our devices with PANI showed visible blue-violet color at room temperature after applying a bias exceeding 8 V.

Interfacial electronic structure of N, N′ -bis(1-naphthyl)- N, N′ -diphenyl-1, 1′ -biphenyl-4, 4′ -diamine/copper phthalocyanine: C60 composite/Au studied by ultraviolet photoemission spectroscopy

The interfacial electronic structures of N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/copper phthalocyanine (CuPc)∕Au, NPB∕C60∕Au, and NPB∕CuPc:C60 composite/Au were investigated by in situ ultraviolet photoelectron spectroscopy to understand the highly efficient hole injection in organic light-emitting diode. The hole-injection barrier of CuPc:C60∕Au was 0.52eV, while those of CuPc∕Au and C60∕Au were 0.96 and 1.62eV, respectively. The lowered injection barrier is attributed to the smaller interface dipole of CuPc:C60 compared to that of pristine CuPc. This small interface dipole pulled up the highest occupied molecular orbital of CuPc in composite, which results in the decreased hole-injection barrier.

Mixing effect of chelate complex and metal in organic light-emitting diodes

Organic light-emitting diodes using thin film dispersing a hole transport material into a soluble polyimide as a hole transport layer and the sublimed molecular film of a chelate complex as an emissive layer were fabricated. In order to improve the injection of electrons into the emissive layer as well as the durability of devices, we have attempted mixing the chelate complex and metal between the emissive layer and the cathodic electrode. The charge injection of the device with the mixed layer was initialized at an applied voltage of 4.19 V. It was observed from the electroluminescent spectra that the oscillator strength was dramatically enhanced with the applied voltage.

Evidence of gap state formed by the charge transfer in Alq3∕NaCl∕Al interface studied by ultraviolet and x-ray photoelectron spectroscopy

Electronic structures of Alq3∕NaCl∕Al and Alq3∕Al were studied by UV and x-ray photoelectron spectroscopy (XPS). The initial energy level of the highest occupied molecular orbital (HOMO) of Alq3∕Al was shifted when the ultrathin NaCl layer was inserted between them, although the vacuum level was not changed. The measured interface dipole was 1.1 eV, identical for both Alq3∕NaCl∕Al and Alq3∕Al. Our experiment shows that the dipole is formed in very short range (less than 0.1 nm) from the interface. The onset of the HOMO level of Alq3 was shifted 0.2 eV toward high binding energy for the additional NaCl layer, which lowered the barrier height and improved injection characteristics of the device. Moreover, a gap state was observed at 1.1 eV below the Fermi level when the NaCl was inserted between Alq3 and Al. The XPS core-level spectra revealed that the interaction on Alq3 and NaCl was very weak, which generated an unusual gap state without breaking or forming chemical bonds. We suggest that the weak interac...

Direct evidence of n-type doping in organic light-emitting devices: N free Cs doping from CsN3

Cesium azide (CsN3) is confirmed to be decomposed during thermal evaporation. Only Cs could be deposited on tris(8-hydroxyquinolinato)aluminum (Alq3) and n-type doping is easily achieved. Organic light-emitting devices with CsN3 show highly improved current density-luminance-voltage characteristics compared to the control device without CsN3. To understand the origin of the improvements, in situ x-ray and UV photoemission spectroscopy measurements were carried out and a remarkable reduction in electron injection barrier is verified with successive deposition of Al on CsN3 on Alq3. CsN3 has a potential as alternative to doping the electron transport layer by replacing the direct deposition of alkali metals.