Jae-Gyoung Lee

Bright pure blue emission from multilayer organic electroluminescent device with purified unidentate organometallic complex

Multilayer organic electroluminescent devices (OELDs) were fabricated with highly pure 2-(2-hydroxyphenyl)benzoxazolato lithium (LiPBO), which was obtained through stepwise purification process, as a blue emission layer. The ionization potential of the carefully purified LiPBO was ∼5.82 eV. The multilayer OELD with a hole-blocking layer (HBL) emitted almost pure blue light with the CIE color coordinate of x=0.15 and y=0.08. However, the emission color was redshifted when an electron-transporting layer (ETL) was introduced instead of the HBL. The device with both the HBL and the ETL showed stable and bright blue emission above 14 000 cd/m2 with the color coordinate of x=0.15 and y=0.11, even though the color purity was slightly poorer than that with only the HBL.

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.

Non-linear charge conduction and emission behaviour of OELD fabricated with Alq3 and TPD-doped soluble polyimide

We have attempted to investigate the non-linear behaviour in the characteristics of organic electroluminescent devices (OELDs) fabricated with polyimide and an organometallic complex as a hole transport layer (HTL) and an emission layer, respectively. The thickness of the polyimide HTL and the kind of metal cathode were varied to study the mechanism of the charge conduction and the electroluminescence (EL). The conventional linear models well explained the voltage-dependent charge injection and the current-dependent EL. New non-linear models proposed here predicted the characteristics almost completely, even though the physical meaning of the included parameters was not clearly defined. The OELD with the Mg cathode showed lower luminous efficiency in the high electric field than that with the Al cathode because of poor adhesion between Mg and Alq3. The maximum luminous efficiency was ca. 1.2 lm/W at 700 cd/m2 in the case of the Al cathode. The emission colour did not change for the OELD with the Mg cathode irrespective of the polyimide HTL, whereas the OELD with the Al cathode and the thick HTL showed a red-shifted EL of ca. 10 nm. Copyright

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.

Hole-transporting polyimide for organic electroluminescent display ☆

Abstract The thermal stability of newly synthesized hole-transporting polyimide, poly[N,N′–diphenyl–N,N′–bis(4–aminobiphenyl)–(1,1′–biphenyl)–4,4′–diamine pyromellitimide] (PMDA-DBABBD PI), via vapor deposition polymerization was investigated with the aid of the capacitance–temperature (C–T) measurement technique. Prior to the examination of the complete organic electroluminescent device (OELD), the single layer devices with the individual materials including tris(8-hydroxyquinolinato) aluminum (Alq3), N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB), copper phthalocyanine (CuPc), and PMDA-DBABBD PI were subjected to the C–T measurement. The relaxation temperatures of the single layer devices with Alq3, TPD, NPB, CuPc, and PMDA-DBABBD PI were 180, 76, 110, 125, and more than 200°C, respectively. The OELD with PMDA-DBABBD PI and Alq3 as a hole-transporting layer and emissive layer was not relaxed up to 150°C, while those containing CuPc/TPD and NPB thin films were catastrophically damaged at ca. 76 and 110°C, respectively. The OELD with the small organic hole-transporting molecule has almost the same relaxation temperature as the single layer device with the respective molecules. The rectifying and electroluminescent characteristics disappeared for the annealed OELD with the small organic hole-transporting molecules, whereas the OELD with the hole-transporting polyimide did still show the rectifying behavior with the green light emission even though the current density and the light intensity became largely reduced.

Photoemission spectroscopy study of Alq3 and metal mixed interfaces

The electronic structures of mixed layers of tris (8-hydroxy-quinoline) aluminum (Alq3) and metal (Au and Al) were studied by ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). The devices with a mixed layer between Alq3 and the cathode were fabricated. The barrier height for electron injection was reduced by doping metals (Au or Al) into Alq3. The doping enhanced the performance of the device. From the XPS study, the doped Au metal did not react with Alq3 and in addition, the doped Al metal reacted slightly with Alq3. From the UPS study, the highest occupied molecular orbit shifted to a higher binding energy for both metal mixed layers. From these studies, it is concluded that the enhanced device characteristics come from the barrier height reduction by the metal doped in Alq3 rather than from the charge transfer complex induced by the reaction of Alq3 and metal.

Red electroluminescent devices utilizing dye-dispersed soluble polyimide thin film as an emission layer

Abstract The dye-dispersed polymer electroluminescent device using the soluble poly(ether imide) (PEI) of relatively high glass transition temperature was successfully prepared. 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4 H -pyran(DCM) was used as a lumophore. The device was turned on at around 12 V. and the brightness at 20 V was very strong. The color of the emitted light was red, which is almost the color of the dispersed dye alone. The stability of the device was also found to be good, even after packaging the cathode side with a polymeric glue at 110 °C.

Balanced charge injection in multilayer polymer light-emitting diode with water soluble nonconjugated polymer dispersed by ionic compounds

The authors have fabricated highly efficient polymeric light-emitting diode (PLED) from ionic compound dispersed water soluble nonconjugated polymer, polyurethane (PU), which was used as an ultrathin hole blocking and electron injection layer (HB-EIL) on the top of commercially available blue-emitting polymer, polyfluorene. The device with HB-EIL showed a maximum quantum efficiency of 1.7%, while the one without HB-EIL showed an efficiency of 0.6%. They propose that the better performance in PLED with PU layer was due to a well balanced charge injection in emitting layer after the enhanced electron injection due to ionic compound in the insulating PU layer.

Contact resistance in interface of metal - light emitting organic thin films

Abstract In this report, the specific contact resistance for the interface between the cathode metals and organic Alq3 thin films were determined using the test pattern with a vertical structure between two contact pads. The samples with the structure of silver/Alq3/silver exhibited a linear current–voltage (I–V) characteristics and the specific contact resistance was calculated be in a range 1×10−1 Ω/cm2. The measurement of specific contact resistance for the samples with the structure of aluminum/Alq3/aluminum was hindered by the formation of the Al2O3 layer in the interface between aluminum and Alq3. The formation of oxide in the interface is believed to give rise to the Schottky characteristics and control the current transport across the cathode/organic interface. The barrier heights of the aluminum/Alq3 contact were calculated to be in a range 0.22 to 0.60 eV, which is lower than the barrier height from the nominally pure aluminum/Alq3 interface.