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Pode, Ramchandra,Ahn, Jeung-Sun,Jeon, Woo Sik,Park, Tae Jin,Kwon, Jang Hyuk Elsevier 2009 CURRENT APPLIED PHYSICS Vol.9 No.5
<P><B>Abstract</B></P><P>Highly efficient red phosphorescent devices comprising a simple bi-layered structure using tris(1-phenylisoquinoline)iridium (Ir(piq)<SUB>3</SUB>) doped in a narrow band-gap fluorescent host material, bis(10-hydroxybenzo [h] quinolinato)beryllium complex (Bebq<SUB>2</SUB>) are reported. The driving voltage to reach 1000cd/m<SUP>2</SUP> is 3.5V in Bebq<SUB>2</SUB>:Ir(piq)<SUB>3</SUB> red phosphorescent device. With a dopant concentration of as low as 4%, the current and power efficiency values of 8.41cd/A and 7.34lm/W are obtained in this PHOLEDs, respectively. External quantum efficiency (EQE) of 14.5% is noticed in this red phosphorescent device, promising to high brightness applications.</P>
Ramchandra Pode,안정선,Woo Sik Jeon,Tae Jin Park,권장혁 한국물리학회 2009 Current Applied Physics Vol.9 No.5
Highly efficient red phosphorescent devices comprising a simple bi-layered structure using tris(1-phenylisoquinoline)iridium (Ir(piq)3) doped in a narrow band-gap fluorescent host material, bis(10-hydroxy-benzo [h] quinolinato)beryllium complex (Bebq2) are reported. The driving voltage to reach 1000 cd/㎡ is 3.5 V in Bebq2:Ir(piq)3 red phosphorescent device. With a dopant concentration of as low as 4%, the current and power efficiency values of 8.41 cd/A and 7.34 lm/W are obtained in this PHOLEDs, respectively. External quantum efficiency (EQE) of 14.5% is noticed in this red phosphorescent device, promising to high brightness applications.
여승준,Ramchandra Balaji Pode,김화민,안정선 한국물리학회 2009 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.55 No.1
In the present paper, results on the polymerization of C60 in benzene-ethanol mixtures are reported. The photoluminescence (PL) of the C60 aggregates prepared by using two different methods was investigated using PL spectroscopy. In the case of benzene-ethanol mixtures of different compositions with ethanol volume fractions less than 50% in the solvent mixture, the PL spectra showed the spectral shape of aggregates at 120 K. The change in the spectral shape of the PL emission due to the formation of C60 aggregates at the liquid-solid transition temperature of the solvent mixture as in case of a single solvent (benzene, toluene, or CS2) is reported. A shift in 0-0 transition energy to a high-energy compared with the C60 crystal was noticed. The 0-0 transition energy for a high ethanol volume fraction (>70%) of the solvent mixture showed no shift and was located at almost the same region as that of the C60 crystal PL spectrum. A significantly big size of C60 aggregates in a mixture of benzene-ethanol with a high ethanol volume fraction compared to that of C60 aggregates in a single solvent (>2.3 nm in benzene) is reported. Furthermore, the high-resolution transmission electron microscopy (HRTEM) image of a C60 cluster produced by irradiating C60 aggregates with on UV pulse-laser reconfirmed these results.
Efficiency Control in Iridium Complex-Based Phosphorescent Light-Emitting Diodes
Diouf, Boucar,Jeon, Woo Sik,Pode, Ramchandra,Kwon, Jang Hyuk Hindawi Limited 2012 Advances in materials science and engineering Vol.2012 No.-
<P>Key factors to control the efficiency in iridium doped red and green phosphorescent light emitting diodes (PhOLEDs) are discussed in this review: exciton confinement, charge trapping, dopant concentration and dopant molecular structure. They are not independent from each other but we attempt to present each of them in a situation where its specific effects are predominant. A good efficiency in PhOLEDs requires the triplet energy of host molecules to be sufficiently high to confine the triplet excitons within the emitting layer (EML). Furthermore, triplet excitons must be retained within the EML and should not drift into the nonradiative levels of the electron or hole transport layer (resp., ETL or HTL); this is achieved by carefully choosing the EML’s adjacent layers. We prove how reducing charge trapping results in higher efficiency in PhOLEDs. We show that there is an ideal concentration for a maximum efficiency of PhOLEDs. Finally, we present the effects of molecular structure on the efficiency of PhOLEDs using red iridium complex dopant with different modifications on the ligand to tune its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies.</P>
Photoluminescence of Nano-Scale Fullerene (C60) Aggregates in a Solvent Mixture
Jeung Sun Ahn,여승준,차정옥,Ramchandra Pode,김화민,이병로 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.5
We present the photoluminescence emission of C60 aggregates in benzene-ethanol mixtures of different compositions. For ethanol volume fractions in the solvent mixture less than 50 %, the photoluminescence spectra show the spectral shape of aggregates at 120 K. A change in the spectral shape of the photoluminescence emission due to the formation of C60 aggregates at the liquid-solid transition temperature of the solvent mixture, as in the case of a single solvent (benzene, toluene, or CS2), is reported. The shift in the 0-0 transition energy to a high energy in comparison with the C60 crystal is noticed. The 0-0 transition energy for a high ethanol volume fraction of the solvent mixture (>70 %) shows no shift and is located at almost the same region as that of C60 crystal photoluminescence spectrum. Sizes of the C60 aggregates in the mixture of benzene-ethanol with a high ethanol volume fraction much bigger than those of C60 aggregates in single crystals (>2.3 nm in benzene) is reported. We present the photoluminescence emission of C60 aggregates in benzene-ethanol mixtures of different compositions. For ethanol volume fractions in the solvent mixture less than 50 %, the photoluminescence spectra show the spectral shape of aggregates at 120 K. A change in the spectral shape of the photoluminescence emission due to the formation of C60 aggregates at the liquid-solid transition temperature of the solvent mixture, as in the case of a single solvent (benzene, toluene, or CS2), is reported. The shift in the 0-0 transition energy to a high energy in comparison with the C60 crystal is noticed. The 0-0 transition energy for a high ethanol volume fraction of the solvent mixture (>70 %) shows no shift and is located at almost the same region as that of C60 crystal photoluminescence spectrum. Sizes of the C60 aggregates in the mixture of benzene-ethanol with a high ethanol volume fraction much bigger than those of C60 aggregates in single crystals (>2.3 nm in benzene) is reported.
Electrical characterization of N- and P-doped hole and electron only organic devices.
Park, Tae Jin,Kim, Sun Young,Jeon, Woo Sik,Park, Jung Joo,Pode, Ramchandra,Jang, Jin,Kwon, Jang Hyuk American Scientific Publishers 2008 Journal of Nanoscience and Nanotechnology Vol.8 No.10
<P>We have fabricated a series of hole only devices with tungsten oxide (WO3) and molybdenum oxide (MoO3) n-doping materials in N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB) hole transport layer, and electron only devices with CsF and Cs2CO3 p-doping materials in 4,7-diphenyl-1,10-phenanthroline (Bphen) electron transport layer. Current-voltage characteristics and conductivity of these devices are investigated. The optimal conditions for ohmic injection and low resistance properties, and process margins of each dopant are reported in this paper.</P>