No Gill Park

Effects of Indium-Tin-Oxide Surface Treatment on Organic Light-Emitting Diodes

Effects of mechanically polished and annealed indium-tin-oxide (ITO) substrates on tris-(8-hydroxyquinoline)aluminium (Alq3)-based light-emitting diodes were studied. It was found that the roughness and oxygen content at the ITO surface were reduced after mechanical polishing. Organic light-emitting diodes (OLEDs) fabricated on polished ITO surfaces showed ten fold better electroluminescence and current injection than OLEDs fabricated on as-received ITO surfaces, due to the increase of the smooth contact area between the ITO surface and the organic layer. The ITO substrate annealed at 300°C did not show much enhancement of electroluminescence, even though it showed high current injection at low voltage. Therefore, mechanical polishing is an effective treatment for enhancing the performance of OLEDs.

Ligand effect on the electroluminescence mechanism in lanthanide (III) complexes

Abstract The lanthanide complexes have been applied as emitting materials and have been anticipated to exhibit high efficiency along with a narrow emission spectrum since both singlet and triplet excitons of ligands are involved in the luminescence process. With increasing current, however, the quantum efficiencies have decreased significantly due to the triplet–triplet annihilation. The effect of the energy levels of singlet and triplet states of ligands have been studied and the efficient energy transfer mechanism to overcome triplet–triplet annihilation is proposed. The calculation shows that the triplet state as well as the singlet state of the ligand affects the quantum efficiency.

Surface treatment effects of indium–tin oxide in organic light-emitting diodes

Abstract Effects of mechanically polished and annealed indium–tin oxide (ITO) substrate in tris-(8-hydroxyquinoline) aluminum (Alq3)-based light-emitting diodes were studied. It was found that roughness and oxygen content at ITO surface were reduced after the mechanical polishing. The organic light-emitting diodes (OLEDs) fabricated on the polished ITO surface showed 10 times better electroluminescence and current injection than the OLEDs based on as-received ITO surface, due to the increase of smooth contact area between the ITO surface and organic layer. The nucleation of TPD on ITO shows different feature according to the surface condition of ITO. As a result, the mechanical polishing is an effective treatment to enhance the performance of the OLEDs.

Theoretical study of Ir(III) complexes of fluorinated phenylbenzoquinoline as red phosphorescent material

In this study, Ir(III) complexes with fluorinated 4-benzoquinoline (pbq) ligands were designed and characterized theoretically. The Hartree–Fock (HF) method with the 3-21G(d) basis set and density functional theory (DFT) utilizing the B3LYP functional with the 6-31G(d) basis set were used for the geometry optimization and the energy level calculation of the ground state of these complexes, respectively. Excited triplet and singlet states are examined using the time-dependent density functional theory (TD-DFT). As a result, it was found that these complexes produced a pure red emission due to the elongated conjugation length. In particular, it is concluded that the Ir(III) complex with pbq-CF3 ligands exhibits the largest emission efficiency and emits light of the purest red wavelength.

Synthesis and Photophysical Studies of Iridium Complexes of Fluorinated Phenylisoquinolines

There have been reported materials emitting green color in both fluorescence and phosphorescence with great success, however efficient red luminescence materials are rare. Recently, it has been reported iridium (III) complexes of 1-(phenyl)isoquinoline ( piq ) and 1-(4′-fluorophenyl) isoquinoline ( piq-F ) show strong electroluminescence (EL) brightness and efficiency, even at high currents. These complexes show the strong intensity of band around 450 nm assigned to the spin-forbidden 3MLCT (metal to ligand charge transfer) transition [1]. In this study, Ir(III) complexes of fluorinated piqs ( piq-F , piq-2F , piq-CF 3 ) as cyclometalated ligand were prepared and photonic properties were investigated to improve the EL efficiency. The Ir(III) complex with piq-CF 3 ligands exhibits the largest emission efficiency with maximum at 612 nm. The result of ab. Initio calculation using the Time-dependent density functional theory (TD-DFT) shows that the strong 3MLCT transition of the complex is due to the strong coupling between the 5d orbital of the Ir atom and the highest occupied molecular orbitals (HOMOs) of ligands.

Time-Dependent Density Functional Theory Study of the Al Complexes with Aromatic Ligands for Blue Organic Light-Emitting Diodes

Aluminum(III) complexes with 8-hydroxyquinoline have attracted much attention because of their potential application in organic light emitting diodes (OLEDs) as green and blue emitting materials with the subtle effect of the substitution on the fluorescence and electroluminescent (EL) properties. The photoluminescence of aluminum(III) complexes [(tri(2′,2-dihydroxybiphenyl) aluminum(III)] have been studied as efficient fluorescence emitting materials [17]. In this paper, geometric and electronic properties of several Al complexes are studied to find the ligand effect related to the intensities of their absorptions and emissions using computational methods. In order to find a new highly efficient blue emitting materials, we have designed new Al complexes with aromatic ligands, [4,5-phenanthrenediol, 3,3′-dihydroxy-2,2′-binaphthalene, 2-(2′-phenol)-2-naphthol, and 1-(2′-phenol)-2-naphthol]. The geometry optimizations of the ground and the lowest excited electronic states were determined using HF/3-21G(d) and configuration interaction with single excitations (CIS) method. The vertical and adiabatic transition energies were calculated using time-dependent density functional theory (TD-DFT) with B1LYP/6-31G(d). As a result, we propose new blue fluorescent Al complexes of high luminescence efficiency.

Ab Initio Study of Substituted Pyrenes for Blue Organic Light-Emitting Diodes

Luminescence efficiency of pyrene molecule is very low because of the aggregation effect of planar pyrene molecules. However, 1,3,6,8-tetra-substituted pyrenes with large electron donating group were reported to give a bright blue fluorescence [1]. In this paper, 1,6-bi-substituted and 1,4,6,9-tetra-substituted pyrenes as well as 1,3,6,8-tetra-substituted pyrenes were studied to find out the possibilities as the blue fluorescent materials of organic light-emitting diodes (OLEDs) [2-4]. Geometrical and electrical calculations were performed by ab initio methods. HF/3-21G(d) basis set was used for the geometry optimization of the ground electronic states of those compounds. The geometry of the low-lying excited electronic state was optimized using configuration interaction with single excitation (CIS) method. The vertical and adiabatic transition energies were calculated by time-dependent density functional theory (TD-DFT) using the B1LYP functional with 6-31G(d) basis set. From calculational results, it was explained that the change in fluorescence wavelength was affected by the position and the number of substituents, through analyzing the change of energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of pyrene. Some of substituted pyrenes showed possibilities as stronger fluorescent materials. New efficient emitting materials for OLEDs were proposed from the calculation results obtained by tuning the position, the number of substitution and the species of substituting moiety.