Young-Seo Park

Low roll-off of efficiency at high current density in phosphorescent organic light emitting diodes

The authors demonstrate that the reduction of quantum efficiency with increasing current density in phosphorescent light emitting diodes (PhOLEDs) is related to the formation of excitons in hole transporting layer based on the analysis of emission spectra and exciton formation zone. Low roll-off of efficiency in a PhOLED was achieved using dual emitting layers (D-EMLs) by confining the exciton formation near the interface between the emitting layers. The external quantum efficiency was maintained almost constant up to 22mA∕cm2 (10000cd∕m2) by adopting the D-EMLs in Ir(ppy)3 based PhOLEDs, resulting in high external quantum efficiency (ηext=13.1%) at high luminance.

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

Excitation Energy Transfer in Organic Materials: From Fundamentals to Optoelectronic Devices

In this review, we discuss investigations of electronic excitation energy transfer in conjugated organic materials at the bulk and single molecule level and applications of energy transfer in fluorescent and phosphorescent organic light emitting devices. A brief overview of common descriptions of energy transfer mechanisms is given followed by a discussion of some basic photophysics of conjugated materials including the generation of excited states and their subsequent decay through various channels. In particular, various examples of bimolecular excited state annihilation processes are presented. Energy transfer studies at the single molecule level provide a new tool to study electronic couplings in simple donor/acceptor dyads and conjugated polymers. Finally, energy transfer in organic electronic devices is discussed with particular emphasis on triplet emitter doped OLEDs and blends for white light emission.

Efficient triplet harvesting by fluorescent molecules through exciplexes for high efficiency organic light-emitting diodes

Efficient triplet harvesting from exciplexes by reverse intersystem crossing (RISC) is reported using a fluorescent molecular system composed of the 4,4′,4″-tris(N-carbazolyl)-triphenylamine and bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine. The exciplex forming material system shows the efficient delayed fluorescence emission. As a result, almost 100% PL efficiency at 35 K and 10% external quantum efficiency at 195 K are achieved from the exciplex. The delayed fluorescence of the exciplex clearly demonstrates that a significant proportion of the triplet exciplexes is harvested through the RISC.

Silane- and triazine-containing hole and exciton blocking material for high-efficiency phosphorescent organic light emitting diodes

One of the important factors for high efficiency phosphorescent organic light-emitting devices is to confine triplet excitons within the emitting layer. We synthesized and characterized a new hole blocking material containing silane and triazine moieties, 2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine (DTBT). Electrophosphorescent devices fabricated using the material as the hole-blocking layer and N,N′-dicarbazolyl-4,4′-biphenyl (CBP) doped with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the emitting layer showed a maximum external quantum efficiency (ηext) of 17.5% with a maximum power efficiency (ηp) of 47.8 lm W−1, which are much higher than those of devices using bathcuproine (BCP) (ηext = 14.5%, ηp = 40.0 lm W−1) and 4-biphenyloxolate aluminium(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) (ηext = 8.1%, ηp = 14.2 lm W−1) as hole-blocking layers.

A highly efficient wide-band-gap host material for blue electrophosphorescent light-emitting devices

We report on an efficient wide-band-gap host material for blue electrophosphorescence devices, namely, 1,2-trans-di-9-carbazolylcyclobutane (DCz). Photophysical studies show that lower-energy excimer formation between the carbazole units can be efficiently suppressed in a DCz film, thus maintaining its high triplet-state energy and inducing an exothermic energy transfer from DCz to iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). Electrophosphorescent devices comprising a FIrpic:DCz emitting layer exhibit a superior performance with a maximum external quantum efficiency of 9.8%, a maximum luminance efficiency of 21.5cd∕A, and a maximum power efficiency of 15.0lm∕W at 0.01mA∕cm2.

Energy transfer from exciplexes to dopants and its effect on efficiency of organic light-emitting diodes

We report that an exciplex is formed at the interface between the N,N′-dicarbazolyl-4-4′-biphenyl (CBP) and the bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM), which are widely used as an emitting layer (EML) host and an electron transporting layer (ETL) for high efficiency, green phosphorescent, organic light-emitting diodes (OLEDs), respectively. The intensity of the exciplex emission is almost proportional to the inverse square of the fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] concentration of the EML. Meanwhile, the efficiency of the OLEDs increases as the concentration of the Ir(ppy)3 increases. This enhancement of the efficiency and the decrease of the exciplex emission originates from the increase in the energy transfer rate from the exciplex to the dopants, due to the decrease in the distance between the exciplex and the dopant. The energy transfer processes were successfully analyzed using the Forster energy transfer mechanism. The high-efficiency OLEDs were obtained through the...

Dendritic Ir(III) complexes functionalized with triphenylsilylphenyl groups: Synthesis, DFT calculation and comprehensive structure-property correlation

We report on the synthesis, DFT calculations and structure-property relationships of phosphorescent Ir(III) complexes with varied number and position of triphenylsilylphenyl substituents. The attachment of the dendritic triphenylsilylphenyl group at the pyridine part of the phenylpyridine ligand induced a stronger metal-to-ligand charge-transfer (MLCT) transition and lower band-gap energy than did the unfunctionalized complex, Ir(ppy)3. On the other hand, the attachment of the triphenylsilylphenyl group at the phenyl part of the phenylpyridine ligand induced a stronger ligand-centered (LC) transition. It was specifically found that the excited state intermolecular interactions, which give rise to non-radiative decay, were more efficiently suppressed when the triphenylsilylphenyl group was attached at the pyridine part of the phenylpyridine ligand and also when the number of substituents was increased. Such site-isolation effects and improved solubility due to the triphenylsilylphenyl group encapsulation made it possible to fabricate wet-processed polymer light-emitting devices from these functionalized Ir(III) complexes. Both the doped poly(vinylcarbazole) (PVK) films and the neat films of our triphenylsilylphenyl based dendritic Ir(III) complexes afforded moderate to high electrophosphorescence efficiencies with excellent phase homogeneity (4.1%/1.7% for Ir(TPSppy)3, 5.9%/2.5% for Ir(ppyTPS)3 and 1.8%/1.8% for Ir(TPSppyTPS)3 (doped polymer film/neat film, respectively)). Moreover, it is noteworthy that the triphenylsilylphenyl substituents greatly enhanced the thermal stability of the dendritic Ir(III) complexes.

Highly efficient orange organic light-emitting diodes using a novel iridium complex with imide group-containing ligands

We report the synthesis and device characterization of a novel iridium complex using a phenylpyridine derivative containing an imide functional group as a bis-cyclometallized ligand ((impy)2Ir(acac)) for orange organic light-emitting diodes (OLEDs). The photoluminescence of the dopant emits orange light with the main peak of 560 nm and a shoulder of 595 nm. The OLEDs made with the synthesized dopant also produce intense orange electroluminescence at 568–572 nm wavelengths depending on the device geometry. Very high peak external quantum efficiencies of 13.8–14.4%, peak power efficiencies of 29.2–32.7 lm/W, and low operation voltages of 5.3–5.4 V at 1000 cd/m2 have been demonstrated for the orange OLEDs.

Effects of Al, Ag, and Ca on luminescence of organic materials

Organic materials have been demonstrated to have the necessary attributes for display applications. In typical organic light emitting devices, metallic electrodes are used to inject charged carriers into the organic electroluminescent (EL) medium. We report severe photoluminescence (PL) quenching of organic thin films comprising the most useful materials, namely 1,4-bis[4-(3,5-di-tert-butylstyryl)styryl]benzene (4PV), upon sub-monolayer deposition of Al, Ag, and Ca in an ultrahigh vacuum environment. The severity of the luminescence quenching, which depends on the type of metal used, can greatly affect the EL device performance. For example, a sub-monolayer coverage of the various metals on a 300 A 4PV thin film can reduce the PL by as much as 50%. Depositing the 4PV layer onto a metal substrate also exhibits PL quenching. An exciton diffusion length of 200 A can be estimated from the quenching data.