Kwon-Hyeon Kim

A Fluorescent Organic Light‐Emitting Diode with 30% External Quantum Efficiency

Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exciplex-forming co-hosts stimulate energy transfer and charge balance in the system. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device.

Highly Efficient Organic Light‐Emitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments

also used as the co-host of the emitting layer (EML) with the molar ratio of 1:1 to exploit the exciplex forming character for low driving voltage, and good electron-hole balance. [ 7,9,10 ] The detailed structure of the OLEDs is; indium tin oxide (ITO) (70 nm)/TAPC (75 nm)/TCTA (10 nm)/TCTA:B3PYMPM: 8.4 mol% dye (30 nm)/B3PYMPM (45 nm)/LiF (0.7 nm)/Al (100 nm). The photoluminescence (PL) spectra of the TCTA:B3PYMPM co-host fi lms doped with the three dyes with the doping concentration of 8.4 mol% are shown in Figure 2 a. The peak wavelength of the PL of Ir(ppy) 3 , Ir(ppy) 2 acac, and Ir(ppy) 2 tmd were 513 nm, 520 nm, 524 nm, respectively. The orientations of the transition dipole moments of the dyes in the host were determined through the analysis of the angledependent PL spectra of the fi lms. [ 7,11 ] Figure 2 b shows the measured angle-dependent PL intensities of the p-polarized light emitted from the 30 nm-thick fi lms composed of TCTA:B3PYMPM:green dyes (0.46:0.46:0.08 molar ratio) at 520 nm close to the PL maxima of the green dyes. The angledependent PL spectra were analyzed using the classical dipole model where the emission from excitons is considered as the dissipated power from oscillating dipoles. [ 12,13 ] Birefringence

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.

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.

Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN)

The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene have been developed for a highly efficient blue fluorescent oragnic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter [4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN)]. We have demonstrated one of the highest external quantum efficiency of 21.8% in blue fluorescent OLEDs, which is identical to the theoretically achievable maximum electroluminescence efficiency using the emitter. Interestingly, the efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intesystem crossing rate rather than charge imbalance.

Single-electron transistor based on a silicon-on-insulator quantum wire fabricated by a side-wall patterning method

We propose and implement a promising fabrication technology for geometrically well-defined single-electron transistors based on a silicon-on-insulator quantum wire and side-wall depletion gates. The 30-nm-wide silicon quantum wire is defined by a combination of conventional photolithography and process technology, called a side-wall patterning method, and depletion gates for two tunnel junctions are formed by the doped polycrystalline silicon sidewall. The good uniformity of the wire suppresses unexpected potential barriers. The fabricated device shows clear single-electron tunneling phenomena by an electrostatically defined single island at liquid nitrogen temperature and insensitivity of the Coulomb oscillation period to gate bias conditions.

Highly Efficient, Conventional, Fluorescent Organic Light‐Emitting Diodes with Extended Lifetime

Highly efficient, yellow-fluorescent organic light-emitting diodes with a maximum external quantum efficiency exceeding 25.0% and extended lifetime are reported using iridium-complex sensitizers doped in an exciplex host. Energy transfer processes reduce the lifetime of the exciplex and excitons on the Ir complexes and enable an excited state to exist in a conventional fluorescent emitter, thereby increasing device lifetime. The device stability depends on the location of the excited state.

Direct formation of nano-pillar arrays by phase separation of polymer blend for the enhanced out-coupling of organic light emitting diodes with low pixel blurring

We have demonstrated a simple and efficient method to fabricate OLEDs with enhanced out-coupling efficiencies and with low pixel blurring by inserting nano-pillar arrays prepared through the lateral phase separation of two immiscible polymers in a blend film. By selecting a proper solvent for the polymer and controlling the composition of the polymer blend, the nano-pillar arrays were formed directly after spin-coating of the polymer blend and selective removal of one phase, needing no complicated processes such as nano-imprint lithography. Pattern size and distribution were easily controlled by changing the composition and thickness of the polymer blend film. Phosphorescent OLEDs using the internal light extraction layer containing the nano-pillar arrays showed a 30% enhancement of the power efficiency, no spectral variation with the viewing angle, and only a small increment in pixel blurring. With these advantages, this newly developed method can be adopted for the commercial fabrication process of OLEDs for lighting and display applications.

An Exciplex Host for Deep-Blue Phosphorescent Organic Light-Emitting Diodes

The use of exciplex hosts is attractive for high-performance phosphorescent organic light-emitting diodes (PhOLEDs) and thermally activated delayed fluorescence OLEDs, which have high external quantum efficiency, low driving voltage, and low efficiency roll-off. However, exciplex hosts for deep-blue OLEDs have not yet been reported because of the difficulties in identifying suitable molecules. Here, we report a deep-blue-emitting exciplex system with an exciplex energy of 3.0 eV. It is composed of a carbazole-based hole-transporting material (mCP) and a phosphine-oxide-based electron-transporting material (BM-A10). The blue PhOLEDs exhibited maximum external quantum efficiency of 24% with CIE coordinates of (0.15, 0.21) and longer lifetime than the single host devices.

Unraveling the orientation of phosphors doped in organic semiconducting layers

Emitting dipole orientation is an important issue of emitting materials in organic light-emitting diodes for an increase of outcoupling efficiency of light. The origin of preferred orientation of emitting dipole of iridium-based heteroleptic phosphorescent dyes doped in organic layers is revealed by simulation of vacuum deposition using molecular dynamics along with quantum mechanical characterization of the phosphors. Consideration of both the electronic transitions in a molecular frame and the orientation of the molecules at the vacuum/molecular film interface allows quantitative analyses of the emitting dipole orientation depending on host molecules and dopant structures. Interactions between the phosphor and nearest host molecules on the surface, minimizing the non-bonded van der Waals and electrostatic interaction energies determines the molecular alignment during the vacuum deposition. Parallel alignment of the main cyclometalating ligands in the molecular complex due to host interactions rather than the ancillary ligand orienting to vacuum leads to the horizontal emitting dipole orientation.Iridium-based phosphors show high photoluminescence quantum yield in organic light-emitting diodes. Here, Moon et al. reveal the mechanism responsible for the preferred orientation of iridium complexes in an organic host and highlight the interaction between phosphor and host molecules at play.