Won-Ik Jeong

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.

High-Performance Flexible Organic Light-Emitting Diodes Using Amorphous Indium Zinc Oxide Anode

We demonstrate a high-performance flexible organic light-emitting diode (OLED) employing amorphous indium zinc oxide (IZO) anode. The amorphous IZO on flexible polycarbonate (PC) substrate shows similar electrical conductivity and optical transmittance with commercial (ITO) glass, even though it was prepared at <50°C. Moreover, it exhibits little resistance change during 5000 bending cycles, demonstrating good mechanical robustness. A green phosphorescent OLED fabricated on amorphous IZO on flexible PC shows maximum external quantum efficiency of η ext = 13.7% and power efficiency of η p = 32.7 1m/W, which are higher than a device fabricated on a commercial ITO on glass (η ext = 12.4% and η p = 30.1 Im/W) and ITO on flexible PC (η et = 8.5% and η P = 14.1 1m/W). The mechanical robustness and low-temperature deposition of IZO combined with high OLED performance clearly manifest that the amorphous IZO is a promising anode material for flexible displays.

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...

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

Large-area organic solar cells with metal subelectrode on indium tin oxide anode

This study examined the effects of the electrode geometry combined with the cell area on the device performance. We systematically investigated the effects of cell area in organic solar cells (OSCs) by introducing of metal subelectrodes to reduce the resistive loss of indium tin oxide. The subelectrode defines the active area and works as the conducting electrode at the same time with very low resistance. The series resistance could be reduced significantly by using the subelectrode, yielding a power conversion efficiency of 2.6±0.3% up to the cell area of 4.08 cm2. This suggests that OSCs with subelectrode geometry can be used for evaluating new materials and processes with accurate measurements on the centimeter scale.

Rhenium oxide as an efficient p-dopant to overcome S-shaped current density-voltage curves in organic photovoltaics with a deep highest occupied molecular orbital level donor layer

Effect of p-dopants in a p-doped hole transport layer inserted between indium tin oxide and a donor layer of α,α′-bis(2,2-dicyanovinyl)-quinquethiophene with a deep highest occupied molecular orbital level is reported to remove the S-shape in the organic photovoltaics (OPV) cell. Among the p-dopants of ReO3, MoO3, WO3, and CuI, ReO3 possesses the largest work function and turns out to be the most efficient p-dopant to remove the S-shape of the current density-voltage curve in the OPV cells. The rest of the dopants could not get rid of the S-shape, even with a doping concentration of 25 mol. %. The difference among the dopants can be understood by the different charge generation efficiency of the dopants.

Enhanced sensitivity to near-infrared with high fill factor in small molecular organic solar cells

High efficiency near-infrared (NIR) absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is due to the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an x-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.