Pyung Eun Jeon

White-light generation through ultraviolet-emitting diode and white-emitting phosphor

White-light-emitting diodes are fabricated by using 375nm emitting InGaN chip with Sr3MgSi2O8:Eu2+ (blue and yellow) or Sr3MgSi2O8:Eu2+, Mn2+ (blue, yellow, and red). At a color temperature of 5892K, the color coordinates are x=0.32, y=0.33, and the color rendering index is 84%; at a color temperature of 4494K, the color coordinates are x=0.35, y=0.33, and the color rendering index is 92%. The blue (470nm) and yellow (570nm) emission bands are originated from Eu2+ ions, while the red (680) emission band is originated from Mn2+ ions in Sr3MgSi2O8 host. The energy transfer among three bands occurs due to the spectral overlap between emission and absorption bands. It is confirmed by the faster decay time of the energy donor. Our white-light-emitting diodes show higher color reproducibility, higher color stability on forward-bias current, and excellent color rendering index in comparison with a commercial YAG:Ce3+-based white-light-emitting diode.

The origin of the hole injection improvements at indium tin oxide/molybdenum trioxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl- 4,4′-diamine interfaces

We investigated the interfacial electronic structures of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) using in situ ultraviolet and x-ray photoemission spectroscopy to understand the origin of hole injection improvements in organic light-emitting devices (OLEDs). Inserting a MoO3 layer between ITO and NPB, the hole injection barrier was remarkably reduced. Moreover, a gap state in the band gap of NPB was found which assisted the Ohmic hole injection at the interface. The hole injection barrier lowering and Ohmic injection explain why the OLED in combination with MoO3 showed improved performance.

GaN-Based White-Light-Emitting Diodes Fabricated with a Mixture of Ba3MgSi2O8:Eu2+ and Sr2SiO4:Eu2+ Phosphors

The photoluminescence (PL) spectra of Ba3MgSi2O8:Eu2+ show one peak at 442 nm and two unresolved peaks at 505 nm. The 442 nm peak is attributed to the 4f→5d transition of the Eu2+ ion doped in the Ba2+(I) site with a weak crystal field, while the 505 nm peak originates from Eu2+ ions on the Ba2+(II) or the Ba2+(III) site with a strong crystal field. The PL spectra of the Sr2SiO4:Eu2+ show two emission peaks at 470 nm and 560 nm. The emission intensity at 470 nm decreases with increasing Eu2+ concentration, while that at 560 nm increases. This can be understood by considering the energy transfer from the 470 nm band to the 560 nm band through multipolar interaction. The GaN-based white-light-emitting diode (LED) fabricated using a mixture of Ba3MgSi2O8:Eu2+ and Sr2SiO4:Eu2+ phosphors has a broad-band spectrum, higher color rendering index and higher color stability against forward bias currents than Y3Al5O12:Ce3+-based white-LEDs.

The interface state assisted charge transport at the MoO 3 /metal interface

The interface formation between a metal and MoO(3) was examined. We carried out in situ ultraviolet and x-ray photoemission spectroscopy with step-by-step deposition of MoO(3) on clean Au and Al substrates. The MoO(3) induces huge interface dipoles, which significantly increase the work functions of Au and Al surfaces. This is the main origin of the carrier injection improvement in organic devices. In addition, interface states are observed at the initial stages of MoO(3) deposition on both Au and Al. The interface states are very close to the Fermi level, assisting the charge transport from the metal electrode. This explains that thick MoO(3) layers provide good charge transport when adopted in organic devices.

Color Tunability and Stability of Silicate Phosphor for UV-Pumped White LEDs

White-light-emitting diodes (LEDs) are fabricated by using 375 nm emissive GaN chip with Sr 3 MgSi 2 O 8 :Eu 2 + (blue and yellow) or Sr 3 MgSi 2 O 8 :Eu 2 + , Mn 2 + (blue, yellow, and red). The white lights from cold to warm color are actualized by controlling the mole fractions of Eu 2 + and Mn 2 + ions in Sr 3 MgSi 2 O 8 . Color tuning is based on the energy transfer from blue emission to yellow absorption, and from blue emission to red absorption due to spectral overlap between them. The energy transfers can be confirmed by faster decay time of the blue emission band. Our white LEDs show warm white light (3600 K), excellent color rendering index (95%), and higher color stability to variation of forward-bias voltage in comparison with a commercial white YAG:Ce 3 + -based diode.

Revised hole injection mechanism of a thin LiF layer introduced between pentacene and an indium tin oxide anode

Hole injection enhancement has been reported for organic thin-film transistors and light-emitting diodes at the indium tin oxide (ITO) anode side by introducing a LiF layer, which is usually used as an electron injection layer at the cathode side to reduce the electron injection barrier. We report a revised mechanism for the hole injection enhancement by studying a prototype interface of pentacene/LiF/ITO anode. Upon deposition of LiF on ITO, the work function of ITO decreases, and energy level realignment occurs between the pentacene and ITO. The hole injection barrier from the ITO to the pentacene highest occupied molecular orbital increases significantly with LiF insertion. Thus, the reduction in the hole injection barrier is not a critical factor for the hole injection enhancement. We suggest that a LiF insulating buffer layer enhances both injection barriers and tunneling through the barrier when a bias is applied.

Energy level alignments at tris(8-hydroquinoline) aluminum/8-hydroquinolatolithium/aluminum interfaces

The electronic structures of tris(8-hydroquinoline) aluminum (Alq3)∕8-hydroquinolatolithium (Liq)/Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopy. We constructed complete energy level diagrams and analyzed chemical interactions at the interface. When Liq was inserted between Al and Alq3, the electron injection barrier was reduced by 0.56eV compared to the structure without Liq. Additionally, a gap state was observed in the gap of Liq, which is related to an interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Liq between them.

The reduction of effective doping with extra dopant: n-Type doping of tris(8-hydroxyquinoline) aluminum with K

We studied the n-type doping effect of K deposited on tris(8-hydroxyquinoline) aluminum (Alq3), which has been used for efficient organic semiconducting devices for the last decade. The K doped or inserted at the interface region of the Alq3/cathode has shown highly enhanced device characteristics and yet, peculiarly, extra doping of K has always deteriorated the device properties. We study the interfacial electronic structures of the Alq3–K system using in situ photoemission spectroscopy and a theoretical model to understand the origin of such deterioration. As the K doping progresses, the lowest unoccupied molecular orbital (LUMO) of pristine Alq3 is gradually filled and it becomes an occupied gap state. This reduction of LUMO density of states makes the electron injection diminished, which is the origin of the device deterioration.