Ho Seong Jang

White Light‐Emitting Diodes with Excellent Color Rendering Based on Organically Capped CdSe Quantum Dots and Sr3SiO5:Ce3+,Li+ Phosphors

White LEDs are fabricated through a combination of blue LEDs with Sr3 SiO5 :Ce(3+) ,Li(+) phosphors and organically capped CdSe quantum dots (QDs), which exhibit a 2D superlattice. The combination of blue emission from the LEDs, greenish-yellow emission from the phosphors, and red emission from the QDs generates white light. As-synthesized white LEDs show excellent color rendering properties and stable color coordinates against increasing forward bias currents.

Yellow-emitting Sr3SiO5:Ce3+,Li+ phosphor for white-light-emitting diodes and yellow-light-emitting diodes

In this letter, a yellow-emitting Sr3SiO5:Ce3+,Li+ phosphor is reported. Through transitions of 5d→4f (F7∕22 and F5∕22) in Ce3+, the phosphor showed a very broad and strong yellow emission under near ultraviolet (UV) or blue light excitation. The energy levels of Ce3+ in Sr3SiO5 were suggested from its absorption and excitation spectra. White light could be obtained by combining this phosphor with 460 or 405nm light-emitting diodes (LEDs) [(x,y)=(0.3086,0.3167) or (0.3173, 0.3103)]. Additionally, a yellow LED was fabricated using a near-UV LED (380nm chip) with Sr3SiO5:Ce3+,Li+.

Tunable full-color-emitting La0.827Al11.9O19.09:Eu2+,Mn2+ phosphor for application to warm white-light-emitting diodes

La0.827Al11.9O19.09:Eu2+,Mn2+ showed three emission bands when excited by ultraviolet light. A blue emission originates from Eu2+, whereas both green and red emissions originate from Mn2+. The luminescent mechanism is explained invoking the energy transfer of Eu2+→Mn2+ and Mn2+(tetrahedralsite)→Mn2+(octahedralsite). This energy transfer was confirmed by faster decay times of the blue and green emissions as energy donors. The emitted color of La0.827Al11.9O19.09:Eu2+,Mn2+ can be easily tailored from blue to red through variations of the Mn2+ content. The white-light-emitting diode fabricated via a combination of an ultraviolet light-emitting diode (λpeak=385nm) with La0.827Al11.9O19.09:Eu2+,Mn2+ showed a warm white light (Tc=3559K).

Effect of phosphor geometry on the luminous efficiency of high-power white light-emitting diodes with excellent color rendering property

High-power white light-emitting diodes (LEDs) are fabricated by combining blue LEDs and green (Ba,Sr)(2)SiO(4):Eu(2+) and red CaAlSiN(3):Eu(2+) phosphors with varying phosphor geometry. The white LED having separated the phosphor layer by the silicone gel layer between green and red phosphor layers shows superior optical properties. The luminous efficiency (eta(L)) is improved by a decrease of reabsorption of green light by red phosphor owing to a difference among refractive indices. The white LED shows very high eta(L) of 51 lm/W and a high color rendering index of 95 under 350 mA. In addition, improved luminous properties of the white LED including a separated phosphor layer are confirmed via ray-trace simulation.

Yellow-emitting γ-Ca 2 SiO 4 :Ce 3+ , Li + phosphor for solid-state lighting: luminescent properties, electronic structure, and white light-emitting diode application

A new yellow-emitting γ-Ca2SiO4:Ce3+,Li+ phosphor was synthesized via a solid-state reaction. The phosphor showed a strong yellow emission with a wide bandwidth of 135.4 nm under blue light excitation. Absorption and photoluminescence measurements and density functional theory calculations suggest that the luminescence of the phosphor can be attributed primarily to the transitions of 5d→4f (2F(7/2) and 2F(5/2)) of Ce3+ ions occupying Ca(1) sites in the host crystal. White light-emitting diodes (LEDs) were fabricated by combining this phosphor with a blue LED, and excellent white light with a high color rendering index of 86 was created owing to the wide emission bandwidth of the phosphor.

Emission Band Change of ( Sr1 − x M x ) 3SiO5 : Eu2 + ( M = Ca , Ba ) Phosphor for White Light Sources Using Blue/Near-Ultraviolet LEDs

The luminescence properties of orange-yellow-emitting Eu 2+ -activated Sr 3 SiO 5 were optimized for application to blue/near ultra- violet (n-UV) light-emitting diodes (LEDs). Sr 2.97 SiO 5 :Eu 2+ 0.03 showed strong orange-yellow emission peaking at ∼580 nm under blue and n-UV light of 450 and 405 nm, respectively, and the effects of Ca and Ba substitutions into Sr 2+ sites on the emission band change were investigated. For the substitution of Ca, Ca 3 SiO 5 :Eu 2+ showed no orange-yellow emission under blue light due to the different crystal structures of Ca 3 SiO 5 (monoclinic) and Sr 3 SiO 5 (tetragonal). The redshift and blueshift of the emission band of (Sr 1-x Ba x ) 3 SiO 5 :Eu 2+ were explained by the competition between crystal field effect and Nephelauxetic effect. The 460 nm-emitting blue LED-pumped white LED with Sr 3 SiO 5 :Eu 2+ or a mixture of Sr 3 SiO 5 :Eu 2+ and green phosphor (Ba 2 SiO 4 :Eu 2+ ) were fabricated and they showed color coordinates of (0.343, 0.281) and (0.349, 0.339), respectively.

White light emission from blue and near ultraviolet light-emitting diodes precoated with a Sr3SiO5 : Ce3+, Li+ phosphor

White-light-emitting diodes (WLEDs) were fabricated by combining a yellow Sr3SiO5:Ce3+, Li+ phosphor with a blue light-emitting diode (LED) (460 nm chip) or a near ultraviolet (n-UV) LED (405 nm chip), respectively. Color temperature (Tc) of Sr3SiO5:Ce3+, Li+-based WLEDs could be tuned from 6500 to 100,000 K (blue LED pumping) and from 4900 to 50,000 K (n-UV LED pumping) without mixing with other phosphors. The blue LED-pumped WLED showed excellent white light (luminous efficiency=31.7 lm/W, Tc=6857 K) at 20 mA. This WLED showed a stable color coordinates property against an increase of the forward current. An n-UV LED-pumped WLED also showed bright white light (25.0 lm/W, 5784 K) at 20 mA.

Bright three-band white light generated from CdSe/ZnSe quantum dot-assisted Sr3SiO5:Ce3+,Li+-based white light-emitting diode with high color rendering index

In this study, bright three-band white light was generated from the CdSe/ZnSe quantum dot (QD)-assisted Sr3SiO5:Ce3+,Li+-based white light-emitting diode (WLED). The CdSe/ZnSe core/shell structure was confirmed by energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy. The CdSe/ZnSe QDs showed high quantum efficiency (79%) and contributed to the high luminous efficiency (ηL) of the fabricated WLED. The WLED showed bright natural white with excellent color rendering property (ηL=26.8 lm/W, color temperature=6140 K, and color rendering index=85) and high stability against the increase in forward bias currents from 20 to 70 mA.

Red-Emitting LiLa2O2BO3 : Sm3 + , Eu3 + Phosphor for Near-Ultraviolet Light-Emitting Diodes-Based Solid-State Lighting

A red-emitting LiLa 2 O 2 BO 3 :Sm 3+ ,Eu 3+ phosphor has been synthesized and characterized optically for white light-emitting diodes (LEDs). Because the oxyborate group (O 2 BO -7 3 ) can contain high Eu 3+ concentration without concentration quenching, LiLa 2 O 2 BO 3 is a suitable host material for red-emitting phosphor for near-ultraviolet (n-UV) LEDs. When LiLa 2 O 2 BO 3 :Eu 3+ was co-doped with Sm 3+ , emission intensity of Eu 3+ was enhanced by ∼ 30% than that of the sample without Sm 3+ owing to the energy transfer from Sm 3+ to Eu 3+ . This energy transfer was confirmed by faster decay times of Sm 3+ as energy donors. LiLa 2 O 2 BO 3 :Sm 3+ ,Eu 3+ showed a relative brightness of 334.7% of that of Y 2 O 2 S:Eu 3+ , commercial red-emitting phosphor for n-UV LEDs.

Mechanism for strong yellow emission of Y3Al5O12:Ce3+ phosphor under electron irradiation for the application to field emission backlight units

This letter reports a strong yellow emission of Y3Al5O12:Ce3+ (YAG:Ce) mixed with ZnS:Ag,Cl under electron excitation. The penetration depths of electron of 1keV and photon of 2.7eV in YAG:Ce were estimated to be approximately 1450A and 4.65mm, respectively. Deeper penetration of blue light from ZnS:Ag,Cl helps to excite a larger number of Ce3+ in a mixture (ZnS:Ag,Cl+YAG:Ce), and YAG:Ce showed strong yellow emission via both cathodoluminescence and photoluminescence. The mixture showed the brightness of 120.5% compared to R,G,B phosphor mixture. This mixture of two phosphors was applied to a carbon nanotube field emission backlight unit.