Ji Hye Oh

Synthesis of narrow-band red-emitting K2SiF6:Mn4+ phosphors for a deep red monochromatic LED and ultrahigh color quality warm-white LEDs

In this study, we synthesized and characterized narrow-band red-emitting K2SiF6:Mn4+ phosphors in order to improve the color qualities of warm white light-emitting diodes (LEDs). The deep red monochromatic LED was realized by fabricating a long wavelength pass dichroic filter (LPDF)-capped phosphor-converted LED (pc-LED) with a synthesized K2SiF6:Mn4+ phosphor. In addition, we introduced four-package white LEDs that combine InGaN blue (B) LED and LPDF-capped green (G), amber (A), and red (R) pc-LEDs to achieve the high color rendition at the warm white correlated color temperatures (CCTs, 2700 K) with the assistance of the narrow-band K2SiF6:Mn4+ red phosphor. We compared the optical properties, including the luminous efficacy (LE), luminous efficacy of radiation (LER), color rendering index (CRI), special CRI for strong red (R9), and color quality scale (CQS), of four-package white LEDs by varying the red pc-LED with one narrow-band red-emitting phosphor and five wide-band red-emitting phosphors. The RAGB four-package white LED using narrow-band red-emitting K2SiF6:Mn4+ phosphor exhibited high LE (107 lm W−1) and ultrahigh color qualities (CRI = 94, R9 = 93, and CQS = 93) at a CCT of 2700 K.

Toward scatter-free phosphors in white phosphor-converted light-emitting diodes

Scatter-free phosphors promise to suppress the scattering loss of conventional micro-size powder phosphors in white phosphor-converted light-emitting diodes (pc-LEDs). Large micro-size cube phosphors (~100 μm) are newly designed and prepared as scatter-free phosphors, combining the two scatter-free conditions of particles based on Mie’s scattering theory; the grain size or grain boundary was smaller than 50 nm and the particle size was larger than 30 μm. A careful evaluation of the conversion efficiency and packaging efficiency of the large micro-size cube phosphor-based white pc-LED demonstrated that large micro-size cube phosphors are an outstanding potential candidate for scatter-free phosphors in white pc-LEDs. The luminous efficacy and packaging efficiency of the Y3Al5O12:Ce3+ large micro-size cube phosphor-based pc-LEDs were 123.0 lm/W and 0.87 at 4300 K under 300 mA, which are 17% and 34% higher than those of commercial powder phosphor-based white LEDs (104.8 lm/W and 0.65), respectively. In addition, the introduction of large micro-size cube phosphors can reduce the wide variation in optical properties as a function of both the ambient temperature and applied current compared with those of conventional powder phosphor-based white LEDs.

Comparisons of the structural and optical properties of o-AgInS2, t-AgInS2, and c-AgIn5S8 nanocrystals and their solid-solution nanocrystals with ZnS

Cubic AgIn5S8 (c-AgIn5S8) as well as orthorhombic AgInS2 (o-AgInS2) and tetragonal AgInS2 (t-AgInS2) nanocrystal (NC) luminophores are successfully prepared by adjusting the stoichiometric [Ag] : [In] ratio at reaction temperatures ranging from 100 to 180 °C. The photoluminescence (PL) quantum yield (QY) of c-AgIn5S8 NCs reached 18.8%, which is much higher than that of o-AgInS2 and t-AgInS2. In this study, the identification and characterization of efficient c-AgIn5S8 luminophore NCs are reported for the first time. Broad PL bands, large Stokes shifts and long PL lifetimes indicate that the PL of c-AgIn5S8 as well as o-AgInS2 and t-AgInS2 NCs can be attributed to donor–acceptor (D–A) pair recombinations. The PL peak shifts and changes in the PL intensity depending on the excitation intensity and temperature confirm that the c-AgIn5S8 NCs comply with D–A pair recombinations. The PL emission of the NCs can be enhanced via a solid-solution with ZnS and the PL wavelength can be tuned by varying the stoichiometric [Ag] : [In] ratio and the preparation temperature. The best c-AgIn5S8–ZnS sample presents a PL emission in the range of 552–587 nm with a maximum QY of 61.3%. The luminous efficacy and color rendering index (CRI) of these AgIn5S8–ZnS NC-based white light-emitting diodes (LEDs) were respectively 53 lm W−1 and 74 at 3700 K under 60 mA. The development of an efficient AgIn5S8–ZnS NC to be applied as a color-converting material in white LEDs as demonstrated in this work provides the possibility of various potential applications in the fields of optic, optoelectronic and bio-related devices.

Synthesis and characterization of green Zn-Ag-In-S and red Zn-Cu-In-S quantum dots for ultrahigh color quality of down-converted white LEDs.

Eco-friendly green Zn-Ag-In-S (ZAIS) and red Zn-Cu-In-S (ZCIS) core/shell-like alloyed quantum dots (QDs) have been synthesized by a facile hot-injection method with a multiple injection approach. Broad full-width at half-maximum (fwhm) of the photoluminescence (PL) emission and tunability of the green ZAIS and red ZCIS QDs were obtained by adopting a low-temperature core growth and high-temperature multiple alloyed reaction. The alloyed green ZAIS and red ZCIS QDs reached PL quantum yields as high as 0.61 and 0.53; fwhm of the PL peaks were as wide as 81 and 106 nm, respectively. This demonstrates the practical realization of white down-converted light-emitting diodes (DC-LEDs), fully covering the whole visible wavelength range and the cyan gap, using two broad fwhm green ZAIS and red ZCIS QDs. We also characterized the vision and color performance using luminous efficacy (LE), color rendering index (CRI), special CRI for strong red (R9), and color quality scale (CQS) of white DC-LEDs incorporated with green ZAIS and red ZCIS QDs at the correlated color temperature (CCT) range of 2700-10 000 K. The tricolor white DC-LED using broad fwhm green-emitting ZAIS and red-emitting ZCIS core/shell-like alloyed QDs exhibits a moderate LE (31.2 lm/W) and ultrahigh color qualities (CRI = 97, R9 = 97, and CQS = 94) with warm white at a CCT of 3500 K.

Surface-Plasmon-Enhanced Band Emission of ZnO Nanoflowers Decorated with Au Nanoparticles

A simple strategy was used to enhance band emission through the transfer of defect emission from ZnO to Au by using the energy match between the defect emission of ZnO and the surface plasmon absorbance of Au NPs through decorating the surface of ZnO nanoflowers with Au nanoparticles (Au NPs). The ZnO nanostructure, which was comprised of six nanorods that were attached on one side in a flower-like fashion, was synthesized by using a hydrothermal method. The temperature-dependent morphology and detailed growth mechanism were studied. The influence of the density of the Au NPs that were deposited onto the surface of ZnO on photoluminescence was investigated to optimize the configuration of the ZnO/Au system in terms of the maximum band emission. The sequential transfer of defect energy from ZnO to Au and electron transfer from excited Au to ZnO was proposed as a possible mechanism for the enhanced band emission.

Evaluation of new color metrics: guidelines for developing narrow-band red phosphors for WLEDs

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are rapidly becoming more popular in the lighting industry due to their energy savings, long lifetimes and environmentally friendly characteristics. The color rendering index (CRI, Ra) and the visual energy efficiency (luminous efficacy of radiation, LER) are the critical criteria to be considered when developing novel red phosphors for use in warm white pc-WLEDs to replace incandescent and fluorescent lamps. In this regard, narrow-band red-emitting materials have been intensively developed in terms of the CRI and LER in an effort to complement the red deficiency of the widely commercialized Y3Al5O12:Ce3+ (YAG) phosphor-based pc-WLEDs. However, CRIs are limited in their inability to guarantee good saturated colors of illuminated objects under a warm white color. Instead of using CRI values as criteria, a two-measure system encompassing the color fidelity score (Rf) and the color gamut score (Rg) was developed and adopted as Illuminating Engineering Society of North America (IES) technical memorandum TM-30-2015 for correct evaluations of the color rendition and to guide the optimization of LED light sources. In this review, we summarize the recent findings on novel narrow-band red phosphors, the improved color and visual energy properties of these phosphors, and their ability to improve the optical properties of corresponding warm white pc-WLED lightings. To solve the complex problem of overestimation of high-CRI values, we discuss the ways in which the narrow-band red phosphors affect the new color metrics (Rf, Rg, and color icon) and the LERs of tri-color pc-WLEDs while varying the narrow scale and the blue-shifted peak position of red phosphors. These new color metrics and LER criteria provide guidelines with which many material and chemistry researchers can develop new red phosphors by optimizing the crystal structure, crystal rigidity, local symmetry, the number of available sites for activators, the selection of the host and activator, and other factors.

Analysis of circadian properties and healthy levels of blue light from smartphones at night

This study proposes representative figures of merit for circadian and vision performance for healthy and efficient use of smartphone displays. The recently developed figures of merit for circadian luminous efficacy of radiation (CER) and circadian illuminance (CIL) related to human health and circadian rhythm were measured to compare three kinds of commercial smartphone displays. The CIL values for social network service (SNS) messenger screens from all three displays were higher than 41.3 biolux (blx) in a dark room at night, and the highest CIL value reached 50.9 blx. These CIL values corresponded to melatonin suppression values (MSVs) of 7.3% and 11.4%, respectively. Moreover, smartphone use in a bright room at night had much higher CIL and MSV values (58.7 ~ 105.2 blx and 15.4 ~ 36.1%, respectively). This study also analyzed the nonvisual and visual optical properties of the three smartphone displays while varying the distance between the screen and eye and controlling the brightness setting. Finally, a method to possibly attenuate the unhealthy effects of smartphone displays was proposed and investigated by decreasing the emitting wavelength of blue LEDs in a smartphone LCD backlight and subsequently reducing the circadian effect of the display.

Realization of InP/ZnS quantum dots for green, amber and red down-converted LEDs and their color-tunable, four-package white LEDs

Eco-friendly InP/ZnS quantum dots (QDs) have been synthesized by the conventional hot injection method using a non-toxic and economic P(N(CH3)2)3 precursor. The carefully controlled synthesis of a series of InP/ZnS QDs was performed by varying the core-growth temperature and time, the [P]/[In] ratio, and the number of ZnS shell coatings. The full-width at half maximum (FWHM) of the photoluminescence (PL) emission peaks from orange-red InP/ZnS QDs can be reduced from 73 to 56 nm by increasing the [P]/[In] ratio from 1.5 to 3.0. This is because there is concurrent formation of InP nuclei during the step in which excess volatile P-precursor is injected. The triple-shell-coated InP/ZnS core–shell QDs of green (G), yellow (Y), and orange-red (OR) colors reached PL quantum yields as high as 0.50, 0.63, and 0.55; and FWHMs of PL peaks as narrow as 55, 76, and 71 nm, respectively. This is the first realization of a variety of efficient green, amber (A), and red (R) monochromatic, down-converted, light-emitting diodes (DC-LEDs) using InP/ZnS QDs. They are fabricated by simply capping a long-wave pass dichroic filter (LPDF) on top of the LED packing associated with each corresponding InP/ZnS QD. In this study, we also characterized the vision and color performance using luminous efficacy, color-rendering index (CRI), special CRI for a strong red (R9) and color quality scale of color-tunable, four-package white LEDs. These consisted of InP/ZnS QD-based G, A, and R monochromatic, LPDF-capped DC-LEDs and a blue InGaN LED. The good optical performance of the InP/ZnS QD-based monochromatic DC-LEDs and their four-package white LEDs could provide the possibility of applying environmentally clean InP/ZnS QDs in monochromatic LEDs in the wavelength ranges of the “green gap”; thereby creating high-quality-color, color-tunable, four-package white LEDs.

Excellent color rendering indexes of multi-package white LEDs

This study introduces multi-package white light-emitting diodes (LEDs) system with the ability to realize high luminous efficacy and an excellent color rendering index (CRI, R a) using the R B,M A B,M G B,M C B (R B,M A B,M G B,M denoted as a long-pass dichroic filter (LPDF)-capped, monochromatic red, amber and green phosphor converted-LED (pc-LED) pumped by a blue LED chip, and C B denoted as a cyan and blue mixed pc-LED pumped by a blue LED) system. The luminous efficacy and color rendering index (CRI) of multi-package white LED systems are compared while changing the concentration of the cyan phosphor used in the paste of a cyan-blue LED package and the driving current of individual LEDs in multi-package white LEDs at correlated color temperatures (CCTs) ranging from 6,500 K (cold white) to 2,700 K (warm white) using a set of eight CCTs as specified by the American National Standards Institute (ANSI) standard number C78.377-2008. A R B,M A B,M G B,M C B white LED system provides high luminous efficacy (≥ 96 lm/W) and a color rendering index (≥ 91) encompassing the complete CCT range. We also compare the optical properties of the R B,M A B,M G B,M C B system with those of the R B,M A B,M G B,M B and RAGB (red, amber, green, and blue semiconductor-type narrow-spectrum-band LEDs) systems. It can be expected that the cyan color added to a blue LED in multi-package white LEDs based on LPDF-capped, phosphor-converted monochromatic LEDs will meet the needs of the high-quality, highly efficient, full-color white LED lighting market in the near future.

Highly-efficient, tunable green, phosphor-converted LEDs using a long-pass dichroic filter and a series of orthosilicate phosphors for tri-color white LEDs

This study introduces a long-pass dichroic filter (LPDF) on top of a phosphor-converted LED (pc-LED) packing associated with each corresponding tunable orthosilicate ((Ba,Sr)2SiO4:Eu) phosphor in order to fabricate tunable green pc-LEDs. These LPDF-capped green pc-LEDs provide luminous efficacies between 143–173 lm/W at 60 mA in a wavelength range between 515 and 560 nm. These tunable green pc-LEDs can replace green semiconductor-type III-V LEDs, which present challenges with respect to generating high luminous efficacy. We also introduce the highly-efficient tunable green pc-LEDs into tri-color white LED systems that combine an InGaN blue LED and green/red full down-converted pc-LEDs. The effect of peak wavelength in the tunable green pc-LEDs on the optical properties of a tri-color package white LED is analyzed to determine the proper wavelength of green color for tri-color white LEDs. The tri-color white LED provides excellent luminous efficacy (81.5–109 lm/W) and a good color rendering index (64–87) at 6500 K of correlated color temperature (CCT) with the peak wavelength of green pc-LEDs. The luminous efficacy of the LPDF-capped green monochromatic pc-LED and tri-color package with tunable green pc-LEDs can be increased by improving the external quantum efficiency of blue LEDs and the conversion efficiency of green pc-LEDs.