Hoo Keun Park

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.

Coaxial RuO2–ITO Nanopillars for Transparent Supercapacitor Application

Supercapacitive properties of ruthenium oxide (RuO2) nanoparticles electrodeposited onto the indium tin oxide (ITO) nanopillars were investigated. Compared to conventional planar current collectors, this coaxially nanostructured current collector-electrode system can provide increased contact for efficient charge transport, and the internanopillar spacing allows easy access of electrolyte ions. The morphological and electrochemical properties depended on the thickness of the RuO2 layers, i.e., the number of electrodeposition cycles. A maximum specific capacitance, Csp, of 1235 F/g at a scan rate of 50 mV/s was achieved for the 30-cycle deposited RuO2-ITO nanopillars. The other capacitive properties such as electrochemical reversibility and Csp retention at high scan rates also improved greatly.

2D SiN x photonic crystal coated Y 3 Al 5 O 12 :Ce 3+ ceramic plate phosphor for high-power white light-emitting diodes

This paper reports the optical effects of a two-dimensional (2D) SiNx photonic crystal layer (PCL) on Y3Al5O12:Ce3+ (YAG:Ce) yellow ceramic plate phosphor (CPP) in order to enhance the forward emission of YAG:Ce CPP-capped high-power white light-emitting diodes (LEDs). By adding the 2D SiNx PCL with a 580 nm lattice constant, integrated yellow emission was improved by a factor of 1.72 compared to that of a conventional YAG:Ce CPP capped on a blue LED cup. This enhanced forward yellow emission is attributed to increased extraction of yellow emission light and improved absorption of blue excitation light through Bragg scattering and/or the leaky modes produced by the 2D PCLs. The introduction of 2D PCL can also reduce the wide variation of optical properties as a function of both ambient temperature and applied current, compared to those of a high-power YAG:Ce CPP-capped LED.

Highly efficient phosphor-converted white organic light-emitting diodes with moderate microcavity and light-recycling filters

We demonstrate the combined effects of a microcavity structure and light-recycling filters (LRFs) on the forward electrical efficiency of phosphor-converted white organic light-emitting diodes (pc-WOLEDs). The introduction of a single pair of low- and high-index layers (SiO(2)/TiO(2)) improves the blue emission from blue OLED and the insertion of blue-passing and yellow-reflecting LRFs enhances the forward yellow emission from the YAG:Ce(3+) phosphors layers. The enhancement of the luminous efficacy of the forward white emission is 1.92 times that of a conventional pc-WOLED with color coordinates of (0.34, 0.34) and a correlated color temperature of about 4800 K.

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.

Bulk Heterojunction Formation between Indium Tin Oxide Nanorods and CuInS2 Nanoparticles for Inorganic Thin Film Solar Cell Applications

In this study, we developed a novel inorganic thin film solar cell configuration in which bulk heterojunction was formed between indium tin oxide (ITO) nanorods and CuInS(2) (CIS). Specifically, ITO nanorods were first synthesized by the radio frequency magnetron sputtering deposition method followed by deposition of a dense TiO(2) layer and CdS buffer layer using atomic layer deposition and chemical bath deposition method, respectively. The spatial region between the nanorods was then filled with CIS nanoparticle ink, which was presynthesized using the colloidal synthetic method. We observed that complete gap filling was achieved to form bulk heterojunction between the inorganic phases. As a proof-of-concept, solar cell devices were fabricated by depositing an Au electrode on top of the CIS layer, which exhibited the best photovoltaic response with a V(oc), J(sc), FF, and efficiency of 0.287 V, 9.63 mA/cm(2), 0.364, and 1.01%, respectively.

Wafer-Scale Growth of ITO Nanorods by Radio Frequency Magnetron Sputtering Deposition

We demonstrate synthesis of tin-doped indium oxide (ITO) nanorods on 2-in. glass wafers via radio frequency (rf)-magnetron sputtering deposition. The nanorods possessed a single-crystal structure of bixbyite, grew along the orientation of the cubic unit cell, and were vertically aligned to the substrates. Height and diameter of the nanorods were as large as ~810 nm and 40-100 nm, respectively. The morphological, structural, compositional, optical, and electrical properties of the ITO nanorods were examined with respect to growth temperature (25-500°C) and growth time (10-60 min). ITO nanorod films synthesized at 500°C exhibited excellent electrical and optical property such as a low sheet resistance (~41 Ω/□) and high transparency in the wavelength range of visible light (i.e., ~87% transmission at 550 nm). The facile approach to synthesize ITO nanorods at a large scale demonstrated in this work may find various applications including the fabrication of high performance optoelectronic devices.

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.