Hee Chang Yoon

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.

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.

Color-by-blue display using blue quantum dot light-emitting diodes and green/red color converting phosphors

We report a novel full-color display based on the generation of full-color by a highly efficient blue QD-LED light approach, or so called color-by-blue QD-LED display. This newly proposed color-by-blue QD-LED display combines a blue CdZnS/ZnS QD-LED blue subpixel and excitation source with front-emitting green/red phosphor subpixels. It is carefully estimated that the detailed display characteristics as well as full color-conversion and reasonable device efficiency of blue, green, and red satisfy the minimum requirements for display application. Also, we would like to emphasize that the proposed blue, green, and red device shows maximum luminance of 1570, 12920, and 3120 cd/m², respectively, luminous efficiency of 1.5, 12.1, and 2.5 cd/A, respectively, and external quantum efficiency of 6.8, 2.8, and 2.0%, respectively. It is expected that full color generation by color-by-blue QD-LED will lead to further technological advancements in the area of efficient and facile display applications.

Sn–P–F containing glass matrix for the fabrication of phosphor-in-glass for use in high power LEDs

In this study, we introduce a low-melting-point (MP) glass ceramic (GlaC) material which can be used to fabricate a phosphor-in-glass (PiG) with Y3Al5O12:Ce3+ (YAG) phosphor for the realization of high-power white-light-emitting diodes (WLEDs). PiG using a low-MP Sn–P-rich GlaC material, with simple fabrication at 230 °C, can address the drawbacks of the phosphor thermal degradation of currently developed PiGs with a high MP of more than 400 °C, and the yellowing of commercialized phosphor-in-silicon binder (PiSB) materials. In addition, the low-MP PiG can serve as a heat sink due to its good thermal conductivity and good stability compared to those of commercialized PiSB materials. The optical properties of a low-MP PiG-based WLED are a luminous efficacy (LE) of 118 lm W−1, an external quantum efficiency (EQE) of 0.33 and a color-rendering index (CRI) of 68 at 4275 K at an applied current of 350 mA. The simply fabricated and stable PiG-based WLED using the low-MP GlaC material will be a competitive candidate in the WLED lighting market.

Color-tunable Ag-In-Zn-S quantum-dot light-emitting devices realizing green, yellow and amber emissions

In this study, we report the creation of heterostructured alloy core/shell Ag-In-Zn-S (AIZS)/ZnS quantum dots (QDs) by sequential core-forming, alloying and shelling processes and the fabrication of color-tunable QD light-emitting diodes (QLEDs) with a standard device architecture. Three different (green, yellow, and amber, denoted as G, Y, and A) AIZS/ZnS QDs were successfully prepared at different core-forming, alloying and shelling temperatures and time periods. The QDs were then incorporated into a solution-processed bottom-emitting QLED structure, where the three-colored QD emissive layers (EMLs) are sandwiched in the same structure with ZnO nanoparticles (NPs) and poly(9-vinlycarbazole) (PVK) as electron and hole-transport layers, respectively. The G, Y, and A AIZS/ZnS QLEDs exhibit the highest ever luminance levels of 999, 698, and 498 cd m−2 and current efficiency (CE) rates of 1.12, 1.17, and 0.36 cd A−1, respectively. Despite the fact that both an appreciable hole barrier and an electron barrier are established in the QD emitting layer (EML) and the hole/electron transport layer, respectively, the three colored QLEDs were successfully activated. Moreover, the electroluminescence (EL) performances of tunable G, Y, and A QLEDs can be distinctly measured using any EL spectrophotometer. The realization of tunable color and moderate brightness as well as the potential all-solution processability using AIZS/ZnS QDs can provide a strong opportunity for future researchers to devise new strategies involving II–VI QD-based QLEDs as a new generation of wide-bandwidth QDs for general lighting applications with high color-rendering indices.

Circadian-tunable Perovskite Quantum Dot-based Down-Converted Multi-Package White LED with a Color Fidelity Index over 90

New metrics of the color and circadian performances of down-converted white light-emitting diodes (DC-WLEDs) are rapidly becoming popular in smart lighting systems. This is due to the increased desire for accurate analytical methods to measure the effects of newly developed quantum dot (QD)-based lighting on the vision, color, and circadian sensors of retina cells in the human eye. In this regard, a two-measure system known as technical memorandum TM-30-2015 (Illuminating Engineering Society of North America), encompassing the color fidelity index (CFI, Rf) and the color gamut index (CGI, Rg), has been developed as a new metrics of color to replace the currently utilized color rendering index (CRI, Ra). In addition, the tunability of the circadian efficacy of radiation (CER) is now more important due to its effect on the control of melatonin suppression/secretion, resetting of the central/local clocks of individuals given their daily cycles, and benefits to human health. In this paper, we developed and analyzed six-colored perovskite (Pe; cyan, green, yellowish green, amber, orange, and red colors) QDs-based multi-package WLED, and optimized the SPDs of tunable PeQD-based multi-package WLEDs in terms of promising human-centric lighting device, given its optimized visual energy, color qualities and health-promoting effects.

Multiple-Color-Generating Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells via Dichroic Film Incorporation for Power-Generating Window Applications

There are four prerequisites when applying all types of thin-film solar cells to power-generating window photovoltaics (PVs): high power-generation efficiency, longevity and high durability, semitransparency or partial-light transmittance, and colorful and aesthetic value. Solid-type thin-film Cu(In,Ga)S2 (CIGS) or Cu(In,Ga)(S,Se)2 (CIGSSe) PVs nearly meet the first two criteria, making them promising candidates for power-generating window applications if they can transmit light to some degree and generate color with good aesthetic value. In this study, the mechanical scribing process removes 10% of the window CIGSSe thin-film solar cell with vacant line patterns to provide a partial-light-transmitting CIGSSe PV module to meet the third requirement. The last concept of creating distinct colors could be met by the addition of reflectance colors of one-dimensional (1D) photonic crystal (PC) dichroic film on the black part of a partial-light-transmitting CIGSSe PV module. Beautiful violets and blues were created on the cover glass of a black CIGSSe PV module via the addition of 1D PC blue-mirror-yellow-pass dichroic film to improve the aesthetic value of the outside appearance. As a general result from the low external quantum efficiency (EQE) and absorption of CIGSSe PVs below a wavelength of 400 nm, the harvesting efficiency and short-circuit photocurrent of CIGSSe PVs were reduced by only ∼10% without reducing the open-circuit voltage (VOC) because of the reduced overlap between the absorption spectrum of CIGSSe PV and the reflectance spectrum of the 1D PC blue-mirror-yellow-pass dichroic film. The combined technology of partial-vacancy-scribed CIGSSe PV modules and blue 1D PC dichroic film can provide a simple strategy to be applied to violet/blue power-generating window applications, as such a strategy can improve the transparency and aesthetic value without significantly sacrificing the harvesting efficiency of the CIGSSe PV modules.

High color rendering index of remote-type white LEDs with multi-layered quantum dot-phosphor films and short-wavelength pass dichroic filters

This paper introduces high color rendering index (CRI) white light-emitting diodes (W-LEDs) coated with red emitting (Sr,Ca)AlSiN3:Eu phosphors and yellowish-green emitting AgIn5S8/ZnS (AIS/ZS) quantum dots (QDs) on glass or a short-wavelength pass dichroic filter (SPDF), which transmit blue wavelength regions and reflect yellow wavelength regions. The red emitting (Sr,Ca)AlSiN3:Eu phosphor film is coated on glass and a SPDF using a screen printing method, and then the yellowish-green emitting AIS/ZS QDs are coated on the red phosphor (Sr,Ca)AlSiN3:Eu film-coated glass and SPDF using the electrospray (e-spray) method.To fabricate the red phosphor film, the optimum amount of phosphor is dispersed in a silicon binder to form a red phosphor paste. The AIS/ZS QDs are mixed with dimethylformamide (DMF), toluene, and poly(methyl methacrylate) (PMMA) for the e-spray coating. The substrates are spin-coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to fabricate a conductive surface. The CRI of the white LEDs is improved through inserting the red phosphor film between the QD layer and the glass substrate. Furthermore, the light intensities of the multi-layered phosphor films are enhanced through changing the glass substrate to the SPDF. The correlated color temperatures (CCTs) vary as a function of the phosphor concentration in the phosphor paste. The optical properties of the yellowish-green AIS/ZS QDs and red (Sr,Ca)AlSiN3:Eu phosphors are characterized using photoluminescence (PL), and the multi-layered QD-phosphor films are measured using electroluminescence (EL) with an InGaN blue LED (λmax = 450 nm) at 60 mA.

Low-Yellowing Phosphor-in-Glass for High-Power Chip-on-board White LEDs by Optimizing a Low-Melting Sn-P-F-O Glass Matrix

We introduce a low-melting-point (MP) Sn-P-F-O glass ceramic material into the phosphor-in-glass (PIG) material to realize an ‘on-chip’ chip-on-board (COB) type of phosphor-converted (pc) white light-emitting diode (WLED) with green (BaSr)2SiO4:Eu2+ and red (SrCa)AlSiN3:Eu2+ (SCASN) phosphors. The optimum Sn-P-F-O-based ceramic components can be sintered into the glass phase with a facile one-step heating process at 285 °C for 1 min. Specifically, these soft-fabrication conditions can be optimized to minimize the degradation of the luminescent properties of the red SCASN phosphor as well as the green silicate phosphor in PIG-based white COB-type pc-LEDs owing to the low thermal loss of the phosphors at low fabrication temperatures below 300 °C. Moreover, the constituents of the COB package, in this case the wire bonding and plastic exterior, can be preserved simultaneously from thermal damage. That is, the low sintering temperature of the glass ceramic encapsulant is a very important factor to realize excellent optical qualities of white COB LEDs. The optical performances of low-MP Sn-P-F-O-based PIG on-chip COB-type pc-WLEDs exhibit low yellowing phenomena, good luminous efficacy of 70.9–86.0 lm/W, excellent color rendering index of 94–97 with correlated color temperatures from 2700 to 10000 K, and good long-term stability.

Efficient and Stable CsPbBr3 Quantum-Dot Powders Passivated and Encapsulated with a Mixed Silicon Nitride and Silicon Oxide Inorganic Polymer Matrix

Despite the excellent optical features of fully inorganic cesium lead halide (CsPbX3) perovskite quantum dots (PeQDs), their unstable nature has limited their use in various optoelectronic devices. To mitigate the instability issues of PeQDs, we demonstrate the roles of dual-silicon nitride and silicon oxide ligands of the polysilazane (PSZ) inorganic polymer to passivate the surface defects and form a barrier layer coated onto green CsPbBr3 QDs to maintain the high photoluminescence quantum yield (PLQY) and improve the environmental stability. The mixed SiN x/SiN xO y/SiO y passivated and encapsulated CsPbBr3/PSZ core/shell composite can be prepared by a simple hydrolysis reaction involving the addition of adding PSZ as a precursor and a slight amount of water into a colloidal CsPbBr3 QD solution. The degree of the moisture-induced hydrolysis reaction of PSZ can affect the compositional ratio of SiN x, SiN xO y, and SiO y liganded to the surfaces of the CsPbBr3 QDs to optimize the PLQY and the stability of CsPbBr3/PSZ core/shell composite, which shows a high PLQY (∼81.7%) with improved thermal, photo, air, and humidity stability as well under coarse conditions where the performance of CsPbBr3 QDs typically deteriorate. To evaluate the suitability of the application of the CsPbBr3/PSZ powder to down-converted white-light-emitting diodes (DC-WLEDs) as the backlight of a liquid crystal display (LCD), we fabricated an on-package type of tricolor-WLED by mixing the as-synthesized green CsPbBr3/PSZ composite powder with red K2SiF6:Mn4+ phosphor powder and a poly(methyl methacrylate)-encapsulating binder and coating this mixed paste onto a cup-type blue LED. The fabricated WLED show high luminous efficacy of 138.6 lm/W (EQE = 51.4%) and a wide color gamut of 128% and 111% without and with color filters, respectively, at a correlated color temperature of 6762 K.