Chun Che Lin

Advances in Phosphors for Light-emitting Diodes

Light-emitting diodes (LEDs) are excellent candidates for general lighting because of their rapidly improving efficiency, durability, and reliability, their usability in products of various sizes, and their environmentally friendly constituents. Effective lighting devices can be realized by combining one or more phosphor materials with chips. Accordingly, it is very important that the architecture of phosphors be developed. Although numerous phosphors have been proposed in the past several years, the range of phosphors that are suitable for LEDs is limited. This work describes recent progress in our understanding of the prescription, morphology, structure, spectrum, and packaging of such phosphors. It suggests avenues for further development and the scientific challenges that must be overcome before phosphors can be practically applied in LEDs.

Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes

Mn(4+)-activated fluoride compounds, as an alternative to commercial (oxy)nitride phosphors, are emerging as a new class of non-rare-earth red phosphors for high-efficacy warm white LEDs. Currently, it remains a challenge to synthesize these phosphors with high photoluminescence quantum yields through a convenient chemical route. Herein we propose a general but convenient strategy based on efficient cation exchange reaction, which had been originally regarded only effective in synthesizing nano-sized materials before, for the synthesis of Mn(4+)-activated fluoride microcrystals such as K2TiF6, K2SiF6, NaGdF4 and NaYF4. Particularly we achieve a photoluminescence quantum yield as high as 98% for K2TiF6:Mn(4+). By employing it as red phosphor, we fabricate a high-performance white LED with low correlated colour temperature (3,556 K), high-colour-rendering index (Ra=81) and luminous efficacy of 116 lm W(-1). These findings show great promise of K2TiF6:Mn(4+) as a commercial red phosphor in warm white LEDs, and open up new avenues for the exploration of novel non-rare-earth red emitting phosphors.

Light Converting Inorganic Phosphors for White Light-Emitting Diodes

White light-emitting diodes (WLEDs) have matched the emission efficiency of florescent lights and will rapidly spread as light source for homes and offices in the next 5 to 10 years. WLEDs provide a light element having a semiconductor light emitting layer (blue or near-ultraviolet (nUV) LEDs) and photoluminescence phosphors. These solid-state LED lamps, rather than organic light emitting diode (OLED) or polymer light-emitting diode (PLED), have a number of advantages over conventional incandescent bulbs and halogen lamps, such as high efficiency to convert electrical energy into light, reliability and long operating lifetime. To meet with the further requirement of high color rendering index, warm light with low color temperature, high thermal stability and higher energy efficiency for WLEDs, new phosphors that can absorb excitation energy from blue or nUV LEDs and generate visible emissions efficiently are desired. The criteria of choosing the best phosphors, for blue (450−480 nm) and nUV (380−400 nm) LEDs, strongly depends on the absorption and emission of the phosphors. Moreover, the balance of light between the emission from blue-nUV LEDs and the emissions from phosphors (such as yellow from Y3Al5O12:Ce3+) is important to obtain white light with proper color rendering index and color temperature. Here, we will review the status of phosphors for LEDs and prospect the future development.

Thermally stable luminescence of KSrPO4:Eu2+ phosphor for white light UV light-emitting diodes

A novel blue phosphor based on phosphate host matrix, KSrPO4 doped with Eu2+, was prepared by solid state reaction. The phosphor invariably emits blue luminescence with a peak wavelength at 424nm under ultraviolet excitation at 360nm. Eu2+-doped KSrPO4 phosphors show higher thermally stable luminescence which was found to be better than commercially available Y3Al5O12:Ce3+ phosphor at temperature higher than 225°C.

Critical Red Components for Next-Generation White LEDs.

Warm white LEDs with a high color rendering index and a low correlated color temperature have undergone rapid development. In this regard, red-emitting materials-such as fluoride phosphors, namely, A2MF6:Mn(4+) (A = K, Na, and Cs; M = Si, Ge, Zr, Sn, and Ti) and XSiF6:Mn(4+) (X = Ba or Zn), nitridoaluminate phosphor (Sr[LiAl3N4]:Eu(2+)), and nanocrystals of cesium lead iodide perovskite (CsPbI3)-have been extensively investigated recently. These compounds generate narrow emissions in the visible red spectral region that are highly perceived by the human eye and lead to excellent chromatic saturation of the red spectra. This paper describes the structure, luminescence properties, morphologies, thermal features, and moisture resistance of critical red components, as well as their limitations for practical applications. This Perspective also provides a basis for future development and scientific challenges in optical research.

Combinatorial approach to the development of a single mass YVO4: Bi3+,Eu3+ Phosphor with red and green dual colors for high color rendering white light-emitting diodes

Instead of developing a novel red phosphor individually, this work proposes the production of white light by combining a near-ultraviolet/ultraviolet diode chip with blue and special yellow phosphors: the yellow phosphor includes the red and green components with high color saturation. The availability of this scheme is demonstrated by preparing a white light-emitting diode (WLED) with color rendering index (Ra) up to 90.3. The desired single-mass yellow phosphor is successfully screened out from the YVO(4):Bi(3+),Eu(3+) system by using a combinatorial chemistry approach. When the emission color and luminous efficiency are both considered, the best composition for producing white light is (Y(1-s-t)Bi(s)Eu(t))VO(4) with 0.040 < or = s < or = 0.050 and 0 < t < or = 0.015. The red component that is required for a high-Ra WLED is obtained through sensitizing luminescence of Eu(3+) by Bi(3+) in a YVO(4) host; meanwhile, both Bi(3+) and Eu(3+) emission are improved by keeping the Bi(3+) and Eu(3+) contents close to the critical concentration.

Emission-tunable CuInS2/ZnS quantum dots: structure, optical properties, and application in white light-emitting diodes with high color rendering index.

Synthesis and application of CuInS2/ZnS core/shell quantum dots (QDs) with varying [Cu]/[In] ratios were conducted using a stepwise solvothermal route. CuInS2 (CIS) core QDs with varying [Cu]/[In] ratios exhibited deep-red emissions result from donor-acceptor pair recombination. The absorption and emission band gap of the CuInS2 QDs increased with the decrease in Cu content. The emission bands of the CuInS2/ZnS were tuned from 550 to 616 nm by controlling the [Cu]/[In] ratio after coating ZnS layer. The CIS QDs model was developed to elucidate the synthesized crystal structure and photoluminescence of the QDs with various [Cu]/[In] ratios. Temperature-dependent photoluminescence spectra of the CIS/ZnS QDs were also investigated. The temperature dependency of the photoluminescence energy and intensity for various CIS/ZnS QDs were studied from 25 to 200 °C. Efficient white light-emitting diodes with high color rendering index values (Ra = 90) were fabricated using CIS/ZnS QDs as color converters in combination with green light-emitting Ba2SiO4:Eu(2+) phosphors and blue light-emitting diodes.

Synthesis of Na2SiF6:Mn4+ red phosphors for white LED applications by co-precipitation

A one-step approach to synthesize Na2SiF6:Mn4+and K2SiF6:Mn4+ red phosphors by co-precipitation is reported in this paper. The phosphors were precipitated from a silicon fluoride solution with NaF and Na2MnO4 (Na2SiF6:Mn4+ preparation) or KF and K2MnO4 (K2SiF6:Mn4+ preparation) using H2O2 to reduce Mn7+ to Mn4+ at room temperature. Na2SiF6:Mn4+ was also prepared through a convenient two-step route with K2MnF6 as a raw material. The obtained Na2SiF6:Mn4+ phosphors have hexagonal structures with space group D32-P321 and no impurity phase when they were examined via X-ray diffraction. Photoluminescence, photoluminescence excitation, thermal luminescence, and luminescence decay time were considered to determine the optical properties of the fluoride complexes. The prepared phosphors exhibited bright red emission under 460 nm light excitation and low-thermal quenching (∼92% of the luminescent intensity at 423 K). Increasing the concentration of Mn4+ enhanced the luminescence intensity. A warm white light LED with high color rendering index (Ra = 86 and R9 = 61) was fabricated by employing Na2SiF6:Mn4+ as red phosphors and commercial Y3Al5O12:Ce3+ as yellow phosphors on a blue-InGaN chip.

Near-ultraviolet excitable orange-yellow Sr3(Al2O5)Cl2:Eu2+ phosphor for potential application in light-emitting diodes

Sr3(Al2O5)Cl2 phosphor doped with Eu2+ was prepared by a soli-state reaction. This phosphor emits a broad orange-yellow luminescence with a peak wavelength of 620nm and a full width at half maximum of about 175nm under near-ultraviolet (NUV) excitation at ∼400nm. Yellow light-emitting diodes (LEDs) for general lighting were fabricated by combining Sr3(Al2O5)Cl2:Eu2+ phosphor with an NUV chip. The phosphor-converted LEDs had a color temperature of about 2300K and their color rendering index was 74.

A low-temperature co-precipitation approach to synthesize fluoride phosphors K2MF6:Mn4+ (M = Ge, Si) for white LED applications

A new class of Mn4+ activated alkali-metal hexafluoride red phosphors are emerging for white light-emitting diodes because of their sharp red line 2Eg → 4A2g emissions (600–650 nm) excited by irradiation of 4A2g → 4T1g (320–380 nm) and 4A2g → 4T2g (380–500 nm) transitions. However, these phosphors have the drawbacks of difficult control of the Mn valence state during synthesis and lack of underlying mechanisms for structure–photoluminescence relationships. In this study, we explore a novel, highly productive route to the quantifiable synthesis of K2GeF6:Mn4+ by the chemical co-precipitation method at room temperature. The prepared yellowish K2GeF6:Mn4+ powders exhibit a hexagonal shape and high crystallinity without significant defects. The photoluminescence thermal stability and white light-emitting diodes applicability of K2GeF6:Mn4+ suggest that it is a promising commercial red phosphor because of its efficient emission intensity, high color purity and excellent thermal stability. Structural analyses and theoretical calculations reveal that the red shift of the K2GeF6:Mn4+ red phosphor compared with K2SiF6:Mn4+ is due to the longer Ge–F distance and lower effective Mulliken charge of F ions in coordination environments of the MnF62− octahedron. The split feature in K2GeF6:Mn4+ is due to the hexagonal distortion in the host. The structure–photoluminescence mechanism is predicted to be general in hexafluoride red phosphors to tune the optical properties through cationic substitutions and crystal structure adjustments.