Remote Phosphor Research Articles

Angular Color Uniformity Enhancement of White LEDs by Lens Wetting Phosphor Coating

In this letter, we proposed a remote phosphor coating method by lens wetting for enhancing the angular color uniformity of phosphor-converted white light-emitting diodes. Simulations based on the volume of fluid (VOF) method were applied to investigate the geometry evolution of a phosphor gel droplet after being dropped onto the lens, and the stable geometry of the phosphor gel was obtained. Monte Carlo ray tracing was used to study the optical performance of LED samples with the stable phosphor geometries. Besides, experiments were conducted to verify the fabrication flexibility of the proposed method. The VOF simulation and experimental results show that the proposed method can realize the hemispherical remote phosphor layers with uniform or non-uniform thickness by adjusting the coating volume. The optical simulations and the experimental measurements show that the LED samples with non-uniform thickness remote phosphor geometry achieves smaller correlated color temperature deviation (<;200 K at 5000 K) than the uniform thickness remote phosphor geometry.

White Light Emission from Planar Remote Phosphor Based on NHC Cycloplatinated Complexes.

We report on the generation of bright white luminescence through solid-state illumination of remote phosphors based on novel cycloplatinated N-heterocyclic carbene (NHC) compounds. Following a stepwise protocol we got the new NHC compound [{Pt(μ-Cl)(C(∧)C*)}2] (4) (HC(∧)C*-κC* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene), which was used together with the related ones 4a (HC(∧)C*-κC*= 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene) and 4b (HC(∧)C*-κC*= 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene) as starting materials for the synthesis of the new ionic derivatives [Pt(R-C(∧)C*) (CNR')2]PF6 (R = -COOEt, R' = t-Bu (5), Xyl (6); R = -CN, R' = t-Bu (7), Xyl (8); R(∧)C = Naph, R' = t-Bu (9), Xyl (10)). The X-ray structures of 6 and 8-10 have been determined. The photophysical properties of these cationic compounds have been studied and supported by the time-dependent-density functional theory (TD-DFT) calculations. The compounds 5, 8, and 9 have been revealed as the most efficient emitters in the solid state with quantum yields of 41%, 21%, and 40%, respectively. White-light remote-phosphors have been prepared just by stacking different combinations of these compounds and [Pt(bzq) (CN) (CN(t)Bu)] (R1) as blue (5, 8), yellow (9), and red (R1) components onto the same substrate. The CCT (correlated color temperature) and the CRI (color rendering index) of the emitted white-light have been tuned by accurately controlling the individual contributions.

Self-Adaptive Conformal–Remote Phosphor Coating of Phosphor-Converted White Light Emitting Diodes

Angular color uniformity (ACU) and luminous efficiency are the most important optical parameters of white light emitting diodes (WLEDs). In this paper, we proposed conformal–remote phosphor coating structure, including conformal silicone layer and phosphor layer, to improve ACU and luminous efficiency of phosphor-converted WLEDs (pc-WLEDs). Due to the effect of conformal silicone layer, the extraction efficiency of blue light increases and the thermal effect of phosphor layer decreases, which improve the luminous efficiency. In addition, the structure composed by conformal silicone layer and self-adaptive phosphor layer can better match spatial intensity distribution of the chip emitting light. The experimental results have verified that the conformal–remote phosphor coating structure shows very high ACU performance, compared with the conventional phosphor coating and self-adaptive remote phosphor coating structure. Besides, for the self-adaptive conformal–remote phosphor coating, the thickness of silicone layer absolutely had no effect on the luminous efficiency and ACU.

Concentric ring phosphor geometry on the luminous efficiency of white-light-emitting diodes with excellent color rendering property.

This study presents a novel remote phosphor design involving a concentric ring remote phosphor layer in which green and red phosphors are separated. The green and red phosphor rings were separately illuminated by blue light emitted from the light-emitting diode (LED), causing low reabsorption in phosphor-converted LEDs (pcLEDs) using green and red phosphors. The experimental results revealed that the pcLEDs with green and red phosphors showed high color rendering, indicating that the LEDs are suitable for certain medical applications and architectural lighting. Moreover, for given green/red phosphor ratio and weights of the green and red phosphors, the output power and luminous flux of the pcLED with a concentric ring remote phosphor layer were greater than those of the pcLED with a mixed remote phosphor layer. The reduction in the reabsorption of green emission by red phosphor in pcLED with a concentric ring remote phosphor layer was responsible for the high luminous flux and indicated a high correlated color temperature of pcLED.

Preparation and properties of the flexible remote phosphor film for blue chip-based white LED

A simple and effective method for larger area flexible and relatively uniform phosphor film layer for blue chip-based white LED is presented in this paper. Besides having a certain ultraviolet resistance and thermal stability, the proposed yttrium aluminum garnet (YAG) phosphor film also achieved applicable color temperature (Tc of 5480K and 4900K), color rendering index (CRI of 68.4 and 70) and luminous efficiency (121.7 and 77lm/W) when simply assembled with single blue chip and high-power COB separately. The preparation process and its test results suggested flexible phosphor film consists of YAG phosphor particles and common silicone without additional converter devices, which has positive practical significance for the packaging of remote phosphor-converted white LEDs.

Glass Remote Phosphor 구조를 갖는 백색 LED 패키지의 형광체 함량과 열처리 온도 최적화

Glass Remote Phosphor 구조를 갖는 백색 LED 패키지의 형광체 함량과 열처리 온도 최적화

Effect of Reassembled Remote Phosphor Geometry on the Luminous Efficiency and Spectra of White Light-Emitting Diodes With Excellent Color Rendering Property

This paper presents a scattered photon extraction (SPE) structure with a reassembled remote phosphor layer. The reassembled remote phosphor layer of the SPE structure was fabricated by combining green and red phosphor pieces with varying phosphor geometries, reducing the reabsorption of green emission by the red phosphor. The experimental results revealed that the SPE structure light-emitting diodes (LEDs) with the green and red phosphors in the remote phosphor layer demonstrated a high color rendering property. Moreover, with the same green and red phosphor weights and ratio in the remote phosphor layers, the output power and the luminous flux of the SPE structure LED with a reassembled remote phosphor layer were higher than those of SPE structure LEDs with mixed remote phosphor layers. The reduction in the reabsorption of green emission by red phosphor in the SPE structure with the reassembled remote phosphor layer was responsible for the high output power, and indicated a high correlated color temperature of the SPE structure with the reassembled remote phosphor layer. It was also found that the green emission with corresponding output power of the SPE structure LED containing the reassembled remote phosphor layer decreased with an increase in the piece number.

White luminescence characteristics of red/green silicate phosphor-glass thick film layers printed on glass substrate

Remote phosphor thick film layers consisting of red/green silicate phosphors embedded in a glass matrix were investigated in terms of the feasibility of a printing process onto a soda lime silicate glass substrate and the resultant performance of white luminescence. Densification of the phosphor layers via a viscous flow of the glass matrix at a low temperature of 400 °C was a key element of the success since the low temperature avoids chemical degradation of the phosphors. Photoluminescence and luminous efficacy were dependent on the relative content of the red/green phosphors. As an optimal example, the thick film containing 30 wt% red and 70 wt% green phosphors demonstrated the best efficiency of ~55.1 lm/W with excellent chromaticity coordinates.

Facile one-step fabrication of 2-layered and 4-quadrant type phosphor-in-glass plates for white LEDs: an insight into angle dependent luminescence

Phosphor-in-glass (PiG) when used in combination with a blue light emitting diode (LED) chip in a remote phosphor configuration offers precise tuning and yields higher luminous efficiencies at elevated temperatures, compared to the conventional conformal LED packaging. However, drawbacks such as spectral overlapping of the constituent phosphors and resultant reabsorption remain unresolved in multi phosphor color conversion plates. These issues were solved up to a desired extent by arranging different color-emitting PiGs, via cutting and reassembly. However, the interface of the multicolored plates acted as a dissipative layer. In this work, a novel fabrication technique was proposed to overcome this drawback by eliminating the interfacial layer through a one-step process. PiGs were fabricated using glass frits at a low softening temperature of 600 °C. As a result, a higher efficacy of the studied prototypes, i.e., a horizontal 2-layered PiG and a 4-quadrant PiG, was obtained as compared with their counterparts. The angular dependency of the luminescence of the segmented 4-quadrant type PiG was studied, and the results were discussed.

Investigation of photo-chromic properties of remote phosphor film and white light emitting diode mixed with TiO2 particles

Based on the hot pressing method, the remote phosphor films are prepared by adding TiO2 particles into YAG:Ce and silicon binder, and then they are packaged into white light emitting diode (WLED) device with chip on board (COB) blue light source. The photo-chromic properties and mechanism are studied and calculated. Based on Mie theory and Henyey-Greenstein function, forward scattering is the main light scattering form of YAG:Ce phosphor powder, while the forward scattering intensity is close to the back scattering intensity of TiO2 particles. The emission spectral intensity and relative luminance of remote phosphor film change with increasing the concentration of TiO2 particles, and the optimum concentration is 0.966 g/cm3. Forward transmission intensity and back reflection intensity are calculated and analyzed, when the concentration of TiO2 is low, the forward transmission intensity of blue light is stronger than that of yellow light and the main transmission form is forward transmission, while the forward and backward intensity of yellow light are similar because of isotropy. With increasing the concentration of TiO2, the forward intensity of blue light gradually decreases, and the transmission intensity is lower than that of yellow light. The forward and backward intensity of yellow light reach their maxima when the TiO2 concentration is 0.966 g/cm3. The main reason for this phenomenon is that the increasing of the utilization ratio between blue light and transmission of yellow light is affected by the strong scattering ability of TiO2. Finally the WLEDs are packaged by remote phosphor films and COB blue light source, the luminous flux of WLED reaches 415.28 lm (at 300 mA and 9.3 V) at a concentration of 0.966 g/cm3, which is increased by 8.15% compared with the concentration in the case of no TiO2 mixing. Besides, the correlated color temperature changes from cool white 6900 K to warm white 3832 K gradually. Consequently, the adding of TiO2 particles can not only improve the emission intensity of remote phosphor film and the luminous flux of WLED, but also regulate the correlated color temperature.

Effective Heat Dissipation from Color-Converting Plates in High-Power White Light Emitting Diodes by Transparent Graphene Wrapping

We have developed a hybrid phosphor-in-glass plate (PGP) for application in a remote phosphor configuration of high-power white light emitting diodes (WLEDs), in which single-layer graphene was used to modulate the thermal characteristics of the PGP. The degradation of luminescence in the PGP following an increase in temperature could be prevented by applying single-layer graphene. First, it was observed that the emission intensity of the PGP was enhanced by about 20% with graphene wrapping. Notably, the surface temperature of the graphene-wrapped PGP (G-PGP) was found to be higher than that of the bare PGP, implying that the graphene layer effectively acted as a heat dissipation medium on the PGP surface to reduce the thermal quenching of the constituent phosphors. Moreover, these experimental observations were clearly verified through a two-dimensional cellular automata simulation technique and the underlying mechanisms were analyzed. As a result, the proposed G-PGP was found to be efficient in maintaining the luminescence properties of the WLED, and is a promising development in high power WLED applications. This research could be further extended to generate a new class of optical or optoelectronic materials with possible uses in a variety of applications.

New design of hybrid remote phosphor with single-layer graphene for application in high-power LEDs

A hybrid remote phosphor with single-layer graphene was developed for the fabrication of warm white LEDs, and with an off-the-shelf blue LED operating at 350mA it produced a high luminous flux of ∼64lm at 100°C. The remote use of a single layer of graphene with a phosphor has a multi-functional role that improves the luminous properties, long-term stability and thermal properties of LEDs. We suggest that it can be expected to play an important role as a promising candidate for next-generation remote phosphors of high-power LEDs.

Sol–gel derived flux assisted synthesis of fine particles YAG:Ce3+ phosphor for remote phosphor converted white light emitting diodes

ABSTRACT YAG:Ce3+ phosphor materials with a higher conversion of blue light to yellow light through its submicron particles are prepared by sol–gel derived liquid phase sintered using Na2CO3-SiO2-NaF flux and its luminescent properties are compared with the solid phase sintered phosphor. The liquid phase sintered phosphor material shows a strong absorbance in the blue light region and its luminescent intensity is about 30% higher with a better CIE color co-ordinate, color rendering index (CRI) and correlated color temperature (CCT) when compared with the solid phase sintered phosphor. This enhancement is possible owing to the improved optical properties of the liquid phase sintered phosphor that indicate high crystallinity, better microstructure with morphological control, and porous free close packing of phosphor particles with good dispersibility in the polymer matrix.

Reliability and Lifetime Prediction of Remote Phosphor Plates in Solid-State Lighting Applications Using Accelerated Degradation Testing

A methodology, based on accelerated degradation testing, is developed to predict the lifetime of remote phosphor plates used in solid-state lighting (SSL) applications. Both thermal stress and light intensity are used to accelerate degradation reaction in remote phosphor plates. A reliability model, based on the Eyring relationship, is also developed in which both acceleration factors (light intensity and temperature) are incorporated. Results show that the developed methodology leads to a significant decay of the luminous flux, correlated colour temperature (CCT) and chromatic properties of phosphor plates within a practically reasonable period of time. The combination of developed acceleration testing and a generalized Eyring equation-based reliability model is a very promising methodology which can be applied in the SSL industry.

Impact of the Geometrical and Optical Parameters on the Performance of a Cylindrical Remote Phosphor LED

Remote phosphor light-emitting diode (LED) modules could offer advantages over intimate white phosphor converted LEDs in terms of phosphor operation temperature, light extraction efficiency, and angular color uniformity. Existing commercial devices show a large variety with respect to the dimensions of the mixing cavity, which raises a question about the optimization of the topology. A simplified simulation model applying a two-wavelength approach and considering the remote phosphor as one virtual surface to which three bidirectional scattering distribution functions are attributed (respectively, describing the blue–blue, blue–yellow, and yellow–yellow interactions) is developed and validated. This model has been used to analyze the impact of the cylindrical mixing cavity parameters such as the absolute reflectance, the diffuse-to-specular reflectance ratio, and the height of the mixing cavity, as well as the pitch and angular full-width at half-maximum of the LEDs on the extraction efficiency, the yellow-to-blue ratio, and the irradiance uniformity. It can be concluded that in order to increase the efficacy substantially, the recuperation of the backward emission of the converted light can only be increased by avoiding further interaction with the phosphor plate. To this extent, topologies other than cylindrical mixing cavities must be considered.

Thermal Remote Phosphor Coating for Phosphor-Converted White-Light-Emitting Diodes

We demonstrated a thermal remote phosphor coating method for realizing high angular color uniformity (ACU) and high efficiency of phosphor-converted white-light-emitting diodes based on thermal control. The proposed phosphor-coating method can fabricate remote phosphor layer geometries through a simple package process. Experimental results show that compared with those samples packaged by conventional dispensing coating, this method can efficiently improve the ACU. Angular color-correlated temperature (CCT) deviation of the test samples by the present method can reduce from 1100 to 90 K for an average CCT of 4300 K from −90° to +90° view angles, and the CCT distributions are 150 and 250 K for average CCTs of 5300 and 6300 K, respectively. In addition, this method can improve the lumen efficiency by 4.45% for an average CCT of about 4300 K, and increased by 4.96% and 5.45% for average CCTs of 5300 and 6300 K, respectively.

Paper No S6.4: K2SiF6:Mn4+ as a Red Phosphor for Remote Phosphor LEDs

AbstractK2SiF6:Mn4+ is a promising red phosphor for the use in LEDs for general lighting and displays. Its narrow red emission and excellent thermal stability make it suitable for improved color rendering. The long millisecond‐range decay and saturation at high excitation power can be overcome by using a remote phosphor light emitting diode (LED) setup.

Luminescent Behavior of the K2SiF6:Mn4+ Red Phosphor at High Fluxes and at the Microscopic Level

Phosphor-converted white light-emitting diodes (LEDs) are becoming increasingly popular for general lighting. The non-rare-earth phosphor K2SiF6:Mn4+, showing promising saturated red d-d-line emission, was investigated. To evaluate the application potential of this phosphor, the luminescence behavior was studied at high excitation intensities and on the microscopic level. The emission shows a sublinear behavior at excitation powers exceeding 40 W/cm², caused by ground-state depletion due to the ms range luminescence lifetime. The thermal properties of the luminescence in K2SiF6:Mn4+ were investigated up to 450 K, with thermal quenching only setting in above 400 K. The luminescence lifetime decreases with increasing temperature, even before thermal quenching sets in, which is favorable to counteract the sublinear response at high excitation intensity. A second, faster, decay component emerges above 295 K, which, according to crystal field calculations, is related to a fraction of the Mn4+ ions incorporated on tetragonally deformed lattice sites. A combined investigation of structural and luminescence properties in a scanning electron microscope using energy-dispersive X-ray spectroscopy and cathodoluminescence mappings showed both phosphor degradation at high fluxes and a preferential location of the light outcoupling at irregularities in the crystal facets. The use of K2SiF6:Mn4+ in a remote phosphor configuration is discussed.

Investigation of remote-phosphor white light-emitting diodes with improved scattered photon extraction structure.

In this study, an improved scattered photon extraction (SPE) package structure with a reflection plate over the remote phosphor layer is presented. The important factors for the remote phosphor layer in the conventional and improved SPE package structures, including the phosphor layer thickness and phosphor concentration, are also studied. The simulation results show that the yellow-blue ratio (YBR) of the conventional and improved SPE package structure increases with the phosphor concentration when the phosphor layer is thin and that the YBR is saturated when the layer is thick. The experimental results show that for both conventional and improved SPE package structure, the correlated color temperature (CCT) decreases as the phosphor concentration increases. However, the YBR of the improved SPE package structure is higher than that of the conventional SPE package structure for a given phosphor layer thickness and phosphor concentration. That is, in the conventional SPE package structure, it is necessary to increase the phosphor concentration and then increase the absorption of blue light to obtain more yellow light. Hence, for the improved SPE package structure, less phosphor is required in the phosphor layer for achieving the chosen CCT.

Evaluation of low-cadmium ZnCdSeS alloyed quantum dots for remote phosphor solid-state lighting technology.

We report on the possibility to enhance color rendering of commercially available remote phosphor light-emitting diode modules by using low-cadmium content ZnCdSeS alloyed quantum dots. The employed numerical simulations showed that the color-rendering index of 90+ at the color-correlated temperature of 3200K can be achieved by application of a single layer of quantum dots onto a neutral-white remote phosphor substrate. The experimental results fully support the numerical calculations, thus revealing the only limiting factor in achieving a higher photometric performance: the self-absorption effect in quantum dots. The presented low-cadmium content quantum dots allow a price-effective upgrade of already existing remote phosphor solid-state lighting technology toward a higher color-rendering capability.