Phosphor-converted White LEDs Research Articles

High‐power single‐chip GaN‐based white LED with 3058 lm

A high-power phosphor-converted white LED with 3058 lm is reported. The high-power white LED was manufactured by utilising a single-blue LED chip with a cerium-doped yttrium aluminium garnet phosphor crystal film, and the LED chip consists of 16 LED cells that are connected in series, the chip dimensions are of 4.5 x 4.5 mm(2). The electrical and optical characteristics of the LED were measured up to a 500 mA injection current under direct operation conditions. Results show that the luminous flux and luminous efficacy at 500 mA reach to 3058 lm and 128 lm/W, respectively.

Effect of the Phosphor Permanent Substrate on the Angular CCT for White Thin-Film Flip-Chip Light-Emitting Diodes

This paper developed a phosphor layer applied for thin-film flip-chip light-emitting diodes (TFFC-LEDs) to produce uniform phosphor-converted TFFC white LEDs (TFFC-WLEDs) by combination of laser liftoff, secondary transferring, and surface roughening process. The spin-coating method was used for phosphor layer fabrication onto a substrate to form the phosphor permanent substrate. The TFFC-LEDs were then bonded onto the permanent substrate. From the results, the blue TFFC GaN-based LED with roughened u-GaN surface on a glass substrate (TFRG-LED) demonstrated a 54.2% (at 350 mA) enhancement in light output power, compared with a blue flip-chip GaN-on-sapphire based LED. As the TFFC GaN/phosphor-/glass-based white LED with roughened u-GaN surface (TFRG-WLED) was operated at a forward-bias current of 350 mA, the enhancement of luminous flux was increased by 75.5%, compared with a TFFC GaN/phosphor template-/glass-based white LED. The angular correlated color temperature (CCT) deviation of a TFRG-WLED can be reduced to 1279 K in the range from −75° to +75° at 5000–6000 K application. The TFRG-WLED was fabricated on the glass substrate with the roughened u-GaN surface and the structure of phosphor layer closed to the u-GaN. These kinds of FRGB-WLED structure contribute to a better light extraction characteristic and a higher CCT stability.

Synthesis and optical properties of red/blue-emitting Sr2MgSi2O7:Eu3+/Eu2+ phosphors for white LED

Phosphor-converted white light emitting diodes (white LEDs) have received great attention in recent years since they have several excellent features such as high lumen output, low power consumption, long lifetime and environmentally friendly. In this work, we report the co-precipitation synthesis of red/blue Sr2MgSi2O7:Eu3+/Eu2+ phosphors with various Eu doping concentration. The results show that the obtained Sr2MgSi2O7:Eu3+/Eu2+ phosphors have good crystallinity and emit strong red (Sr2MgSi2O7:Eu3+) and blue (Sr2MgSi2O7:Eu2+) emissions under near UV light excitation. The sharp emission peaks at 577, 590, 612, 653, and 701 nm corresponded to the typical 5D0 → 7Fj (j = 0,1,2,3,4) transitions of Eu3+, and the blue emission peaking at 460 nm is attributed to the typical 4f65d1-4f7 transition of Eu2+ in the same Sr2MgSi2O7 host lattice. Both phosphors can be well excited in the wavelength range of 260–400 nm where the near UV-LED is well matched. The above results suggest that the Sr2MgSi2O7:Eu3+/Eu2+ phosphors are promising red/blue-emitting phosphors for the application in near UV pumped phosphor-converted white LEDs.

Estimation of the average junction temperature of two phosphors-converted white LED array based on (B + Y + R)/B ratio

The method of non-contact measurement of the junction temperature (Tj) for phosphor-converted white LEDs based on W/B ratio, the ratio of the total radiant energy (W) to the radiant energy of blue emission (B), is verified firstly. It is shown that for two phosphors (Y3Al5O12:Ce and CaAlSiN3:Eu)-converted white LEDs, an significant uncertainty is introduced into the linearity between Tj and W/B ratio. Then a new approach is proposed which uses (B+Y+R)/B ratio, the ratio of the sum of radiant energies of blue emission (B), yellow emission (Y) and red emission (R) to the radiant energy of blue emission, to establish the correlation with Tj. Result shows that the proposed approach is of a satisfactory linearity between Tj and (B+Y+R)/B ratio, with R-square equal to 0.9906 and RMSE equal to 2.27°C. It is also demonstrated that the proposed method is applicable to actual LED lighting system composed of large number of LEDs.

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.

Efficiency Drop in Green InGaN/GaN Light Emitting Diodes: The Role of Random Alloy Fluctuations.

White light emitting diodes (LEDs) based on III-nitride InGaN/GaN quantum wells currently offer the highest overall efficiency for solid state lighting applications. Although current phosphor-converted white LEDs have high electricity-to-light conversion efficiencies, it has been recently pointed out that the full potential of solid state lighting could be exploited only by color mixing approaches without employing phosphor-based wavelength conversion. Such an approach requires direct emitting LEDs of different colors, including, in particular, the green-yellow range of the visible spectrum. This range, however, suffers from a systematic drop in efficiency, known as the "green gap," whose physical origin has not been understood completely so far. In this work, we show by atomistic simulations that a consistent part of the green gap in c-plane InGaN/GaN-based light emitting diodes may be attributed to a decrease in the radiative recombination coefficient with increasing indium content due to random fluctuations of the indium concentration naturally present in any InGaN alloy.

Structural and photoluminescence investigations of Sm3+ doped BaY2ZnO5 nanophosphors

BaY2ZnO5:Sm3+ nanophosphor was successfully synthesized using solution combustion process. XRD and photoluminescence (PL) techniques were used to analyze the structural and photoluminescence properties. Morphological study of the thermally stable powder was carried out using transmission electron microscope (TEM). Rietveld refinement technique has been used to analyze the samples qualitatively as well as quantitatively. X-Ray diffraction analysis confirms that the highly crystalline single phased Sm3+ doped BaY2ZnO5 nanophosphor crystallizes in orthorhombic lattice with Pbnm space group. The average particle size lies in the range 80–90nm with spherical morphology. The photoluminescence excitation at 411nm yields an intense orange–red emission centered at 610nm due to 4G5/2–6H7/2 transition. The concentration dependent luminescent behavior of BaY2(1−x)Sm2xZnO5 nanophosphor shows that the optimum concentration for best luminescence is 3mol%. These results indicate that these nanophosphors find potential applications in the field of phosphor-converted white LED systems.

A promising novel orange–red emitting SrZnV2O7:Sm3+ nanophosphor for phosphor-converted white LEDs with near-ultraviolet excitation

A novel trivalent samarium doped SrZnV2O7 nanophosphors was developed via urea assisted solution combustion method using metal nitrates as initial raw materials. The qualitative and quantitative phase analysis was carried out using Rietveld refinement technique. It was found to crystallize in monoclinic lattice with the P121/n1 (14) space group. The photoluminescent spectral study of SrZnV2O7:Sm3+ revealed that the excitation of 405nm yields the characteristic emission peaks at 569, 599, 640 and 702nm due to 4G5/2→6H5/2, 4G5/2→6H7/2, 4G5/2→6H9/2 and 4G5/2→6H11/2 respectively. The optimum concentration of Sm3+ ion in SrZnV2O7 for best luminescence was found to be 2mol%. The luminescence intensity was further enhanced by incorporating compensator charge R+ (R=Li, Na, and K) into the SrZnV2O7:0.02Sm3+ nanophosphor. The critical distance for non-radiative energy transfer was calculated to be 26.64Å. Dipole–dipole (d–d) interactions were ascribed as the major factor responsible for concentration quenching arising from the over-doping of the activator ions. The results indicate that these nanophosphors are suitable candidate for PC-WLEDs using near UV excitation.

Real-time optical wireless transmissions of digital TV signals using white InGaN LEDs grown with an asymmetric quantum barrier

The feasibility of using InGaN LEDs grown with asymmetric barrier layer (ABL) as transmitters in visible light communications is investigated experimentally. Compared with normal LEDs, the improvement in the spontaneous emission rate due to enhanced carrier localization and better uniformity of carrier distribution in ABL-containing MQWs leads to the fabricated LEDs can exhibit a 32.6% (@ 350 mA) increase in emission intensity and a 10.5% increase in modulation bandwidth. After eliminating the slow-responding phosphorescent components emitting from the phosphor-converted white LEDs, an open eye-diagram at 180 Mb/s is demonstrated over a distance of 100 cm in directed line-of-sight optical links. With the use of proposed LEDs, real-time transmissions of digital TV signals over a moderate distance (~100 cm) in free space is shown to be available in a 150 Mbit/s white LED-based optical link with conventional on-off keying modulation.

Probing the origin of green emission in 1D ZnS nanostructures

In this paper, the origin of the green emission has been addressed by systematically probing at nanoscale the change of surface morphology, chemical composition and cathodoluminescence spectrum of an individual ZnS microrod and of ZnS nanowires upon increasing oxidation temperature in air. We found that the emergence of the green emission is directly related to the formation of the ZnO layer and the imperfect surface/interface between the newly formed ZnO and the ZnS backbone. A simple approach for fabrication of a broad-band white luminescence ZnO/ZnS-based material has also been proposed and experimentally tested demonstrating high potential for application in UV-excited phosphor-converted white LED.

Cymbal-shaped phosphor structure for phosphor-converted white LEDs.

High efficiency phosphor-converted white light-emitting diodes with superior color uniformity have been investigated. It is proposed that the cymbal-shaped phosphor structure can improve the uniformity of the angle-dependent correlated color temperature (CCT) and also increase the luminous intensity compared to the conventional dispensing phosphor structure. In this experiment, we form the cymbal-shaped structure, which features a bump upon the central surface of the bottom layer, by employing an injection process after the dispensing coating. The upper bump phosphor layer not only enhances the extraction efficiency of lights, but also compensates the difference of the excitation optical path in the dispensing bottom layer between the normal-concentrated forward-scattered blue rays and those emitted with larger angles. This considerably eliminates the "blue center" phenomenon. The CCT deviation have been reduced from 315 K to 120 K using the cymbal-shaped phosphor coating method, and the light extraction efficiency (LEE) is enhanced by 8.5% compared with conventional dispensing phosphor-converted white LEDs. This new cymbal-shaped design was verified both experimentally and theoretically.

Study of Temperature Distribution Within pc-WLEDs Using the Remote-Dome Phosphor Package

Both theoretical and simulation approaches were used to obtain the thermal power and temperature distribution in phosphor-converted white LEDs (pc-WLEDs) using a remote-dome phosphor package (RDPP). Different material and geometric parameters were systematically and thoroughly studied. An experiment was performed to measure the temperature distribution in an RDPP pc-WLED, which confirmed the simulation results. Given practical limitations of the material parameters, the most feasible method to reduce the phosphor temperature is to extend the dome radius and the thickness of the phosphor region.

Precise Spatial-Color Optical Modeling in Phosphor-Converted White LEDs

White light LEDs are playing a more and more important role in display in various structures. Here, a new modeling algorithm is proposed for use in phosphor-converted white light-emitting diodes (pcW-LEDs); the algorithm can be used to accurately simulate color appearances, potentially enabling optical designers to remove yellowish/bluish spots in LED lighting. The proposed modeling method was applied to simulate a pcW-LED without an encapsulation lens, and subsequently, a pcW-LED with a hemispherical lens. Both simulations accurately predicted the blue and yellow light distribution. The model was further verified by applying a total internal reflector lens to the pcW-LED. In the midfield region, the blue and yellow light distribution exhibited large variations as the observation distance changed; this varying light pattern for both the blue and yellow lights can be accurately predicted using the proposed model. The developed optical model should facilitate designing a pcW-LED that features high-quality illumination and increased color uniformity.

Process dependent luminescence characteristics of low-temperature Ce3+:YAG doped glass for phosphor-converted white-light-emitting diodes

The glass particle size dependence of luminescence and optical characteristics were investigated for low-temperature Ce3+:YAG (LT-CeYDG) on high-power phosphor-converted white LEDs. With transition temperature 567°C of low-temperature glass powder, 4wt.% LT-CeYDG doped sample held phosphor particle distribution uniformity and quantum efficiency 85% and 34.5%, respectively, better than those of silicone counterparts in addition to improved thermal stability and durability.

Exploring packaging efficiency in phosphor-converted white LEDs

Exploring packaging efficiency in phosphor-converted white LEDs

Self-Adaptive Structure of Remote Phosphor for Phosphor-Converted White LEDs

High-efficiency white light-emitting diodes (LEDS) with superior spatial color homogeneity (SCH) have been investigated. In this paper, a new method of packaging of phosphor-converted white LEDs (pc-WLEDs) that employs a self-exposure technique as well as a remote phosphor configuration is presented. It is suggested that this remote phosphor configuration reduces the backscattering of emission light of LEDs as well as the operating temperature of the phosphor, which improve the luminous efficiency and reliability of pc-WLEDs. The slurry method with self-exposure process was applied to achieve a self-adaptive remote phosphor coating layer surrounding the LED chips which could map the angular intensity distribution of blue light emitted from LED chips every direction and therefore improve the spatial color homogeneity. The experimental results show that pc-WLEDs with self-adaptive remote phosphor coating has remarkable advantages over conventional dispensing coating and conformal coating in terms of efficiency and color homogeneity.

Enhanced luminous efficiency of phosphor-converted LEDs by using back reflector to increase reflectivity for yellow light.

To obtain high reflectivity over a broad range of green, yellow, and red light as well as blue light incidents from a particular angular range and further increase the luminous efficiency of a phosphor-converted white LED packaging module, a novel back hybrid reflector including a SiO(2) total internal reflection layer (TIR), five-pair SiO(2)/TiO(2) double distributed Bragg reflector (DBR) stacks, and a gold (Au) metallic mirror was designed and fabricated. The double DBR stacks have layers configured to reflect green, yellow, and red light as well as blue light, which includes a first portion where the thickness of the layers are relatively larger, and also includes a second portion where the thickness of the layers is relatively smaller. Light that is passing toward the hybrid reflector at angles greater than the critical angle (56°) is reflected by the SiO(2) TIR layer at the sapphire/SiO(2) interface, whereas the light that passes through the SiO(2) TIR layer with incident angles between 0° and 56° is reflected by the double DBR stacks. The overall hybrid reflector can ensure a reflectivity of more than 95% in both the blue light wavelength region and the yellow light wavelength region. The obtained higher reflectivity in the yellow light wavelength region will benefit the phosphor-converted LEDs because yellow light backscattered by phosphor particles is reflected upward.

Packaging efficiency in phosphor-converted white LEDs and its impact to the limit of luminous efficacy

Abstract In this paper, we start from the study on the packaging efficiency of the phosphor-converted white LED via a new way to measure and calculate the blue light from the blue die to the encapsulation lens. Then we try to estimate the limit of luminous efficacy of a pcW-LED with Type VII structure. In the calculation, with the EQE of 81% of the blue die and Stokes loss, we obtain the optimal limit of luminous efficacy. The largest one reaches 300 lm/W, and occurs for green-white light with CCT from 4000K to 5000K. More practical limit is calculated in considering phosphor quantum loss and geometry loss, and the practical limit of luminous for CRI around 60 is around 210 lm/W, and for CRI larger than 80 is around 175 lm/W. Luminous efficacy will be sacrificed to obtain higher CRI. In order to know the real optical flux on the illuminated target, we introduce the optical utilization factor (OUF). Three application cases are discussed. The OUFs for light bulb, automotive head lamp, and street light are 90%, 60% and 45%, respectively. In considering human factors, it is interesting to find that a light source with lower luminous efficacy can perform higher illumination luminous efficacy (ILE). Therefore it is important to use ILE rather than LE when a light source is practically applied to lighting.

Curved remote phosphor structure for phosphor-converted white LEDs.

A phosphor-converted light emitting diode with curved remote phosphor layer was obtained in this study. By changing the geometry of the flat remote phosphor layer in a conventional remote phosphor structure, the package extraction efficiency and luminous efficacy of the curved remote phosphor structure was improved by reducing the total internal reflection at the interface of the air and phosphor layer. Ray tracing simulations were performed to analyze the package extraction efficiency of the curved remote phosphor structure. Experimental results showed that the convex remote phosphor structure had high luminous efficacy with high color uniformity. The convex remote phosphor structure would be a suitable structure if the remote phosphor structure is used in lighting applications.

Abnormal anti-quenching and controllable multi-transitions of Bi3+ luminescence by temperature in a yellow-emitting LuVO4 :Bi3+ phosphor for UV-converted white LEDs.

Phosphors with an efficient yellow-emitting color play a crucial role in phosphor-converted white LEDs (pc-WLEDs), but popular yellow phosphors such as YAG:Ce or Eu(2+) -doped (oxy)nitrides cannot smoothly meet this seemingly simple requirement due to their strong absorptions in the visible range. Herein, we report a novel yellow-emitting LuVO(4) :Bi(3+) phosphor that can solve this shortcoming. The emission from LuVO(4) :Bi(3+) shows a peak at 576 nm with a quantum efficiency (QE) of up to 68 %, good resistance to thermal quenching (T(50 %) =573 K), and no severe thermal degradation after heating-cooling cycles upon UV excitation. The yellow emission, as verified by X-ray photoelectron spectra (XPS), originates from the ((3)P(0),(3)P(1))→(1) S0 transitions of Bi(3+). Increasing the temperature from 10 to 300 K produces a temperature-dependent energy-transfer process between VO(4)(3-) groups and Bi(3+), and further heating of the samples to 573 K intensifies the emission. However, it subsequently weakens, accompanied by blueshifts of the emission peaks. This abnormal anti-thermal quenching can be ascribed to temperature-dependent energy transfer from VO(4)(3-) groups to Bi(3+), a population redistribution between the excited states of (3)P(0) and (3)P(1) upon thermal stimulation, and discharge of electrons trapped in defects with a trap depth of 359 K. Device fabrication with the as-prepared phosphor LuVO(4) :Bi(3+) has proved that it can act as a good yellow phosphor for pc-WLEDs.