Phosphor Temperature Research Articles

Thermal and optical investigations of a laser-driven phosphor converter coated on a heat pipe

Heat accumulation on phosphor converter is one of the most serious bottlenecks that impede the development of the laser illumination. In this work, a copper powder sintered flattened heat pipe coated with high reflectivity silver was utilized to dissipate the heat from the phosphor stimulated by three high power blue lasers (450 nm). In comparison, a conventional aluminum heat pipe with the same size of the heat pipe substrate was investigated either. The thermal and optical performances were experimentally evaluated. It was found that the heat pipe can lower the phosphor temperature and maintain the output radiant power constantly. Moreover, the heat pipe substrate can reduce the chromatic shift during the temperature rising. The correlated color temperature (CCT) deviation is only 292 K during a 1200 s measurement at an input current of 2.5 A. The highest driven power that the heat pipe substrate can sustain is 34.4 W, which is 14.7% higher than the aluminum substrate. In addition, the carbonization failure process of the phosphor and silicone mixture were revealed in this work. Some further research suggestions were also proposed on improving the thermal and optical performances of the laser illumination.

The phosphor temperature measurement of white light-emitting diodes based on magnetic nanoparticle thermometer.

There is much debate about whether the junction temperature or phosphor temperature is higher in white light-emitting diodes (LEDs). The main reason is that the junction and phosphor temperatures cannot be measured directly using traditional temperature measurement tools. In this study, a magnetic nanoparticle thermometer, a nondestructive and precise temperature measurement tool, is introduced and described in detail. The model, measurement principle, and experimental setup are described. The temperature of the phosphor layer and the top surface of the P-N junction in white LEDs was measured directly using the magnetic nanoparticle thermometer, and the results show that the phosphor layer temperature was higher than the chip top surface temperature at different input voltages, providing a reference for relative researchers.

Tunable correlated color temperature of NaSrPO4 phosphors via Dy3+ and Eu3+ co-doping for warm white light-emitting diodes

Correlated color temperature (CCT) of NaSrPO4 phosphors were tuned through the co-doping of Dy3+ and Eu3+. The pure and doped NaSrPO4 phosphors were synthesized by a high temperature solid-state sintering method. The crystal structure and luminescent properties of the phosphors were studied. NaSrPO4:Dy3+ phosphors exhibit two emission peaks at 481 and 576 nm, originating from the transitions of 4F9/2→6H15/2 and 4F9/2→6H13/2 of Dy3+, while two new emission peaks at 590 and 612 nm originating from the transitions of 5D0→7FJ (J = 1, 2) of Eu3+ appear for NaSrPO4:Dy3+, Eu3+. With the changing Eu3+ doping concentration from 0 to 0.07, the emission light of NaSrPO4:Dy3+, Eu3+ changes from bluish white (CCT: 10,150 K) to reddish white (CCT: 5186 K). Furthermore, the energy transfer phenomenon from Dy3+ ions to Eu3+ ions in Dy3+, Eu3+ co-doped NaSrPO4 phosphors was also investigated. All in all, the NaSrPO4:Dy3+, Eu3+ phosphors are a new kind of full color single phase warm white emitting phosphors for near ultraviolet pumped warm white light-emitting diodes.

Mechanochemical synthesis of binary phosphorus telluride: Short range structure and thermal properties

A binary phosphorus telluride sample (20 at% Te) was synthesized by mechanochemical solid state reaction from elemental powders. The X-ray diffraction pattern and neutron total structure factor show no detectable Bragg peaks, indicating that the sample is amorphous with respect to X-ray and neutron diffraction. The neutron and X-ray total pair distribution functions show correlation peak arising from PTe heteropolar bonds in addition to those arising from PP and TeTe bonds. The TeTe bond length is slightly shorter than that in the crystal structure of trigonal tellurium, suggesting a different environment from a polymeric chain structure. The thermal analysis results show that the binary phosphorus telluride is thermally unstable, that is, it decomposes above 200 °C into trigonal tellurium crystals and gaseous phosphorus, where phosphors begins to gasify at a lower temperature than the sublimation temperature of red phosphors.

Noncontact and instant detection of phosphor temperature in phosphor-converted white LEDs

Phosphor-converted white light-emitting diodes (pc-WLEDs) have become a major light source in general lighting. To stabilize the photometric characteristics of pc-WLEDs, much effort has been made to manage the heat dissipation of the LED dies. The thermal problems of the phosphor parts, a critical reliability concern for pc-WLEDs, have recently attracted academic interest. This study proposed a practical approach for measuring phosphor temperature in an operating pc-WLED using a noncontact, instant detection method to remotely monitor the emission spectrum. Conventionally, an infrared camera or thermocouples have been used to measure temperature. An IR camera requires good calibration on the emissivity and is usually blocked by the lens or other components covered on the phosphors. Moreover, a thermocouple requires time to reach the thermal equivalence between the detector and the sample under testing, and this approach is destructive when used for inner detection. Our approach has advantages over the conventional methods because it is noninvasive, noncontact, and instant, and inner detection. The approach is also independent of the peak wavelength of pumping lights, the concentration and thickness of phosphor, and correlated color temperatures.

Evaluation of Dy-doped phosphors (YAG:Dy, Al2O3:Dy, and Y2SiO5:Dy) as thermographic phosphors

Phosphor thermometry has been viewed as a promising method for non-contact temperature measurement. This article explores the temperature dependence of the Dy-doped phosphors: YAG: Dy, Al2O3:Dy, and Y2SiO5:Dy. The phosphors were energized at 355nm from a 3rd-harmonic-generation Nd:YAG laser. The intensity of YAG:Dy was much stronger than the other phosphors, and the I458nm/I497nm intensity ratios had high temperature resolution over a wide range of temperatures. Although Al2O3:Dy and Y2SiO5:Dy had weaker intensity, the temperature resolution of intensity ratios was as substantial as YAG:Dy. The uncertainty in measured temperature of Dy-doped phosphors was evaluated using linearized error propagation theory, based on the noise-to-signal ratio and temperature sensitivity, revealing that the measurement uncertainty of YAG:Dy was less than the other two phosphors, and thus was the most reliable thermographic phosphor. In addition, the optimization of the bandpass filter combination was explored at each temperature range with YAG:Dy phosphor. Consequently, each temperature can be precisely measured by dividing the integrated intensity between 441 and 475nm as a counter peak range and 476–500nm as a main peak range. Thus, ideal bandpass filters are relatively constant throughout all temperatures.

An optical-thermal model for laser-excited remote phosphor with thermal quenching

Laser-excited remote phosphor (LERP) has been reported to be an effective approach to produce high-luminance white light based on laser diodes (LDs). However, the local phosphor temperature may easily reach thermal quenching point due to the local high light power density, resulting in a significant drop/deterioration of efficiency, reliability and lifetime. In this paper, we focused on the phosphor thermal quenching and developed an optical-thermal coupling model to predict the high phosphor temperature of LERP. From this model, both accurate phosphor heating and temperature can be obtained by iteration. For validation, experiments were performed to verify the model and good agreement was observed between the measurements and the theoretical predictions. Based on the validated model, the critical incident power against thermal quenching under various factors was systematically studied. It was found in the experiments that when a 680mW laser spot with a diameter of 1.0mm was projected onto a phosphor layer, the phosphor temperature was as high as 549.0°C, which would result in severe thermal quenching and even silicone carbonization. It was also found that increasing pump spot from 0.5mm to 3.0mm can dramatically enhance critical power by 19 times. The effect of decreasing phosphor layer thickness on critical power enhancement was explained by the model. Some suggestions were also provided to prevent thermal quenching and improve the optical/thermal performance of LERP.

On the determination of the temperature distribution within the color conversion elements of phosphor converted LEDs

We present an iterative optical and thermal simulation procedure which enables the determination of the temperature distribution in the phosphor layer of a phosphor converted LED with good accuracy. Using the simulation both the highest phosphor temperatures, which are mostly relevant to material degradation as well as the temperatures of those phosphor particles which mainly contribute to converted light emission can be determined. We compare the simulations with experimental studies on the phosphor temperature. While infrared thermography only gives information on the phosphor layer surface temperature, phosphor thermometry provides temperature data on the volume temperature of the phosphor layer relevant to color conversion.

Large-scale synthesis of ZnO nanoparticles and their application as phosphors in light-emitting devices

A simple one-pot route to large-scale synthesis of ZnO nanoparticles (NPs) has been demonstrated, and the ZnO NPs can be produced more than 34 grams in one synthesis process. The ZnO NPs show bright yellowish fluorescence under ultraviolet illumination with a quantum yield (QY) of 42%, which make them suitable for application as phosphors in light-emitting devices (LEDs). Yellowish LEDs have been fabricated by employing the ZnO NP powder as phosphors, and the luminous efficiency of the LEDs can reach 64.2 lm/W. Additionally, the fluorescence intensity of the phosphors shows little degradation when the ambient temperature reaches 100 °C, and the correlated color temperature of the LEDs remains constant when the driving current reaches 100 mA, indicating the good temperature and injection-current stability of the ZnO NP phosphors.

Spincoat-fabricated multilayer PDMS-phosphor composites for thermometry

Phosphor thermometry offers unique advantages over traditional forms of temperature sensing. Polymer-encapsulated phosphor powders provide versatility and flexibility not achievable when using the thermographic phosphors in powder form. By encapsulating the powder in a polymeric sleeve custom devices with unique properties can be created. Here, the authors report on the design, synthesis, and characterization of the first multilayer thermographic phosphor structure. A thin layer of neat PDMS, Sylgard 184, was sandwiched between two layers of La2O2S:Eu phosphor-doped PDMS. The thicknesses ranged from 0.15 to 4 mm depending on spin speed. The temperature dependent luminescence of the structure was characterized from −40 °C to 75 °C, in a low humidity environmental chamber. Results show suitability for thermometry in this range. In addition, for design guidance, quantitative values for thermal conductivity and stress/strain characteristics versus phosphor loading percentage and temperature were measured. Thermal conductivities ranged from 0.15 W mK−1 for the Sylgard 184 to a value between 0.3 and 0.4 W mK−1 for pure phosphor powder for temperatures from −55 °C to 195 °C. Tensile properties for a strain of up to 1 revealed differences between the different phosphor loadings and phosphor batches. Young’s modulus for the spincoat layered materials was between 1.2 and 1.4 N mm−2 and 0.8 for drop casted samples.

Effect of phosphor layer’s location on LED’s luminous depreciation under elevated temperature

High power white light-emitting diode (LED) has been widely used in our daily life due to its extraordinary characteristics of high luminous efficiency, low power consumption, long lifetime and environment protection. Among all the technologies to produce white light, phosphor converting scheme is most popular. In this mode, part of the blue light emitted by LED chip is converted into yellow light by phosphor layer. Then a white output light of blue and yellow light mixture is obtained. The conversion ability of phosphor layer can determine LED’s performance parameters including luminous efficiency, correlative color temperature, angular color uniformity, etc. When one of the parameters does not meet the requirement (like relative light output drops below 70%), the LED fails. Therefore, LED’s reliability is greatly affected by phosphor layer’s property. Previous study shows that the conversion efficacy of phosphor exhibits exponential degradation as temperature increases. The decrease of phosphor’s conversion efficacy can induce light output decrease and deteriorate the reliability. Thus, for phosphor converted LED (pc-LED), thermal management is of great importance. In conventional phosphor coating process, phosphor gel is directly dispensed onto the LED chip. When the LED is working, heat generated by the chip is accumulated due to the poor heat conduction of phosphor layer, resulting in the phosphor temperature rise. Besides, the phosphor’s self-heating can also cause phosphor temperature rise. To solve this problem, some researchers used remote phosphor coating to reduce the influence of heat produced by chip. The phosphor layer was moved away from the chip to avoid contact. It means that the phosphor layer’s location can have an effect on pc-LED’s reliability. In this paper, the phosphor layer’s location was further controlled by phosphor slice. The phosphor slices were prepared by mold to keep specific size. An online testing method was adopted to investigate the effect of phosphor layer’s location on LED’s lumen maintenance under accelerated temperature life test. Three cases were aged under temperature of 120oC, namely blue LED module without phosphor layer, white LED module by conventional phosphor coating and white LED module by combining blue LED module and phosphor slice. The lumen maintenance was monitored and recorded every 30 s. After 30 h of aging, the light output of three cases of modules got stabilized. The results showed that the relative light output of conventional packaged white LED module had a 30% drop, while that of the white LED module by using phosphor slice only dropped 2.7%. The reason behind the phenomenon was well explained. Compared with the conventional phosphor layer, phosphor slice was away from the chip so that the heat generated by chip would not affect the phosphor temperature. Apart from this, the phosphor slice was in the air and the good heat dissipation can reduce the influence of phosphor self-heating. Thus, the temperature of phosphor slice was much lower than that of the conventional one and the reliability was somewhat enhanced. To sum up, the effect of phosphor layer’s location on LED’s luminous depreciation was studied by online testing system in this paper. Phosphor slices were prepared to achieve various phosphor location and comparison experiments were conducted. The results showed that white LED module with phosphor slice had better reliability. The reason was attributed to the good heat dissipation property of phosphor slice. This work presented an attempt for improving LED’s reliability. For conventional packaged LED, better heat dissipation design was needed to improve the performance.

Phosphor Temperature Overestimation in High-Power Light-Emitting Diode by Thermocouple

Phosphor temperature in high-power white light-emitting diodes can greatly affect the optical properties, reliability, and lifetime. Accurate estimation of phosphor temperature is the first step for enhancing thermal management inside the package. In this paper, the phosphor temperature was measured by a plug-in method with thermocouple. The thermocouple’s bead was inserted into the phosphor layer, and then the silicone matrix got cured. The phosphor temperature was measured under various input currents. Under 350 mA, the temperature difference between the measured value and the junction temperature was about 17 °C higher than those in the references. The reason for this overestimation was attributed to light energy absorption and conversion of the bead, which was confirmed by thermal simulations.

A modified bidirectional thermal resistance model for junction and phosphor temperature estimation in phosphor-converted light-emitting diodes

Besides the junction temperature, phosphor temperature is another key parameter to characterize the thermal behavior of phosphor-converted light-emitting diodes (pc-LEDs). However, the measurement of phosphor temperature remains a challenge. In this paper, we proposed a modified bidirectional thermal resistance model for the junction and phosphor temperature estimation. Compared with the conventional thermal resistance model, both the heat generation of the phosphor layer and the heat flow through the phosphor layer were further considered in this model. Three LED packaging structures were fabricated and measured to complete the model. The heat generation of the chip and phosphor layer was measured. With varying driving current from 0.05A to 0.65A with an increment of 0.1A, the maximum deviation of the predicted and measured junction and phosphor temperature is less than 1% and 9.2%, respectively, which proves the feasibility of the proposed model for the junction and phosphor temperature estimation.

Dy3+/Eu3+ co-doped CsGd(MoO4)2 phosphor with tunable photoluminescence properties for near-UV WLEDs applications

A series of color tunable Dy3+/Eu3+ co-doped CsGd(MoO4)2(CGM) phosphors were prepared by the conventional solid state reaction. The crystal structure, morphology, vibrational modes, luminescence properties and energy transfer mechanism were investigated. Upon near ultraviolet (n-UV) excitation, Dy3+/Eu3+ co-doped phosphors exhibit the characteristics blue (4F9/2 → 6H15/2), yellow (4F9/2 → 6H13/2) and red (5D0→7F2) emission corresponding to transitions of Dy3+and Eu3+ions. By changing the doping concentration of Dy3+ and Eu3+, the emission hue could be controlled via the energy transfer mechanism. The energy transfer efficiency and critical distance of Dy3+ and Eu3+ have been calculated. The calculated chromaticity coordinates and color correlated temperature of co-doped phosphors lie at the F4 warm white points of standard illuminants.

Discovery of Novel Narrow-Band Red Phosphors Using High-Throughput First Principles Descriptors

As a red component, nitride phosphors are widely used in achieving warm-light-emitting diodes (LEDs) with high color rendition. There is therefore a huge impetus for the development of new nitride phosphors to further improve the luminous efficacy and color rendition of phosphor-converted LEDs. However, the harsh synthesis conditions (high temperature and/or high pressure) of nitride phosphors have severely hindered this development. In this talk, we outline a computationally efficient multi-property screening framework for identifying promising narrow-band red-emitting phosphors utilizing the Eu2+ activator based on first principles calculations. This screening relies on the use of first principles descriptors that correlate with desired red phosphor properties, such as high thermal/chemical stability, high quantum efficiency, near-UV absorption wavelength and narrow red emission band. We will demonstrate the applicability of our approach through its ability to identify existing well-known narrow-band red phosphors. Finally, we will propose several promising novel red phosphor host materials using this approach. Our study presents a highly efficient way to discover and design novel phosphors for illumination-grade lighting.

Synthesis and characterization of flexible thermographic phosphor temperature sensors

The temperature dependence of the emission characteristics of thermographic phosphors has been used extensively for surface temperature measurements of systems where thermal management is critical for the safe operation of the system. The instantaneous, remote, and highly accurate nature of this form of temperature measurement makes it a very attractive measurement technique. However, the destructive nature of depositing phosphors directly onto the surface of interest and the complications of working with fine powders has limited the use of this technique in all areas. This work focuses on the design and characterization of polymer-encapsulated thermographic phosphor flexible sensors for surface temperature assessment. La2O2S:Eu powder was embedded in an elastomeric sleeve at concentrations of 10%, 25%, and 50% wt. and fully characterized. The effect of spin-coating on emission characteristics of La2O2S:Eu was tested and the decay times were compared to results obtained from bulk-doped samples previously created by the authors.

Preparation and luminescent properties of the coating of phosphor in lead-free glass by multilayer screen-printing

The coating of phosphor in glass (PiG) for white light emitting diodes (WLEDs) has been prepared by multilayer screen-printing and low sintering-temperature. The lead-free and low-melting glass was synthesized, and the main compositions were P2O5-ZnO-B2O3 (PZB). The microstructure, luminescence spectra and lifetime behaviours of coating were investigated. The results of microstructure revealed that decreasing content of phosphor and increasing sintering temperatures were benefit for the coating because of better sintering. Based on the luminescence spectra, the suitable content of phosphor was 25% and sintering temperature was 600 °C. Under blue light (460 nm) exciting, the broad yellow band around 550 nm was obtained. It’s indicated that the white light can be achieved. The coating showed longer lifetime with increasing content of phosphor and lower sintering temperature. The obtained coating of PiG is expected to be an excellent material instead of organic resin for high-power WLEDs packaging.

Effect of Thinning Encapsulant Layer on Junction and Phosphor Temperature of White Light-Emitting Diodes

Thinning of an encapsulant layer for light-emitting diodes (LEDs) is a current major technological trend. The influence of encapsulant thickness on the junction temperature as well as the maximum phosphor temperature for blue and white LEDs is studied systematically for the first time. Using the finite-element method and forward voltage method, it is demonstrated that, in contrast to common belief, a thinned encapsulant layer for relatively low-power LED emitters with an input electrical power of 0.5 W will lead to an increase in the junction temperature for the white LEDs fabricated using different methods, including the phosphor volume distributed in-cup method and chip-coating method, as well as for monochromatic blue LEDs. It is also demonstrated that, different from the effect of encapsulant thickness on the junction temperature, a reduction in the encapsulant thickness results in a decrease in the maximum phosphor temperature for the in-cup white LEDs and an increase in the phosphor temperature for the chip phosphor-coating white LEDs. For example, the largest phosphor temperature drop of the in-cup white LED can be as high as 5 °C. The effect of thermal conductivity of leadframe and phosphor concentration under different encapsulant thicknesses is also indicated.

Laser excitation of red, green, blue and trichromatic white rare-earth phosphors for solid-state lighting applications

Phosphors for solid-state lighting, based on a number of different compositional schemes and driven by light from blue light-emitting diodes (LEDs) are widely used for lighting applications. The low efficiency of pump LEDs at high drive currents (droop effect) limits the amount of light produced by conventional LED and phosphor combination. Pumping with laser diodes (LDs) can circumvent this limitation. In this paper we report on the characterisation of individual and a mixture of KEu(WO4)2 (red), BaMg2Al16O27:Eu, Mn (green) and (Sr,Mg)2SiO4:Eu (blue) phosphors, excited by a CW-driven LD operating at 404nm. The activator ion in each of these phosphors is, respectively, Eu3+, Mn2+ and Eu2+. Characteristics of light produced with each phosphor, variations with incident light power and phosphor temperature as well as effects from phosphor ageing are described. Results of comparison between pumping with coherent and incoherent light at the same wavelength are also described.

Thermal Behavior of Remote Phosphor in Light-Emitting Diode Packages

This letter reports on the detailed thermal behavior of remote type light-emitting diode (LED) packages with different positions of a phosphor layer from the substrate. The temperatures of phosphor layers were directly measured using microthermocouples. It was confirmed by both measurement and simulation, for the investigated LED package structure, the phosphor temperatures are always higher than the junction temperature. Both temperatures are found to change as a function of the phosphor-substrate distance. It is demonstrated that the distance determines the effect of back-scattered photons and the thermal resistance between the phosphor layer and the substrate. From both the experiment and simulation results, it is proposed that there exists an optimal distance of phosphor layer in the remote-type LED package, which is about 320 $\mu \text{m}$ for the package structure utilized in this study. The package with the optimal position exhibits an optical output which is 6% higher than that from a package where the phosphor layer directly covers LED chip. It is shown that the heating load at the phosphor layer changes with the position of the phosphor layer and must be implemented for the precise thermal analysis of whole LED package.