Phosphor Conversion Efficiency Research Articles

Microstructured interface modification of laser-driven phosphor-in-glass-film for ultra-high-efficiency white lighting

Phosphor-converted white laser lighting has been regarded as a potential high-brightness light source in lighting and display. Nevertheless, the traditional phosphor-in-glass-film (PiGF) converter suffers the roadblock of low luminous efficiency owing to the low phosphor conversion from small laser spot and the light scattering loss from film-substrate interface. Herein, a laser-driven PiGF with microstructured interface modification was proposed for ultra-high-efficiency white lighting. The effects of PiGF thickness and laser power on the optical performance of PiGF-MS based laser lighting were assessed and compared with the conventional PiGF based laser lighting. The PiGF-MS converter has higher phosphor conversion efficiency and suitable thermal stability originating from the light field strengthening effect of microstructured interface and the outstanding thermal conductivity of sapphire. At the PiGF thickness of 80 µm, the luminous efficacy of PiGF-MS based laser lighting is enhanced by 11% compared with that of PiGF based laser lighting. At this thin PiGF thickness, the PiGF-MS converter adequately emits a cold white light while the PiGF converter still yields a blue light due to its insufficient blue-yellow light conversion. The PiGF-MS converter enables an ultra-high luminous efficacy of 249.1 lm/W and a suitable correlated color temperature (CCT) of 6500 K under a laser power of 3.15 W. The results indicate that the presence of microstructure interface in the PiGF significantly reduces the blue intensity and enhances yellow intensity in the emitted light. The PiGF-MS converter provides a promising roadmap to enhance the light conversion and efficiency of white laser lighting.

Optimizing the spontaneous-emission of far-UVC phosphors

Far-UVC light can enable virus-deactivation while remaining harmless to human tissues. This triggered great efforts to create far-UVC light sources with sufficient emission power and efficiency. However, current sources, such as mercury lamps, KrCl excimer lamps, and LEDs, are made from hazardous chemicals or are limited by low efficiency. Consequently, an alternative approach for reaching the far-UVC is now receiving renewed interest: using phosphors for converting higher frequencies to the desired range of far-UVC. However, this concept is limited by the phosphor's conversion efficiency. In this paper, we propose to utilize principles of nanophotonics to create far-UVC sources. Specifically, we design a phosphor-dielectric multilayer that increases the efficiency of far-UVC light conversion and controls the intrinsic emission properties, including the angular spectrum and emission rate, by shaping the local density of photonic states. To exemplify our approach, we design an aperiodic multilayer nanostructure made of the phosphor material YPO4:Pr3+, showing an increase in light extraction by a factor of 3 compared to naïve bulk structures. Our approach can be applied to any phosphor material and any emitter geometry, opening avenues for engineering nanophotonic light sources in the far-UVC and other spectral regimes.

Mixed Detailed and Compact Multi-Domain Modeling to Describe CoB LEDs

Large area multi-chip LED devices, such as chip-on-board (CoB) LEDs, require the combined use of chip-level multi-domain compact LED models (Spice-like compact models) and the proper description of distributed nature of the thermal environment (the CoB substrate and phosphor) of the LED chips. In this paper, we describe such a new numerical solver that was specifically developed for this purpose. For chip-level, the multi-domain compact modeling approach of the Delphi4LED project is used. This chip-level model is coupled to a finite difference scheme based numerical solver that is used to simulate the thermal phenomena in the substrate and in the phosphor (heat transfer and heat generation). Besides solving the 3D heat-conduction problem, this new numerical simulator also tracks the propagation and absorption of the blue light emitted by the LED chips, as well as the propagation and absorption of the longer wavelength light that is converted by the phosphor from blue. Heat generation in the phosphor, due to conversion loss (Stokes shift), is also modeled. To validate our proposed multi-domain model of the phosphor, dedicated phosphor and LED package samples with known resin—phosphor powder ratios and known geometry were created. These samples were partly used to identify the nature of the temperature dependence of phosphor-conversion efficiency and were also used as simple test cases to “calibrate” and test the new numerical solver. With the models developed, combined simulation of the LED chip and the CoB substrate + phosphor for a known CoB LED device is shown, and the simulation results are compared to measurement results.

Enhancement of efficiency and uniformity for green remote phosphor films and laminated white LEDs based on ZrO2 microparticles

Green remote phosphor films (RPFs) with different ZrO2 microparticle concentrations for blue LEDs were prepared based on their strong scattering effect; the laminated white LED (WLED) with blue LED and red RPF was packaged further. XRD, SEM and double integrate sphere were used to test the RPFs. Results showed that ZrO2 microparticles were evenly dispersed in the silicon resin, the scattering effect significantly improves the utilization rate of excitation light and the transmission intensity of emission light, and the phosphor conversion efficiency (PCE) had a maximum value of 79.35% when ZrO2 concentration was 0.88 wt%. The laminated WLEDs test showed that the luminous efficiency (LE) reached a highest value of 150.38 lm/W at the same concentration, which has increased by 6.37%; ZrO2 microparticles can adjust the color rendering index (CRI) changed from 86.5 to 89.1, the correlated color temperature (CCT) gradually decreased from 4613 to 4219 K, and the CCT uniformity increased by 5.59%. Research shows that ZrO2 microparticles have potential application value in the preparation of high-quality WLEDs.

Characterization, packaging, and optical model of single-primary color phosphor thin films and LEDs

Blue, green, and red phosphor thin films (PTFs) used for NUV LED were prepared by hot pressing method. The excitation spectrums of PTFs have broadband absorption characteristics owing to the 4f7 → 4f65d transition of Eu2+; the emission peaks are 445 nm, 520 nm, and 627 nm, respectively. The phosphor conversion efficiency (PCE) gradually increases and finally remains unchanged with phosphor mass fractions because of absorption saturation effect. Different color LEDs are packaged by NUV chips and PTFs; spectral decomposition and data fitting methods are used to achieve the fitting of the relationship between the phosphor mass fraction and the emission light intensity of the NUV chip and phosphor. The double Gaussian model is used to express the spectral power distribution (SPD) of a single-primary color LED and compare it with the measured results. White LEDs (WLEDs) were prepared by two methods, the first is the combination of multi-LEDs, the second is NUV chip pumps three-color-mixed phosphor directly. Experimental results showed that the WLEDs prepared by first method with the light color parameters of CCT = 5342 K, LER = 274 lm/W, show small parameter difference of ΔCCT = 24 K, ΔLER = 15 lm/W, NCC = 98.94% compared with the calculated results. The WLEDs prepared by second method with the light color parameters of CCT = 5292 K, LER = 263 lm/W shows a 4.0% decrease in LER.

A laser current control algorithm to avoid color distortion in laser projection displays

AbstractLaser display technology has gradually become an important solution for large screen display. As a mature lighting technology in recent decades, phosphor has been widely used in the field of lighting and display. As a solid‐state light source, it has the advantages of low power consumption, environmental protection, rich color, and dynamic modulation. The luminescent efficiency of phosphors varies with temperature, which results in the nonlinearity of phosphor emission when the light source is dynamically modulated and the color temperature drift of the display image. In this paper, a control model of color temperature of laser TV display changing with laser current and temperature is established. The color temperature of laser TV is corrected by using current compensation control algorithm. The problem of color temperature drift caused by the change of phosphor conversion efficiency with t7emperature is solved. The color temperature deviation is 1118 K in the luminance range of the light source before compensation. By compensating and controlling the laser current, the ratio of blue light and phosphor is constant when the luminance changes, thus ensuring the color temperature constant when the luminance of the laser light source changes. The color temperature deviation is 50 K in the luminance range. By analyzing the different luminance of the display image, the stability of the color temperature is improved.

29.4: A laser current control algorithm to avoid color distortion in laser projection displays

Laser display technology has gradually become an important solution for large screen display. As a mature lighting technology in recent decades, phosphor has been widely used in the field of lighting and display. As a solid‐state light source, it has the advantages of low power consumption, environmental protection, rich color and dynamic modulation. The luminescent efficiency of phosphors varies with temperature, which results in the non‐linearity of phosphor emission when the light source is dynamically modulated and the color temperature drift of the display image. In this paper, a control model of color temperature of laser TV display changing with laser current and temperature is established. The color temperature of laser TV is corrected by using current compensation control algorithm. The problem of color temperature drift caused by the change of phosphor conversion efficiency with temperature is solved. The color temperature deviation is 1118 K in the brightness range of the light source before compensation. By compensating and controlling the laser current, the ratio of blue light and phosphor is constant when the brightness changes, thus ensuring the color temperature constant when the brightness of the laser light source changes. The color temperature deviation is 50K in the brightness range. By analyzing the different brightness of the display image, the stability of the color temperature is improved.

Determining Temperature-Dependent Optical Characteristics of Remote Phosphor Plates by Transmission-Type Measurement System

We propose a transmission-type measurement system for determining temperature-dependent optical characteristics (TDOCs) for phosphors or remote phosphor plates which are applied in high-power white light-emitting diodes, related to their stabilities and reliabilities. The proposed measurement system possesses remote-excitation characteristics and is able to separately control the temperature of tested phosphors or remote phosphor plates and that of excitation light sources (with the emission wavelength of 360-470 nm), to avoid the heat interference between them. This system is finally examined by fabricating two types of remote phosphor plates, i.e., yellow Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> :Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and red CaAlSiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> remote phosphor plates. A comparison between the proposed transmission-type system and the reflection-type system at various temperatures is also carried out. Results show that both the phosphor quantum efficiency and phosphor conversion efficiency of remote phosphor plates under various temperatures are possibly overestimated by the traditional reflection-type system. Employing the proposed method and carrying out experiments, we discover that the fraction of silicone in the silicone-phosphor mixture affects TDOCs of remote phosphor plates remarkably. Finally, via both the proposed system and the microhyperspectral imaging technology, thermal-quenching effects in the microcosmic level have also been investigated.

Failure Mechanism of Phosphors in GaN‐Based White LEDs

The degradation and failure mechanisms of nitride phosphor in GaN‐based white LEDs are studied using an in situ multi‐functional accelerated life test and different analytical technologies. The electroluminescence results show that both the intensities of the blue emission (BL) from the LED chips and the yellow emission (YL) from the phosphors decrease with the stress time increasing. However, the YL/BL intensity ratio increases with a yellow shift for the chromaticity coordinates. The reason is not only due to the degradation of the LED chip but the nitride phosphors. The generated microcracks and delamination of phosphor layer lead to the increase of blue light absorption. Except for that, we find that the reason for the decrease of the phosphor conversion efficiency is related to the oxidization of the phosphor's luminescence center Eu2+ ions instead of phosphor thermal quenching. These findings help further understand failure mechanism of phosphors during the operation, and improve the long‐term color quality for the white LEDs.

Phosphor-based white LED by various glassy particles: control over luminous efficiency.

Generating white light through a mainstream remote phosphor design suffers from phosphor conversion efficiency loss due to a backscattering of light. Such a loss also reduces luminous efficiency of the resulting white light. To overcome this issue, various glassy scatterers with different morphologies such as glass bubbles, glass beads, and nanosized silica particles were employed as scatterers, together with a fixed amount of yellow phosphor (YAG:Ce3+) and a poly(dimethylsiloxane) (PDMS) matrix. In addition, the simulation of the system validates the rigorous multiple scattering of the incoming light most probably due to refractive index mismatch between the glass bubbles and surrounding PDMS matrix along with the internal reflections.

Enhancement of efficiency and CCT uniformity for red phosphor thin films, red LEDs and laminated white LEDs based on near-ultraviolet LEDs using MgO nanoparticles.

Red phosphor thin films (PTFs) with different MgO nanoparticle concentrations for near-ultraviolet (NUV) LEDs were prepared based on their strong scattering effect; red LEDs and laminated white LEDs were packaged further. SEM and XRD showed that CaAlSiN3:Eu2+ (CASN) and MgO nanoparticles were uniformly distributed in silicone resin and their crystal structure remained unchanged. The phosphor conversion efficiency (PCE) had a maximum value of 83.15% when the MgO nanoparticle concentration was 15 wt%. An increase of concentration can improve the spatial distribution uniformity of photonics for 410 nm, 627 nm and 660 nm. Fluorescence lifetime showed that the value is positively correlated with concentration change. The packaged red LED luminous flux reaches a maximum of 20.337 lm at a concentration of 15 wt%. The laminated white LED showed that the MgO nanoparticle concentration can be used to adjust the correlated color temperature (CCT) from 4322 K to 1987 K. Under similar CCT, the red phosphor concentration is only 1.83 wt%, the dosage is reduced by 56.12%, and the corresponding luminous efficiency of radiation (LER) and luminous efficiency (LE) are 296.03 lm W−1 and 73.72 lm W−1 respectively. The increase was 11.42%, the decrease was 10.14%, the color rendering index (CRI) increased from 90.6 to 91.8, and CCT uniformity increased from 82.04% to 89.27% with an increase of 8.81%. Research shows that MgO nanoparticles have potential application value in the preparation of high-quality white LEDs.

Novel Method for Estimating Phosphor Conversion Efficiency of Light-Emitting Diodes

This study presents a novel method for estimating the phosphor conversion efficiency of white light-emitting diodes (WLEDs) with different ratios of phosphors. Numerous attempts have been made for predicting the phosphor conversion efficiency of WLEDs using Monte Carlo ray tracing and the Mie scattering theory. However, because efficiency depends on the phosphor concentration, obtaining a tight match between this model and the experimental results remains a major challenge. An accurate prediction depends on various parameters, including particle size, morphology, and packaging process criteria. Therefore, we developed an efficient model that can successfully correlate the total absorption ratio to the phosphor concentration using a simple equation for estimating the spectra and lumen output. The novel and efficient method proposed here can accelerate WLED development by reducing costs and saving fabrication time.

Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses

We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e.g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h~384 h and 768 h~1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.

Investigation of Saturation Effects in Ceramic Phosphors for Laser Lighting.

We report observations of saturation effects in a Ce:LuAG and Eu-doped nitride ceramic phosphor for conversion of blue laser light for white light generation. The luminous flux from the phosphors material increases linearly with the input power until saturation effects limit the conversion. It is shown that the temperature of the phosphor layer influences the saturation power level and the conversion efficiency. It is also shown that the correlated color temperature (CCT), phosphor conversion efficiency and color rendering index (CRI) are dependent both on the incident power and spot size diameter of the illumination. A phosphor conversion efficiency up to 140.8 lm/W with CRI of 89.4 was achieved. The saturation in a ceramic phosphor, when illuminated by high intensity laser diodes, is estimated to play the main role in limiting the available luminance from laser-based lighting systems.

Stabilizing CCT in pcW-LEDs by self-compensation between excitation efficiency and conversion efficiency of phosphors

A novel method to stabilize the correlated color temperature in pcW-LEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pcW-LED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. When the operating temperature unavoidably becomes higher in the LED die quickly after the initial turn-on, the phosphor conversion efficiency degrades and the LED blue light performs red shift. With the positive matching, the red shift actually helps enhance the excitation efficiency of the phosphor to compensate the thermal quenching and efficiency degradation. Therefore, the ratio of the blue light to the yellow light can keep almost constant, as well as the CCTs. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. Finally, we introduce a new factor, the so-called guide number, which is used to count the total change of the enhancement in equivalent excitation efficiency and the relative reduction of the phosphor light emission. The guide number essentially helps in designing the matching blue LED die and phosphor pair for good CCT stabilization.

Analysis of a remote phosphor layer heat sink to reduce phosphor operating temperature

The remote phosphor method has provided significant improvement in overall LED lighting system efficiency by reducing the number of photons absorbed at the LED chip. However, increased demand for higher light output from smaller light engines has resulted in high radiant energy and heat densities on the phosphor layer. The problem is exacerbated by the phosphor conversion efficiency decreasing with increased operating temperature in the remote phosphor layer. A higher operating temperature can negatively affect performance in terms of luminous efficacy, color shift, and life. In cases such as this, the system’s performance can be improved through suitable thermal management that reduces the phosphor layer temperature. In this study, we present the first investigation to experimentally quantify the operating temperature and optical performance effects of using a dedicated phosphor layer heat sink solution as a thermal management strategy to reduce phosphor layer operating temperature. The effects of heat sink geometry and material parameters on phosphor layer operating temperature and optical performance were investigated. The experimental results showed a decrease in phosphor layer operating temperature with an increase in phosphor layer heat sink interface area, while the total radiant power decreased. Ray-tracing simulations identified the low surface reflectance of the heat sink interface area as the cause of this decrease in radiant power. A finite element model was developed from the experimental results to understand the decrease in phosphor layer operating temperature with increased heat sink interface area. This simulation work was used in identifying the causes affecting observed optical and thermal performance in the short-term experiments. The study also investigated the long-term performance of phosphor layer heat sinks and the findings are reported.

Optical Degradation Mechanisms of Indium Gallium Nitride-Based White Light Emitting Diodes by High-Temperature Aging Tests

We have measured five major mechanisms that govern light-emitting diode (LED) degradation, calculated respective percentages, and identified the most influential mechanism during the time evolution of aging. After 480 h, due to the reduction of phosphor-conversion efficiency, the increase of non-radiative centers, the deterioration of thermal properties, the darkening of silicone resin lenses, and the degradation of Ag reflective layers, approximately 42% of the average optical power has decayed. Within this 42%, Ag degradation contributes approximately 28% (with the remaining 14% contributed by the degradation of silicone resin lenses), primarily because other chemical elements have deposited on the surface of Ag reflective layers, thus lowering the reflectivity of these layers. The present work can help draw the community's attention to lowering LED junction temperatures, and to the optimization of packaging designs.

Analysis of the Temperature Dependence of Phosphor Conversion Efficiency in White Light-Emitting Diodes

We investigate the temperature dependence of the phosphor conversion efficiency (PCE) of the phosphor material used in a white light-emitting diode (LED) consisting of a blue LED chip and yellow phosphor. The temperature dependence of the wall-plug efficiency (WPE) of the blue LED chip and the PCE of phosphor are separately determined by analyzing the measured spectrum of the white LED sample. As the ambient temperature increases from 20 to $80^{\circ}C$ , WPE and PCE decrease by about 4.5% and 6%, respectively, which means that the contribution of the phosphor to the thermal characteristics of white LEDs can be more important than that of the blue LED chip. When PCE is decomposed into the Stokes-shift efficiency and the phosphor quantum efficiency (QE), it is found that the Stokes-shift efficiency is only weakly dependent on temperature, while the QE decreases rapidly with temperature. From 20 to $80^{\circ}C$ the phosphor QE decreases by about 7% while the Stokes-shift efficiency changes by less than 1%.

Performance Improvement of GaN-Based Flip-Chip White Light-Emitting Diodes with Diffused Nanorod Reflector and with ZnO Nanorod Antireflection Layer

The GaN-based flip-chip white light-emitting diodes (FCWLEDs) with diffused ZnO nanorod reflector and with ZnO nanorod antireflection layer were fabricated. The ZnO nanorod array grown using an aqueous solution method was combined with Al metal to form the diffused ZnO nanorod reflector. It could avoid the blue light emitted out from the Mg-doped GaN layer of the FCWLEDs, which caused more blue light emitted out from the sapphire substrate to pump the phosphor. Moreover, the ZnO nanorod array was utilized as the antireflection layer of the FCWLEDs to reduce the total reflection loss. The light output power and the phosphor conversion efficiency of the FCWLEDs with diffused nanorod reflector and 250 nm long ZnO nanorod antireflection layer were improved from 21.15 mW to 23.90 mW and from 77.6% to 80.1% in comparison with the FCWLEDs with diffused nanorod reflector and without ZnO nanorod antireflection layer, respectively.

Light distribution and light extraction improvement mechanisms of remote GaN-based white light-emitting-diodes using ZnO nanorod array

To improve the light uniform distribution, light extraction efficiency, and phosphor conversion efficiency, both the remote phosphor coating method and the ZnO nanorod array were simultaneously utilized for fabricating the remote GaN-based white light-emitting diodes with ZnO nanorod array. Compared with the luminous flux of 1.23lm and phosphor conversion efficiency of 74.0% of the traditional GaN-based white light-emitting diodes, the light output power and phosphor conversion efficiency of the resulting remote GaN-based white light-emitting diodes with ZnO nanorod array operated at the same injection current were improved to 1.96lm and 89.1%, respectively. According to the measured angular distribution of the light output, the remote coating phosphor layer could make light diffusion to lead uniform light distribution. Furthermore, the ZnO nanorod array could improve the light extraction efficiency and could guide the blue light to pump the remote coating phosphor layer to convert blue light into yellow emission.