Abstract
In this work, we report on the characterization and reliability/stability study of phosphorescent materials for lighting applications. More specifically, we investigated (a) phosphors directly deposited over light-emitting diodes (LED) chip, (b) remote phosphor (RP) solutions encapsulated in plastic medium for LED lighting, and (c) phosphors without binder for extreme high-intensity laser diode white lighting. The optical and thermal properties of phosphors were studied to develop a sample based on a mix of phosphor compounds in order to achieve different correlated color temperatures (CCT) and high color rendering index (CRI) LEDs. Thermal properties of cerium-doped YAG (Yttrium Aluminum Garnet) phosphor materials were evaluated in order to study thermal quenching. A maximum phosphor operating temperature of 190–200 °C was found to cause a sensible efficiency degeneration. Reduced efficiency and Stokes shift also caused a localized temperature increase in the photoluminescent materials. In the case of remote phosphors, heat did not find a low thermal resistance path to the heatsink (as occurred through the GaN LED chip for direct phosphor-converted devices) and thermal analysis indicated that material temperature might therefore increase to values in excess of 60 °C when a radiation of 435 mW/cm2 hit the sample template. Reliability was also investigated for both plastic-encapsulated materials and binder-free depositions. Pure thermal reliability study indicated that phosphors encapsulated in polycarbonate material were stable up to temperature of approximately 100 °C, while binder-free phosphor did not show any sensible degradation up to temperatures of 525 °C.
Highlights
IntroductionThe use of light conversion phosphors in solid-state lighting (SSL) has become the preferred method in recent years to achieve a good spectrum quality (in terms of correlated color temperature (CCT), color rendering index (CRI) and white point accuracy) in blue-pumped light sources
The use of light conversion phosphors in solid-state lighting (SSL) has become the preferred method in recent years to achieve a good spectrum quality (in terms of correlated color temperature (CCT), color rendering index (CRI) and white point accuracy) in blue-pumped light sources.Photoluminescent materials are a primary component of white solid-state lighting systems
We report on phosphor in three different applications: (i) remote phosphor for light-emitting diodes (LED) application, (ii) silicone-encapsulated phosphors in mid-power LED
Summary
The use of light conversion phosphors in solid-state lighting (SSL) has become the preferred method in recent years to achieve a good spectrum quality (in terms of correlated color temperature (CCT), color rendering index (CRI) and white point accuracy) in blue-pumped light sources. Photoluminescent materials (phosphors) are a primary component of white solid-state lighting systems. Being based on rare-earth-doped aluminate, silicate, garnets, or nitride compounds, they offer green, yellow, and red photoluminescence (PL) when excited with a blue radiation (445–455 nm), typically emitted by a gallium nitride-based light-emitting diodes (LED). Conversion efficiency, saturation, and operating temperature have a strong impact on the efficiency of the final system, while the stability of the emission over time has a crucial effect on LED color temperature and luminous flux stability [1]. Due to much higher incident power density and Proceedings 2018, 2, 1158; doi:10.3390/ecms2018-05233 www.mdpi.com/journal/proceedings
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