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Description
Since the introduction of high brightness and high efficiency light emitting diodes (LEDs), solid state lighting is ready to take over the lighting and display market. While the original concept of white LEDs (wLEDs) is based on a blue-emitting chip on which a light-converting phosphor is coated, a new approach has gained interest in the last decade [1] (see figure 1). In this so-called remote approach, a phosphor plate is placed at a certain distance from the LED chip. This lowers the phosphor temperature since the conductive heating by the LED chip is eliminated. Moreover the lowered excitation flux on the phosphor results in less self-heating during the downshifting of the blue light by the phosphor. Due the reduced reabsorption of light by the LED chip, the inherent scattering properties of the phosphor particles and the freedom to optimize the light mixing chamber, the remote approach yields improved light out-coupling and a more uniform light distribution.
Despite the benefits described above, research on the stability and durability of remote phosphors is still scarce and inconsistent, while of most importance from a commercial point of view. Standard accelerated aging methods are available for lumen depreciation in on-chip LED lifetime prediction [2], but no established standard methods exist for the remote- approach.
This work investigates the influence of three acceleration factors for aging: (1) heat, (2) humidity and (3) irradiance, ultimately providing a way to investigate the aging of phosphors in a remote configuration. Phosphor plates are mounted on teflon mixing chambers with a blue pumping LED at the bottom and placed in a climate chamber. Integrating sphere measurements, XRD and SEM(-EDX) studies are performed at predefined aging-times to retrieve the influence of each acceleration factor on the phosphor stability with respect to photoluminescence (PL), particle crystallinity and morphology respectively. As a proof of concept, commercial phosphors are tested first, providing optimized accelerated aging conditions. The approach is then transferred to investigate in-house prepared phosphors. Summarizing we present a technique which allows us to study the degradation of phosphors in the remote wLED configuration.
