Abstract
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.
Highlights
The solid-state lighting (SSL) technology is regarded as the next-generation lighting approach, and exhibits great advantages of energy-saving, environmental-friendliness and smart lighting among others [1,2]
To illustrate the change of the correlated color temperature (CCT) [26], we desire to introduce a parameter, δCCT, which can be written as δCCT = CCT [ a f ter ] − CCT [be f ore], (1)
We calculate the ratio of the yellow optical power to the blue optical power, namely Y/B = Pyellow /Pblue, which indirectly describes the phosphor-conversion efficiency (PCE) of hppc-WLEDs [27]
Summary
The solid-state lighting (SSL) technology is regarded as the next-generation lighting approach, and exhibits great advantages of energy-saving, environmental-friendliness and smart lighting among others [1,2] It is typically represented by white light-emitting diodes (LEDs), especially for the high-power phosphor-coated white (hppc-W) LEDs, which enjoy additional advantages, such as long lifetime, color-tunable property and high luminous efficiency [3,4]. Sci. 2018, 8, 610 resistive loss [9], the electrostatic discharge stability [10], and the color-rendering property via the color mixing [11]. Aside from these improvements, various stress-inducing degradation tests, by means of temperature, current, static charge, optical radiation, and moisture, have been developed [12,13,14]
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