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Open AccessArticle

Reliability of Blue-Emitting Eu2+-Doped Phosphors for Laser-Lighting Applications

1
Department of Information Engineering, University of Padova, via Gradenigo 6/b, 35131 Padova, Italy
2
Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova, Italy
*
Author to whom correspondence should be addressed.
Materials 2018, 11(9), 1552; https://doi.org/10.3390/ma11091552
Received: 15 June 2018 / Revised: 27 July 2018 / Accepted: 1 August 2018 / Published: 28 August 2018
(This article belongs to the Special Issue Light Emitting Diodes and Laser Diodes: Materials and Devices)
This paper investigates the reliability of blue-emitting phosphors for Near-UV (NUV) laser excitation. By means of a series of thermal stress experiments, and of stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of Eu2+-activated alkaline-earth halophosphate phosphors under typical and extreme operating conditions. In particular, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the Photo-Luminescence (PL), which was attributed to the thermally induced ionization of the Eu2+ optically active centers. Several analytical techniques, including spatially and spectrally resolved PL, Electron Paramagnetic Resonance (EPR) and X-ray Photo-emission Spectroscopy (XPS) were used to support this hypothesis and to gain insight on the degradation process. By means of further tests, evidence of this degradation process was also found on samples stressed under a relatively low power density of 3 W/mm2 at 405 nm. This indicated that the optically (and thermally) induced ionization of the optically active species is the most critical degradation process for this family of phosphorescent material. The operating limits of a second-generation Eu-doped halophosphate phosphor were also investigated by means of short-term stress under optical excitation. The experimental data showed that a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state PL performance and non-recoverable degradation of the material were found to take place. This behavior is a consequence of the extremely harsh excitation regime, mainly due to the thermal management capabilities of the substrate material employed for our experimental purposes rather than from intrinsic properties of the phosphors. View Full-Text
Keywords: phosphors; laser-lighting; degradation; europium phosphors; laser-lighting; degradation; europium
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Buffolo, M.; De Santi, C.; Albertini, M.; Carbonera, D.; Rizzi, G.A.; Granozzi, G.; Meneghesso, G.; Zanoni, E.; Meneghini, M. Reliability of Blue-Emitting Eu2+-Doped Phosphors for Laser-Lighting Applications. Materials 2018, 11, 1552.

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