A lack of efficient red luminecscent line emitters which exhibit a high absorption between 400 and 500 nm of the electromagnetical spectrum was the decisive factor to start this work. The invention of the blue LED (light emitting diode) enables the production of white light by combining the blue emitting LED with a yellow emitting phosphor.
Advances in chip technology lead to high power LEDs with a power consumption of several watts. Long lifetimes (>50 000 working hours) and their high energy efficiency make them very interesting for the lighting market, but further problems have to be solved for the application in general lighting. A big issue for warm-white LEDs are colour rendering and colour temperature due to the lack of red parts in the spectrum. Another important feature of light sources is a stable colour over time. Therefore the applied converter material has to be chemically and photochemically stable. The chip-phosphor package is operated at working temperatures of about 150 to 200¡C Ð that also should not influence performance of the phosphor material. High absorption at the emission maximum of the LED and a high conversion efficiency (>90%) are important properties for designing a marketable converter material.
In this work tungstates were studied due to their high chemical stability and their high alkaline character. Their use as a host lattice for lanthanide ions was investigated. LiLaW2O8 with 50 mol-% and La2W3O12 with 40 mol-% EuIII show the best performance in converting the blue light of the chip with quantum efficiencies of up to 90% at high absorption cross-setctions. Further investigations of temperature behaviour, long term durability under high photon fluxes and their conversion efficiency of the blue emitted to red converted light have to be done to allow a statement about their use in LED applications.
| Identifer | oai:union.ndltd.org:uni-osnabrueck.de/oai:repositorium.ub.uni-osnabrueck.de:urn:nbn:de:gbv:700-201002185438 |
| Date | 18 February 2010 |
| Creators | Bettentrup, Helga |
| Contributors | Prof. Dr. Markus Haase, Prof. Dr. Thomas Jüstel |
| Source Sets | Universität Osnabrück |
| Language | German |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf, application/zip |
| Rights | http://rightsstatements.org/vocab/InC/1.0/ |
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