Aluminoborate powders as rare earth-free phosphors for LED lighting – LUMINOPHOR-LED

Solid-state lighting (SSL) using light emitting diodes (LEDs) is recognized as a major disruptive technology expected in the near future to dominate the public lighting market. The advantages of SSL sources are the energy saving (50 % compared to typical lighting devices) and their potential stability to produce long lifetime devices. The important requirements for industrial development of SSL devices are low cost prices, good color rendering indexes and the reduction of bluish “cold” lightings. The down-conversion of blue or near-UV lights coming from LEDs to white emission (WLED) is essentially done through phosphors based on rare-earth (RE) activators. Since 2011, reducing the use of rare earth elements is a crucial point in all countries due to the soaring prices resulting from the near-monopoly held by China on these strategic elements in the field of materials for energy: permanent magnets, magnetocaloric, photonics, etc... Accordingly, this study is directly related to the current challenge of SSL devices to meet the huge market demand for better lighting.
At Néel Institute, we develop a new type of phosphors based on metal aluminoborate powders without any rare earth as doping. The innovation of these phosphors is to produce broad emission bands throughout the visible spectrum, from color centers (structural defects) in a very stable amorphous matrix. It only takes one phosphor to generate white light from the initial issue of a LED emitting in the near UV (370-390 nm). Moreover, these phosphors are composed of non-toxic and abundant elements, with no rare earth, thus making it much less expensive.
The main objective of this proposal is to optimize the chemical compositions, synthesis procedures and thermal treatments for these micrometric powders to enhance their luminescence properties. For that, it is necessary to increase the absorption of the excitation light by adjusting the chemical routes and compositions or by doping the phosphors with metal ions to increase the overall photoluminescence (PL) efficiency. This will be facilitated by the great versatility of “chimie douce” process such as polymeric precursor methods or sol-gel routes. This chemistry will be combined with several techniques of forming powders: ball milling, supercritical drying in autoclave and spray pyrolysis. This will allow reducing the grain size (1-3 µm) to facilitate the phosphor dispersion in conventional silicone matrices and should avoid problems with PL reabsorption. The powders will also be in the form of pellets by sintering fine powders around their glass transition temperature. This completely new setting form will allow to remove the silicone binder, which currently limits the lifetime of the devices, especially for lighting applications with high power.
The complementary spectroscopic studies (EPR, NMR, FTIR, PL) and structural methods (X-ray Diffraction, Pair Distribution Functions (PDF), NMR, coupledwith thermal analyses (DSC, DTA) - Thermogravimetry (TG) - Mass Spectrometry (MS)) will enable to specify the chemical nature and structural environment of emitting centers. This will be a crucial point to improve then the phosphor syntheses (chemical routes and nature of precursors) and associated thermal treatments in order to optimize the concentration of color centers, while avoiding PL quenching impurities. The efficiency optimization of these new phosphors will be followed by a complete characterization of their performance (PL yields, lifetimes, color coordinates, thermoluminescence, cathodoluminescence). All these studies will allow us to propose PL mechanisms. Finally, the design and development of a SSL prototype is also planned to test the real potentialities of applications of these promising rare earth-free phosphors (overall efficiency (lm / W), photostability, color coordinates ...).