DS03 - Stimuler le renouveau industriel

Garnet-type nanophosphors for white LED lighting – NanophosforLED

Garnet-type nanophosphors for white LED lighting

Synthesis of inorganic cerium-doped oxide nanocrystals for the development of nanophosphors allowing for a better coupling with blue LEDs and more efficient white LED devices.

Garnet-type nanocrystals for enhanced white LED performances

the objective of this project is to increase the final EQE of wLEDs by elaborating and shaping nanophosphors with high iQY, high photostability and appropriate spectral properties. We will focus on garnet-type nanophosphors: YAG:Ce will be used as a model system for developing new synthesis methods; then our study will be extended to original nano-garnets such as Ce3+-doped Y3Al5-2xMgxSixO12, Y3Al5-xScxO12, Y3Al5-xGaxO12. Three steps will be followed: (1) the development of a new synthesis route to obtain YAG:Ce nanophosphors with high crystalline quality, (2) the development of new garnet-type nanophosphors to increase the doping concentration and to optimize spectral emission, and (3) the shaping of nanophosphors as binder-free layers with controlled light propagation.

The key factor is to elaborate and shape nanophosphors with high iQY, high photostability and appropriate spectral characteristics, through the following steps:
(1) We will develop an original synthesis method based on the combination of solvothermal conditions and high pressure (up to 400 bars) to obtain well-crystallized NCs with the targeted size (50-100 nm). This technique, recently set up using the extended technical skills of our laboratory in this field, is very promising as it leads to well-crystallized YAG:Ce small NCs (~20 nm). Various experimental parameters (temperature, pressure, precursor concentration, etc) will be adjusted to achieve high crystal quality with the targeted size. With the help of in situ photoluminescence (PL) and X-ray Absorption Spectroscopy (XAS, to specify Ce oxidation state), we will study YAG:Ce nanocrystallization and optimize the synthesis conditions to achieve high PL intensity, by minimizing the concentration of Ce4+, a strong PL quencher.
(2) In addition to YAG:Ce NCs, we will develop new nanophosphors, more appropriate for wLEDs applications in terms of spectroscopic properties and absorption. The goal is to realize a genuine nanophosphor crystal engineering to obtain a redshifted emission to achieve comfortable warm white LEDs and to increase Ce doping concentration for a better absorption of the incident blue LED emission and enhanced luminous efficiency. Based on results published on micron-sized crystals, we propose to substitute Al3+ cations by other elements (Mg2+/Si4+, Sc3+, Ga3+) to modify the crystal field for emission shifts and/or enlarge the lattice parameter for better Ce incorporation.
(3) The most promising nanophosphors will be shaped to propose binder-free phosphor layers with finely controlled scattering rate. Nanoceramics will be prepared under various conditions (isostatic pressure, annealing temperature, NC size) to control material transparency. Nanophosphors will also be deposited by spray.

Ce3+-doped Y3Al5O12 (YAG:Ce) nanocrystals were synthesized by a unique solvothermal method, Under sub-critical conditions. A home-made autoclave was used, operating in a larger pressure and temperature range than that with conventional commercial equipment and allowing direct in situ photoluminescence (PL) and X-ray absorption characterizations. The study of various synthesis conditions (pressure, temperature, precursor concentration, reaction time) allowed the best reaction conditions to be pinpointed to control YAG:Ce nanocrystal size, as well as crystal quality, and to get efficient optical properties. Without any post thermal treatment, we succeeded in obtaining wel-lcrystallized YAG:Ce nanocrystals (30–200 nm), displaying typical PL properties of YAG:Ce with a maximal
emission at 550 nm. The pristine 100 nm-sized YAG:Ce nanoparticles present an internal quantum yield of about 40 %. In situ X-ray absorption near edge spectroscopy demonstrates the presence of Ce4+
in nanocrystals elaborated at high temperature, resulting from the oxidation of Ce3+ during the crystallization process.

The future prospects are the development of other garnet-type phases et the shaping of YAG:Ce NCs, either as ceramics or as thin films.

G. Dantelle et al. RSC Advances 2018, 8, 26857
G. Dantelle et al. SPIE 2018, 10533, 153322

Commercial white LEDs (wLEDs) combine a 450-nm emitting LED chip with micrometer-sized Ce3+-doped Y3Al5O12 powder (YAG:Ce, called phosphor) encapsulated inside an epoxy or silicone resin. This garnet-type phosphor presents high internal quantum yield (iQY~90%), high photostability and good, though not optimal, spectroscopic properties (?exc=450 nm and ?em=550 nm). However, wLEDs present reduced external quantum efficiency (EQE), which does not outreach 70%, bad ageing properties and uncomfortable “cold white” emission. These drawbacks are related to the use of YAG:Ce micron-sized phosphors: (1) blue and yellow lights are scattered in all directions, and in particular towards the chip, inducing light losses by re-absorption and gradual chip deterioration; (2) phosphors need to be encapsulated inside a resin, which has limited ageing properties, deteriorating wLED color rendering index over time and contributing to reduce wLED lifetime, especially when operating at high power. (3) YAG:Ce emission lacks of a red-component, leading to cold-white LEDs. The objective of this project is to achieve higher EQE, better spectral emission and better ageing properties of wLEDs by using nanosphosphors with a nanocrystal (NC) targeted size between 50 and 100 nm. At this scale, backscattering should be avoided and light propagation could be controlled independently. Moreover, nanopowders could be shaped without any binder, allowing a better control of the device ageing. We will focus our study on garnet-type NCs, chemically stable and for which detailed characterizations already exist at the micron scale, especially for the YAG reference material.
The key factor is to elaborate and shape nanophosphors with high iQY, high photostability and appropriate spectral characteristics, through the following steps:
- First, we will develop an original synthesis method based on the combination of solvothermal conditions under high pressure (up to 400 bars) to obtain well-crystallized NCs with the targeted size (50-100 nm). This technique, recently set up using the extended technical skills of our laboratory in this field, is very promising as it leads to well-crystallized YAG:Ce small NCs (~20 nm). Various experimental parameters (temperature, pressure, precursor concentration, etc) will be adjusted to achieve high crystal quality with the targeted size. With the help of in situ photoluminescence (PL) and X-ray Absorption Spectroscopy (XAS, to specify Ce oxidation state), we will study YAG:Ce nanocrystallization and optimize the synthesis conditions to achieve high PL intensity, by minimizing the concentration of Ce4+, a strong PL quencher.
- Second, in addition to YAG:Ce NCs, we will develop new nanophosphors, more appropriate for wLEDs applications in terms of spectroscopic properties and absorption. The goal is to realize a genuine nanophosphor crystal engineering to obtain a redshifted emission to achieve comfortable warm white LEDs and to increase Ce doping concentration for a better absorption of the incident blue LED emission and enhanced luminous efficiency. Based on results published on micron-sized crystals, we propose to substitute Al3+ cations by other elements (Mg2+/Si4+, Sc3+, Ga3+) to modify the crystal field for emission shifts and/or enlarge the lattice parameter for better Ce incorporation.
- Finally, the most promising nanophosphors will be shaped to propose binder-free phosphor layers with finely controlled scattering rate. Nanoceramics will be prepared under various conditions (isostatic pressure, annealing temperature, NC size) to control material transparency. Nanophosphors will also be deposited by spray. In both cases, for a better control of transparency and thus light propagation, we will add scattering centers (Al2O3, TiO2…), whose nature, size and quantity will be adjusted. The relationship between scattering rate and PL intensity will be studied to propose the best phosphor layer for enhanced EQE of the final device.

Project coordination

Géraldine Dantelle (Institut Néel - CNRS)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

INEEL Institut Néel - CNRS

Help of the ANR 208,999 euros
Beginning and duration of the scientific project: September 2017 - 48 Months

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