Improving the efficiency of the phosphor improves the efficiency of the LED

Film thickness versus number of coats for screen printed Cabot phosphor layers. Note the uniformity of the films in the SEM images.
The overarching goal of this project was to develop luminescent materials using aerosol processes for making improved LED devices for solid state lighting. In essence this means improving white light emitting phosphor-based LEDs by improvement of the phosphor and phosphor layer. The efficiency of these LEDs is based on the combined efficiency of the LED, phosphor, and the interaction between the two.

The structure of these types of light sources is a blue or UV LED under a phosphor layer that converts the blue or UV light to a broad visible (white) light. Traditionally, this is done with a blue emitting diode combined with a blue absorbing, broadly yellow emitting phosphor such as Y3Al5O12:Ce (YAG). A similar result may be achieved by combining a UV emitting diode and at least three different UV absorbing phosphors: red, green, and blue emitting. These emitted colors mix to make white light.

External quantum efficiency values determined for of YAG:Ce phosphors from emission in an integrating sphere.
Cabot’s spray-based process for producing phosphor powders is able to improve the brightness of the powder itself by increasing the activator (the species that emits the light) concentration without adverse quenching effects compared to conventional synthesis. This will allow less phosphor powder to be used, and will decrease the cost of the light source; thus lowering the barrier of entry to the lighting market. The process also allows for chemical flexibility of the phosphor particles, which may result in tunable emission spectra and so light sources with improved color rendering. Another benefit of Cabot’s process is the resulting spherical morphology of the particles. Less light scattering results when spherical particles are used in the phosphor layer compared to when conventional, irregular shaped phosphor particles are used. This spherical morphology will result in better light extraction and so an improvement of efficiency in the overall device.

Cabot has produced a number of different compositions in a spherical morphology that may be useful for solid state lights, as well as demonstrated processes that are able to produce particles from 10 nanometers to 3 micrometers. Towards the end of the project Cabot demonstrated that the process produces YAG:Ce powder that has both higher internal quantum efficiency (0.6 compared to 0.45) and external quantum efficiency (0.85 compared to 0.6) than the commercial standard. However, these highly bright materials were only produced in research and development quantities, not in a reproducible manner at a commercial scale.

This work was done by Cabot Corp., Boston, MA. DE-FC26-04NT42276

This Brief includes a Technical Support Package (TSP).

Development of Advanced LED Phosphors by Spray-based Processes for Solid-State Lighting (reference GDM0002) is currently available for download from the TSP library.

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