Characterization, Modeling and Optimization of Remote Phosphor LED Modules

The invention of the GaN-based blue-emitting LED in the early 90's enabled a new generation of white light based on blue emitting LEDs. Presently, the combination of a blue-emitting LED with a suitable phosphor material, conventionally known as a white phosphor-converted LED, produces white light with a higher luminous efficacy than any other existing light source. Yet the luminous efficacy value is currently only the half of the theoretical value. Among the loss mechanisms associated to the white phosphor-converted LEDs, the package extraction efficiency, i.e. the efficiency to extract the white light from the package/system, is one of the most important issues. Under the conformal configurations of white phosphor-converted LEDs, the phosphor is located adjacent to the LED chip, whose reflectivity is limited (~ 50 %). Nearly 50 % of the scattered and wavelength converted light by the phosphor, after illuminating it with blue light, is directed backwards, which turns into a loss of 25 %. In an attempt to address this issue, the remote location of the phosphor has been proposed in the mid-00s.

In this thesis, the characteristics and loss mechanisms of white phosphor-converted LEDs with the phosphor element at a remote location from the blue-emitting LED, are explored experimentally and by simulation models implemented in ray tracing. The main loss mechanisms are the significant backward scattered light by the phosphor and the absorption of this light by the non-ideally reflective surfaces in the mixing cavity, as well as the absorption of the converted light by the phosphor element. Novel architectures based on these findings are designed to achieve unique luminous efficacy and color quality results. 

The first model, a black box model, represents the phosphor element by a bi-spectral BSDF. With the black box model, the impact of geometrical and optical variations of the mixing cavity and the pump blue-emitting LEDs on the light extraction ratio, the yellow-to-blue ratio and the irradiance uniformity of the remote phosphor LED module are evaluated. In the second model, a gray box model, the scattering and the fluorescence of the phosphor element is represented by the absorption and scattering bulk parameters, as well as by the absorption, quantum yield and emission spectra of the phosphor. The baseline concept consists of a cylindrical mixing cavity and a central blue-emitting LED array. Using the gray box model, more striking configurations of white LEDs are formulated, integrating novelties in the optical properties of the phosphor element, such as the nanoparticle phosphors, and in the pumping source blue-emitting laser diodes. 

The edge concept exhibited an increase of 5 % in the luminous efficacy with respect to the baseline at a CCT of 6800 K and can be considered as the best performing architecture, while the phosphor mass was reduced to 5 % of the baseline phosphor mass. 

In addition to the optical aspects of the white LEDs, this thesis also reports on some thermal issues of the white phosphor-converted LEDs based on experimental characterization. For the same CCT and test current, an intimate pc-LED exhibited a larger the junction temperature than the remote phosphor counterpart in a light engine. Besides, it was demostrated separately the effect of both convection and radiation on the junction temperature increase of the blue-emitting LED in the remote phosphor configuration.