Analysis and reliaiblity study of luminescent materials for white lighting

With this work we report on the characterization and reliability/stability study of photoluminescent materials for lighting applications. Photoluminescent materials (phosphors) are a primary component of the white solid state lighting systems. Being based on rare earth doped Alluminate, Silicate, Garnets or Nitride compounds they offer green, yellow and red photoluminescence, when excited with a blue radiation (445 – 455 nm), typically emitted by a Gallium Nitride based LED.

With this work we report on a characterization of Phosphor materials for lighting, particularly: a) phosphors directly deposited over LED chip, b) remote phosphor solutions encapsulated in plastic medium for LED lighting, C) phosphors without binder for extreme high intensity Laser Diode white lighting.

The optical and thermal properties of phosphors have been studied to develop solution based on mix compounds to achieve different Correlated Color Temperatures and high Color Rendering Index LEDs. Thermal properties of Cerium Doped YAG phosphors materials have been evaluated in order to study the thermal quenching. A maximum phosphor operating temperature of 190-200 °C has been found to cause a sensible efficiency degeneration. Reduced efficiency and Stokes shift, also cause a localized temperature increase in the photoluminescent materials. In the case of remote phosphors, heat does not find a low thermal resistance path to the heatsink (as occurs through the GaN LED chip, for direct phosphor converted devices), thermal analysis indicate that material temperature might therefore increase up to values in excess of 60°C when a radiation of 435 mW/cm² hits the sample template.

Reliability has also been investigated both for plastic encapsulated materials and binder free depositions: pure thermal reliability study indicate that phosphors encapsulated in Polycarbonate material are stable up to temperature of approximately 100 °C, while binder free phosphor do not show any sensible degradation up to temperatures of 525 °C.