Hermetic Polymer-Free White LEDs for Harsh Environments
AlGaInN-based white LED can achieve lifetimes of up to 100000 h depending on junction temperature and current density. However state of the art silicone encapsulated LEDs with powder-based phosphors reveal a strong dependence of field lifetimes on environmental conditions such as humidity, corrosive gas and air pollutant exposure. The reason behind this is the design of current white LEDs. They consist of blue light emitting chips and yellow phosphor, which is excited by the blue light and in total produces white light. The yellow phosphor powder is dispersed in a polymer or silicone matrix and is either placed in a layer directly on, or in a lens above the chip. Continuous improvements of the LED chips enable current and luminance densities, which lead to an extreme photo-thermal stain of the phosphor converter even without external influences. Furthermore, the polymer matrix itself has low mechanical and thermal reliability and is permeable for humidity and gases. High ambient temperatures or penetrating pollutants can then accelerate browning and corrosion inside the LED package. This reduces the lifetime and causes color shifts, which the eye can easily recognize when comparing different LEDs. The silver reflectors inside the package and chip are particularly sensitive in this respect.
An interesting alternative to polymer-dispersed phosphors are luminescent ceramics. They can be manufactured impermeable, provide a 30 times higher thermal conductivity and are chemical, mechanical and temperature resistant. Furthermore they can be manufactured by sintering in desired size and shape. Beyond that, optical functionalization of the surfaces and volume scattering allow the conversion efficiency and emission characteristic to be tailored to LED applications. This provides entirely new opportunities for the manufacturing of white LEDs. To facilitate these Fraunhofer has developed high-power LEDs which use luminescent ceramics as hermetic, light converting cap material. Since pollutants cannot penetrate inside and give rise to corrosion, a direct application of these new LEDs without elaborate external housing in chemical burdened environments is possible. As a further advantage the inevitable heat generated by the light conversion is dissipated through the thermally conductive ceramics itself, rather than further burdening the LED chip. This allows high ambient temperature operation. Furthermore, the developed wafer-level package technology allows long device lifetimes with low color drift by completely avoiding polymers.