Philips photonic crystal LED light extraction efficiency of 73%

Researchers at Philips Lumileds (San Jose, Canada) and Philips Research (Eindhoven, The Netherlands) have developed an electronic crystal structure when moving toward maximum light extraction efficiency for high-power, high-brightness LEDs. Blue light LEDs are expected to have a light extraction efficiency of 73% without being packaged.

In fact, this light extraction efficiency is very important for an unpackaged LED for two reasons. First, the organic materials used in LED packaging are used for high-intensity degradation and, therefore, are not suitable for high-power LEDs.

Second, the use of packaging materials can reduce the brightness of the LED; this is because the packaging material has a higher refractive index than air and can be directly in contact with the LED emitter. This means that the light emitted from the emitter can enter the encapsulating material at a higher angle than the presence of the air gap, increasing the "optical angle" of the LED. (For a typical packaged LED, the reduction in brightness is due to one of these two factors.)

Gallium nitride (GaN)-based LEDs are very thin (700 nm) and have a hexagonal photonic crystal mode on the surface layer with a lattice constant of 455 nm and a depth of 250 nm. This new version of the LED is at least somewhat better than other photonic crystal LEDs because it is so thin that the LEDs only allow very few optical modes, so researchers can do very little optical mode. optimization.

The LED is grown from a multi-quantum well on sapphire with a maximum wavelength of 450 nanometers (nm) and a size of 200 x 200 microns. The reflector under the transmitter is a low loss silver product. To figure out the whereabouts of the remaining light (non-extracted), the researchers did a simulation experiment and found that about half of the light was absorbed by the quantum well and the other half was absorbed by the reflector.