Even-brighter LEDs set to emerge from research labs

Researchers from Nichia Corp. in Japan say they are on the road to replacing every interior and exterior light in use today with with an LED. Prompting this prediction is research results recently published in the online journal Journal of Physics D: Applied Physics. The Nichia team fabricated white LEDs that exhibit high-luminous efficacy and brightness while simultaneously operating from a lower voltage.

The history of efficacy in incandescent lamps, halogen lamps,fluorescent lamps, sodium-vapor lamps and commercial white LEDs.

In one case they fabricated a white LED operating from 20 mA that put out 249 lm/W, about three times that  of a tri-phosphor fluorescent lamp (90 lm/W). They also devised a single-chip high-power white LED that put out 203 lm at 350 mA with an efficacy of 183 lm/W. Another four-chip high-power white LED reached 1,913 lm with an efficacy of 135 lm/W at 1 A. This white LED had a higher flux than a 20 W-class fluorescent lamp with 1.5 times the luminous efficacy of a tri-phosphor fluorescent lamp (90 lm/W).

Top, the structure of an ordinary LED. Bottom, the configuration used by Nichia researchers.

One way researchers boosted LED efficacy was through use of an ITO contact as a p-type electrode. The transmittance of this electrode was higher than 90%, allowing much more of the light generated in the LED's active layer to exit without being absorbed. Researchers also used a patterned sapphire substrate, which changed the direction of the propagating light from a specular direction to a nearly Lambertian one. Compared with the case of an ordinary flat sapphire substrate, the propagating light can more efficiently exit. This significantly reduces the density of the propagating light in the nitride films and reduces the quantity of light reflected at the electrode.

The report is free for a limited time: http://iopscience.iop.org/0022-3727/43/35/354002/pdf/0022-3727_43_35_354002.pdf

Nichia Corp., http://www.nichia.co.jp/en/about_nichia/info.html