Takahiko Sakamoto

Ultra-High Efficiency White Light Emitting Diodes

We fabricated the high luminous efficiency white light emitting diode (LED) and the high power white LED by using the patterned sapphire substrates and an indium–tin oxide (ITO) contact as a p-type electrode. The high luminous efficiency white LED was the yellow YAG-phosphors-coated small-size (240 ×420 µm2) high efficiency blue LED with the quantum efficiency of 63.3% at a forward-bias current of 20 mA. The luminous flux (Φ), the forward-bias voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency (ηL), and the wall-plug efficiency (WPE) of the high luminous efficiency white LED are 8.6 lm, 3.11 V, 5450 K, 138 lm/W, and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W). The high power white LED was fabricated from the larger-size (1 ×1 mm2) blue LED with the output power of 458 mW at 350 mA. Φ, Vf, Tcp, ηL, and WPE of the high power white LED are 106 lm, 3.29 V, 5200 K, 91.7 lm/W, and 27.7%, respectively, at 350 mA. The WPE is greater than that of a fluorescent lamp (25%) in the visible region. Moreover, the luminous flux of the high power white LED reaches to 402 lm at 2 A, which is equivalent to the total flux of a 30 W incandescent lamp.

Improvement of Luminous Efficiency in White Light Emitting Diodes by Reducing a Forward-bias Voltage

We fabricated three types of high luminous efficiency white light emitting diodes (LEDs). The first is the white LED, which had a high luminous efficiency (ηL) of 161 lm/W with the high luminous flux (v) of 9.89 lm at a forward-bias current of 20 mA. The blue LED had a high output power (e) of 42.2 mW and a high external quantum efficiency (ηex) of 75.5%. The second is the high luminous efficiency white LED with a low forward-bias voltage (Vf) of 2.80 V, which was almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) were 169 lm/W and 50.8%, respectively, at 20 mA. They were approximately twice higher than those of a tri-phosphor fluorescent lamp (90 lm/W and 25%). The third is the high power white LED fabricated from the high power blue LED with high e of 651 mW at 350 mA. v, ηL, and WPE of the high power white LED were 145 lm, 134 lm/W, and 39.6% at 350 mA, respectively.