Optoelectronic Devices

• Text, Chapter 11

• Sze is a good reference

WASHINGTON, DC – U.S. Department of Energy (DOE) Assistant Secretary for Energy Efficiency and Renewable Energy Alexander Karsner today announced that with DOE funding, a concentrator solar cell produced by Boeing-Spectrolab has recently achieved a world-record conversion efficiency of 40.7 percent, establishing a new milestone in sunlight-to-electricity performance.  This breakthrough may lead to systems with an installation cost of only $3 per watt, producing electricity at a cost of 8-10 cents per kilowatt/hour, making solar electricity a more cost-competitive and integral part of our nation’s energy mix. Attaining a 40 percent efficient concentrating solar cell means having another technology pathway for producing cost-effective solar electricity.  Almost all of today’s solar cell modules do not concentrate sunlight but use only what the sun produces naturally, what researchers call “one sun insolation,”  which achieves an efficiency of 12 to 18 percent.  However, by using an optical concentrator, sunlight intensity can be increased, squeezing more electricity out of a single solar cell.The 40.7 percent cell was developed using a unique structure called a multi-junction solar cell.  This type of cell achieves a higher efficiency by capturing more of the solar spectrum. In a multi-junction cell, individual cells are made of layers, where each layer captures part of the sunlight passing through the cell.  This allows the cell to get more energy from the sun’s light.

2006 Solar Cell Record

Narrowly edging out the previous record set by Spectrolab late last year, two scientists at the University of Delaware have just created a new device that can convert 42.8% of the light striking it into electricity. The solar cell, built by Christina Honsberg and Allan Barnett, splits light into three components — high, medium and low energy light — and directs it to several different materials which can then extract electrons out of its photons.One of the device's key elements is an optical concentrator — a lens-type component that increases the cell's efficiency by directing more sunlight to it than would happen naturally. It measures in at just below 1 cm thick, a major improvement over the Spectrolab model which featured a concentrating lens about 1 foot thick. Unlike most concentrators that use a two-axis tracking system to follow the sun, this optical concentrator is also stationary — a major feat.The Defense Advanced Research Projects Agency (DARPA) — which has been funding this and similar efforts through its Very High Efficiency Solar Cell (VHESC) program — hopes to eventually incorporate this technology into portable solar cell battery chargers for American troops. It will now fund a newly formed DuPont-University of Delaware VHESC Consortium to shift production from a lab-scale model to a full-on manufacturing prototype model.

Spectral Response

LightEmittingDiode

Demonstration using nine pieces of the latest white LEDs: a luminous flux of 90 lm was achieved at an input power of 0.6 W.Nichia Corp. has developed a white light emitting diode (LED) with a luminous efficiency of 150 lm/W at a forward current of 20 mA (photo). The efficiency is 1.5 times that of the company's current product. When compared to other light sources in terms of the efficiency alone, it is approximately 1.7 times that of a high-color rendering fluorescent lamp (90 lm/W) and approximately 11.5 times that of an incandescent lamp (13 lm/W). Its efficiency is even higher than that of a high pressure sodium lamp (132 lm/W) which is regarded as the most efficient light source possible. As with the common products, the white LED is a combination of a blue LED chip and a YAG yellow phosphor. It is contained in the same package as the one used for Nichia's NICHIARAIKOH. The output luminous flux at 20 mA is 9.4 lm, when the color temperature is set to 4,600 K. The average color rendering index (Ra) is 95

Nichia Unveils White LED with 150 lm/W Luminous Efficiency12 21, 2006 16:39Satoshi Ookubo, Nikkei Electronics

Currently, the degree of luminous flux achieved from the input power, when the light source is placed in lighting equipment, is evaluated for comparison of brightness between the white LED and other light sources, in addition to the luminous efficiency. Nichia made the comparison with this index, i.e. luminaire efficiency, between the latest white LED and an incandescent lamp. Assume that the white LED loses about 20% of the input from the power source; its luminous efficiency lowers by about 25% when the luminescent color is changed to the light bulb color; and all the light emitted from the LED can be taken out of the lighting equipment because of the LED's high directivity. Then, the LED reportedly achieved a luminaire efficiency of 90 lm/W. In the case of the incandescent lamp, only about 70% of the emitted luminous flux could be taken out of the equipment, resulting in a luminaire efficiency of 9.1 l m/W. Nichia claims that the luminaire efficiency of its new white LED is 10 times higher than that of an incandescent lamp.

The company says that the high luminous efficiency was achieved through a combination of elemental technologies developed so far by its own staff. Although Nichia did not reveal the details of improvements added to these technologies, "we have revised the light emitting layer as well as the package to enhance light extraction efficiency," says a spokesperson. The white LED is in the prototyping stage and the schedule for its commercial launch is yet to be decided. The company plans to improve manufacturing techniques in order to launch a 150 lm/W product as early as possible.

GreenSource LED Lamp Sets Efficiency Record03/24/08, By Alex Wilson

A prototype 4.75-inch parabolic aluminized reflector (PAR-38) lamp using light-emitting diodes (LEDs) has shattered the efficacy record for reflector lamps by delivering 659 lumens using just 5.8 watts of electricity—for an efficacy of 113.6 lumens per watt. The lamp was made by LED Lighting Fixtures (LLF). The lamp’s performance, tested by the National Institute of Standards and Technology (NIST), is dramatically better than that of the best reflector-style compact fluorescent lamps (CFLs) on the market. The color temperature was measured at 2,760 kelvins (similar to incandescent) and the color rendering index (CRI) at 91.2—also significantly better than CFLs. (Incandescent lamps have a CRI of 100.)

Gary Trott, LLF’s vice president for market development, is quick to admit that this product isn’t ready for commercialization. “It was really a technology demonstration to show what’s possible with our technology taken to the absolute max,” he said. The prototype lamp uses state-of-the-art LEDs from both Cree and Osram. Trott predicts that commercially available LED products from his company will achieve 80 lumens per watt by the end of 2008.Although current LED fixtures average 30–40 lumens per watt (lpw), the Solid-State Lighting Program at the U.S. Department of Energy (DOE) estimates that LED fixtures are capable of achieving an efficacy of 160 lpw. By comparison, incandescent lamps typically produce 10–18 lpw and compact fluorescent lamps (CFLs) 35–60 lpw.

Two white muti-chip LED emitters reach record lumen efficiency31 Mar 2009

An 18 W warm-white emitter achieved an effiency of 77.4 lm/W, while a neutral-white 72 W emitter was measured at 89.8 lm/W, according to NIST reports.The Optoelectronics Packaging & Materials Labs at the University of California, Irvine, has developed neutral-white and warm-white multichip emitters that are claimed to have record performance in terms of lumen efficiency. Both were made from blue LED chips available on the open market.

The performance of the two emitters was confirmed in reports issued in December 2008 by the National Institute of Standards and Technology (NIST).

Both measurements were conducted after a thermal equilibrium was reached between the emitter and its heat sink with a steady-state temperature of 37 deg.C.

Frank Shi, professor of Optoelectronics Packaging & Materials Labs at UCal, said that a combination of innovations in package design, packaging process development, white LED phosphors, and packaging materials processing optimization was used to achieve this result.

Also, the results were achieved without using special prototype blue LED chips.

Cree press release:

DURHAM, N.C., February 3, 2010 — Cree, Inc. (Nasdaq: CREE), a market leader in LED lighting, announces another industry-best reported efficacy record of 208 lumens per watt for a white power LED. This R&D result passes a significant milestone within the solid-state lighting industry as well as demonstrates Cree’s relentless drive to increase the performance of its LEDs.

Cree’s tests confirmed that the LED produced 208 lumens of light output and achieved 208 lumens per watt efficacy at a correlated color temperature of 4579 K. The tests were conducted under standard LED test conditions at a drive current of 350 mA at room temperature.

“We have now broken the elusive 200-lumen-per-watt efficacy barrier for a single white power LED,” said John Edmond, Cree co-founder and director of advanced optoelectronics. “This is a result of improvements in blue optical output power, lower operating voltage and higher conversion efficiency. We continue to push the envelope in white LED technology to enable the highest efficiency white lighting products in the marketplace.”

While this level of performance is not yet available in Cree’s production LEDs, Cree continues to lead the industry with the broadest family of high-performance LEDs.

Mar 16, 2011

From the OSRAM Laboratory: Efficiency Record for Warm WhiteOSRAM Opto Semiconductors has set a new laboratory record of 142 lm/W for the efficiency of a warm white LED light source. With a correlated color temperature (CCT) of 2755 K the LED achieves a good color rendering index (CRI) of 81. Measurements were taken under standard conditions: room temperature and pulsed mode at an operating current density of 350 mA/mm².

Laboratory setup for a warm white LED achieves a peak value of 142 lm/W directly on the Planckian curve at 2700 K; an optimized setup at 3000 K could achieve 160 lm/W

www.led-professional.com/

absorption

stimulated

Lasers

Optical joint density of states

~0.05 eV

~10-9 eV!!

Temperature Source

1,700 K Match flame

1,850 K Candle flame, sunset/sunrise

2,700–3,300 K Incandescent light bulb

3,350 K Studio "CP" light

3,400 K Studio lamps, photofloods, etc.

4,100 K Moonlight, xenon arc lamp

5,000 K Horizon daylight

5,500–6,000 K Vertical daylight, electronic flash

6,500 K Daylight, overcast

9,300 K CRT screen

Note: These temperatures are merely characteristic;considerable variation may be present.

The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins, or alternatively in mired (micro-reciprocal Kelvin).[2] This permits the definition of a standard by which light sources are compared. An incandescent light bulb's light is thermal radiation and the bulb approximates an ideal black-body radiator, so its color temperature is essentially the temperature of the filament.

Many other light sources, such as fluorescent lamps, emit light primarily by processes other than thermal radiation. This means the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp. Daylight has a spectrum similar to that of a black body with a correlated color temperature of 6500K (D65 viewing standard) or 5500K (daylight-balanced photographic film standard).

x y

D65 0.3127 0.3290

Sun 0.3233 0.3326

5780 K 0.3264 0.3357

The Sun has an effective surface temperature of 5780 K. However, the Sun's spectrum is not a precise blackbody. The graph at right shows the spectrum of the Sun as seen from above the Earth's atmosphere, together with the spectrum of a blackbody at 5780 K with the same total flux as the Sun, and the spectrum of CIE D65 `daylight' normalized to the solar

flux at 560 nm. The Sun's spectrum was taken from R. L. Kurucz, I. Furenlid, J. Brault & L. Testerman (1984) Solar Flux Atlas from 296 to 1300 nm. A table of the spectral energy distribution of CIE D65 is available at the UCSD Color and Vision database.

The table at left gives the chromaticities of the Sun and of a blackbody at 5780 K, along with the chromaticity of D65. The chromaticities were computed in the usual way by integrating

the monochromatic tristimuli X, Y, Z from the CIE 2° tables over the corresponding spectrum. Each entry is correctly coloured relative to the D65 white point.

Test colors

R1 Old rose R5 Turquoise

R2 Mustard yellow R6 Sky blue

R3 Yellow-green R7 Violet

R4 Light green R8 LilacAdditional test colors with saturated colors

R9 Red R12 Blue

R10 Yellow R13 Skin tone

The highest possible colour rendering index (Ra=100) is assigned to the black body.

Test colors for the colour rendering index  As specified in DIN 5035, the colour rendering indices are assigned to colour rendering properties and colour rendering groups as follows: