New Alloy Targeted as Satellite Photovoltaic Power Source
EarthVision Reports
03/17/00
ALBUQUERQUE, NM, March 17, 2000 - Scientists at Sandia National Laboratories are researching ways to use a new semiconductor alloy, indium gallium arsenide nitride
(InGaAsN), as a photovoltaic power source for space communications satellites and for lasers in fiber optics. The addition of one or two percent nitrogen in gallium
arsenide, a standard semiconductor material, dramatically alters the alloy's optical and electrical properties and causes what they call "crazy physics" to occur, giving it characteristics suitable for satellite photovoltaics and laser applications, says Eric Jones, a Sandia physicist who has been working with the material for three years.
A news release from the National Lab says nitrogen has a large effect on gallium arsenide's bandgap structure, which is the minimum energy necessary for an electron to transfer from the valence band into the conduction band and create current. In fact, the addition of the nitrogen reduces the material's bandgap energy by nearly one-third.
"In the semiconductor world, this is unheard of," Jones says. "The new material allows designers to tailor properties for maximum current production with different bandgaps. This is what makes the material unique."
InGaAsN has captured the interest of the satellite communications industry that sees it as a potential power source for satellites and other space systems. The new material, which may be used as part of an electricity-generating solar cell, has a potential 40 percent efficiency rate when put into a state-of-the-art multi-layer cell. That is nearly twice the efficiency rate of a standard silicon solar cell.
InGaAsN was developed in Japan about 10 years ago. Sandia got involved with it in the mid-1990s when Hong Hou, now chief technology officer of EMCORE Corp. Albuquerque Operations, joined the Labs from AT&T Bell Labs. His PhD dissertation at the University of California, San Diego, was on the material. It was about this time that the DOE Center of Excellence for the Synthesis and Processing of Advanced Materials, headed by George Samara at Sandia, selected InGaAsN as the focus of a new line of research in photovoltaic material.
Jones says an InGaAsN solar cell that could provide power to a satellite will ultimately have four layers. The top layer would consist of the alloy indium gallium phosphide; the second of gallium arsenide; the third of two percent nitrogen with indium in gallium arsenide; and the fourth, germanium.
Each layer absorbs light at different wavelengths of the solar spectrum. Existing satellite systems use either silicon for solar cells or a two-layered solar panel made up of the indium gallium phosphide layer and the gallium arsenide layer. Silicon space solar cells have a maximum theoretical efficiency around 23 percent, while the dual-layer indium gallium phosphide/gallium arsenide solar cell is around 30 percent. That compares to the 40 percent efficiency rate predicted for the layered solar cell containing InGaAsN.
Sandia says the trick is to get these theoretical gains to happen in the real world. Commercial application becomes interesting if the addition of the InGaAsN junction can add 4 or 5 percent of overall cell efficiency compared to the best commercial devices available today the scientists say.
"You get two times the power from the new material as from silicon," Jones says. "With InGaAsN, the size of the solar collecting package can be smaller, meaning the satellite will weigh less, come in a smaller package, and be cheaper to launch."
But before InGaAsN can realistically be used in a photovoltaic system, researchers must better understand the material, and a higher quality alloy must be developed.
"We are doing a lot of tweaking to try to make the material viable," Sandia researcher Andy Allerman says. "This includes changing some things in the growth process - like temperature - and then measuring its effects after the InGaAsN is grown. We're trying to understand the optical and electrical properties."