|A nearly perfect solar cell, part 2|
|Contact: Paul Preuss, [email protected]|
Clues from blue
In lasers and light-emitting diodes, photons are emitted when holes recombine with electrons. Just as gallium arsenide absorbs long wavelengths -- red light -- as a solar cell, it emits long wavelengths when used as an LED. Red LEDs have been familiar for decades, but it was only in the 1990s that a new generation of wide-bandgap LEDs emerged, capable of radiating light at very short wavelengths.
At 3.4 eV, gallium nitride emits invisible, highly energetic ultraviolet light. When some of the gallium is exchanged for indium, however, colors like violet, blue and green are produced. The Berkeley Lab researchers surmised that the same alloy might emit even longer wavelengths if the proportion of indium was increased. But the failure of simple indium nitride to emit any light at its supposed bandgap of 2 eV frustrated their attempts.
The 2 eV bandgap had been measured on samples of indium nitride created by sputtering, a technique in which atoms of the components are knocked off a solid target by a beam of hot plasma. If such a sample were to be contaminated with impurities like oxygen, its bandgap would be displaced.
In order to get the best possible samples of indium nitride , the Berkeley Lab researchers worked with the group at Cornell headed by William Schaff, renowned for their expertise at molecular beam epitaxy (MBE), and with another at Ritsumeikan University headed by Yasushi Nanishi. In MBE the components are deposited as pure gases in high vacuum at moderate temperatures under clean conditions.
The Berkeley Lab studied these high quality crystals using optical absorption, photoluminescence, and photomodulated reflectance measurements. They established that the indium nitride bandgap was dramatically lower than thought, only 0.7 eV. And because their collaborators could precisely control the relative amounts of indium and gallium in the crystals, the group soon learned that bandgap width increases smoothly and continuously as the proportions shift away from indium in favor of gallium, until reaching the well-established value of 3.4 eV for simple gallium nitride.
This extraordinary range of bandgaps in a single kind of alloy immediately suggested its use for solar cells: alloys with varying proportions of two of these three elements could bracket the entire solar spectrum from the near infrared to the deep ultraviolet, and every color of light between.
Indium gallium nitride, however, is not an obvious choice for solar cells. Gallium nitride itself is very hard to grow, and there is no easy way to dope it to create p-type material. Japanese researchers overcame these difficulties in the late 1980s, growing gallium nitride on a sapphire substrate, but the addition of indium to the mix created new problems. The resulting crystals are riddled with defects, hundreds of millions or even tens of billions per square centimeter.
Ordinarily, defects ruin the optical properties of a semiconductor, trapping charge carriers and dissipating their energy as heat. In studying LEDs, the Berkeley Lab researchers found that indium does not mix evenly with gallium in the alloy. Instead it peppers the material with myriad tiny indium-rich clusters that, remarkably, emit light efficiently. Any other kind of LED with so many defects would be opaque!
Joel Ager, one of the MSD researchers on the project, says, "We were inspired that indium gallium nitride works as an LED. Its defect tolerance should be a great advantage in solar cells."
To exploit the alloy's near-perfect correspondence to the spectrum of sunlight will require a multijunction cell with layers of different composition. Walukiewicz explains that "lattice matching is normally a killer" in multijunction cells, "but not here. Optoelectonic properties of these materials show amazing insensitivity to defects created by the lattice mismatch. "
Two layers of indium gallium nitride, one tuned to a bandgap of 1.7 eV and the other to 1.1 eV, could attain the theoretical 50 percent maximum efficiency for a two-layer multijunction cell. Currently, no materials with these bandgap can be grown together.
Indium gallium nitride solar cells could be made with more than two layers, perhaps a great many layers with only small differences in their bandgaps, for solar cells approaching the maximum theoretical efficiencies of better than 70 percent.
It remains to be seen if a p-type version of indium gallium nitride suitable for solar cells can be made, but here too success with LEDs of the same material gives hope. A number of other parameters remain to be settled, like how far charge carriers can travel in the material before being reabsorbed.
Indium gallium nitride's advantages are many. It can be grown on a transparent substrate, including sapphire (aluminum oxide) or silicon carbide. It has tremendous heat capacity. Like other group III nitrides, it is extremely resist to radiation. These properties are ideal for the solar arrays that power communications satellites and other spacecraft.
And what about cost? "If it works, the cost should be on the same order of magnitude as traffic lights," Walukiewicz says. "Maybe less."
Solar cells so cheap and efficient could not only revolutionize space
applications but could play a key role in the use of solar power on Earth.