Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


An unexpected discovery could yield a full spectrum solar cell


A newly established low band gap for indium nitride means that the indium gallium nitride system of alloys (In1-xGaxN) covers the full solar spectrum

Light emitting diodes made of indium gallium nitride held clues to the potential new solar cell material

Researchers in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory, working with crystal-growing teams at Cornell University and Japan’s Ritsumeikan University, have learned that the band gap of the semiconductor indium nitride is not 2 electron volts (2 eV) as previously thought, but instead is a much lower 0.7 eV.

The serendipitous discovery means that a single system of alloys incorporating indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight -- from the near infrared to the far ultraviolet -- to electrical current.

"It’s as if nature designed this material on purpose to match the solar spectrum," says MSD’s Wladek Walukiewicz, who led the collaborators in making the discovery.

What began as a basic research question points to a potential practical application of great value. For if solar cells can be made with this alloy, they promise to be rugged, relatively inexpensive -- and the most efficient ever created.

In search of better efficiency

Many factors limit the efficiency of photovoltaic cells. Silicon is cheap, for example, but in converting light to electricity it wastes most of the energy as heat. The most efficient semiconductors in solar cells are alloys made from elements from group III of the periodic table, like aluminum, gallium, and indium, with elements from group V, like nitrogen and arsenic.

One of the most fundamental limitations on solar cell efficiency is the band gap of the semiconductor from which the cell is made. In a photovoltaic cell, negatively doped (n-type) material, with extra electrons in its otherwise empty conduction band, makes a junction with positively doped (p-type) material, with extra holes in the band otherwise filled with valence electrons. Incoming photons of the right energy -- that is, the right color of light -- knock electrons loose and leave holes; both migrate in the junction’s electric field to form a current.

Photons with less energy than the band gap slip right through. For example, red light photons are not absorbed by high-band-gap semiconductors. While photons with energy higher than the band gap are absorbed -- for example, blue light photons in a low-band gap semiconductor -- their excess energy is wasted as heat.

The maximum efficiency a solar cell made from a single material can achieve in converting light to electrical power is about 30 percent; the best efficiency actually achieved is about 25 percent. To do better, researchers and manufacturers stack different band gap materials in multijunction cells.

Dozens of different layers could be stacked to catch photons at all energies, reaching efficiencies better than 70 percent, but too many problems intervene. When crystal lattices differ too much, for example, strain damages the crystals. The most efficient multijunction solar cell yet made -- 30 percent, out of a possible 50 percent efficiency -- has just two layers.

A tantalizing lead

The first clue to an easier and better route came when Walukiewicz and his colleagues were studying the opposite problem -- not how semiconductors absorb light to create electrical power, but how they use electricity to emit light.

"We were studying the properties of indium nitride as a component of LEDs," says Walukiewicz. In light-emitting diodes and lasers, photons are emitted when holes recombine with electrons. Red-light LEDs have been familiar for decades, but it was only in the 1990s that a new generation of wide-band gap LEDs emerged, capable of radiating light at the blue end of the spectrum.

The new LEDs were made from indium gallium nitride. With a band gap of 3.4 eV, gallium nitride emits invisible ultraviolet light, but when some of the gallium is exchanged for indium, 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 even though indium nitride’s band gap was reported to be 2 eV, nobody could get light out of it at 2 eV," Walukiewicz says. "All our efforts failed."

Previously the band gap had been measured on samples 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, the band gap would be displaced.

To get the best possible samples of indium nitride, the Berkeley Lab researchers worked with a group at Cornell University headed by William Schaff, renowned for their expertise at molecular beam epitaxy (MBE), and also with a group 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.

When the Berkeley Lab researchers studied these exquisitely pure crystals, there was still no light emission at 2 eV. "But when we looked at a lower band gap, all of a sudden there was lots of light," Walukiewicz says.

The collaborators soon established that the alloy’s band-gap width increases smoothly and continuously as the proportions shift from indium toward gallium, until -- having covered every part of the solar spectrum -- it reaches the well-established value of 3.4 eV for simple gallium nitride.

Promising signs

At first glance, indium gallium nitride is not an obvious choice for solar cells. Its 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, however, the Berkeley Lab researchers found that the way indium joins with gallium in the alloy leaves indium-rich concentrations that, remarkably, emit light efficiently. Such defect-tolerance in LEDs holds out hope for similar performance 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. These materials can accommodate very large lattice mismatches without any significant effect on their optoelectronic properties."

Two layers of indium gallium nitride, one tuned to a band gap 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 band gaps can be grown together.) Or a great many layers with only small differences in their band gaps could be stacked to approach the maximum theoretical efficiency 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. Here too success with LEDs made of the same alloy gives hope. A number of other parameters also 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 has tremendous heat capacity and, like other group III nitrides, is extremely resist to radiation. These properties are ideal for the solar arrays that power communications satellites and other spacecraft. But 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 efficient and so relatively cheap could revolutionize the use of solar power not just in space but on Earth.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

Additional information

"Effects of the narrow band gap on the properties on InN," by J. Wu, W. Walukiewicz, W. Shan, K. M. Yu, J. W. Ager III, E. E. Haller, Hai Lu, and William J. Schaff, appears in the journal Physical Review B, 15 November 2002.

Investigations of indium gallium nitride have also been reported in "Unusual properties of the fundamental band gap of InN," by Wu, Walukiewicz, Yu, Ager, Haller, Lu, Schaff, Yoshiki Saito, and Yasushi Nanishi, Applied Physics Letters, 27 May 2002, and in "Small band gap bowing in In1-xGaxN alloys," by Wu, Walukiewicz, Yu, Ager, Haller, Lu, and Schaff, Applied Physics Letters, 24 June 2002.

Paul Preuss | EurekAlert!
Further information:

More articles from Power and Electrical Engineering:

nachricht Linear potentiometer LRW2/3 - Maximum precision with many measuring points
17.05.2017 | WayCon Positionsmesstechnik GmbH

nachricht First flat lens for immersion microscope provides alternative to centuries-old technique
17.05.2017 | Harvard John A. Paulson School of Engineering and Applied Sciences

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Wafer-thin Magnetic Materials Developed for Future Quantum Technologies

Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.

Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...

Im Focus: World's thinnest hologram paves path to new 3-D world

Nano-hologram paves way for integration of 3-D holography into everyday electronics

An Australian-Chinese research team has created the world's thinnest hologram, paving the way towards the integration of 3D holography into everyday...

Im Focus: Using graphene to create quantum bits

In the race to produce a quantum computer, a number of projects are seeking a way to create quantum bits -- or qubits -- that are stable, meaning they are not much affected by changes in their environment. This normally needs highly nonlinear non-dissipative elements capable of functioning at very low temperatures.

In pursuit of this goal, researchers at EPFL's Laboratory of Photonics and Quantum Measurements LPQM (STI/SB), have investigated a nonlinear graphene-based...

Im Focus: Bacteria harness the lotus effect to protect themselves

Biofilms: Researchers find the causes of water-repelling properties

Dental plaque and the viscous brown slime in drainpipes are two familiar examples of bacterial biofilms. Removing such bacterial depositions from surfaces is...

Im Focus: Hydrogen Bonds Directly Detected for the First Time

For the first time, scientists have succeeded in studying the strength of hydrogen bonds in a single molecule using an atomic force microscope. Researchers from the University of Basel’s Swiss Nanoscience Institute network have reported the results in the journal Science Advances.

Hydrogen is the most common element in the universe and is an integral part of almost all organic compounds. Molecules and sections of macromolecules are...

All Focus news of the innovation-report >>>



Event News

Dortmund MST Conference presents Individualized Healthcare Solutions with micro and nanotechnology

22.05.2017 | Event News

Innovation 4.0: Shaping a humane fourth industrial revolution

17.05.2017 | Event News

Media accreditation opens for historic year at European Health Forum Gastein

16.05.2017 | Event News

Latest News

New approach to revolutionize the production of molecular hydrogen

22.05.2017 | Materials Sciences

Scientists enlist engineered protein to battle the MERS virus

22.05.2017 | Life Sciences

Experts explain origins of topographic relief on Earth, Mars and Titan

22.05.2017 | Physics and Astronomy

More VideoLinks >>>