Full-light spectrum solar cell soon?
By Jeff Havens, Staff Writer
Researchers announced Nov. 19 a serendipitous discovery that means it may be possible to manufacture a new solar cell that can convert the full spectrum of sunlight from the near infrared to the far ultraviolet to electrical current.
Even the best existing solar cells do not have the ability to convert the whole visible light spectrum into electrical energy. This discovery could revolutionize solar cells by making them more efficient, durable and cheaper, said scientists.
The new solar cell would incorporate the chemical elements of indium, gallium, and nitrogen to form a single unit.
If solar cells can be made with this alloy, they promise to be rugged, relatively inexpensive, and the most efficient ever created, say researchers at Lawrence Berkeley National Laboratory.
Specifically, the scientists at Berkeley National Laboratory, working with Cornell University and Japans Ritsumeikan University, learned that the semiconductor indium nitride has a much wider energy band gap than previously thought. Its as if nature designed this material on purpose to match the solar spectrum, said Wladek Walukiewicz, who led the collaborators in making the discovery
Many factors limit the efficiency of solar cells. For example, silicon is cheap, 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, such as aluminum, gallium, and indium, with elements from group V (nitrogen or arsenic).
One of the most fundamental limitations of solar cell efficiency is the energy band gap on the semiconductor within the solar cell. In a solar cell, negatively charged material with extra electrons forms a junction with a positively charged material. Incoming light (photons), with the correct amount of energy, knock electrons loose from the negatively charged material. This process sets off a cascade of events, which leads to the formation of an electric field, which creates a current, which can then be used to run machines.
Photons with less energy than the energy band gap slip through the charged material in the solar cell. For example, low-energy, red-light photons are absorbed by high energy band-gap semiconductors. However, the semiconductor emits a photon at the exact same frequency and energy as the photon that was absorbedthe net effect is that the same type of photon that was absorbed is also emitted. The result is that the energy from the low-energy photons are not captured to run a machine.
Photons with higher energy than the energy band gap are briefly absorbed by the charged material in the solar cell, but the energy from the photons are emitted as low-energy, heat photons. For example, high-energy, blue-light photons entering a low-energy, band-gap semiconductor in a solar cell waste the excess energy as heat rather than using the energy to run a machine.
The maximum efficiency in converting light to electrical power, in a single material solar cell, is about 30 percent. The best efficiency actually achieved is about 25 percent. Through the researchers discovery, it is hoped that dozens of layers of elements can be stacked and used to catch photons at all energy levels and frequencies. The goal of such stacking would be to reach and exceed efficiencies of better than 70 percent.
The scientists believe is that the new generation of solar cells, which was sparked by this discovery, will use all types of visible light photons to run machines. However, obstacles must be overcome before there is mass production of these solar cells. The primary obstacle is how far charge carriers can travel before the energy they carry is reabsorbed by other materials the carrier encounters in its journey. The researchers are optimistic that the obstacles will be overcome.