Silicon is hardly the most efficient material for use in solar cells, but it is used because it’s cheaper to make silicon solar cells. Gallium arsenide, for example, is a much more efficient semiconductor than silicon, but has a high manufacturing cost. Now, a team of scientists from the University of Illionis, devised a technology that builds gallium arsenide solar cells in a much cheaper fashion, making them more cost-efficient.
John Rogers and Xiuling Li focused on cheap methods to manufacture thin films of gallium arsenide: “If you can reduce substantially the cost of gallium arsenide and other compound semiconductors, then you could expand their range of applications,” said Rogers, the Lee J. Flory Founder Chair in Engineering Innovation, and a professor of materials science and engineering and of chemistry.
There are two ways gallium arsenide are being deposited on a wafer. Either the desired device is made directly on the wafer, or the semiconductor-coated wafer is cut up into chips of the desired size. The Illinois group decided to deposit multiple layers of the material on a single wafer, creating a layered, “pancake” stack of gallium arsenide thin films.
“If you grow 10 layers in one growth, you only have to load the wafer one time,” said Li, a professor of electrical and computer engineering. “If you do this in 10 growths, loading and unloading with temperature ramp-up and ramp-down take a lot of time. If you consider what is required for each growth – the machine, the preparation, the time, the people – the overhead saving our approach offers is a significant cost reduction.”
To make the process more efficient, the researchers built stacks of alternate layers of aluminum arsenide and gallium arsenide, which they then bathed in an acid and an oxidizing agent. The aluminum arsenide has been dissolved by these and the sheets of gallium arsenide broke free, ready to be transferred to another substrate (glass, plastic, silicon) by a soft stamp-like device. The wafer that initially held the gallium arsenide can then be used for another grow.
“By doing this we can generate much more material more rapidly and more cost effectively,” Rogers said. “We’re creating bulk quantities of material, as opposed to just the thin single-layer manner in which it is typically grown.”
Freeing the material from the wafer also opens the possibility of flexible, thin-film electronics made with gallium arsenide or other high-speed semiconductors. “To make devices that can conform but still retain high performance, tha’s significant,” Li said.
An obvious advantage of this multilayer technology is that the producers aren’t stressed by area constraints, an important factor in the fabrication of solar cells. The layers can be laid out side-by-side on another substrate to produce a much larger surface area. Old technologies using single layers have been limited to the size of the wafer.
Taking off the limitations and lowering prices on super-efficient solar cells that have only been used in space applications since their birth could finally make the solar technology more affordable for smaller companies and could make the price of coal rival with that of solar, which is the best case possible for the environment.