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Researchers Create More Productive Solar Cells

Despite the fact that silicon is the market standard semiconductor material, which includes photovoltaic (solar electric) cells, it is hardly the most efficient product on the market. For example, the semiconductor gallium arsenide and similar compound semiconductors give close to twice the performance of silicon in photovoltaic (PV) products. However, gallium arsenide is rarely used in utility-scale applications because of its high production cost. Researchers may have found a way to reduce production costs to make this thin-film semiconductor less expensive.

University of Illinois professors J. Rogers and X. Li discovered lower-cost ways to create thin films of gallium arsenide that also offered versatility in the ways they can be used.

By lowering the expense of gallium arsenide and other compound semiconductors, many new applications are possible.

Typically, gallium arsenide is deposited in a single thin layer on a small wafer. Either the desired unit is made directly on the wafer, or the semiconductor-coated wafer is cut up into chips of the preferred size. The Illinois team decided to put numerous 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 operation, you simply have to load the wafer one time. If you do this in ten operations, loading and unloading with temperature ramp-up and ramp-down, it takes a lot of time. Reducing the resources necessary for each growth – the equipment, the research, the time, the people –saves overhead and reduces cost.

Next, the researchers separately peel off the layers and transfer them. To accomplish this, the stacks swap layers of aluminum arsenide with the gallium arsenide. Bathing the stacks in a solution of acid and an oxidizing agent dissolves the layers of aluminum arsenide, freeing the individual small sheets of gallium arsenide. A soft stamp-like system picks up the layers one at a time from the top down and moves them to a substrate – glass, plastic or silicon, depending on the application. The wafer may be reused for another growth.

This operation makes it possible to increase the quantity and speed of production, and to reduce the cost. This process could generate large amounts of material, as opposed to the single-layer method.

Freeing the material from the wafer also opens the chance of flexible, thin-film electronics produced with gallium arsenide or other high-speed semiconductors.

In a document released, in the May 20th online edtion of the journal Nature, the group describes its methods and displays three kinds of devices utilizing gallium arsenide chips manufactured in multilayer stacks: light units, high-speed transistors and solar cells. The creators also offer a detailed price comparison.

One more benefit of the multilayer approach is the release from area constraints, especially important for solar cells. As the layers are removed from the stack, they can be laid out side-by-side on an additional substrate in order to make a much larger surface area, whereas the typical single-layer method limits area to the size of the wafer.

For solar panels, you want large area coverage to catch as much sunshine as possible. In an extreme case, ten times the area of the standard wafer can be grown.

Next, the team plans to explore more potential product applications and additional semiconductor materials that might adapt to multilayer growth.

About the author

Shannon Combs writes for the Residential Solar Power Systems blog, her personal hobby weblog centered on ideas to aid home owners to save energy with sun power.

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