Cheaper High-Efficiency Solar Panels

Suntech Power has developed a better way to make high-grade silicon wafers.

Chinese solar-panel manufacturer Suntech Power has developed a new process for making silicon wafers for solar cells that could cut the cost of solar power by 10 to 20 percent.

Solar crystals: A new solar panel from Suntech incorporates cells made using a new silicon-wafer casting process. The cells—the smaller squares inside the panel—are half monocrystalline (the dark areas of the cells) and half multicrystalline (the variegated areas).

The most efficient silicon solar cells use wafers consisting of a single crystal of silicon. When made by the new process, these high-quality “monocrystalline” wafers cost about the same as  lower-quality multicrystalline wafers, or potentially half as much as monocrystalline wafers made by conventional processes. (Wafer cost is only part of the cost of solar power, which is why a process that may cost half as much only reduces the overall cost by 10 to 20 percent.)

The idea underlying the process was patented more than 20 years ago but never commercially developed by the patent owners. The patents expired about three years ago, and several companies—JA Solar, LDK Solar, and Renesola, in addition to Suntech—recently announced that they had succeeded in making the process work.

Stuart Wenham, Suntech’s CTO, described the advance at a solar conference this week in Seattle, and said the company has already started selling solar panels made using the process.

This news may spell trouble for businesses in the United States and elsewhere hoping to commercialize new thin-film solar technologies. In theory, thin-film technology is cheaper per watt than silicon technology. But its makers have found it hard to compete with Chinese makers of conventional silicon solar panels, which have steadily cut costs in part by improving manufacturing techniques and in part because government support has allowed them to scale up production quickly.

Making high-quality monocrystalline wafers ordinarily involves heating silicon to over 1,400 ° C (higher than its melting point), and then dipping a seed crystal into the melt. An ingot from which the wafers will be cut is formed by gradually pulling the seed up as the silicon crystallizes around it. This happens over the course of one to two days, during which time the pool of silicon must be kept hot—which takes a lot of energy. Both the energy consumption and the slow rate of production make the process expensive. Making multicrystalline ingots is faster and less energy-intensive—the silicon is melted and then cooled. There is no need to keep the silicon hot, saving energy, but cells made from these materials are much less efficient.

Suntech has developed a way to form monocrystalline material using a modified version of the multicrystalline process.

It uses seed crystals, but instead of being gradually drawn out of the silicon (as with the conventional monocrystalline process), they are arranged at the bottom of a crucible and completely covered with melted silicon. Then heat is extracted through the bottom of the crucible, ensuring that crystallization begins at the bottom, where the seeds are.

This is essentially the process patented decades ago. Refinements Suntech has made help overcome one of the main challenges the process presents: the molten silicon in contact with the edges of the container forms its own seeds, and as a result, the final slab of silicon is monocrystalline on the inside and multicrystalline toward the outside.

Suntech figured out how to keep the multicrystalline area to a minimum: the resulting ingot is 70 percent monocrystalline. Pure monocrystalline wafers are made from the center of the ingot. The material on the edges, which is half monocrystalline and half multicrystalline, is also used. Cells made of this turn out to be about 10 percent more efficient than ordinary multicrystalline cells, which is almost as efficient as purely monocrystalline cells.

Wenham says the process can use existing wafer-processing equipment, so it can be scaled up quickly. “The process could be quite a game-changer in photovoltaics, as it offers much higher performance at reduced costs,” he says. 

Wenham says that Suntech expects much of the industry to adopt similar technology in the next two years. Because the basic principle is no longer under patent, many companies have been able to develop their own versions of it. He says Suntech is looking to patent technologies related to using the new materials to make solar panels.

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