Twin Creeks Technologies—a startup that has been operating in secret until today—has developed a way to make thin wafers of crystalline silicon that it says could cut the cost of making silicon solar cells in half. It has demonstrated the technology in a small, 25-megawatt-per-year solar-cell factory it built in Senatobia, Mississippi.
Siva Sivaram, the CEO of Twin Creeks, says the company’s technology both reduces the amount of silicon needed and the cost of the manufacturing equipment. He claims the company can produce solar cells for about 40 cents per watt, which compares to roughly 80 cents for the cheapest solar cells now. Twin Creeks has raised $93 million in venture capital, plus loans from the state of Mississippi and other sources that it used to build its solar factory.
The conventional way to make the crystalline silicon wafers—which account for the bulk of solar cells—involves cutting blocks or cylinders of silicon into 200-micrometer-thick wafers, a process that turns about half of the silicon into waste. The industry uses 200-micrometer wafers because wafers much thinner than that are brittle and tend to break on the manufacturing line. But in theory, they could be as thin as 20 to 30 micrometers and still be just as efficient, or more efficient, at converting sunlight into electricity.
Twin Creeks’ process makes 20-micrometer-thick wafers largely without waste. It involves applying a thin layer of metal that makes them durable enough to survive conventional solar-cell processing equipment. Sivaram says that by greatly reducing the use of wire saws and related equipment and making thinner wafers, Twin Creeks reduces the amount of silicon needed by 90 percent and also greatly reduces capital costs. He says the technology can be added to existing production lines. The company’s primary plan is to sell manufacturing equipment, rather than produce solar cells. “I expect that by this time next year, we’ll have a half a dozen to a dozen of these tools in the field,” he says.
The process begins in a vacuum chamber, where a high-energy beam of hydrogen ions bombards three-millimeter-thick disks of crystalline silicon. The ions accumulate at a precise depth of 20 micrometers, which is controlled by the voltage of the beam. Once enough ions accumulate, a robotic arm quickly removes the wafers, which are then placed inside a furnace, where the ions in the silicon form microscopic bubbles of hydrogen gas that expand, creating tiny fractures within the silicon wafer and causing a 20-micrometer-thick layer of silicon to flake off. The company then applies a metal backing to the thin silicon. (The proprietary process it uses sets it apart from another company, Astrowatt, which makes wafers that are similarly thin. But Astrowatt’s wafers are slightly curved, which could make them difficult to handle in conventional production equipment.)
The Twin Creeks wafers are compatible with conventional solar-cell production equipment, and with processes now being used to make advanced solar-cell designs, such as heterojunction cells. Sivaram says the hydrogen-ion process works with single-crystal materials other than silicon, including gallium arsenide, a semiconductor that has been used to produce world-record efficiency solar cells.
Using an ion beam to create thin wafers of crystalline silicon has been considered before, but it was far too expensive to be a practical manufacturing method. It required a particle accelerator that could produce ion beams that are both very high current and very high energy, and “such a beast did not exist,” Sivaram says. To make the technology viable, Twin Creeks developed an ion accelerator that is “10 times more powerful” than any commercially available accelerator, he says.
While the company emphasizes that the technology is compatible with existing production lines, it does require at least one change. Ordinarily, wafers are treated to create a rough surface texture that helps them absorb light rather than reflect it. The texture is made of pyramids that are about as tall as the Twin Creeks wafers are thick, so it isn’t practical to use with the new wafers. Sivaram says the company has implemented an alternative anti-reflection technology that allows its solar cells to perform as well as ones made with the conventional process.