Solar-powered electricity prices could soon approach those of power from coal or natural gas thanks to collaborative research with solar start-up Ampulse Corporation at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL).
Silicon wafers account for almost half the cost of today’s solar photovoltaic (PV) panels, so reducing or eliminating wafer costs is essential to bringing prices down.
Current crystalline silicon technology, while high in energy conversion efficiency, involves processes that are complex, wasteful, and energy intensive. First, half the refined silicon is lost as dust in the wafer-sawing process, driving module costs higher. A typical 2-meter boule of silicon loses as many as 6,000 potential wafers during sawing. Second, the wafers produced are much thicker than necessary. To efficiently convert sunlight into electricity, they need only one-tenth the typical thickness.
NREL, DOE’s Oak Ridge National Laboratory (ORNL), and Ampulse have teamed on an approach to eliminate this waste and dramatically lower the cost of the finished solar panels. The aim is to create a less expensive alternative to wafer-based crystalline silicon solar cells.
By using a chemical vapor deposition process to grow the silicon on inexpensive foil, Ampulse is able to make the solar cells just thick enough to convert most of the solar energy into electricity. No more sawdust — and no more wasting refined silicon materials.
Straight from Pure Silicon to High-Quality Crystal Silicon Film
NREL developed the technology to grow high-quality silicon.
ORNL developed the metal foil that has the correct crystal structure to support that growth.
Ampulse is installing a pilot manufacturing line in NREL’sProcess Development Integration Laboratory (PDIL), where solar companies can work closely with NREL scientists on integrated equipment to answer pressing questions related to their technology development, as well as rapidly overcoming R&D challenges and risk. NREL’s PDIL program is focused on transformative innovation in the domestic PV industry.
With knowledge and expertise acquired from the PDIL pilot production line tools, Ampulse plans to design a full-scale production line to accommodate long rolls of metal foil.
The Ampulse process “goes straight from pure silicon-containing gas to high-quality crystal silicon film,” said Brent Nelson, who is the operational manager for the PDIL at NREL. “The advantage is you can make the wafer just as thin as you need it — 10 microns or less.”
Most of today’s solar cells are made out of wafer crystalline silicon, though thin-film cells made of more exotic materials like copper, indium, gallium, arsenide, cadmium, tellurium, and others are making a strong push into the market.
The advantage of silicon is its abundance, as it is derived from sand. Its disadvantage is that purifying it into wafers suitable for solar cells is expensive and energy intensive.
Manufacturers add carbon and heat to sand to produce metallurgical-grade silicon, which is useful in other industries, but not yet suitable for making solar cells. This metallurgical-grade silicon is then converted to pure trichlorosilane (SiCl3) or silane (SiH4) gas.
Typically, the purified gas is converted to create a silicon feedstock at 1,000 degrees Celsius (°C). This feedstock is melted at 1,414°C and recrystallized into crystal ingots that are finally sawed into wafers. Think of it as the Rube Goldberg approach to creating a solar cell.
Instead, the Ampulse process backs up two steps. Rather than create a feedstock, it works with the silane directly and grows just the needed silicon right onto a foil substrate.