Bright idea: This furnace uses lightbulbs, not heating elements, to treat silicon wafers.
Making solar cells involves subjecting silicon wafers to temperatures in excess of 1,000 °C. The process normally involves the use of heating elements, and requires a lot of energy.
A new optical furnace developed by researchers at the National Renewable Energy Laboratory in Golden, Colorado, heats up solar wafers by focusing light on them—a much more efficient process that uses about half the energy of a conventional furnace. More importantly, the new design also uses light to remove certain impurities from the silicon wafers, a step that can improve the power output of finished cells.
The work is at an early stage—so far the researchers have only improved the efficiency of the resulting solar cells by half a percentage point. But based on lab tests, they think they can increase the efficiency by four percentage points, from about 16 percent efficient to 20 percent, which would be a big deal in the solar industry, which celebrates even half-a-percent increases.
High temperatures are needed at more than one step during solar-cell manufacturing. Furnaces are used to introduce dopants into the silicon to create electric fields within the material, to create electrical contacts, and to oxidize surfaces to improve efficiency. The new furnace also allows for better control of some of these processes, which can improve a solar cell’s efficiency.
NREL’s design isn’t the only one that uses light to process silicon. Rapid thermal processing furnaces, used in the microelectronics industry, also use light to heat up semiconductors. But the new furnaces use highly reflective and heat-resistant ceramics to ensure that the light is absorbed only by a silicon wafer, not by the walls inside the furnace. “That makes it many times more efficient,” says Bhushan Sopori, the researcher in charge of the furnace project at NREL.