By precisely designing the shape of the interior of the furnace, the researchers can control exactly where the light is focused, ensuring the wafers are heated evenly. It’s not enough to make sure the wafer is evenly illuminated—the edges have to receive more light because they lose heat more rapidly than the rest of the wafer.
The process reduces thermal stress on the wafers, and it allows for precise control over the chemical reactions that heating enables. Precise control of the rates and timing of the heating can also improve the electrical contacts on the solar cell, improving its efficiency. And it makes it practical to introduce an oxidation step. Oxidation has typically been used by only a few manufacturers for high-end solar cells, but the new process would make it cheaper and thus allow more manufacturers to use it.
Sopori says NREL has developed processes that take better advantage of photonic effects than the rapid thermal processing furnaces. As photons interact with the silicon, they can cause deleterious impurities such as iron to move out of the material, while keeping advantageous ones such as boron, which is needed for the solar cell to perform properly.
The researchers haven’t yet realized the complete four percentage point improvement in efficiency in part because the new processing steps aren’t all compatible with other steps in conventional manufacturing. Sopori says they are working to modify the other steps to take full advantage of the optical furnace.
NREL is also working with Advanced Optical Systems to develop a machine that can process not just one wafer at a time, as with the lab version, but up to 2,000. Such high throughput will be necessary if the furnaces are to compete with conventional ones, which are cheap to operate.