The Optical Cavity Furnace is a relatively new type of furnace that uses light and optics rather than other sources to create silicon-based photovoltaic (PV) cells. The new process uses only half the amount of energy to make conventional PVs.
The recent innovation uses a series of lamps in a reflective chamber to create temperature uniformity at high-heat levels throughout the chamber. It’s so uniform that, when heated up to 1,000 degrees Celsius, the entire furnace interior only varies by a few degrees. The heat is used to convert silicon wafers into fully functional photovoltaic cells.
Light Has Multiple Advantages in Furnaces. Photons have special qualities that prove useful in creating solar cells. When light is shined on silicon atoms that are bonded electronically to each other it changes their potential.
The Optical Cavity Furnace shines visible and near-infrared light to heat the solar cell, and also shines ultraviolet light to take advantage of photonic effects that occur deep within the atomic structure of the cell material. This combination offers unique capabilities that lead to improved device quality and efficiency.
Iron and other impurities can degrade the silicon quality quickly. But shining the right light on it can remove that impurity from the silicon. Optics can also make a lot of things happen at the interfaces in a cell, where, for example, metal can reflect the light and speed the diffusion of impurities. The lamps in the furnace help fool the impurities in the silicon into moving out of the way, by creating vacancies.
Bhushan Sopori, NREL Principal Engineer said “We call it injecting vacancies.” A vacancy refers to the lack of a silicon atom. “If the atom is missing, you have a vacancy here, an empty space.” Those spaces prompt the impurities such as iron to feel much more like moving – and they do so at a much lower temperature than would otherwise be required. The iron moves in with the aluminum, creating an aluminum-iron mix that, happily, is needed anyway as a contact point.
Removing impurities can change a cell’s efficiency from 13 percent to 17 percent. What that means is that 17 percent of the photons that hit the improved cell are converted into usable electricity.
The absence of cooling water and confinement of energy in the OCF proves to be a big advantage for lowering the energy payback time of solar cells.
Other advantages of the photonic approach:
Silicon cells often have silver contacts in front and aluminum contacts in back. They usually are fired simultaneously as the cell is being formed. The OCF by selectively heating the interfaces of silicon and metal can better control the process, and thus create stronger field surfaces and improved cell performance.
The Optical Cavity Furnace uses photons of light to remove weak, cracked wafers from the processing line. Photons can more easily produce a thermal stress in a wafer and screen out bad wafers. The photon process tests the wafers’ integrity right after they are cut. The conventional method requires physical twisting and bending of the wafers to test for weakness.
“Its main purpose is to process the wafers into solar cells. We have developed the furnace configurations for major steps used in silicon solar cell fabrication, junction formation, oxidation, and metallization firing,” Sopori said.
Consider the Source
