Sometimes, the most significant and innovative technologies do not occur with many bells and whistles hailing a totally unique concept or product. Such is the case with the work of Department of Energy's National Renewable Energy Laboratory (NREL) researchers. Twenty years, and 12 patents later, NREL researchers have finally made progress in thermal processing with the creation of the Optical Cavity Furnace (OCF). The OCF innovation is a variation on the furnace technology that processes wafers into solar cells.
The OCF relies on optics, which heat and purify wafers with the highest degree of accuracy. In addition, the oven creates a significant increase in solar cell efficiency. The breakthrough technology requires rigid engineering processes to enhance efficiency while simultaneously reducing heating and cooling costs.
Employing a standard Rapid-Thermal-Processing (RTP) furnace, solar cell wafers go through a five-step manufacturing process, including a radiative heating method that utilizes infrared lamps. The lamps provide heat and ultraviolet (UV) light for processing wafers. The infrared lampsraise the temperature of the wafer as high as 1,000 degrees Celsius (C), in just a matter of seconds.
Below are the five steps for RTP wafer manufacturing:
·Heat the crystalline silicon solar cells
·Ease the stress on silicon cells
·Form junctions – positive (p-type) and negative (n-type)
·Diffuse dopants -- boron and phosphorous used to increase cells’ efficient.
Each step requires the technician to retool and reconfigure the furnace. It also entails modifying the temperature. Another problem with the standard thermal processing method-- wafers do not heat evenly, tend to cool and lose heat around the edges.
The New Technology
The OCF works by a series of lamps enclosed within a chamber covered with highly reflective material. The reflective skin allows the chamber to attain a uniform temperature level, which other methods have yet to match. By installing a superior insulating liner on the wall cavity, high quality reflective ceramics, and using a sophisticated geometric pattern to design the oven, it practically does away with energy loss.
This innovative cavity design requires about 50 percent of the energy of standard thermal furnaces. The Optical Cavity Furnace works under the same theory a microwave oven. Optics focus visible and infrared lights directly on the target—in this case the wafer—not the enclosure.
The optic cavity furnace solves these issues by heating the wafers in a uniform manner without the material touching the lamps. The OCF relies on proprietary software to change the temperature profile, which eliminates the need to retool or rearrange as required with thermal furnaces.
NREL colleague inserts solar cells into OCF, as Principal Engineer Bhushan Sopori gets ready to read its analysis on his computer screen
Source: NREL - Dennis Schroeder
Rapid-Thermal-Processing furnace uses radiative heat to boost the temperature of every part of a silicon wafer up to 1,000 degrees Celsius, within several seconds. An OCF heats wafers at a slower rate. This method not only takes advantage of the photonic effects, but also reduces the power requirements and lowers energy loss.
According to Dr. Bhushan Sopori, NREL’s Principal Engineer on the project, the ends of the one-kilowatt and two-kilowatt lamps used in the furnace are the only parts that require cooling—using nitrogen. Sopori goes on to say, the use of photons enable the researchers to form faster at lower temperatures.
Unmatched Production Capacity
The National Renewable Energy Laboratory researchers continue to work on enhancing the OCF’s efficiency. Researchers expect to soon raise the furnace’s efficiency another four percent—a significant accomplishment, when one considers small efficiency gains typically experienced in the industry. Dr. Sopori said, “Our calculations show that some material that is at 16 percent efficiency now is capable of reaching 20 percent if we take advantage of these photonic effects."
Collaborating with its private-industry partner AOS Solar Inc., the NREL has started the construction of a manufacturing prototype OCF. It will have the capacity to produce 1,200 wafers per hour. The researchers expect to shave about 25 to 50 percent off the cost of processing wafers when compared to the RTP furnace. The OCF hits the “trifecta” of solar cell manufacturing—boost solar cell quality, enhance efficiency and reduce costs.
The OCF completes the process in less time than rapid thermal processing furnaces--requiring just minutes to process a single wafer. Solar manufacturers are clambering to gain access to the technology. NREL has R&D agreements with some of the larger solar panel manufacturers.
Government Collaboration with Private Sector
The work of the NREL serves as a testament to the role of government laboratories, such as NREL, National Institute of Standards and Technology, and the Manufacturing Extension Partnerships to finance and develop innovative technology to foster advances in solar energy manufacturing and other renewable energy technologies.
Source: NREL- Dennis Schroeder
Along with working with NREL to expand the production capacity of the first commercial Optical Cavity Furnace, AOS Solar Inc is working to increase the furnace’s capacity to manage other wafer manufacturing processes, including oxidation and diffusion processes. Solar cells comprise the main cost of any solar energy system.
Bringing down the cost of solar cells, drives the systems cost even lower. Improvements in solar cell efficiency and technological advances help drive prices lower. This technology goes a long ways towards making solar energy, “cheap energy.”