Date: Jul 15, 2010 Author: Joe Singleton Source: MDA (
click here to go to the source)
by Joe Singleton/jsingleton@nttc.edu
A new manufacturing process could cut in half the cost of solar panels for homes and offices, resulting in photovoltaic (PV) systems that offer enough in electric bill savings to pay for themselves within six years of installation.
International Solar Electric Technology, Inc. (ISET; Chatsworth, CA), manufactures copper-indium-gallium-selenide (CIGS) thin-film solar cells using a patented process that reduces the need for expensive equipment and labor, thereby resulting in lower manufacturing costs compared with conventional solar cells. ISET uses what's known as an "ink-based" process—rather than the physical vapor deposition process that is common among many thin-film photovoltaic system manufacturers.
MDA funded ISET through a 2005 SBIR Phase II contract to develop lightweight, flexible solar cells capable of achieving high power densities for use in space-borne ballistic missile defense technologies. The MDA contract helped ISET develop valuable knowhow for improving solar cells, and after the MDA work concluded, the company began considering the technology's commercial potential.
Solar cells have been around for more than 50 years. Solar cells based on crystalline silicon wafers are most commonly used for powering terrestrial residential and commercial projects as well as for powering satellites in space. However, such cells are expensive and therefore require subsidies for commercial PV installations to be affordable. Thin-film CIGS cells were first produced in the 1980s and can potentially provide high performance for a much lower cost. Researchers at the National Renewable Energy Laboratory have recently fabricated a small-area CIGS cell with conversion efficiency of 20 percent. This level of performance for CIGS nearly matches crystalline silicon solar cells having 23 percent efficiency.
But is there a way to produce—and mass-produce—inexpensive, efficient, durable, and environmentally friendly thin-film CIGS solar cells, while keeping all production within the United States? ISET President Vijay Kapur says "yes" on all counts.
To make solar cells, ISET uses water-based inks that are printed onto flexible foils and glass in various patterns to fabricate complete solar modules. The printing capability offers the company the flexibility to design solar panels of all sizes, which cannot readily be achieved using other solar cell technologies.
ISET's process starts with the metallization of a non-conducting substrate by the sputter deposition of a 0.5-micron molybdenum layer. Next, an aqueous precursor metal-oxide ink formulated from nanoparticles of copper-, indium-, and gallium-oxides is coated onto the metallized substrate. The thickness of the wet precursor coating as deposited ranges between 15 microns and 20 microns. The dried precursor oxide layer is reduced under a hydrogen gas atmosphere to obtain a copper-indium-gallium alloy layer. The alloy layer is then annealed under a hydrogen selenide atmosphere to form the desired CIGS absorber layer with a thickness between 1 micron and 2 microns. Solar cell formation is completed by deposition of a cadmium sulfide window layer by chemical-bath deposition, followed by a layer of zinc oxide by low-pressure organometallic chemical vapor deposition. Top contacts made of silver are used for current collection. CIGS modules are formed by monolithic integration of the cells using metal contacts followed by encapsulation.
Unlike its CIGS competitors, ISET uses copper-indium-gallium-oxide nanoparticles for formulating the ink. In ISET's proprietary oxide-based approach, controlled composition and stable inks are formulated, thereby making them easier for engineers to work with in a solar cell manufacturing operation. High-quality oxide-based inks already are widely accepted for products in the electronic, printing, paint, food, and cosmetic industries, Kapur said.
Kapur further noted that the biggest advantage of his product is cost. The equipment and materials ISET uses in making the solar cells in a nonvacuum process are relatively inexpensive and require minimal manufacturing floor space. The machines used in the manufacturing process are robotic and reduce the need for direct labor. Savings derived from efficiently using inexpensive equipment and materials, as well as a limited workforce, can be passed on to the consumer.
Within the next few years, ISET plans to market a solar energy system for homes using its patented thin-film cell technology. The system, capable of generating up to 4 kilowatts, will tie into the utility grid and be offered for about $10,000 with all components installed—including rooftop-mounting. This price represents one-third the average cost of a $30,000 installed silicon unit today with the same power output. Another advantage: The $10,000 investment could be recovered in about six years through savings on electricity bills along with the currently available incentives, said Kapur. The savings estimate is based on sun-prone areas such as Southern California. In less sunny places, like the Pacific Northwest, it might take longer for a solar system to pay for itself. But by paying a little extra for independent battery storage, users can make a clean break with their local electricity supplier. After recouping the cost of the solar system through savings on energy bills, they effectively will be powering their homes for free.
Efficiency is another advantage of ISET's thin-film solar cells. Kapur said his company can design multi-junction thin-film solar cells with at least 25-percent efficiency—surpassing the best performance of current CIGS technology. The company's ultimate goal is to develop thin-film cells that not only outperform competitive CIGS technologies, but exceed the 23-percent efficiency standard of silicon solar cells, he said.
Adaptability is another advantage of this technology. Using ISET's technology, solar panels of different sizes can be fabricated both on rigid and flexible substrates without making major changes for tooling in the manufacturing plant. ISET's extra-thin cells will allow for lightweight, flexible modules—finding use in more products, such as next-generation personal electronics.
And given the current state of the national economy, ISET's solar-cell technology brings an added bonus: The technical development, engineering, manufacturing, and selling of the company's products will remain in the United States. Kapur says his company will employ a small group of highly qualified, well-paid U.S. workers to develop American-made alternative-energy products.
Besides the obvious markets in home power and personal electronics, Kapur plans to explore use of his technology in the areas of rural electrification, water purification, and cell-phone charging stations, which would specifically be built for export to developing nations where many people still lack reliable electric service.
Kapur plans for ISET to become a full-scale production company, moving away from its R&D roots. To enable this plan, the company recently started operating a pilot plant in a 24,000-square-foot facility in California. While Kapur plans to grow ISET, the company looks for partnerships and opportunities to obtain private equity funding.
