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Progress in solar cells and high cost materials lead to developments in specialty films

Photovoltaic market is growing explosively, offering an opportunity for specialty film. Solar cell and module production, which was forecasted before the recession hit to grow at 50% pa for the next few years, is now expecting a slightly more modest 30% annual growth. A solar cell with special films is just one component of a fully assembled and functional solar module. The market for new modules is expected to grow from 6 gig watts a year in 2008 to 34.7 GW/year by 2015.
Solar modules have always been high in cost. Many module makers are trying to convert from batch production to continuous roll-to-toll methods to achieve higher volumes and lower cost. They are making new second-generation “thin film” solar cells, which are expected to grow from 10% of today’s market to about 40% in a decade. Thin-film solar modules are less expensive and lighter weight than previous modules made with crystalline silicon solar cells, but thin-film modules may not last as long or be as efficient. Solar module production is largest in Europe (where installations are heavily government subsidized) and in Asia, where many new module plants are being built. The US, however, is ahead in building plants for the new thin-film cells, which can be built into both flexible and rigid solar modules. Flexible solar modules show promise. For flexible solar modules to be widely used in roofing, however, they will have to provide a product with life of 20-25 years. High growth, new cell technologies, and a big push to cut costs is encouraging development of new film technologies.
There are two types of commercial solar cells. Crystalline silicon (c-Si), produced for about three decades, constitutes about 90% of the market, and “Thin-film,” introduced 10 years ago, makes up the remaining 10%. Older-style c-Si solar cells all share similar construction, with slight variations. They use silicon wafers sawed from rods of pure silicon to convert light waves into electricity. Silicon is expensive, and the wafers are fragile, so modules require special handling throughout production. Crystalline silicon modules consist typically of a glass top sheet, followed by EVA encapsulation film, silicon-wafer solar cells, another EVA film, and finally a back sheet of laminated fluoropolymer and other films. All these components make c-Si modules expensive, but they have an operating life of 20 to 25 years and electrical efficiency of 14% to 23%. The world’s largest producer of c-Si solar modules is Suntech Power Holding Co. in China.
Thin-film solar cells are the second generation, holding the promise of lower cost. Many are made by continuous roll-to-roll production, whereas crystalline silicon is a batch process. “Thin-film” refers to the light-absorbing semiconductor layer, which is indeed a thin film of conductive metal, vapor-deposited in a vacuum on a thin substrate like glass, aluminum, stainless steel or polyamide. Thin film modules are much thinner, less expensive and less fragile than crystalline silicon, but they have a shorter track record since most are recently developed. Their electrical efficiency ranges widely from 6% to 20%, depending on the semiconductor. Thin film cells use one of three major semiconductor materials: a-Si (amorphous silicon), CdTe (cadmium telluride), or CIGS (copper-indium-gallium selenide). They also have one or two protective encapsulation layers of plastic films, a front sheet of glass, and sometimes a back sheet of glass or plastic. Amorphous silicon (a-Si) is the oldest and most widely used thin-film semiconductor. Though not as efficient as crystalline silicon, it is less expensive. It can also be deposited on glass and heated to convert it into micro-crystalline silicon. The biggest commercial producer of a-Si thin-film solar cells is Sharp Corp. of Japan, also a large producer of conventional c-Si solar cells. Sharp pairs a-Si with micro-crystalline silicon to get higher efficiency. Power Film Inc. (formerly Iowa Thin Film Technologies), started by former 3M scientists in Ames, Iowa, makes plastic-based solar cells by depositing a-Si on a 1-mil flexible polyimide substrate. Power Film makes custom modules, typically with an ETFE front sheet and fluoropolymer/polyester back sheet. Individual cells are laser-cut from a continuous sheet with electrical connections printed on the surface of the panel, reportedly increasing reliability and reducing cost. Power Film targets low-voltage navigational aids for the military and will launch a “20-year” module for building integration later this year. Nanotechnology is also being used in thin-film semiconductors for solar cells. Innovalight in Sunnyvale, Calif., has a technology to print light-absorbing inks that contain silicon nano-crystals.
CdTe semiconductors account for about 30% of thin-film solar cells. CdTe is similar in efficiency to a-Si, but more sensitive to moisture. First Solar Inc. in Tempe, Ariz., the world’s largest producer of thin-film modules, uses CdTe cells, starting with a glass front sheet, a thin layer of tin oxide, a semiconductor on a metal substrate, EVA encapsulate, and glass back sheet. CIGS is the newest type of thin-film metal semiconductor. It’s potentially the most efficient and least expensive, but also the most sensitive to moisture. Early entrants in CIGS solar cells use different production methods, and most are not fully commercial yet, so they account for only about 1% of the market. Nanosolar in San Jose, Calif., prints CIGS as an ink (like printed circuits) onto a metal substrate in a roll-to-roll process. Commercial cells have up to 15% efficiency, developmental cells up to 20%. Global Solar Energy, a 10-year-old firm in Tucson, Ariz., and Helio Volt in Austin, Texas, both started up plants last year to make CIGS solar cells and modules.
Thin-film solar cells with metal or mineral semiconductors are still in their infancy. Organic semiconductor materials are dissolved in solvents or inks and printed or coated onto a plastic substrate in a continuous roll-to-roll process. Substrates for organic semiconductors can be plastic because organic coatings do not need to withstand high temperatures like metal. Organic conductive materials are less expensive than silicon, cadmium, or tellurium, the metals used in current thin-film solar cells, but organics are also less efficient. The first organic thin-film solar modules are in prototype production now. Power Plastic modules from Konarka, for example, have a clear plastic top layer, a coating of prepolymer adhesive, then ink containing fullerene conductive nano-carbon clusters (or “buckyballs”), and another liquid polymer coating, applied onto DuPont Teijin Mylar polyester film with SiOx coating. The efficiency of Konarka’s test cells is now up to 6%. Longevity is also short (3-5 years), but Power Plastic is intended to generate power for portable electronic devices that typically would not be used for more than a few years, like laptops and cell phones. Plextronics Inc. in Pittsburgh has developed and is licensing organic inks for solar cells. It makes solar modules only for R&D, not commercial sale. A Plextronics cell consists of a transparent anode layer that lets sunlight into a layer pigmented with photoactive ink, which passes electrons to an electrically conductive polymer layer on a plastic substrate. Efficiency of test samples is up to 6%.
The advent of flexible thin-film modules has created a new market for plastic front sheets. Most are clear monolayer fluoropolymer films like ETFE or its derivatives. DuPont, Asahi Glass, and Saint-Gobain Performance Plastics Corp. make the fluoropolymer films. Saint-Gobain is developing new coextruded multi-layer photovoltaic products that could combine ETFE top sheets and encapsulation layers or glass top sheets with encapsulants. Other new optical films, most based on other fluoropolymers, are being introduced for this market. Ten months ago, Rowland Technologies Inc. in Wallingford, Conn., introduced the first PVDF front sheet, called Rowlar, using Arkema’s Kynar PVDF. Rowlar, which took 18 months to develop, has light transmission >93% and haze <9%. Another new optical front-sheet material is Corin XLS polyimide, made by ManTech NeXolve Corp. (formerly ManTech SRS Technologies) in Huntsville, Ala. It comes as a sprayable material or film and is used for photovoltaic arrays in space.
All solar cells use encapsulation films, typically EVA, to protect the electrode. The two largest producers are Specialty Technology Resources Inc. (STR) in Enfield, Conn., and Etimex Primary Packaging GmbH in Germany (formerly part of BP Plastics). Mitsui and Bridgestone in Japan also make EVA encapsulation films. And 20 or 30 new companies are getting into EVA film extrusion in China and Korea, where conventional module manufacturing is growing the fastest. For the module maker, crosslinked EVA is costly and slow to laminate and cure (10 to 20 min). Photovoltaic grades of EVA have high vinyl acetate ratios and special crosslinking aids for faster curing. Etimex offers ultra fast EVA film that cures in 7-10 min, but that is still considered slow. Alternative thermoplastic encapsulant films are being introduced to reduce lamination time. Thin-film a-Si modules are sometimes sandwiched between glass front and back sheets using PVB (polyvinyl butyral) encapsulation films, because of PVB’s long use in automotive safety glass. Du Pont and Kuraray make PVB encapsulation films.Several companies have introduced TPU encapsulation films. Bemis Worldwide, a formulator of specialty adhesives in USA is enhancing production of several new encapsulation films, including a TPU and several polyolefin films. The development of new materials in back sheets is extremely fast. The standard material for 20 years has been a lamination of DuPont’s biaxially oriented Tedlar PVF (polyvinyl fluoride) film on either side of biaxially oriented PET film. PET film provides dielectric and mechanical strength, while PVF protects the PET film against UV. DuPont is the only commercial producer of PVF and also makes Tedlar PVF film. BioSolar Inc., a start-up in Santa Clarita, Calif., is challenging Tedlar with a new lower-cost back sheet made entirely of biomaterials. It includes nylon 11 derived from castor beans, cotton-fiber paper, and polylactic acid (PLA) film, and is as effective as Tedlar/PET/Tedlar and much less expensive. rkema has introduced a 30-micron Kynar PVDF blown film for back sheets of c-Si modules, replacing 25-micron Tedlar film. 3M Company has developed new Scotchshield Film 17 as an alternative back sheet for c-Si modules. It is made of THV-PET-EVA. The THV fluoroplastic is made by 3M’s Dyneon subsidiary. The PET was especially developed by 3M for the application. Potentially the least expensive of all is a special EPDM film from BRP Manufacturing (formerly Buckeye Rubber Products) in USA which was co-developed over five years with researchers at the National Renewable Energy Laboratory. The crosslinked EPDM film replaces a lamination of three higher-cost films—EVA sealant, PET dielectric, and Tedlar PVF—with one inexpensive film. It is formulated to cure at the same temperature as the EVA top encapsulant layer in c-Si modules.
(Source Courtsey : Plastics Technology)
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