Hard on the heels of the news that renewable energy now leads fossil fuel production in terms of investment, companies like Xunlight are demonstrating thin-film silicon solar manufacturing capability in the hopes of capturing market share from rigid silicon solar panel manufacturers. For Xunlight, the manufacturing process involves imprinting amorphous silicon solar cells on sheets of stainless steel so thin you can roll them up like a blueprint. And, in a sense, they are – a blueprint for building integrated photovoltaics (BIPV) that promises more solar surfaces faster and cheaper.
Xunlight’s manufacturing process creates solar “sheets” 36 inches wide and up to a mile long, using a nine-layer, chemical vapor deposition process for the semiconductor materials, plus a bottom reflective layer and a surface electrode layer via the sputtering process. The resulting sheet, one hundred times thinner than a typical piece of paper, uses only a limited amount of silicon to create triple-junction solar cells.
These amorphous (without crystalline configuration) solar cells are much cheaper to manufacture, thus offering truly affordable solar energy in a lightweight format that can be used on many surfaces, from backpacks to buildings, to generate electricity.
Their drawback is an 8-percent efficiency rating, which is significantly lower than traditional, crystalline silicon panels at about 20 percent. Xunlight reportedly achieved the efficiency by incorporating three different materials (amorphous silicon, amorphous silicon germanium, and nanocrystalline silicon) that capture light in different wavelengths, overcoming bandgap deficiencies in other thin films and crystalline solar cells using only one primary material.
In fact, efficiency ratings are the single drawback of thin-film solar – a drawback that tends to overshadow their usefulness in so many applications given their flexibility. This is due to the inherent nature of amorphous silicon, which is less structurally reliable, resulting in degradation in the presence of sunlight or at high temperatures. Some concentrating solar technologies which promise to overcome this inefficiency are still on the drawing board.
Xunlight now joins the thin-film race with several other companies (Heliovolt, Nanosolar and United Solar Ovonic) to achieve thin film with efficiencies that match crystalline solar. Xunlight has an advantage in its high volume, roll-to-roll technique, but the two former companies are using copper indium gallium selenide (CIGS), which is competitive with crystalline solar in terms of efficiency. Unfortunately, no one has yet been able to make a cheap product in volume because CIGS is so difficult to work with.
Thin film as it currently stands also carries the inherent risk of easier permeability and combustibility, but solar installers envision a day when they can carry rolls of thin film up to a roof, install it, and have a watertight roof that is also aesthetically pleasing to homeowners. And thanks to the American Recovery and Reinvestment Act of 2009, millions in funding await entrepreneurs who think they can achieve that goal.
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