Lightness and flexibility are two advantages of thin film.
Thin-film solar technology has come a long way from the little strips that have powered calculators for decades. In recent years, an increasing number of manufacturers have begun to produce thin-film products for use on buildings as a viable competitor to more traditional mono- and polycrystalline silicon wafer photovoltaic (PV) arrays. This growing market share has been fueled by increases in thin film’s efficiency and decreases in its cost, but also by specific niches that traditional PV cannot fill, as well as the technology’s greater adaptability to architectural design.
Thin film has a lot to recommend it to the built environment. For one, it is much more durable than silicon wafers, which break easily. This makes silicon wafers unsuitable for regions such as Florida and the Gulf Coast, where hurricanes frequently put all building materials through the most severe of stresses. Even in the most clement environments, silicon wafers must be encased in glass-and-steel panels for their protection, adding weight to the modules and requiring elaborate armatures for their support. These arrays transmit wind and gravity loads to their host buildings and also create roof penetrations, which open avenues for leaks.
Most thin-film products today are produced in glass modules. “The cheapest thin film is made on glass and used in solar arrays in very large fields,” says Ken Zweibel, director of the GW Solar Institute at George Washington University. But the photovoltaic substance—usually either amorphous silicone, cadmium telluride, or copper indium gallium selenide (CIGS)—can be laid down on nearly any substrate in a process much like that of printing with ink. Several companies, such as Uni-Solar, Global Solar Energy, and Advanced Green Technologies (AGT), are producing flexible, stick-down solar strips by laying the PV substance down on stainless steel foil. “We are launching a 6-meter-long flexible module,” says Jean-Noel Poirie, the vice president of marketing at Global Solar. “It is meant to go on roofs that have weight limitations, where you can’t put heavy glass and steel arrays.”
“I went and toured a large installation of AGT’s stick-down PV strips out at the Nike corporate campus,” says Craig Briscoe, an associate at Portland, Ore., architecture firm Zimmer Gunsul Frasca (ZGF) Architects. “There were a lot of great things about it. We were walking around on the solar array. It’s very tough, not fragile like crystalline panels. It adds no extra weight from a structural perspective. And it has a really simple application—you just roll it out and plug it in.” First Solar’s FS Series 3 module, which is glass-enclosed, is 6.8 mm thick; Uni-Solar’s PowerBond laminate is 4 mm thick.
Another significant feature of thin-film technology that separates it from silicon wafer is that it functions under ambient daylight conditions and doesn’t require direct sun exposure. Crystalline PV cells work by taking direct UV rays and multiplying them inside the crystallized silicon. Thin film generates electricity by absorbing solar heat and radiation. This means that thin film will function independent of orientation and under cloudy conditions. It also opens up greater flexibility for the integration of the technology into architecture. Thin film can be produced in a variety of colors, including blue, red, brown, green, and yellow (though the lighter colors sacrifice efficiency), and can be integrated with relative ease into glass curtainwalls, and not just as spandrel units. Processes of laser etching mean that even vision panels can be outfitted with thin-film PV in much in the same way that ceramic fritting is currently used.
In terms of the price of purchase, crystalline solar modules and thin film are now approximately on par, costing between $1 and $3 per watt of energy that the modules will produce. This is a huge improvement for thin film, which just five years ago cost in the $4 to $7 per watt range. However, there is another important calculation to consider—watts per square foot. Currently, you get fewer watts per square foot with some thin film than you do with crystalline, a major factor given that the roof of any building has a limited amount of real estate to house a solar array. But compared with crystalline wafers, thin film is cheaper to produce. It uses a fraction of the material and requires less energy in the manufacturing process: crystalline wafers must be heated to extreme temperatures, whereas thin-film processes demand much less heat.
This illustration shows a Global Solar Energy CIGS (copper indiu gallium selenide) cell on a stainless steel foil substrate. Molybdenum is deposited as the back contact of the cell; transparent conductive oxide (TCO) is the top contact. A collection grid on top provides a low-resistance path for the electric current.
In addition to dropping its price, thin film must close the watt-per-square-foot gap, which will require a leap in the technology’s efficiency. ZGF, for example, has considered using thin film for a few projects, but in each case the firm chose traditional crystalline panels for their greater efficiency. “Typically, more traditional mono- and polycrystalline panels convert 16 to 20 percent of the sun’s energy into electricity,” explains Sean Quinn, AIA, a sustainable design specialist at HOK who formerly worked for the U.S. Department of Energy. “In thin film right now, we’re looking at 6 to 10 percent conversion. They’re going to have to push the efficiency up to 15 to 20 percent in next five years. Once they do that, we’re looking at a much more commercially viable technology.” Zweibel is bullish about the future of films using CIGS, a relatively new technology. These films are relatively hard to make but most efficient, Zweibel says.
Thin film may make most sense in dense urban environments and on mid- and high-rise buildings, where shading and the relatively meager amount of roof space to building square footage makes rooftop PV arrays of negligible value. For example, HOK is currently designing a 60-story office tower in Abu Dhabi, United Arab Emirates, that will include thin-film solar panels integrated into the full elevation of the façade. “As we move forward and try to meet 2030 goals and the GSA’s net-zero requirement, thin film is going to get us closer,” says Anica Landreneau, AIA, the sustainable design practice leader at HOK. “In order to meet those goals you have to generate power on site, which is so hard to do in an urban environment. Anything that can help us is great.”