As efficiencies continue to improve, more applications are found for flexible photovoltaics
For a long time, it was common knowledge that thin film and flexible photovoltaics (PV) were not practical because of their lower efficiencies compared with more traditional glass-encased PV. That is no longer the case. Today efficiencies are comparable (approaching or exceeding 20 percent) to traditional CIGS (copper indium gallium diselenide) or CdTe (cadmium telluride) technologies, and with their lighter weight-per-output flexible PV can at last claim advantages over traditional technologies due to growing commercial applications.
It is the usual cost-per-unit equation: the lower manufacturing costs are, and the lower installation costs become, the more people request the technology and the cycle fuels itself to reduce overall costs even more, thereby increasing market share.
“In the early days of flexible thin-film PV, efficiencies were around 4–8 percent,” says Colin Touhey, cofounder and CEO of Pvilion, Brooklyn, N.Y., a design and product development company specializing in flexible PV products. “Every year gets much better in terms of efficiency. We are constantly evaluating the materials and they are currently reaching 20–24 percent for architectural applications; military applications have always been at the very high end of this technology at approximately 30 percent efficiency.”
Perhaps another factor playing into this improved scenario is the consolidation of companies that have survived the shakeout after 2010, when the world economy caught up with the solar industry and many of the hundreds of PV solar companies that emerged in the early 2000s went bankrupt or were acquired by larger players.
Choose your technology
There are three main material technologies used in thin-film PV: amorphous silicon (a-Si), CdTe and CIGS, which is perhaps the largest category in commercial markets. Each has its advantages and passionate advocates.
“The most diffused PV cell technology in the marketplace is based on crystalline silicon cells, the cells most commonly used in rigid glass solar panels,” says Luca Bonci, director of Solbian Energie Altrenative Srl, Avigliana, Italy. Thin film technologies, based on new materials technologies, became a market player in the early 2000s with significant advancements in manufacturing capabilities.
An industry market report by GBI Research in 2009 estimated that more than 100 companies entered the thin-film market between 2001 and 2009 and went from a minimal 14 megawatts (MW) production to 2,141MW or almost 25 percent of the total PV market. However, the industry went through a major downturn beginning in 2010 and has been slowly recovering its share of the PV market since the global economic downturn. Many of the companies from the boom stage went bankrupt, were acquired by larger companies or were bought by Chinese companies that secured the technologies.
“Thin-film technology is ideally suited for manufacturing flexible PV,” says Bonci, “but suffered in the past both for technical reasons (it is generally more sensitive to moisture and is less efficient) and for its [at the time] higher prices.”
Much of the R & D on thin film stems from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) photovoltaics research into polycrystalline thin-film materials, primarily CIGS and CdTe, which it shares with major industrial players through cooperative agreements.
According to the NREL published information found on its PV research website, CIGS have typical substrate thin-film layers that are “deposited onto a glass, metal or polymer substrate. Sunlight enters through the top layer of the device (the transparent conducting oxide) and produces electrical current and voltage in the lower layers.” These assembly structures can produce a conversion efficiency of about 20 percent.
NREL’s description of the CdTe schematic states the layers of the device are “deposited onto a glass ‘superstrate’ that allows sunlight to enter. The sunlight passes through the glass and produces electrical current and voltage in the lower layers.” This configuration produces a conversion efficiency of about 17 percent.
Recent improvements up the ante
It’s all in the numbers. The biggest challenge to growth for thin-film or flexible PV is in the efficiencies, which affect the cost-per-wattage ratios and, in turn, the market potential for higher rates of adoption by commercial markets. Early December 2015 an online white paper from a German consortium1 was published, announcing an efficiency for crystalline silicon, thin flexible solar cells with an efficiency of 11.5 percent that are fabricated on welded silicon foils of 50μm thickness, making them well suited for roll-to-roll manufacturing.
Another recent announcement by the trade show IDTechEx2 reported that the Johannes Kepler University in Austria developed organolead halide perovskite (PbI) PV solar cells with highly flexible, ultrathin and stretchable capabilities yielding 12 percent. The report says the cells are 3μm thick and weigh only 5.2g/meter cubed. However, it is reported that if the cell should break down, its constituent parts are both toxic and carcinogenic, and “the stability of the cells under ambient conditions is a persistent problem.” Nevertheless, researchers predict the technology has the promise of more than 20 percent efficiency.
Results now, improvements to come
Several companies are putting their money on the line with real projects and production, including Solbian, a partner on several projects with Pvilion, as well as others. Solbianflex panels cannot be rolled out like true flexible thin-film PV, but can flex enough for most applications.
“What Solbian did,” says Bonci, “[and it was] among the first to do so, was to find a way to encapsulate the cells in a semiflexible package that is much lighter than rigid panels, but shares the advantage of using the best cell technology for the cost, efficiency and availability.”
Solbian collaborated with Pvilion on the solar panels that were laminated to the vinyl fabric cladding of the multiuniversity team producing the “Techstyle Haus” entry in the 2014 International Solar Decathlon. [See “Fabric Solar House Competes at Versailles” on this site.]
As efficiency improves more people will find a reason to use flexible PV, but there continues to be skepticism about the reliability of some thin-film technologies based on past promises about organic thin-film developments.
“In theory,” says Bonci, “organic PV could give a really flexible panel, nice aesthetic, ease of applications and low cost, but the fact is that we have been waiting for that for too long now. I think we already have the technology, both with thin-film and flexible crystalline. What is missing are the ‘solutions,’ ‘products’ [and] ‘ready-to-use applications.’ These are in initial stages of development.”
Bonci cites the application of thin, flexible PV to backpacks and camping tents, but says, “Even if you sell a million backpacks, PV volume is [still] small.” What we need, he says, are major manufacturers interested in selling ready-to-use solar canvas, which would boost flexible PV volumes significantly. According to Bonci, Pvilion is pioneering this field, offering solutions to many new markets.
“Most people are familiar with the rain fly on camping tents,” says Todd Dalland, cofounder and president of Pvilion. “What if buildings could take the concept of a fly and apply flexible PV? We think that every building should have a solar ‘fly,’ a skin covering the entire building that becomes a power generator.”
Systems like this can be modular and retrofitted to buildings of any shape or size, according to Pvilion’s directors. “The advantage with these systems is that they can easily be moved,” says Robert Lerner, cofounder and vice president of Pvilion. “The wraps or skins are prewired and come out of the crate ready for installation. They also can be used for parking lot applications where shade and power are simultaneously provided.”
According to Lerner, it would take less than half a day to set up a parking shade with integrated PV to cover a section of a typical lot. Five years down the road, Lerner predicts they will be able to provide coverings for hundreds of vehicles across the country using this system.
“As we scale up the technology,” says Dalland, “designers will have greater flexibility and many more options. With flexible PV you can design solar arrays on curved surfaces, and with free-form boundaries. This will happen, and when the cost of flexible PV panels is the same or less than glass enclosed PV, it will be integrated into building designs as commonly as thin, compact batteries are in personal electronic devices now.”
Bruce N. Wright, AIA, is a consultant to designers and architects and a frequent contributor to Fabric Architecture, Specialty Fabrics Review and Advanced Textiles Source.
1 Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, Germany; Dept. of Physics, University of Konstanz, Konstanz, Germany; BLZ-Bavarian Laser Center, Erlangen, Germany; i-MEET: institute Materials for Electronics and Energy Technology, University of Erlangen-Nuremberg, Erlangen, Germany. Published online: Dec. 11, 2015, by EDP Sciences. EPJPhotovoltaics.
2 Published June 2010; GBI Research.