Tinted Windows that Generate Electricity

Light power: A flexible, lightweight solar panel from Heliatek.
Credit: Heliatek

A startup in Germany has developed a new kind of solar panel made of small, organic molecules deposited on polyester films. The technology is similar to what's used for OLED displays for phones and flat-screen TVs. The panels are flexible, and far lighter than conventional solar panels, yet in some locations—particularly where it's hot or cloudy—they can generate just as much electricity as a conventional solar panel.
Heliatek, based in Dresden, is funded by Bosch, BASF, and others, and has raised 28 million euros so far. The company, which recently started making its panels on a small, proof-of-concept production line, hopes to raise an additional 60 million euros, part of which will be used to build a 75-megawatt factory. This is fairly small for a solar panel factory—at such a small scale, Heliatek's panels will cost more per watt than conventional solar panels, says CEO Thibaud de Séguillon. But in four to five years, by which time Heliatek should reach large-scale production, the cost could drop to around 40 to 50 cents per watt, which would make them competitive with conventional solar panels, he says.
Meanwhile, Heliatek will need to find a way to sell its solar panels at a premium to fund its expansion. It plans to do this by selling products that take advantage of its solar panels' unusual  light weight and flexibility. In one case, it's working with a building materials company to integrate its solar panels into forms for concrete facades. At a construction site, forms will be filled with concrete, and the panels will become a part of the façade.
Heliatek is also working with another manufacturer to incorporate its solar panels, which can be semitransparent, into windows. "It's like tinted windows, only these windows generate electricity," Séguillon says.
Builders might be willing to pay a premium for the solar panels because they're cheaper to integrate into a building; they wouldn't have to buy hardware to anchor the panels to a roof, for example. Policies in Europe that will soon require buildings to produce as much electricity as they consume could also drive builders to integrate solar panels into windows and facades, says Séguillon.
Heliatek's key innovations are the active materials in its solar cells and the process for making the cells. Organic solar cells have been around for decades. The idea behind them is that certain organic molecules—typically types of long polymers—could be cheaply printed, leading to very-low-cost solar cells. But such cells have proven inefficient and have had relatively short lifetimes, so they are used only in niche applications.
Heliatek's panels are more efficient than the polymer-based ones, and are expected to last as long as a conventional silicon solar cell. The company uses short molecules called oligomers instead of polymers. Oligomers are inherently more stable, and can be deposited using a vacuum-deposition process that allows for precise control over the thickness and uniformity of the resulting films. That uniformity increases efficiency, and makes it easy to make multilayer solar cells that contain materials tuned to particular wavelengths of light—making the cells even more efficient.
Heliatek's complete panels (a panel is a collection of cells wired together) convert 8 percent of the energy in light into electricity (polymer solar panels are 3 to 5 percent efficient). Conventional silicon solar panels are 14 to 15 percent efficient, but the Heliatek technology's good performance in low light  and high heat can make up for the lower efficiency, Séguillon says. In recent tests in Singapore, for example, Heliatek panels generated slightly more electricity over the course of a month than conventional silicon solar panels, he says.
Vacuum deposition is more expensive than printing, but Heliatek has introduced another innovation to help lower costs. Rather than making solar cells in batches on sheets of glass, it makes them continuously in a roll-to-roll process that deposits the materials on polyester.
To reach its ultimate cost goals, Heliatek will need to substantially improve its panels' efficiency from 8 percent to 12 percent. This is possible because the company's manufacturing process has allowed them to make what's called a tandem solar cell, which has two layers for absorbing light and producing electrons. In the current design, both layers are tuned to convert the same wavelengths of light. But the company could also tune layers to different wavelengths and thus convert more of the solar spectrum. Heliatek is working with BASF to develop new absorber materials for such cells.
If the company does meet its cost goals, it still may be hard for it to compete head to head with companies making conventional solar panels. For one thing, banks are more willing to lend money for large solar power projects that use conventional panels, since they have proved workable in the field. In the long term, new solar companies may fare better by developing radically different solar panels that produce far more electricity than