Thin Film Could Soon Make Solar Glass and Facades a Practical Power Source

The possibilities for building-integrated photovoltaics are about to get much more interesting. Future buildings, even some in design today, could use a new, semi-transparent solar glass. Thin-film solar technologies may eventually make it practical for the entire building envelope to generate power.

Denis Du Bois
May 22, 2006

A new technology makes it possible to manufacture windows that generate electricity and still allow 70 percent of the light to pass through. The developer of this technology, XsunX, calls it Power Glass. They are bringing their technology out of the lab this year, while they work on a solar cell that could be applied to a building's façade.

The transparency of Power Glass sets it apart from existing building-integrated photovoltaics (BIPV) products on aesthetics alone, and it could be considerably less expensive. Large-scale manufacturing techniques, combined with the use of thin-film photovoltaic materials, promise a lower cost on a per-watt or per-foot basis.

XsunX reel-to-reel cassette system allows high-volume production. Inside a sealed deposition chamber, solar cells are applied in a thin layer onto large rolls of supporting material. The process happens at 150 degrees C, low enough to use a plastic or polyester substrate.

Thin film offers diverse applications
Power Glass is made using amorphous silicon, the non-crystalline form of silicon. What's different is that amorphous silicon can be deposited in a very thin film and remains flexible.

By comparison, the crystalline silicon used in conventional solar cells is a thousand times thicker, requiring more silicon. And crystalline silicon must be deposited on a rigid substrate that can withstand high manufacturing temperatures.

The applications of amorphous silicon thus far have been primarily in liquid-crystal displays and thin-film transistors; its photovoltaic applications have been limited by its relatively low power-producing efficiency compared to crystalline silicon.

The efficiency gap is closing, thanks to continued private development, and to research at the National Renewable Energy Laboratory. While the four percent conversion efficiency of Power Glass won't knock anyone's socks off, efficiency is not always the highest metric of effectiveness.

At the Power-Gen Renewable Energy conference I spoke with XsunX CEO Tom Djokovich, who offered a fresh perspective: "Maybe the opportunity is not in more efficient solar cells, but in more efficient use of solar cells," he suggests. "That's why BIPV is smart. It lets us use these vast, previously unused building surfaces. And we're not altering the aesthetics of the environment -- we simply wrap the building with an invisible capacity to produce energy."

Lawrence Gasman, principal analyst at NanoMarkets, writes: "Lower costs are likely to play a role in driving thin-film into the numerous markets already served by PV." However, he adds, marketers should not stress cost alone because, in the long run, "what will drive the thin-film PV market as much as anything else is its unique characteristics in terms of flexibility, weight and ability to integrate into other products."

Reel-to-reel reduces manufacturing cost
XsunX has exclusively licensed a reel-to-reel cassette system that it says allows high-volume production of thin films with low risk of contamination. Solar cells are applied in a thin layer -- about 0.2 microns thick -- onto large rolls of supporting material.

"This system provides our technology enough scalability and throughput to bring the price point down below two dollars per watt on power glass," Djokovich says.

The process happens at 150 degrees C, low enough to use a plastic or polyester substrate, but it requires extremely clean operating conditions.

"Roll to roll manufacturing suffers from various maladies," says Djokovich. "The innovation of our cassette system is that it allows you to place the scalability of roll to roll processing inside a vacuum deposition cluster tool."

In the sealed deposition chamber, multiple cassettes of film are processed simultaneously. The result is rolls of photovoltaic film.

Alternative to mosaics
The flexible film is then applied, much like a low-e coating, to the surface of a multi-pane window. The film allows edge-to-edge coverage, making the entire window an active energy conversion area.

Architectural solar glass companies such as Scheuten Solar arrange opaque solar cells in glazing, separated by clear spaces, resulting in a visible mosaic or stripes. Power Glass looks more like tinted solar control glass, without a pattern. It blocks 30 percent of the incoming light, uniformly across the window opening.

Related articles:
"Solar Power Outside Promotes Collaboration Inside" Scheuten Solar Glass Case Study
"Solar Glass in a Nutshell"

Timing depends on licensees
XsunX has begun marketing its technology to potential licensees this year. Manufacturers will license the technology, and purchase the manufacturing equipment, from XsunX.

"We focus on the development of proprietary thin film solar cells and their manufacturing processes," explains Djokovich. "We sell technology, and the manufacturing processes are a delivery method that provides commercial scalability to our design."

A handful of product integrators have beta-tested sample films in concept Power Glass products for glazing applications. Tests like these help manufacturers build a business case for the new technology.

Once signed, licensees will take some time to integrate the reel-to-reel process into their plants. Look for the first commercial Power Glass products as early as 2007.

What next?
In January 2006 XsunX began developing a high-efficiency opaque solar cell with exciting potential to make the non-glass portions of buildings productive. It, too, is flexible, so it can be applied to contoured surfaces on a roof or façade.

The technology combines concepts from Power Glass and a nanocrystalline solar cell to make a two-sided solar cell. Stacking techniques like this have been the subject of research to increase power output by trapping more of the light spectrum.

Stacked solar cells were initially built as a string with one positive and one negative terminal, similar to a storage battery. And, as with batteries, degradation in one cell degrades the entire string. This is because of a requirement known as current matching: The cell with the lowest current limits the current of the complete unit.

The XsunX stacked cell will have four terminals, instead of two. Djokovich predicts this design will eliminate degradation by overcoming the need for current matching. He explains:

"We can have a solar cell on one side of an insulator working to its maximum potential, and a solar cell on the other side working to its maximum potential, and combined they provide significantly more power. Out the back we run two positive and two negative terminals. With that we eliminate a condition that's plaguing other thin film devices, the need for current matching."

The market for opaque photovoltaics overshadows that of solar glass. With this technology, XsunX is aiming squarely at that larger market. The company's researchers expect to achieve at least 12 percent conversion efficiency, if not 15 percent (comparable to today's silicon wafer technology), at a lower cost.

"This is a product that will go right after the crystalline wafer market and offer an alternative, a lower price point, and more diversity of applications," says Djokovich.

He sees the next BIPV opportunity in the form of roofing materials and facades, particularly in prefabricated construction.

"The trend is toward modular panel construction -- where entire façades of a building are prefabricated offsite -- and integrating these PV components into the façades at the factory level. Then when they're installed they're all part of an engineered, integrated structure."

This panacea of BIPV may be two steps closer to reality.

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