Micro technology powers low-cost solar energy

When Martha Symko-Davies, solar business manager for the National Renewable Energy Laboratory in Golden, Colo., was searching for companies worthy of U.S. Department of Energy investment a couple of years ago, she was not looking for a research project. She wanted “innovative, disruptive technology” that could also rapidly go from prototype to pilot-scale manufacturing. She found it at Semprius Inc., a Durham, N.C.-based developer of high-concentration photovoltaic (CPV) modules, which earned a $3 million grant to help speed development of its manufacturing technology.

Semprius uses transfer printing to manufacture solar cells. All images courtesy Semprius.

Two years after that investment, Semprius is ramping up to pilot-scale production after breaking ground on a new plant in North Carolina last fall. And the renewable energy division of the German manufacturer Siemens AG recently took a 16 percent stake in the company, further validating what Symko-Davies called “an amazingly novel process” that meets her three main criteria of cost, efficiency and reliability. Symko-Davies is on a mission to make U.S. solar-cell manufacturing competitive with Chinese manufacturers, and Semprius’ technology is a step in that direction, she said.

Russell Kanjorski, vice president of business development for Semprius, said the core innovation is the company’s microtransfer-printing process. In the process, a sacrificial layer is grown on a gallium arsenide (GaAs) substrate, followed by epitaxial (or crystalline) growth of multijunction solar cells. Then, in a massively parallel process, the 600µm × 600µm solar cells are released and lifted off with a rubber stamp—thousands at a time—and printed on 6" ceramic interposer wafers. The wafers, along with primary and secondary lenses, are used to create CPV modules. A 6" GaAs wafer contains cells for as many as 40 to 50 modules.

The use of 600µm cells—each about the size of the tip of a ballpoint pen—offers a number of benefits that address the cost and performance of CPV modules, according to the company. For example, it enables the use of high-performance and inexpensive primary and secondary optics. The primary optic is a silicone-on-glass lens array; the secondary optic is a ball lens. The use of microcells reduces the optical path of the module, so, instead of being a 14"- or 16"-deep CPV module, Semprius’ module is only 2.5" deep. The CPV module has a geometric concentration of 1,100×, which means solar cells are less than 0.1 percent of the module area, which further reduces cell cost. Most competing technologies have a concentration of 400× to 600×, according to the company.

Cost advantages

Once scaled up, the total cost of CPV module manufacturing in the Semprius process will be lower than traditional solar module manufacturing, reducing the cost per kWh of electricity, Kanjorski said.

A cell-on-interposer, composed of one 600µm solar cell on a ceramic chip.

Semprius’ CPV module, with primary lenses on top and solar cells on a backplane on the bottom.

Symko-Davies confirmed that the Semprius printing process will allow the company to produce modules at a much lower cost than traditional methods. But cost wasn’t the only aspect of the process that piqued her interest, she said, noting that optics are also included in the unit. Often, solar cell companies contract with a separate optical-systems supplier. Semprius offers an all-in-one solution, further reducing costs. The company produces the entire CPV module, including the optics.

Another advantage, Kanjorski said, is that other solar companies typically use the entire wafer in their solar cells. “This microtransfer-printing process enables us to use a rubber stamp and we just release the very top layers of the wafer,” Kanjorski said. After the solar cells are removed, the GaAs wafer is cleaned and re-used to grow more solar cells.

Small modules, high concentration and high efficiency represent the future of CPV module manufacturing, Kanjorski said, and the Siemens investment—part of a $23 million investment from venture capitalists—shows that the big players are taking notice. “We are working with them, as well as other large customers, on the commercial side of the business,” Kanjorski said.

Right now, the company is manufacturing solar cells on an R&D prototype basis, but the new manufacturing plant, which will be brought online in stages, will include a pilot line to demonstrate high-volume manufacturing. Semprius will be automating significant portions of the process, he said.

Kanchan Ghosal, head of applications engineering at Semprius, said that there are many possible microtransfer-printing technology applications, but the decision to go with solar was made because the company saw an opportunity for a near-term market and for obtaining funding. However, the company will still examine licensing the technology for other applications, such as in LEDs, medical devices, displays and flexible electronics.

Finding the ‘sweet spot’

The current “sweet spot” of the solar cell market is commercial, utility-scale operations located in the desert or other dry regions, according to Kanjorski. “We’re trying to drive lower costs through some of the novelties that are part of our technology, [like] high efficiency, higher concentration and high energy yield, all of which tend to lower the cost. And we want to focus on where it works best, which tends to be the sunnier regions.”

The reason Semprius’ cells work best in high-temperature areas is because of their low temperature coefficient compared to traditional crystalline-silicon cells. As the ambient temperature rises and the module heats up during operation, Semprius’ cells are able to operate much closer to their peak efficiency than traditional cells. However, the optics in Semprius cells require direct sunlight to work effectively. That’s found in areas like southern Europe, North Africa, India, the Middle East, southwest U.S., Australia and parts of China.

Kanjorski was brought onboard in September to help Semprius take research into lower-cost solar processes to the next level, through what is known as the “valley of death” and into a viable business.

Symko-Davies said Semprius is well on its way toward that goal. It’s not only Semprius’ technology, but the “out-of-the-box” thinking at Semprius that impressed her. To expand the use of solar energy, “it’s going to take something more disruptive than what we already have,” she said, adding that when she’s deciding on an investment for the National Renewable Energy Laboratory, 50 percent of her decision is about the technology, 25 percent is the team and 25 percent is the business plan.

Stick with that formula, she said, and solar energy’s recent PR problems might go away. The problem with Solyndra—a now-infamous example of government aid to a solar company that eventually went out of business—was that the team and the business plan were not there to support the technology.

With Semprius and companies like it, she said, there is hope that solar energy could graduate beyond its current 1 percent share of the energy market.

They’re pushing innovation,” she said. “They’re looking at disruptive technology that can make a difference in the United States.” µ