New Approaches Could Make Solar a Better Business
Advances in laser drilling, coatings and polygon scanner technology promise to speed up photovoltaic panel production – and drive down costs, too.
The closer you get to Germany by plane, the more the country seems to glitter. Shiny solar panels cluster on rooftops and installations in and around cities, and they crop up in more agricultural areas as well. Many of these solar collectors are used for water heating, but more and more, the systems are converting sunlight directly into electrical current.
Researchers from Fraunhofer-Gesellschaft, an international application-oriented research organization, were busy this summer preparing to present methods for improving solar cell efficiency and for driving down costs to attendees at the European Photovoltaic Solar Energy Conference, held Sept. 5 to 9 in Hamburg, Germany. And the photonic technologies they have developed for solar run the gamut from lasers to coatings.
Not surprisingly, lasers figure heavily into their methods. “Laser technology permits contact-free, precise and quick processing,” said Dr. Malte Schulz-Ruhtenberg of Fraunhofer Institute for Laser Technology (ILT). Speeding up processes can naturally lower costs.
When light hits a traditional solar cell, negative charge carriers are released from their bonds, and electrical current results. Until now, the contacts that draw the electric current had been placed on the front and rear of the cell. But the contacts on the front cast shadows on the cell, interfering with some light absorption. Moving all of the contacts to the rear, a technology known as emitter wrap-through (EWT), cut out these shadows and increased the level of generated energy.
And high-rate laser drilling creates the tiny holes needed for EWT very quickly and precisely. “We [have] developed a fast drilling process with 10,000 holes per second, suitable for an EWT production chain,” Schulz-Ruhtenberg said. “Unfortunately, the EWT cell concept is very complex. To my knowledge, there are no cell producers actively pursuing this technology at the moment.”
The researchers incorporated two coupled JenLas IR 70 lasers from Jenoptik AG of Jena, Germany, into the process. “The laser has the advantage of a programmable pulse length in the range of hundreds of nanoseconds to some microseconds, which was important for the process development,” Schulz-Ruhtenberg said. “To achieve 10,000 holes per second, it was necessary to couple two of these lasers, since together a repetition rate of 60 kHz is possible – a single laser only reaches 30 kHz at the desired pulse energy of >2 mJ.”
Polygon laser scanners
A prototype polygon scanner system in development by ILT researchers could enable faster laser processing of solar cells and other materials. In such a scanner, rotating polygonal mirrors would precisely deflect millions of laser pulses per second, providing higher speeds and throughput rates, enabling very quick processing of large areas.
“The original idea of such a mirror is used, as far as I know, in the printing industry – for example, for offset printing of newspapers where speeds of hundreds of meters per second are required,” Schulz-Ruhtenberg said. “The idea to transfer this to laser processing is also rather old. But due to the current trend to go to high-repetition-rate ultrashort-pulse lasers, these scanners become more and more important for laser materials processing.”
Any type of laser can be used with a polygon scanner. High-repetition-rate lasers operating at several megahertz particularly benefit from scanning speeds of hundreds of meters per second, Schulz-Ruhtenberg said.
The scanners could improve any large-area process, such as surface treatment of large samples, he added. “For example, it is possible to treat the full surface of a solar cell – 156 x 156 mm2 = 24,336 mm2 – in a matter of seconds, if a suitable laser is used. In addition, this type of scanner allows [for separation of] the pulses of a high-repetition-rate laser with up to 20 MHz, making it possible to fully exploit the potential of these lasers.”
One specific solar-cell-processing application could be full-area-laser doping. “Currently, the selective-emitter process is becoming state of the art, but it still requires a standard oven process to apply the full-area emitter,” he said. “Polygon scanners, in combination with high-repetition-rate lasers, can enable a laser-doping process of the full area, combining the selective-emitter laser doping and the oven doping into a single laser-based process.”
One of these is the hot-wire chemical vapor deposition (CVD) method for production of the semiconductor layers at the heart of a solar cell. In conventional CVD, which is plasma-activated, the material is bombarded with high-energy particles during coating. In the hot-wire CVD approach, the gases that create the film are activated on hot wires instead of in plasma, a gentler approach “because no (ionized or neutral) energetic particles hit the surface, since there are no electrical fields and no plasma,” said Dr. Markus Höfer of IST. “The activation of the gas phase is solely done thermally, and energy is transported ‘chemically’ by means of radicals.” The approach also allows for better use of the silane gas required for production.
“With the hot-wire CVD method, we convert up to 90 percent of the gas used [into] film material,” said the IST’s Dr. Volker Sittinger. The current literature reports that the silane gas-conversion efficiency of hot-wire CVD can be five to 10 times higher than that of plasma-enhanced CVD, which decomposes less than 20 percent of the precursor gases, Höfer said.
The principle behind the method dates to the 1910s and is used for research in a wide range of fields and for various applications, Höfer said. “However, most of this work is done in universities in small laboratory systems and on small areas. The novelty is that Fraunhofer IST has developed an industrial-like coating system for developing and demonstrating the capabilities of hot-wire CVD on larger areas and under production-like conditions.”
These technologies and more could usher in an even brighter age for solar energy – a brighter, cheaper and more efficient age.