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New generation of solar cells-More efficient.


Typically, silicon PV cell manufacturers add a grid of thin silver lines to the cell via a screen-printing process to form the front contacts.

The TetraSun cell instead loads 50-micron-wide copper electrodes on its front contacts in a way that prevents diffusion of the metal — which can degrade performance. The new process exceeds the performance of traditional heterojunction cells without the need of any special equipment, complicated module assembly, or costly transparent conductive oxides. That adds up to a significant cost advantage when it comes to high-volume manufacturing.

"It's a potentially disruptive technology, and that's why we decided to work with TetraSun," said NREL's Martha Symko-Davies, who headed the Energy Department's SunShot Initiative PV Incubator program when TetraSun received a grant from it back in 2010. "The Incubator program supports potentially disruptive innovations from small startups.

"This shows we still have innovation in the United States. People thought there was nothing left to be done in silicon, but there is something left to be done."

Symko-Davies was referring to the Shockley-Queisser limit, which postulates that the efficiency of silicon solar cells can't exceed 29 percent; that is, no more than 29% of the photons that hit the cell can be converted into electricity. Modern monocrystalline solar cells don't achieve much higher than 22 percent conversion efficiency due to practical considerations such as reflection off the cell and light blockage from the thin wires on its surface. That's why analysts are enthusiastic about the TetraSun cell, which comes in at 21 percent efficiency even as copper replaces silver to lower the cost.

TetraSun had a unique idea, but NREL's measurements and characterization capabilities made it practical. "As the margins go down with silicon, the cost of every component becomes significant, especially when you're talking about square miles of this material," said NREL Principal Scientist Mowafak Al-Jassim. "We're trying to make enough of these solar panels to generate gigawatts of power. That's a lot of silver. We needed to replace silver with an equally good conductor, but one that was much cheaper."


Maximizing Savings More Essential Than Ever


The importance of squeezing every last penny of savings from a solar cell has grown rapidly the past five years, as manufacturers try to ramp up to gigawatt scale and as the Chinese industry has lowered costs and margins.

Lately, companies almost always have had to increase costs to improve the efficiency of their solar cells. "TetraSun was able to keep the costs down while improving efficiency by two absolute percentage points, which is very significant," Al-Jassim said.

NREL's Harin Ullal, who managed the research with TetraSun, said achieving 21 percent efficiency in just 18 months is unusual — and indicative of the cell's disruptive technology. "That compares to 17 percent to 19 percent efficiency for screen-printed silicon cells," he said.

Ullal noted that the new technology, besides replacing expensive silver with abundant copper on the front contact grid, uses n-type silicon wafers, which have been doped with impurities that give them an excess of conductive electrons. (By contrast, p-type cells are doped to have more electron holes than electrons.) The n-type wafers can improve cell efficiency compared to the more common p-type wafers, which can suffer light-induced degradation. The innovations in the TetraSun cell structure design, corrosion resistance, and choice of n-type silicon material doping all added up to the efficiency gain of more than two absolute percent.

"By 2020, this technology could potentially reach the Energy Department's SunShot target of one dollar per watt for PV systems and about 6 cents per kilowatt hour for electricity generation," Ullal said.

Leading the team for TetraSun were Oliver Schultz-Wittmann, Denis DeCeuster, Adrian Turner, and Doug Crafts. In addition to Al-Jassim, Symko-Davies, and Ullal, NREL's team included Peter Hacke, Chunsheng Jiang, and Richard Mitchell.

First Solar built its worldwide reputation on cadmium telluride solar cells, which come from the second and sixth columns of the periodic tables. Those elements are costly by the pound, but require such thin layers that they can challenge silicon on price. But today, silicon continues to grab the lion's share of the solar market. With the acquisition of TetraSun, First Solar now is poised to have a presence in both thin films and silicon.

First Solar Chief Executive Officer Jim Hughes said in April that his company acquired TetraSun because its crystalline silicon cells achieve high efficiency using a simpler design, require fewer process steps, and have wider tolerances than conventional multicrystalline silicon solar cells. Hughes noted that the TetraSun technology also saves costs with its large-format wafers and by eliminating the need for expensive silver and transparent conductive oxides.

"This breakthrough technology will unlock the half of the PV market that favors high-efficiency solutions, which has been unserved by First Solar to date," Hughes said at the time of the R&D 100 Award announcement. He said that adding the TetraSun option to First Solar's market-leading cadmium telluride cells "gives us a unique end-to-end suite of solutions" to serve the spectrum of commercial applications.

"There has been tremendous focus in the solar industry on improving cell efficiency and cost," Ullal said, no.

ting that often technical advancements that offer efficiency improvements are more complex and costly. "This technology is special because it offers improvements in both performance and production cost at the same time."