Did Scientists Just Crack The Solar Code?

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By Irina Slav

Scientists the world over have been trying to take a space-grade, super-efficient process for creating spray-on solar cells and make it economically feasible–on Earth.

Everyone said it was impossible.

Until now.

There are two things that have kept solar from exploding on the market: efficiency and cost. Never have the two been able to meet.

Scientists achieved the ultimate in solar-cell efficiency with a space-employed process called MOVPE. But it was far too expensive to translate into anything practical back on the home planet.

Now, scientists from the National Renewable Energy Laboratory (NREL) say they have cracked the code with a new process of D-HVPE-grown solar cells.

While both processes more or less involve spraying chemical vapors onto a substrate, MOVPE always had the efficiency advantage; but the materials used made the process itself hardly feasible from a cost standpoint.

In a nutshell, MOVPE’s advantage was its ability to produce the right aluminum-containing materials that underpin the solar cell efficiency. The previously ditched HVPE version couldn’t grow these materials. Now, they can, with D-HVPE.

D-HVPE has now achieved efficiency parity with MOVPE, which may sound redundant at first glance, but the kicker is this: It’s done it at a cost that could disrupt the entire industry.

Efficiency: The Holy Grail of Solar

Efficiency is the Holy Grail of solar because it’s what’s keeping it from trouncing the energy market.  At less than 20% on average (15%-18%, to be precise), the efficiency of solar cells is much lower than that of gas-fired power plants, which sport efficiency levels of between 42% and 60%. The maximum efficiency achieved for solar cells is just 23%—a poor showing for the environmentally superior power source.

In an ideal world, greater efficiency means lower costs–but only when it doesn’t cost a massive amount of money to produce in the first place, paring any gains that efficiency might have made in end-game energy costs.

Greater efficiency solves some other pretty big problems as well, including how much space utility-scale solar panels suck up. And space is, after all, the most finite of the finite resources. More efficient solar panels will reduce the number of panels required to generate the same amount of power.

D-HVPE appears to be on track to resolve all of these issues–at once.

‘Impossible’ Is a Fleeting Notion These Days

This is not the first time the space cell processes have been used.

NASA used them for its Mars rover back in 2013. At 27%, their efficiency was even at the time higher than the maximum for commercial-grade batteries. This was because the space cells were made of several semiconductor materials. They were what experts call multi-junction solar cells.

Multi-junction solar cells have the potential to be immensely more efficient than single-junction cells because their various layers can absorb multiple wavelengths of light. This allows the cells to convert more of it into electricity. Depending on the number of layers—or junctions—efficiency rates could range from about 45 percent to more than 70 percent. This is an efficiency rate that would impress even the hardest renewables sceptic.

The higher efficiency of space cells came with a big drawback, though: An astronomical cost.

This dominant method, is called metal organic vapor phase epitaxy, or MOVPE.  Here’s how one scientist from the NREL’s solar cell team explains it: “You essentially dose pre-engineered chemicals onto a hot wafer, and they will deposit as thin-film layers with the same lattice spacing as the wafer.”

“MOVPE can grow very complicated structures—or devices like solar cells—but it’s expensive and slow,” David Young, a senior scientist and expert in silicon solar cells, added in a 2018 news release.

So, what have NREL scientists actually achieved?

They claim to have basically revolutionized the process of growing high-efficiency, space-grade material (also used in space-grade batteries), for prices that are … down to earth.

In other words, they succeeded in growing a new material for solar cells using a method believed to be obsolete and a method that is much cheaper than the one that is more widely used today.

The challenger method that the NREL researchers used for their breakthrough is called dynamic hydride vapour phase epitaxy, or D-HVPE.

The original HVPE method was also slow and less accurate in the layering of semiconductor materials than MOVPE, which was why it fell out of favour. But with its dynamic version, the NREL scientists managed to shorten times and increase accuracy to deposit chemicals on the wafer to make a solar cell.

$100/watt vs $1/watt

As it stands, high-efficiency solar cells cost about $100 per watt of energy in 2018 because of the costly process of their production. This compares with about $1 per watt for ordinary commercial cells.

At 100 times the cost, there has clearly been great incentive to revisit a space-based efficiency process that had earlier been rendered impossible.

Does that mean we know how much D-HVPE will cost per watt of energy? No. It’s too early for that, but NREL scientists assure us that this is where solar technology starts to make sense.

“The HVPE process is a cheaper process,” said Ptak, a senior scientist in NREL’s National Center for Photovoltaics. “Now we’ve shown a pathway to the same efficiency that’s the same as the other guys, but with a cheaper technique. Before, we were somewhat less efficient but cheaper. Now there’s the possibility of being exactly as efficient and cheaper.”

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