How Low (And How Green) Can Solar Cells Go

Jun 07, 2023

The silicon wafer NexWafe says buh-bye kerf, hello low-cost, lightweight, flexible solar cells.

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The economic case for fossil energy is teetering on the edge of collapse, and the German company NexWafe GmbH is about to give it one final shove into the abyss. NexWafe is building a new factory that will send new low-cost silicon wafers flying into the solar cell market. To gild the green lily, part of the cost savings involves kerf, a funny word for the serious problem of materials waste.

Silicon wafers form the heart of silicon solar cells, and manufacturers have been falling over themselves to find more efficient ways to produce wafers. The current manufacturing method of choice involves melting silicon into a mass of polycrystalline silicon (aka polysilicon among other names) and weeding out impurities. A seed-crystal growing process called the Czochralski method is then used to “pull” monocrystalline silicon ingots from the mass. Finally, the ingots are sliced into wafers.

Silicon wafers can also be made from polycrystalline ingots. Solar cells made with polycrystalline wafers tend to have a lower solar conversion efficiency. On the plus side, they tend to cost less and are considered to be a reasonable option for some use cases.

Either way, the whole wafer manufacturing process takes time, energy, and money. Part of the money involves the kerf, which refers to the silicon waste produced when ingots are sliced. It’s like sawdust, but for silicon. The equipment manufacturer Meyer Burger introduced a new kerf-reducing diamond cutter for silicon ingots back in 2014, but there is still room for improvement.

For that matter, the Czochralski method dates back to 1915 and is named after its inventor, the Polish scientist Jan Czochralski, who came up with the process when he went to dip his pen in an inkwell and hit molten tin instead (so sayeth Wikipedia). Clearly the time is ripe for a change.

That brings us to NexWafe. The company has just received an infusion of €30 million (a bit more than US$32 million) to speed up work on a commercial-scale production facility, to be located in Bitterfeld, a town in the southeast part of the German state Saxony-Anhalt. Another factory is to follow in Saudi Arabia if all goes according to plan.

The Bitterfeld project is the latest step in a long R&D journey. NexWafe spun out of the Fraunhofer Institute for Solar Energy Systems ISE back in 2015 with an assist from Franhofer Venture. The startup was tasked with deploying a new, zero-kerf production method developed by the research team of Stefan Rebar over the previous 15 years.

Rebar is the co-founder of NexWafe, and all that hard work is about to pay off. The new “EpiNex” kerfless wafer is based on epitaxy, which is a method of growing crystals on a substrate. If the substrate is removable, you end up with an ultra-thin layer of crystals in the desired shape and size, with no need for a kerf-producing cutting step.

“The process focus is on atmospheric pressure chemical vapor deposition (APCVD) at temperatures up to 1300°C. This process is well-known from microelectronics, but had to be radically adapted for photovoltaic applications in terms of throughput of the equipment,” Fraunhofer explained in a 2015 press release.

“Stefan Reber and his team of 30 people have developed different generations of deposition reactors, ranging from very flexible batch-type laboratory setups to large multi-chamber in-line systems with the option of continuously depositing p- and n-doped epitaxial layers,” Fraunhofer continued, with p- and n- referring to two different solar cell variations.

According to NexWafe, the new kerfless silicon wafer will cost 30% less than conventional wafers, so whoa if true.

“Fully compatible with conventional solar cell manufacturing, NexWafe offers a 70% reduction in energy consumption during manufacturing. NexWafe’s continuous, direct gas-to-wafer manufacturing process also minimizes waste, resulting in wafers that are 30% less expensive than conventional wafers,” the company affirms.

NexWafe has already demonstrated its process on EpiNex wafers that are only 50 microns across, or about the diameter of a human hair. They are also less than one-third as thick as conventional wafers. That opens up a wider range of solar cell applications that call for lighter weight and increased flexibility.

Once the factory is up and running, NexWafe expects to hit a goal of 90 microns.

There being no such thing as a free lunch, silicon production is an industrial process with industrial impacts. Still, an estimated 70% reduction in energy consumption is no small potatoes. As calculated by NexWafe, that savings translates into a savings of more than 6 million tons of carbon dioxide for every 10 gigawatts of wafers produced.

That’s good for the planet, and it’s also good for fossil energy stakeholders on the prowl for carbon credits.

“The future value of carbon credits is predicted to reach $60 per ton of avoided emissions. Looking ahead, a large manufacturing facility using EpiNex™ technology would be capable of generating more than $360M a year in carbon credits,” NexWafe explains.

That may explain why Aramco Ventures is behind the €30M setup for the Bitterfeld facility, along with Reliance New Energy Limited and ATHOS Venture GmbH, among others. The new investment joins the gravity-based energy storage startup Energy Vault, among other notches on Aramco Ventures’s growing clean tech belt.

All else being equal, Aramco Ventures’s parent company, Aramco, could continue to squeeze petroleum out of the ground under a carbon market scenario, by deploying carbon credits for investments in green startups like NexWafe.

That depends on partly on policymakers, and partly on consumer demand for petroleum products. After some early fits and starts in electric vehicle technology, the transportation market is finally beginning to drift into a more sustainable model. Plastics and other synthetic products are also showing signs of deserting the petroleum ship as new bio-materials surface.

On the solar side, driving down the cost of silicon wafers is just one piece of an expanding puzzle. Keep an eye on perovskites, see-through photovoltaic windows, green hydrogen, and agrivoltaics for more signs that solar power is lifting a more sustainable economy from the ashes of the fossil energy era.

Here in the US, even so-called “red” states have been succumbing to the siren call of decarbonization. Of particular interest is the Aramco Ventures-backed energy storage startup Form Energy, which is setting up shop in West Virginia (more CleanTechnica coverage is here).

If all goes according to plan, Form’s long duration, iron-air technology will help accelerate the pace of renewable energy development across the US, regardless of state level partisan politics.

No more Trainwreck Twitter. Find me on Spoutible: @TinaMCasey or LinkedIn @TinaMCasey or Mastodon @Casey or Post: @tinamcasey

Photo: New method to reduce the cost of silicon wafers for solar cells (photo courtesy of NexWafe).

Tina specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Views expressed are her own. Follow her on Twitter @TinaMCasey and Spoutible.

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