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Breakthrough Furnace Can Cut Solar CostsOctober 21, 2011Solar cells, the heart of the photovoltaic industry, must be tested

for mechanical strength, oxidized, annealed, purified, diffused,etched, and layered.

Heat is an indispensable ingredient in each of those steps, and

that's why large furnaces dot the assembly lines of all the solarcell manufacturers. The state of the art has been thermal or rapid-

thermal-processing furnaces that use radiant or infrared heat toquickly boost the temperature of silicon wafers.

Now, there's something new.

A game-changing Optical Cavity Furnace developed by the U.S.Department of Energy's National Renewable Energy Laboratory

uses optics to heat and purify solar cells at unmatched precisionwhile sharply boosting the cells' efficiency.

The Optical Cavity Furnace (OCF) combines the assets thatphotonics can bring to the process with tightly controlledengineering to maximize efficiency while minimizing heating and cooling costs.

NREL's OCF encloses an array of lamps within a highly reflective chamber to achieve a level of temperatureuniformity that is unprecedented. It virtually eliminates energy loss by lining the cavity walls with super-

insulating and highly reflective ceramics, and by using a complex optimal geometric design. The cavity designuses about half the energy of a conventional thermal furnace because in the OCF the wafer itself absorbs what

would otherwise be energy loss. Like a microwave oven, the OCF dissipates energy only on the target, not on thecontainer.

Different configurations of the Optical Cavity Furnace use the benefits of optics to screen wafers that aremechanically strong to withstand handling and processing, remove impurities (called impurity gettering), form

junctions, lower stress, improve electronic properties, and strengthen back-surface fields.

Making 1,200 Highly Efficient Solar Cells per HourNREL researchers continue to improve the furnace and expect it tobe able soon to hike the efficiency by 4 percentage points, a large

leap in an industry that measures its successes a half a

percentage point at a time. "Our calculations show that somematerial that is at 16 percent efficiency now is capable of reaching20 percent if we take advantage of these photonic effects," NRELPrincipal Engineer Bhushan Sopori said. "That's huge."

Meanwhile, NREL and its private-industry partner, AOS Inc., arebuilding a manufacturing-size Optical Cavity Furnace capable of

processing 1,200 wafers an hour.

At about a quarter to half the cost of a standard thermal furnace,the OCF is poised to boost the solar cell manufacturing industry inthe United States by helping produce solar cells with higher qualityand efficiency at a fraction of the cost.

The furnace's process times also are significantly shorter thanconventional furnaces. The Optical Cavity Furnace takes only a fewminutes to process a solar wafer.

NREL has cooperative research and development agreements with several of the world's largest solar-cellmanufacturers, all intrigued by the OCF's potential to boost quality and lower costs.

R&D 100 Award WinnerNREL and AOS shared a 2011 R&D 100 Award for the furnace. The awards, fromR&D Magazine, honor the most important technological breakthroughs of theyear.

Billions of solar cells are manufactured each year. A conventional thermal furnaceheats up a wafer by convection; a Rapid-Thermal-Processing furnace usesradiative heat to boost the temperature of a sil icon wafer up to 1,000 degreesCelsius within several seconds.

In contrast to RTP furnaces, the Optical Cavity Furnace processing involves waferheating at a relatively slower rate to take advantage of photonic effects. Slower

heating has an added advantage of significantly lowering the power requirementsand the energy loss, so it can boost efficiency while lowering costs.

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Enlarge imageThe cavity inside the Solar Optical Furnace glows

white hot during a simulated firing of a solar cell.

Credit: Dennis Schroeder

Enlarge image

Bhushan Sopori, a principal engineer at NationalRenewable Energy Laboratory, discusses the

capabilities of the Optical Cavity Furnace withcolleagues Vishal Mehta and Peter Rupnowski.Credit: Dennis Schroeder

of 3 NREL: News Feature - Breakthrough Furnace Can Cut Solar Costs

11/14/2011http://www.nrel.gov/news/features/feature_detail.cfm/feature_id=1629



"With all solar cells, optics has a big advantage because solar cells are designedto absorb light very efficiently," NREL Principal Engineer Bhushan Sopori said."You can do a lot of things. You can heat it very fast and tailor its temperatureprofile so it's almost perfectly uniform."

In fact, the OCF is so uniform, with the help of the ceramic walls, that when the

middle of the wafer reaches 1,000 degrees Celsius, every nook and cranny of it isbetween 999 and 1,001 degrees.

"The amazing thing about this is that we don't use any cooling, except somenitrogen to cool the ends of the 1-kilowatt and 2-kilowatt lamps," Sopori said.That, of course, dramatically lowers the energy requirements of the furnace.

The use of photons also allows junctions to be formed quicker and at lowertemperatures.

As America strives to reach the goal of 80 percent clean energy by 2035, the

White House and the U.S. Department of Energy are challenging the solarindustry to reach the goal of $1 per watt for installed solar systems. To reach that

goal, manufacturers need better, less expensive ways to make solar cells. At$250,000, the Optical Cavity Furnace can do more, do it quicker, and do i t at alower capital cost than conventional furnaces.

Twenty Years of Great IdeasFor more than two decades, Sopori had great ideas for making a better furnace.

He knew that incorporating optics could produce a furnace that could heat solarcells, purify them, ease their stress, form junctions and diffuse just the rightamount of dopants to make them more efficient.

"It's always easy on paper," Sopori said recently, recalling the innovations that worked well on paper and in thelab, but not so well in the real world. "There are moments … you realize that no one has ever done something likethis. Hopefully it will work, but there are always doubts."

Trouble was, he'd come up with some elegant theoretical solutions involving optics, but wasn't able to combinethem with the optimal geometry and materials of a furnace. "We've had a whole bunch of patents (12) to do

these things, but what we were missing was an energy-efficient furnace to make it possible," Sopori said.

And then, combining his expertise in optics with some ingenious engineering with ceramics, he had his ah-hamoment:

NREL's Optical Cavity Furnace uses visible and infrared light to uniformly heat crystalline silicon wafers, especiallyat the edges, which are prone to cooling or heat loss, at unprecedented precision. The rays heat the sample, butthe wafer never physically contacts the lamps.

The Optical Cavity Furnace is versatile. Each step in the solar cell manufacturing process typically requires a

different furnace configuration and temperature profile. However, with the OCF, a solar cell manufacturer simplytells a computer (using NREL proprietary software) what temperature profile is necessary for processing a solarcell.

So, the OCF can perform five different process steps without the retooling and reconfiguration required by thefurnaces used today, all the while incrementally improving the sunlight-to-electricity conversion efficiency of each

solar cell.

— Bill Scanlon

Enlarge imageAn NREL colleague inserts a multi-

crystaline silicon solar cell sampleinto the Optical Cavity Furnacewhile Principal Engineer Bhushan

Sopori, gets ready to read itsanalysis on his computer screen.Credit: Dennis Schroeder

Light Has Multiple Advantages in FurnacesPhotons have special qualities that prove useful in creating solar cells.

When light is shined on silicon atoms that are bonded electronically to each other it changes their

potential. The work by Bhushan Sopori and his colleagues ensures that that change is for the better.

The Optical Cavity Furnace shines visible and near-infrared light to heat the solar cell, and also shinesultraviolet light to take advantage of photonic effects that occur deep within the atomic structure of the cell material. This combination offers unique capabilities that lead to improved device quality andefficiency.

Iron and other impurities can degrade the silicon quality quickly, noted Sopori, the principalinvestigator for the Optical Cavity Furnace (OCF), developed by the U.S. Department of Energy'sNational Renewable Energy Laboratory and its private industry partner AOS Inc. "But shining the rightlight on it can remove that impurity from the silicon," Sopori said. "We've shown that it is possiblewith the photonic effects to getter these impurities during solar cell fabrication."

Optics can make a lot of things happen at the interfaces in a cell, where, for example, metal canreflect the light and speed the diffusion of impurities, Sopori said.

The lamps in the furnace help fool the impurities in the silicon into moving out of the way, by creatingvacancies.

"We call it injecting vacancies," Sopori said. A vacancy is a lack of a silicon atom. "If the atom ismissing, you have a vacancy here, an empty space." Those spaces prompt the impurities such as ironto feel much more like moving – and they do so at a much lower temperature than would otherwisebe required. The iron moves in with the aluminum, creating an aluminum-iron mix that, happily, isneeded anyway as a contact point.

Removing impurities can change a cell's efficiency from 13 percent to 17 percent. What that means is

that 17 percent of the photons that hit the improved cell are converted into usable electricity.

The absence of cooling water and confinement of energy in the OCF proves to be a big advantage forlowering the energy payback time of solar cells.


11/14/2011http://www.nrel.gov/news/features/feature_detail.cfm/feature_id=1629



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Other advantages of the photonic approach:

• Silicon cells often have silver contacts in front and aluminum contacts in back. They usually arefired simultaneously as the cell is being formed. The OCF by selectively heating the interfaces of

silicon and metal can better control the process, and thus create stronger field surfaces andimproved cell performance.

• The Optical Cavity Furnace uses photons of light to remove weak, cracked wafers from theprocessing line. Photons can more easily produce a thermal stress in a wafer and screen out badwafers. The photon process tests the wafers' integrity right after they are cut. The conventionalmethod requires physical twisting and bending of the wafers to test for weakness.


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