NREL's new robots scrutinize solar cells

The race to build a better solar cell is looping through the National Renewable Energy Laboratory where new robots are fabricating thin-film cells and analyzing glitches faster and with more precision than ever before.

How much faster? The robot working with silicon can build a semi-conductor on a six-inch-square plate of glass, plastic or flexible metal in about 35 minutes. It pivots and dishes like a point guard, sifts like a master chef, analyzes like a forensics expert and does it all while maintaining a vacuum seal on the entire process.

Simultaneously, it can analyze glitches and measure light absorption, while preparing the next half-dozen plates.

"It used to require us to go to, let's see, one … two … three … four … five labs to do the same thing," NREL scientist Ingrid Repins said.

And the silicon robot is one of just six such robots in six bays in NREL's Process Development and Integration Laboratory (PDIL), the place where industry is starting to turn to test their newest cells.

The bay that uses silicon as the semiconductor for solar cells was the first to begin operating and holds all the speed and performance records so far.

Next to go on line were bays devoted to stand-alone characterization, integrated characterization and atmospheric processing.

The latest bay to start operating is the one that uses Copper Indium Gallium diSelenide (CIGS) as the semi-conductor in solar cells. Still being installed is the final bay, which will work with cadmium-telluride cells.

In each bay, the central transfer robot is the hub, operating like a jukebox, delivering the plate to chambers that can deposit micron-thin layers of chemicals to build the semi-conductors, or test and measure the growth of the crystals that make the cells.

Solar Companies Can Test Samples, Use Their Own Tools

Solar companies will be able to hook their own tools to the central robot and discover how their newest formulas compare. A vacuum transport tool can take the sample plates to the different, yet compatible, bays to see how an unusual process might bolster the power of a cell.

Solar companies know how to make solar cells in a dozen different ways — as shingles, as windows, as fanny packs, as attachments to space vehicles — but they constantly are searching for ways to lower costs and gain efficiency.

"The whole goal is dollars per watt," Repins said.

President Obama has set a goal that solar energy become cost-competitive with coal and other fossil fuels by 2015.

"The gap is closing," Repins said. "We're getting closer. Already, First Solar is saying that for a large installation in southern California where electricity prices are relatively high, they are at parity now."

NREL scientists are hoping their PDIL facility will help industry close that gap sooner by bringing lab-like precision to industrial-type processes.

R&D Agreement with Climax Molybdenum

For example, NREL last month signed a cooperative agreement with Climax Molybdenum of Empire, Colorado, which wants the lab to help test a new process of building sodium into the molybdenum layer of solar cells and then sputtering that sodium onto the CIGS layer.

Traditionally, the sodium leaches into the solar cell from the glass plate, but that's not really a good way to do it because there is little quality-control in the glass-making procedure, Repins said.

For Climax Molybdenum, NREL will measure how well the company uses its tools to sputter the sodium from the molybdenum into the semiconductor, and how precisely it gets there.

"The assumption is that there will be more control getting sodium from the molybdenum than from the glass," she said.

If it's perfected, that's another step toward lowering the cost of solar energy.

Solar cells are like mini-batteries, with three layers of thin films representing the two terminals and the current in between. The three layers together are about one-seventh the thickness of a human hair.

The middle layer, which absorbs the sun's rays and acts as the current, is where the action is.

Some companies are sure CIGS will emerge as the best semi-conductor; others pin their hopes on cadmium telluride or the venerable silicon.