Tandem Solar Cell May Boost Electricity from Sunlight

Scientists have teamed up two materials to soak up more sunlight in a new solar cell.

The dynamic duo in this case was silicon, the workhorse of conventional photovoltaics, and a mineral called perovskite. First discovered in the Ural Mountains and named for Russian mineralogist Lev Perovski, the mineral is a crystal made of calcium titanium oxide that has useful photovoltaic properties.

In a conventional solar cell, only part of the solar spectrum is put to use, with the rest reflected or wasted as heat. This inherent barrier is known as the Shockley-Queisser limit, and for silicon tops out at 33.7 percent efficiency in converting sunlight to electricity in theory, though the limit is much lower in an actual device.

One loophole through this is to use several photovoltaic materials, each tuned to a specific part of the solar spectrum. These multi-junction solar cells have set world records in efficiency, approaching 50 percent, but they require expensive manufacturing techniques, limiting them to niche applications like satellites or requiring mirrors to concentrate sunlight on tiny cells (, Oct. 20, 2014).

Perovskite, however, is cheap and semitransparent, and soaks up sunlight in the ultraviolet and visible parts of the spectrum, so it pairs nicely with silicon photovoltaics that absorb in the infrared.

"The challenge has always been how to combine the two," said Tonio Buonassisi, an assistant professor of mechanical engineering at the Massachusetts Institute of Technology.

Buonassisi explained that he and his collaborators previously demonstrated a cell that stacked the perovskite layer on top of the silicon layer, but in which the materials functioned as separate cells.

A combination approach

The cell was 1 square centimeter in size, consisting of a 200-micron-thick silicon layer topped with a 1-micron-thick perovskite layer.

The researchers structured the device to exploit quantum tunneling, an effect where an electronic potential can pass through a barrier that would ordinarily limit its movement. When the right wavelength of light strikes the perovskite, it mobilizes an electrical charge that passes into the silicon and through the circuit.

"In this tunnel junction, holes from the silicon solar cell recombine with electrons flowing from the perovskite solar cell using quantum mechanical tunneling," said Jonathan Mailoa, a graduate student at MIT and co-author of the report, in an email. "This enables our device to work as a monolithic perovskite-silicon tandem cell."

Sunlight also generates a current in the silicon layer, so the net result is a cell that generates a higher voltage than either of the layers could do by themselves. "The voltage we got out of the tandem was almost spot on the linear sum of the two cells, 1.65 volts," Buonassisi said.

The demonstration cell only had a power conversion efficiency of 13.7 percent, but with further optimization, researchers expect to top 25 percent efficiency. Higher efficiency means more electricity produced for a given size of solar cell, which increases power output and lowers overall system costs.

The tandem cell still falls far short of the efficiencies demonstrated in top-tier multi-junction cells. But with lower manufacturing costs, the researchers aren't aiming to compete with sports-car solar cells. Instead, they want perovskite-silicon solar cells to be everyday drivers, replacing conventional panels on roofs and in solar farms.

"While the silicon requires some very high temperatures and very fancy deposition techniques, the perovskite part is quite simple, and therein lies the advantage," said Colin Bailie, another co-author and a graduate student in materials science and engineering at Stanford University. "All the layers of the perovskite can be printed in a process similar to printing a newspaper."

There are still some engineering challenges: The high-performance perovskite layer degrades fast, sometimes within hours, which is bad for a device that must withstand the elements for decades. The tandem device also requires a window layer on top, which acts as an electrode, but limits the amount of light passing through.

In order for their idea to catch on, researchers will also have to demonstrate that the improved performance of these cells will offset the costs of adding perovskite. "On paper, it looks like there is a window of opportunity, but history shows that practical losses can quickly tilt the scales the other way," Mailoa said. "It'll require excellent engineering design."

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