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Published on March 27th, 2015 | by Tina Casey


Silicon Solar Cells Hit A Wall, Perovskite Leaps It

March 27th, 2015 by  

Evolve, already! That’s what our friends over at the American Institute of Physics have to say about silicon solar cells. Although silicon controls about 90% of today’s global solar cell market, researchers have had a tough time teasing more efficiency out of the stuff.

That’s where perovskite comes in. Perovskite is our most favorite material next to graphene, and a team of researchers from Stanford University and the Massachusetts Institute of Technology (MIT) are aiming to blow up the silicon market with a new tandem perovskite solar cell.

silicon perovskite solar cell

A New Perovskite Solar Cell

For those of you new to the topic, perovskite refers to a class of easily synthesized crystalline materials, first discovered in a natural state back in the 19th century, in the Ural Mountains.

We’re already beginning to see promising new developments in perovskite solar cells, including tinted, transparent solar cells. The MIT/Stanford team is pretty confident that their new research will shake things up even more.

The new research represents a step up from an earlier tandem perovskite solar cell report from Stanford in January, which involved a “temporary tattoo” of perovskite on a silicon layer.

The perovskite absorbs the visible layers, while the silicon absorbs the infrared layers, thus enabling the combined cell — called a tandem solar cell — to harvest more light.

As described by MIT writer David Chandler, that solar cell consisted of stacked layers, each with their own electrical connections. The new MIT/Stanford collaboration involves stacked layers that are electrically integrated.

Here’s how the American Institute of Physics (AIP) explains it:

Another key part of the tandem’s design is that it uses a serial connection, which means that the two solar cells are connected in a manner so that the same amount of current passes through each of the solar cells. In other words, the same amount of light is absorbed in each solar cell and their voltage is added together.

If you want all the details, you can go to AIP and look for “A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction” in the journal Applied Physics Letters, but for those of you on the go, here is a schematic of the new solar cell showing all the layers:

Stanford MIT tandem perovskite solar cell

The dark image on the right of Figure A is an image of the cell at a 45 degree angle, showing off a nanowire mesh.
Figure B illustrates the charge-transport mechanism around the silicon (Si) junction.

So… What’s The Problem?

Not to go all Debbie Downer on this, but using a single integrated circuit has its limitations. The chief one is that the electrical current is limited by the lowest-performing material, so you’re back to square one.

However, the research team was able to demonstrate — at least in the proof-of-concept stage — that this hurdle can be overcome.

The initial version only has an efficiency of 13.7%, which is actually a bit less than you’d expect, given that perovskite solar cells can reach the 16% range or more. However, the concept opens up a pathway to achieving a much more impressive efficiency of 30%. Getting up to the 35% range is also a possibility.

Here’s how the new research handles the efficiency bottleneck:

To address that limitation, the team aims to match the current output of the two layers as precisely as possible. In this proof-of-concept solar cell, this means the total power output is about the same as that of conventional solar cells; the team is now working to optimize that output.

So… What’s It Gonna Cost?

Tandem solar cells are more efficient but they tend to be pricey, so it’s not surprising that they account for a negligible fraction of the global market — 0.25% according to AIP. However, AIP is pretty excited about the commercial viability of the new research.

Part of the savings involves using proven deposition methods to stack the layers, along with off-the-shelf semiconductor materials.

Another money-saving factor is the use of a single electrical circuit, instead of a separate system for each layer.

Next steps for the research involve improving the efficiency of the perovskite layer and making some improvements in the silicon layer, along with other tweaks that will bring the cost of the solar cell down.

It looks like version 2.0 is a long way off, so don’t hold your breath for that new tandem perovskite solar cell, but in the meantime, US taxpayers can go ahead and give ourselves a nice big group hug because the MIT/Stanford research was funded by the Energy Department along with the public/private Bay Area Photovoltaic Consortium based at Stanford.

On the other hand, for all you electric vehicle fans out there, it looks like perovskite-juiced EV batteries could be rolling down your block sooner rather than later.

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Image Credits: (top) courtesy of the researchers, edited by Jose-Luis Olivares/MIT; (bottom) courtesy of AIP. 


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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.

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