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Page added on March 28, 2015

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New Kind of “Tandem” Solar Cell

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Researchers at MIT and Stanford University have developed a new kind of solar cell that combines two different layers of sunlight-absorbing material in order to harvest a broader range of the sun’s energy. The development could lead to photovoltaic cells that are more efficient than those currently used in solar-power installations, the researchers say.

solar tech layers

Schematic diagram shows the layered structure of the hybrid solar cell. The top sub-cell, made of pervoskite (blue), absorbs most of the high-energy (blue arrow) photons from the sun, while letting lower energy (red arrow) photons pass through, where they are absorbed by the lower sub-cell made of silicon (gray). The other layers serve to provide electrical connections between the two sub-cells, and to carry the electricity generated by the cells out to wires where it can be harnessed. Courtesy of the researchers (edited by Jose-Luis Olivares/MIT)

The new cell uses a layer of silicon — which forms the basis for most of today’s solar panels — but adds a semi-transparent layer of a material called perovskite, which can absorb higher-energy particles of light. Unlike an earlier “tandem” solar cell reported by members of the same team earlier this year — in which the two layers were physically stacked, but each had its own separate electrical connections — the new version has both layers connected together as a single device that needs only one control circuit.

The new findings are reported in the journal Applied Physics Letters by MIT graduate student Jonathan Mailoa; associate professor of mechanical engineering Tonio Buonassisi; Colin Bailie and Michael McGehee at Stanford; and four others.

“Different layers absorb different portions of the sunlight,” Mailoa explains. In the earlier tandem solar cell, the two layers of photovoltaic material could be operated independently of each other and required their own wiring and control circuits, allowing each cell to be tuned independently for optimal performance.

By contrast, the new combined version should be much simpler to make and install, Mailoa says. “It has advantages in terms of simplicity, because it looks and operates just like a single silicon cell,” he says, with only a single electrical control circuit needed.

One tradeoff is that the current produced is limited by the capacity of the lesser of the two layers. Electrical current, Buonassisi explains, can be thought of as analogous to the volume of water passing through a pipe, which is limited by the diameter of the pipe: If you connect two lengths of pipe of different diameters, one after the other, “the amount of water is limited by the narrowest pipe,” he says. Combining two solar cell layers in series has the same limiting effect on current.

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.

Perovskites have been studied for potential electronic uses including solar cells, but this is the first time they have been successfully paired with silicon cells in this configuration, a feat that posed numerous technical challenges. Now the team is focusing on increasing the power efficiency — the percentage of sunlight’s energy that gets converted to electricity — that is possible from the combined cell. In this initial version, the efficiency is 13.7 percent, but the researchers say they have identified low-cost ways of improving this to about 30 percent — a substantial improvement over today’s commercial silicon-based solar cells — and they say this technology could ultimately achieve a power efficiency of more than 35 percent.

They will also explore how to easily manufacture the new type of device, but Buonassisi says that should be relatively straightforward, since the materials lend themselves to being made through methods very similar to conventional silicon-cell manufacturing.

One hurdle is making the material durable enough to be commercially viable: The perovskite material degrades quickly in open air, so it either needs to be modified to improve its inherent durability or encapsulated to prevent exposure to air — without adding significantly to manufacturing costs and without degrading performance.

This exact formulation may not turn out to be the most advantageous for better solar cells, Buonassisi says, but is one of several pathways worth exploring. “Our job at this point is to provide options to the world,” he says. “The market will select among them.”

“I think this work is very significant,” says Martin Green, a professor at the University of New South Wales, in Australia, who was not connected with this research. “The work is important in establishing a proof-of-concept and will stimulate higher efficiencies with this approach. … It’s an excellent starting point for further work in this area.”

The research team also included Eric Johlin PhD ’14 and postdoc Austin Akey at MIT, and Eric Hoke and William Nguyen of Stanford. It was supported by the Bay Area Photovoltaic Consortium and the U.S. Department of Energy.

MIT



5 Comments on "New Kind of “Tandem” Solar Cell"

  1. paulo1 on Sat, 28th Mar 2015 8:59 am 

    Now, if they could just make cells that work efficiently in a cloudy environment? I am sure that as time goes by efficiencies in design will increase. Ghung?

  2. Kenz300 on Sat, 28th Mar 2015 10:07 am 

    Every year wind and solar technology improves and the prices continue to get cheaper.

    The transition to safer, cleaner and cheaper alternative energy sources is growing around the world.

    What Country Powered Itself Entirely On Renewable Energy For 75 Days?

    http://www.greencarreports.com/news/1097457_what-country-powered-itself-entirely-on-renewable-energy-for-75-days

    ————————

    Germany’s renewable energy sector is among the most innovative and successful worldwide. Net-generation from renewable energy sources in the German electricity sector has increased from 6.3% in 2000 to about 30% in 2014.[1][2] For the first time ever, wind, biogas, and solar combined accounted for a larger portion of net electricity production than brown coal.[3] While peak-generation from combined wind and solar reached a new all-time high of 74% in April 2014,[4] wind power saw its best day ever on December 12, 2014, generating 562 GWh.[5] Germany has been called “the world’s first major renewable energy economy”.[

    Renewable energy in Germany – Wikipedia, the free encyclopedia

    https://en.wikipedia.org/wiki/Renewable_energy_in_Germany

  3. dave thompson on Sat, 28th Mar 2015 11:09 am 

    This green washing of our industrial civilizations downfall is despicable. The sooner we face our demise the sooner humans might have a chance at understanding what mother nature is doing. Not to dismiss that solar is a viable option to set up on your house if you can afford it. I would love to be the first on my block of suburbia to stick it to the man and disconnect from the grid.

  4. peakyeast on Sat, 28th Mar 2015 3:51 pm 

    @paulo1: I believe CIGS cells are efficient in low light conditions.

    Also: Spectrolab has been producing double and triple junction solar cells for years. – They are normally used for concentrator solar-cells since they actually get more efficient at higher concentrations of light (the recommended concentration is 500x) and can take upto 100C temperatures during operation. They have efficiencies upto 40+% of the light.

  5. peakyeast on Sat, 28th Mar 2015 3:54 pm 

    Here is a link to the 40% concentrator cell:

    http://www.spectrolab.com/DataSheets/PV/CPV/C4MJ_40_Percent_Solar_Cell.pdf

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