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Researchers develop ‘tandem’ solar cell to harness more sunlight



March 27, 2015 – Researchers at MIT and Stanford University have developed a new kind of solar cell that combines two different layers of sunlight-absorbing material to harvest a broader range of the sun’s energy, and say this could lead to PV cells that are more efficient than those currently used in solar-power installations.

The new cell uses a layer of silicon 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 (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 explained. 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 said. “It has advantages in terms of simplicity, because it looks and operates just like a single silicon cell,” he said, with only a single electrical control circuit needed.

One trade-off is that the current produced is limited by the capacity of the lesser of the two layers. To address that limitation, the team aimed 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, says MIT. Now the team is focusing on increasing the power efficiency—the percentage of the sun’s energy that gets converted to electricity—that is possible from the combined cell. In this initial version, the efficiency is 13.7%, but researchers say they have identified low-cost ways of improving this to about 30%, adding that the technology could ultimately achieve a power efficiency of more than 35%.

The researchers will also explore how to easily manufacture the new type of device, but Buonassisi said this 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… all 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 said, but is one of several pathways worth exploring. “Our job at this point is to provide options to the world,” he said. “The market will select among them.”

— With files from David Chandler, MIT News Office

Photo 1: Test sample of a monolithic perovskite-silicon multi-junction solar cell produced by the MIT-Stanford University team. Image Felice Frankel.

Photo 2: 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 (grey). 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).