New Solar Cell Is More Efficient, Costs Less than Its Counterparts

Exposed in step-like formation, layers of new photovoltaic cell harvest more of sun’s energy.

The cost of solar power is beginning to reach price parity with cheaper fossil fuel-based electricity in many parts of the world, yet the clean energy source still accounts for just slightly more than 1 percent of the world’s electricity mix.

Solar, or photovoltaic (PV), cells, which convert sunlight into electrical energy, have a large role to play in boosting solar power generation globally, but researchers still face limitations to scaling up this technology.  For example, developing very high-efficiency solar cells that can convert a significant amount of sunlight into usable electrical energy at very low costs remains a significant challenge.

A team of researchers from MIT and the Masdar Institute of Science and Technology may have found a way around this seemingly intractable trade off between efficiency and cost.  The team has developed a new solar cell that combines two different layers of sunlight-absorbing material to harvest a broader range of the sun’s energy.  The researchers call the device a “step cell,” because the two layers are arranged in a step wise fashion, with the lower layer jutting out beneath the upper layer, in order to expose both layers to incoming sunlight.  Such layered, or “multijunction,” solar cells are typically expensive to manufacture, but the researchers also used a novel, low-cost manufacturing process for their step cell.

The team’s step-cell concept can reach theoretical efficiencies above 40 percent and estimated practical efficiencies of 35 percent, prompting the team’s principal investigators — Masdar Institute’s Ammar Nayfeh, associate professor of electrical engineering and computer science, and MIT’s Eugene Fitzgerald, the Merton C. Flemings-SMA Professor of Materials Science and Engineering — to plan a startup company to commercialize the promising solar cell.
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Beyond silicon
Traditional silicon crystalline solar cells, which have been touted as the industry’s gold standard in terms of efficiency for over a decade, are relatively cheap to manufacture, but they are not very efficient at converting sunlight into electricity.  On average, solar panels made from silicon-based solar cells convert between 15 and 20 percent of the sun’s energy into usable electricity.

Silicon’s low sunlight-to-electrical energy efficiency is partially due to a property known as its bandgap, which prevents the semiconductor from efficiently converting higher-energy photons, such as those emitted by blue, green, and yellow light waves, into electrical energy.  Instead, only the lower-energy photons, such as those emitted by the longer red light waves, are efficiently converted into electricity.

To harness more of the sun’s higher-energy photons, scientists have explored different semiconductor materials, such as gallium arsenide and gallium phosphide.  While these semiconductors have reached higher efficiencies than silicon, the highest-efficiency solar cells have been made by layering different semiconductor materials on top of each other and fine-tuning them so that each can absorb a different slice of the electromagnetic spectrum.

These layered solar cells can reach theoretical efficiencies upward of 50 percent, but their very high manufacturing costs have relegated their use to niche applications, such as on satellites, where high costs are less important than low weight and high efficiency.

The Masdar Institute-MIT step cell, in contrast, can be manufactured at a fraction of the cost because a key component is fabricated on a substrate that can be reused.  The device may thus help boost commercial applications of high-efficiency, multijunction solar cells at the industrial level.

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