Nanometer-Thick Solar Cells Designed with Two-dimensional Materials

by Prachi Patel

Materials Research Society | Published: 15 July 2013

thin-photovoltaics-220 The MIT team found that an effective solar cell could be made from a stack of two one-molecule-thick materials: Graphene (a one-atom-thick sheet of carbon atoms, shown at bottom in blue) and molybdenum disulfide (above, with molybdenum atoms shown in red and sulfur in yellow). The two sheets together are thousands of times thinner than conventional silicon solar cells. Credit: Jeffrey Grossman and Marco Bernardi. Click image to enlarge. 

Stacking two layers of atomically thick semiconducting materials could give photovoltaic devices that are less than 1 nm thick, researchers at MIT have found. The researchers predict that such solar cells could be up to 1.5% efficient at converting sunlight into electricity.

This efficiency is small compared to the typical 15-16% efficiencies of conventional silicon solar cells. But those cells are also hundreds of thousands of times thicker. The nanometer-thick solar cells boast up to a thousand times more power per weight than conventional devices. And because they are ultrathin and light, they could be useful for flexible devices or in applications where weight is a crucial factor, such as in aerospace. 

“There’s a lot of push for high efficiencies,” says Jeffrey Grossman, a professor of power engineering at MIT. “But we shouldn’t ignore potential applications with potentially moderate efficiencies where weight is important or where thinness of just a few nanometers allows you different choices in substrate.” 

Two-dimensional materials such as graphene and molybdenum disulfide have shown promise for optoelectronic devices. A graphene sheet that’s just three angstroms thick, for instance, can absorb 2.3% of the sunlight falling on it. Yet researchers have so far focused on the materials’ electronic applications and haven’t explored optical or optoelectronics applications, Grossman says. 

“A single layer of molybdenum disulfide can absorb 10–15 percent of energy from sunlight,” he says. “That’s really remarkable given how thin the material is. Given how good these thin materials are at absorbing sunlight, their potential for photovoltaic devices is great.” 

Grossman and his colleagues wanted to study the feasibility and predict the performance of solar cells made by stacking two layers of 2D materials such as graphene, molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), and tungsten disulfide (WS2). The researchers initially used first principal calculations to compute the light absorbance of these 2D materials. They found that the numbers accurately matched experimental work that has been done in the past with the materials. 

Then they computationally examined two different photovoltaic devices. The first device, composed of stacked MoS2 and graphene single layers, was 0.9 nm thick, and showed an efficiency up to 1%, corresponding to a power density of 2,500 kW/kg. The second device, designed by stacking WS2 and MoS2 monolayers, was up to 1.5% efficient with a thickness of 1.2 nm and a corresponding power density of 1,800 kW/kg. 

Stacking several devices made of different combinations of these and other 2D materials could lead to solar cells with higher efficiencies, Grossman says. While the work so far has been based on computer modeling, he says that his group is now trying to produce the ultrathin photovoltaic devices. 

Read the abstract in Nanoletters  here. 


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