Monday, November 26, 2012

A solar funnel that could lead to more efficient cells

The reality of solar panels is that those on the market today aren't very efficient - most of the solar cells, which make up an entire panel, convert less than a fifth of the sunlight into electricity. But researchers at MIT said on Monday they have come up with a funnel-like design that will manipulate the incoming electrons to engineer more efficient solar cells.

The research, just published in the journal, Nature Photonics, used computer modeling to look at how to stretch the semiconductor molybdenum disulfide to change its physical properties to make use of a broader spectrum of sunlight than what silicon, the most common solar cell material, can manage today. Whether the design will work as well in real life will require further research.

Improving cell efficiency is important for lowering the cost of producing solar electricity. One way to do that is to extract more energy from the same amount of materials. That also will reduce the amount of land needed to generate the same amount of electricity. As it stands, photovoltaic power plants are more land-intensive compared with fossil fuel power plants with a similar energy output. Building solar farms on large swath of land has prompted fierce debates over their environmental impact on wildlife and prompted developers to agree to set aside wildlife corridors in exchange for permits or to avoid lawsuits.

What some scientists have been working on is to manipulate the band gap in a material. A band gap describes the amount of energy that electrons need to move around and generate electricity. If you can manipulate band gaps, then you can control the amount of electricity produced. Band gap engineering is not a new concept and is already used by solar cell developers and academic researchers in their search for more efficient solar cell designs.

What the MIT researchers proposed is more novel: strain a material to create specific and varying band gaps within a single material to capture different portions of the light spectrum. They imagined creating that strain by using a microscopic needle to poke at the material down the center and create that funnel. The pressure on the needle would cause different degrees of strain and band gaps.

Knowing how to stretch a material is only part of the solution. Finding materials that can withstand the pressure is another hurdle. Conventional solar materials would break or warp undesirably under the straining process proposed by the research. But there is a more recently minted class of ultra-strength materials" that could be suitable. MIT researchers settled on molybdenum disulfide.

The research, which received support from U.S. and Chinese institutions, is only a start in exploring the idea of using ultra-strength materials to engineer more efficient solar cells. The MIT research team, which includes Ju Li, Xiaofeng Qian and Cheng-Wei Huang, hopes to conduct lab work to verify the results of their computer modeling. Ji Feng of Peking University in China rounds up the research team.


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