US/China research team proposes “solar energy funnel” to harness photons for electricity; using elastic strain to capture a wider spectrum

A visualization of the broad-spectrum solar
energy funnel. Image: Yan Liang

Researchers from Peking University in China and MIT are proposing using elastic strain as a viable agent to create an optoelectronic material with a spatially varying bandgap that is tunable for use in photovoltaics, photocatalysis and photodetection. In a paper published in Nature Photonics, they propose that a photovoltaic device made from a strain-engineered MoS2 monolayer will capture a broad range of the solar spectrum and concentrate excitons or charge carriers.

The “funnel” is a metaphor: electrons and their counterparts, holes—which are split off from atoms by the energy of photons—are driven to the center of the structure by electronic forces. However, the material actually does assume the shape of a funnel—a stretched sheet of thin material, nano-indented at its center by a microscopic needle that produces a curved, funnel-like shape.

The pressure exerted by the needle imparts elastic strain, which increases toward the sheet’s center. The varying strain changes the atomic structure just enough to tune different sections to different wavelengths of light—including not just visible light, but also some of the invisible spectrum, which accounts for much of sunlight’s energy.

We’re trying to use elastic strains to produce unprecedented properties.
—Ju Li, corresponding author

Li, who holds joint appointments as the Battelle Energy Alliance Professor of Nuclear Science and Engineering and as a professor of materials science and engineering at MIT, and with Xi’an Jiaotong University in China, sees the manipulation of strain in materials as opening a whole new field of research.

Elastic strain has the potential to be used to achieve rapid and reversible tuning of the bandgap. However, as a result of plasticity or fracture, conventional materials cannot sustain a high enough elastic strain to create sufficient changes in their physical properties. Recently, an emergent class of materials—named ‘ultrastrength materials’—have been shown to avoid inelastic relaxation up to a significant fraction of their ideal strength. Here, we illustrate theoretically and computationally that elastic strain is a viable agent for creating a continuously varying bandgap profile in an initially homogeneous, atomically thin membrane.
—Feng et al.

Strain—defined as the pushing or pulling of a material into a different shape—can be either elastic or inelastic. Xiaofeng Qian, a postdoc in MIT’s Department of Nuclear Science and Engineering who was a co-author of the paper, explains that elastic strain corresponds to stretched atomic bonds, while inelastic, or plastic, strain corresponds to broken or switched atomic bonds. A spring that is stretched and released is an example of elastic strain, whereas a piece of crumpled tinfoil is a case of plastic strain.

The new solar-funnel work uses precisely controlled elastic strain to govern electrons’ potential in the material. The MIT team used computer modeling to determine the effects of the strain on the thin layer of molybdenum disulfide (MoS2), a material that can form a film just a single molecule thick.

The elastic strain, and therefore the change that is induced in electrons’ potential energy, changes with their distance from the funnel’s center—much like the electron in a hydrogen atom, except this “artificial atom” is much larger in size and is two-dimensional. In the future, the researchers hope to carry out laboratory experiments to confirm the effect.

Unlike graphene, another prominent thin-film material, MoS2 is a natural semiconductor: It has a crucial characteristic (a bandgap), that allows it to be made into solar cells or integrated circuits. But unlike silicon, now used in most solar cells, placing the film under strain in the “solar energy funnel” configuration causes its bandgap to vary across the surface, so that different parts of it respond to different colors of light.

In an organic solar cell, the electron-hole pair (exciton), moves randomly through the material after being generated by photons, limiting the capacity for energy production. “It’s a diffusion process,” Qian says, “and it’s very inefficient.” But in the solar funnel, he adds, the electronic characteristics of the material “leads them to the collection site [at the film’s center], which should be more efficient for charge collection.”

The convergence of four trends, Li says, has opened up the field of elastic strain engineering:

– the development of nanostructured materials, such as carbon nanotubes and MoS2, that are capable of retaining large amounts of elastic strain indefinitely;

– the development of the atomic force microscope and next-generation nano-mechanical instruments, which impose force in a controlled manner;

– electron microscopy and synchrotron facilities, needed to directly measure the elastic strain field; and

– electronic-structure calculation methods for predicting the effects of elastic strain on a material’s physical and chemical properties.

The work was done with Ji Feng of Peking University and Cheng-Wei Huang, and was supported by the US National Science Foundation, the US Air Force Office of Scientific Research, and the National Natural Science Foundation of China.


Ahoy! Cargo Ships Go Retro

Every day, cargo ships criss-cross the world’s oceans to bring goods from here to there, burning up fossil fuels along the way, but an Irish company is testing a time-tested method of propulsion that some might say is retro, some might say is incredibly innovative.

Green Energy Tips

Guest Post

When powering your home we often use the local power grid, however we rarely think about where that energy comes from. There are plenty of ways to save money, power and to be as ecologically-friendly as possible. We have covered some tips on how you can switch to another, alternate power source which will improve your life and possibly make you self-sufficient as you disconnect yourself from the local power grid.

1. Switch to green energy
The most logical thing to do in this case is to contact your service provider and to ask for information pertaining such options. Companies across the world are slowly adopting green energy such as wind power and solar power. The prices may vary since a lot of companies will charge you some for working with an alternate power source. Make sure you research what options your company has available before working on the task yourself.

2. Solar power
Solar power can split into two categories – active and passive. Solar cells are what makes active power which absorb and store the energies of the sun for later use or as a backup or supplemental system for your standard power grid. Passive solar systems are actually a way of heating and lighting up your home through the clever use of mirrors and positioning of the roofs and walls of the house without the use of collectors. Before you decide to setup your own solar power array make sure you are aware of what the rules for your city or town may be since this will be the determining factor for setting them up.

3. Solar hot water
While you are at it why not use the rays of the sun to heat up your own water? This has been done for thousands of years in one form or another and today you have the chance to do the same by installing your own solar hot water system. If the temperatures allow it you can work with a circulation system for a more efficient water supply and if that is a problem you can always go with a passive solar heater. Whatever the case though you should always consider having a backup water heater that works on electricity in case the sun is hidden by clouds on rainy days.

4. Harness wind power
Depending on the nature of your home and its location you could use the strong currents in your area to food power to your home. Although this is a less than ideal variant it still works great as it is proven by people living in the Mongolian steppes and other windy places where wind power is used excessively today. Depending on the winds you could cut down your power bill to more than 80% so consider this as well as an alternate source.

5. Hydro power
If you are lucky enough to have water running through your property such as a creek, stream or a river then you have the perfect source of energy right there, though that largely depends on the speed and magnitude of the water flow. There are systems you can use to do exactly that and in the worst case scenario you’ll at least be able to power up a room or two or have the system act as a backup or a supplement to your conventional power grid.

About the Author

Nicole is a passionate writer, dreamer and adventurer. She enjoys writing about green lifestyle and home organizing.

Google Invests $75M In Iowa Wind Farm, Bringing Its Total Green Energy Investments To Almost $1B

Google just announced that it has invested $75 million in a 50 MW wind farm in Rippey, Iowa, a small town an hour outside of Des Moines. This is Google’s second wind energy investment in the state. In 2010, Google entered a long-term contract to buy green energy for its Iowa data center, but this is the company’s first direct investment into an Iowa wind project.

The wind farm, Google notes, is already in operation and has been contracted to sell all of its energy to the Central Iowa Power Cooperative, which will use it to provide electricity to about 15,000 homes. It looks like none of this electricity will be used for Google’s local data center.

With this investment, Google has brought its total committed investment into green energy projects to just over $990 million. The company’s other wind energy investments include a $157 million investment in a Southern California wind energy farm and a $100 million investment into a wind farm in Oregon that is anticipated to be the world’s largest at 845 MW. Google’s first wind energy investment was worth $38.8 million and involved two North Dakota wind farms. Besides the wind farms themselves, Google also owns a 37.5 percent stake in the Atlantic Wind Connection backbone, a project that aims to build transmission lines for green energy along the mid-Atlantic coast.


Morocco Leading the World Toward a Green Energy Future

Moroccowas the first country in the world to recognize the fledgling American Republic in 1777. Now, they are seeking clean energy independence and asking the world to join them in a green revolution.

This North African country, a constitutional monarchy about the size of California, has recently set a royal goal to ensure that 40 percent of its electricity demand is met from renewable energy sources by 2020. This is an extremely aggressive goal considering more than 90 percent of its current energy use is fossil fuel based and imported.

Astoundingly, with the cooperation of European and worldwide partners, Morocco has even grander plans to power itself entirely by renewable energy and potentially, in cooperation with other Northern African countries, export excess clean energy to Europe.

With these aggressive goals, it shouldn’t be surprising that King Mohammed VI of Morocco isn’t afraid of what anyone thinks when he talks about the reality of climate change. And talk he does.

In a message to world participants at the September 2102 conference in Morocco, Energy Challenges in the Euro-Mediterranean Region, King Mohammed said:

“Convinced of the vital importance of protecting and preserving the environment, and having realized at an early stage—thanks to its geographical location—the potential impact of climate change, my country resolutely opted for sustainable development which, needless to say, goes hand in hand with human development.”

In 2009, King Mohammed announced at a ceremony attended by U.S. Secretary of State Hillary Clinton a $9 billion solar project with the target of creating Read more »