March 9, 2010

New Light Shines on Nanocrystal Solar Cells

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The Hanrath Energy laboratory, also known as the Hanrath Group, combats one of the biggest obstacles that marketable solar energy technology faces: cost. Led by Prof. Tobias Hanrath, chemical and biomolecular engineering, the team of interdisciplinary engineer graduate and undergraduate students investigates the capabilities of more cheaply produced solar cells constructed from semi-conductor nanocrystals.

“Silicon is the ‘poster child.’ That’s what people use because everyone is using it,” said Hanrath. However, he commented that crystalline silicon is not a strong absorber of light, the silicon solar cells need a thick film layer to be effective and panels are cost and energy intensive to produce. The production entails heating extremely pure sand to approximately 1500 degrees Celsius.

In contrast, the nanocrystal solar cells, approximately 400 nanometers thick, are roughly 300 times thinner than the current technology. The greater capacity of nanocrystals to absorb light energy enables such thinness, which translates into a lower material cost to produce a solar cell.

Another distinct advantage of the nanocrystal solar cell is the low cost to actually make it.

“You don’t need to grow huge silicon crystals,” said Hanrath Group member, Will Baumgardner grad. You can make it in a beaker and then scale that process up.”

To make the nanocyrstals, the Hanrath Group uses semi-conductors, such as lead and selenium, in a process called the “hot injection method.” In the hot injection method, lead is dissolved in a solvent with a nitrogen tube over the beaker to prevent contact with air. Selenium is then injected into the solvent and reacts with the lead to form groups of lead selenide. A “surfactant,” a carbon based compound, is then added to prevent the lead selenide nanocrystals from growing too large.

The lead selenide nanocrystal solution, the basis of the solar cell, can then be applied to a surface, such as a glass panel, and be configured to convert light energy into electricity. As these solar cells are able to come in liquid form, Hanrath noted that the manufacturing cost of panels could be quite small. He envisioned solar cells sprayed onto a flexible substrate like ink at newspaper print facilities.

Aside from cost, nanocyrstals carry the unique property that their size may be controlled by the solvent’s temperature and the ratio of the semi-conductors. This enables nanocrystals to be adjusted to absorb different wavelengths of sunlight whereas crystalline silicon absorbs at only one.

“The ability to tune the electronic properties of the materials opens a door towards low cost advanced solar cell architectures, which breach the efficiency limits of current devices,” said Hanrath.

However, despite being more effective absorbers of energy than silicon crystals, nanocrystals are less effective as conductors. In solar technology, energy in the form of light hits solar cells and releases an electron that will fall back into place unless otherwise harnessed. Nanocrystal technology grapples to retain the energy that is lost when the electron travels between particles.

The Hanrath Group’s best performance reached 3.6% efficiency in the conversion of light energy into electricity. Their efficiency falls short compared to the current solar cells on the market, which generally operate in the 15% to 25% range, but are also more expensive to produce.

Hanrath commented on the dramatic increase in knowledge of nanocrystals compared to 15 years ago. He described the growing field as fast moving and competitive, with similar research projects being conducted at other Ivy League institutions.

Cornell’s Hanrath Group continues to investigate how individual nanocyrstals behave as solar cells, how to assemble particles in an organized way to increase the efficiency of inter-particle conduction and how to merge their progress on these two fronts to create more efficient energy conversion.

While admitting to the daunting task of extracting an electron from a confined system, Hanrath added, “we have the right people, the right tools and the right mind set to get it done.”

Further research of The Hanrath Group involves investigating new nanocrystals that can be made of less toxic materials, such as tin.

While Hanrath and Baumgardner commented that the world’s most pressing problem is over population, they both saw energy consumption as a close second that stems from the first. Baumgardner pointed to solar energy as the best long term solution to limit climate change, to reduce use of diminishing resources and to improve many standards of living.

“Other technologies still use resources that can be used up. The most efficient way is to take energy being inputted. The sun is the only source of input to the earth,” said Baumgardner.

“As a chemical engineer, working with these materials is one way I can contribute,” said Hanrath.

Original Author: Michelle Winglee