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Posted on January 21, 2009 by  & 

New photovoltaic material that captures the full spectrum of sunlight

One of the limiting factors with solar energy is that it is only able to capture a small range of frequencies from sunlight.
Many organizations are striving to create solar cells that efficiently convert light into electricity at a cost equal or cheaper than current forms of power.
Sunlight is made up of a range of frequencies called the solar spectrum - some can be seen with the naked eye when passed through a prism which separates all the light frequencies while others are invisible. Currently solar cell materials are only able to use a limited amount of these frequencies.
Now led by Malcolm Chisholm, Distinguished University Professor and Chair of the Department of Chemistry, researchers at Ohio State University have created a new material that absorbs all the energy contained in sunlight at once and generates electrons in a way that makes them easier to capture. Chisholm explains, "There are other such hybrids out there, but the advantage of our material is that we can cover the entire range of the solar spectrum,"
Most solar cells generate electricity by energizing the atoms of the material with light and in the process knock some of the electrons in the atoms loose. The electrons only stay loose for a tiny fraction of a second before they sink back into the atoms from which they came.
Ideally the electrons need to be captured during the time they are free (called charge separation) which is difficult but with the new hybrid material that the researchers are working on, the electrons stay free for longer.
After the team (with help from colleagues at National Taiwan University) synthesized molecules of the new material in a liquid solution, measured the frequencies of light the molecules absorbed, and measured the length of time that excited electrons remained free in the molecules - they noticed something strange. The molecules didn't just fluoresce as some solar cell materials do. They phosphoresced as well. Both luminous effects are caused by a material absorbing and emitting energy, but phosphorescence lasts much longer.
To their surprise, the chemists found that the new material was emitting electrons in two different energy states - one called a singlet state, and the other a triplet state. Both energy states are useful for solar cell applications, but the triplet state lasts much longer than the singlet state - in this case electrons in the triplet state stayed free 7 million times longer - up to 83 microseconds, or millionths of a second.
When they deposited the molecules in a thin film, similar to how they might be arranged in an actual solar cell, the triplet states lasted even longer - 200 microseconds.
Chisholm believes that the long-lived excited state should allow them to better manipulate charge separation. He adds although the experiment provided proof of concept that hybrid solar cell materials such as this one can offer unusual properties, the material is years from commercial development.
For more attend PHOTOVOLTAICS BEYOND CONVENTIONAL SILICON, April 7-8, Dresden, Germany.

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Business Development Director, Research

Posted on: January 21, 2009

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