New solar energy material captures every color of the rainbow

October 16, 2008

Researchers have created a new material that overcomes two of the major obstacles to solar power: it absorbs all the energy contained in sunlight, and generates electrons in a way that makes them easier to capture.

Ohio State University chemists and their colleagues combined electrically conductive plastic with metals including molybdenum and titanium to create the hybrid material.

"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," explained Malcolm Chisholm, Distinguished University Professor and Chair of the Department of Chemistry at Ohio State.

The study appears in the current issue of the Proceedings of the National Academy of Sciences (PNAS).

Sunlight contains the entire spectrum of colors that can be seen with the naked eye -- all the colors of the rainbow. What our eyes interpret as color are really different energy levels, or frequencies of light. Today's solar cell materials can only capture a small range of frequencies, so they can only capture a small fraction of the energy contained in sunlight.

This new material is the first that can absorb all the energy contained in visible light at once.

The material generates electricity just like other solar cell materials do: light energizes the atoms of the material, and some of the electrons in those atoms are knocked loose.

Ideally, the electrons flow out of the device as electrical current, but this is where most solar cells run into trouble. The electrons only stay loose for a tiny fraction of a second before they sink back into the atoms from which they came. The electrons must be captured during the short time they are free, and this task, called charge separation, is difficult.

In the new hybrid material, electrons remain free much longer than ever before.

To design the hybrid material, the chemists explored different molecular configurations on a computer at the Ohio Supercomputer Center. Then, with colleagues at National Taiwan University, they synthesized molecules of the new material in a liquid solution, measured the frequencies of light the molecules absorbed, and also measured the length of time that excited electrons remained free in the molecules.

They saw something very unusual. 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, and the triplet state lasts much longer than the singlet state.

Electrons in the singlet state stayed free for up to 12 picoseconds, or trillionths of a second -- not unusual compared to some solar cell materials. But 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.

"This long-lived excited state should allow us to better manipulate charge separation," Chisholm said.

At this point, the material is years from commercial development, but he added that this experiment provides a proof of concept -- that hybrid solar cell materials such as this one can offer unusual properties.

Source: Ohio State University

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3 / 5 (2) Oct 16, 2008
i wonder what the efficiency would be of a total cell.
2.5 / 5 (2) Oct 16, 2008
This sounds like it has huge potential. The ingredients are low cost. Manufacturing sounds easy. Efficiency can be poor so long as the cost is low. The critical metric is $/W. Another is the half life for the material i.e. how many years before it drops 50% in efficiency.
1.5 / 5 (2) Oct 16, 2008
a few months ago i did read about a high-school student that was the FIRST to come up with some kind of solar cell to capture the entire range of the solar spectrum. I doubt these guys are the first ones as they claim, unless they paid the kid off to shut up, in which case yes, these guys are the first ones with this this solar panel technology.
4 / 5 (2) Oct 16, 2008
Efficiency can be poor so long as the cost is low. The critical metric is $/W. Another is the half life for the material i.e. how many years before it drops 50% in efficiency.

Another is the W/m^2, or output per unit of surface area. Even if it's cheap, if you're limited to (for instance) your roof area, and the total output is too low to make a difference, you just won't bother.
3 / 5 (2) Oct 17, 2008
They may not be the first to capture the full range but they are probably the first to properly measure what they are capturing.

But keeping the electrons in the triplet state for 83 to 200 ms is impressive.
5 / 5 (1) Oct 17, 2008
Catching more of the spectrum is great, but if you're doing it at a low efficiency then the technology isn't an advancement.

You're advancing the potential energy captured but not the amount. It's like putting great tires on a Pinto. Sure the tires can stand up to 200 miles an hour but will it make any difference whatsoever.
4.5 / 5 (2) Oct 17, 2008
It's like putting great tires on a Pinto. Sure the tires can stand up to 200 miles an hour but will it make any difference whatsoever.

Yes. Great tires will burn better when the Pinto explodes.
4.5 / 5 (2) Oct 17, 2008
There's a big difference between 'catching' and 'using' the entire energy spectrum. After reading the paper, I'm not sure why this article claims it as a solar material. They are making photo-absorbing metallorganic complexes. Early in the paper they reference an extensive body of literature using similar chemicals for photonics. Dye lasers would be a good example of light-emitting organics. They also have short-lived singlet states and long-lived triplet states. The long life of the triplet states is detrimental in producing light because they either decay non-radiatively or limit population inversion.

As for the other comments, 80ms is 1000 times longer than the measured 80us. Triplet lifetimes of organics often go up into actual seconds! No surprise that the thin film had longer lifetimes. Solutions are much more likely to suffer from excited state quenching. For more info on photochemistry, definitely check out Montalti's Handbook of Photochemistry.

The kid who 'invented' a solar cell that absorbs all the types of light was basing his literature search on work done previously at universities. It certainly was not his own work but those at the university were very friendly and encouraging to their credit.

Absorption is very different from effectively using light. The energy of photons differs with their wavelength such as E=h*c/lambda. Shorter wavelengths (more blue) have higher energy. So in our famous silicon photovoltaics, any energy greater than the band gap (analagous to HOMO/LUMO gap) will be ABSORBED. This energy promotes an electron across the band gap. The excess photon energy is just wasted. Silicon absorbs anything with a wavelength less than about 1.1um. Sunlight can be approximated around 400-700nm. Sure it is absorbing it, but the cell isn't using all the energy. Getting higher efficiencies with multiple wavelengths requires special techniques like multi-junction cells and non-linear avalanche-type effects.

I'm all for solar power but this article really doesn't have much at all to do with it.

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