Discovery paves way for development of efficient, inexpensive plastic solar cells

Oct 10, 2010

Physicists at Rutgers University have discovered new properties in a material that could result in efficient and inexpensive plastic solar cells for pollution-free electricity production.

The discovery, posted online and slated for publication in an upcoming issue of the journal , reveals that energy-carrying particles generated by packets of light can travel on the order of a thousand times farther in organic (carbon-based) semiconductors than scientists previously observed. This boosts scientists' hopes that based on this budding technology may one day overtake silicon solar cells in cost and performance, thereby increasing the practicality of solar-generated electricity as an alternate energy source to .

"Organic semiconductors are promising for solar cells and other uses, such as video displays, because they can be fabricated in large plastic sheets," said Vitaly Podzorov, assistant professor of Physics at Rutgers. "But their limited photo-voltaic has held them back. We expect our discovery to stimulate further development and progress."

Podzorov and his colleagues observed that excitons – particles that form when semiconducting materials absorb photons, or light particles – can travel a thousand times farther in an extremely pure crystal called rubrene. Until now, excitons were typically observed to travel less than 20 nanometers – billionths of a meter – in organic semiconductors.

"This is the first time we observed excitons migrating a few microns," said Podzorov, noting that they measured diffusion lengths from two to eight microns, or millionths of a meter. This is similar to exciton diffusion in inorganic solar cell materials such as silicon and gallium arsenide.

"Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion," he said.

Excitons are particle-like entities consisting of an electron and an electron hole (a positive charge attributed to the absence of an electron). They can generate a photo-voltage when they hit a semiconductor boundary or junction, and the electrons move to one side and the holes move to the other side of the junction. If excitons diffuse only tens of nanometers, only those closest to the junctions or boundaries generate photo-voltage. This accounts for the low electrical conversion efficiencies in today's organic solar cells.

"Now we lose 99 percent of the sunlight," Podzorov noted.

While the extremely pure rubrene crystals fabricated by the Rutgers physicists are suitable only for laboratory research at this time, the research shows that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors. Continuing development could result in more efficient and manufacturable materials.

The scientists discovered that excitons in their rubrene crystals behaved more like the excitons observed in inorganic crystals – a delocalized form known as Wannier-Mott, or WM, excitons. Scientists previously believed that only the more localized form of excitons, called Frenkel excitons, were present in organic semiconductors. WM excitons move more rapidly through crystal lattices, resulting in better opto-electronic properties.

Podzorov noted that the research also produced a new methodology of measuring excitons based on optical spectroscopy. Since excitons are not charged, they are hard to measure using conventional methods. The researchers developed a technique called polarization resolved photocurrent spectroscopy, which dissociates excitons at the crystal's surface and reveals a large photocurrent. The technique should be applicable to other materials, Podzorov claims.

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User comments : 6

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scenage
not rated yet Oct 10, 2010
Can recycled plastic bags be used?
fixer
not rated yet Oct 10, 2010
I note that the military are also going down this path,
looks like we should all have free electricity within the next decade.
Bob_Kob
5 / 5 (1) Oct 10, 2010
Why is it that I hear something like this concerning solar cells every week but nothing ever comes from it.
Caliban
not rated yet Oct 11, 2010
Probably, most recycled plastic would be far too "dirty" for use in this application. As a bonus, though- they can be digested to produce methanol feedstock for any number of endproducts.

The same biogenic processes that are being developed to produce fuels and whatnot can be adapted to produce the precursor materials to manufacture plastics.

No need for Petro in order to produce any of those things, anymore.

Bob_B
not rated yet Oct 11, 2010
Why would Oil companies NOT just buy up these developers/developments and squash them, as usual?

@Bob_Kob: I'm with you, it is tiring to read the same "could provide the breakthrough" or "may give us free energy" and so on.

The news is: Free energy is available and being connected to the grid.
fixer
not rated yet Oct 11, 2010
Yup, two weeks and my solar panels go up, why wait until it's perfected because there will always be "next years model".
In AU solar is down below $1800 for 1.5kw now and that's good enough for me!

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