Discovery paves way for cheap plastic solar cells

A new discovery at Rutgers University looks likely to lead to the creation of efficient and inexpensive solar cells made of plastic.

The researchers discovered that excitons – energy-carrying particles generated by packets of -can travel on the order of a thousand times farther in organic semiconductors than previously thought.

It means that solar cells based on this technology could plausibly overtake silicon solar cells in cost and performance.

“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 conversion efficiency has held them back. We expect our discovery to stimulate further development and progress.”

Podzorov found that excitons can travel a thousand times farther in an extremely pure crystal organic semiconductor called rubrene than in other organic semiconductors.

 The team measured diffusion lengths from two to eight microns – similar to exciton diffusion in inorganic solar cell materials such as silicon and gallium arsenide.

“This is the first time we observed excitons migrating a few microns,” said Podzorov. “Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion.”

By contrast, most organic solar cells today lose 99 percent of the sunlight.

The scientists discovered that excitons in their rubrene crystals behaved more like those seen in inorganic crystals – a delocalized form known as Wannier-Mott, or WM, excitons. WM excitons move rapidly through crystal lattices, resulting in good opto-electronic properties.

It was previously believed that only the more localized form of excitons, called Frenkel excitons, were present in organic semiconductors.