Rubrene  (Source:
Rubrene crystal raises hope for the use of organic semiconductors and cheaper, more efficient solar cells

Rutgers University physicists have found new properties within a material that could lead to the production of less expensive and more efficient plastic solar cells.

Vitaly Podzorov, co-author of the study and assistant professor of physics at Rutgers University, along with his research team have discovered that organic semiconductors allow energy-carrying particles -- which are created by "packets" of light -- to journey a thousand times farther than researchers previously thought. 

"Organic semiconductors are promising for solar cells and other uses, such as video displays, because they can be fabricated in large plastic sheets," said Podzorov. "But their limited photovoltaic conversion efficiency has held them back. We expect our discovery to stimulate further development and progress.

Podzorov and his team came to these conclusions by observing excitons, which are particles that consist of an electron and an electron hole where a positive charge is attributed to the absence of an electron. Excitons form when semiconducting materials absorb photons, which are light particles.

The problem with organic semiconductors up until this point was that they were observed to travel less than 20 nanometers. This is an issue because electrons and the holes move to the two opposite sides when they "hit" a semiconductor junction or boundary. If these excitons only diffuse "tens of nanometers," the only ones that generate photo-voltage are those closest to the boundaries or junctions. 

"Now we lose 99 percent of sunlight," said Podzorov. 

But now, Podzorov and his team have observed that excitons can journey a thousand times farther than previously thought in rubrene, which is an extremely pure crystal organic semiconductor

"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."

Excitons within the rubrene crystal acted like excitons in inorganic crystals, which means better opto-electronic properties, increased efficiency and lower costs. Podzorov and his team hope, with further development, that solar cells based on this technology can replace silicon solar cells.

In the midst of this research Podzorov also discovered a new way of measuring excitons based on optical spectroscopy. Excitons are hard to measure because they are not charged, so the Rutgers University research team created a new method called polarization resolved photocurrent spectroscopy, which "dissociates" excitons at the surface of the crystal and exposes large photocurrent. 

This study will be in an upcoming issue of Nature Materials, where Podzorov has submitted other relevant and recent research on organic semiconductors. 

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