Solar Energy- Plastic Solar Cells

Physicists at Rutgers University in New Jersey have found new properties in an organic semiconductor material that could lead to efficient and inexpensive plastic solar cells.
“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 the university.
“But their limited photovoltaic conversion efficiency has held them back. We expect our discovery to stimulate further development and progress,” he continued.
Mr. Podzorov and his colleagues learned that excitons can travel a thousand times farther in a pure crystal organic semiconductor called rubrene.

Excitons are particles that consist of an electron and an electron hole and are formed when a semiconducting material absorbs photons or light particles. They generate a photo-voltage when they hit a semiconductor boundary or junction, which is how power is generated in a solar device.

Typically, excitons can only travel less than 20 nanometers in organic semiconductors, placing them at a disadvantage with inorganic solar cell materials such as silicon or gallium arsenide.
If excitons diffuse only tens of nanometers, only those closest to the boundaries would generate photo-voltage. This accounts for the low electric conversion efficiencies of current organic solar cells.
However, the excitons in the rubrene crystals are capable of travelling farther, thus generating more photo-voltage for a higher electric conversion efficiency.
Since excitons are not charged, they are hard to measure using traditional methods. The researchers developed a technique based on optical spectroscopy called polarization resolved photocurrent spectroscopy to dissociate excitons at the rubrene’s surface, revealing a large photocurrent.
The researchers speculate that the technique can be applicable to other possible organic semiconductor materials.