A Silver Bullet for Organic Solar Cells

From the 2007 Research Review.

Centuries ago, stained-glass makers developed a technique for creating luminous colors by mixing minuscule droplets of metals into molten glass. Today, scientists at NREL are using a related concept to boost the efficiencies of organic solar cells.

Made largely from plastic, organic solar cells have the potential to be manufactured more easily and less expensively than solar cells made of silicon and other materials. They can be made of lower-cost materials in processes that are amenable to high-throughput manufacturing and that do not require high-temperature or high-vacuum conditions.

To explore this potential, Jao van de Lagemaat and his colleagues at NREL and the University of Colorado at Boulder are studying organic solar cells known as polymer-fullerene cells. The active layer of these cells consist of a mixture of plastic and fullerene molecules, which are molecules of 60 carbon atoms arranged in the shape of a soccer ball.

The two materials form interpenetrating phases within the active layer, and when blue light is absorbed by that layer, it forms "excitons," which are weakly bound pairs of a newly freed electron and the positive "hole" left behind in the material when the electron was freed. As the excitons diffuse to a boundary between the two materials, they split, and electrons move through the fullerenes to the back contact, while holes move through the plastic to the front contact. The result is a flow of electrical current.

There is one big roadblock to achieving high conversion efficiencies with these types of cells, however: The active layer does not absorb the low-energy red and infrared portions of the solar spectrum. But van de Lagemaat and his associates have overcome that obstacle. Their solution was to add a layer of silver with a thickness on the scale of a billionth of a meter (a nanometer).

"We are using a nanoscale layer of discontinuous silver particles, which boosts cell performance by exploiting a phenomenon called surface-plasmon resonance," van de Lagemaat says. "Surface plasmons are free electrons that move in unison on the surface of the silver layer when excited by light. These plasmons greatly enhance the electromagnetic energy transmitted to the underlying active layer. The result is a boost in current generation and hence, improved conversion efficiency of the cell," van de Lagemaat says.

The team has demonstrated that building a nanoscale silver layer within an organic solar cell can increase the output current of the device at long wavelengths. In experiments using a silver layer one nanometer thick, the solar energy conversion of the solar cell improved by a factor of 1.7. Fine-tuning will produce devices with even higher efficiencies.

Organic solar cells might not look quite as beautiful as a stained-glass window. But the performance improvement in these plastic cells—gained by using this silver "bullet"—will be very much appreciated by those wanting more affordable, clean electricity.