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Sweet Spot Found in Organic Solar Cells

GAITHERSBURG, Md., March 8, 2012 — A better fundamental understanding of how to optimize a cell’s performance could bring organic solar cells a step closer to market.

Prototype solar cells made of organic materials currently lag far behind conventional silicon-based photovoltaic cells in terms of electricity output. However, if efficient organic cells can be developed, they would have distinct advantages of their own: They would cost far less to produce than conventional cells, could cover larger areas and, conceivably, could be recycled far more easily.

Now, scientists at the National Institute of Standards and Technology (NIST) and the US Naval Research Laboratory (NRL) are studying cells made up of hundreds of stacked thin layers that alternate between two organic materials — zinc pthalocyanine and C60, the soccer-ball-shaped carbon molecules known as “buckyballs.” When light strikes the multilayered film, all of the layers are excited, causing them to give up electrons that flow between the buckyball and pthalocyanine layers and creating an electric current.

Light that strikes this organic solar cell causes electrons to flow between its layers, creating an electric current. Measurements made by the NIST/NRL research team determined the best thickness for the layers, a finding that could help optimize the cell’s performance. (Image: NIST)

The researchers discovered that, by varying the thickness of each layer — each of which is only a few nanometers thick — the amount of electrical current the overall cell puts out changes dramatically. In this sense, determining the ideal thickness of the layers is crucial to making the best-performing cells, said NIST chemist Ted Heilweil.

“In essence, if the layers are too thin, they don’t generate enough electrons for a substantial current to flow, but if too thick, many of the electrons get trapped in the individual layers,” Heilweil said. “We wanted to find the sweet spot.”

To find the “sweet spot,” the team explored the relationship between layer thickness and two different aspects of the material. Although the layers generated an initial “spike” in current when struck with light, the current decayed fairly quickly. The ideal cell would generate electrons as steadily as possible. They researchers discovered that changing the layer thickness affects the initial decay rate, but also affected the overall capacity of the material to carry electrons.

The investigators set out to find the optimum combination of these two factors by measuring a number of films, grown by Paul Lane of NRL, that had layers of varying thickness. They found that layers ~2 nm thick give the best performance.

Heilweil said the results encourage him to think that prototype cells based on this geometry can be optimized, although one engineering hurdle remains: finding the best way to get the electricity out.

“It’s still unclear how to best incorporate such thin nanolayers in devices,” he said. “We hope to challenge engineers who can help us with that part.”

The findings appeared in Physical Review Letters.

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