Organic solar cells may be a step closer to market because
of measurements taken at NIST and the United States Naval Research Laboratory
(NRL), where a team of scientists has developed a better fundamental understanding
of how to optimize the cells’ performance.
Prototype solar cells made of organic materials currently
lag far behind conventional silicon-based photovoltaic cells in terms of
electricity output. But if even reasonably 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.
The cells the team studied are made by stacking up hundreds
of thin layers that alternate between two different organic materials—zinc
pthalocyanine and C60, the soccer-ball shaped carbon molecules
sometimes called buckminsterfullerenes, or buckyballs. Light that strikes this
multilayered film excites all its layers from top to bottom, causing them to
give up electrons that flow between the buckyball and pthalocyanine layers,
creating an electric current.
Each layer is only a few nanometers thick, and varying their
thickness has a dramatic effect on how much electrical current the overall cell
puts out. According to NIST chemist Ted Heilweil, determining the ideal
thickness of the layers is crucial to making the best-performing cells.
“In essence, if the layers are too thin, they don’t generate
enough electrons for a substantial current to flow, but if they are too thick,
many of the electrons get trapped in the individual layers,” says Heilweil. “We
wanted to find the sweet spot.”
Finding that “sweet spot” involved exploring the
relationship between layer thickness and two different aspects of the material.
When light strikes the film, the layers generate an initial spike in current
that then decays fairly quickly; the ideal cell would generate electrons as
steadily as possible. Changing the layer thickness affects the initial decay
rate, but it also affects the overall capacity of the material to carry
electrons, so the team wanted to find the optimum combination of these two
of NRL grew a number of films that had layers of different thickness, and the
team made measurements at both laboratories that took the two factors into
account, finding that layers of roughly 2 nm thick give the best
performance. Heilweil says the results encourage him to think prototype cells
based on this geometry can be optimized, though 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 says. “We hope to challenge engineers who can help
us with that part.”