Plant-inspired solar cell design promises improved efficiency
The new approach to solar cell design was inspired by the way plants use photosynthesis to convert light energy into chemical energy that can be later released to fuel activities. Well-defined structures within plant cells rapidly separate positive and negative charges, which, according to Sarah Tolbert, a UCLA professor of chemistry and one of the senior authors of the research, is the key to making the process so efficient.
"Biology does a very good job of creating energy from sunlight," said Tolbert. "Plants do this through photosynthesis with extremely high efficiency."
This is not the case with man-made solar cells, especially the low-cost plastic (or polymer) solar cells that are being looked to as a replacement for cells made from expensive silicon. Plastic solar cells are known to be relatively inefficient – in large part due to their much cruder structures (compared to plants) that allow positive and negative electric charges to recombine before they can become useful electrical energy.
If synthetic organic photovoltaic materials could mimic the approach used by plants, the UCLA scientists figured, charge separation and collection could be significantly enhanced. The key was having the right structure.
The researchers’ system uses two components: a polymer donor, which absorbs sunlight, and a nano-scale fullerene acceptor, which accepts electrons passed to it from the donor, generating electrical energy. The components are neatly arranged – like "small bundles of uncooked (polymer) spaghetti with precisely placed (fullerene) meatballs" – to allow for retention and separation of electrons for a period of days or even weeks.
UCLA scientists have devised a new arrangement of solar cell ingredients – with bundles of polymer donors (green rods) and neatly organized fullerene acceptors (purple, tan) – that promises to greatly improve inexpensive photovoltaic materials’ ability to retain energy from sunlight. (Image courtesy of UCLA Chemistry)
"When the charges never come back together, the system works far better," said Benjamin Schwartz, a UCLA professor of chemistry and another senior co-author. "This is the first time this has been shown using modern synthetic organic photovoltaic materials."
In the new system, the materials are designed to self-assemble into the desired structures when placed into a solution, so there is no additional work once the materials are made. The researchers are currently working on how to incorporate the system into actual solar cells. For more see the paper in the journal Science: Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies.
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