Understanding Plastic Solar Cells

Monday, February 9, 2015 @ 01:02 PM gHale

Plastic solar cells, also referred to as organic solar cells, use polymers instead of the usual silicon to convert the energy from sunlight into electricity.

The use of plastic as basic material helps reduce the cost and weight of these solar cells, while also making them flexible, but their efficiency of around 10 percent still remains below that of commercial silicon solar cells, which have efficiencies of between 15 and 20 percent.

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Around ten years ago, it was found by chance that the efficiency of plastic solar cells increased by a factor of two to three times by adding an extra solvent (‘co-solvent’) during the production process. “These co-solvents are now used in all plastic solar cells”, said Eindhoven University of Technology TU/e professor René Janssen. “But nobody knew exactly why they have such a favorable effect on the efficiency.”

They did know there was a connection with the “morphology” of the solar cell, where the exact structure of two mixed plastic components in the cell between where electrons move under the influence of sunlight. These components, which are both organic materials, end up dissolved during the production process, after which they evaporate and harden. The mysterious co-solvent always adds in to the solvent before evaporation.

Eindhoven researchers led by René Janssen used a combination of optical technologies to find a definitive explanation. They said if they did not add a co-solvent, they found large droplets formed during the hardening of the plastic mixture. These have an adverse effect on the electron transport, and as a result on the efficiency of the solar cell.

“The more co-solvent you add to the solution, the smaller the bubbles turn out to be, until they disappear completely when a specific content is reached,” Janssen said.

They also found the reason for that.

“There are two effects that arise during the hardening process,” Janssen said. “On the one hand the solution evaporates, and as well as that polymers take on a ‘folded’ structure. We saw that the co-solvent makes this ‘folding’ process start at a much earlier stage, which means the bubbles are ultimately no longer formed at all.” In this way the co-solvent acts as a kind of “baking powder:” It improves the structure of the mixture, but the agent in itself is not enough.

Taking this to the next level, the researchers hope their findings will make the development of plastic solar cells more effective.

“Up to now it was mainly a question of trial-and-error,” Janssen said. “But now we can predict much more accurately what is likely to work, and what isn’t.”

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