Why 'baking powder' triples efficiency of plastic solar cells

Plastic or organic solar cells use polymers instead of silicon to convert the energy from sunlight into electricity. The use of plastic as basic material reduces the cost and weight of these solar cells, and makes them flexible. However, the efficiency (10%) of plastic solar cells is lower compared to that of commercial silicon solar cells (between 15 and 20%).

The efficiency of plastic solar cells can be doubled or tripled by adding an extra solvent during the production process. “These co-solvents are now used in all plastic solar cells,” said TU/e professor René Janssen. “But nobody knew exactly why they have such a favourable effect on the efficiency.”

However, researchers at TU/e have come up with the answer in a publication in Nature Communications. This new understanding will now enable focused development of plastic solar cells.

Morphology

It was known that there was a connection with the ‘morphology’ of the solar cell, in other words the exact structure of two mixed plastic components in the cell between which electrons move under the influence of sunlight. These components - both of them organic materials - are dissolved during the production process, after which they evaporate and harden. The mysterious co-solvent is always added to the solvent before evaporation.

Droplet size

The Eindhoven researchers led by René Janssen used a combination of optical technologies to find a definitive explanation. They say that if they did not add a co-solvent, they found that large droplets were 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,” said Janssen.

Baking powder

They also found the reason for that. “There are two effects that arise during the hardening process,” said Janssen. “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.

Infographic that shows the effect of the co-solvent on the solution of two plastic components (polymer and fullerene molecules) during the production process of the plastic solar cell. Without the co-solvent droplets of fullerene, molecules are formed which restrict the efficiency of the solar cell. With the co-solvent, the polymer molecules fold much more quickly, so that droplets can't be formed. Source: TU Eindhoven/Hans van Franeker

More effective

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,” said Janssen. “But now we can predict much more accurately what is likely to work, and what isn’t.”