‘Baking powder’ effect to boost solar cell efficiencies?
The extra solvent plays a comparable part to that of baking powder in a dough mixture. Exactly how the addition works has been unclear for the last ten years but now the TU/e researchers have explained the mechanism in a paper published in Nature Communications which points to a new understanding that will enable a more focused development of plastic solar cells.
Plastic solar cells, or 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 reduces the cost and weight of these solar cells, and makes them flexible. But their efficiency of about 10 percent still remains below that of commercial silicon solar cells, which reach efficiencies of between 15 and 20 percent.
Around ten years ago, it was found by chance that the efficiency of plastic solar cells was 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,” explained TU/e professor René Janssen. “But nobody knew exactly why they have such a favorable effect on the efficiency.”
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. The 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.
The Eindhoven researchers led by René Janssen used a combination of optical technologies to find a definitive explanation. If they did not add a co-solvent, they found that large droplets were formed during the hardening of the plastic mixture. The droplets 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.
The researchers also found the reason for that. “There are two effects that arise during the hardening process,” explained 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".
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