Small molecules love the sun

C&I Issue 22, 2011

For the first time, US scientists have produced a small-molecule solar cell that is able to convert sunlight into electricity as efficiently as the best polymer solar cells.

Polymer solar cells are cheaper to produce and less rigid than conventional cells based on silicon, but are much less efficient at producing electricity. The best silicon solar cells are able to convert over 20% of the light energy falling on them into electricity, whereas the best polymer solar cells only achieve 8% conversion.

Small-molecule solar cells offer all the advantages of polymer cells, but are even easier to produce and don’t suffer from the structural variability that besets polymers. However, conversion efficiencies are only around 5%.

Now, scientists from the University of California, Santa Barbara, have developed a small-molecule solar cell able to convert light energy into electricity with almost 7% efficiency (Nature Materials, doi: 10.1038/NMAT3160). The team includes Alan Heeger, who shared the Nobel prize for chemistry in 2000 for discovering conductive polymers.

In both polymer and small-molecule solar cells, a donor material is mixed with an acceptor material, often based on a fullerene molecule such as C70. On exposure to light, the donor material produces electron-hole pairs, which quickly divide at the junction between the donor and acceptor materials. The electrons then travel through the acceptor material to the positive electrode, while the holes (essentially just missing electrons) travel through the donor material to the negative electrode, generating an electric current.

The UCSB scientists have produced a novel donor molecule that is more efficient at generating electron-hole pairs and transporting holes than any previously developed. A mixture of this donor molecule and a C70-based acceptor molecule can convert 6.7% of the light energy falling on it into electricity.

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