Image by North Carolina State University
This is a twofer: Scientists in the United States and Hong Kong have not only discovered the key to improving the efficiency of organic solar cells, but they've done so in a way that makes them easy and inexpensive to produce on a commercial scale.
The upshot is that the research has led to an organic – i.e., carbon-based – solar polymer cell that is up to 10.8 percent efficient, meaning that it converts 10.8 of the sunshine it absorbs into electricity. Until now the record efficiency has been 9.8 percent.
Plus, the new solar polymer cells are thin and flexible enough that they can be printed repeatedly on a thin, flexible surface that can be furled like a roll of paper towels.
Scientists at North Carolina State University carefully mixed a polymer – a heavy molecule created by evaporating its liquid – with a fullerene – a class of roughly spherical carbon molecules. They then added enough of a solvent to the mix to create a liquid and spread a thin layer of it on a flexible.
As the solvent evaporated, the thin layer of polymer-fullerene solidified into hard, evenly distributed "clumps" that are connected to one another by other polymer molecules. Meanwhile, the fullerene in the mixture snaked around the clumps.
Harald Ade, a physicist at NC State, and a postdoctoral researcher, Wei Ma, studied how the neighboring polymers and fullerenes interact – a process known as aggregation – and discovered that the polymer clumps determined the efficiency of an organic solar polymer cell.
Ade and Ma then teamed up with chemists from the Hong Kong University of Science and Technology. Together they discovered that the clumps' size and aggregation in a solar polymer cell can be adjusted by changing the temperature at which the cells are manufactured.
They also found that the record efficiency of 10.8 percent can be reached only by using many different kinds of fullerenes. Until now, the most efficient organic solar polymer cells have been made using only two kinds of fullerenes. These discoveries led them to a variety of options for testing the cells' efficiency. It also allowed them to adjust the cells' thickness to improve production.
"Once we saw how temperature affected the aggregation and morphology of these solar cells, it allowed the chemists more freedom to play with different chemical compositions in the active layer," Ade told the NC State News Department.
"[T]hese solar cells could be compatible with existing methods of mass production, like slot die casting and roll-to-roll processing similar to newspaper printing, rather than the more expensive production methods currently in use that are required for thickness control."
Ade said the research of the two teams should lead to more experimentation that could increase the cells' efficiency even more, and make them easier and less costly to manufacture. The results of their work were published Nov. 10 in the journal Nature Communications.