A new and somewhat unexpected way to create very high-efficiency organic solar cells has been found. Rather than working solely to increase the purity of a solar cell’s two domains — acceptor and donor — researchers found that if impure domains are made smaller, this also increases efficiency levels.
“We’ve shown that impure domains if made sufficiently small can also lead to improved performances in polymer-based organic photovoltaic cells,” says Harald Ade, a physicist at North Carolina State University, who led this research. “There seems to be a happy medium, a sweet-spot of sorts, between purity and domain size that should be much easier to achieve than ultra-high purity.”
The conversion efficiency of polymer/organic photovoltaic cells is extremely dependent upon excitons, which are electron/hole pairs that are energized by light, making their way quickly to the interfaces of the donor and acceptor domains, and losing less energy in the form of heat. It was assumed that the primary way to increase the speed at which this occurs was by increasing the purity of the domains, leaving less to slow down the excitons. But the new research clearly shows that it can also be increased by making the domain size smaller.
This research is also the first time that the domain size, composition, and crystallinity of an organic solar cell has been measured all at once. It was this knowledge that allowed this breakthrough.
By using a solvent to shrink down the dominant acceptor domain size from 177 nanometers to 34 nanometers, the researchers were able to increase the efficiency of the material by a substantial amount, 42%.