10-10/11 :: October/November 2010
efficiency of the solar cell. This sets the limit on how much electrical energy can be yielded from a given amount of solar energy.
Using electrically detected magnetic resonance (EDMR), the scientists demonstrated that the polarons always get in one another’s way when their magnetic moment (spin) is identical. The researchers’ EDMR method involved manipulating the spin of the polarons using an external magnetic field and a microwave pulse. Using a resonance effect, the randomly distributed spin could be turned and aimed like a compass needle. Measurements revealed that current flows freely when the tiny magnets are oppositely aligned, but is blocked when they are aligned in the same direction.
The researchers demonstrated these current losses in plastic solar cells at room temperature, having redesigned an experimental method originally developed for silicon. “With this important finding, we should soon see advance- ments in organic solar cell technology as new plastics are introduced that develop no spin blockades,” says project leader Dr. Klaus Lips.
image cover: contact fingers for electrical measurements; back- ground: extrem thin, flexible solar cell on titanium foil as carrier layer, © HZB
J. Behrends, A. Schnegg, K. Lips, E. A. Thomsen, A. K. Pandey, I. D. W. Samuel, and D. J. Keeble: Bipolaron Formation in Organic Solar Cells Observed by Pulsed Electrically Detected Magnetic Resonance, In: Physical Review Letters, Vol. 105(2010), Issue 17, October 22, 2010, Article 176601 [4 pages], DOI:10.1103/ PhysRevLett.105.176601: http://dx.doi.org/10.1103/PhysRevLett.105.176601