Issue Date: March 30, 2009
Solar Cells Move Further Into The Red
Current solar cells do not efficiently harvest red and near-infrared photons at the long-wavelength end of the visible spectrum, but electrical engineer Craig A. Grimes and coworkers at Pennsylvania State University show that using Förster-type resonance energy transfer (FRET) can improve the efficiency of solar cells at those wavelengths (ACS Nano, DOI: 10.1021/nn900090x). In FRET, a donor molecule transfers fluorescence energy to a nearby acceptor molecule when the donor's emission spectrum overlaps the acceptor's absorption spectrum. In Grimes's nanowire solar cells, ruthenium polypyridine complex acceptors are anchored to the surfaces of TiO2 nanowires, and dissolved red-absorbing zinc phthalocyanine donor molecules are trapped in the spaces between the nanowires. Confinement of the donors ensures that a significant fraction of donors and acceptors are within FRET distance. The researchers chose as the donor a bulky molecule containing four tertiary butyl groups that disrupt π-π stacking and improve solubility. FRET improved the quantum yield of the devices in the 675- to 680-nm range by a factor of 1.5 for Black Dye acceptor (N-749) and by a factor of 4 for N-719 acceptor.
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