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.