September 11, 2013
FRONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES
Dr. Arthur J. Nozik
Department of Chemistry and Biochemistry, University of Colorado, Boulder and
National Renewable Energy Laboratory, Golden, Colorado
"Multiple Exciton Generation in Quantum Dots, Quantum Dot Arrays, Quantum Dot Solar Cells, and via Molecular Singlet Fission: Application to Next Generation Solar Photon Conversion"
4:10 pm, 5326 Stevenson Center (Refreshments at 3:45)
Abstract. In quantum dots (QDs), quantum rods (QRs) and unique molecular chromophores that undergo singlet fission (SF) the relaxation pathways of photoexcited states can be modified to produce efficient multiple exciton generation (MEG) from single photons . We have observed efficient MEG in PbSe, PbS, PbTe, and Si QDs and efficient SF in molecules that satisfy specific requirements for their excited state energy levels. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport while still maintaining quantization and MEG. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and respectable power conversion efficiencies via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. These solar cells also show for the first time external quantum yields (QYs) for photocurrent that exceed 100% in the photon energy regions of the solar spectrum where MEG is possible (i.e.,energy conservation is satisfied); the photocurrent internal QYs from MEG as a function of photon energy match those determined via time-resolved spectroscopy and settles controversy concerning MEG. We have also observed very efficient SF in thin films of molecular crystals of 1,3 diphenylisobenzofuran with quantum yields of 200% at the optimum SF threshold of 2Eg (HOMO-LUMO for T1-S0 ), reflecting the creation of two excited triplet states from the first excited singlet state. Various possible configurations for novel solar cells based on MEG in QDs and SF in molecules that could produce high conversion efficiencies will be presented, along with progress in developing such new types of solar cells. Recent analyses of the dramatic effects of MEG or SF combined with solar concentration on the conversion efficiency of solar cells will also be discussed. The properties required for nanocrystals and SF molecules to achieve the high solar conversion efficiencies predicted by theory will be discussed.
Host: Dr. Rizia Bardhan
Dr. Arthur J. Nozik