Plenary Lecture 1: Arthur Nozik
National Renewable Energy Laboratory & University of Colorado
TITLE: Multiple Exciton Generation in Semiconductor Quantum Dots and Novel Molecules: Applications to Third Generation Solar Photon Conversion
ABSTRACT: In order to utilize solar power for the production of electricity and fuel on a massive scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential, long-term approach to high efficiency is to utilize the unique properties of quantum dot nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient photogenerated electron-hole pair multiplication or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of hot electron and hole relaxation, cooling, charge transport, and interfacial charge transfer of the photogenerated carriers with femtosecond (fs) to ns time resolution. At NREL, we have been studying these fundamental dynamics in various bulk and nanoscale semiconductors (quantum dots (QDs), quantum rods/wires, and quantum wells) for many years using fs transient absorption, photoluminescence, and THz spectroscopy. Recently, we predicted that the generation of more than one electron-hole pair (which exist as excitons in QDs) per absorbed photon would be an efficient process in QDs. This prediction has been confirmed over the past several years in several classes of QDs. We have observed very efficient and ultrafast multiple exciton generation (MEG) from absorbed single high energy photons in Group IV-VI and recently in Si QDs. Efficient MEG has the potential to greatly enhance the conversion efficiency of solar cells that incorporate QDs for both solar electricity and solar fuel (i.e. H2) production. Selected aspects of this work will be summarized and recent advances will be discussed; a unique quantum mechanical model to explain efficient and ultrafast MEG based on the coherent superposition of multiple excitonic states (in collaboration with Al. L. Efros and A. Shabaev at NRL) will also be discussed. Various possible configurations for quantum dot solar cells that could produce ultrahigh conversion efficiencies for the production of electricity and solar fuels (e.g. H2 from H2O) will be presented, along with progress in developing such new types of solar cells. Finally, we have also predicted an analogous MEG effect in molecules (called singlet fission) that could be used in molecular chromophore-sensitized nanocrystalline TiO2 solar cells and new preliminary evidence for this effect will also be presented.
BIOGRAPHY: Dr. Arthur J. Nozik is a Senior Research Fellow at the U.S. DOE National Renewable Energy Laboratory (NREL) and Professor Adjoint in the Department of Chemistry and Biochemistry at the University of Colorado, Boulder. In 2007 he was appointed the Scientific Director of the new Center for Revolutionary Solar Photoconversion under the Colorado Renewable Energy Collaboratory. Nozik received his BChE from Cornell University in 1959 and his PhD in Physical Chemistry from Yale University in 1967. Before joining NREL in 1978, then known as the Solar Energy Research Institute (SERI), he conducted research at the Materials Research Center of the Allied Chemical Corporation (now Honeywell, Inc). Dr. Nozik's research interests include size quantization effects in semiconductor quantum dots and quantum wells, including multiple exciton generation from a single photon; the applications of unique effects in nanostructures to advanced approaches for solar photon conversion; photogenerated carrier relaxation dynamics in various semiconductor structures; photoelectrochemistry of semiconductor-molecule interfaces; photoelectrochemical energy conversion; photocatalysis; optical, magnetic and electrical properties of solids; and Mössbauer spectroscopy. He has published over 200 papers and book chapters in these fields, written or edited 5 books, holds 11 U.S. patents, and has delivered over 240 invited talks at universities, conferences, and symposia. He has served on numerous scientific review and advisory panels, chaired and organized many international and national conferences, workshops, and symposia, and received several awards in solar energy research, including the 2008 Eni Award and the 2002 Research Award of the Electrochemical Society. Dr. Nozik has been a Senior Editor of The Journal of Physical Chemistry from 1993 to 2005. A Special Festschrift Issue of The Journal of Physical Chemistry honoring Dr. Nozik’s scientific career appeared in the December 21, 2006 issue. Dr. Nozik is a Fellow of the American Physical Society and a Fellow of the American Association for the Advancement of Science; he is also a member of the American Chemical Society, the Electrochemical Society, and the Materials Research Society.
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