Dr. Franz Himpsel - University of Wisconsin
“Using Soft X-ray Spectroscopy to Design New Types of Solar Cells,”
Abstract: Solar cells are the ideal solution to the energy problem, converting abundant solar energy directly into electricity, which is fully convertible into all other types of energy. An area of 100×100 square miles is sufficient to satisfy the electricity needs of the US with currently available solar cells. But that would cost about a trillion dollars. This talk illustrates how spectroscopy with soft X-rays can assist the development of new materials and new designs for solar cells with better price/performance ratio. The starting point is the most general layout of a solar cell, which consists of a light absorber sandwiched between an electron donor and an electron acceptor. The relevant energy levels can be measured by a combination of X-ray absorption spectroscopy and photoelectron spectroscopy. Examples will be given, such as the combination of all three components in one molecule (donor-π-acceptor complexes ,). A future dream experiment will be discussed where the movement of photo-generated carriers through such a complex is followed in real time by pump-probe techniques at the latest generation of X-ray light sources (compare  for such an experiment in the UV/vis).
Biography: Franz received a Diploma in Physics at the University of Munich with a thesis in quantum electrodynamics under F. Bopp. After a summer at CERN he went to Munich for a PhD in condensed matter physics 1976/77 with W. Steinmann. As postdoc he joined D.E. Eastman at IBM Yorktown Heights in 1977 to work on surface science with synchrotron radiation. He became staff member in 1980, manager in 1982, and senior manager in 1985, heading the Surface Physics Department. In 1995 he moved to the Physics Department of the UW Madison as full professor, where he also served as the Scientific Director of the Synchrotron Radiation Center (SRC) from 1997 to 2002.
His scientific interests have been in the area of surfaces and nanostructures. Utilizing his experience with synchrotron radiation he devised methods for mapping energy bands of solids and surfaces by angle-resolved photoemission. As complement he developed inverse photoemission with tunable photon energy for the study of unoccupied states. For identifying the bonding configurations at semiconductor surfaces/interfaces he took advantage of surface-sensitive core level spectroscopy with synchrotron radiation. Self-assembled nanostructures at surfaces have been a common thread throughout his career, such as magnetic quantum wells, atomic chains for the study of low-dimensional electrons, and an atomic scale memory for testing the limits of data storage. Most recently, he has ventured into the attachment of bio-molecules to surfaces and the design of new materials for solar cells. He has 470 publications in refereed journals and is among the 100 most-cited physicists.
Host: Faisal Alamgir