Andrew Rappe Seminar
12:30 pm - 1:30 pm, plus 30 minutes of Q & A
SERC Building, room 408
Title: The bulk photovoltaic effect in polar oxides for robust and efficient solar energy harvesting
Speaker: Andrew Rappe, Chemistry and Materials Science and Engineering, University of Pennsylvania
For individuals who will not be on Temple's campus, Dr. Rappe's lecture can be accessed through Webex. The meeting number is 641 859 413
Solar energy is the most promising source of renewable, clean energy to replace the current reliance on fossil fuels. Ferroelectric (FE) materials have recently attracted increased attention as a candidate class of materials for use in photovoltaic devices. Their strong inversion symmetry breaking due to spontaneous polarization allows for excited carrier separation by the bulk of the material and voltages higher than the band gap (Eg), which may allow efficiencies beyond the Shockley-Queisser limit. Ferroelectric oxides are also robust and can be fabricated using low cost methods such as sol-gel thin film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and FE-electrode interfaces can dramatically increase the current harvested from FE absorber materials. Further improvements have been blocked by the wide band gaps (Eg =2.7-4 eV) of FE oxides, which allow the use of only 8-20% of the solar spectrum and drastically reduce the upper limit of photovoltaic efficiency.
In this talk, I will discuss new insight into the bulk photovoltaic effect, and materials design to enhance the photovoltaic efficiency. We calculate from first principles the current arising from the "shift current" mechanism, and demonstrate that it quantitatively explains the observed current. Then, we analyze the electronic features that
lead to strong photovoltaic effects. Finally, we present new oxides that are strongly polar yet have band gaps in the visible range, offering prospects for greatly enhanced bulk photovoltaic effects.
Andrew Rappe's research group creates and uses new theoretical and computational approaches to study complex systems in materials science, condensed-matter physics, and physical chemistry.
The group looks for new phenomena that occur when different components are brought together. For example, it examines molecules adsorbing on metal surfaces, in order to understand the effect of surface composition and structure on preferred adsorption sites, dissociation pathways, and vibrational dynamics. They also study how the compositions of oxide solid solutions lead to Angstrom-scale chemical structure, nanometer scale structural disorder, and long-range ferroelectric and piezoelectric properties. These studies find real-world applications in catalysis, corrosion, SONAR, fuel cells and other important technologies. Whenever possible, they model systems analytically, in order to extract general principles and simple pictures from complex systems. The group recently derived general expressions for the vibrational lifetimes of molecules on surfaces, revealing the dependence of lifetime on molecular coverage and arrangement. The recent exploration of quantum stress fields has helped to link chemical and mechanical effects in materials.