Abstract
The solar-to-electric power conversion efficiency of a conventional solar cell based on a semiconductor material is fundamentally constrained by the so-called Shockley Queisser limit of ~32%. This is because photon energy in excess of the semiconductor bandgap is lost as waste heat. In this lecture, the speaker will describe his and his research group’s recent efforts to exceed this limit based on exciton fission. The absorption of one photon by a semiconductor material usually creates one electron-hole pair, but this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs in a process called singlet exciton. Recent measurements in the speaker’s lab provided the first spectroscopic signatures in exciton fission of a critical intermediate known as the multiexciton state or triplet pair state. More importantly, population of the multiexciton state is found to rise concurrently with that of the singlet state on the ultrafast time scale upon photo excitation. This observation provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately following optical excitation. The speaker will demonstrate the feasibility of harvesting the multiexciton state for two electron-hole pairs. The speaker will also outline a set of design principles for molecular materials with high singlet fission yield and for the implementation of singlet fission in solar cells with power conversion efficiency beyond the Shockley Queisser limit.
About the speaker
Prof Xiaoyang Zhu obtained his PhD from the University of Texas at Austin (UT Austin) in 1989 and started his postdoctoral research at the same university and Fritz Haber Institute from 1990 to 1993. He then joined Southern Illinois University Carbondale as Assistant Professor. In 1997, he moved to University of Minnesota and became Merck Endowed Professor of Chemistry in 2008. During 2010 to 2012, he returned to UT Austin and was appointed as L.N. Vauquelin Regents Professor of Chemistry, the Director of DOE Energy Frontier Research Center and the Director of Center for Materials Chemistry. He joined Columbia University in 2013 as Professor of Chemistry.
Prof Zhu’s long standing research interest is in light-matter interaction, solar energy conversion, and ultrafast spectroscopy. A main research thrust is to establish new photophysical mechanisms that may be utilized to revolutionize solar energy conversion or light emission. Recent discoveries in his lab showed how an electron and a hole is bound by the Coulomb potential across an organic semiconductor interface, how one can extract hot electrons from a photoexcited quantum dot, how an exciton can split into two via the singlet fission process, and how charge carriers can recombine in semiconductor nanostructures for laser emission.
Prof Zhu received several awards including Cottrell Scholar Award and Friedrich Wilhelm Bessel Award. He is a Fellow of the American Physical Society.
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