Topic：Solar Energy Conversion Beyond the Limit
Speaker: Prof. X.-Y. Zhu,
Department of Chemistry, Columbia University, e-mail: firstname.lastname@example.org
Time: 2014.11.7 14:00-15:00
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% [J. Appl. Phys. 32(1961) 510.]. This is because photon energy in excess of the semiconductor bandgap is lost as waste heat. In this lecture, I will describe our 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 my lab provided the first spectroscopic signatures in exciton fission of a critical intermediate known as the multiexciton state or triplet pair state [Science 334 (2011) 1541; Nature Chem. 4 (2012) 840]. 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 [Acct. Chem. Res. 46 (2013) 1321]. I will demonstrate the feasibility of harvesting the multiexciton state for two electron-hole pairs [J. Am. Chem. Soc. 134 (2012) 18295; Nature Commun. 4 (2013) 2679]. I 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.
Light-matter interaction, solar energy conversion, ultrafast spectroscopy. A main research thrust is to establish new photophysical mechanisms that may be utilized to revolutionize solar energy conversion or light emission. As examples, recent discoveries in our 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.
Honors & Awards
Alexander-von-Humboldt Fellowship, Humboldt Foundation, 1992.
Camille and Henry Dreyfus New Faculty Award, Dreyfus Foundation, 1993.
Cottrell Scholar Award, Research Corporation, 1996.
Friedrich Wilhelm Bessel Award, Humboldt Foundation, 2006.
Fellow of the American Physical Society, 2011.
Brian Bent Memorial Lecturer, Columbia University, 02/15/2007.
Plenary Lecturer, 31st DOE Solar Photochem. Meeting, Annapolis, MD, 06/07/2009.
Welch Lecturer, Welch Conference, Houston, TX, 10/ 25/2010
Milton Kahn Lecturer, University of New Mexico, Albuquerque, NM, 04/22/2011
Plenary/public lecturer, I-CAMP Summer School, Boulder, CO, 08/10/2012
Plenary lecturer, Southeast Ultrafast Conference, Atlanta, GA, 01/11/2013
Plenary lecturer, ICSFI-14, Gyeongju, Korea, 07/03/2013
R.B. Woodward Lecturer in the Chemical Sciences, Harvard University, 10/10/2013
Sample Publications (total 200+)
· M. Muntwiler, Q. Yang, W. A. Tisdale, X.-Y. Zhu, “Coulomb barrier for charge separation at an organic semiconductor interface,” Phys. Rev. Lett. 2008, 101, 196403.
· W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, X.-Y. Zhu, “Hot electron transfer from semiconductor nanocrystals,” Science 2010, 328, 1543-1547.
· W.-L. Chan, M. Ligges, A. Jailaubekov, L. Kaake, L. Miaja-Avila, X.-Y. Zhu, “Observing the Multi-Exciton State in Singlet Fission and Ensuing Ultrafast Multi-Electron Transfer,” Science 2011, 334, 1541-1545.
· L. Miaja-Avila, J. Tritsch, A. Wolcott, W.-L. Chan, C. A. Nelson, X.-Y. Zhu, “Direct mapping of hot electron relaxation and multiexciton generation dynamics in PbSe quantum dots,” Nano Lett. 2012, 12, 1588-1591.
· W.-L. Chan, M. Ligges, X.-Y. Zhu, “The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain,” Nature Chem. 2012, 4, 840-845.
· C. A. Nelson, X.-Y. Zhu, “Reversible electronic traps in PbS quantum dot solids induced by an order-disorder phase transition in capping molecules,” J. Am. Chem. Soc. 2012, 134, 7792-7795.
· W.-L. Chan, J. R. Tritsch, X.-Y. Zhu, “Harvesting singlet fission for solar energy conversion: one versus two electron transfer from the quantum mechanical superposition,” J. Am. Chem. Soc. 2012, 134, 18295–18302.
· A. Jailaubekov, A. P. Willard, J. Tritsch, W.-L. Chan, N. Sai, R. I. Gearba, L. G. Kaake, K. J. Williams, K. Leung, P. J. Rossky, X.-Y. Zhu, “Hot charge transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics,” Nature Mater. 2013, 12, 66-73.
· M. Tuan Trinh, M. Y. Sfeir, J. J. Choi, J. S. Owen, X.-Y. Zhu, “A hot electron-hole pair breaks the symmetry of a semiconductor quantum dot,” Nano Lett. 2013, 13, 6091-6097.
· J. R. Tritsch, W.-L. Chan, X. Wu, N. R. Monahan, X.-Y. Zhu, “Harvesting singlet fission for solar energy conversion via triplet energy transfer,” Nature Commun. 2013, 4, 2679.
· Journal Editorial/Advisory Boards: Prog. Surf. Sci. (2006- ); Acc. Chem. Res. (2009-); Chem. Phys. (2013-)
· Symposium Organizer: 236th, 235th, 221st ACS National Meeting; MRS Spring Meeting (2009) and Fall Meeting (2005); 49th SPIE. Symposium (2004); 4th International Symposium on Ultrafast Surface Dynamics (2003);
· Guest Lecturer: Asian Pacific Regional College on Science at the Nanoscale (2006).
· Co-founder & Chief Technology Officer, MicroSurfaces, Inc. (2001-present).
· International Steering Committee: Symp. on Ultrafast Surface Dynamics (2004-present)