Exciton Fission & Solar Energy Conversion Beyond the Limit
Frontiers of CMP Lecture: September 29 (Tuesday)
Columbia Pupin 329 4:10 PM live
Rice SST 301A 3:10 PM CST simulcast from Columbia U Oregon Eugene 1:10 PM PST simulcast from Columbia
Speaker: Prof. Xiao Yang Zhu (Columbia, Chemistry) http://www.columbia.edu/cu/chemistry/groups/zhu/
Title: Exciton Fission & Solar Energy Conversion Beyond the Limit
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 andtetracene, 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. We provide experimental foundation for a quantum coherent mechanism in which the vibronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately following optical excitation.
I will demonstrate the feasibility of harvesting the multiexciton state for two electron-hole pairs. 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.