CaSTL Seminar with Xiaoyang Zhu

The maximum solar-to-electric power conversion efficiency of a conventional solar cell is determined by the Shockley-Queisser limit of ~31%. One viable approach to exceed this limit is to create two or more electron-hole pairs from the absorption of one photon in a process called singlet fission or multiple exciton generation.  Recent measurements in our group by time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton 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 [Science 2011, 334, 1541-1545; Nature Chem. 2012, 4, 840-845; Acct. Chem. Res. 2013, 46, 1321-1329]. We demonstrate the feasibility of harvesting the multiexciton state for multiple charge carriers or the triplets [J. Am. Chem. Soc. 2012, 134, 18295–18302; Nature Commun. 2013, 4, 2679.]. We 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.