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 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. We demonstrate the feasibility of harvesting this quantum superposition state for multiple charge carriers. This approach may be implemented for the solar energy conversion beyond the Shockley-Queisser limit.  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 334 (2011) 1541-1545.  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. 4 (2012) 840-845.  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. 134 (2012) 18295–18302.  W.-L. Chan et al. “The quantum coherent mechanism for singlet fission: experiment and theory,” Acct. Chem. Res. 46 (2013) 1321-1329.
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