Singlet fission is known to improve solar energy utilization by circumventing the Shockley-Queisser thermodynamic limit. In the process of singlet fission, two essential steps are the formation of a correlated triplet pair, and its subsequent quantum decoherence. Here we examined both the essential steps in hexacene with transient absorption spectroscopy under different temperatures. The triplet pair which formation is facilitated by electronic coupling and phonon relaxation, emerges on the same ultrafast timescales of ca. 200fs upon photo-excitation, thus making quantum decoherence the critical step for directional anisotropy of singlet fission. Temperature-dependent experiments show that the formation rate of the triplet pair is independent of temperature. On the other hand, the quantum decoherence is controlled by electron-phonon coupling. The quantum decoherence occurs rapidly under high temperature, while it lasts much longer below 200 K. These results will help us design new singlet fission materials for high solar energy exploitation.

Electron-phonon coupling in singlet fission of hexacene

ACS Fall 2020 Virtual Meeting

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