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Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors

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journal contribution
posted on 2021-01-28, 14:11 authored by Anton Matthijs Berghuis, T. V. Raziman, Alexei Halpin, Shaojun Wang, Alberto G. Curto, Jaime Gómez Rivas
Using diffraction-limited ultrafast imaging techniques, we investigate the propagation of singlet and triplet excitons in single-crystal tetracene. Instead of an expected broadening, the distribution of singlet excitons narrows on a nanosecond time scale after photoexcitation. This narrowing results in an effective negative diffusion in which singlet excitons migrate toward the high-density region, eventually leading to a singlet exciton distribution that is smaller than the laser excitation spot. Modeling the excited-state dynamics demonstrates that the origin of the anomalous diffusion is rooted in nonlinear triplet–triplet annihilation (TTA). We anticipate that this is a general phenomenon that can be used to study exciton diffusion and nonlinear TTA rates in semiconductors relevant for organic optoelectronics.