posted on 2021-01-28, 14:11authored byAnton 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.