Effective Negative Diffusion of Singlet Excitons in Organic Semiconductors

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.

Automated summary

Using diffraction-limited ultrafast imaging techniques, the study investigated the propagation of singlet and triplet excitons in single-crystal tetracene, revealing a narrowing distribution of singlet excitons after photoexcitation. This narrowing led to a negative diffusion effect where singlet excitons migrated towards high-density regions, ultimately resulting in a distribution smaller than the laser excitation spot. Modeling of the excited-state dynamics attributed this anomalous diffusion to nonlinear triplet-triplet annihilation (TTA), which may have broad implications for studying exciton diffusion and TTA rates in semiconductors relevant to organic optoelectronics.