Strongly Anisotropic and Rapid Excitons Transport in Organic Crystal.

Exciton diffusion plays a crucial role in the photoelectric conversion process. The increased diffusion length is beneficial to simplify device design and enhance photoelectric conversion efficiency. The isotropic exciton diffusion may lead to reduced transport efficiency directed to the charge separation site. Therefore, it's not only needed to increase the transport rate of excitons, but also needed to restrict the transport direction of excitons. Here we have successfully achieved strongly anisotropic and rapid excitons transport in thienopyrrole-dicarbonyl fluoroboron organic crystal. Two organic crystals of SNH-FB and SNEt-FB have been synthesized to control the exciton transport rate and directions. The transient absorption imaging results indicate that no obvious exciton diffusion is observed in SNH-FB crystals, but ethyl substituted SNEt-FB crystals have a significantly enhanced exciton diffusion constant (∼2 cm2/s) along only one axis on the ab plane of the crystal. Theoretical calculations demonstrate that the coupling strength of molecules along different directions of the SNEt-FB crystals is significantly different, making important contributions to the obvious anisotropy of exciton transport.