Ultrafast Electron Transfer in All-Small-Molecule Photovoltaic Blends Promoted by Intermolecular Interactions in Cyanided Donors

Cyano substitution has been established as a viable approach to optimize the performance of all-small-molecule organic solar cells. However, the effect of cyano substitution on the dynamics of photo-charge generation remains largely unexplored. Here, we report an ultrafast spectroscopic study showing that electron transfer is markedly promoted by enhanced intermolecular charge-transfer interaction in all-small-molecule blends with cyanided donors. The delocalized excitations, arising from intermolecular interaction in the moiety of cyano-substituted donor, undergo ultrafast electron transfer with a lifetime of \begin{document}$ \sim $\end{document}3 ps in the blend. In contrast, some locally excited states, surviving in the film of donor without cyano substitution, are not actively involved in the charge separation. These findings well explain the performance improvement of devices with cyanided donors, suggesting that manipulating intermolecular interaction is an efficient strategy for device optimization.