Tunable Charge Transfer Dynamics at Tetracene/LiF/C60 Interfaces

Ultrafast optical spectroscopy was utilized to investigate charge transfer dynamics across organic semiconductor tetracene/C60 interfaces in the presence of a LiF barrier layer. Photoinduced absorption spectra in the 1.6−2.3 eV range reveal a strong effect of the intermediate LiF barrier layer on dynamics of the charge transfer excitons (CTE) creation and recombination. Increasing thickness of the LiF film from 0 to 1 nm significantly suppresses CTE recombination while CTE generation remains practically unaltered. Further increase of LiF thickness to 2 nm prevents creation of CTE by diffusion from tetracene but does not affect direct CTE excitation by incident photons. Unlike thin films studied here, direct CTE photogeneration at the interface between thick organic films accounts for a small fraction (as compared to diffusion-induced) of total CTE population, resulting in a larger contribution of the LiF barrier to charge separation efficiency.