Sub-bandgap photoexcited dynamics at an organic donor/acceptor photovoltaic interface.

Although sub-bandgap light absorption signals in organic donor/acceptor (D/A) photovoltaic systems have been studied extensively, the underlying origins, as well as the impacting factors, are still elusive. By theoretically constructing an organic D/A interface under a femtosecond electric pulse pumping, we obtain an insightful understanding of this issue. First, a careful comparison between the absorption spectra of the D/A interface and the individual donor (acceptor) demonstrates the existence of two weak absorption signals below the donor (acceptor) optical gap. Furthermore, we clarify that the lower-energy signal originates from "cold" charge transfer (CT) absorption, while the higher-energy signal is from "hot" CT absorption. Finally, effects of several key factors, such as the interface structure and the photoexciting condition, on CT absorptions are discussed. These findings should be of vital importance both to understand the sub-bandgap excited states and to recognize their roles in organic photovoltaic devices.