Photoexcitation Dynamics in Strongly Interacting Donor/Acceptor Blends Probed by Time-Resolved Photoluminescence Spectroscopy

In organic semiconductors, strong acceptor molecules represent versatile building blocks to tailor their electronic properties and it is of particular interest how they modify the transport of charge carriers or excitons. Here, we study the low-temperature photoexcitation dynamics of a donor/acceptor system comprised of tetracene (Tc) and the strong acceptor F4-TCNQ using time-resolved photoluminescence (PL) spectroscopy. Tc/F4-TCNQ blends reveal both relatively pure domains of Tc and a mixed phase, where strong interactions in the ground state lead to the formation of charge transfer complexes. The mixed phase can efficiently quench excitons from Tc domains either by charge or by energy transfer, which becomes apparent as a faster PL decay with respect to neat Tc. We analyze the low-temperature dynamics by means of global and target analysis and find that the photoexcited population relaxes into lower-energetic states on a time scale of 10 to several 100 ps. This relaxation process is efficient in neat Tc but seems to be suppressed to a certain extent in the Tc domains of the mixtures, potentially due to lower exciton mobility. Our study emphasizes the important role of thermal energy for efficient exciton transport in organic devices with high dopant concentrations.