Diffusion-limited energy transfer in blends of oligofluorenes with an anthracene derivative

Organic semiconductor devices such as light-emitting diodes and solar cells frequently comprise a blend of molecular or polymeric materials. Consequently, resonant energy transfer between the components plays a major role in determining device performance. Energy transfer may take place through either single-step donor-acceptor transfer, realized for example as Förster transfer, or as a sequence of donor-donor transfers toward the acceptor site. Here we use a well-defined model system comprising an oligofluorene trimer, pentamer, or heptamer as the donor in combination with an anthracene derivative as the acceptor in order to study the rate and mechanism of energy transfer in thin films by time-resolved photoluminescence spectroscopy. We find the transfer process to be entirely dominated by sequential donor-donor transfer. In addition, we observe a strong dependence on oligomer length with an optimum energy transfer rate for the pentamer.