Mechanistic study of excitation energy transfer in dye-doped PPV polymers

The mechanism of excitation energy transfer is studied using host-guest systems consisting of green and yellow emitting poly(phenylene vinylene) (PPV) based polymers into which red emitting dyes are dispersed. The photoluminescence from such polymer-dye systems is studied in the steady-state and time resolved. Furthermore, the electroluminescence from devices containing these polymer-dye systems as emissive layer is measured. It is shown that in such disordered polymers, characterized by dispersive exciton transport, energetic resonance between the polymer and the dye is not the only requisite for efficient energy transfer. In addition, the exciton kinetics of the combined polymer-dye system has to be taken into account. Efficient transfer of excitation energy from a disordered polymer to a dye can only occur if, at a certain energy, the polymer-to-dye exciton transfer rate is higher than the intrapolymer exciton migration rate. A consequence of the mechanism described here is that when a dye is dispersed into a disordered polymer, always a residual polymer emission remains, both in photoluminescence as well as in electroluminescence, provided that excitons are created on the polymer. This knowledge is important when constructing a device employing such a polymer-dye system as the emissive layer.