Excitation energy migration in highly emissive semiconducting polymers

The process of excitation energy migration (EEM) in conjugated polymers, which is relevant both for light emitting diodes and laser applications is probed by doping the blue light emitting methyl-substituted ladder-type poly(para-phenylene) with small concentrations of a highly fluorescent (pi) -conjugated macromolecule. The experimentally attained temperature and concentration dependence of the steady state photoluminescence are modeled and discussed by means of a two step EEM process: (1) a thermally activated migration within the host and (2) transfer from the host to the guest. In particular we show that a Forster type mechanism alone cannot account for the experimental facts in such a guest host system. We have used the same materials to tune the emission color of organic- light-emitting diodes (OLEDs). Electroluminescence (EL)- current characteristics of the fabricated devices are presented. We found that the EEM process in OLEDs is more efficient for the EL process due to charge carrier trapping on the guest material compared to PL. For dopand concentrations as low as 1 w% one observes an increase of the photoluminescence as well as the electroluminescence quantum efficiency ((eta) EL) by approximately 50% with respect to the pure host polymer efficiencies. We realized a white light emitting diode with an external (eta) EL of up to 1.8% with the emission color being independent of the applied bias.

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