Electric-field-induced luminescence quenching in an electroluminescent organic semiconductor

We report on the effect of a static electric field on the photoluminescence (PL) of laddertype poly(paraphenylene) (m-LPPP) in devices of polymer-light-emitting diodes. External electric fields of 4.5 MV/cm reduce the integral PL intensity down to 24% of the zero-field value and alter the shape of the PL spectra. The field-induced PL quenching in the m-LPPP polymer is ascribed to field dissociation of the emitting species. Time-resolved spectroscopy showed that the emission is composed of contributions from the radiative recombination of free singlet excitons and of a second emitting species, which we describe as self-trapped excitons. The observed field-induced changes of the shape of the PL emission can be attributed to the different field response of these two emitting species. Thus the PL emission color can be changed with the electric field in the blue-green spectral range. The magnitude of the binding energy of free singulet excitons is discussed.