Effect of dispersive transport and partial trap filling on thermally stimulated current in conjugated polymers

Abstract

Thermally stimulated current (TSC) controlled by thermal release of excess charge carriers (holes) from intrinsic traps in a conjugated polymer poly(9,9-bis(2-ethylhexyl) fluorene-2,7-diyl) has been studied at different readout voltages (VRO) applied during the heating run. We found that the TSC peak measured at low applied electric field is located at somewhat higher temperature then the thermally stimulated luminescence (TSL) detected in this material; however, the TSC peak shifts towards the lower temperature with increasing VRO and at high electric fields approaches the temperature spectra of TSL. We found that the above shift of the TSC peak is not accompanied with a substantial increase of TSC intensity usually observed in the TSC of organic solids, and is considered as an indication that the recombination of liberated charge carriers plays no role. Whereas the observed behavior of the TSC peak is due to the thermally stimulated dispersive transport of holes in the polymer. We formulated a model of dispersive TSC, which considers the partial filling of deep traps. Results of the calculations agree well qualitatively with the experiment. The possible implication of the dispersive character of TSC for evaluation of the energetic distribution and spatial concentration of traps from the TSC data is analysed.