Space charge and internal electric field distribution in poly(2,5-pyridinediyl)

We present measurements of the spatial distribution of space charge and the internal electric field in 3–4 μm thick films of a luminescent conjugated polymer by means of the laser intensity modulation method. The internal electric field is spatially modified by the existence and persistence of space charge layers induced by the external bias. We observe a double layer of homo-charges and hetero-charges, which is explained by a trapping model that implements the presence of traps near the surface of the film. Within this model, space charge layers are built up by a combination of carrier injection from the electrodes, not into the valence or conduction bands, but directly into intra-gap traps near the interface, as well as the separation of bulk electrons and holes in the external field and subsequent trapping. A trap density of about 1023 m−3 is concluded from the results. In addition, the conjugated polymer poly(2,5-pyridinediyl) is found to have strong electret properties with local charge storage of up to 500 C/m3.