Distribution of space charges in luminescent conjugated polymers

We present investigations of the distribution of space charges in thin films of conjugated polymers. A thermal wave method (LIMM) has been used to resolve the depth profile of space charges and the distribution of the internal electric field in 3-4 mum films of poly(2,5-pyridinediyl), the poly(p-phenylenvinylene) derivative poly[2-methoxy,5-(2'-ethyl-hexyloxy)-1,4-phenylenvinylene] (MEHPPV) and polyfluorine (P170). The results demonstrate that in conjugated polymers, space charges cannot only be created but can also be stored permanently. The electric field in the bulk of the film is significantly reduced because of the existence of two space charge layers: a narrow, positively charged layer at the front electrode, and a broader, negatively charged layer at about 500nm. We assign the positive layer to the injection of positive charge carriers from the anode into near surface traps, and the second hetero-charge layer to bulk charges that have been separated within the external electric field and accumulated in the trapping zone. The dynamics of the accumulation and poling of the space charge layers has been investigated by means of measurements of the temporal evolution and temperature dependence of the pyroelectric response. (C) 2002 Society of Chemical Industry.