The formation of very thin layers of conjugated polymers on poly(ethylenedioxithiophene) doped with polystyrenesulfonic acid, PEDOT-PSS, known as interlayers, has been explored to improve polymer light-emitting diodes, PLEDs, efficiency and lifetime. The thickness of these interlayers can vary from less than 1 nm up to about 10nm, depending on the annealing conditions and on the conjugated polymer molecular weight. The nature of the interaction between PEDOT-PSS and the conjugated polymer that leads to the formation of the insoluble interlayer is still a matter of debate. The role it plays in PLEDs characteristics has been mainly related to hole transport, injection barriers and reduction of exciton quenching by PEDOT-PSS due to its separation from the luminescent layer.

We have recently reported on the use of cross-linkable polymers to form thicker interlayers. The properties of PLEDs with these thicker interlayers could be related to the frontier levels energy of the polymers forming the interlayers. However, this relation was not found for PLEDs containing very thin (<1nm) interlayers.

To obtain further insight into the conjugated polymer distribution on top of PEDOT-PSS we have carried out AFM and current sensing AFM (CS-AFM) studies of the interlayers formed with poly(9,9-dioctylfluorene), PFO, of two different molecular weights. Scaling laws analysis was performed on the microroughness results obtained from non-contact AFM.

Phase separation is evidenced at the PEDOT:PSS surface, with the valleys showing higher current, which we associate to doped PEDOT. CS-AFM studies on the PFO-based interlayers showed the presence of electrical current domains, with current values and domain sizes depending of the PFO molecular weight. The results suggest a preferential deposition of PFO in the valleys, i.e. it is mainly anchored in the PEDOT rich regions.

Acknowledgments: The authors acknowledge financial support from FCT-Portugal under the contract PTDC/FIS/72931/2006 and Instituto de Telecomunicações.