Organic electronics and optoelectronics have emerged as new application areas for organic materials. Pi conjugation in polymeric materials and/or pi-pi intermolecular interactions in low molecular weight molecules were shown to lead to conducting materials. These semiconducting properties are being explored in organic electronic devices (such as field-effect transistors, FETs, and diodes). Some of these materials are also luminescent and are being used in optoelectronics devices (such as light-emitting diodes, LEDs, and light-emitting electrochemical cells, photovoltaics, and optically pumped lasers). Low density, mechanical flexibility and control of properties via molecular design are at the origin of this new technological area. Furthermore, as some materials can be solution processable, they are leading to the development of new manufacturing technologies (such as inkjet or roll-to-roll printing), which can bring fabrication prices down, and allow the fabrication of large active areas.
Interfaces, either between electrodes and organics or between organics, play a key role in the performance of devices where charge injection, transport or charge extraction are important processes. We have been studying the role of such interfaces and ways to control them.
In this communication, I will address two main topics: i) modification of indium-tin oxide, ITO, surface via self assembly [1,2]; and ii) polymer interlayers formed on top of the hole-injection polymer, poly(3,4-ethylenedioxithiophene) doped with polystyrene sulfonic acid, PEDOT:PSS [3,4]. Their influence on the performance of polymer LEDs will be discussed. In particular, we find that, in the case of SAM modified ITO, the adequate choice of molecules can significantly affect ITO’s charge injection ability in LEDs. In the second type of interface modification, we find that the interlayer thickness and nature of the conjugated polymer used to form it control device performance. The fabrication of sub-monolayers and their characterisation by AFM and current-sensing AFM, has allowed us to establish the binding sites of the conjugated polymer on the surface of PEDOT:PSS.
Acknowledgments_ This work was partially supported by FCT under the contract PTDC/FIS/72831/2006.

References

[1] J. Morgado, A. Charas, and N. Barbagallo, Appl.Phys. Lett., 81, 933 (2002)
[2] J. Morgado, N. Barbagallo, V. Bodrozic, A. Teixeira, A.C. Fernandes, R. Güntner, U. Scherf, F. Cacialli, L. Alcácer, Synthetic Metals 154, 153-156 (2005).
[3] G. Bernardo, A. Charas, L. Alcácer, J. Morgado, Appl. Phys. Lett. 91, 063509 (2007).
[4] G. Bernardo, A. Charas, L. Alcácer, J. Morgado, J. Appl. Phys., 103, 084510 (2008).