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Patterned films formed via spin coating blends of semiconducting and insulating polymers

Farinhas, J. ; Ferreira, Q. ; Matos, M. ; Morgado, J. ; Charas, A.

Patterned films formed via spin coating blends of semiconducting and insulating polymers, Proc Encontro Nacional de Química-Física da SPQ, Aveiro, Portugal, Vol. 1, pp. 111 - 111, June, 2009.

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In the field of organic electronics, the fabrication of integrated and/or pixelated systems requires the patterning of the organic materials at reduced dimensions: from tens of micrometers for display pixels, down to micro- or submicrometers, to achieve high integration density of electronic/optoelectronic components. Furthermore, the morphology of the organic films plays a significant role on the characteristics of the devices. This is particularly relevant in Organic Photovoltaic cells (OPVs), combining electron donor and electron acceptor materials. The most efficient OPVs are based on interpenetrating networks of the two components, segregated into domains, whose dimensions, down to the nanoscale, are relevant for charge generation and transport [1].

In this context, there has been an increasing interest on methods and techniques to pattern electroactive organic materials at small scales, in particular semiconducting polymers, due to their wide range of applications in organic electronic devices. This led to the development of several techniques, such as screen printing [2], inkjet printing [3], and soft lithography [4]. Such approaches are, however, expensive and complicated with respect to the numerous parameters required to achieve good resolution over large substrates.

Here, we report on the fabrication of patterned films of a cross-linkable semiconducting polymer, F8T2Ox2, by controlling the phase separation taking place during the deposition, by the spin coating method, of a solution containing a mixture of F8T2Ox2 and an «inert» insulating polymer. F8T2Ox2 contains side chains with cross-linkable end-groups, which can polymerize in the presence of photo-acid generator and under UV illumination, yielding an insoluble (cross-linked) network. Thus, after film deposition, the subsequent cross-linking of F8T2Ox2 allows the easy removal of the insulating polymer by dissolution, leaving a patterned film of cross-linked F8T2Ox2. Figure 1 shows a topographical image obtained by Atomic Force Microscopy of one of those films deposited onto Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) covered quartz substrate after removal of the insulating polymer. The PEDOT:PSS layer was introduced to reproduce the surface where the patterned film is deposited when is applied to electronic devices, such as Light-Emitting Diodes (LEDs) or OPVs. We show that upon variation of some parameters, such as spin speed, solution concentration and composition, the type of morphology, patterns dimensions and film thicknesses can be tailored. We also demonstrate film morphologies with polymer patterns dimensions of ca. 100 nm with potential applications in the fabrication of nanostructured layers for efficient OPVs.