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Columnar-grain Films of Conjugated Polymers

Farinhas, J. ; Ferreira, Q. ; Fonseca, S. ; Burrows, H. ; Matos, M. ; Alcácer, L. ; Morgado, J. ; Charas, A.

Columnar-grain Films of Conjugated Polymers, Proc International Symp. on Flexible Organic Electronics - IS-FOE, Halkidiki, Greece, Vol. 1, pp. 90 - 90, July, 2009.

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Abstract
Organic Photovoltaic cells (OPVs) based on conjugated polymers represent the most promising technology in solar cells for being flexible, light-weight, low cost, and compatible with large-area manufacturing processes. However, the efficiencies are still low comparing to Si-based cells. The most efficient OPVs are composed of blends of electron-donor and electron-acceptor components with appropriate redox levels, forming a bulk heterojunction [1]. In these systems, the morphology of the active layer is determinant to the efficiency of the charge generation and charge collection processes. Columnar films, where domains rich in the donor and in the acceptor are segregated into columns perpendicular to the substrate surface, have been pointed out as the optimal morphologies, providing columns radius are within the range of the exciton diffusion length of the active components (ca. 10 nm for conjugated polymers) [2].
In this communication, we report on columnar-grain films, with submicron columns diameters (less than 400 nm), based on a conjugated polymer, namely a cross-linkable polyfluorene derivative [3], as the donor component, and fullerene (C60) or a soluble derivative (PCBM) as the acceptor component (Fig. 1). These films were fabricated using a two step method: i) firstly, columnar films of the cross-linked conjugated polymer are prepared onto PEDOT:PSS covered quartz substrates and ii) the electron-acceptor component is deposited on top by thermal evaporation or by spin coating from solution. We found that the photoluminescence of the polymer is strongly reduced when the acceptor is deposited on top of the columnar films. Moreover, comparing with planar bilayer configurations based on the same donor/acceptor pairs, PL quenching values are significantly higher for the columnar films indicating that charge generation should be a more efficient process in the films with structured interfaces. These results suggest that the prepared columnar films have potential applications in the fabrication of efficient OPV cells.