The influence of the microstructure upon the photovoltaic performance of MDMOPPV: PCBM bulk hetero-junction organic sola
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The influence of the microstructure upon the photovoltaic performance of MDMOPPV:PCBM bulk hetero-junction organic solar cells T. Martens1*, J. D’Haen2, T. Munters1, L. Goris2, Z. Beelen1, J. Manca2, M. D’Olieslaeger2, D. Vanderzande1,2, L. De Schepper1,2, R. Andriessen3 AFFILIATIONS 1
Instituut voor Materiaal Onderzoek (IMO), Limburgs Universitair Centrum (LUC), Wetenschapspark 1, 3590 Diepenbeek, Belgium 2 IMEC, Division IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium 3 Agfa-Gevaert NV, Septestraat 27, 2640 Mortsel, Belgium ABSTRACT
In this paper, a clear view on the bulk microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is obtained by means of TEM (Transmission Electron Microscopy). Using TEM, 3-dimensional information is acquired on phase separated regions, formed during casting. Particle statistics illustrate quantitatively that a.o. drying conditions and choice of solvent dramatically influence the blend structure. More information about the lateral blend structure and distribution is obtained in cross-sectional view. Since blend morphology is strongly related to photovoltaic performance, TEM can be a powerful tool for understanding today’s photovoltaic performances and screening new sets of materials. Keywords: TEM; solar cells; PCBM; MDMO-PPV INTRODUCTION
In the recent wave of breakthroughs in the field of ‘plastic’ electronics, promising results are being obtained in the development of solar cells based on organic materials. Particular interest in this development goes out to the concept of the interpenetrating bulk donor-acceptor heterojunction, in which the active organic layer consists of a blend of a donor material and an acceptor material. Current state-of-the-art plastic solar cells are based on an active layer of a soluble fullerene derivative (PCBM) as acceptor and an alkoxy-poly(para-phenylene vinylene) derivative (MDMO-PPV) as a donor. Starting from the first conjugated polymeric photovoltaic devices, major increases in efficiency have been made. The discovery of a femto-second photo-induced charge transfer[1-2] from the conjugated polymer to buckminsterfullerene (C60) and the replacements of the C60 by the solubilized derivative PCBM[3-4], yielded 1% efficient solar cells. Recently, control over molecular morphology by casting conditions[5] resulted in 2.5% efficient plastic solar cells. These devices consist of an indium tin oxide (ITO) layer / a spincoated poly(3,4-ethylene-dioxy-thiophene)/polystyrene sulphonic acid (PEDOT/PSS) layer / the spincoated active layer of MDMO-PPV:PVBM in a 1:4 ratio by weight / an evaporated LiF layer / and an Al electrode[6]. Most recent devices, consisting of so-called sulphinyl-MDMO-PPV have a power-conversion efficiency under AM 1.5 illumination of 2.9%[7]. *
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