R. Mezzengaa,b, N. Sarya, C. Brochonc, G. Hadziioannouc, J. Ruokolainend
aDepartment of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Ch. Musée 3, CH-1700, Fribourg, Switzerland
bNestlé Research Center, Vers-Chez-Les-Blanc, 1000 Lausanne 26 (Switzerland)
cLaboratoire d'Ingénierie des Polymères pour les Hautes Technologies, UMR 7165, Université Louis Pasteur, Ecole Européenne de Chimie, Polymères et Matériaux, 25, Rue Becquerel, 67087 Strasbourg, France
d Helsinki University of Technology, Physics Laboratory, Helsinki, 02015 Finland
We describe a new route towards design of block copolymer photovoltaic precursors with sharp interfaces. The specific morphology arises from the self-assembly of π-conjugated poly(diethylhexyloxy-p-phenylene vinylene) rod homopolymers (PPV) and poly(diethylhexyloxy-p-phenylene vinylene)-polystyrene (PPV-PS) rod-coil block copolymers. In order to demonstrate this concept, two PPV-PS block copolymers, one with low rod volume fraction (~17%) and the other with high volume fraction (~50%) were synthesized. In the first case, given the low volume fraction of the rod block, the pure asymmetrical rod-coil block copolymer formed an isotropic homogeneous phase. However, ordered clusters of alternating PS and PPV domains with characteristic length of the order of several μm, appeared when PPV rod homopolymers were blended to the PS-PPV diblock. Furthermore, the long-range order of the clusters as well as their volume fraction could be greatly increased when the symmetric rod-coil PPV-PS was blended to PPV homopolymer. Tomographic reconstruction from transmission electron micrographs allowed demonstrating that the clusters were organized in lamellar phase, while wide angle x-ray scattering showed that within the PPV domains the PPV blocks and PPV homopolymer rods were closely packed. The study of the spacing widths of the PPV and PS domains showed that clusters are organized in a smectic C configuration with large tilt angles of the rods (54°) and stretching of the coil blocks which is typical of weakly-segregated block copolymers organized in a lamellar phase. The stability of the rod-to-rod interaction peak at high temperatures (190°C), well beyond the order-disorder transition temperature of the clusters (130°C), suggests that aggregation of the rods is mediated by π-π interactions and that clusters are thermodynamically stable structures. Based on these results we give an energetic driving force rational towards the formation of these clusters and propose it as a general route towards design of interfaces in organic photovoltaic materials.