Self-assembling of novel fullerene-grafted donor-acceptor rod-coil block copolymers

Fullerene-grafted rod-coil block copolymers, designed for being used as active polymer layers in donor-acceptor bulk heterojunction photovoltaic devices, have been synthesized. The copolymer synthesis was monitored by H-1 nuclear magnetic resonance, size exclusion chromatography and infrared absorption spectroscopy, while the material proper-ties were explored by X-ray diffraction, atomic force microscopy, UV-vis absorption, and photoluminescence spectroscopy. Structural properties of bulk materials and thin films were investigated to study the influence of the coil block molecular weight as well as that of the grafted fullerenes on the molecular self-assembling process. The results show that the rod homopolymer is in the crystalline state at ambient temperature and undergoes a phase transition into a smectic-type liquid-crystalline phase at 55 degrees C. The rod-coil microphase separation enhances interlamellar ordering but destabilizes the intralamellar order, leading to the appearance of a liquid-crystalline phase for the non-fullerene-grafted copolymer at ambient temperature. Adding the fullerene moieties considerably affects the polymer assembling through the growth of fullerene nanocrystals. The latter hinder the formation of the lamellar phase by pinning the coil segments. The identification of the major driving forces that control the molecular self-assembling process allows us to suggest different alternative strategies that can be used as guidelines for the design of new photovoltaic polymer self-assembling materials.