Photocrosslinkable liquid–crystalline polymers for stable photovoltaics by adjusting side-chains spacing and fullerene size to control intercalation

Abstract

We report a novel copolymer system with high crystallinity and photocrosslinkable building blocks for π–π intermolecular interactions that is, an alternating copolymer with liquid–crystalline nature, and heat/solvent resistance. By copolymerization of 2,5-bis(3-bromododecylthiophen-2-yl)thieno[3,2-b]thiophene (BbTTT) monomer with thiophene and thieno[3,2-b]thiophene via Stille reaction, two novel copolymers PBbTTT-T and PBbTTT-TT have been synthesized. The balanced space between fullerene size and the side-chains of the polymer is crucial to determine the optimum polymer:fullerene blending ratios and the formation of intercalated nanostructure, in which the lamellar arrangement can be controlled by adjusting the fullerene size. This pre-optimum bimolecular crystal morphology can be frozen and preserved with long term performance after UV treatment, a clear advantage for the photo-crosslinking strategy. Furthermore, the free space in the intercalation impacts greatly on the stability of the donor–acceptor bicontinuous network, especially after photocrosslinking. The bulk-heterojunction organic photovoltaics based on PBbTTT-TT:PC71BM at 1:3.5 by weight shows an stable, well-ordered and intercalated nanostructure with an efficiency higher than 2.4% after 40h annealing at an elevated temperature of 150°C.