Cyclic alkyl chains promote the polymer self-assembly and packing orders for solar cells

The micro-structure ordering of the pi-conjugated polymer film self-assembling from solution governs its electrical characteristics and performances for organic electronics. The side chains appended on the conjugated backbone could impact the self-assembling characteristics of the polymer, which show us how to design novel substituents to optimize the polymer arrangement and packing in its solid state. Here, cyclic alkyl chains are proven to be excellent ones for designing of narrow bandgap polymers, the design feature of which with preferential conformations can promote the polymer self-assembly and result in higher degree of lamellar order as well as tighter lamellar packing in the active layer, comparing with linear alkyl chains. The linear ones have poorer ordering in the polymer: fullerene blends as well as in the single crystals of the monomers. The well-organized polymer micro-structures facilitate polymer: fullerene phase separation together with balanced hole/electron transporting, and ultimately, improve the power conversion efficiency impressively compared to linear chains.