Conformational and aggregation properties of PffBT4T polymers: atomistic insight into the impact of alkyl-chain branching positions

Abstract

Recently, a series of PffBT4T-based polymers were developed and their photovoltaic performances depended much on the branching positions of the alkyl side chains. Herein, we investigated the backbone conformations and aggregation behaviors of three PffBT4T polymers with different branched alkyl chains by means of atomistic molecular dynamics simulations. The results show that the modification of the alkyl-chain branching positions not only changes the steric hindrance, but also results in different degrees of backbone twists. The first-position branched alkyl chains bring about both a big steric hindrance and a highly twisted backbone, thus resulting in very weak polymer aggregation in solution. In contrast, the third-position branched alkyl chains bring about a much smaller steric hindrance and a decreased backbone twist, and therefore very strong polymer aggregation appears at both room temperature and an elevated temperature. Notably, the polymer with the second-position branched alkyl chains shows a moderate backbone twist and alkyl-chain steric hindrance, thus exhibiting desirable temperature-dependent aggregation. Our work provides the rationalization of experimental observations and is helpful for the establishment of the structure–property relationship for polymer solar cells.