The effect of molecular geometry on the polymer/fullerene ratio in polymer solar cells

Abstract

In order to reveal the correlation between molecular structures and the corresponding film morphology as well as polymer/fullerene blend ratio, a new donor–acceptor (D–A) polymer poly-(benzodithiophene-naphthothiophenedione) (PTN8) based on and was synthesized, which was intentionally designed to further increase the molecular structure bulkiness of our previously reported polymer poly-(benzodithiophene-thienopyrroledione) (PTP8). The experimental results and theoretical calculations indicate that both polymers have similar solubilities, optical properties and energy levels, while the structural modification has a significant impact on the polymer intermolecular packing and crystallinity by altering the backbone coplanarity. The different molecular geometries lead to significantly different device performances, optimal fullerene loadings and efficiency dependence on blend ratios. Therefore molecular geometry should be carefully considered when rationally designing polymers for reducing the use of fullerene.