The impact of ester groups on 1,8-naphthalimide electron transport material in organic solar cells

Abstract

Recently, 1,8-naphthalimide (NI) molecules have emerged as a new category of organic electron transport materials (ETMs) employed in organic solar cells (OSCs). Among them, NI molecules with a "NI-π-NI" molecular structure commonly exhibit over aggregation in films owing to their planar molecular structures, which is unfavorable for solution-processed film fabrication in OSC devices. In this work, we successfully synthesized a new NI molecule named NDT-2, employing a strategy that incorporates a solubilizing ester group modification to induce twisted molecular planarity, thereby reducing intermolecular over-aggregation and optimizing quality of the film. Interestingly, the NDT-2 modified by ester groups as a π bridge exhibits an enlarged molecular dihedral angle, enhanced alcohol solubility, a smoother film surface morphology, and improved film forming quality. Ultimately, OSCs incorporating NDT-2 as the ETM achieved a notable power conversion efficiency (PCE) of 17.1%. This study introduces a design strategy that utilizes ester functional groups to improve film forming quality of ETM, offering valuable insights for the development of advanced, alcohol-soluble NI molecules as efficient ETMs in OSCs.