Side chain engineering of mono-fluorinated acceptors for efficient non-halogenated solvent-processed organic solar cells

Abstract

Organic solar cells (OSCs) processed from non-halogenated solvents often suffer from excessive aggregation of non-fullerene acceptors (NFAs) and suboptimal phase separation, which undermine device performance. In this work, we modified the flexible alkyl chains of the mono-fluorinated NFA, BTz-δ-C₄C₈ (also referred to as BTzC₄IC-2F-δ), and synthesized BTz-δ-C₄C₁₂, which differs in the internal side-chain length (2-ethylhexyl vs. 2-butyloctyl). Molecular dynamics simulations and experimental analyses demonstrate that longer inner alkyl chains slightly increase the backbone torsion and reduce the dipole moment, which helps suppress excessive aggregation and improve miscibility with the donor polymer PFBT4T-T20. These effects promote an optimized balance between molecular packing and phase separation, resulting in improved nanoscale morphology and enhanced charge carrier mobility. As a result, toluene-processed OSCs based on BTz-δ-C₄C₁₂ deliver an impressive power conversion efficiency (PCE) of 16.6% accompanied by a high fill factor (FF) of 76.7%.