Suppressing Exciton–Vibration Coupling and Reducing Nonradiative Energy Loss in Conjugated Polymers Through Fluorine Substitution in Side Chains

Abstract

Fluorine (F) substitution in polymers modulates both molecular energy levels and film morphology; however, its impact on exciton–vibrational coupling and molecular reorganization energy is often neglected. Herein, we systematically investigated F-modified polymers (PBTA-PSF, PBDB-PSF) and their nonfluorinated counterparts (PBTA-PS, PBDB-PS) through simulations and experiments. We found that F atoms effectively lower the vibrational frequency of the molecular skeleton and suppress exciton–vibration coupling, thereby reducing the nonradiative decay rate. Moreover, introducing F atoms significantly decreases the reorganization energy for the S₀ → S₁ and S₀ → cation transitions while increasing the reorganization energy for the S₁ → S₀ and cation → S₀ transitions. These changes facilitate exciton dissociation and reduce the energy loss caused by dissociation and nonradiative recombination of excitons. Additionally, introducing F atoms into polymers enhances the π–π stacking strength and the crystal coherence length in both neat and blended films, ultimately resulting in improvements in the power conversion efficiency of PBTA-PSF:L8-BO and PBDB-PSF:L8-BO are 16.51% and 17.59%, respectively. This study provides valuable insights for designing organic semiconductor materials to minimize energy loss and achieve a higher power conversion efficiency.