18.66% Efficiency of Polymer Solar Cells Employing Two Nonfullerene Acceptors with Fluorine or Chlorine Substitution

Organic semiconducting materials with fluorine or chlorine substitution are commonly designed to prepare efficient polymer solar cells (PSCs) through finely modulating the energy levels and absorption spectra. Herein, two small molecular acceptors with fluorine substitution L8‐BO or chlorine substitution eC9‐2Cl are selected to prepare ternary PSCs with PM6 as polymer donor. The optimal ternary PSCs exhibit a power conversion efficiency of 18.66%, benefiting from the simultaneously increased open‐circuit voltage ( V OC ) of 0.89 V, short‐circuit current density ( J SC ) of 26.63 mA cm −2 , and fill factor of 78.74% when the content of chlorine substitution eC9‐2Cl is about 20 wt% in acceptors. Fluorinated and chlorinated substitutions can improve the V OC or J SC of the corresponding binary PSCs, which can be recombined into efficient ternary PSCs through optimizing their content in acceptors. Chlorinated materials have special crystalline conditions due to the specific features of the chlorine atoms with large atomic radius and the empty 3 d orbits compared to the corresponding fluorinated materials. Adding an appropriate amount of the chlorine substitution eC9‐2Cl further enhances the crystallization and intermolecular interaction of the ternary PSCs, which is beneficial for charge transport in the active layer and for device performance improvement.