Abstract
Non-fullerene acceptor (NFA)-based organic solar cells have outperformed fullerene-based devices, yet their photophysics is less well understood. Herein, changes in the donor polymer backbone sidechain substitution and backbone fluorination in benzodithiophene-thiophene copolymers are linked to the photo-physical processes and performance of bulk heterojunction (BHJ) solar cells using ITIC as the NFA. Increased geminate recombination is observed when the donor polymer is alkoxy-substituted in conjunction with faster nongeminate recombination of free charges, limiting both the short-circuit current and device fill factor (FF). In contrast, thienyl-substitution reduces geminate recombination, albeit nongeminate recombination remains significant, leading to improved short-circuit current density, yet not the FF. Only the combination of thienyl-substitution and polymer backbone fluorination yields both efficient charge separation and significantly reduced nongeminate recombination, resulting in FFs in excess of 60%. Time-delayed collection field measurements ascertain that charge generation is field independent in the thienyl-substituted donor polymer:ITIC systems, whereas weakly field dependent in the alkoxy-substituted polymer:ITIC blend, indicating that the low FFs are primarily caused by nongeminate recombination. This work provides insight into the interplay of donor polymer structure, BHJ photophysics, and device performance for a prototypical NFA, namely, ITIC. More specifically, it links the donor polymer chemical structure to quantifiable changes of kinetic parameters and the yield of individual processes in ITIC-based BHJ blends.