Thienyl Sidechain Substitution and Backbone Fluorination of Benzodithiophene-Based Donor Polymers Concertedly Minimize Carrier Losses in ITIC-Based Organic Solar Cells

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 photophysical 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.