Stable Postfullerene Solar Cells via Direct C-H Arylation Polymerization. Morphology-Performance Relationships

The scope of the environmentally benign direct C-H arylation polymerization (DARP) process is validated and significantly extended in the synthesis of a high-performance benzodithiophene-based copolymer series, PBDT(Ar)-FTTE, with previously untested and systematically varied heteroaryl (Ar) substituents. Bulk-heterojunction (BHJ) polymer solar cells (PSCs) containing the high-performance nonfullerene acceptor (NFA) ITIC-Th and DARP-derived donors are fabricated and evaluated, yielding PCEs as high as 8%. The relationships between Ar-sensitive copolymer structure, BHJ morphology, and PSC performance are elucidated through in-depth characterization of structural order, phase separation, and charge transport using SCLC, AFM, GIWAXS, R-SoXS, and NEXAFS measurements, which conclusively demonstrate the important effects of Ar-tunable, dimensionally smaller, and well-blended copolymer domains for maximum PSC performance. Smaller BHJ copolymer domains having greater ITIC-Th miscibility definitively correlate with enhanced J_SC, FF, and PCE metrics. Surprisingly regarding cell performance durability, while unencapsulated PBDTT-FTTE:ITIC-Th PSCs deliver the highest initial PCE, the unencapsulated PBDTTF-FTTE:ITIC-Th devices exhibit the optimum combination of high initial photovoltaic metrics and stability, retaining nearly 90% of the initial PCE after 51 days in ambient conditions and 83% of initial PCE after 180 min under simulated solar illumination. Importantly, for this PBDT(Ar)-FTTE:ITIC-Th series, PSC photovoltaic stability correlates with the presence of large pure BHJ domains, and moreover rivals or exceeds the stability of the analogous fullerene-based PSCs. Together, these results argue that solar cells prepared with the environmentally benign DARP process and NFAs are promising for both greener and more stable solar energy generation.