To Fluorinate or Not to Fluorinate in Organic Solar Cells: Achieving a Higher PCE of 15.2% when the Donor Polymer is Halogen-Free

Fluorination of the donor and/or acceptor blocks of photoactive semiconducting polymers is a leading strategy to enhance organic solar cell (OSC) performance. Here, the effects are investigated in OSCs using fluorine-free (TPD-3) and fluorinated (TPD-3F) donor polymers, paired with the nonfullerene acceptor Y6. Interestingly and unexpectedly, fluorination negatively affects performance, and fluorine-free TPD-3:Y6 OSCs exhibit a far higher power conversion efficiency (PCE = 14.5%) than in the fluorine-containing TPD-3F:Y6 blends (PCE = 11.5%). Transmission electron microscopy (TEM) analysis indicates that the TPD-3F:Y6 blends have larger phase domain sizes than TPD-3:Y6, which reduces exciton dissociation efficiency to 81% for TPD-3F:Y6 versus 93% for TPD-3:Y6. Additionally, grazing incidence wide-angle X-ray scattering (GIWAXS) reveals that the TPD-3F:Y6 blends are less textured than those of TPD-3:Y6, while space-charge limited currents reveal lower and unbalanced hole/electron mobility in TPD-3F:Y6 versus TPD-3:Y6 blends. Charge recombination dynamic, transient absorption, and donor-acceptor miscibility assays additionally support this picture. Furthermore, conventional architecture TPD-3:Y6 OSCs deliver a PCE of 15.2%, among the highest to date for halogen-free polymer donor OSCs. Finally, a large-area (20.4 cm(2)) TPD-3:Y6 blend module exhibits an outstanding PCE of 9.31%, one of the highest to date for modules of area >20 cm(2).

Automated summary

Fluorination of donor and/or acceptor blocks in OSCs can negatively affect device performance, as seen in the lower efficiency of fluorinated compared to non-fluorinated systems. TEM analysis reveals larger phase domain sizes and decreased exciton dissociation efficiency in fluorinated blends, while GIWAXS shows less textured structures and SLC testing indicates lower and unbalanced hole/electron mobility in fluorinated blends.