Fluorine Substituents
Reduce Charge Recombination
and Drive Structure and Morphology Development in Polymer Solar Cells
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Abstract
Three structurally identical polymers, except for the
number of
fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT),
are investigated in detail, to further understand the impact of these
fluorine atoms on open circuit voltage (<i>V</i><sub>oc</sub>), short circuit current (<i>J</i><sub>sc</sub>), and fill
factor (<i>FF</i>) of related solar cells. While the enhanced <i>V</i><sub>oc</sub> can be ascribed to a lower HOMO level of
the polymer by adding more fluorine substituents, the improvement
in <i>J</i><sub>sc</sub> and <i>FF</i> are likely
due to suppressed charge recombination. While the reduced bimolecular
recombination with raising fluorine concentration is confirmed by
variable light intensity studies, a plausibly suppressed geminate
recombination is implied by the significantly increased change of
dipole moment between the ground and excited states (Δμ<sub>ge</sub>) for these polymers as the number of fluorine substituents
increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly
more scattering in the in-plane lamellar stacking and out-of-plane
π–π stacking directions, observed with GIWAXS.
This indicates that the addition of fluorine leads to a more face-on
polymer crystallite orientation with respect to the substrate, which
could contribute to the suppressed charge recombination. R-SoXS also
reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains.
The higher domain purity is correlated with an observed decrease in
PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to
12% (PBnDT-DTffBT). The disclosed “fluorine” impact
not only explains the efficiency increase from 4% of PBnDT-DTBT (0F)
to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution
should be generally considered in the future design of new polymers