The
synthesis of four different diketopyrrolopyrrole (DPP) low
bandgap polymers by direct arylation polymerization (DArP) is reported.
These materials were designed for use in organic photovoltaic (OPV)
and organic field-effect transistor (OFET) devices. While the DPP
conjugated unit was held constant for each of the materials, the alternating
unit of the copolymer was varied from thiophene (TTT), to phenyl (TPT),
to 3,4-difluorothiophene (TTfT), to 2,5-difluorophenyl (TPfT) creating
a series of DPP materials that can be used to study structure–property-performance
relationships. Molecular weights (M
w)
of 17–110 kg/mol were achieved by DArP and the resulting polymers
displayed excellent optical and electrical properties, comparable
to previous reports of similar materials synthesized by Stille or
Suzuki polycondensation. The fluorinated TTfT and TPfT materials had
similar absorption profiles, but exhibited reduced E
homo levels (by 0.1–0.2 eV) relative to TTT and
TPT, which is due to the incorporation of the highly electron withdrawing
fluorine atoms. OPVs fabricated with the TTT and TPT materials reached
average power conversion efficiencies of nearly 4%. Additionally,
OFET hole mobilities on the order of 10–2 cm2 V–1 s–1 were achieved
and the fluorine substituted TTfT and TPfT materials exhibited a 2-
to 3-fold improvement in hole mobility versus their nonfluorinated
analogues.
We report the design, synthesis, and characterization of a series of thieno [3,4-b]thiophene acceptor blocks with octyl (T8), phenyl (TP), perfluorooctyl (TF8), and perfluorophenyl (TFP) side groups. Their subsequent copolymerization with dithienylbenzodithiophene by direct arylation polymerization afforded novel low bandgap poly(thienothiophene-alt-dithienylbenzodithiophene) (PTB) polymers. The strongly electron withdrawing TF8 and TFP groups were shown to significantly lower both E HOMO and E LUMO levels and gave computed copolymer ground-to-excited state dipole changes (Δμ ge ) that were relatively higher than for the nonfluorinated analogues. These materials show favorably aligned energy levels relative to conventional fullerenetype acceptors, which should allow them to perform well in organic photovoltaics.
We present a strategy
for tuning physical properties of P3HT-based
copolymers by incorporating a fluorinated thiophene repeat unit. The
synthesis and characterization of a series of fluorinated polythiophene
P(3HT-co-3H4FT) materials are described, where the
percentage of fluorinated repeat units in the polymer backbone is
systematically varied from 0 to 100%. These P(3HT-co-3H4FT)s (P0, P25, P50, P75, and P100) were synthesized via direct arylation
polymerization (DArP) methods. By varying the feed ratio of the two
monomers, the percent of fluorinated repeat units (3H4FT) could be
precisely controlled. As fluorination is increased, there is a strong
effect on the electronic properties of the polymers, evidenced by
a 0.4 eV drop in the E
HOMO level for P100 when compared to P0. GIWAXS and TEM were
used to determine the crystallinity and morphology. TEM analysis of
thin film polymer/PCBM bulk-heterojunction blends indicates that increased
fluorination does not result in stronger phase separation. Organic
photovoltaic devices were fabricated to evaluate changes in device
performance as a result of fluorination.
The performance of organic photovoltaics (OPVs) is heavily dependent on the structure and functionalization of the conjugated polymer used in the active absorbing layer. Using a set of materials based on poly(thieno-[3,4-b]thiophene-alt-benzodithiophene) with different alkyl, aryl, perfluoroalkyl, and perfluoroaryl pendant functionalities, we have studied the correlation between absorbance, morphology, crystallinity, charge mobility, and the OPV performance in an effort to identify structure-performance relationships. The perfluorinated pendants on PTF8B and PTFPB were shown to significantly enhance the V oc in the OPV devices (by ∼0.2 V), but also induced the formation of larger phase separated PCBM-rich domains. PT8B and PTFPB devices reached average efficiencies of ∼3.2%.
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