1 research outputs found
Synthesis of PCDTBT-Based Fluorinated Polymers for High Open-Circuit Voltage in Organic Photovoltaics: Towards an Understanding of Relationships between Polymer Energy Levels Engineering and Ideal Morphology Control
The
introduction of fluorine (F) atoms onto conjugated polymer backbone
has verified to be an effective way to enhance the overall performance
of polymer-based bulk-heterojunction (BHJ) solar cells, but the underlying
working principles are not yet fully uncovered. As our attempt to
further understand the impact of F, herein we have reported two novel
fluorinated analogues of PCDTBT, namely, <b>PCDTFBT</b> (1F)
and <b>PCDT2FBT</b> (2F), through inclusion of either one or
two F atoms into the benzothiadiazole (BT) unit of the polymer backbone
and the characterization of their physical properties, especially
their performance in solar cells. Together with a profound effect
of fluorination on the optical property, nature of charge transport,
and molecular organization, F atoms are effective in lowering both
the HOMO and LUMO levels of the polymers without a large change in
the energy bandgaps. <b>PCDTFBT</b>-based BHJ solar cell shows
a power conversion efficiency (PCE) of 3.96 % with high open-circuit
voltage (<i>V</i><sub>OC</sub>) of 0.95 V, mainly due to
the deep HOMO level (−5.54 eV). To the best of our knowledge,
the resulting <i>V</i><sub>OC</sub> is comparable to the
record <i>V</i><sub>OC</sub> values in single junction devices.
Furthermore, to our delight, the best <b>PCDTFBT</b>-based device,
prepared using 2 % v/v diphenyl ether (DPE) additive, reaches the
PCE of 4.29 %. On the other hand, doubly-fluorinated polymer <b>PCDT2FBT</b> shows the only moderate PCE of 2.07 % with a decrease
in <i>V</i><sub>OC</sub> (0.88 V), in spite of the further
lowering of the HOMO level (−5.67 eV) with raising the number
of F atoms. Thus, our results highlight that an improvement in efficiency
by tuning the energy levels of the polymers by means of molecular
design can be expected only if their truly optimized morphologies
with fullerene in BHJ systems are materialized