Influence of the backbone conformation of conjugated polymers on morphology and photovoltaic properties

Abstract

Two conjugated polymers, PBDT–TBT and PBDT–TTBT, were designed and synthesized. PBDT–TBT has a zigzagged chain conformation (Z-type), while PBDT–TTBT has a linear chain conformation (L-type). The results show that the polymer chain conformation significantly affects the optoelectronic properties of the polymers. With the conformation change from Z-type to L-type, the J-aggregation of the polymer was enhanced greatly, i.e. PBDT–TTBT shows a much more prominent absorption shoulder peak at the long wavelength direction compared to PBDT–TBT. In the X-ray diffraction analysis, it was found that the L-type polymer shows much stronger crystallinity than the Z-type polymer. Benefiting from the enhanced inter-chain π–π stacking effect, the blend of PBDT–TTBT/PC₇₁BM showed higher hole mobility compared to the blend of PBDT–TBT/PC₇₁BM. The power conversion efficiencies (PCEs) of the polymer solar cells based on PBDT–TTBT/PC₇₁BM and PBDT–TTBT/PC₇₁BM are 6.3% and 4.9%, respectively. These results clearly reveal that the backbone conformation is an important issue for improving the intermolecular interaction of conjugated polymers, and hence provides a guideline for the molecular design of conjugated polymers.