Thiazolothienyl Imide-Based Wide Bandgap Copolymers for Efficient Polymer Solar Cells
Two new wide bandgap copolymers PTzTIBDTT and PTzTIBDTT-S based on the thiazolothienyl imide (TzTI) and benzodithiophene with distinct side chain were synthesized and characterized for applications in polymer solar cells (PSCs). The single crystal of TzTI model compound showed a highly planar backbone with a close π-stacking distance of 3.65 Å, a desired structural feature for efficient charge transport. Moreover, the TzTI incorporation can trigger intramolecular noncovalent N…S interaction to yield self-planarized polymer backbone, which should be beneficial for achieving ordered molecular packing and efficient charge transport. Due to its strong electron-withdrawing effect, the incorporation of TzTI unit largely lowers the polymer HOMO level to -5.65 and -5.69 eV for PTzTIBDTT and PTzTIBDTT-S, respectively. The PSCs containing PTzTIBDTT:PC71BM active layer exhibited a promising power conversion efficiency (PCE) of 8.00% with a large Voc of 0.90 V. To the best of our knowledge, the PCE is among the highest values for the fullerene PSCs based on an imide-containing polymer donor. This work not only demonstrates thiazolothienyl imide is a promising building block for constructing high-performance wide bandgap photovoltaic polymer semiconductors, but also reveals that noncovalent N⋯S conformational lock is an effective molecular design approach for enabling polymer semiconductors with planar backbone for efficient PSCs.