A highly crystalline face-on π-conjugated polymer based on alkoxythiophene-flanked benzobisthiazole for organic photovoltaics

The use of noncovalent intramolecular interactions constitutes a powerful design strategy for preparing π-conjugated polymers featuring high backbone coplanarities and thereby high crystallinities. Herein, we report the design and synthesis of an alkoxythiophene-flanked benzobisthiazole (BBTz) as a new building unit for π-conjugated polymers, which was subsequently copolymerized to give a simple BBTz-bithiophene copolymer with alkyl and alkoxy groups ( PDBTz2 ). Owing to the S···O noncovalent intramolecular interactions between the alkoxy oxygens and thiazole sulfurs in BBTz, PDBTz2 showed greater coplanarity and crystallinity than its alkyl counterpart, PDBTz1 . Interestingly, the backbone orientation was completely altered from the edge-on orientation observed for PDBTz1 to a face-on orientation for PDBTz2 , which is preferable for organic photovoltaics (OPVs). In addition, the electron-donating nature of the alkoxy group increased the HOMO energy level of PDBTz2 compared to that of PDBTz1 , which enabled photoinduced hole transfer from a nonfullerene acceptor, Y6, to the polymer. As a result, the short-circuit current density of an organic photovoltaic cell based on PDBTz2 and Y6 was significantly greater than that of a cell based on PDBTz1 and Y6. This study confirmed that alkoxythiophene-flanked BBTz is a promising building unit for high-performance π-conjugated polymers.