High photovoltaic performance of as-cast devices based on new quinoxaline-based donor-acceptor copolymers

Two new donor-acceptor copolymers, namely PBDT-DFQX-TTSEH and PBDT-DFQX-TTSC8, based on difluorinated quinoxaline (Qx) acceptor units and alkylthiothienyl benzo[1,2-b:4,5-b']dithiophene (BDT) donor units with different alkylthio side chains were designed and synthesized. The two copolymers exhibit a lower-lying HOMO energy level and closer interchain packing than their analogues with alkyl side chains attributed to the more polarizable S atom inducing stronger intermolecular interactions. Of these two polymers, PBDT-DFQX-TTSC8 with linear alkylthio side chains shows closer molecular stacking with PC71BM than PBDT-DFQX-TTSEH with branched alkylthio side chains. As a result, the polymer solar cell based on the blend of PBDT-DFQX-TTSC8 and PC71BM exhibited a higher PCE of 8.51% than that based on PBDT-DFQX-TTSEH and PC71BM, which was only 7.02%. More importantly, both copolymers show higher photovoltaic performances in as-cast devices than in those treated by additive or thermal annealing because an appropriate bulk morphology was formed during the process of spin-casting due to the strong interchain interactions of the polymers, which eliminates the need for solvent additives or thermal annealing.