Effect of furan π-bridge on the photovoltaic performance of D-A copolymers based on bi（alkylthio-thienyl）benzodithiophene and fluorobenzotriazole

The medium band gap donor-acceptor （D-A） copolymer J61 based on bi（alkylthio-thienyl）benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit and thiophene as n-bridge has demonstrated excellent photovoltaic performance as donor material in nonfullerene polymer solar cells （PSCs） with narrow bandgap n-type organic semiconductor ITIC as acceptor. For studying the effect of n-bridges on the photovoltaic performance of the D-A copolymers, here we synthesized a new D-A copolymer J61-F based on the same donor and acceptor units as J61 but with furan n-bridges instead of thiophene. J61-F possesses a deeper the highest occupied molecular orbital （HOMO） level at -5.45 eV in comparison with that （-5.32 eV） of J61. The non-fullerene PSCs based on J61-F：ITIC exhibited a maximum power conversion efficiency （PCE） of 8.24% with a higher open-circuit voltage （Voc） of 0.95 V, which is benefitted from the lower-lying HOMO energy level of J61-F donor material. The results indicate that main chain engineering by changing n-bridges is another effective way to tune the electronic energy levels of the conjugated D-A copolymers for the application as donor materials in non-fullerene PSCs.