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 π-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 π-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 π-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 π-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.