New m-alkoxy-p-fluorophenyl difluoroquinoxaline based polymers in efficient fullerene solar cells with high fill factor

Abstract

Abstract Two new donor (D) - acceptor (A) copolymers, named m -O- p -F-DFQx-BDT (OFQx-T) and m -EH- p -F-DFQx-BDT (EHFQx-T), which were based on meta -octyloxy- para -fluorophenyl and meta -ethylhexyloxy- para -fluorophenyl difluoroquinoxaline as acceptor units (O-DFQx/EH-DFQx) and alkylthienyl substituted benzodithiophene (BDT) as a donor unit, were designed and synthesized. EHFQx-T had higher absorption coefficient than OFQx-T which contributed to larger short-circuit current density ( J sc ). EHFQx-T showed a lower the highest occupied molecular orbital (HOMO) which is beneficial for the voltage open-circuit ( V oc ). The polymer solar cells (PSCs) based OFQx-T:PC 71 BM and EHFQx-T:PC 71 BM blended film as active layer showed high power conversion efficiency (PCE) of 7.60% and 8.44%, respectively, with 1,8-diiodooctane (DIO) solvent additive treatment. More importantly, OFQx-T:PC 71 BM and EHFQx-T:PC 71 BM had good fill factor (FF), especially the FF of OFQx-T:PC 71 BM was over 70%. The high FF contributed to obtain high PCEs for OFQx-T and EHFQx-T. The more balanced and higher charge mobility, smaller geminate recombination and suitable nanoscale phase separation size of EHFQx-T demonstrate that changing octyl chain to ethylhexyl chain in DFQx acceptor unit is efficient to improve photovoltaic properties in fullerene solar cells.