Enhanced photovoltaic properties of di(dodecylthiophene)-alt-2,3-di(3-octoxylphenyl)-5,8-dithieno[3,2-b]thiophene 6,7-difluoroquinoxaline copolymer by fluorination

A novel di(dodecylthiophene)-alt-2,3-di(3-octoxylphenyl)-5,8-dithieno[3,2-b]thiophene-6,7-difluoroquinoxaline copolymer (PBT-TTQ-F) and its counterpart of PBT-TTQ-H without the substituted fluorine atom were designed and synthesized. Significant effect of the attaching fluorine atoms in the polymer backbone on photovoltaic properties of copolymers was observed. In the optimized bulk hetero-junction polymer solar cells with a structure of ITO/PEDOT: PSS/copolymers: PC71BM (1:2, w/w)/Ca/Al, PBT-TTQ-F exhibited a higher power conversion efficiency (PCE) of 3.05% with a short-circuit current (Jsc) of 9.48 mA/cm2 and a fill factor (FF) of 48.6% under an illumination of AM 1.5, 100 mW/cm2. The PCE, Jsc and FF values are 1.2-1.7 times higher than those of the PBT-TTQ-H-based devices. Our results demonstrated that photovoltaic properties could be significantly enhanced by copolymeric fluorination in PCSc.