Synthesis and photovoltaic properties of quinoxaline-based semiconducting polymers with fluoro atoms

Abstract

A new accepter unit, 6,7-difluoro-2,3-dihexylquinoxaline, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor-acceptor pair for OPVs. New series of copolymers with dioctyloxybenzodithiophene as the electron rich unit and 6,7-difluoro-2,3- dihexylquinoxaline as the electron deficient unit are synthesized. The solid films of poly[2,6-(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)- alt-5,8-(6,7-difluoro-2,3-dihexylquinoxaline)] (PBQxF) and poly[2,6-(4,8-bis(2- ethylhexyloxy) benzo[1,2-b:4,5-b']dithiophene)-alt-5,8-(6,7-difluoro-2,3- dihexyl-5,8-di(thiophen-2-yl) quinoxaline)] (PBDTQxF) show absorption bands with maximum peaks at about 599 and 551 nm and the absorption onsets at 692 and 713 nm, corresponding to band gaps of 1.79 and 1.74 eV, respectively. The devices comprising PBQxF with PC71BM (1:2) showed open-circuit voltage (VOC) of 0.64 V, short-circuit current density (JSC) of 1.58 mA/cm2, and fill factor (FF) of 0.39, giving power conversion efficiency (PCE) of 0.39%. To obtain absorption in the longer wavelength region, thiophene units without any alkyl group are incorporated as one of the monomers in PBDTQxF, which may result in low solubility of the polymers to lead lower efficiency.