Step-by-step improvement in photovoltaic properties of fluorinated quinoxaline-based low-band-gap polymers

Abstract

A series of fluorinated quinoxaline-based polymers with typical donor-π-acceptor configurations has been synthesized by Stille coupling reaction. The electron-donating dialkoxy-substituted benzodithiophene (BDT) is connected to the electron-withdrawing 2,3-diphenylquinoxaline (DPQ) acceptor through a thiophene bridge. To investigate the effect of strong electron-withdrawing moieties, fluorine atoms have been systematically incorporated at various positions on the DPQ units such as the 6,7-positions, the para-positions of the phenyl substituent on the 2,3-positions, and both locations to afford PBDT-QxF, PBDT-FQx, and PBDT-FQxF, respectively. Because of significant contributions from the fluorine atoms, these polymers display quite differentiated optical and electrochemical properties in comparison with those of their non-fluorinated counterpart, PBDT-Qx. In addition, a gradual improvement in power conversion efficiencies (PCEs) is observed in the order of PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF, when inverted-type polymer solar cells (PSCs) with a configuration of ITO/ZnO/polymers:PC71BM/MoO3/Al were fabricated. In particular, PBDT-FQxF, with four fluorine atoms on the 6,7-positions and the para-positions of the phenyl substituent on 2,3-positions of DPQ, exhibits the highest PCE of 6.60% with an open-circuit voltage (Voc) of 0.91 V, a short-circuit current density (Jsc) of 10.15 mA/cm−2, and a fill factor (FF) of 71.5%.