Influence of fluorine substituents on the film dielectric constant and open-circuit voltage in organic photovoltaics

Abstract

Conjugated polymers with fluorine substituents on their backbone have exhibited improved performance over their un-fluorinated analogues by lowering the polymer HOMO level, thereby increasing the open-circuit voltage (VOC). To further investigate how fluorine substituents improve device performance, three polymers with the same donor and acceptor co-monomers, but differing by the number of fluorine atoms on the acceptor unit, were synthesized. Although the HOMO levels of the mono-(P1F) and di-fluorinated (P2F) polymers are essentially the same, an increase in VOC was still observed in the OPV device incorporating P2F. This implies that correlating the VOC to the donor polymer HOMO level is inadequate to fully explain the improvement in VOC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in VOC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.