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

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 (V-OC). 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 V-OC was still observed in the OPV device incorporating P2F. This implies that correlating the V-OC to the donor polymer HOMO level is inadequate to fully explain the improvement in V-OC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in V-OC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.