The Effect of Fluorinated Benzothiadiazole-dicyanovinyl Acceptors on the Dielectric Constants of Organic Photovoltaic Materials

Abstract

It has been proposed that organic semiconductors with a dielectric constant of over 10 could undergo spontaneous free charge carrier generation upon photoexcitation. To explore the effect of variations in the low and high (optical) frequency dielectric constants on free charge generation in homojunction devices, we have prepared a series of monomeric donor−acceptor (D-A) and dimeric acceptor−donor−donor−acceptor (A-D-D-A) compounds, with cyclopentadithiophene donors and benzothiadiazole-dicyanovinyl acceptors, where the benzothiadiazole units are either fluorinated or protonated. Fluorination had a major effect on the low and high frequency dielectric constants. The ascast fluorinated dimer film was found to have a low frequency dielectric constant of over 10 at 0.1 MHz (the regime where charge recombination could occur), but an optical frequency dielectric constant of only 4.0 at 10 14 Hz (the regime where exciton dissociation occurs). In contrast, the protonated version had a low frequency dielectric constant of only around 6 but an optical frequency dielectric constant of 4.6. Thermal annealing of the film led to an increase in the optical frequency dielectric constant of the fluorinated dimer to 4.7 but a decrease for the protonated material to 3.9. Homojunction devices composed of the as-cast fluorinated dimer had low maximum photoconversion efficiencies (less than 1%) despite a low frequency dielectric constant of 10 and relatively balanced hole and electron mobilities of order 10 −5 cm 2 V −1 s −1 . The results provide further evidence that the optical frequency dielectric constant is a critical parameter for designing materials for efficient single chromophore homojunction devices.