Reaching a Double-Digit Dielectric Constant with Fullerene Derivatives

The dielectric constant (ϵ_r) of organic semiconductors is a key material parameter for improving device performance in the field of organic electronics. However, the effect of the dielectric constant on the electronic and optoelectronic properties of materials remains unclear due to the scarcity of known organic semiconductors with an ϵ_r value higher than 6. Herein, the optical and electronic properties of a homologous series of fullerene derivatives with high ϵ_r are studied. The low frequency (<10⁶ Hz) ϵ_r is extracted from the capacitance measured using impedance spectroscopy, and the effect of length (n) and geometrical arrangement of the polar ethylene glycol (EG) side chains is investigated. The ϵ_r is found to correlate with length for the symmetrical Bingel adducts, whereas for the unsymmetrical branched-EG chain adducts there is no significant difference between the two EG chain lengths. For BTrEG-2, the ϵ_r reaches 10, which is an unprecedented value in monoadduct fullerene derivatives. These materials open up new possibilities of studying the effect of ϵ_r in organic electronic devices such as organic photovoltaics, organic thermoelectrics, and organic field-effect transistors.