Optical properties of nondegenerate ground-state polymers: Three dioxythiophene-based conjugated polymers

We report the optical properties of three dioxythiophene-based conjugated polymers: poly(3,4-ethylenedioxythiophene) poly(3,4-propylenedioxythiophene), and poly(3,4-(2,2-dimethylpropylenedioxy)thiophene). Films of ca. 200 nm thickness of these polymers were prepared on indium-tin-oxide coated glass substrates using a potentiostatic electropolymerization method. The reflectance and transmittance of the samples were measured over a broad energy range from the midinfrared through the ultraviolet. To extract the optical constants of the polymers, we modeled all of the layers of this multilayer thin-film structure using a Drude-Lorentz model. From the parameters obtained, we compute the optical constants, such as absorption coefficient and frequency-dependent conductivity. These functions yield information about the electronic structure of the neutral and doped polymers, which show evidence for polaron states at low doping and bipolarons at the maximum doping level. We observed doping-dependent changes in the electronic structure of these polymers as well as doping-induced infrared-active vibrational modes.