Electrochemical Doping for Absorption and Conductivity Tuning of P(NDI2OD-T2) Films

Abstract

Electrochemical doping of thin films of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) is shown as straightforward method to achieve different degrees of doping both during in situ electrochemical experiments as well as in the solid state. Results obtained from cyclic voltammetry and absorption spectroscopy upon reduction can be explained by the presence of the neutral state as well as polaron and bipolaron species, including neutral/polaron and polaron/bipolaron mixed valence states. The UV-vis-NIR spectra are analyzed and explained based on the calculated electronic structure and the corresponding transitions between different states, this includes features such as numbers and positions of the peaks and their evolution during reduction. Most intruingly, doped films are stable after transfer in the solid state, as evidenced by absorption spectroscopy. Conductivity measurements of films with different degrees of doping show a bell-shaped conductivity profile, which underlines the classification of P(NDI2OD-T2) as a conjugated redox polymer with mixed valence transport. Maximum conductivities of up to 2 × 10⁻⁴ S cm⁻¹ are obtained at intermediate doping levels under the coexistence of neutral state and polarons. Conductivity measurements of blade-coated films point to anisotropic charge transport with the highest charge transport along the blade /polymer chain direction and an anisotropic conductivity ratio of 4.1.