Effects of dopant solution solvent on the stability of doped P3HT films

Abstract

Understanding the factors affecting stability is crucial for achieving commercial success with doped conjugated polymers (CPs). In this work, we sequentially doped poly(3-hexylthiophene) (P3HT) films with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) from chlorobenzene/acetonitrile (CB/AN) solvent blends and stored the doped P3HT films under inert conditions at room temperature. The sequential doping of the P3HT film was accompanied by solution doping when CB was used to prepare the F4TCNQ solution. By combining UV–visible–near infrared (UV–vis–NIR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and grazing-incidence wide-angle X-ray diffraction (GIXRD) measurements, we found that with prolonged storage time, the microstructures of doped P3HT produced by sequential doping could retain crystalline structures; however, the crystalline aggregates produced by solution doping would recover back to amorphous chains. The diffidence between the microstructures leads to the conductivity σ, the carrier density n and mobility μ of the P3HT film doped with F4TCNQ from CB/AN blend decreased faster than that doped from pure AN. Two doping mechanisms, namely, integer charge transfer (ICT) and charge transfer complexes (CTCs), occurred in each freshly doped P3HT film. The order of the ICT phase influenced its stability, regardless of its formation from solution doping or sequential doping. The lower the order of the ICT phase is, the better its stability. Both the ICT and CTC phases were unstable at room temperature, yet no interconversion between them was observed.