Optical measurement of doping efficiency in poly(3-hexylthiophene) solutions and thin films

The doping mechanism of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ) doped poly(3-hexylthiophene) (P3HT) thin films and solutions is studied by UV-visible and IR absorption spectral analysis. We show that integer charge transfer between the two molecules is observed in this system with 63% of the dopants producing a fully transferred charge in thin film. The primary P3HT doped species in solution and thin film are shown to be bipolarons and polarons, respectively. The absorption signatures of polarons and bipolarons are identified and their cross sections are measured, which can be used to evaluate the influence of processing conditions on the density of impurity dopant-induced polarons in nominally "neat" P3HT films.