Enhanced doping and structure relaxation of unsubstituted polythiophene through oxidative chemical vapor deposition and mild plasma treatment

We report on the enhancement of electrical properties of unsubstituted polythiophene (PT) through oxidative chemical vapor deposition (oCVD) and mild plasma treatment. The work function of p-type oCVD PT increases after the treatment, indicating the Fermi level shift toward the valence band edge and an increase in carrier density. In addition, regardless of initial values, nearly the same work function is obtained for all the plasma-treated oCVD PT films as high as ~5.25 eV, suggesting the pseudo-equilibrium state is reached in the oCVD PT from the plasma treatment. This increase in carrier density after plasma treatment is attributed to the activation of initially not-activated dopant species (i.e., neutrally charged Br), which is analogous to the release of trapped charge carriers to the valence band of the oCVD PT. The enhancement of electrical properties of oCVD PT is directly related to the improvement of the thin film transistor performance such as drain current on/off ratio, ~103 and field effect mobility, 2.25 x 10-2 cm2/Vs, compared to untreated counterparts of 102 and 0.09 x 10-2 cm/Vs, respectively.