Doping of polythiophene by microwave plasma deposition

The work represents an efficient method in deposition of electrical conducting polythiophene thin films utilizing a well-assembled microwave plasma system in combination with an in situ doping with iodine. Microwave (2.54 GHz at 150–250 W) plasma polymerization method has been used to fabricate dense and pinhole-free polythiophene films in high uniformity as evidenced by Scanning Electron Microscopic analysis. Moreover, a much improved electrical conductivity of the otherwise insulating polythiophene has been achieved by in situ doping with iodine (I2). Although initial conductivities (1.4 × 10− 5 to 1.0 × 10− 4 S/cm) are lower than those of plasma-polymerized films obtained from regular ex situ doping (1.5 × 10− 4 to 1.9 × 10− 3 S/cm), the latter decreases rapidly and reaches an undoped value in 24 h. The conductivity of the in situ-doped material, on the other hand, decreases at a rate approximately seven-fold more slowly and does not reach the undoped value. Characterization of plasma-polymerized polythiophene films with various spectroscopic methods has been carried out. Incorporation of iodine is evident in Infrared spectroscopy. Ultraviolet–visible spectra of the doped polythiophene are at longer wavelengths (440–535 nm) than the undoped (374–535 nm) suggesting a longer conjugating framework. Energy-dispersive X-ray spectroscopic results are also supportive of iodine incorporation of up to 10%.