Synthesis of doped polythiophene film by microwave plasma polymerization

A microwave plasma reactor for the synthesis of polythiophene thin films has been designed and assembled. Plasma polymerization parameters were studied. Microwave power in the range of 150-300W was employed for 1-2 minutes. Polythiophene films were characterized by various spectrophotometric methods. Infrared analyses showed absorption frequencies of important functional groups mostly similarly observed in the case of chemically-synthesized materials. Fabricated polythiophene films exhibited UV-Vis spectra indicative of increased conjugative systems as the microwave power increased although at high microwave power partial fragmentation was evident. Surface analysis by Scanning Electron Microscopy revealed a uniformly fabricated globular particle morphology. In addition, these polythiophene thin films were found to be dense and pinhole-free. Results from Energy-dispersive X-ray spectroscopy analysis were suggestive of partial fragmentation of the films at high microwave powers as some degree of sulfur content was decreased. This is in good agreement with a possibility of partial cleavage of a C-S bond in the thiophene rings. Electrical conductive measurements revealed that the undoped films exhibit higher conductivity (5.4x10⁻⁷ to 1.9x10⁻⁶ S/cm) than polythiophene typically prepared from electrochemical methods. As for the doped materials, initial conductivities (1.4x10⁻⁵ to 1.0x10⁻⁴ S/cm) were lower than plasma-polymerized films which were doped with conventional method (1.5x10⁻⁴ to 1.9x10⁻³ S/cm). However, it was found that conductivity of the latter decreased more rapidly and reached an undoped value in a short time (48 hours). In contrary, the decaying rate of conductivity of in situ doped material could be sustained for a longer period of time (more than 96 hours).