RAPID ION-EXCHANGE DURING REDOX SWITCHING OF POLY(3-METHYLTHIOPHENE) STUDIED BY X-RAY PHOTOELECTRON-SPECTROSCOPY

The ionic composition of oxidatively doped and redox cycled poly(3-methylthiophene) was analyzed by using X-ray photoelectron spectroscopy (XPS) survey scans, quantitative multiplex experiments, and atomic composition depth profiling to determine ion-exchange properties. Thin films of poly(3-methylthiophene tetrafluoroborate), 1000-13500 Angstrom thick, were synthesized electrochemically onto a platinum electrode from an acetonitrile solution containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF(4)) as a supporting electrolyte. The films were subsequently potential cycled in tetrabutylammonium perchlorate (TBAClO(4)) and tetrabutylammonium perrhenate (TBAReO(4)) electrolytes to examine ion exchange. Surface analysis of perchlorate-treated films, 1500 Angstrom thick, indicated a complete exchange of dopants on the film surface. The extent of ion exchange into the film could not be determined with perchlorate due to deterioration of the chlorine and fluorine signals upon sputtering with an argon ion beam. Surface analysis of perrhenate-treated films, 1000-13 500 Angstrom thick, indicated a complete exchange of dopant ions at the surface, and through the bulk, after one redox cycle. Spontaneous ion exchange studies were carried out at 10 times greater exposure time to electrolyte than in switching experiments. While essentially complete exchange occurs in this time frame, some residual tetrafluoroborate could be detected. This indicates that ion transport in poly(3-methylthiophene) during redox cycling is rapid and complete.