Fourier transform infrared study of lightly doped polythiophene under d.c. electric fields

We have studied the effect of a strong d.c. field on the i.r. absorbance bands of lightly FeCl4−-doped polythiophene (PT) thin films. The electrochemically prepared PT-BF4− films were first electrochemically reduced and then chemically doped with FeCl3. The d.c. conductivities of the films were in the range 10−6-10−4ω−1cm−1. Application of a d.c. electric field < 4 kV/cm caused large changes in the absorbances of the i.r. modes, especially in the CH stretching, CH in- and out-of-plane vibrational modes and in the bands corresponding to the thiophene ring stretching. Also the absorbances of the doping-induced bands changed substantially, but they relaxed close to the original values faster than the other modes after the field was switched off. Special care was taken to study the contribution of heating of the film to the observed results by measuring the temperature of the films both by using a thermocouple and by using the films itself as a sensitive temperature sensor. Various mechanisms including, e.g., structural changes in the disordered PT material, charge injection-induced polaron or bipolaron formation and doping continuation may contribute to the observed large changes in the i.r. modes of PT.