Conductivity and Conjugation Length in Poly(3-methylthiophene) Thin Films

The evolution of the morphology and electrical conductivity of poly(3-methylthiophene) thin films has been analyzed at various stages of the polymerization reaction. It is shown that the progress of the electropolymerization is accompanied with an increasing morphological disorder and by a steep decrease of the conductivity. Conversely, when the polymer growth is limited to films of a few nanometers thickness, more compact and more ordered materials are obtained while the conductivity increases strongly, reaching values in the range of 2 X 10³S cm⁻¹. A parallel analysis of the variation of the electrochemical and spectroscopic properties as a function of film thickness shows that the anodic current peak E_P, shifts nearly 100 mV toward less positive values while the absorption maximum shifts bathochromically ca. 40 nm, confirming that the mean conjugation length is noticeably more extended in ultrathin polymer films. These results show that both the molecular structure and the chains stacking order are subject to an increasing disorder as the polymerization progress.