New insights into the characterization of poly(3-chlorothiophene) for electrochromic devices

Abstract

Poly(3-chlorothiophene) has been prepared by anodic polymerization in boron trifluoride diethyl etherate with 0.1 M tetrabutylammonium tetrafluoroborate. The potential applied for the generation of the polymer has been found to affect considerably both physical (e.g. the thickness and roughness of the films, and the current efficiency) and electrochemical properties (e.g. the electroactivity, the doping level and the specific capacitance) whereas the density and the electrical conductivity remain practically unaltered in the interval ranging from 1.60 to 1.90 V. The dependence of the electrochemical behaviour with the generation potential has been explained by the morphological and topographical characteristics of the material, which have been investigated not only using scanning electron microscopy and atomic force microscopy but also electrochemical impedance spectroscopy. On the other hand, experimentally determined electronic properties have been reproduced by quantum mechanical calculations on model oligomers formed by up to 16 chemical repeating units that were arranged through a repetitive sequence of head-to-tail linkages. Oxidized and reduced poly(3-chlorothiophene) were found to be blue and Bordeaux red, respectively, the intensity of such two colors depending on the potential used for the generation of the film. Moreover, morphological changes induced by oxidation–reduction processes indicated that this polymer is a potential candidate for the fabrication of very stable electrochromic devices. Accordingly, an all-thiophene electrochromic device made of poly(3,4-ethylenedioxythiophene) and poly(3-chlorothiophene) has been successfully fabricated. This device has been proved to exhibit very remarkable redox stability: the loss of electroactivity is 65% of the initial value after 390 cycles and remains stable at such a value for more than 500 cycles.