Development of Poma/Ptaa Layer-By-Layer Films As Active Material for Electrochromic Devices
Authors: Christinelli Wania Aparecida , Pereira Ernesto Chaves 
Published: February 2020

Besides those need concerning the energy conversion and storage, there are many different forms to save energy in the modern society. Among then, the use of smart windows which control the amount of light in modern buildings has become an important issue in the last years. Besides, the same kind of devices, in a different configuration, are used, for many years, as rear drive mirror in automobiles. A large class of optically active materials have been proposed to be applied in electrochromic windows, and among them, conducting polymers are a promising choice. These materials change their color upon reversible electrochemical redox cycles, which changes also their electronic properties, such as electronic conductivity. Coupled to the electronic redox reactions, there are ion intercalation (deintercalation) to compensate the generated charges and which are described as the slow step during the redox reaction. As consequence, there are several papers in the literature aiming at to optimize the redox reaction rate to improve the electrochromic device properties. In the present work, we investigate the electrochromic properties of layer-by-layer poly(o-methoxyaniline)-poly(3-thiophene acetic acid), POMA/PTAA, films and compare then with POMA films prepared by casting. The main technique used to characterize the materials has been the potentiostatic steps, between -0.2 V and 0.5 V, and collecting both, the current density and the optical density, measured at 700 nm, during the experiments. The results have shown that the changes in the optical densities for the LBL POMA/PTAA films are larger than those observed in POMA casting ones. Besides, the rate of the change is faster for the LBL material. In this last sense, the optical density change period has been 1 s for the pure POMA film while it is 0.4 s for the LBL one.  A second effect is that, considering the same charge, there is a variation of 26 % in the transmittance for POMA film while it is 46 % for the LBL POMA/PTAA one. A third important effect is an increase of the change in the optical density as the number of bilayers is increased for the LBL films. Unexpected, for pure POMA sample, there is a decrease in the optical change as the film mass is increased. Probably, this last effect is a consequence of the decrease in the ion intercalation rate or even a deactivation of the deep portion of the film, under the experimental conditions used during the potentiostatic step. As described above, LBL film presents a different behavior. It is described in previous papers that this material, LBL POMA/PTAA, present self-doping effect. In this sense the ion intercalation total amount decreases significantly due to the self-doping effect which means an interaction between H+ and COO- groups in the PTAA polymer, and the amine (or imine) groups in POMA chains. Finally, there is an important increase in the optical and electrochemical results upon continuous cycling. Under this condition, the LBL film remains optically and electrochemically stable for at least 3000 cycles.