Dynamic and quasi-stationary electrochromic response of amorphous tungsten oxide thin films : In situ combined electrochemical and optical measurements during lithium intercalation

Abstract: Electrochromic (EC) materials can adjust their optical properties, reversibly, by means of an external electrical stimulus. They have relevant technological applications; for example, energy-efficient smart windows, which can adapt dynamically—according to the given environmental conditions—to control the heat and visible light fluxes between the interior and exterior of a building. EC applications are currently available on the market. However, there are still questions concerning the fundamental processes responsible for the EC effects.This thesis focuses on EC inorganic oxide materials in the thin film form; particularly, amorphous tungsten oxide. In this case, the electrochromism is induced by the intercalation of small ions (such as lithium ions) into the material and the insertion of electrons from an external circuit due to charge neutrality requirements within the film. These electrons are the ones causing the optical changes. This work centers its attention to the quasi-equilibrium and dynamic EC processes. They were studied by in situ simultaneous electrochemical and optical measurements at different conditions—that is, for a wide range of intercalation levels and bias potentials.The experimental results from quasi-equilibrium measurements were in accordance with a phenomenological optical absorption model for amorphous tungsten oxide that is based on electronic transitions between states localized on neighboring tungsten atoms. In this case, the consideration of W4+ sites in the model was needed to properly reproduce the experimental results.The dynamic measurements used an experimental setup which can acquire simultaneously the frequency-dependent electrical and optical responses of the EC system.  In the frequency domain, different mechanisms with various characteristic times and responses can be isolated. Here, the coloration was mainly assigned to the ion and electron diffusion within the film. However, adsorption-related phenomena were also found to contribute to the coloration, especially at high bias potentials—corresponding to low intercalation levels. Interestingly, the dynamic optical response was in-phase with the electrical one at high bias potentials. Nevertheless, a delay between the former and the later was noticed as the bias potential decreased—that is, increasing intercalation level.The methods and results from this thesis provide new perspectives into the fundamental coloration mechanisms in EC systems. In addition, studies like those presented here can be readily extended to different materials at diverse conditions—for example, at various optical wavelengths, material compositions, and film thicknesses.