Electrical and optical behavior of tungsten oxide based electrochromic devices

Abstract

A broad set of phenomenological analysis tools, aimed at isolating generic characteristics of electrochromic (EC) device behavior from measured data with minimal reference to specific models, have been developed. The tools, which involve both directly-measured and derivative parameters, are applied in a demonstrative manner to specific EC devices; step-potential and step-current excitations are considered, along with variations in applied potential, imposed current, film thickness, insertion species, and series resistance. The step-current methods are extended and applied in appreciable detail to EC devices involving both H- and Li-electrolytes. In the H-based devices, a spontaneous (open-circuit) deintercalation process has been observed; this process is absent in the Li-based devices. In both types of devices, the present work exposes an inherent asymmetry between bleaching (deintercalation) and coloring (intercalation), most prominent at the onset of deintercalation. This asymmetry, and the resulting hysteresis, are explained in terms of a two-phase model. From a corresponding equivalent circuit, it is shown that the bleaching behavior may be well-predicted from the coloring behavior with essentially a single adjustable parameter. Behavior, in terms of measured and derived parameters, of devices using an aqueous H-electrolyte are compared with that of devices using a non-aqueous Li-electrolyte. The curves representing the various aspects of behavior for each device are generally similar in shape and may be made approximately to coincide through a linear scaling relation, suggesting that similar fundamental processes govern behavior in both types of devices. Preliminary work towards a model to predict the effect of the size of the insertion species on the intercalation/deintercalation behavior is performed. Additionally, a phenomenological site-saturation model for the optical efficiency is proposed and shown to predict well the observed behavior with no adjustable parameters. The tungsten-oxide films used in the present study, prepared through a novel wet-chemical synthesis and processing procedure involving the use of a transient additive, have been characterized in terms of their resulting microstructures, stoichiometries, and EC behavior. Additionally, the effect of the additive on these properties has been assessed. Finally, identification of particularly fertile avenues for investigation of the means to construct made-to-order devices of the future is also attempted.