Electrochromic Materials and Devices

Roger J. Mortimer, David R. Rosseinsky, Paul M. S. Monk, Roger J. Mortimer, David R. Rosseinsky, Paul M. S. Monk

• Publisher: Wiley-VCH
• Year: 2015
• ISBN:
  9783527679874
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Electrochromic materials can change their properties under the influence of an electrical voltage or current. Different classes of materials show this behavior such as transition metal oxides, conjugated polymers, metal-coordinated complexes and organic molecules. As the color change is persistent, the electric field needs only to be applied to initiate the switching, allowing for applications such as low-energy consumption displays, light-adapting mirrors in the automobile industry and smart windows for which the amount of transmitted light and heat can be controlled.

The first part of this book describes the different classes and processing techniques of electrochromic materials. The second part highlights nanostructured electrochromic materials and device fabrication, and the third part focuses on the applications such as smart windows, adaptive camouflage, biomimicry, wearable displays and fashion. The last part rounds off the book by device case studies and environmental impact issues.

Part I
Electrochromic Materials and Processing

Chapter 1
Electrochromic Metal Oxides: An Introduction to Materials and Devices

Claes-Göran Granqvist

1.1 Introduction

Electrochromic materials are able to change their properties under the action of an electrical voltage or current. They can be integrated in devices that modulate their transmittance, reflectance, absorptance or emittance. Electrochromism is known to exist in many types of materials. This chapter considers electrochromic metal oxides and devices based on these.

Figure 1.1 shows a generic electrochromic device comprising five superimposed layers on a single transparent substrate or positioned between two transparent substrates [1]. Its variable optical transmittance ensues from the electrochromic films, which change their optical absorption when ions are inserted or extracted via a centrally positioned electrolyte. The ion transport is easiest for small ions, and protons (H⁺) or lithium ions (Li⁺) are used in most electrochromic devices. Transparent liquid electrolytes as well as ion-containing thin oxide films were employed in early studies on electrochromics [2], but polymer electrolytes became of interest subsequently [3, 4] and paralleled the developments in electrical battery technology.

Figure 1.1 Generic five-layer electrochromic device design. Arrows indicate movement of ions in an applied electric field. From Ref. [1].

The ions are moved in the electrochromic device when an electrical field is applied between two transparent electrical conductors, as illustrated in Figure 1.1. The required voltage is only of the order of 1 V DC, so powering is, in general, easy and can be achieved by photovoltaics [5]. In small devices, the voltage can be applied directly to the transparent conductors but large devices – such as ‘smart’ windows for buildings – require ‘bus bars’, that is, a metallic frame partly or fully around the circumference of the transparent conducting thin film in order to achieve a uniform current distribution and thereby sufficiently fast and uniform colouring and bleaching. The transparent substrates are often of flat glass, but polymers such as polyethylene terephthalate (PET) or polycarbonate can also be used. The permeation of gas and humidity through foils may or may not be an issue for devices; barrier layers can be applied if needed [6].

An electrochromic device contains three principally different kinds of layered materials: The electrolyte is a pure ion conductor and separates the two electrochromic films (or separates one electrochromic film from an optically passive ion storage film). The electrochromic films conduct both ions and electrons and hence belong to the class of mixed conductors. The transparent conductors, finally, are pure electron conductors. Optical absorption occurs when electrons move into the electrochromic film(s) from the transparent conductors along with charge-balancing ions entering from the electrolyte. This very simplified explanation of the operating principles for an electrochromic device emphasises that it can be described as an ‘electrical thin-film battery’ with a charging state that translates to a degree of optical absorption. This analogy has been pointed out a number of times but has only rarely been taken full advantage of for electrochromics.

Electrochromic devices have a number of characteristic properties that are of much interest for applications. Thus, they exhibit open circuit memory, just as electrical batteries do, and can maintain their optical properties and electrical charge for extended periods of time without drawing energy (depending on the quality of electrical insulation of the electrolyte). The optical absorption can be tuned and set at any level between states with minimum and maximum absorption. The optical changes are slow and have typical time constants from seconds to tens of minutes, depending on physical dimensions, which means that the optical changes can occur on a timescale comparable with the eyes' ability to light-adapt. Furthermore, the optical properties are based on processes on an atomic scale, so electrochromic windows can be without visible haze; this latter property has been documented in detailed spectrally resolved measurements of scattered light [7]. By combining two different electrochromic films in one device, one can adjust the optical transmittance and reach better colour neutrality than with a single electrochromic film. Finally, the electrolyte can be functionalised, provided it is a solid and adhesive bulk-like polymer, so that the smart window combines its optical performance with spall shielding, burglar protection, acoustic damping, near-infrared damping and perhaps even more features.

This chapter is organised as follows: Section 1.2 serves as a background and gives some notes on early work on electrochromic materials and devices. Section 1.3 provides an in-depth discussion of EC materials and covers optical and electronic effects and, specifically, charge transfer in tungsten oxide. It also treats ionic effects with foci on the inherent nanoporosity in electrochromic oxides and on possibilities to augment the porosity by choosing appropriate thin-film deposition parameters. A number of concrete examples on the importance of the deposition conditions are reported, and Section 1.3 ends with a discussion of the electrochromic properties of tungsten–nickel oxide films across the full compositional range. Section 1.4 surveys properties of transparent conducting electrode materials as well as transparent electrolytes. Section 1.5 gives a background to electrochromic devices, specifically delineating a ...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Table of Contents
• In memoriam
• Preface
• Acknowledgements
• List of Contributors
• Part I: Electrochromic Materials and Processing
• Part II: Nanostructured Electrochromic Materials and Device Fabrication
• Part III: Applications of Electrochromic Materials
• Part IV: Device Case Studies, Environmental Impact Issues and Elaborations
• Appendix: Definitions of Electrochromic Materials and Device Performance Parameters
• Index
• End User License Agreement