H2020-MCSA-RISE project N. 734834

Infographics

Molecules | Nanotechnology | Electrochromism | Self-assembly | The colors | Graphene et al | Functionalization | Devices

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Electrochromism

Introduction

Transition metal oxides

Polycyclic aromatic hydrocarbons

Viologens

Conducting polymers

Polythiophenes

Introduction

Electrochromism refers to the ability some materials have to change their optical properties --transparency, color-- after application of an electric voltage or current. The underlying mechanism can be physical: the transparency of a liquid crystal layer depends on the orientation of the crystals, which can be controlled by an electric field. It can be chemical as well: the color of a material can be modified reversibly by exchanging ions or electrons or both (electrochemical mechanism). Some inorganic oxides, organic molecules, conducting polymers, and metallopolymers are different classes of electrochromic compounds [1]. Advantages of organic materials over inorganic materials are their lower cost and ease of processing.

Organic molecules have other advantages as electrochromes: high optical contrast between the colorless and the color states, fast response to an electric stimulus, good optical memory. The latter property means that the color stays for a long time after the application of the stimulus, which implies low power consumption. When the same molecule has several accessible redox states, like with viologen-derived compounds, it may present different colors. A redox reaction changes the molecular energy levels and, hence, the absorption spectrum. The part of the molecule responsible for its color is called chromophore. Organic compounds offer therefore the possibility to tune the color through proper chromophore design, which is truly important for display applications. When the chromophore absorbs radiations in the UV, the molecule is transparent. By opposition, the chromophore is black (or dark grey) in case it has a broad absorption band covering the entire visible spectrum, an example being oxydized NiO. The latter property is important for devices requiring transparent-to-black transitions [2].

Organic electrochromic molecules can be attached in huge quantity on nanoparticles thanks to the large surface area the latter present. With such hybrid nanostructures, excellent efficiency and chromatic contrast can be achieved.

The figure on the left shows highly emissive chromophores of different colors in solution.

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