What are Electrochromic Materials?

Jump to:Transition Metal Oxides | Metal Coordination complexes | Small Organic Molecules | Electrochromic Polymers

Electrochromic materials change appearance in response to electricity and can be found across a huge range of material categories. The two main categories are organic materials such as small molecules and polymer and inorganic materials such as metal oxides and metal complexes. Electrochemical oxidation and reduction reactions lead to changes in a material’s absorption characteristics in the UV-Vis-NIR region. As a result, electrochromic materials experience reversible color changes.

The color changing properties of all these materials are consistent but the way in which they can change color varies. The color change process itself can be used to classify electrochromic materials:

Transition Metal Oxides

Transition metal oxides exhibit electrochromism when they possess a suitable band gap, redox-active metal centers, sufficient ionic mobility, and strong electron–lattice interactions. Transition metal oxide electrochromes can be broadly classified into anodically coloring and cathodically coloring materials. Oxides of tungsten, molybdenum, vanadium, niobium, and titanium exhibit color changes upon reduction (cathodic coloring), whereas oxides of iridium, rhodium, nickel, and cobalt undergo color changes upon oxidation (anodic coloring). Electrochromic effects occur due to electron–ion separation. Metal atoms (eg. Li+) are introduced into the TMO, and the valence electrons move to the d-levels of the transition metal ion, reducing it. For this to work the injected ions should possess a high diffusion coefficient and a high solubility in the lattice of TMO.

These color changes arise from redox reactions that alter the oxidation states of the metal centers. The associated changes in the electronic structure and band gap modify light absorption properties, leading to the observed differences in color. The visible color change in electrochromic oxides results from small-polaron absorption and intervalence charge transfer between different metal oxidation states (W⁶⁺ → W⁵⁺). A polaron is an electron (or hole) that becomes localized on a metal site and is accompanied by a local lattice distortion, effectively forming a "trapped" charge carrier. As more electrons are inserted during reduction, these polarons can pair up to form bipolarons (two electrons localized in close proximity on adjacent or even the same metal sites).

Polaron hopping involves single electrons moving between adjacent metal sites (W⁵⁺ ↔ W⁶⁺). This is a thermally activated process that directs charge transport during electrochromic switching. At higher levels of electron insertion, bipolaron hopping can occur, where paired electrons migrate between sites. This leads to additional absorption bands (often in the near-infrared) and modifies the optical spectrum, explaining the dual-band absorption sometimes observed in highly reduced oxides. Together, these processes alter the electronic structure and narrow the effective band gap, producing the characteristic color shifts seen in cathodic and anodic electrochromic oxides.

Tungsten Trioxide

Tungsten trioxide (WO₃) was the first electrochromic material to be used in a controlled and reversible color changing device in 1969 and is still widely investigated. It has high functionality, high staining efficiency, high contrast, high chemical stability and a long life cycle. A WO₃ film can be different colors depending on the amount of ions that have intercalated into the film. This is due to the partial reduction tungsten to oxidation state 5⁺ and the changes in the band gap observed from the intercalation of Li⁺. The performance of electrochromic devices based on WO₃ films is dependent on the atomic structure, nanoparticle size, pore size and absorption properties.

Metal Coordination complexes

Many metal coordination complexes are also electrochromic materials because of their intense coloration and redox reactivity. Redox activity can occur at either the metal cations or bonded organic ligands. Color changing properties are linked to low-energy metal-to-ligand charge-transfer (MLCT), ligand-to-metal charge transfer (LMCT) intervalence charge-transfer (IVCT), intra-ligand excitation. These electronic transitions, involving valence electrons, fall in the visible-region and lead to different optical characteristics upon oxidation or reduction of the complex.

Transition metals, such as copper, iron and iridium, are often used as the coordinating metal due to their redox activity. Like with transition metal oxides, new absorption bands at different parts of the visible or near infrared regions are generated by switching between different redox state. In cases where the central cation undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals. It is less clear for ligands and extra charge lost or gained is usually indicated by a superscripted + or −. Typical electrochromic organic ligands include oligopyridines and phthalocyanines, but there are many options out there for further investigation.

Small Organic Molecules

Small organic molecules are also capable of electrochromism as they contain redox active cores with various substituents that can control the band gap. The band gap in this case is between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Small molecule materials offer superior features, such as high contrast, low toxicity, rich colors, simple synthetic routes, and low cost. The advantage of small organic molecules is that they can be easily tuned by simple changes to their structure. They exist as salts as they are ionic materials that can undergo redox reactions.

Viologens

Small organic molecules are also capable of being electrochromic. Small molecule materials offer superior features, such as high contrast, low toxicity, rich colors, simple synthetic routes, and low cost. The advantage of small organic molecules is that they can be easily tuned by simple changes to their structure. They exist as salts as they are ionic materials that can undergo redox reactions:

The electrochromic properties of these materials can be modulated by varying the nitrogen substituents on the pyridyl ‘N’. The counter ion used within the salt can also be modified with specific functionalities to enhance electrochromic behaviour. Electrochromic devices based on viologens have advantages of low operational voltages. Efforts have been made to increase their low cycle life and poor device efficiency long term.

Other small molecule have been investigated as electrochromic materials and are based on redox active cores including:

Redox Active Core	Colors
Carbazoles	Dark green
Methyoxyphenyls	Blue
Quinones	Purple
Tatrathiafulvalenes	Green
Phenylene diamines	Blue/green
Pyrazolines	Red
Thiophenes	Dark red to purple to navy blue
Furans	Wine-red to orange to orange-pink

Electrochromic Polymers

Electrochromic polymers are renowned for their low cost, easy processing, easy control over color and other properties via synthetic and processing techniques. Electrochromic devices based on polymers offer long lifetimes, high optical contrast, stable oxidation states, excellent switching reproducibility and flexibility so they can be applied to areas that are uneven or foldable compared to their inorganic and small organic counterparts.

Many electrochromic polymers have multiple redox states and are multichromatic. Classic examples of electrochromic polymers include poly(thiophenes), poly(pyrroles), poly(anilines) and other π-conjugated polymers. Poly(imides), poly(amides) and poly(norbornenes) are high performance polymers that can contain electroactive chromophores. These polymers are based on the same redox active cores found in small organic molecules are mentioned in the table above.

PEDOT:PSS

PEDOT:PSS is a cathodic electrochromic material which possesses thiophene redox active sites throughout the polymer backbone. When a negative voltage is applied, doping of electrolyte ions occurs between the polymer backbone, resulting in a change in the energy band gap of the material and thus achieving electrochromism. While the de-doping process occurs under the condition of applied positive voltage, the appearance of color returns to the original form.

Further electrochromic polymers have been developed based on the poly(thiophene) backbone. Different thiophene units, such as thienothiophene (TT) and 2,2'-bithophene (BT), and different substituents, such as triethylene glycol (TEG) have been used to harness reversible electrochromism. p(g2T-TT) films can uptake a much higher density of ions than PEDOT:PSS while demonstrating high volumetric capacitance at low biases.

Organic-Inorganic Composites

Organic-inorganic nanocomposites were also developed to combine the advantages of both organic and inorganic electrochromic materials. Such hybrid materials could be prepared from the use of either only electrochromic-active organic or inorganic materials or both. Modifying the interfacial interactions between organic and inorganic parts has received a lots of research attention because they are so important for structure strength, mass transport, electron conduction and electrochromic performance.

Examples of organic-inorganic composites include many electrochromic polymer and transition metal oxides composites such as PEDOT–WO₃ and polyaniline-TiO₂. As well as metal-organic frameworks which are 3D versions of metal coordination complexes with electroactive polymers. Graphene oxide is also widely used in electrochromic composites.

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