Polymers and Electronics

What is electrochromism

Chromism is the phenomenon displayed by proper materials to change their colour reversibly when they are placed in different environments. One of the most useful forms of chromism is electrochromism, which is the reversible colour change of an appropriate material when it alters between its different electronic states. More precisely, the electrochromic (EC) effect is defined as a visible and reversible variation of optical properties shown by a material upon its electrochemical oxidation/reduction. It results from the generation of different absorption bands in the visible region of the spectra (390-750 nm) upon switching between redox states, i.e. the material changes its colour by accepting electrons (reduction) or by ejecting them (oxidation). The colour change is achieved when a burst of charge is applied. The particular transmission or reflection of light from an EC material accounts to its specific colour.

Electrochromic Devices (ECDs): Structure, operation and applications

An ECD is essentially an electrochemical cell in which the EC electrode is separated by an electrolyte from the charge-balancing counter electrode. The colour changes occur by charging and discharging the cell by applying a potential. After the resulting pulse of current has decayed and the colour change has been effected, the new redox state persists. The EC electrode in these devices constitutes of a conductive, transparent glass (like indium tin oxide, ITO) coated with the EC material. Various types of EC materials and structures can be used to construct ECDs. Commercial applications of EC materials in devices include anti-glare car rear-view mirrors, strips as battery state-of-charge indicators and sunglasses. Proposed applications include „smart windows“, re-usable price labels, devices for frozen-food monitoring, spacecraft thermal control, electronic paper displays and computer data storage. Specifically, smart windows are used in building and automotive applications in order to control the incident daylight and glare. The control of sunlight can be achieved by electrically switchable chromogenic materials. Smart windows have the advantage that the electric field must be applied only during the switching operations.

A material is considered suitable to be applied in ECDs when it displays a good optical contrast between its various colour states and a fast colour response to the polarity change. For efficient operation of an ECD, it is necessary to take a number of properties into consideration: electrochromic efficiency, optical memory, response time (i. e. the colouration–decolouration switching time), stability and durability (i. e. the number of colouration–decolouration cycles). The difficulty in achieving satisfactory values for all these parameters at the same time stimulates the development of new methods for preparation of electrochromic films, new materials and components for the devices.

Polymer electrochromic materials (ECs) and their advantages

A variety of conducting polymers, CPs, have colours both in the oxidized and reduced states, since their band gap is in the visible region. After oxidation, the intensity of the π-π* transition decreases, and two low energy transitions emerge to produce a second colour. This, combined with the fact that CPs can repeatedly undergo electrochemical doping/undoping processes, makes them the most promising class of materials to be used in ECDs. Polymer ECs can be prepared by chemical or electrochemical polymerization and they can be easily formed in films by techniques such as printing. On the other hand, inorganic ECs need complicated preparation techniques (e. g. vacuum evaporation) and they have poor processibility. As a result, polymer ECDs have lower cost than the corresponding inorganic devices. Moreover, in polymer ECDs the colour of the material depends on many parameters, such as the monomers used, the operation potential etc, thus a large number of colours are available with these materials, in stark contrast with inorganic ECDs where the number of colours available for a given material is limited. Additionally, polymer ECDs have better operational characteristics, such as high contrast, faster switching times and higher lifetime.

Research objectives

One of the main challenges is to develop novel CPs for multi-coloured ECDs, capable for large scale production. To date, the multicolour EC polymers reported have several important disadvantages, such as low stability and poor process ability. The aim of the NEMEDES project is to produce copolymers with proper absorption spectra through the entire visible region of the electromagnetic spectra, study their electrochromic behaviour and apply them in multi-coloured ECDs. The main objective is to manufacture ECDs with exceptional operating characteristics and enhanced stability by designing the monomers and the polymerization procedure in order to produce copolymers with defined structures.

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