Organic molecular metal that change colour and electrical properties with an electric field

Scientists at the Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) have demonstrated the capacity of an organic material to quickly and reversibly change its colour and electrical properties upon the application of an electric field. The reported results constitute a proof-of-concept that opens up new possibilities for the design and fabrication of organic electrochromic and rectifying devices for electronics.

With the application of a voltage, some iodine species migrate to the positive regions and the material is chemically transformed, changing its color and the electrical properties, from metallic to semiconductor. Pictured, an experiment of the research.  Wearable electronics, cell phones, electronic papers, smart windows, sensors, and wireless communication devices need mechanically flexible electronic components that are able to change their colour and the degree of charge transfer. The development of lightweight flexible organic conducting materials with these properties has a great technological interest in this field.

A group of researchers from the Institute of Materials Science of Barcelona (ICMAB-CSIC) led by Jaume Veciana has proven the ability of an organic molecular metal [(BEDO-TTF)2,4I3] to reversibly change its color (electrochromic) and its degree of charge transfer (rectifying) upon the application of an electric field and have explained its mechanism.

With the application of a voltage, some iodine species migrate to the positive regions and the material is chemically transformed, changing its color and the electrical properties, from metallic to semiconductor, forming a diode-type device. This is a reversible process: when the voltage is removed, the original properties come back.

The scientists have tested the response of the material to cyclic voltages from –8 to +8 V. The discoloration and coloration is fast, compared to other materials, say the scientists.

Both colour change and the time scale depends on the voltage applied. If the material changes colour and properties in 0.9 seconds upon a voltage of 5 V, the material will recover its previous state in 0.9 seconds upon a voltage of -5V.

This is a reversible process: when the voltage is removed, the original properties come back

The team has demonstrated also that the changes depend on the amount of water within one of the material layers. For the transformation, a high relative humidity high is necessary (around  85%). Therefore, the scientists have added an hydrophilic PVA (polyvinyl alcohol) layer on the top of the film’s surface to increase and keep the water content. This method works remarkably well, allowing the electrochromism to be realized at ambient conditions”, say the scientists.

The reported results, published in the Flexible Electronics journal, constitute a proof-of-concept that opens up new possibilities for the design and fabrication of organic electrochromic and rectifying devices that operate with a very simple working principle. Furthermore, the basic requirement for achieving such electrochromic behavior is attained without the need of using a complicated pre-processing or post-processing.

The material chosen is is exceptionally robust because maintain its properties even when it has grain boundaries and small defects in its structure. Also, it is obtained under ambient conditions via a very simple and scalable procedure.

As other thin films materials, this technology provides some advantages over the existing ones: low cost, flexibility, easy working principle, reliability and lower power consumption. All of these could help towards their implementation in real life applications.