Changing Color with the Application of Charge

Title: Layer-by-Layer Assembled Solid Polymer Electrolyte for Electrochromic Devices

Journal: Chemistry of Materials

Authors: Chien A. Nguyan,† Avni A. Argun, Paula T. Hammond, Xuehong Lu,†‡ and Poo See Lee†‡§

Affiliation: †Temasek Laboratories, Nanyang Technological University; ‡School of Materials Science and Engineering, Nanyang Technological University; §Department of Chemical Engineering, Massachusetts Institute of Technology

Pictorial representation of a LPEI/PAA/PEO/PAA tetralayer

Layer-by-layer (LbL) has become a widely used procedure to make coatings with no defects and largely ordered surfaces. One way to employ LbL is to dip a substrate into a solution of what you want to attach to your film, rinse, and repeat. The breadth of materials made so far using LbL is huge.  Electrochromic devices (materials that change color when hit with an electrical charge), proton-exchange membranes for fuel cells, loading/delivery materials for biological applications, humidity sensors, and many other materials have been reported.  This paper discusses the synthesis of an electrochromic device.

Super cool video of electrochromism

The authors used a tetralayer composed of a layer of linear poly(ethylene imine) (LPEI), poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO), and another layer of PAA. The researchers built up these tetralayers using LbL. They studied the film growth, thickness profile, formation process, thermal properties and the electrical characteristics of the tetralayer films. Although all of these were interesting, I will focus on the electrical properties and the production of an electrochromic device.

The tetralayer films were found to be conductive in both wet and dry states, with the wet states more conductive than the dry states. From these results, the researchers concluded that water plasticization (the presence of water increasing the plasticity) was very important.  Using this electrolytic film the authors then fabricated electrochromic devices.  The schematics for two devices are shown below:

Schematics of the electrochromic devices

In both types of electrochromic devices, when a negative voltage is applied, the gold grid acts as a working electrode and the polyaniline doped with dodecylbenzene sulfonic acid (PANI-DBSA) layer is oxidized and becomes transparent.  In the transmissive device this allows the light to pass through the transparent indium tin oxide coated polyethylene terephthalate (ITO-PET) substrate, while in the reflection device the light was reflected by the gold grid.  When the reaction was reversed by applying a positive voltage the PANI-DBSA layer became absorptive.  The change in the ability to transmit light was driven by the exchange of ions between the tetralayer electrolyte and the PANI-DBSA layers.

In this paper the researchers successfully demonstrate the use of LbL to make the electrolyte in an electrochromic device.

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