Electronic structure electrochromic devices

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

Periodic nanostructured layers promise wide applications in electronic and optoelectronic devices. Photoelectrochemical and electrochromic structures are among them [1]. The most suitable process for formation of such layers is electrochemical anodization of tantalum-aluminum multi-layer thin-fllm compositions. This process is inexpensive and permits to form nanostructured pillar layers of Ta205 with large surface area. Details of Ta20s pillar formation from two-layer Al/Ta thin film compositions were described in our previous papers [2,3]. The main purpose of our further investigations was to investigate the processes of anodization of a multi-layer Al/Ta/Al structure. It was found that application of the bottom A1 layer improves uniformity of nanostructured pillar layers due to more homogeneous current supply. Besides, this layer serves as an electrode of a metal/dielectric/metal (MDM) sfructure. Furthermore, such metals as Nb and Ti may be used instead of Ta layer. 

The electrochromic properties of polymers and their applications in optical devices are described in Chapter 7 of this book. Therefore, attention here will be mainly restricted to ion insertion compounds, namely to mixed ionic-electronic conductors having a soft lattice into which ions can be rapidly and reversibly inserted. To assure electroneutrality, the insertion of one ion, e.g. a monovalent cation M", into the compound s lattice must be accompanied by the injection of a balancing electron in the electronic structure of the same compound ... 

In a series of papers [128-133], the Reynolds group recently reported an impressive set of monomers that straddle various aryl groups with the electron-rich 3,4-ethylenedioxythiophene system, LXVI. These monomers offer almost unprecendented control over the gap and redox properties of their resultant polymers. Furthermore, since the majority of these polymers are soluble, the ambiguities regarding their structure are lifted. These (monomers and) polymers oxidize at relatively low potentials, making them extremely stable in their doped state. Electrochromic devices made from them can be cycled >10 times without noticeable degradation. 

The electrochromic device consists of an alternating layer structure comprising a transparent substrate coated with an ITO coating, a cathode, an ionic conductor, an anode, and an another ITO-coated substrate. Both anode and cathode undergo redox reactions upon application of electric current between the conductive glass electrodes. The redox reactions lead to the reversible formation of colored species in the specific layer while at the same time electrons and cationic species (usually Li or H" ) move through the ionic conductor toward the appropriate electrode. 

While an invaluable tool in producing conjugated polymers on conducting substrates, electropolymerization has limitations that include a lack of primary structure verification and characterization along with the inability to synthesize large quantities of processable polymer. To overcome the insolubility of PEDOT, a water-soluble polyelectrolyte, poly(styrenesulfonate) (PSS) was incorporated as the counterion in the doped PEDOT to yield the commercially available PEDOT/PSS (Baytron P) (39), which forms a dispersion in aqueous solutions [140]. While this polymer finds most of its application as a conductor for antistatic films, solid state capacitors, and organic electronic devices, its electrochromism is distinct and should not be ignored. 

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