Mixed conductors as electrochromic materials

The presence of a multicolour electrochromic effect in rare-earth diphthalo-cyanines was first reported in 1970 by Moskalev et al. . Lutetium diphthalocyanine, LuH(Pc)2, has been studied extensively at Rockwell International Corporation by Nicholson and co-workers . This material may display different colours when polarized either anodically (red, orange) or cathodically (blue, violet) from its initial rest potential. The initially green complex film is obtained by vacuum evaporation. The anodic reaction occurs by the insertion of anions into the film and extraction of electrons rather than by loss of protons. The cathodic reaction, leading to blue and violet products, occurs by the insertion of cations. When protons are present in the electrolyte, the reaction is the following . 

The dark violet product appears to be a protonic conductor. It has a hydrogen-ion mobility of 8 x 10 1800 (Q cm) at room temperature. 

The mechanism of coloration and bleaching in thin films of cathodic materials may be written as follows 

As well as optical properties, the method of thin film preparation also influences the electrical properties. As-deposited WO3 films are n-type semiconductors. The room temperature conductivity usually lies in the range (Q.crn) , as shown in Table 38.1. Moreover, it 

Electrochromism has also been observed in either amorphous or crystalline films of M0O3. The mechanism of coloration is thought to be similar to that of WO3 films. To improve electrochromism, mixed oxides WO3/M0O3 have been studied. The absorption band of these materials extends to higher energies, i.e. to a region of higher display efficiency . 

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