Photo-electrochromic devices

The formation of colored radical cations (and accompanying radical anions) and the possibility of oxidation (or reduction) by an electrochemical process indicates the possibility of erasable photo-electrochromic devices. 

In a development of the conventional electrochromic device structure, it has been proposed that the rear electrode, which is normally a transparent indium tin oxide film for transmission devices or a metallic film for reflective devices, be replaced by a semiconductor to enable a so-called photo-electrochromic device to be fabricated. In this device reduction can only be carried out on an n-doped electrode and oxidation on a p-type electrode. However, when the device is illuminated, minority charge carriers are generated, and both oxidation and reduction can be performed. This extra degree of freedom has led to the possibility of building a photo-electrochromic display or memory device [118]. 

Other materials being investigated include ferrocene with a bipyridinium salt,234 niobium oxide,235 nickel oxo-hydroxide,236 and cobalt oxohydroxide.237 The last is pale yellow in the reduced state and dark gray in the oxidized state. A typical electrolyte is lithium perchlorate in propylene carbonate. Solid electrolytes, such as a lithium salt (perchlorate, tetrafluoroborate, or triflate), in a polyepoxide238 or in a polyvinyl chloride gel in ethylene carbonate-propylene carbonate,239 lithium iodide in polyvinyl bu-tyral,240 and Naflon H (a polymeric perfluorocarbon-sulfonic acid),241 have also been tested. Some other systems use suspended particles between two panes of glass.242 When the particles are aligned by an electric field, the window becomes transparent. Combination photo-voltaic-electrochromic devices are under study.243... 

Electro-photoswitching devices possess a dual mode operation resulting from different electrochemical properties in the two photo-interconvertible states. They may display both photochromism and electrochromism. They allow the photomodulation of electrochemical properties as was shown to occur in photochromic dihydroazulene derivatives [8.247]. Separate photochemical and electrochemical interconversion has been realized in systems containing a photoactive azo [8.248, 8.249] or dihydroazulene [8.250] unit appended with an anthraquinone moiety. 

The applications of TCO materials are enormous. They can be used for deicing, heating, and, particularly, as electrical contacts in optical devices, such as photo-voltaic cells [5, 6], electrochromics [7] or spectroelectrochemical set-ups [8]. 

Fig. 7.11 Photos of a PANl-based flexible electrochromic display device containing 25 pixels. The display region and the connections were made by depositing gold on a plastic sheet using an appropriate mask and an evaporation technique. Each pixel can be driven separately. Left. PANI is in its oxidized state in all pixels. Right. PANI is reduced in two pixels (the bleached ones)...

Organic thin films have been extensively investigated in connection with the attempts to apply organic materials to electronic and electrochemical devices. Phthalocyanine and its metallo-derivatives are particularly of interest in this field a wide variety of physical and physicochemical properties applicable to such devices, i.e., photo-conductivity, photovoltaic effect a, electrochromism, electrocatalysis , have been reported for their films. 

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