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A novel electrophotographic system is described which consists of a thin film of a transition metal oxide and a thin film photoconductive layer sandwiched between a pair of electrodes. When an electric field is applied across this composite structure and an optical image is projected on it, the resulting modulation of the conductivity pattern in the photoconductive layer causes a similar coloration pattern in the oxide layer, thereby forming a visible image. The formation of color centers and the associated optical and electrical properties are discussed.

The optical absorption spectrum of thin films of MoO3 has been measured over the temperature range from 77° to 290°K. The fundamental absorption edge occurs at 3300 Å with an absorption coefficient of 105 cm−1. A stoichiometric film of MoO3 shows three absorption peaks at 3350, 2880, and 2700 Å, respectively, and these are suggested to be due either to exciton formation or to transitions involving a split valence band. Values for the refractive index have been measured in the wavelength range 0.42–1.2μ. The low-frequency dielectric constant (18.0±1) has been measured and the comparison with the high-frequency dielectric constant (5.70) indicates that the bonding is predominantly ionic. The electrical conductivity has been measured on a polycrystalline sample in the temperature range 298° to 900°K and the activation energies of conduction found to be 1.83 eV (intrinsic) and 0.56 eV (impurity). On irradiating a thin film (0.2μ) of MoO3 with light of wavelength shorter than 3300 Å, three color-center bands having maxima around 5000, 6250, and 7700 Å are formed. The same type of color centers are also formed on heating the film in oxygen in inert atmosphere. The color centers thus formed cannot be bleached optically. However, they can be bleached thermally in oxygen at 300°C. The growth and decay of color centers has been studied in detail. The formation of color centers is associated with increased dark conductivity of the MoO3. Very weak photoconductivity and photovoltaic effects with threshold energy corresponding to the fundamental absorption edge at 3300 Å have been observed. An ESR signal characteristic of Mo5+ state has been observed in a film of MoO3. There is a small increase of ESR signal on color-center formation which disappears completely on heating in air at 300°C. An energy-level diagram has been proposed to account for the optical and electrical properties as well as the color-center formation in MoO3.

A novel electrophotographic system is described which consists of a thin film of a transition metal oxide and a thin film photoconductive layer sandwiched between a pair of electrodes. When an electric field is applied across this composite structure and an optical image is projected on it, the resulting modulation of the conductivity pattern in the photoconductive layer causes a similar coloration pattern in the oxide layer, thereby forming a visible image. The formation of color centers and the associated optical and electrical properties are discussed.

We report on how electrochromic coloration is affected by oxygen deficient stoichiometries in sputtered amorphous tungsten oxide (a-WO3−y) films. The electrochromic coloration efficiency increases with increasing oxygen deficiency in (a-WO3−y) films. No coloration is observed in nearly stoichiometric WO3 films. Raman spectroscopic studies reveal that the number of W5+ states generated with lithium insertion increases with the oxygen deficiency. Furthermore, there are no Raman peaks resulting from W5+ states in lithiated a-WO3−y films with near perfect stoichiometry, which is consistent with the absence of electrochromic coloration in those films. We conclude that the coloration efficiency of a-WO3−y films depends on the number of the W5+ states generated by lithium insertion and that the oxygen deficiency plays an important role in generating the W5+ states with lithium insertion.

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The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the W6+–O and W6+=O bonds, respectively, and a weaker peak at 220 cm−1 that we attribute to the W4+–O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. Comparison of the Raman spectra of sputtered isotopic a-W16O3−y and a-W18O3−y films confirms these assignments. We conclude that the as-deposited films contain mainly the W4+ and W6+ states, and the W5+ states are generated as a result of reduction of the W6+ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the W6+ and W5+, and W5+ and W4+ states.