Kohei Kiyota

The Effect of Repeated Operation on Electrochromism in an All-Solid-State Device Using Li-Doped MgF2 Thin-Film Electrolyte

We have fabricated a thin-film electrochromic device consisting of WO3 and Li-doped MgF2 films. Although the device exhibited excellent coloration and bleaching response (coloration period: ~100 ms, bleaching period: ~200 ms), the following changes in the device characteristics were observed upon retreated operation: 1) bleaching was impaired, 2) spontaneous coloration increased while spontaneous bleaching decreased and 3) the threshold voltage for coloration decreased. We have found that these changes can be attributed to the change in the free energy or the Li+ ion.

Effect of surface states of WO3 on the operating characteristics of thin film electrochromic devices

Abstract The effect of gas adsorption at the WO 3 -MgF 2 interface on the operating characteristics of a thin film electrochromic device consisting of WO 3 and MgF 2 were investigated, and the results were discussed from the viewpoint of surface states of the WO 3 film. As a result of gas adsorption the coloration speed and efficiency are increased, the threshold characteristics for coloration become sharp, the open- circuit memory becomes long and the leak current is reduced. These effects were obvious only when the adsorption was performed at the WO 3 -MgF 2 interface and water adsorption is essential to the effects. The film thickness dependence of the conductivity of as-grown WO 3 film revealed that the depletion layer of WO 3 , whose thickness is about 5000 A in vacuum, vanishes when MgF 2 film is deposited onto it. We concluded that the effect of gas adsorption can be attributed to the surface states and the depletion layer induced by the surface states of WO 3 . Using the analogy of a metal/oxide/semiconductor device, we proposed an energy band model for electrochromic devices which can explain the effects of gas adsorption systematically.

Enhancement in oscillator strength of color centers in electrochromic thin films deposited from WO2 powder

Color centers in amorphous films evaporated from WO2 powder and WO3 powder were investigated. It was found that the device using ‘‘WO2’’ film, which has a WO3‐like structure, exhibits a high‐speed response and a high optical density, which is attributed to the enhancement in oscillator strength of the color centers in ‘‘WO2’’ films. The use of a configuration coordinate model revealed that the enhancement in the oscillator strength is caused by the increase in the degree of extension of the polaron wave function in the ‘‘WO2’’ film. The absorption band of the color center in ‘‘WO2’’ exhibits a low‐energy shift as compared with that in WO3, which supports the increase in the degree of the small polaron extension in ‘‘WO2.’’ We also studied the electrical conductivity, density, ir absorption spectra, and voltammograms of the ‘‘WO2’’ and WO3 films. These results suggest that the increase in the degree of the extension of the polaron wave function is caused by some change of the lattice condition which is int...

Electrolysis in Electrochromic Devices Consisting of WO3 and MgF2 Thin Films

The role of moisture in the ambient atmosphere on the operation of all-solid-state electrochromic devices has been investigated, and electrolysis of the water in electrochromic devices studied. Because of the dependence of the current passing through MgF2 film on the polarity of the applied voltage, it is suggested that during operation of the devices, water should be supplied only near the surface of the MgF2 film. IMA measurement and direct observation of gas discharge revealed that the water near the surface of the device is mainly dissociated, and that protons flow into the WO3 film through the MgF2 film, resulting in colouration. The colouration behaviour of devices with two different structures leads us to believe that moisture in the atmosphere contributes to more than half the optical density of the device. We also found that bleaching is not always accompanied by dissociation of the water.

Electrochromism in a Thin-Film Device Using Li2WO4 as an Li-Electrolyte

A thin-film electrochromic device using Li2WO4, which is shown to be an Li electrolyte by the fact that the coloration response is independent of the atmosphere, is found to exhibit quick response (coloration: 50 ms, bleaching: 200 ms). The spontaneous coloration is suppressed by the deposition of a WO3 film at low pressure (10-6 Torr), and by using a metal with a small work function as the counter-electrode. The threshold voltage for coloration increases as the work function of the counter-electrode material decreases. Supressions of the spontaneous coloration and the threshold voltage shift are induced by the potential difference between the WO3 and the counter-electrode introduced by the difference in the Fermi levels. Complete bleaching of the device is achieved in a device with a WO3 film deposited at 10-6 Torr at a substrate temperature of 160°C, while it is not achieved in a device with a WO3 film deposited at 10-4 Torr. This suggests that Li2WO4 is colored by proton injection from WO3 into Li2WO4, which arises from the electrolysis of water in the WO3 film deposited at high pressure.

Fixing Time Of Ink-Jet Inks On Plain Paper

We investigated water-based ink-jet inks which fix quickly on plain paper. Ink-jet inks have liquid components which evaporate slowly to prevent the print head nozzles from clogging. The fixing time of these inks is too long for printing on plain paper. Inks cannot penetrate paper which contains a sizing agent, which increases the papers resistance to liquids. The most widely used sizing agent is rosin. We attempted to shorten fixing time by including in the ink, a component for dissolving rosin. The glycols used in the ink are useful because they evaporate slowly. We determined the relationship between the solubility of the ink for sizing agents, rosin, and the time needed for fixing and penetration. The higher the solubility of a glycol for rosin, the faster the penetration of the ink containing it. Hexylene glycol dissolves rosin over 70 wt%, and ink containing over 15 wt% hexylene glycol penetrated plain paper within 5 seconds.