Window glass becomes a memory device?  – A new information display / storage device –

We have developed a new information display / storage device using three terminal thin-film transistor structure on an electrochromic material, which have been attracted attention as ‘Electric curtain’. The device shows color change (colorless transparent <–> black) together with electrical conductivity change (insulator <–> metal) by applying gate voltage. Since the present device can be fabricated at room temperature, low cost fabrication is possible. Thus, larger area device is easily fabricated. For example, the present device is applicable as information display / storage on a window glass. This result has been published in the following paper (open access journal).

T. Katase, T. Onozato, M. Hirono, T. Mizuno, and H. Ohta, “A transparent electrochromic metal-insulator switching device with three-terminal transistor geometry”, Sci. Rep. 6, 25819 (2016).

Proton and hydroxyl ion play an essential role for tuning functionality of oxides because their electronic state can be controlled by modifying oxygen off-stoichiometry and/or protonation. Tungsten trioxide (WO₃), a well-known electrochromic (EC) material for smart window, is a wide bandgap insulator, whereas it becomes a metallic conductor H_xWO₃ by protonation. Although one can utilize electrochromism together with metal-insulator (MI) switching for one device, such EC-MI switching cannot be utilized in current EC devices because of their two-terminal structure with parallel-plate configuration. Here we demonstrate a transparent EC-MI switchable device with three-terminal TFT-type structure using amorphous (a-) WO₃ channel layer, which was fabricated on glass substrate at room temperature. We used water-infiltrated nano-porous glass, CAN (calcium aluminate with nano-pores), as a liquid-leakage-free solid gate insulator. At virgin state, the device was fully transparent in the visible-light region. For positive gate voltage, the active channel became dark blue, and electrical resistivity of the a-WO₃ layer drastically decreased with protonation. For negative gate voltage, deprotonation occurred and the active channel returned to transparent insulator. Good cycleability of the present transparent EC-MI switching device would have potential for the development of advanced smart windows.