Smart technology

Within the Winsmart project, different technologies of optically switchable windows are being developed. The "state of the art" electrochromic window with oxide electrochromic and oxide counter electrode, separated by an ion conductor will be further improved.

As alternative, electrochromic windows with redox electrolyte are being developed.

Schematic configuration of an electrochromic device with WO₃ as electrochromic layer on a transparent electrode (TE) in contact with a redox electrolyte comprising Li⁺, I^-  and I₃^- ions. The counter electrode is realized by sputtering a very thin layer of platinum on a second transparent electrode.

Here, an electrochromic layer is deposited on a transparent conducting electrode (TE) being in contact with a redox electrolyte comprising for example Li⁺, I^-  and I₃^- ions, I^-and I₃^- acting as redox couple.

The counter electrode is prepared by sputtering a very thin layer of platinum as a catalyst on another TE. Under negative polarization, electrons are shifted from the counter electrode, where I^- is oxidized to I₃^-, to the electrode with WO₃, where Li⁺ ions are injected out of the electrolyte, which leads to a coloration. For bleaching, the polarization is reversed. By this, high optical contrasts can be achieved. Especially, colourless redox couples can be applied instead of the I^-/I₃^- couple, which may create a pale yellow. Therefore, high transparencies can be reached.

Electrochromic device with transparent redox electrolyte in bleached (left) and coloured state (right).

Another development closely related to the electrochromic device with redox electrolyte is the photochromic device based on a combination of a dye solar cell and electrochromic tungsten oxide.

Schematic configuration of a photochromic device with WO₃ as an electrochromic layer, TiO₂ and dye as photoactive layer, in contact with a redox electrolyte.

Here, a dye is adsorbed on a porous surface of a layer out of TiO₂, which is in contact with an electrochromic layer of WO₃. Upon illumination, the dye is excited, and the excited electron is transferred to the WO₃ via the TiO₂, creating a deep blue colour. In parallel, the dye is reduced to the ground state by oxidation of I^- ions, forming I₃^-, and Li⁺ ions are intercalated into WO₃ out of the redox electrolyte. The bleaching process is realized by adding a catalyst like Pt, which catalyzes the back reaction of the electrons from WO₃ to I₃^- in the redox electrolyte. In contrast to previous works based on sol-gel chemistry, now at Fraunhofer ISE it has been achieved to prepare photoelectrochromic devices by sputtering.

Transmittance spectra of sputtered photoelectrochromic device in bleached and coloured state (upon illumination).

Sputtered photoelectrochromic device in bleached and coloured state (upon illumination).

Furthermore, at the University of Ljubljana,  it was possible to prepare a photochromic coating based on a special solgel-synthesis of tungsten oxide without redox electrolyte or dye on TiO₂, which colours upon illumination and bleaches in the dark.

Photochromic WO₃ layers without dye and redox electrolyte in bleached (left) and after colouring by illumination for 60 min (right).

This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 314407.