Switchable glazing for the regulation of light transmission and energy transmission are increasingly in demand for reasons of comfort and increasing energy costs. This particularly concerns large-area glazing of buildings as well as the automotive sector. Such systems can be realized, especially, using electrochromic layers. The basic structure of such a system corresponds to that of a thin-film battery.
An electrochromic device includes two transparent electrodes, an active layer, a lithium-containing electrolyte and a counter electrode. By applying a voltage, lithium is intercalated in the active layer, usually tungsten oxide (WO3), whereby it turns dark. By reversing the voltage, the lithium is transferred from the active layer via the electrolyte into the counter electrode. The active layer becomes lighter again. The counter electrode ideally shows a contrary behavior, i.e. bright for lithium intercalation and dark in the case of lithium deintercalation. Such behavior is known only for a number of oxides of the transition metals: Cr, Co, Mn, Ni, V, Ir, Fe, and Ru. Here, iridium and ruthenium cannot be used for a large-scale application on the grounds of costs. Iridium oxide (IrO2) is used only on small areas, for example in switchable car mirrors. Further boundary conditions are the achievable light / dark switching stroke, the cycle stability, the color impression and health aspects of the materials.
The search for alternatives and modifications of the above materials for the counter electrode and the active layer are the subject of our research. The Fraunhofer IST has participated in the following joint project:
Smart windows of second generation "ECWin2.0", Subproject: New color matched efficient counter electrodes for electrochrome glazing, FKZ: 13N13375.
The project partners were EControl-Glas GmbH & Co. KG and GFE Fremat GmbH.
Two central project results are the identification of TiNb2O7 as an alternative material for the active layer and the identification of yet another combination of elements for the counter electrode (see the publications below)
- Development of active layer and counter electrode materials
- Development of deposition processes for active layer and counter electrode materials
- Suitability tests of sputtering targets and sputtering processes for the deposition of electrochromic layers
- Characterization of electrochromic layers by means of combined cyclovoltammetry and photometry as well as other methods of layer analysis (XRD, XPS, SIMS, EPMA, FIB-SEM, SEM)
- Design and testing of prototypes of electrochromic systems