Innovative electrochromic window and display technology to the European NTBF and SME sectors

• Research unit: EU RFP
• Project number: 98.0181

Key words (English)

Electrochromics; nanocrystalline; display devices; rear mirrors

Alternative project number (English)

EU project number: IN20732I

Research programs (English)

EU-programme: 4. Frame Research Programme - 9.1 Dissemination and optimisation of results

Short description (English)

See abstract

Partners and International Organizations (English)

Coordinator: University College Dublin (IRL)

Abstract (English)

This project concerns the use of mesoporous films for electrochromic display and window applications. Electrochromic switching of color mesoscopic films occurs via electron injection from an external source into a nanocrystalline TiO2 film, the space charge being compensated rapidly due to ready compensation by ion movement in the electrolyte present in the mesopores. The surface of the nanocrystals is derivatized with a redox active molecule, such as a viologen endowed with an anchoring group to attach it firmly to the TiO2 surface. The viologens form a group of redox indicators which undergo drastic color changes upon oxidation/reduction. The reduced form of methylviologen, for example, is deep blue while the oxidized form is colorless. Reduction of the anchored viologen relay by conduction band electrons of TiO2 can be used for the amplification of the optical signal. The amplification is due to orders of magnitude of higher molecular extinction coefficients of these relays as compared to that of the TiO2 conduction band electrons. Upon electron injection, transparent nanocrystalline films of TiO2 derivatized by viologen develop a strong color. The films can be decolorized by reversing the potential. By varying the chemical structure and redox potentials of the viologens, it is possible to tune the color and hence build multi-color electrochromic display devices. Such surface-derivatized nanocrystalline oxides accomplish a performance which in terms of figure of merit, i.e. the electric charge required to achieve an optical density change of one, can compete with conventional electrochromic systems. Hence, they show great promise for practical applications. The exponential dependence of the change in the optical density on the applied potential allows a simple multiplexing method to be envisaged for addressing individual pixels of the nanocrystalline film, which is important for display applications.
The work accomplished in the final project period is in keeping with the tasks assigned to EPFL as technology provider within the consortium. The goal is to develop large area prototypes, as well as to demonstrate color switching and stability under cycling. In particular, a reflective film of ZrO2 was developed which is placed between the working electrode and the counterelectrode. Meanwhile, this invention has been protected by a patent application. The surface was derivatized with a silane function to avoid spill over of the viologen onto the reflector. The chemical attachment of the viologen to the TiO2 surface via a silyl group was also achieved. The results are reported in detail in the two publications which are attached. Finally, a method was developed for multiplexing the device. This will be disclosed once the patent which is presently being prepared has been filed.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 98.0181