Clean Technology 2010

Smart windows” transition metal oxide films: technology and electrochromic device performance

K.A. Gesheva, T. Ivanova, M. Kozlov, S. Bojadjiev
Bulgarian Academy of Sciences, BG

Keywords: transition metal oxides

Abstract:

Transition metal oxides exhibit electrochromic effect, under a small voltage they change their optical transmittance from collored to bleach state. They are envisaged for application as smart windows in future architectural low-energy building. Smart window is a device controlling the energy of solar flux entering the building or car and make the interiors comfortable and at the same time energy utilization more effective. We have developed a whole optimized technology for deposition of thin films on conductive glass capable to change their transmittance if a small voltage (-1 - +2 volts) is applied across the film. Visually the film colours in a nice dark blue colour, and if the polarity of the voltage is changed the films bleaches up back to the initial transparency. Transition metal oxides due to their specific electronic structure are capable to exhibit this effect, called by Satyen Deb in 1968 electro chromic effect. He observed it in WO3 for the first time. Today the research is in the way to decrease the prise, to a few hundreds of dollars. The effect is well known, and the technology is necessary to be cost-effective. We offer the atmospheric pressure chemical vapour deposition as a very promising due to the fact that being an atmospheric pressure process it permits a flow-through technological manner, once the film growth-rate is enough high. The presentation deals with the optimization of a device, based on working electrode of WO3-MoO3 mixed films and counter electrode of MoO3-CrO3 mixed film. The extensive research done on the film materials leads to the conclusion, that WO3-MoO3 mixed structure possess the high electro chromic performance of WO3 and the excellent kinetic properties of MoO3 growth process, namely its higher growth-rate. Colour Efficiency approaching 200 cm2/C combined with optical modulation in the range of 65-70% down to around 20-25% are practically acceptable electrochromic performance characteristics. As counter electrode we have developed by the same technology Mo/Cr - based mixed oxide films, which do not possess electrochromic effect in cathodic regime, when employed as working electrodes. We employed these films as counter electrodes of the same device. Details about the electrochromic device made of the electrodes metal oxide materials and solid electrolyte containing optimized concentration of lithium ions, as well as data about the scan rate of potential applied for optimal device performance will be the subject of paper presentation. Results from the voltammetry measurements of the electrochromic performance will be referred to film materials properties characterized with high sensitivity characterization techniques like Field Emission Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, as well as FTIR and Photospectroscopy measurement.
 
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