Nitrogen doped W oxide films for electrochromic applications

• Electrochromic (EC) materials are able to change their optical properties such as transmission, absorption and reflection reversibly by application of an external voltage. EC metal oxides are divided into two groups: cathodic (coloring under ion insertion) and anodic (coloring under ion extraction). W oxide is a well-known cathodic EC material and its color changes from transparent to dark blue when ions are inserted.A desirable electrochromic material must have and maintain a high optical modulation, high coloration efficiency, fast coloration/bleaching switching kinetics and a stable charge/ discharge reversibility.  In this study, W oxide films with different nitrogen levels were deposited by using reactive DC sputtering onto glass and ITO coated glass in Ar+O2+N2 atmosphere. For all films, the total gas pressure was set to 4.0 Pa, the Ar flow rate was kept at 50 ml/min, and the O2+N2 flow rate was kept at 7.5 ml/min. The optical, structural and electrochromic properties of undoped and N-doped W oxide films were investigated. The optical studies revealed that the average optical transmittance and band gap decreased (from 3.43 to 3.08 eV) due to N doping.  It is shown that a small amount of nitrogen has promising effects on the EC performance (i.e. charge/discharge reversibility, optical modulation, coloration efficiency) of the WO3 films. It is observed that CE values increased by increasing N2 flow rate and its maximum value was 33.8 cm2/C. The maximum ΔT at 537 nm was 73.6% for an optimized N doped W oxide film.

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Engineering Science with specialization in Solid State Physics

Teknisk fysik med inriktning mot fasta tillståndets fysik

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