Durability studies of annealed electrochromic tungsten oxide films

• In electrochromic (EC) applications, annealing is a crucial parameter not only for an individual layer but also for a full device. For the fabrication of a complete EC device, indium tin oxide (ITO) is often preferred as a transparent conductor layer. ITO films with high transparency and low electrical resistance are usually obtained by sputtering at high substrate temperatures. Consequently, the effect of high temperature on the EC layers can be very significant during sputtering of the ITO top layer for EC devices consisting of five sputtered layers on a single substrate. The role of annealing of a single layer of WO3 may also be important for EC performance. In the present work, we studied the effects of annealing on the durability of WO3 films. Thin films of WO3 were deposited by reactive DC magnetron sputtering in a mixture of Ar and O2 gases using an oxygen to argon ratio of 0.15. The total gas pressure was set to 4.0 Pa, and the sputtering power was 200 W. The WO3 films were deposited onto (i) unheated glass plates, (ii) such plates pre-coated with transparent and electrically conducting ITO with a sheet resistance of 60 Ω/square, and (iii) glass plates pre-coated with fluorine-doped tin oxide (FTO) with a sheet resistance of 14 Ω/square. Film thicknesses were 300±10 nm. After deposition of the films, the samples were annealed at 150, 300, 450, and 600 °C in ambient air for one h using a heating rate of 10 °C min-1. Cyclic voltammetry (CV) was performed for up to 500 cycles between 2.0 and 4.0 V vs. Li/Li+ at a scan rate of 20 mV s–1. Annealing at temperatures at and above 300 °C resulted in deteriorated electrochromic properties of the WO3 films i.e., a decreased transmittance variation. Charge density and coloration efficiency changes during extended electrochemical cycling were also observed as a function of cycle number and annealing temperature. It was found that the maximum optical transmittance modulation at a wavelength of 528 nm after 500 CV cycles was 69.3% for the film annealed at 150 °C.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

Nyckelord

Engineering Science with specialization in Solid State Physics

Teknisk fysik med inriktning mot fasta tillståndets fysik

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