Optical properties and stability of near-optimum WO3/Ag/WO3 multilayers for electrochromic applications

• Near-optimum glass/WO3/Ag/WO3 transparent conductors with electrochromic WO3 were fabricated.
• A Bruggeman effective medium code was used to model the interfacial surface roughness of the coatings.
• Electrochromic devices with WAW coatings exhibit high reflectance and are suitable for solar gain control.
• A “safe” operating region of 12.6 mC/cm2 of inserted charge was established for the fabricated devices.

Near-optimum glass/WO3/Ag/WO3 coatings have been fabricated by electron beam gun deposition. Such coatings are suitable for electrochromic (EC) applications to serve as transparent conductors that incorporate the active EC film. They were tested with regard to their optical properties and electrochemical stability.The coatings exhibit luminous transmittance (~ 0.70), low sheet resistance (9–10 Ω) and low thermal emissivity (0.09–0.10). EC devices incorporating these coatings have significant transmittance modulation (6:1), high coloration efficiency (32 cm2/s at 550 nm) and considerable open circuit memory. Due to the presence of the Ag layer, high reflectance in the infrared (~ 70% at 2000 nm) can be maintained both in the bleached and in the colored state, unlike typical ECs that exhibit high absorptance when colored. This feature renders the fabricated devices suitable for heat gain control of buildings in warm climates.As regards the stability of the EC devices, it was found that deep coloration is not reversible when it is effected by an electric field, while self-bleaching is. This could be due to a reaction of Ag with Li ions that reach the Ag layer through the WO3 film. A “safe” region for reversible operation was established limiting the allowable intercalated charge density to 12.6 mC/cm2 and reducing the transmittance modulation to 3:1. In the “safe” region more than 1000 voltammetric coloration–bleaching cycles are possible without any signs of degradation.The Monte Carlo-Characteristic Matrix optical code that has been used for the design of the coatings was enhanced by an effective medium model with the Bruggeman formulation, in order to simulate roughness effects. It was found that the proposed method gives a good approximation of the interfacial surface roughness as confirmed by independent measurements.