Effect of Gd-doping on electrochromic properties of sputter deposited WO3 films

With distinct ion insertion-induced tinting behavior, tungsten oxide (WO3) has been intensively studied as a representative electrochromic (EC) material for many years. To enhance its EC properties, strategies including nanostructuring, plasmonic engineering, and metal ion doping have been adopted. Here, we show that rare earth element of gadolinium (Gd) doping can effectively improve the coloration efficiency (η) of a sputtered amorphous WO3 (∼30% enhancement at the maximum of eye-sensitive wavelength of 520 nm for 0.57 wt% doping). By combining the film structural characterization and electrochemical analysis, such an increment can be attributed to the monoclinic Gd2O3 nanoislands formed on WO3 surface which enhanced charge capacity and reaction kinetics of the film. The large transmittance modulation (76% at 800 nm), high coloration efficiency (68.3 cm2 C−1 at 633 nm) and fast switching speed (7.8 and 10.4 s for coloration and bleach time, respectively) were achieved for the modest Gd-doped WO3 film (0.9%). However, further increasing of Gd doping results in decreasing of η, which could be attributed to the excess amount of Gd2O3 that inhibits the penetration of electrolyte into the active material. We believe that the understanding of the Gd-doping effect on the electrochemical behaviours of WO3 may shed light on selecting the proper doping elements and their concentration for energy-saving smart windows.