WO3 and NiO thin films for electrochromic devices were fabricated by a radio frequency (RF) sputtering deposition method. These thin films were used as working and counter electrodes, respectively. We investigated the performance degradationphenomenon and established the degradation mechanism for both WO3 and NiO thin films. Based on the long-term cyclingtests, the transmittance variation (ΔT) was observed to depend on the cycling number. The electrochromic properties of thefull cell with WO3 and NiO thin films degraded significantly as the cycle number increased. The ΔT increased in the earlystages and showed the highest value (25%) at the 200th cycle before gradually decreasing and becoming saturated at the 800thcycle with the lowest value of 5%. Interestingly, while WO3 thin films showed gradual degradation, NiO thin films weredelaminated after 700 cycles, which led to an abrupt degradation in performance. The degradation of WO3 and NiO thin filmsis mostly caused by dissolution by H2O. While WO3 is directly attacked by H2O, NiO is chemically converted into Ni(OH)2,which then converts to NiOOH through an electrochromic reaction during the cycle test; NiOOH is easily dissolved in H2O.

Keywords: Tungsten trioxide, Nickel oxide, Electrochromism, RF sputtering, Degradation mechanism, Thin films