Degradation mechanism of the complementary electrochromic devices with WO₃ and NiO thin films fabricated by RF sputtering deposition

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 degradation phenomenon and established the degradation mechanism for both WO₃ and NiO thin films. Based on the long-term cycling tests, the transmittance variation (ΔT) was observed to depend on the cycling number. The electrochromic properties of the full cell with WO₃ and NiO thin films degraded significantly as the cycle number increased. The ΔT increased in the early stages and showed the highest value (25%) at the 200^thcycle before gradually decreasing and becoming saturated at the 800^th cycle with the lowest value of 5%. Interestingly, while WO₃ thin films showed gradual degradation, NiO thin films were delaminated after 700 cycles, which led to an abrupt degradation in performance. The degradation of WO₃ and NiO thin films is 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.