Effects of active layer thickness on the performances of inorganic all-solid-state electrochromic(EC) devices WO₃/Ta₂O₅(H+)/NiO prepared by DC magnetron sputtering are investigated. Beer-Lambert law is used to predict the maximum optical contrast (~76%, significantly higher than previously reported) and the corresponding thickness(~260 nm) of the cathodic(WO₃) EC layer. Studies by cyclic voltammetry suggest that the optimum thickness of the anodic(NiO) EC layer should be confined close to 140 nm, in order to maintain charge balance within the devices. Furthermore, the optical switching behaviors for the devices with 100-nm-thick electrolyte[Ta₂O₅(H⁺)] layer are demonstrated to be qualitatively and quantitatively different from those with 700-nm-thick electrolyte layer, though all of them are sensitively dependent on the number of protons in the devices. Besides, the work principle scheme of the proton−based solid−state EC device was illustrated for the

first time. The obtained results could enlighten mechanistic understanding, and enable rational design for EC device with outstanding EC performances.