EC (Electrochromic) devices typically consist of five thin-film layers sandwiched between two glass substrates: a transparent conductive layer, an electrochromic layer, an electrolyte layer, an ion storage layer, and a second transparent conductive layer. The working principle is as follows: when an external voltage is applied across the electrodes of the electrochromic cell, ions migrate into (or out of) the electrochromic layer under the electric field induced by the applied voltage. This reduces (or increases) the valence state of the electrochromic material. Before equilibrium is reached, the electrochromic material undergoes color change; once equilibrium is established, the color of the electrochromic material remains stable.