The core working principle of electrochromic glass is that by applying a tiny voltage, the ions inside the glass are driven to move, causing reversible and stable changes in the optical properties (color and transparency) of the material.
Detailed working principle (Step-by-step analysis
You can imagine it as a very thin and transparent "battery" that changes color when charged and becomes transparent again when discharged.
1. Core structure (sandwich structure
A typical electrochromic glass is composed of five layers of functional films, sandwicketed between two glass substrates:
Outer glass: Substrate.
Transparent conductive layer (TCO) : Typically indium tin oxide (ITO), serves as an electrode for applying voltage.
Electrochromic Layer (EC Layer) : Core layer, containing electrochromic materials (such as tungsten trioxide WO₃). When ions and electrons are injected, it changes color (usually turns blue).
Ionic conductor layer (electrolyte layer) : It stores ions (such as Li⁺) and allows ions to pass through under the action of an electric field, but insulates electrons.
Ion storage layer (counter electrode layer) : When the electrochromic layer changes color, it needs to provide or receive ions to maintain charge balance, and usually has complementary color-changing properties itself.
Transparent conductive layer (TCO) : The electrode on the other side.
Outer glass: Substrate.
The structure is simplified as: glass, conductive layer, electrochromic layer, ion conductor layer, ion storage layer, conductive layer, and glass
2. Working Process (Taking tungsten trioxide WO₃ and lithium ions Li⁺ as examples)
a. Coloration - When voltage is applied
When a DC voltage (usually very low, 1-5V) is applied to the transparent conductive layers on both sides:
Electric field drive: The electric field drives lithium ions (Li⁺) in the ionic conductor layer to move towards the electrochromic layer (WO₃).
Ion and electron injection: Meanwhile, electrons (e⁻) are also injected into the electrochromic layer through an external circuit.
Chemical reactions occur: In the electrochromic layer, ions and electrons undergo reduction reactions (intercalation) with the material.
WO₃ (transparent) + xLi⁺ + xe⁻ ⇌ LixWO₃ (blue)
Discoloration: The product LixWO₃ is a deep blue compound that absorbs visible light, thereby darkening the color of the glass and reducing its transparency. Meanwhile, the ion storage layer is oxidized and loses ions to maintain the charge balance of the entire system.
b. Fading (Bleaching) - when the voltage is reversed or disconnected
When the voltage is reversed or short-circuited:
Process reversal: The direction of the electric field changes or disappears, and ions and electrons are extracted from the electrochromic layer and return to the ion conductor layer and the ion storage layer.
Reverse reaction occurs: The above chemical reaction proceeds to the left, with LixWO₃ decomposing back into transparent WO₃ and releasing Li⁺ and e⁻.
Restore transparency: The glass returns to a state of high transparency.
c. Memory Effect
One key advantage of electrochromic glass is that it has Bistable properties. Once the discolored or faded state is formed, it can be maintained for a long time (for several hours or even days) even if the power is completely cut off. Because the above-mentioned chemical reaction is in a stable state when not driven by an external electric field. It only consumes electricity when it needs to change its state, which makes it very energy-efficient.
