What Is Electrochromic Smart Glass?

Electrochromic (EC) Smart Glass

Electrochromic smart glass changes its transmittance (i.e. how much light it passes) if stimulated by an electrical signal. This reversible change alters the state of the glass between transparent and opaque (or any state in between).  Below we see the general arrangement of an electrochromic cell:-

Electrochromic glass layers; Image attribution: Keyur.tithal, CC BY-SA 3.0, via Wikimedia Commons

Like a Battery, Sort of…

As you can see from the diagram, an electrochromic glass panel comprises a stack of layers. This stack is normally a few microns thick (a few thousandths of a millimetre) and is created using the same technique of physical vapour deposition (PVD) as is used in the semiconductor manufacturing industry.

• On the outer glass panels there are transparent conductive layers, normally of indium tin oxide (ITO), converting the whole structure into something similar to a battery, where the ITO comprises the electrodes.
• In the centre of the structure are the ion storage layer, the ion conductor (electrolytic) layer and the electrochromic (EC) layer which together are responsible for the change in transmittance.

When you apply a direct (DC) voltage to the structure, charged particles (normally lithium ions) migrate from the ion storage layer, through the electrolyte to the electrochromic layer (often tungsten oxide, which is transparent in its inactive state). This causes the electrochromic layer to undergo electrochemical reduction-oxidation (i.e. redox) which results in absorption of light and this causes its colouration. When you reverse the voltage, the lithium ions migrate back from the electrochromic (EC) layer, through the electrolyte, and back to the ion storage layer, returning the glass to its transparent state. This change of state can take place in the order of minutes. If you are looking for a deep-dive into this subject, check out our article on What Is Electrochromism.

Why does Electrochromic Glass change Transmittance?

When a voltage is applied across the electrochromic stack, lithium ions will ‘intercalate’ (i.e. insert themselves into) the electrochromic layer. The inserted lithium ions reduce the ‘band gap’ of tungsten oxide to roughly 2 electron-Volts (eVolts), which means that incident photons having at least that energy can be absorbed by the tungsten oxide, energising electrons into a higher energy state. Since visible light photons do have at least this energy, they are absorbed by the intercalated electrochromic layer, and the solar radiation which reaches the human eye by passing through the glass can be seen to be missing those wavelengths, i.e. it lacks visible light and thus appears tinted.

Electrochromic Device Architectures and Memory

There are various architectures to electrochromic devices:

• • Hybrid Electrochromic

Hybrid electrochromic glass can retain its state for up to 4-5 days, thus exhibiting a memory function [3] and uses an inorganic electrochromic layer with an organic (polymer) electrolyte. The charge retention is limited by leakage though, causing the EC glass to eventually return to its transparent state. The memory capability requires power only when it changes state. SPD and PDLC smart glass technologies, on the other hand, must be powered continuously as long they are maintained in their transparent states.

• • Solid State Electrochromic

Solid-State EC glass does not have any memory capability unfortunately, but has been proven to be very resilient in cyclic tests under extreme temperature conditions and UV exposure, affording a lifespan of around 20-30 years, which is ideal for building facades.

Why is Low Voltage So Important?

If a product operates below 60Vdc it is considered a ‘Safety Extra-Low Voltage’ (SELV) device by the IET, as defined in European standard EN 60335. This is important as the electrochromic glass will often be installed into a building facade or a vehicle (think: aircraft, yacht or automobile) and will need wires routed to it. The lower the voltage (and current), the lower the risk to safety and the cheaper the cables. For a large smart glass installation you can imagine how lower-voltage technologies can reduce installation costs and operational maintenance costs.

Applications

• Architects, interior designers and building services engineers who specify electrochromic smart glass in the built environment should bear in mind that electrical wiring regulations may be different in each country.
• Vehicle OEMs must bear in mind the cost and weight of cabling needed to power the electrochromic smart glass, as well as the voltage transformations needed from 48Vdc (aircraft) or 24Vdc (automotive) down to the operating voltage of the electrochromic glass (typically 3Vdc).
• Consumer appliance OEMs must bear in mind the power requirements and the carbon footprint when mass manufacturing devices based on electrochromic glass.

Another factor: the extra-low voltage makes it feasible to directly power the electrochromic smart glass from photovoltaic (PV) solar panels on the building facade (or vehicle), facilitating an eco-friendly solution in every respect.

It’s The Environment, Stupid

One obvious benefit is a substantial reduction in air conditioning costs, thanks to the infrared (i.e. solar heat) rejection by electrochromic smart glass when installed in building facades (and vehicles). The fact that the behaviour can be dynamically controlled allows owners, managers and users to tune the electrochromic smart glass facade to their needs, which can change over the course of a day and of course seasonally.

One Last Thing: Time

It is worth repeating that electrochromic smart glass takes in the order of minutes to change state, which is markedly different from SPD and PDLC smart glass technologies (which take a matter of seconds). In architectural applications, this gradual change can be an advantage since it allows our eyes to adjust to the slow change in light level. Also, buildings can benefit from a slow, gradual change in the facade, avoiding step changes which may distract people, traffic or wildlife. For transportation, this slow change could be a disadvantage or even a safety issue, since visibility depends on being able to see out of the vehicle. However, this factor may be offset by the other benefits of controlling glare and heat using a safe low voltage with low-cost and low-weight cabling.

So, Is Electrochromic Glass What I Need?

Well, this will depend on the cost vs benefits, both in the construction of the building, vehicle or consumer device, as well as the operational costs of running them. Electrochromic smart glass needs to reduce substantially in cost and also in switching times in order to gain mass market approval. The US Dynamic Glass Act, thanks to its 30% tax credits, will help of course, but this is just a temporary measure (until 1 Jan 2025). More important may be the dynamic heat rejection capabilities, coupled with low energy consumption and safe voltages, which could tip the scale for many applications.

Manufacturers

Please note that a Google search for ‘electrochromic glass’ turns up many manufacturers who are not making electrochromic smart glass, but other types of smart glass. Here is a partial list of electrochromic glass manufacturers, enumerated in alphabetical order:-

• Halio
• Miru
• Sage Electrochromics (part of Saint Gobain)
• View Inc.

Smartglass World – Marketplace

If you are looking for more electrochromic smart glass manufacturers, distributors or installers, look no further than our parameterised Search. The screenshot below shows that we have 19 such companies at the moment, which can be filtered down further if you specify product attributes, such as minimum transmittance, electrical power or haze:

Smartglass World Marketplace search for electrochromic manufacturers, distributors and installers

If you press the ‘Show Results’ button, this will produce a list of companies, which you can contact by posting a request on the Smartglass World Marketplace.

References

[1] “Smart Switchable Glazings for the New Millennium” – Carl M. Lampert, Star Science [2] “Colour and the Optical Properties of Materials”, Richard Tilley [3] https://www.commercialwindows.org/electrochromic.php