1. Electrochromic materials and smart windows (the short version)

• Materials that change how light passes, by voltage: EC materials change their optical properties reversibly and persistently when a voltage is applied. Built into multilayer devices, they can modulate optical transmittance between widely separated extremaSourced [Source 1]. Mortimer’s review describes the colour change as effected either by an electron-transfer (redox) process or by a sufficient electrochemical potentialSourced [Source 2]
• Different uses ask for different things: a window can switch over minutes but needs large area and long life; a display needs a response of seconds or milliseconds. The Chem. Rev. review states that for smart windows or energy storage the acceptable switching times are on the order of minutes, while displays usually need seconds (or even milliseconds)Sourced [Source 6]
• The biggest difficulty is “how to measure lifetime”: a 2023 paper points out that, despite the long history of electrochromism, there is no universally accepted method for quantitatively comparing cycling stability between different materials or devicesSourced [Source 13]. The international standard ISO 18543 specifies an accelerated ageing test for electrically switched EC glazing, but its numbers alone do not rank materialsSourced [Source 19]

2. Mechanism — electrons and ions go in together, and the colour changes

In a typical inorganic EC device, the colour changes when electrons and small cations (H⁺, Li⁺ and so on) enter the colouring layer (tungsten oxide, for example) together. According to the review by Niklasson and Granqvist, today’s research on inorganic EC is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). In charged tungsten oxide the electrons enter localised states, and the optical absorption can be modelled phenomenologically as a superposition of transitions between different charge states of the W ions (6+, 5+ and 4+)Sourced [Source 1]. The same review says that devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductorSourced [Source 1].

View the original SVG schematic and exact annotations

Niklasson and Granqvist’s review indicates that the colouration of nickel oxide appears to be a surface phenomenon confined to the outer parts of the grains; once hydrated Ni oxide has been transformed into hydroxide and oxy-hydroxide phases, electrochromism is consistent with reversible changes between Ni hydroxide and oxy-hydroxide (the Bode reaction scheme), and it was demonstrated to be associated solely with proton transfer. The films are made by sputtering, and mixing Ni oxide with other oxides of wide band gap is treated as a way to enhance bleached-state transmittanceSourced [Source 1].

3. Families of materials — inorganic oxides, viologens, conductive polymers

Mortimer’s review sorts the main classes of EC materials into metal oxides, viologens (in solution and as adsorbed or polymeric films), conjugated conducting polymers, metal coordination complexes (as polymeric, evaporated or sublimed films), and metal hexacyanometallates. It adds that other materials aspects important for building devices include optically transparent electrodes, electrolyte layers and device encapsulationSourced [Source 2]. The table below is our own arrangement.

The families cannot be ranked from this table, since wavelength, layer structure and conditions differ. For conductive polymers themselves, see our explainer on conductive polymers and ionic conductors.

Byker’s patent (US 4,902,108, filed 31 March 1986 and granted 20 February 1990; the patent record lists Gentex Corp as applicant) proposes single-compartment, self-erasing, solution-phase EC devices. The solution consists of a solvent, a cathodic compound (including the known viologens) whose absorption in the visible increases on reduction, and an anodic compound whose absorption increases on oxidation. It states that the devices are surprisingly stable to cycling between light and dark states and that transmittance can be varied over more than a factor of 10 in several seconds. The obstacle to commercial use was the instability of the solution, and minimising oxygen and water raises stability, it addsSourced [Source 5].

4. A materials engineer’s view 1: the electrolyte and counter electrode set colour and lifetime

5. Reading the performance metrics — coloration efficiency, response time, cycle life, memory effect

The Chem. Rev. review (organised for displays) defines the main EC metrics as followsSourced [Source 6].

These definitions are the review’s arrangement for displays; for evaluation standards for windows, see Chapter 12.

As concrete examples, the film of W₁₈O₄₉ nanowires and gold nanorods of Sheng et al. (a laboratory test piece) coloured in 4.5 s (613.5 nm) and 6.5 s (900 nm) and bleached in 4.0 s and 5.0 s on a step voltage (−1.0 V and +0.2 V for 30 s), with coloration efficiencies of 48.46 cm²/C (613.5 nm) and 86.23 cm²/C (900 nm), and the transmittance difference ΔT stayed at about 96% (613.5 nm) and 93% (900 nm) after 300 switching cyclesSourced [Source 7]. Llordés et al. report that a composite film of tin-doped indium oxide nanocrystals in niobium oxide glass blocks near-infrared and visible light independently over a 2.5 V range and retained 96% of its charge capacity after 2,000 cyclesSourced [Source 4]. Both are numbers for small laboratory samples and bear no direct relation to a window’s lifetime (decades outdoors).

6. Our calculation: the charge needed to darken 1 m² of window

Coloration efficiency (CE) is defined per unit area of injected charge (optical modulation ΔOD = log₁₀(bleached transmittance / coloured transmittance); CE = ΔOD / (charge / area)). The charge needed to darken 1 m² of window by ΔOD = 1 (transmittance one-tenth) is “1 / CE”Our calculation. With the CE of the film of Sheng et al. (48.46 cm²/C) it is about 206 C/m²; with the same film’s CE at 900 nm (86.23 cm²/C), about 116 C/m². With the CE of a display device presented in the Chem. Rev. review (440 cm²/C), it is about 23 C/m²Our calculation. If the window switches in 5 minutes (300 s), the average currents are about 0.69 A, 0.39 A and 0.076 A respectivelyOur calculation (1 C = 1 A·s; the charge entering and leaving the counter electrode is assumed equal to this).

Commentary: if current is fed from the edge of a large window, the voltage drops across the resistance of the transparent conductor, and the colouring speed may differ between the edge and the centre. Window size, conductor resistance and current become constraints on the uniformity of colouring and on the switching time. Quantitative figures for this could not be confirmed in our primary sources. For transparent conductive materials, see our explainer on transparent conductive films.

7. How they are used — buildings, auto-dimming mirrors, aircraft

Windows in buildings: energy saving and comfort

A 2004 report by LBNL (Lawrence Berkeley National Laboratory) used DOE-2.1E simulation on a three-story prototypical office building in five US climates and 16 California climate zones. It compared an EC window controlled to hold daylight illuminance at a set level with a spectrally selective low-e window (with daylighting controls, no interior or exterior shading) and reports that, in a building with a large window-to-wall area ratio of 0.60, perimeter-zone primary energy use in east, south and west zones falls by 10–20%, and peak demand under the same condition by 20–30%Sourced [Source 10] (simulation results, not a guarantee for real buildings). A 2006 report also includes a 2.5-year outdoor test of prototype large-area EC windows. Compared with an efficient low-e window with the same daylighting control, it states, annual peak cooling load fell by 19–26% and lighting energy use by 48–67% when controlled for visual comfort. Subjects strongly preferred the EC window, with preferences related to perceived reductions in glare, reflections on the computer monitor and window luminanceSourced [Source 9]. LBNL’s 2006 design guide says it compiles, for designers, basic information on what an EC window looks like, how fast it switches and what product offerings then existed, from data drawn from simulations, laboratory tests and a 2.5-year outdoor testSourced [Source 11].

On the manufacturing side, Soladigm’s (later View) final report to DOE (2011) says it moved from a 2-inch research prototype to inline manufacture of full-scale commercial insulating glass units and that its technology exceeds DOE’s 2020 performance and reliability targets (a company report; the content and figures of those targets were not checked here)Sourced [Source 12]. SageGlass’s product page lists “SageGlass Classic”, which tints the full pane uniformly, and “SageGlass Harmony”, which tints with an in-pane gradient, and says tinting is controlled by a predictive algorithm and weather data (the product page’s description)Sourced [Source 18].

Cars: from anti-glare mirrors to dimmable components

Gentex describes its auto-dimming mirrors (interior and exterior) as using light sensors, proprietary gels and microprocessor-based algorithms to detect glare and darken to the level needed. Its “Dimmable Glass” page lists, as uses of the same EC technology, self-dimming sunroofs and moonroofs, sun visors, head-up-display combiners and sensor shrouds (the company’s description)Sourced [Source 16]. Byker’s patent lists, among uses of EC devices, variable-transmittance light filters and variable-reflectance mirrors such as anti-glare rear-view mirrors of automotive vehiclesSourced [Source 5].

Aircraft: windows that “darken electrically”

Gentex presents its electronically dimmable windows (EDW) for aircraft as windows that darken on demand to cut sunlight and glare while keeping the view. It cites no moving parts; that the windows fail clear, making them FAA compliant for emergency-exit integration; a wide visible-transmittance range with light-blocking efficiency above 99.999% and low haze; a lower cabin heat load; and reduced acoustic transmission into the cabin (the company’s description; measurement conditions, standards and independent confirmation are not given)Sourced [Source 17].

Company descriptions are the companies’ claims, not independent confirmation.

8. A materials engineer’s view 2: controlling visible light and near-infrared separately

9. How thermochromic and photochromic systems differ

Smart-window materials fall into three groups by what “triggers” the colour change. The review by Wang et al. sets out the basic principles and recent developments of electrochromic, photochromic and thermochromic materials for smart windowsSourced [Source 3].

Thermochromic: the review by Cui et al. says vanadium dioxide (VO₂) is a promising material for energy-saving smart windows because of its reversible metal-to-insulator transition near room temperature with large changes in optical properties. It surveys the effects of atomic defects, doping and lattice strain and nano- and micro-scale morphology engineering, and sets out the challenges of bridging the gap between laboratory research and large-scale practical applicationSourced [Source 14]. Sheng et al. note that VO₂ has a fixed transition temperature (Tc) of 68 °C and co-assembled tungsten-doped W–VO₂ nanowires to show a window responding over a wide range of 30–50 °CSourced [Source 7]. Photochromic: the IUPAC technical report defines organic photochromism as a reversible colour change by light and identifies fatigue (chemical degradation) and the number of cycles, cyclability and half-life as the main evaluation parametersSourced [Source 15]. The difference from EC: ISO 18543 states that it is applicable to chromogenic glazings that switch by electrical stimulus and is not applicable to photochromic and thermochromic glazingsSourced [Source 19].

10. Laboratory performance versus deployment

11. Limits, and what could not be confirmed

(1) Performance figures of products

The colouring and bleaching times, cycle life, warranty, cost and in-pane uniformity of the products of SageGlass, Gentex and Soladigm (View) could not be confirmed numerically in the primary sources we checked. Company descriptions are quoted as they stand.

(2) The texts of reviews

Granqvist’s reviews (Thin Solid Films 2014; Sol. Energy Mater. Sol. Cells 2012) and the review of conductive polymers by Beaujuge and Reynolds (Chem. Rev. 2010) were not cited, because neither text nor abstract could be checked. We checked the abstracts of Niklasson and Granqvist, Mortimer, Wang et al. and Cui et al., and the open full texts on PubMed Central of the Chem. Rev. review (2022) and of Sheng et al. (2023).

(3) Reports of the US Department of Energy and LBNL

The DOE and LBNL reports were checked through OSTI’s catalogue records (abstracts). The full texts (PDF) could not be read because the service did not respond when we checked. The figures are limited to those given in the abstracts.

(4) The texts of standards

We have not checked the texts (which are paid) of ISO 18543, ISO 9050, JIS R 3106 and JIS D 5705. Test conditions, cycle counts and pass criteria are not described.

(5) Other applications

EC displays (electronic paper), devices combined with energy storage, metal-electrodeposition types (reversible metal electrodeposition), other electro-optical dimming methods such as liquid crystals and SPDs, wearables and fibre-shaped devices are not covered here.

12. How to read evaluations and standards

What is measured: EC materials and devices are evaluated for optical modulation (the difference in transmittance), coloration efficiency (modulation per injected charge), response time (time to 90% of the modulation), cycle life and durability, from transmission and reflection spectra in the coloured and bleached states and from electrochemical measurements (potential, current, charge). For window glass, the visible transmittance, solar transmittance and solar heat gain coefficient are calculated from spectral measurements for each coloured state.

Which standard we match to which item is our own arrangement (Our calculation). Numbers and titles were checked on the official ISO page for ISO and the JISC JIS search for JIS. For ASTM E2141 we give only the number, edition and subject as written in a non-ASTM public source (the Chem. Rev. review); the ASTM website was not opened.

How the standards differ in standing

• ISO is an international standard; JIS is Japan’s national standard. The correspondence between JIS R 3106 and ISO 9050, and whether JIS D 5705 applies to EC mirrors, were not checked here
• ASTM E2141 is a US industry standard. It is a separate standard from ISO 18543, and we did not check whether the content of the tests (temperatures, cycles and so on) matches
• ISO 18543 is a standard for glazing, not for a material by itself. It looks at the durability of the assembled EC window and does not compare the merits of materials such as WO₃ or viologens (our commentary)

Cautions in reading the standards

• A standard is a test method and requirements, not a guarantee of lifetime. We did not check the text of ISO 18543 (test conditions, number of cycles, pass criteria). If you see a claim that a product conforms to ISO 18543, check the edition and what the test involved
• The same named metric can differ in method and conditions. Response time is defined as the time to 90% of the modulation, yet some reports do not reach the full modulation [6]. Cycle life changes with voltage, wavelength and end point [13]
• Read a manufacturer’s value together with the standard, edition and test conditions it follows. Gentex’s “light-blocking efficiency above 99.999%” and “fail clear” for aircraft windows come with no measurement conditions or standards [17]
• The gap between a standard’s range and conditions of use: ISO 9050 covers luminous and solar characteristics with angular properties excluded [20]. In-plane non-uniformity of colouring, oblique incidence and long-term outdoor degradation are not in that standard’s values (our commentary)

13. Glossary

14. References (primary sources)

1. Niklasson G. A., Granqvist C. G. “Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these”, J. Mater. Chem. 17, 127–156 (2007), DOI 10.1039/B612174H — doi.org/10.1039/B612174H
2. Mortimer R. J. “Electrochromic Materials”, Annu. Rev. Mater. Res. 41, 241–268 (2011), DOI 10.1146/annurev-matsci-062910-100344 — doi.org/10.1146/annurev-matsci-062910-100344
3. Wang Y., Runnerstrom E. L., Milliron D. J. “Switchable Materials for Smart Windows”, Annu. Rev. Chem. Biomol. Eng. 7, 283–304 (2016), DOI 10.1146/annurev-chembioeng-080615-034647 — doi.org/10.1146/annurev-chembioeng-080615-034647
4. Llordés A. et al. “Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites”, Nature 500, 323–326 (2013), DOI 10.1038/nature12398 — doi.org/10.1038/nature12398
5. Byker H. J. (applicant: Gentex Corp) “Single-compartment, self-erasing, solution-phase electrochromic devices, solutions for use therein, and uses thereof”, US Patent 4,902,108 (filed 31 March 1986, granted 20 February 1990) — patents.google.com/patent/US4902108A
6. Gu C. et al. “Emerging Electrochromic Materials and Devices for Future Displays”, Chem. Rev. 122, 14679–14721 (2022), DOI 10.1021/acs.chemrev.1c01055 (checked in the open full text on PubMed Central) — doi.org/10.1021/acs.chemrev.1c01055
7. Sheng S.-Z. et al. “Nanowire-based smart windows combining electro- and thermochromics for dynamic regulation of solar radiation”, Nat. Commun. 14, 3231 (2023), DOI 10.1038/s41467-023-38353-4 (open access) — doi.org/10.1038/s41467-023-38353-4
8. Boykin C. et al. “Energy Efficient Electrochromic Windows Incorporating Ionic Liquids” (report to the US Department of Energy, OSTI, November 2008) — doi.org/10.2172/1001225
9. Lee E. S. et al. (LBNL) “Advancement of Electrochromic Windows” (LBNL report, OSTI, April 2006) — doi.org/10.2172/899732
10. Lee E. S., Yazdanian M., Selkowitz S. E. (LBNL) “The energy-savings potential of electrochromic windows in the US commercial buildings sector” (LBNL report, OSTI, April 2004) — doi.org/10.2172/891618
11. Lee E. S. et al. (LBNL) “A Design Guide for Early-Market Electrochromic Windows” (LBNL report, OSTI, May 2006) — doi.org/10.2172/898284
12. Rozbicki R. (Soladigm) “Soladigm DOE Final Scientific/Technical Report” (OSTI, December 2011; a company report) — doi.org/10.2172/1060493
13. Padilla J. et al. “Quantitative Assessment of the Cycling Stability of Different Electrochromic Materials and Devices”, ACS Appl. Opt. Mater. 1, 1174–1183 (2023), DOI 10.1021/acsaom.3c00087 — doi.org/10.1021/acsaom.3c00087
14. Cui Y. et al. “Thermochromic VO₂ for Energy-Efficient Smart Windows”, Joule 2, 1707–1746 (2018), DOI 10.1016/j.joule.2018.06.018 — doi.org/10.1016/j.joule.2018.06.018
15. Bouas-Laurent H., Dürr H. “Organic photochromism (IUPAC Technical Report)”, Pure Appl. Chem. 73, 639–665 (2001), DOI 10.1351/pac200173040639 — doi.org/10.1351/pac200173040639
16. Gentex Corporation “Dimmable Glass” (automotive product page; the company’s description) — gentex.com/products-technology/automotive/dimmable-glass
17. Gentex Corporation “Electronically Dimmable Windows” (aerospace product page; the company’s description) — gentex.com/products-technology/aerospace/electronically-dimmable-windows
18. SageGlass (Saint-Gobain) Product page (descriptions of SageGlass Classic and Harmony) — sageglass.com/en
19. ISO “ISO 18543:2021” page — iso.org/standard/75536.html
20. ISO “ISO 9050:2026” page — iso.org/standard/88642.html
21. Japanese Industrial Standards Committee (JISC) JIS search listing for “JIS R 3106” — jisc.go.jp (JIS R 3106)
22. Japanese Industrial Standards Committee (JISC) JIS search listing for “JIS D 5705” — jisc.go.jp (JIS D 5705)

15. Claim-to-source audit

Last updated 10 October 2026. Sources are limited to primary material (peer-reviewed papers, a patent, reports of the US Department of Energy and LBNL, official pages of ISO and JISC, and company product pages). The texts of standards are paid and could not be checked. The ASTM International website was not opened. Because the article includes structural readings and materials-design interpretations, those are marked as Commentary and kept separate from sourced fact. The colouring and bleaching times, cycle life, warranty and cost of products, the title and test content of ASTM E2141, a JIS standard for EC glass and the texts of standards could not be confirmed and are not stated here. The figures are results for particular samples and conditions, simulation results or company statements, and cannot be generalised to other devices, products or real buildings. All figures are explanatory concept graphics. Figs. 2, 3 and 5 are vector drawings; Figs. 1 and 4 are AI-generated images, and none of them shows a real photograph, measured data or a physical product.