[Green Energy and Technology] Energy Efficiency and Renewable Energy Through Nanotechnology || Window Glass Coatings

Window Glass Coatings

Jitka Mohelníková

Abstract Thin film coatings modulate optical and thermal and other propertiesof window glass. Coated window glasses influence indoor climate and energyefficiency in buildings. They can be classified into groups of spectrally selectiveglazings, chromogenic glazed systems for switchable smart window technolo-gies and light-enhancing transparent materials. Low-emissivity glazings aretypical of spectrally selective window glasses. Chromogenics have applicationsin light control and switching technologies that are optically, thermally,chemically or electrically activated. Transparent materials with micro-structuredor holographic films and antireflective coatings serve to direct light and controllight functions. An overview of several types of window glazings and coatingswill be presented.

1 Introduction

Energy savings and motivation toward solar energy utilization in buildings broughtforth the development of advanced glazing materials [1–3]. These glazings withthin film coatings and selective surfaces have ability to modulate the properties ofwindow glass. Special windows glazing influences the climate in the buildings inwhich they are used. This is why they have found wide applications in architecture[4, 5]. They can be used for:

J. Mohelníková (&)Faculty of Civil Engineering, Brno University of Technology,Veverí 95, 602 00 Brno, Czech Republice-mail: [email protected]

L. Zang (ed.), Energy Efficiency and Renewable Energy Through Nanotechnology,Green Energy and Technology, DOI: 10.1007/978-0-85729-638-2_26,� Springer-Verlag London Limited 2011

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• limiting heat loses and maximizing solar gains and natural lighting in buildingsto reduce energy consumption for heating and artificial lighting,

• reducing glare and lowering the energy demands for cooling and air-conditioning,

• reducing the amount of cleaning water and solvents due to reduction in main-tenance requirements in glazed windows.

Glazings can be classified into three main groups with respect to their appli-cations in buildings [6–8]:

• spectrally selective glazings (e.g., glazings with modulated transmittance,reflectance and absorbance in the visible or infrared spectral range),

• chromogenic glazings (i.e., switchable glazings for dynamic light control),• light-enhancing glazings (e.g., light reflectors, collectors or wave-guiding trans-

parent materials, antireflective glass and glazings with self-cleaning surfaces).

2 Spectrally Selective Glazings

Spectrally selective coatings modulate the spectral properties of glass substrates.For window glazing applications they are represented mainly by thin films thatreduce absorption of infrared radiation within glass panes. They have very lowemissivity (between 0.2 and 0.05) compared to the emissivity of common windowglass 0.84 [9–11]. These thin films are called low-emissivity (low-e) coatings.The major requirements for these coatings are:

• high transmittance in visible spectral range,• high reflectance in infrared spectral region.

Low-emissivity coatings are classified into two main groups [9–15]:

• Coatings reflective in infrared spectral range (between 2 and 10 lm). This typeof low-e glazing is also called a heat mirror or a winter film. They serve toeliminate heat radiation losses of windows or collect thermal energy.

• Coatings reflective in near-infrared part of the solar radiation spectrum inspectral range between 0.78 and 2.5 lm. They are called solar controlling low-emissivity coatings or summer films. Their main application is reduction of solarthermal loads.

2.1 Low-Emissivity Glazings

Low-emissivity (low-e) glazings have special coatings deposited on their surfaces.Low-e coatings operate as transparent heat mirrors. They can be deposited as[11, 14, 15]:

914 J. Mohelníková

• doped oxides semiconductor coatings,• conducting micro-grid coatings,• multilayer thin films consisting of metal and transparent dielectric layers.

Doped oxide semiconductor coatings consist of dielectric layers with mobilecharge carriers. The coatings have high reflectance in the infrared spectral rangeand good transmittance of visible wavelengths. Materials such as SnO2:F,In2O3:Sn, SnO2:Sb, ZnO:Al or Cd2SnO4 are used for these coatings [11, 16–19].

The transparent heat mirror coatings can be also designed as micro-grid thinconducting films with small openings (around 2.5 lm). These openings transmitsolar radiation and the rest parts of the coating reflect infrared radiation. This typeof coatings has not found wide applications for window glass panes [19].

Multilayer coatings consisting of a metal film sandwiched between transparentdielectric layers are recommended for window glazings. Metals have highreflectance and low-emissivity in the infrared spectral range [20]. Figure 1 showsspectral reflectance of selected thin metal films (Aluminium-Al, Silver-Ag,Gold-Au and Copper-Cu) of equal thicknesses deposited on a glass substrate [21].The reflectance curves are compared in the visible spectral range (380–780 nm)and in the part of infrared range between 1,000 and 15,000 nm.

It is obvious that the most apt materials are silver and gold layers. The thinsilver and gold layers both have high visible transmittance and high reflectance inthe infrared spectral range. Silver is the most adept metal for low-e coatingsbecause of its high infrared reflectance and low light absorbance [21]. Aluminiumis ill suited as a glazing component due to its higher reflectance and reducedtransmittance in the visible range.

Dielectric coatings protect the thin metal layer and influence light transmittanceof the low-e coatings. Spectral transmittance of a thin metal film (M) comparedwith transmittance of the same metal film with single and double dielectric (D)layers is presented in Fig. 2 (computer simulation [22]). Dielectric layers have ananti-reflective property. The top dielectric layer on the metal film has higherinfluence on visible transmittance.

Dielectric materials with high refractive indices, such as TiO2, SnO2, SnBO2,In2O3, ZnO, ZnS, Si3N4 and Bi2O3 [23, 24], can be used in low-emissivity

Fig. 1 Spectral reflectance of silver (Ag), aluminum (Al), gold (Au) and copper (Cu) thin filmson glass, a visible spectral range 380–780 nm, b infrared spectral range 1,000–15,000 nm

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coatings. Multilayer low-e coatings use combinations of dielectric materials withhigh and low refractive indices, for example, TiO2 and SiO2.

The materials selected as layers in a low-e coating influence its visible trans-mittance. Figure 3 presents a spectral transmittance of a low-e coating with metal(M) layer and dielectric layers of materials with high (Dh) and low (Dl) refractiveindices (computer simulation [22]).

The light transmittance of the five-layer Dh/Dl/Dh/M/Dh/Dl/Dh coating is veryhigh but relatively narrow-band compared to the three-layer Dh/M/Dh coating ofthe same material composition, Fig. 3.

An optimized design of the multilayer composition of dielectric and metallayers yields visible transmittance and infrared reflectance coatings [23–26].Examples of low-e coatings include [14, 27–33]:

• a central silver layer between two dielectric layers:glass/ZnO/Ag/ZnOglass/ZnS/Ag/ZnSglass/TiO2/Ag/TiO2

glass/SnO2/Ag/SnO2

glass/Bi2O3/Ag/Bi2O3

Fig. 2 Spectraltransmittance of thin filmswith metal (M) and dielectric(D) layers deposited on glass

Fig. 3 Influence of dielectriclayers on transmittance ofa thin low-e coating (M-metallayer, Dh-dielectric layerwith high refractive index,Dl-dielectric layer withlow refractive index)


• a silver layer and three dielectric layers of two different materials: glass/TiO2/Ag/TiO2/SiO2

• a copper layer between two dielectric layers: glass/SnO2/Cu/SnO2

• a titanium nitride layer between two dielectric layers: glass/TiO2/TiN/TiO2

• a zirconium nitride layer between two dielectric layers: glass/ZrO2/ZrN/ZrO2

• a gold layer between two dielectric layers: glass/In2O3/Au/In2O3.

The thin gold film is inert but metal coatings of silver or copper are notchemically stable. Their optical properties degrade by thermal oxidation andcorrosion caused mainly by atmospheric pollutants such as chlorine and sulphur[33]. The metal layer’s chemical stability and durability is increased with a pro-tective layer [34]. The metal layer is protected by a barrier layer or blocker layer[23]. A barrier layer should be deposited on one or both metal-dielectric interfaces.Materials that can be used to protect the metal layers against corrosion include Si,Ti, TiN, TiAOx, NiCr, NiCrOx, Cr, Zr, Mo, W and ZrSi [23, 35–37].

Examples of characteristic compositions of low-e window coatings with pro-tective layers:

• glazings with a single silver and a single blocker layer [35–37]:glass/SnO2/Ag/NiCrOx/SnO2 or glass/ZnO/Ag/TiAOx/SnO2, glass/SnBO2/ZnO/Ag/NiCrOx/SnBO2 or glass/TiO2/ZnO/Ag/NiCrOx/Si3N4

• glazings with a single silver and double blocker layer:glass/SnO2/TiAOx/Ag/TiAOx/SnO2 [37].

Low-e window coatings can be designed as single, double or even triple metallayer films [38, 39]. Multi-layered dielectric-metal coatings serve as the broadbandinfrared (IR) reflectors for low-emissivity glazings. It means these glazings caneliminate thermal radiation losses, provide near-IR solar control and substituteconvenient transmittance in the whole visible spectrum [40]. Compositions of suchmultifunctional coatings include:

• glass/SnO2/ZnO/Ag/NiCrOx/SnO2/Ag/NiCrOx/SnO2 [37]• glass/ZnO/Ag/Ti/ZnO/Ag/Ti/ZnO/TiO2/ZnO [40]• glass/TiOx/ZnOx/Ag/NiCrOx/TiOx/ZnOx/Ag/NiCrOx/SiNx [41, 42], the better

thermal and mechanical stability of this coating is achieved by dividing the firstlayer and middle layer of titanium oxide and also two silver layers in thefollowing way glass/TiOx/NiCrOx/TiOx/ZnOx/Ag/NiCrOx/Ag/NiCrOx/TiOx/NiCrOx/TiOx/ZnOx/Ag/NiCrOx/Ag/NiCrOx/SiNx [41, 42]

• glass/Si3N4/Ni:Cr/Ag/Ni:Cr/Si3N4/Ni:Cr/Ag/Ni:Cr/Si3N4 [43]• glass/TiOx/Si3N4/NiCr/Ag/NiCr/Si3N4/SnO2/ZnO/Ag/NiCrOx/SnO2/Si3N4 [44]

Figure 4 shows spectral transmittance of three different compositions of thelow-e coatings with single, double and triple silver layer and TiO2/SiO2 layers inthe following compositions (computer simulation [22]):

• single silver coating: glass/TiO2/Ag/TiO2

• double silver coating: glass/TiO2/Ag/TiO2/SiO2/TiO2/Ag/TiO2

• triple silver coating: glass/TiO2/Ag/TiO2/SiO2/Ag/SiO2/TiO2/Ag/TiO2


2.2 Spectrally and Angularly Selective Solar Control Glass

Solar control glazings reduce overheating and glare [45]. Glazings with spectrallyselective coatings are reflective or absorptive in the near-IR range of solar radiationspectrum. Special coatings can also influence angular selectivity of window glass.General requirements for the design of solar control reflective coatings are visibletransmittance and high near-IR reflectance. These properties are achieved by fab-ricating thin dielectric/metal/dielectric films with metal layers, such as Ag, Cu, Au,or TiN and dielectric layers of Bi2O3, In2O3, SnO2, TiO2, ZnO or ZnS [9, 14].

Solar control coatings can also consist of absorbing and reflecting materials,such as CrNx, TiNx, and FeNx, embedded in layers of a material with a highrefractive index [46]. A thin film of CuxS can be also used for solar controlpurposes [47].

The near-IR absorbing thin film and a low-e layer within one coating is anotherpossibility [48, 49]. The coating consists of SnO2 layer (deposited on the glasssubstrate) with a dopant of antimony, tungsten, vanadium, iron, chromium,molybdenum, niobium, cobalt or nickel and the low-e layer of SnO2 containing adopant of fluorine or phosphorus [48, 49].

Special near-IR absorbing glazings include a light absorptive resin laminatedbetween two glass panes [50]. The transparent acrylic resin contains copper ions,which reduce solar transmittance in the near-IR spectrum and allow for hightransmittance of visible light.

Angular selective coatings can also be designed for solar control glazings.Angle selective coatings allow for seasonal self-regulation of window glazings.They reflect part of direct solar rays affecting glass panes at high angles ofincidence, which happens in the summer, and transmit diffusive and low-angleincidence direct solar radiation characteristic of winter sunlight [51–54]. Coatingswith angularly selective oblique columnar microstructures are used for this pur-pose. These coatings are based on materials such as Al, Cr, Ta and cernet [54–57].Affected solar radiation is scattered or directly transmitted by the columnar micro-structured coating, dependent upon the angle of incidence. The scattering of directsolar radiation eliminates glare. Scattering surface patterns and thin films are

Fig. 4 Spectraltransmittance of single,double and triple silverlow-e coatings


applied for special glare protection glazings [58, 59]. Such coatings reduce theneed for cooling in summer seasons.

3 Chromogenic Glazings

Chromogenic glazing materials offer solar control switching activated optically,thermally, chemically or electrically [7, 60, 61]. Switchable glazings have manyapplications including architectural glazed roofs and façades, automotivewindscreens and sunroofs or aircraft windows and displays [61].

Window chromogenic glazings allow dynamic changes in solar transmittancedependent upon external conditions, such as solar radiation intensity or tempera-ture variations. Chromogenic glazings fall into two categories:

• non-electrically activated devices: photochromic (optical activation), thermo-chromic and thermotropic (thermal activation) and gasochromic glazings(chemical activation). Some of the glazings can also be connected to an externalswitch, allowing for user-defined operation with electronic control.

• electrically activated devices, such as electrochromic, electrochromic-photo-voltaic glazings, photoelectrochromic glazings (they can also be applied in anelectrically non-activated variation) and glazing devices with liquid crystals orsuspended particles.

3.1 Photochromic Glazings

The photochromic phenomenon is a reversible change between two energy statesof a material due to light absorption [62]. Photochromism has been known since1880s [62–64]. Photochromic glass color centers are activated under radiationexposure, causing glass coloration (darkening). The destruction of the color cen-ters of the photochromic glass occurs in the state without radiation present, whenthe glass is in the initial transparent state. Color centers are activated in response toexposure to ultraviolet and short-wave visible radiation. The destruction of colorcenters is influenced by long-wave visible and short-wave infrared radiation.

Photochromic materials are metal halides, such as silver, copper halides, cad-mium halides, europium or cerium. Chromium, molybdenum or tungsten and themineral hackmanit can be also used [62–65]. Investigations in the kinetics ofphotochemical reactions of photochromic glasses confirm desirability of silverhalides [64, 65] as AgCl or AgBr. Another group of photochromics is organicmaterials. Examples include certain dyes, stereoisomers and polynuclear aromatichydrocarbons [66, 67].

The best-known variation is ophthalmic photochromic glass. Photochromicglasses have few applications for large-area glazings in buildings [62–68].A glazed photochromic element with special thin film based on magnesium–nickel


alloy has been developed [68] and is expected to be implementable as a windowcoating [69]. The diffuse-reflection type of photochromic thin film (e.g. SnO2)forms pyramidal crystal grain projections. This irregular surface is coated withMg6Ni layer. In a double glass unit configuration, the photochromic film faces aninternal cavity. The cavity is filled with diluted hydrogen which influencesreflective and transparent state variations of the unit.

The aforementioned photochromic material, SnO2, implemented in the follow-ing device, glass/SnO2:F/electrolyte/Pd/Mg6Ni/irregularly-shaped SnO2:F/glass,can be activated electrically when connected to an appropriate controller [69].

Another special photochromic device features user-controllable light trans-mittance. This device bleaches at the user’s discretion [70]. Such a device iscomprised of a radiation sensitive electrode/interconnecting medium/ion interca-lative layer, embedded between two panes of glass with deposited transparentconductive oxide (TCO) layers. An electrical connection exists between the twoTCO electrodes. Radiation sensitive electrode materials include zinc–cadmiumsulfide and titanium oxide with dopants Ta, Nb, Sb, V and Ru [70].

3.2 Thermochromic and Thermotropic Glazings

Thermochromic glass alters its optical properties with response to temperaturechanges [71–73]. Thermochromic materials’ colors are modified by a thermallyinduced chemical reaction or by a phase transformation. The phase transformationfrom semiconductor state (high visible transmittance) to metallic state (highinfrared reflectance) occurs in transition metal oxides. Thermochromic coatings inthe metallic state have similar properties as low-emissivity coatings.

Organic and inorganic compounds are known for their thermochromic prop-erties [72, 73]. Such organic compounds are anil, spiropyrans, polyvinyl acetalresins and hydroxide groups. Examples of inorganic thermochromic compoundsinclude AgI, Ag2HgI4, Cd3P3Cl, HgI, HgI2 and SrTiO3. Vanadium dioxide VO2

has been used for thermochromic window applications [7, 74–78]. Reversiblemetal-to-semiconductor phase transformation can be achieved by doping VO2.Tungsten is a frequently used dopant [76, 77]. Thin films VO2:Mg [78] andmultilayer coatings TiO2/VO2/TiO2/VO2/TiO2 [7] have also been investigated forwindow coating applications.

Thermotropic glazings change their translucence with temperature changesfrom clear states to light-scattering, non-transparent states. Thermotropic devicescontain special gels embedded between two glass panes. The gels influence theradiation and conductive heat transfer through the glazing. They consist of ther-motropic materials of two components with different refractive indices [79–83].These components are homogenous for low temperatures (transparent state).If temperature rises above the limit value (between 20 and 50�C) the componentsare phase-separate, forming very small nanoparticles. The separation causesscattering of solar radiation, leaving a non-transparent, white-colored glazing [83].


3.3 Gasochromic Glazings

Dynamic transmittance changes of gasochromic glazings are caused by chemicalreactions of hydrogen with wolfram oxide [83–88]. Gasochromic devices consistof a double-glazed unit with connections to a gas supply unit and an electroniccontrol unit. An example of a gasochromic double-glazed unit compositionis: glass/WO3/Pt/cavity filled with gas (e.g. H2, O2)/glass [86, 87]. The opticallyactive component of the gasochromic glazing is a thin porous WO3 film. The Ptlayer serves as a catalyst. The glazed unit changes color when the gasochromicfilm is exposed to a low concentration of H2. The bleaching process that producesa transparent state occurs with exposure of diluted O2. The following chemicalreaction WO3 (colorless) ? xH2(Cat) ? 2xH+ ? WO3 ? H2xWO3 (blue) causesblue coloration of the gasochromic glazing [88]. Color intensity depends on theWO3 film thickness and hydrogen concentration. The effect of the gasochromiccoloration is also used in the aforementioned photochromic reflective light controlelement [69].

A switchable gasochromic mirror device based on Pd/Ni-WO3 anodic doublelayer coating deposited on the glass has been developed [89]. The hydrogen gasdissociates from the Pd catalyst into H atoms, which diffuse into the Ni-WO3 film,causing coloration.

3.4 Electrochromic Glazings

The electrochromic phenomenon has been the subject of many scientific researchprograms and investigations [90–110]. Electrochromic (EC) smart window glaz-ings are just one possible application of this phenomenon.

EC glazings reversibly change their optical properties under an applied voltage.The presence of electric current causes variation of the material’s chemicalcomposition and its optical properties. EC glazings require only a small voltage(between 1 and 5 V) for switching. They are transparent when no voltage isapplied. A typical composition of EC glass is the following [93–95]: glass/trans-parent electrode (TCO)/EC layer/central electrolyte-ion conductor/ion-storagelayer/transparent electrode (TCO)/glass.

The switchable process of glazing coloration and bleaching is caused by ionmigration between the EC and ion-storage layer. Ions of the EC layer move towardthe ion-storage layer through the central electrolyte when voltage is appliedbetween two transparent electrodes. The ions transport to the ion-storage layercauses EC glazing coloration. A change in electric polarity causes ion flow in theopposite direction, causing bleaching of the EC device.

Many materials have been tested for EC applications. Typical materials whichare used for EC devices are [90–105], for example:


• EC layer: inorganic materials, such as WO3, LixWO3, HxWO3, NiO or organicmaterials, such as viologen,

• ion conduction layer: LiClO4 ? PC, Ta2O5, PMMA ? organic,• ion-storage layer: WO3, WO3:MO, Nb2O5, NiO, Prussian Blue, LiyV2O5,• transparent electrode: In2O3:Sn or SnO2:F.

The coloration process is achieved by the oxidation–reduction reaction inorganic EC materials [103]. The EC effect occurs in inorganic metal oxides due tothe dual injection (cathodic) or ejection (anodic) of ions and electrons (e-).A typical reaction for the cathodic coloring material is:

WO3 (transparent) ? yH+ ? ye- $ HyWO3 (blue) [103, 104]Ions such as Li+, Na+ and Ag+ can also cause EC coloration.A typical coloring anodic reaction is, for example:

LiyV2O5 light yellowð Þ� yLiþ � ye� $ V2O5 blueð Þ [103]

or Ir OHð Þx transparentð Þ $ IrOx blackð Þ + XHþ + xe� [104]

Possibilities of enhancing an EC glazing visible transmittance in its transparentstate can be achieved through adding Al or Mg to the Ni oxide [106] or antire-flective layers [107]. EC safety glass laminated with ion-conducting PVB (Poly-vinyl butyral) sheets [108] and flexible polyester-based foils with EC coatings[109] have also been developed.

3.5 Photoelectrochromic Glazings

Photoelectrochromic glazings combine electrochromic and photochromic princi-ples [110–114]. Such glazing units consist of an electrochromic material (WO3,IrOx, V2O5 or NiO) and a semiconductor film (TiO2, CdS, ZnS, ZnO or WO)[110]. The composition of the photoelectrochromic glazing can be [111, 112]:glass/transparent electrode/WO3 layer/nanoporous TiO2 layer/dye monolayer/electrolyte LiI/Pt layer/transparent electrode/glass.

The thin film of TiO2 is porous and covered with a dye monolayer. The dye isexcited under illumination and donates electrons to the TiO2, which conduct themto the WO3 layer. This reduces the tungsten and changes its color from transparentto blue. The Pt thin film catalyzes the reverse reaction. The process takes placeunder illumination. This means that the whole system operates as a passivephotochromic element. The device can also have transparent electrodes connectedto an external switch. Electrons from the WO3 layer can flow to the counter-electrode in case of the closed external switch (darkening state). The device istransparent in the electrically non-activated state.


3.6 Electrochromic and PV Glazing

Photovoltaics can be used as power sources for switchable glazings, creating acomplete smart window system [115]. Another interesting application of photo-voltaic (PV) technology for windows is the integration of PV thin films of specialcolors into the glazings [116]. An electrochromic (EC) device powered by anintegrated amorphous silicon carbide (a-SiC:H) PV film is an example of aphotovoltaic/electrochromic (PV/EC) glazing [117–119]. The PV part serves as asemitransparent power supply and the EC thin film serves as a modulator oftransmittance. These two parts of the PV/EC coating consists of:

• PV part: TCO/a-SiC:H-n/a-SiC:H-i/a-SiC:H-p• EC part: TCO/V2O5 (ion-storage layer)/LiAlF4 (ion conductor layer)/Liy-

WO3(EC layer)/TCO

The whole composition of a PV/EC glazing can be as follows: glass/TCO/a-SiC:H-n/a-SiC:H-i/a-SiC:H-p/TCO/V2O5/LiAlF4/LiyWO3/TCO. The device isdarkened when the top and bottom TCO layers are electrically connected viaexternal switch. The middle TCO layer is used for battery charging and usercontrol [119].

3.7 Liquid Crystal and Suspended Particle Glazings

Liquid crystals (LC) are materials comprised of thin, needle-shaped organicmolecules that are randomly distributed and flow like liquids. Under certainconditions they could be aligned and ordered. They exist in several liquid crys-talline phases, such as [120]:

• nemanic phase (LC orientation order but no position order),• cholesteric phase (LC local orientation in a helical or spiral configuration),• smetic phase (LC orientation and position order).

LC orientation alters with electric field. Orientation influences the light trans-mittance of the LC glazing. There are several types of LC used in switchabledevices. Polymer dispersed liquid crystals (PDLC) and nemanic curvilinearaligned phase of encapsulated liquid crystals (NCAP) are used in large-areaglazing applications [63, 120, 121]. Guest-host LC represents another possibilityfor windows [63]. PDLC have wide applications for architectural and automotiveglazings [122].

These window glazings have LC within an index matched polymer matrixbetween two transparent conductive oxides TCO electrodes. The composition ofthe LC glazing can be: glass/TCO/dielectric layer/PDLC layer/dielectric layer/TCO/glass [14, 124]. LC molecules are dispersed between two conductive elec-trodes. Activation of the electric field causes alignment of the LC. This allows forenhanced light transmittance. The device appears translucent and white in the


off-state. LC glazings require electrical activation to be in the transparent state[14, 121–126].

A new LC glazing technology which does not require constant power has beendeveloped. A reverse mode of PDLC glazing is achieved by doping with photo-conductive molecules in the polymer LC matrix [127, 128, 129].

Suspended particle glazings also require electrical activation to reach trans-parency. They are not transparent in the non-activated state [130–136]. A sus-pended particle (SP) switchable device consists of glass/TCO/dielectric layer/SPlayer/dielectric layer/TCO/glass [130, 131]. Polyester sheets can be also usedinstead of the glass panes.

The active SP layer has needle-shaped dipole particles (less l lm long). Theyare suspended randomly in an organic fluid or gel in the electrically non-activatedstate, causing absorption and scattering of incident light. The suspended particlesalign if an electric field is applied, causing bleaching of the device.

SP glazing with modified particles could provide darkening in several colors.SP glazings with encapsulated particles in polymers and laminated the compositebetween polyester sheets show promise for large glazed areas applications [136].

4 Light Enhancing and Controlling Glazings

Modern solar technologies offer light enhancement and control systems. Thesesystems serve to transport and redirect light into the interiors of buildings toimprove indoor visual comfort. Optical fibers, transparent materials with micro-structured surfaces or holographic films, fluorescent concentrators and mirrors canguide, redirect and concentrate light [137–140].

These materials integrated into façades and glazed roofs can eliminate glare andoverheating problems in building interiors. They can also be used for solar con-version systems [140, 141]. Combinations of prismatic materials and switchabletechnology provide smart solar control transparent devices [116, 142–149].

Enhancement of light and solar transmittance in window glass can be achievedthrough antireflective coatings [150–158]. Special self-cleaning functional coat-ings also positively influence light transmittance of windows [159–169].

4.1 Switchable Devices and Prismatic Surfaces

Microprismatic glass coatings and retro-reflecting prisms serve for angle selectivesolar control. They allow variation of transmittance dependent upon the angle ofsolar radiation incidence [138–140]. High-angle incident light is retro-reflected butlow-angle incident and diffuse light is transmitted.

The microprismatic surface patterns are produced by the interference holog-raphy or micro-machining technology. The microstructure can be periodic


(triangular, sinusoidal and parabolic) or random (periods between 0.2 and 50 lm)[140, 141]. The surface is laminated on the inner surfaces of double-glazed units inbuilding applications.

Transparent materials with micro and/or macro-structured surfaces can becombined with switchable devices to achieve a user-controlled system, Fig. 5c[116, 142–144]. This combination provides dynamic regulation of solar trans-mittance. Parts of the prismatic surfaces exposed to high solar incidence have thinchromogenic coatings. These coatings reduce transmittance due to a switchablecoloration process. Glare protection is achieved through prismatic surface retro-reflecting and light-scattering effects.

Another possible application of switchable glazing technology is the combi-nation of laminated glass with light emitting diodes (LEDs), with a liquid crystalinterlayer embedded between two glass panes with transparent conductive oxidelayers [145–147].

A switchable glazing system with surface micro-blinds has been developed, asshown in Fig. 6. The device controls light transmittance in response to appliedvoltage. Thin metal micro-blinds are deposited on a glass substrate with a trans-parent conductive oxide layer and an insulator layer. The micro-blinds are rolled inthe transparent electrically non-activated state. When electric field is activated apotential difference between the thin rolled metal and transparent conductive layercauses the micro-blinds to stretch and limit light transmittance (shading effect)[148, 149].

4.2 Antireflective Glazings

Special low-iron glasses and glazings with antireflective coatings can be used toimprove visible and solar transmittance.

Generally an antireflective (AR) coating consists of alternating dielectric layersof materials with high (as ZrO2, Ta2O5 or TiO2) and low (as MgF2, SiO2 and

Fig. 5 Prismatic transparentmaterials, a transmission oflow-angle incident solar rays,b retro-reflection of high-angle incident solar rays,c retro-reflection andabsorption of solar radiationon switchable coatings(deposited on the obliqueparts of the prisms)[116, 142–144]


Al2O3) refractive indices [150, 151]. Devices comprised of TiO2 and SiO2 layersare commonly used [152]. Multilayer dielectric AR coatings can enhance thetransmittance of visible light. A single AR layer with low refractive index and highporosity is recommended for solar energy applications. A third type of the ARcoating can improve both visible and solar near-IR transmittance [152].Non-porous materials can also achieve the AR effect through surface etching,nano-patterns or gratings [152–155].

An example of a practical application of AR coatings for windows is theenhancement of the visible transmittance of low-e glazings. An AR coating ofporous SiO2 can be deposited on both sides of a glazing with a pyrolytic low-etin-oxide coating [156]. Antireflective coatings are also useful in switchableglazing technologies, such as electrochromic glazings [157, 158].

4.3 Self-Cleaning Glazings and Coatings

Special coatings can create self-cleaning or easy-to-clean surfaces [159–161].These coatings can provide:

• photocatalytic function (decomposition of organic materials on the glasssurface),

• hydrophilic function (strongly wettable surface which allows water sheeting,which cleans the surface),

• hydrophobic function (weakly wettable surface).

A porous TiO2 thin film coating can exhibit both the photocatalytic andhydrophilic effects [159–161]. Incident solar radiation (UV part) generates thecatalytic activity within the TiO2 surface coating. The excited titanium transfers itsenergy to the oxygen molecules, which loses an electron. Electrons migrate to the

Fig. 6 Function of micro-blinds, a in the electricallynon-activated state(transparent), b in theelectrically activated state(shading), 1-glass substrate,2-adhesion layer andtransparent conductive layer,3-insulator, 4-reflectiveresilient metal micro-blind(4a-rolled blind, 4-stretchedblind) [148, 149]


organic material on the glass surface and decompose it. This process also changesthe glass surface to hydrophilic by creating oxygen vacancies, which react withwater. Due to this activity water droplets wet the surface. The water washes awaythe decomposed organic materials. The thin TiO2 photo-catalytically activecoating should be covered by a special corrosion protective coating [162, 163] forlarge façade or roof glazing applications.

Another self-cleaning system is a hydrophobic surface coating. It consists of awater polymer matrix which creates a surface with nanoscale pores. The self-cleaning effect is achieved due to water surface tension. Water drops form almostspherical droplets on the hydrophobic surface. They fall off the surface due togravity and roll dust and small dirt with them. Water drops are removed beforethey can evaporate on the surface [164], as shown in Fig. 7.

Special smart switchable coatings provide protective, hydrophobic or hydro-philic and other properties. Such smart coatings can also reversibly alter thewetting properties of surfaces [165–169] in response to external stimuli.

5 Conclusion

An overview of several types of window glazings and coatings has been presented.Implementations of thin film coatings for multifunctional glazed window unitsprovide integrated systems with spectrally selective glazings of high visibletransmittance and low-emissivity, which can be completed with solar control andswitchable glazings with light retro-reflecting and guiding units.

The development of glazing systems continues to interest researchers. There isa trend toward applications of chromogenic glazings for large glazed areas ofbuilding envelopes. Improvements in durability, maintenance-free service andlow-cost production will be the focal points of future work.

Photovoltaics offer self-powered glazed devices. Systems based on nanotechnol-ogy and advanced materials provide new types of self-activated switchable coatingsfor multifunctional smart glazing devices with reversibly modified properties.

Fig. 7 Self-cleaningsurfaces, a water drop rolledon the hydrophobic surface,b water drop spread on thehydrophilic surface [164]


Integrated multi-function glazings are representatives of systems that are ableto dynamically modulate their properties in the response to external climaticconditions. Such changes can increase energy efficiency, and indoor visual andthermal comfort in buildings.

Acknowledgments The window glazings and coatings overview was elaborated within theframe of research projects MŠMT MEB 080804 and GACR 101/09/H050. The authoracknowledges Dr. Pavel Pokorny, ISI AS CR, Brno for consultations on the thin film design andcomputer simulations.

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