Recent Progress on Preparation Strategies of Liquid Crystal Smart Windows

: Liquid crystal (LC) smart windows that are able to regulate natural light by changing the optical transmittance in response to external stimulus have become an effective way to reduce building energy consumption. The rapid development of technology has brought out a variety of responsive smart windows suitable for daily life, including electrical, thermal, and photo-responsive ones. In this review, the recent progress in LC smart windows that switch between transparent and opaque states by different stimuli is overviewed. The preparation strategies for single-/dual-responsive smart windows are outlined, exclusively concentrating on the functional design and working principle. Furthermore, the advantages and current drawbacks of smart windows for each response mode are brieﬂy described. Finally, a perspective on the direction of future responsive LC smart windows is discussed.


Introduction
In developed countries, energy used in residential buildings accounts for up to 40% of energy consumption, which has surpassed the energy consumed by industry and transportation [1,2].Among all building components, windows are usually reckoned as the most energy-wasting part [3,4].Nevertheless, the proportion of window space in buildings has been growing as a result of modern architectural aesthetics and human needs [5,6].Smart windows, windows that can dynamically adjust light transmittance on demand, have been considered an extremely crucial and promising technology to economize building energy consumption by controlling the indoor light environment [7][8][9][10].Smart windows are gaining immense popularity as they can maintain the building temperature and improve personal comfort in a novel and exciting way.
Smart windows have already attracted research interest from academia and industry, originating from the enormous commercial market and achieving the incredible development of smart energy-efficient buildings [11][12][13].According to a Globe Newswire report [14], in 2021, the global smart windows market is valued at $960 million and is expected to roughly triple to $2259.7 million by 2027, demonstrating a compound annual growth rate of almost 14.9% during 2022-2027.Compared with traditional windows, smart windows are able to realize the management of natural daylight by themselves, which is considered a more efficient choice for managing energy [15][16][17].Especially, smart windows are incorporated with lighting and air conditioning control systems, to further create a comfortable living environment [18,19].
To date, there are different materials available for the manufacturing of smart windows, including electrochromic materials [20,21], thermochromic materials [22,23], phase changing materials [24,25], suspended particle [26,27], hydrogel [8,28], and perovskites [29,30].Among them, liquid crystal (LC), due to its inherent responsive properties [31,32], is one of Crystals 2022, 12, 1426 2 of 13 the most attractive materials for smart windows.The studies of LC smart windows have been largely developed in the past few decades, and several LC smart windows are already hitting the market [18,33,34].In particular, LC smart windows which switch between transparent and opaque states can be used not only for building energy conservation, but also as a functional device in private spaces, sunroofs of cars, portholes of airplanes, greenhouses, and augmented reality displays [35][36][37][38][39].
In this review, the research progress of LC smart windows that switch between transparent and opaque states is overviewed, from the electrical-, thermal-, and photo-responsive approaches.We outline the distinct mechanisms of each stimulus and systematically present the preparation strategies for various types of smart windows.Additionally, recent achievements of dual-responsive LC smart windows are summarized, including thermal-/electrical-and thermal-/photo-responsive smart windows.Lastly, the aim of this review is to broaden research interests and enlighten further innovative approaches to prompt the development of multi-responsive smart windows that meet human needs.

Single-Responsive Liquid Crystal Smart Windows
LC is a stimuli-responsive material.It is characterized by the anisotropy of dielectric, optical, and mechanical properties [40][41][42].These properties are responsive to a range of stimuli, including the electric field, temperature, light, etc., which make LCs suitable for application in the field of smart windows [43,44].Single-responsive LC smart window refers to a smart window that dynamically modulates light transmittance solely by one external stimulus, typically referring to electrical-, thermal-, or photo-responsive approaches.

Electrical-Responsive Liquid Crystal Smart Windows
An electrical-responsive LC (ERLC) smart window is an active smart window that can be adjusted according to people's desires [45].In general, an ERLC smart window is a component consisting of the LC mixture sandwiched between two transparent conductive electrodes that regulates the optical properties by the absence/presence of an electric field [17,46].Unlike typical electrochromic smart windows, ERLC smart windows commonly switch between the transparent and opaque states through light scattering by electrically controlled reorientation of the LC director [47].Therefore, the response time is relatively short (~ms), and the ERLC smart window gets more attention and is rapidly commercialized [48,49].Here, we introduce several strategies for producing ERLC smart windows and elaborate on each working mechanism in detail.
It is a simple way to realize light management by taking advantage of electrohydrodynamic instability (EHDI) in LCs [50,51].Doping the nematic LCs (NLCs) with a suitable species of ions has proven to be a very practical way to achieve the dynamic scattering phenomenon [52].Under the action of the alignment layer, LC mixtures are aligned in the same direction, and the pattern behind the device is visible.When applying the electric field, the movement of ions generates the vortex, and the LCs director disorder, resulting in a strong light scattering (Figure 1a,b).Cetyltrimethylammonium bromide (CTAB) cationic surfactant is a commonly used ionic material.In recent years, the charge-neutral zwitterion molecules with multiple ionic groups have become better options.It has been shown that the use of zwitterion molecules can further improve the long-term stability of EHDI smart windows [53,54].In addition to NLCs, doping the ion materials into the cholesteric liquid crystals (CLCs) or smectic liquid crystals (SmLCs) can be another practical approach for fabricating bistable smart windows, which has attracted intense attention due to the low energy consumption [55][56][57].As shown in Figure 1c, in the absence of an electric field, the helical axis of CLCs is aligned orthogonally to the substrate through the anchoring force by the homogenous alignment, exhibiting a planar state.When a low-frequency voltage is applied, ions move freely, and the window switches from planar texture to focal conic (FC) texture, which scatters incident light in random directions and leads to the scattering state, and CLCs will maintain the metastable FC state without applied fields.When the high-frequency voltage is applied, the ions inside barely move, therefore, the negative dielectric CLCs respond to the electric field and self-assemble to a planar state, and the window returns to the initial transparent state (Figure 1d) [58].als 2022, 12, 1426 3 of leads to the scattering state, and CLCs will maintain the metastable FC state without a plied fields.When the high-frequency voltage is applied, the ions inside barely mov therefore, the negative dielectric CLCs respond to the electric field and self-assemble to planar state, and the window returns to the initial transparent state (Figure 1d) [58].Another popular ERLC smart window is based on the polymer/LCs composite sy tem.Polymer dispersed liquid crystal (PDLC) is the most well-known representativ [59,60].During PDLC preparation, low-molar-mass LCs are usually not soluble in the po ymer network, phase separation happens, and thus the nano-or micrometer size LC dro lets are embedded in a continuous polymer matrix.PDLC shows a milky scattering sta due to the refractive index mismatch between the LCs and polymer in the initial stat Under the applied sufficient voltage, LC director aligns in the direction of the field, resu ing in the loss of the refractive index mismatch, and therefore allowing the incident lig to pass through (Figure 2a) [61,62].However, PDLC usually shows a fuzzy state when th electric field is not applied because the polymer matrix lacks the LC orientation contro which is unsuitable for traditional buildings.In addition, the amount of polymer conte is higher than 40 wt.%, and a higher working voltage (~100 V) is required to achieve th transparent state [63].To solve these problems, polymer stabilized liquid crystal (PSLC which can switch between transparent and scattering states by removing and applying a electric field, has become an alternative to PDLC [36].In the OFF state, thanks to the alig ment layer on both sides of the substrate, LCs and the polymer network are aligned in th same direction.In this case, the LC effective index matches that of the polymer matri and PSLC shows a clear state.When the voltage is applied, LCs are reoriented in respon to the field.As the polymer network hinders the rotation of LCs, a multi-domain formed, which results in the difference in the refractive index, and the incident light ca be strongly scattered (Figure 2b) [64,65].As there is a smaller quantity of LC polym network (<10 wt.%) in PSLC, the working voltage is relatively low (~30 V).Besides, it worth mentioning that Yang's group has developed a novel coexistent system of polym dispersed and polymer stabilized liquid crystal (PD&SLC) in response to the problem high working voltage of PDLC [66,67].Taking advantage of the difference in polymeriz tion rates of non-liquid crystalline monomers and liquid crystalline monomers, they co structed a unique system in which the homeotropically polymer network structure PSLC is randomly distributed in each cavity of the porous polymer matrix of PDLC (Fi ure 2c).The interactions between the homeotropically polymer network and LCs can e fectively reduce the confinements from the porous polymer matrix (Figure 2d), thereb reducing the working voltage (~35 V, Figure 2e) [68].Moreover, many studies have bee focused on ways to further enhance the functionalities of polymer/LCs composite ERL Another popular ERLC smart window is based on the polymer/LCs composite system.Polymer dispersed liquid crystal (PDLC) is the most well-known representative [59,60].During PDLC preparation, low-molar-mass LCs are usually not soluble in the polymer network, phase separation happens, and thus the nano-or micrometer size LC droplets are embedded in a continuous polymer matrix.PDLC shows a milky scattering state due to the refractive index mismatch between the LCs and polymer in the initial state.Under the applied sufficient voltage, LC director aligns in the direction of the field, resulting in the loss of the refractive index mismatch, and therefore allowing the incident light to pass through (Figure 2a) [61,62].However, PDLC usually shows a fuzzy state when the electric field is not applied because the polymer matrix lacks the LC orientation control, which is unsuitable for traditional buildings.In addition, the amount of polymer content is higher than 40 wt.%, and a higher working voltage (~100 V) is required to achieve the transparent state [63].To solve these problems, polymer stabilized liquid crystal (PSLC), which can switch between transparent and scattering states by removing and applying an electric field, has become an alternative to PDLC [36].In the OFF state, thanks to the alignment layer on both sides of the substrate, LCs and the polymer network are aligned in the same direction.In this case, the LC effective index matches that of the polymer matrix, and PSLC shows a clear state.When the voltage is applied, LCs are reoriented in response to the field.As the polymer network hinders the rotation of LCs, a multi-domain is formed, which results in the difference in the refractive index, and the incident light can be strongly scattered (Figure 2b) [64,65].As there is a smaller quantity of LC polymer network (<10 wt.%) in PSLC, the working voltage is relatively low (~30 V).Besides, it is worth mentioning that Yang's group has developed a novel coexistent system of polymer dispersed and polymer stabilized liquid crystal (PD&SLC) in response to the problem of high working voltage of PDLC [66,67].Taking advantage of the difference in polymerization rates of non-liquid crystalline monomers and liquid crystalline monomers, they constructed a unique system in which the homeotropically polymer network structure of PSLC is randomly distributed in each cavity of the porous polymer matrix of PDLC (Figure 2c).The interactions between the homeotropically polymer network and LCs can effectively reduce the confinements from the porous polymer matrix (Figure 2d), thereby reducing the working voltage (~35 V, Figure 2e) [68].Moreover, many studies have been focused on ways to further enhance the functionalities of polymer/LCs composite ERLC smart windows, such as liquid crystal type (CLCs, SmLCs, dual-frequency LCs) [69,70], chemical structure of monomers [71], polymerization type [72,73], doping nanomaterials [74,75], etc.Take the doping of nanoparticles in PDLC as an example.Some metal nanoparticles (ZnO, CuO, Fe 3 O 4 , TiO 2 , etc.) can reduce the driving voltage of PDLC due to the fact that nanoparticles can affect the dielectric constant of LCs or create the local field effects.Especially Ag nanoparticles, with which the surface plasmon excitations at metal-LC interfaces increase the local electric fields, and the driving voltage of PDLC drops from 77 V to 40 V [63].
Crystals 2022, 12, 1426 [74,75], etc.Take the doping of nanoparticles in PDLC as an example.Some metal particles (ZnO, CuO, Fe3O4, TiO2, etc.) can reduce the driving voltage of PDLC due fact that nanoparticles can affect the dielectric constant of LCs or create the local fi fects.Especially Ag nanoparticles, with which the surface plasmon excitations at LC interfaces increase the local electric fields, and the driving voltage of PDLC drop 77 V to 40 V [63].

Thermal-Responsive Liquid Crystal Smart Windows
Thermal-responsive LC (TRLC) smart windows use LC materials whose pro change when the temperature reaches a critical value transition temperature [76 TRLC smart window mechanism is generally considered to be a passive way witho additional control system, and the windows can adaptively adjust the light transm according to the ambient temperature.Therefore, TRLC smart windows are one most attractive options for energy-efficient buildings.
Exploiting the reversible phase transition properties of LCs is the most used m for preparing TRLC smart windows, especially the reversible phase transition be the smectic A (SmA) and the chiral nematic phases [77].Sun et al. constructed a PSL based on the above characteristic [78].Within the temperature range of the SmA, t vice presents a strong light-scattering state resulting from the FC domains caused interaction between the elastic force of the polymer network and the self-configura SmA.When the temperature is higher than 35.5 °C (chiral nematic phase), due to t choring effects of the planar alignment layers and the crosslinked polymer netwo device shows a transparent state (Figure 3a,b).To improve the shear strength of the film and solve the problem that the film is hazy around room temperature, a PD film was developed [79].By introducing the PDLC system, the maximum shear st of the film is increased by nearly 30 times.At room temperature (25 °C), the Sm molecules are oriented perpendicular to the surface of the substrate because of the a

Thermal-Responsive Liquid Crystal Smart Windows
Thermal-responsive LC (TRLC) smart windows use LC materials whose properties change when the temperature reaches a critical value transition temperature [76].The TRLC smart window mechanism is generally considered to be a passive way without any additional control system, and the windows can adaptively adjust the light transmittance according to the ambient temperature.Therefore, TRLC smart windows are one of the most attractive options for energy-efficient buildings.
Exploiting the reversible phase transition properties of LCs is the most used method for preparing TRLC smart windows, especially the reversible phase transition between the smectic A (SmA) and the chiral nematic phases [77].Sun et al. constructed a PSLC film based on the above characteristic [78].Within the temperature range of the SmA, the device presents a strong light-scattering state resulting from the FC domains caused by the interaction between the elastic force of the polymer network and the self-configuration of SmA.When the temperature is higher than 35.5 • C (chiral nematic phase), due to the anchoring effects of the planar alignment layers and the crosslinked polymer network, the device shows a transparent state (Figure 3a,b).To improve the shear strength of the above film and solve the problem that the film is hazy around room temperature, a PD&SLC film was developed [79].By introducing the PDLC system, the maximum shear strength of the film is increased by nearly 30 times.At room temperature (25 • C), the SmA LC molecules are oriented perpendicular to the surface of the substrate because of the anchoring of the homeotropically aligned polymer network, and the film shows a clear state.When the film Crystals 2022, 12, 1426 5 of 13 is heated to a higher temperature into the chiral nematic phase, LC molecules rearrange to form the FC texture, and the film exhibits an opaque state (Figure 3c,d).Theoretically, the more phase transitions of LCs are fully utilized, the more the effectiveness of TRLC smart windows will be obtained.Oh et al. have reported on smart windows with three switching states through the smectic-nematic-isotropic phase transition (Figure 3e,f) [80].In the SmA phase, LCs and black dichroic dye molecules are homeotropically aligned due to the orientation force of the alignment layer, the light absorption and scattering are minimized, and the pattern behind the device is visible.When the temperature is increased to 31 • C, LCs transfer to the chiral nematic phase and form the FC domain, and the synergistic effect of black dyes and LC molecules enhance the absorption.The film simultaneously produces the scattering state and deep black color, showing a high-haze opaque state, and the transmittance is only 4.3% (Figure 3g).With further increase in the temperature to the isotropic phase, the light scattering disappears, the transmittance is slightly higher (19.4%), and the film presents a haze-free opaque state.
2022, 12, 1426 5 molecules rearrange to form the FC texture, and the film exhibits an opaque state (Fi 3c,d).Theoretically, the more phase transitions of LCs are fully utilized, the more th fectiveness of TRLC smart windows will be obtained.Oh et al. have reported on s windows with three switching states through the smectic-nematic-isotropic phase tr tion (Figure 3e,f) [80].In the SmA phase, LCs and black dichroic dye molecules are ho otropically aligned due to the orientation force of the alignment layer, the light absorp and scattering are minimized, and the pattern behind the device is visible.When the perature is increased to 31 °C, LCs transfer to the chiral nematic phase and form th domain, and the synergistic effect of black dyes and LC molecules enhance the absorp The film simultaneously produces the scattering state and deep black color, showi high-haze opaque state, and the transmittance is only 4.3% (Figure 3g).With furthe crease in the temperature to the isotropic phase, the light scattering disappears, the t mittance is slightly higher (19.4%), and the film presents a haze-free opaque state.

Photo-Responsive Liquid Crystal Smart Windows
Light is clean and renewable energy.It is fascinating to use light to manage change of transmittance of smart windows [81].Commonly, a photo-responsive (PRLC) smart window is produced by doping photo-responsive materials to LC mix [82].Similar to TRLC smart windows, PRLC smart windows are also considered co fective, stimuli-rational, and energy-efficient for their simple structures, passive modulations, and zero-energy input characteristics.
Azobenzene derivatives are some of the most used photo-responsive material preparing the PRLC smart windows [83,84].In the initial state, the azobenzene chira pant induces LCs to form CLCs, and the window scatters the incident light through FC state.By exposing to ultraviolet (UV) light in a specific switching temperature ra photoisomerization of azobenzene reduces the order parameter of LCs, the win switches from FC state to isotropic state, and becomes transparent (Figure 4b,c) [85].O the UV light is removed, the push-pull azobenzene (2-(4-hydroxyphenylazo)benzoic) (HABA) has the advantage of quick thermal relaxation (Figure 4a), and the window  [79].(e) DSC curve of the 8CB/S811 mixtures [80].(f) Schematic representation and photographs of the TRLC film [80].(g) Transmittance of the film with a change in temperature [80]."*" means the chiral LC phase.

Photo-Responsive Liquid Crystal Smart Windows
Light is clean and renewable energy.It is fascinating to use light to manage the change of transmittance of smart windows [81].Commonly, a photo-responsive LC (PRLC) smart window is produced by doping photo-responsive materials to LC mixtures [82].Similar to TRLC smart windows, PRLC smart windows are also considered cost-effective, stimulirational, and energy-efficient for their simple structures, passive light modulations, and zero-energy input characteristics.
Azobenzene derivatives are some of the most used photo-responsive materials for preparing the PRLC smart windows [83,84].In the initial state, the azobenzene chiral dopant induces LCs to form CLCs, and the window scatters the incident light through the FC state.By exposing to ultraviolet (UV) light in a specific switching temperature range, photoisomerization of azobenzene reduces the order parameter of LCs, the window switches from FC state to isotropic state, and becomes transparent (Figure 4b,c) [85].Once the UV light is removed, the push-pull azobenzene (2-(4-hydroxyphenylazo)benzoic) acid (HABA) has the advantage of quick thermal relaxation (Figure 4a), and the window can return to the opaque state immediately [84].To develop an original transparent PRLC smart Crystals 2022, 12, 1426 6 of 13 window, Talukder et al. has developed a clever approach to using an azobenzene dimmer PSC−01, whose helical twist powers (HTP) of trans and cis states are 64.18µm −1 and 16.31 µm −1 , respectively, as shown in Figure 4d [86].In the initial homeotropic alignment cell, another chiral dopant S811 was added in extra portion to counteract the HTP of trans-PSC−01 (right-handed), so LCs are aligned perpendicular to the substrate in the unwound state, and the absorption of black dye is minimal (Figure 4e,f).When the cell is exposed to UV light, the trans-cis photoisomerization of azobenzene takes place and the HTP changes.Thus, left-handed CLCs are generated and enter the FC state, and the cell turns into a dark opaque state.Further, using visible light or heating, the LCs will require reversal from FC state to homeotropic state.Besides, overcrowded alkenes molecular motor is another promising photo-responsive molecule [87].The repeated rotation of the molecular motor is based on the carbon-carbon double bond between the upper half rotor and the lower half stator which serves as an axis (Figure 4g).Doping the molecular motor as a chiral dopant to LCs, CLCs form and show a planar texture under the boundary condition imposed by the planar alignment, the helical axis of CLCs is homogeneously aligned perpendicular to the substrate, and the device shows a transparent state.Upon the UV irradiation, the molecular motor switches from a stable state to an unstable state, and the HTP is changed.A large number of defects and disclinations are generated between LC domains and the free energy density increases with the HTP change, causing a light-scattering state due to the FC state, and a milky star pattern appears on the smart window by photomask (Figure 4h,i).
2022, 12, 1426 6 trans-PSC−01 (right-handed), so LCs are aligned perpendicular to the substrate in the wound state, and the absorption of black dye is minimal (Figure 4e,f).When the ce exposed to UV light, the trans-cis photoisomerization of azobenzene takes place and HTP changes.Thus, left-handed CLCs are generated and enter the FC state, and the turns into a dark opaque state.Further, using visible light or heating, the LCs will req reversal from FC state to homeotropic state.Besides, overcrowded alkenes molecular tor is another promising photo-responsive molecule [87].The repeated rotation of the lecular motor is based on the carbon-carbon double bond between the upper half r and the lower half stator which serves as an axis (Figure 4g).Doping the molecular m as a chiral dopant to LCs, CLCs form and show a planar texture under the boundary dition imposed by the planar alignment, the helical axis of CLCs is homogeneo aligned perpendicular to the substrate, and the device shows a transparent state.U the UV irradiation, the molecular motor switches from a stable state to an unstable s and the HTP is changed.A large number of defects and disclinations are generated tween LC domains and the free energy density increases with the HTP change, causi light-scattering state due to the FC state, and a milky star pattern appears on the sm window by photomask (Figure 4h,i).Apart from the photo-isomerized materials, the photothermal materials that cha the LC phase are a practical strategy for preparing PRLC smart windows [82,88].Men al. have reported a photothermal material isobutyl-substituted diammonium borate ( which is transparent in the visible area with a light brown color and good compatib with LCs (Figure 4j).On a cloudy day, the high transparency of the window is obta based on the chiral SmLC uniform perpendicular to the surface, and light can easily  Apart from the photo-isomerized materials, the photothermal materials that change the LC phase are a practical strategy for preparing PRLC smart windows [82,88].Meng et al. have reported a photothermal material isobutyl-substituted diammonium borate (IDI), which is transparent in the visible area with a light brown color and good compatibility with LCs (Figure 4j).On a cloudy day, the high transparency of the window is obtained based on the chiral SmLC uniform perpendicular to the surface, and light can easily pass through the windows.On sunny days (over 100 mW/cm 2 ), IDI converts light to heat causing the chiral SmLC to enter the CLC phase, CLCs exhibit an FC state by the helical torque force from the chiral dopant R5011, so the window becomes opaque and reduces the light entrance of the room (Figure 4k,j) [88].

Chapter Summary
Recent preparation strategies for single-responsive LC smart windows are covered in this section.Stimulus-responsive LC smart windows have their own pros and cons.
ERLC smart windows can switch states at any time according to the user's requirements, making it the first choice for commercialization.However, the high working voltage and complicated preparation process are the major disadvantages, and efforts are still needed for improving ERLC smart windows considering the cost-effectiveness and reliability.TRLC smart windows have developed rapidly in the past few years and relying on the ambient temperature to adjust the indoor light is their highlight.It is a meaningful direction to seek LC material systems that can undergo phase transition changes in a suitable room temperature range, taking into account a diversiform latitude environment.The PRLC smart window is another potential passive smart window.The most crucial point is to solve the problem of photo-responsive material by itself with light color.

Dual-Responsive Liquid Crystal Smart Windows
There has been exciting attention to multi-responsive LC smart windows that integrate various driven methods for future applications [89].A dual-responsive LC smart window is a window that realizes switching states (transparent and opaque states) through two independent stimuli, such as temperature and electric field or temperature and light.

Thermal-/Electrical-Responsive Liquid Crystal Smart Windows
If a system can achieve both TR and ER, then it has the potential to realize thermal-/electrical-responsive LC (TR/ER LC) smart windows.PD&SLC is a good example [79,90].Utilizing the polymer network structure of PD&SLC, and LC materials with SmA-N phase transition, a dual-responsive window can be obtained (Figure 5a) [91].SmA LCs are homeotropically aligned at room temperature due to the formation of the vertical polymer network structure, and the film is transparent as a result of the perfectly matched refractive index.When the temperature is higher than the SmA-N phase transition, the temperature induces the formation of CLC and achieves an FC texture.The film exhibits a lightscattering state (Figure 5b).Moreover, the scattering film can still be powered electrically, as Section 2.1 mentioned (Figure 5c).Another excellent strategy for preparing TR/ER LC smart windows has been proposed by Sung et al. [92] A suitable concentration of chiral dopant S811 was added to the negative dielectric LC to form a CLCs phase.Because of the impurity ions of LCs and chiral dopant, the EHDI effect can drive the device away from the naturally planar state to a dynamic scattering state by applying a certain voltage and frequency (Figure 5d), and the transmittance decreases (Figure 5e).In the instance of temperature control, the author found that EHDI production is temperature dependent, the threshold electric field can be decreased by increasing the cell temperature, and the EHDI smart window can be controlled by the temperature (Figure 5d).More importantly, at different temperatures, the switching state corresponds to various voltages (Figure 5f), which benefits human inhabitants at all latitudes.[92].(e) Transmission spec of smart window driven by 100 V at three different frequencies [92].(f) At different temperatur transmittance as a function of voltage and frequency [92]."*" means the chiral LC phase.

Thermal-/Photo-Responsive Liquid Crystal Smart Windows
Incorporation of photo-responsive chiral dopants into LCs with the SmA-N pha transition is the most straightforward strategy for fabricating thermal-/photo-responsi liquid crystal (TR/PR LC) smart windows (Figure 6a) [93].The working principle of az benzene materials as a chiral dopant in TR and PR LC smart windows is described Sections 2.2 and 2.3, respectively, and will not be repeated here.Furthermore, photosponsive materials are not only used as the chiral dopant, but also can be grafted as functional group in polymer for alignment layers [94].A dual-responsive homeotrop alignment PSLC was produced by copolymer MAzo-co-GMA and LCs with phase tran tion (Figure 6b).As mentioned above, the TR mode mainly depends on the phase chan ing of LCs.In PR mode, the PSLC shows a transparent state due to the MAzo-co-GM homeotropic alignment layer without irradiation.Turning on the UV light (365 nm), t photo-isomerization of the azobenzene group takes place, and the azobenzene group pr duces a photothermal effect as well.The photothermal effect dominates, causing the L to undergo a phase transition, and the smart window switches from a transparent state an opaque state (Figure 6c).[92].(e) Transmission spectra of smart window driven by 100 V at three different frequencies [92].(f) At different temperatures, transmittance as a function of voltage and frequency [92]."*" means the chiral LC phase.

Thermal-/Photo-Responsive Liquid Crystal Smart Windows
Incorporation of photo-responsive chiral dopants into LCs with the SmA-N phase transition is the most straightforward strategy for fabricating thermal-/photo-responsive liquid crystal (TR/PR LC) smart windows (Figure 6a) [93].The working principle of azobenzene materials as a chiral dopant in TR and PR LC smart windows is described in Sections 2.2 and 2.3, respectively, and will not be repeated here.Furthermore, photoresponsive materials are not only used as the chiral dopant, but also can be grafted as a functional group in polymer for alignment layers [94].A dual-responsive homeotropic alignment PSLC was produced by copolymer MAzo-co-GMA and LCs with phase transition (Figure 6b).As mentioned above, the TR mode mainly depends on the phase changing of LCs.In PR mode, the PSLC shows a transparent state due to the MAzo-co-GMA homeotropic alignment layer without irradiation.Turning on the UV light (365 nm), the photo-isomerization of the azobenzene group takes place, and the azobenzene group produces a photothermal effect as well.The photothermal effect dominates, causing the LCs to undergo a phase transition, and the smart window switches from a transparent state to an opaque state (Figure 6c).

Thermal-/Photo-Responsive Liquid Crystal Smart Windows
Incorporation of photo-responsive chiral dopants into LCs with the SmA-N pha transition is the most straightforward strategy for fabricating thermal-/photo-responsi liquid crystal (TR/PR LC) smart windows (Figure 6a) [93].The working principle of az benzene materials as a chiral dopant in TR and PR LC smart windows is described Sections 2.2 and 2.3, respectively, and will not be repeated here.Furthermore, photo-r sponsive materials are not only used as the chiral dopant, but also can be grafted as functional group in polymer for alignment layers [94].A dual-responsive homeotrop alignment PSLC was produced by copolymer MAzo-co-GMA and LCs with phase tran tion (Figure 6b).As mentioned above, the TR mode mainly depends on the phase chan ing of LCs.In PR mode, the PSLC shows a transparent state due to the MAzo-co-GM homeotropic alignment layer without irradiation.Turning on the UV light (365 nm), t photo-isomerization of the azobenzene group takes place, and the azobenzene group pr duces a photothermal effect as well.The photothermal effect dominates, causing the L to undergo a phase transition, and the smart window switches from a transparent state an opaque state (Figure 6c).

Chapter Summary
The recent advances in the preparation of dual-responsive LC smart windows ha been reported, covering the last 2-4 years.While the current techniques are far from m

Chapter Summary
The recent advances in the preparation of dual-responsive LC smart windows have been reported, covering the last 2-4 years.While the current techniques are far from mature, these works are indeed inspiring.In general, dual-response smart windows and single-response windows complement each other, and the fabrication methods are more dependent on the technological development of single-response smart windows.Therefore, it is urgent to develop more single-response preparation strategies.

Conclusions
The review mainly summarizes the recent progress of LC smart windows switching between transparent and opaque states for electrical-, thermal-, and photo-responsive ones.We describe the preparation strategies and working mechanisms for each category focusing on single-and dual-responsive techniques.A brief comparison between these LC smart windows is presented in Table 1.Smart windows, which aim to achieve carbon neutrality and high-efficiency energysaving buildings, still face many challenges and opportunities in future development.Obviously, a multi-responsive smart window is superior to a single-responsive smart window in functionality and practicality.However, single-responsive smart window preparation technology is the driving force behind the development of dual-responsive smart windows.Therefore, how to promote the development of single-responsive smart window technology is the fundamental problem.In terms of materials, it is meaningful to further explore new LC materials and optimize existing material systems.Improving the dielectric properties of LCs enables ERLC smart windows to have a lower driving voltage; introducing photo-responsive groups into LC molecules can form photo-driven LC materials while maintaining liquid crystalline.Material changes will enrich fabrication strategies.As for preparation techniques, on the one hand, we need to carry out novel preparation methods to improve the production toolbox, and on the other hand, future smart windows should be integrated with various response modes and continue to break the boundaries between each simulation mode, to form a smart window that better suits personal demand.
In short, we provide an up-to-date overview of smart windows and promote the development of preparation strategies through this review.Smart windows will have the opportunity to enter thousands of households in the near future.

Figure 1 .
Figure 1.(a) Schematic diagrams and (b) image of the EHDI smart window based on the NLCs the ON/OFF states [53].(c) The operating principle of the bistable CLCs smart window.[58].(d) T pictures and polarizing optical microscope (POM) images of EHDI smart window in scattering sta and transparent state [58].

Figure 1 .
Figure 1.(a) Schematic diagrams and (b) image of the EHDI smart window based on the NLCs in the ON/OFF states [53].(c) The operating principle of the bistable CLCs smart window.[58].(d) The pictures and polarizing optical microscope (POM) images of EHDI smart window in scattering state and transparent state [58].

Figure 3 .
Figure 3. (a) DSC curve of the nCB/SLC-1717 mixtures [78].(b) Schematic representation of the working principle and images of the PSLC film in the cooling/heating states [78].(c) DSC curve of the SmA-LC/SLC-1717/S811 mixtures [79].(d) Schematic diagrams and images of the PD&SLC film in the cooling/heating states[79].(e) DSC curve of the 8CB/S811 mixtures[80].(f) Schematic representation and photographs of the TRLC film[80].(g) Transmittance of the film with a change in temperature[80]."*" means the chiral LC phase.

Figure 5 .
Figure 5. (a) Schematic diagram of preparation of PD&SLC film [91].Digital photographs of flexi PD&SLC film controlled by (b) temperature and (c) electric field, respectively [91].(d) Schema explication of TR/ER LC smart window under different driving modes [92].(e) Transmission spec of smart window driven by 100 V at three different frequencies [92].(f) At different temperatur transmittance as a function of voltage and frequency [92]."*" means the chiral LC phase.

Figure 6 .
Figure 6.(a) Schematic diagram and images of TR/PR LC smart window in two working modes [9 (b) Synthetic route of the copolymer MAzo-co-GMA alignment agent [94].(c) Working principle TR/PR LC smart window based on the MAzo-co-GMA alignment layer [94].

Figure 5 .
Figure 5. (a) Schematic diagram of preparation of PD&SLC film [91].Digital photographs of flexible PD&SLC film controlled by (b) temperature and (c) electric field, respectively [91].(d) Schematic explication of TR/ER LC smart window under different driving modes [92].(e) Transmission spectra of smart window driven by 100 V at three different frequencies [92].(f) At different temperatures, transmittance as a function of voltage and frequency [92]."*" means the chiral LC phase.

Figure 5 .
Figure 5. (a) Schematic diagram of preparation of PD&SLC film [91].Digital photographs of flexi PD&SLC film controlled by (b) temperature and (c) electric field, respectively [91].(d) Schema explication of TR/ER LC smart window under different driving modes [92].(e) Transmission spec of smart window driven by 100 V at three different frequencies [92].(f) At different temperatur transmittance as a function of voltage and frequency [92]."*" means the chiral LC phase.

Figure 6 .
Figure 6.(a) Schematic diagram and images of TR/PR LC smart window in two working modes [9 (b) Synthetic route of the copolymer MAzo-co-GMA alignment agent [94].(c) Working principle TR/PR LC smart window based on the MAzo-co-GMA alignment layer [94].

Figure 6 .
Figure 6.(a) Schematic diagram and images of TR/PR LC smart window in two working modes [93].(b) Synthetic route of the copolymer MAzo-co-GMA alignment agent [94].(c) Working principle of TR/PR LC smart window based on the MAzo-co-GMA alignment layer [94].

Table 1 .
A brief comparison of single-and dual-responsive LC smart windows.."Statechange" means the smart windows from the initial state (0 V, room temperature) to the working state.T represents the transparent state.O represents the opaque state.2."_" means data not available. 1