Recent Progress in Liquid Crystal-Based Smart Windows with Low Driving Voltage and High Contrast

Abstract

Smart windows based on liquid crystal (LC) have made significant advancements over the past decade. As critical mediators of outdoor light entering indoor spaces, these windows can dynamically and rapidly adjust their transmittance to adapt to changing environmental conditions, thereby enhancing living comfort. To further improve device performance, reduce energy consumption, and ensure greater safety for everyday use, scientists have recently focused on reducing driving voltage and enhancing contrast ratio, achieving notable progress in these areas. This article provides a concise overview of the fundamental principles and major applications of LC smart windows. It systematically reviews recent advancements over the past two years in improving these two key optical properties for variable transmittance LC smart windows, both internally and externally, and highlights the remaining challenges alongside potential future directions for development.

1. Introduction

In recent years, global warming and climate change have become major topics of concern, closely tied to human activities and increasingly posing a threat to human survival [1,2]. In response to environmental and energy crises, many governments have set ambitious ‘dual carbon’ goals, including ‘carbon neutrality’ and ‘carbon peaking’ [3]. Buildings are a significant source of carbon emissions, particularly in developed countries, where energy consumption in buildings accounts for more than 30% of total energy use, surpassing the combined energy consumption of industry and transportation [4]. Among the various energy-consuming components, heating, ventilation, and air conditioning systems account for nearly 50% of building energy use [5]. This leads to significant energy loss through doors and windows, as windows are not only crucial for regulating indoor temperature and lighting during the day but are also the least energy-efficient component [6]. To mitigate the increasing energy consumption, there is an urgent need to design low-cost, efficient devices to block excessive sunlight and reduce indoor temperatures.

Smart windows represent a promising application of energy-saving technologies, gaining significant attention in both academic and industrial sectors due to their multifunctionality and environmental benefits [7]. These windows hold substantial potential in providing comfortable indoor environments and reducing energy consumption [8,9]. Compared to traditional physical windows and blinds, smart windows offer additional functionalities, including esthetic enhancement, optical control, intelligent regulation, and energy conversion potential [10,11,12,13,14]. They can modulate light transmittance (from opaque to transparent states) by applying external stimuli, addressing privacy concerns while reducing energy consumption by up to 39%. For instance, during cold weather, sunlight incident on the smart window surface serves not only as a source of illumination but also as an infrared heating source. Conversely, in hot weather, the smart window adjusts its transmittance to block a portion of light and heat from entering the indoor space [15]. This dynamic light control technology allows for the regulation of transmitted light to enhance visual comfort and work–life conditions indoors while selectively blocking or controlling heat transfer, thereby reducing air conditioning energy consumption. In addition to applications in architecture, smart windows can be broadly utilized in various fields, including displays, privacy protection, switchable eyewear, and medical devices. Smart windows can be fabricated using materials such as electrochromic materials, suspended particle devices, polymer-dispersed liquid crystal (PDLC), and polymer-stabilized cholesteric liquid crystal (PSCLC) [16,17,18,19,20,21,22]. Among these, LC is considered an ideal material for smart windows due to its flexible controllability and fast response time.

LC represents a phase intermediate between liquids and solids, possessing the fluidity of liquids and the ordered alignment characteristics of solids [23,24]. Unlike the three conventional phases of matter—solid, liquid, and gas—LC exhibits remarkable responsiveness to external stimuli such as electric fields, light, heat, magnetic fields, and mechanical friction [25,26,27,28]. Due to their mesogenic orientation, LC exhibits unique optical properties, including birefringence, scattering, and reflection [29,30]. Cholesteric liquid crystal (CLC) forms a subgroup of LC materials, created by the combination of nematic liquid crystal (NLC) with chiral dopants, and therefore, they it is also referred to as chiral nematic LC [31,32]. In CLC, the orientation of LC molecules relative to neighboring molecules is slightly twisted, distinguishing them from nematic and smectic LC. Chiral dopants are compounds derived through microbial fermentation; as a kind of optically active substance, they can impart left-handed and right-handed properties to the CLC and create a helical structure [33]. This helical arrangement modulates the refractive index periodically along the helical axis in response to the polarization of incident light, thereby exhibiting the characteristics of a one-dimensional photonic crystal. Furthermore, CLC demonstrates bistability with both planar and focal conic textures, which can be switched via electric field pulses, and by manipulating electrohydrodynamic instabilities, uniform helical structures can also be achieved [34,35,36]. These remarkable properties of CLC have led to their application in various optoelectronic devices.

In recent years, to enhance the performance of smart windows, various smart optoelectronic devices have been proposed and explored, including electrochromic, thermochromic, and photochromic devices, all of which can change color under external stimuli to adapt to environmental changes, thereby regulating light and temperature. Figure 1 shows a schematic illustration for the modulation mechanism and construction of different chromic devices as smart windows and their advantages and limitations [37].

Photochromic smart windows are typical passive modulation devices that alter their color when exposed to light of a specific wavelength, with the effect being particularly noticeable in the ultraviolet (UV) range. Inorganic materials based on photochromic mechanisms, such as WO₃, are based on photon-induced redox reactions and light-triggered electron transfer, offering many advantages, including low cost, good stability, and ease of operation. However, photochromic materials are typically limited by a low contrast ratio, slow bleaching rates, and poor stability, particularly when exposed to intense UV radiation, rendering them unsuitable for energy-efficient smart windows [5,37].

Thermochromic smart windows represent another type of passive modulation device, attracting considerable attention due to their ability to directly regulate temperature. Thermochromic materials operate under temperature stimuli, generally remaining transparent at low temperatures and changing to a colored state at high temperatures, making them suitable for energy consumption regulation in buildings. Most thermochromic materials, including some metal oxides like VO₂, change color through reversible phase transitions at certain temperatures [38,39]. However, because smart windows in typical buildings do not usually operate under excessively high or low temperatures, thermochromism still does not fully meet the requirements for energy-efficient and high-performance smart windows.

In contrast to photochromic and thermochromic smart windows, which passively adapt and regulate based solely on environmental light stimuli, electrochromic smart windows employ active electrochemical regulation, enabling dynamic modulation across a broad spectral range. This active adjustment can cater to individual preferences. Although electrochromic windows tend to have higher costs and more complex structures, electricity provides the most precise and controllable driving force compared to other forms of actively adjustable stimuli. The combination of electricity with a wide range of electrochromic materials and device architectures makes electrochromism a suitable option for smart color regulation [40].

Electrochromism is a widely studied phenomenon where the optical properties of materials, such as reflectance, transmittance, and absorbance, can be altered by applying small electrical signals, ions, and electrons. During the electrochromic process, conjugated polymers adsorb on the polymer surface and shallow ionic traps, forming a dense microstructure that prevents ion doping, thereby reducing electrochemical activity [41]. Electrochromic materials, as key components of electrochromic devices, have been developed for various potential applications due to their unique optical modulation behavior and ultra-low power consumption. These applications include displays, smart windows, wearable and portable electronics, and electro-reconfigurable optics. Electrochromic materials can be classified into two main categories based on their composition: organic and inorganic materials. Most organic electrochromic materials are either organic small molecules [42,43] or conductive polymers [44,45]. Small organic molecules are characterized by high color purity, high coloring efficiency, and rich color tunability [46], but they exhibit poor thermal diffusion and low cycling stability in devices. Conductive polymers, due to their electrical conductivity, are easily processed via solution-based techniques [47]. By using different reduction dyes or various functional terminal group modifications, multi-colored electrochromic devices, including red, green, blue (RGB), and black, have been developed to achieve an extremely wide color range in the visible spectrum [43,48]. Despite the potential for full-color adjustment in these RGB-to-black multi-colored electrochromic devices, the mixed systems of various organic electrochromic materials used in these devices significantly increase the complexity of device fabrication and integration, as well as the cost. Additionally, some organic electrochromic materials require relatively high electrochemical potentials to produce distinct color contrasts, which leads to poor cycling stability and longevity during color-switching processes [49,50]. Inorganic electrochromic materials mainly include WO₃ [51], NiO [52], MnO₂ [53,54], vanadium oxides [55,56], TiO₂ [57,58], and Prussian blue [59]. Most of these materials are at the forefront of practical applications and commercialization due to their excellent optical contrast, light stability, high cycle life, good thermochemical stability, and durability [60]. However, compared to organic materials, inorganic electrochromic materials exhibit very undesirable drab colors and suffer from issues such as low electrical conductivity, slow switching speeds, and limited color tunability. These limitations restrict their development in high-quality displays and complex optical devices for advanced applications [52,59,61,62]. For instance, tungsten trioxide (WO₃) exhibits a single blue hue under different applied voltages, which affects its range of applications [63]. Furthermore, during the electrochromic redox process, the embedding of metal oxides leads to ion trapping at binding sites, making it difficult to remove ions when a reverse voltage is applied [40]. These shortcomings represent challenges in developing electrochromic devices.

To advance the development of electrochromic LC smart windows, enhancing their optoelectronic performance is essential. For smart windows intended for everyday architectural use, most current studies ensure a response time in the millisecond range. However, the most critical factors are minimizing the driving voltage and maximizing the contrast ratio. Reducing the driving voltage can significantly save energy and extend the service life, while excessively high voltages pose substantial safety risks. Therefore, it is crucial to stabilize the voltage below the safe threshold for human exposure. As a fundamental performance metric, the contrast ratio directly affects user comfort; superior contrast indicates a wider adjustable range, thus broadening the application potential of smart windows. Therefore, improving the contrast ratio is a key step in expanding the application prospects of LC smart windows.

Our group published a review paper on LC smart windows in 2023 [7]. This paper provides a brief overview of the basic structure and principles of transparency-changing electrochromic LC smart windows, with a particular focus on the recent advancements in electrochromic LC smart windows over the past two years. These improvements include enhancing contrast, as well as reducing threshold and saturation voltages. The article also discusses the current challenges and issues in this field and offers insights into the prospects for future development.

2. Transparency-Changing LC-Based Smart Windows

As can be seen from the previous introduction, the unique properties of cholesteric phase liquid crystals allow them to be widely used in optoelectronic devices. Light is divided into left and right circularly polarized components. CLC, due to its helical structure, exhibits Bragg reflection for the light component with the same handedness as the helix when the incident light satisfies the Bragg condition and transmits light opposite to its helical chirality and light of other wavelengths [64]. With the deepening of research, scientists no longer focused only on pure CLC; they have turned their attention to combining organic small-molecule polymers (such as polyvinyl alcohol, ethylene-vinyl acetate copolymer, and acrylate copolymers) with LC, leading to the widely studied field of PDLC [65]. PDLC is phase-separated composite film formed by dispersing LC droplets within a continuous polymer matrix, as shown in Figure 2a. PDLC can control light scattering through the LC materials and polymers in it to achieve haze regulation, which allows it to switch back and forth between low-haze transparent and high-haze opaque states, thus changing the transmittance of liquid crystals. PDLC/PSLC can provide privacy by having a high haze, which reduces optical clarity (please note that this is different than energy saving, which requires rejection of the light through reflection or absorption). In the absence of high voltage, the polymer matrix and randomly oriented LC molecules exhibit different refractive indices, causing incident light to scatter at the interface due to the refractive index mismatch, resulting in a cloudy appearance. When a sufficiently high voltage is applied, the LC molecules, which possess positive dielectric anisotropy, align parallel to the external electric field, and their refractive index gradually approaches that of the polymer matrix. As the refractive index mismatch diminishes, the film becomes transparent [66,67]. There are mainly three methods for fabricating PDLC, including polymer-induced phase separation (PIPS), thermal-induced phase separation (TIPS), and solvent-induced phase separation (SIPS) [11,68,69]. The most commonly used method is PIPS, where monomers compatible with the LC and photoinitiators undergo ultraviolet (UV)-induced photopolymerization. During this process, the medium is initially mixed into the polymer matrix, and the immiscibility of the low-molecular-weight LC in the polymer matrix leads to phase separation, generating nanoscale LC droplets embedded along the matrix and resulting in a milky white mixture. In fact, the use of LC and polymer-based smart windows is of paramount significance. Both LC and polymer materials are well-known for their low production costs, ease of fabrication, and excellent mechanical properties.

With the initial application of PDLC, it was found that the smart window based on LC with an initial transparent state is more suitable for the situations faced in daily life. Therefore, reverse PDLC has gradually become the focus of research. From the word ‘reverse’, it can be inferred that reverse PDLC is transparent when the voltage is OFF and opaque when the voltage is ON [15]. Some of the more successful methods so far include the use of large LCs and a polymer matrix formed out of reactive mesogenic network precursors [70], the surface energy modification of polymer droplets [71], the utilization of dual-frequency responsive LCs [72], the technique of nematic emulsion polymerization [73], the use of rough and supportive alignment layers [74], and, finally, the introduction of an anisotropic polymer matrix [75]. The underlying rationale of these approaches has been to alter the surface energy of positive/negative dielectric anisotropy-type LCs using active monomers, to disperse double frequency-responsive LCs in a polymer matrix, to vertically align LC molecules along rough glass substrates, to disperse LCs into an anisotropic polymer matrix, and to replace passive polymeric matrix elements with low amounts of mesogenic networks (PSLC-based systems) [15]. In practical applications, reverse-mode PDLC has greater advantages in energy saving and is more promising.

Although PDLC exhibits excellent mechanical properties and high-speed switching capabilities under an electric field (in the range of a few milliseconds to tens of milliseconds), their high polymer content (≥40.0 wt%) [76] necessitates high driving voltages, which significantly exceed the safety voltage standards for human exposure, thereby presenting notable safety risks. To address this issue, researchers have reduced the prepolymer content to below 5 wt% [77]. By dissolving a small amount of monomer and photoinitiator in low-molecular-weight LCs, a precursor mixture is obtained, which is then cured via ultraviolet (UV) irradiation or heating. During this process, the crosslinked monomers form numerous LC clusters enclosed within a polymer network. To facilitate alignment, an alignment layer is employed to ensure that the uncured mixture remains in an ‘electric field-on’ orientation between the two conductive electrodes. This alignment layer, pre-melted onto the electrode surface, imparts a micron-scale surface topography that allows for interaction with the LC layer and provides the appropriate orientational properties for the LC layer, which directly arranges along the electrode surface. Upon contact with the alignment layer, the initially disordered LCs in the uncured mixture align according to the direction of the orientational LC structures, with the alignment propagated through a templating effect until the entire coating reaches a uniform alignment direction. Finally, UV radiation or heating is applied to crosslink the small amount of reactive mesogens in the uncured mixture, forming a crosslinked polymer network that extends from one plastic surface to another. During UV curing, by applying a bias to the substrate or using polyimide [78] or nylon alignment layers on the ITO surface, an oriented polymer network formed by the reactive mesogens can be achieved. Once oriented and formed into the desired configuration, the reactive mesogens undergo polymerization to form the required PSLC structure [79,80], imparting the targeted functional properties, as shown in Figure 2b. Similarly to the alignment layers, the presence and quality of the mesogen polymer network determine the local alignment of the LC molecules, which in turn affects the operational mode and performance of the PSLC system [15]. The PSLC fabricated using this method offers higher viewing angle transparency compared to conventional PDLCs, as the polymer network is relatively sparse, significantly reducing the effects of refractive index mismatch between the polymer and LC [81,82]. The lower polymer content also limits the total area defined by the polymer-LC interface, resulting in fewer anchoring points and reduced factors, such as anchoring energy, which hinder LC reorientation. Consequently, this device can operate at a much lower driving voltage compared to traditional PDLCs. Despite these advancements, challenges remain, including the lack of contrast and the pressure to further reduce driving voltages. A series of recent efforts by researchers to address these critical performance metrics will be discussed in the following section.

3. Enhanced Electrical and Optical Performance: Low Driving Voltage and High Contrast

The driving voltage of a variable transmittance electrochromic LC consists of two parameters: the threshold voltage (V_th) and the saturation voltage (V_sat). V_th and V_sat are defined as the voltage required for the transmittance to approach 10% and 90% of the maximum value, respectively. For typical PDLCs, the threshold voltage can be expressed using Equation (1) [83,84]:

$${\mathrm{V}}_{\mathrm{t}\mathrm{h}}=\frac{\mathrm{d}}{3\mathrm{a}}\left(\frac{{\mathrm{\sigma }}_2}{{\mathrm{\sigma }}_1}+2\right){\left(\frac{\mathrm{K}\left({\mathrm{l}}^2-1\right)}{\Delta \mathrm{\epsilon }}\right)}^{\frac{1}{2}},$$

(1)

where d is the cell thickness, l = a/b is the ratio of the major axis to the minor axis of the droplet (anisotropic ratio), K is the effective elastic constant, Δε is the dielectric anisotropy, σ₁ and σ₂ are the conductivities of the polymer matrix and the LC. The value of σ₂ can be calculated using Equation (2) [84]:

$${\mathrm{\sigma }}_2={\mathrm{\sigma }}_{\parallel }{\sigma }_{\perp }{\left({\mathrm{\sigma }}_{\parallel }{\mathrm{s}\mathrm{i}\mathrm{n}}^2\theta +{\mathrm{\sigma }}_{\perp }{\mathrm{c}\mathrm{o}\mathrm{s}}^2\theta \right)}^{-1},$$

(2)

where σ_‖ and σ_⊥ are the conductivities parallel and perpendicular, respectively, to the droplet director, and θ is the angle between the applied electric field and the droplet director. Based on the theoretical equations, it can be observed that the driving voltage is inversely proportional to the dielectric anisotropy of the LC and proportional to both the droplet radius and the elastic constant.

The contrast, as a key parameter of the optoelectronic properties in LC systems, characterizes the difference between the transparent and opaque states. It is defined as the ratio between the maximum and minimum transmittance values. From an experimental perspective, it can be intuitively interpreted as the ratio of the maximum to minimum light power received by the power meter. In general, the contrast of an LC film can be expressed using Equation (3):

$$\mathrm{C}\mathrm{R}=\frac{{\mathrm{T}}_{\mathrm{s}\mathrm{a}\mathrm{t}}\left(\%\right)}{{\mathrm{T}}_0\left(\%\right)},$$

(3)

where T_sat represents the maximum transmittance of the LC film and T₀ represents the minimum transmittance. Building on these equations, a series of methods have been developed to enhance the optoelectronic performance of LCs.

3.1. Doping with General Organic/Inorganic Compounds

It is well-known that PDLC films, as electrically controlled switches, have long faced the limitations of high driving voltage and low contrast. To address these issues, PSLC with a low polymer content has gradually emerged as a popular research subject. Zhang et al. successfully fabricated PSLC devices on flexible PET substrates [85]. However, the resulting contrast remained relatively low. Yun et al. improved the polymer network morphology of reflective PSLCs (RPSLCs) by adjusting the ratio of azobenzene acrylate, achieving high-contrast RPSLC films [86]. Nevertheless, the required driving voltage for these films remains high, and an increase in the monomer concentration leads to a notable decrease in transmittance in the initial state. Therefore, further improvements in these two critical parameters are essential.

To reduce the driving voltage and improve contrast, Yin et al. employed small phenylacetylene molecules with linear structures [87]. They found that doping linear structures of small phenylacetylene molecule materials can significantly increase the dielectric anisotropy of the LC mixture. As shown in Equation (1), dielectric anisotropy has a substantial impact on both V_th and V_sat. The samples A0’, A4’, B6’, and C5’ in Figure 3a represent 20 μm thick samples with 1.5 wt% DEB, 1 wt% ETB, 1.5 wt% EMB, and 1.25 wt% DEB, respectively. Additionally, these molecules effectively alter the polymer network morphology, creating a denser, coral-like network and more distinct spherical structures, functioning as modifiers within the system, as shown in Figure 3e. Samples A0, A4, B6, and C5 are samples A0’, A4’, B6’, and C5’ with 8 μm thick films. This porous network structure lowers the driving voltage of the sample, while the spherical network positively contributes to contrast enhancement. The combination of these two network structures ultimately results in superior optoelectronic properties for the LC film. However, although the contrast has been improved, its value is still only about 50 in this case, so there is a need to propose a better method.

Table 1. Recent studies on general organic/inorganic compound doping in LCs for enhanced optoelectronic performance.

Materials	Methods	Features	Refs
2 wt% PFPMA, BA, BDDA, LMA, NLC E8	20 μm thick PDLC, UV light (12.5 mW/cm2, 365 nm)	Vth: from 12 V to 7.5 V Vsat: from 18 V to 14 V, compared to that without PFPMA	[88]
21.3 wt% CHMA, 10.7 wt% HPMA, 1.6 wt% BDDA, 6.4 wt% PEGDA600, 2 wt% Irg651, 60 wt% E8	20 μm thick PDLC, UV light (15 mW/cm2, 365 nm)	CR: reach to 262	[89]
2 wt% BDDA, 2 wt% TCDDA, IBOA, Irg651, NLC E8	20 μm thick PDLC, UV light (20 mW/cm2, 365 nm)	Vth: from 9 V to 8.2 V Vsat: from 23 V to 21.2 V CR: from 106 to 203, compared to TCDDA-only case	[90]
1.25 wt% DEB, BPEFDA, RM257, S811, NLC E7	20 μm thick RPSLC, UV light (3.5 mW/cm2, 365 nm)	Vth: from 14.1 V to 7.6 V Vsat: 16.7 V to 10.4 V CR: from 40.9 to 49.2, compared to that without DEB	[87]
0.3 wt% AABM, EDDET, NLC-HNG74100-100	25 μm thick RPSLC, UV light (10 mW/cm2, 365 nm)	Vth: from 18.2 V to 11.5 V CR: from 115.6 to 185.9, compared to that without AABM	[86]

provides an overview of recent studies on general organic/inorganic compound doping in LCs for enhanced optoelectronic performance.

To reduce the threshold voltage while also enhancing contrast, researchers have begun incorporating isotropic or dichroic dyes into LCs, combining the effects of light absorption and light scattering, including dye-doped chiral LC and dye-doped polymer LC. Previous studies have shown that adding dyes with high solubility and strong dichroism can provide a higher contrast ratio. In the theoretical context, without considering the solubility limit, the contrast of dye-doped LCs can also be expressed from the perspective of the dye’s absorption coefficient, as given by Equation (4) [91]:

$$\mathrm{C}\mathrm{R}=\frac{{\mathrm{T}}_{\mathrm{s}\mathrm{a}\mathrm{t}}}{{\mathrm{T}}_0}={\mathrm{e}}^{\frac{\Delta \mathrm{\alpha }}{3}\mathrm{c}\mathrm{d}},$$

(4)

where Δα is the dichroism of the dye, Δα = α_‖ − α_⊥, α_‖ is the absorption coefficient of the sample in the OFF state, α_⊥ is the absorption coefficient of the sample as the saturation voltage is applied, c is the weight-percent concentration of dye, and d is the cell gap. Dye-doped LC offers the advantage of operating without the need for polarizers. When dyes are incorporated into LCs, the orientation of the dichroic dye molecules is guided by the surrounding LC molecules due to the guest–host effect, significantly enhancing the optical efficiency [92,93,94,95]. Guest–host LCs can be viewed as a composite of dye dissolved in a host LC [96]. Usually, geometrically anisotropic dichroic dye molecules are used to absorb light of a specific range of wavelength. Due to anisotropy, absorption is maximized when the light polarization is parallel to the dye molecules’ long axis and minimized when perpendicular. Considering the close host–guest type of interaction between the dye and the LC molecule, both are therefore reoriented together when an external electric field is applied. The dye rotating with the host LC will absorb light to favorably modulate light transmission [15]. However, the improvement in transmittance does not result in a dramatic enhancement in contrast, which often remains limited to values around tens. To address this limitation, Li et al. introduced black dyes into the CLCs [97,98]. In the voltage-off state, the dye molecules follow the random helical arrangement of chiral LC molecules, forming a focal conic texture, which results in a scattering state. Since the dye molecules are randomly oriented, they can absorb light omnidirectionally. Upon the application of voltage, the dye molecules align along the electric field direction, transitioning to an isotropic state (H state) (Figure 4) [99]. This LC configuration is also referred to as guest–host LC. The introduction of dye molecules increases additional light absorption, scattering, and absorption effects, further strengthening the overall performance. As a result, the average transmittance in the dark state decreases sharply, while the transmittance in the transparent state remains largely unaffected, leading to an ultra-high contrast ratio (approaching about 400). Furthermore, the saturation voltage appeared at a low value (about 20 V), significantly optimizing the optoelectronic performance of LC devices and contributing to cost control and improved energy efficiency.

Table 2. Recent studies on dye doping in LCs for enhanced optoelectronic performance.

Materials	Methods	Features (Compared to That Without Dye)	Refs
0.2 wt% AMCA, 0.5 wt% IRG651, SLC-1717	20 μm thick PDLC	Vsat: from 39.7 V to 25.5 V CR: from 58.4 to 96.6	[100]
3 wt% Anthraquinone dye (26B3OH), NLC (D5AOB)	20 μm thick	Vth: from 2.75 V to 2.45 V	[101]
0.3 wt% ZnO, 0.0625 wt % orange azo dye-doped HALC	10 μm thick	Vth: from 1.81 V to 1.79 V Vsat: from 2.05 V to 1.96 V CR: from 203 to 220	[102]
0.3 wt% dye CoPc, NLC E7, chiral dopant S811	7.7 μm thick	Reserve mode Operating voltage: from 9 V to 6.9 V	[103]
A few weight percent black dye, chiral dopant, E7	20 μm thick	The simplest structure and method Vsat: maintaining about 20 V CR: from about 100 to 400	[97,98]
1 wt% black dye (BD), chiral dopant R5011, polymer monomer LC242, E7	12 μm thick PDLC	Vth: maintaining 20 V Vsat: maintaining 26 V CR: from 100 to 286	[104]
0.248 wt% orange azo dye, 0.2983 wt% ZnO NPs, NLC (MC98468)	10 μm thick	Vth: from 1.81 V to 1.71 V Vsat: from 2.05 V to 1.90 V CR: from 203 to 255	[105]
0.05 wt% azo dye DR1, NOA-65, NLC HPC21300-000	23 μm thick PDLC	Vth: from 34.87 V to 10.19 V Vsat: from 46.96 V to 25.54 V CR: from 132.29 to 253.5	[106]
0.5 wt% azo dye: orange 3, chiral dopant R1101, NLC-MDA003969	10 μm thick	Reserve mode Operating voltage: from 29 V to 27 V CR: from 13.7 to 16.4	[94]

provides an overview of recent studies on dye doping in LCs for enhanced optoelectronic performance.

3.2. Doping with Nanoparticles

Nanomaterials with sizes ranging from 1 to 100 nm have attracted much attention in the development of the LC field over the past decade due to their photonic and physical properties that are size-dependent and related to dopant characteristics [107,108,109,110]. With their unique properties, these materials can be incorporated into tunable dielectric anisotropy LC materials to enhance their optoelectronic performance [111,112,113]. As demonstrated by previous studies, a lower polymer content allows LC molecules to more easily reorient under an applied electric field, thereby reducing the driving voltage of PDLC. However, an excessively low polymer content can lead to instability in the alignment of LC molecules. The doping of nanoparticles can reduce the driving voltage without lowering the polymer content, and the driving voltage can be further controlled and optimized by adjusting the type and concentration of nanoparticles [114,115].

Previous studies have demonstrated that incorporating nanomaterials to increase free volume and disrupt specific interactions can significantly enhance the optoelectronic properties of PDLC films. By introducing nanoparticle dopants directly into the PDLC system or combining them with metal layers, the photonic performance of PDLC films can be improved. He et al. introduced polyhedral oligomeric silsesquioxane (POSS) into PDLC and found that the low surface free energy of POSS effectively reduced the driving voltage [116,117,118]. Subsequently, their team proposed a hybrid PDLC film containing cesium tungsten bronze (Cs_xWO₃) and POSS [119]. Initially, by doping only POSS, they observed that as the POSS content increased, both the V_th and V_sat significantly decreased. When the POSS concentration reached 12 wt%, the threshold voltage dropped from approximately 70 V to 17 V, and the saturation voltage dropped from nearly 100 V to below 25 V, while simultaneously achieving a dramatic increase in contrast (up to 400). Although the contrast showed a decreasing trend with further increases in POSS concentration, it only returned to the original level (without POSS) when the concentration exceeded 15 wt%. To investigate the cause of the reduced threshold voltage, the researchers examined the morphology of the polymer matrix using scanning electron microscopy (SEM, Figure 5a). The microstructure of the polymer matrix in the pure PDLC sample was dense, resulting in a high driving voltage and weak scattering effects. After doping with POSS, the sample exhibited a porous polymer matrix, and as the doping concentration increased, the pore size also increased. According to Equations (1) and (2), both V_th and V_sat decrease as the droplet size increases. The addition of POSS, which lowers the surface free energy of the polymer matrix, reduces the anchoring energy of the LC droplets, making them more susceptible to reorientation. Furthermore, due to the pronounced phase separation, the introduced POSS also lowered the transmission at the voltage-off state (T_off), thereby improving the contrast.

Building on this, the researchers introduced the nanomaterial Cs_xWO₃ into the PDLC containing 12 wt% POSS. They found that as the Cs_xWO₃ concentration increased, the driving voltage of the sample decreased. When the doping concentration reached 10 wt%, the sample exhibited ultra-low driving voltages (V_th = 7 V, V_sat = 11 V), while the contrast first increased and then decreased. Similarly, SEM images revealed that when the Cs_xWO₃ concentration ranged from 0 wt% to 4 wt%, the polymer matrix maintained a porous ‘Swiss cheese’ structure, with pore sizes gradually decreasing. This led to enhanced light scattering, reduced T_off, and improved contrast. However, unusually, the driving voltage did not increase due to the reduction in pore size; instead, it gradually decreased. This phenomenon could be attributed to the presence of Cs_xWO₃. Of course, the doping concentration of Cs_xWO₃ is not beneficial beyond a certain point. When the concentration exceeded 6 wt%, the polymer matrix transitioned into a fibrous structure with larger pores. Although the larger pores reduced the anchoring energy at the polymer matrix surface and lowered the driving voltage, they also increased T_off, thereby reducing the contrast. Therefore, selecting the appropriate concentration of Cs_xWO₃ is crucial (Figure 5b).

Inspired by the experimentation with nanomaterial-doped PDLC [120], scientists have integrated the currently popular material, graphene oxide, into the composite with LCs. Graphene oxide is a two-dimensional honeycomb-structured material that possesses unique mechanical properties, such as high flexibility and mechanical strength, as well as exceptional optoelectronic characteristics, including high polarizability, due to the presence of oxygen functional groups [121]. The increased oxidation level of graphene oxide facilitates its effective dispersion in polar liquids and establishes a strong π-π stacking interaction with the LC [122,123]. Previous studies have confirmed the applicability of graphene oxide in LC smart windows. For instance, Dalir et al. found that the addition of 1% graphene oxide to nematic LCs significantly reduced the response time from 9.97 ms to 1.85 ms [124]. Research by Yadev et al. and Mrukiewicz et al. indicated that doping 0.3 wt% graphene oxide into LC could lower the transition temperature from 32.9 °C to 31.2 °C while also reducing the threshold voltage from 0.8 V to 0.74 V [122,123]. In a recent study, Malik’s team optimized the doping concentration of graphene oxide, further enhancing the eco-friendly and energy-saving functionalities of GO-PDLC composite smart windows [125]. They observed that in the absence of an electric field, graphene oxide flakes tend to align along the ITO-coated substrate and follow the orientation of LC molecules (Figure 6a). Due to the strong anchoring force between the benzene rings of the LC and the graphene honeycomb structure, the LC molecules near the graphene oxide flakes tend to align along the surface of the graphene oxide. When the applied electric field reaches a certain level, the graphene flakes in the GO-PDLC composite also align in the direction of the electric field, which promotes the orientation of the LC molecules along the applied electric field direction, resulting in a lower operating voltage for the GO-PDLC composite compared to pure PDLC (Figure 6b). Further investigations revealed that after doping with 0.05 wt% GO, the sample’s V_sat significantly decreased, dropping from 125 V to 11.5 V at 30 °C, and from 105 V to 5 V at 40 °C (Figure 6c). These changes are attributed to the strong π-π stacking interactions between the LC benzene rings and the GO structure, further demonstrating the critical role of the interaction between the LC molecules and graphene oxide in reducing the driving voltage. However, a notable drawback is that the threshold voltage of the GO-PDLC sample at 30 °C is slightly higher than that of pure PDLC, and a reduction in contrast may occur at this temperature.

From the aforementioned studies, it is evident that the incorporation of nanoparticles (NPs) can sometimes lead to a reduction in the transmittance of LC devices [126]. However, the introduction of carbon nanotubes (CNTs) has addressed this issue. CNTs, with their elongated axial shape similar to that of LC molecules, align more easily with LC molecules, thus preventing the reduction in optical transmittance [127,128]. Previous research has demonstrated that the inclusion of CNTs and CNT composites into PDLC can significantly reduce the threshold voltage while maintaining a satisfactory contrast ratio [129,130]. The primary aim of reducing the driving voltage is to minimize the energy consumption of LC smart windows. Therefore, in addition to conventional PDLC, Li’s team has also explored the addition of multi-walled carbon nanotubes (MWCNTs) into reverse-mode polymer network LC (R-PNLC) [131]. Since, in practical applications, the transparent state of smart windows is typically required to be maintained for extended periods, R-PNLC inherently possesses an advantage in energy efficiency. Their studies revealed that the doping of MWCNTs not only reduced the V_th and V_sat but also enhanced the contrast ratio as the concentration of MWCNTs increased. Unlike other dopants, the size of the LC domains did not significantly change with varying MWCNT concentrations. To further investigate the impact of MWCNTs on the driving voltage, the researchers measured the conductivity and the impedance of R-PNLC films with different MWCNT concentrations. They found that the addition of MWCNTs enhanced the conductivity of the polymer matrix, leading to a decrease in V_th. However, it is important to note that an excessive amount of MWCNTs can increase the ionic impurities within the LC, generating counteracting electric fields that weaken the effective electric field, thereby raising the driving voltage. Therefore, determining the optimal concentration of nanomaterials is a critical issue that warrants careful investigation.

The incorporation of carbon nanotubes (CNTs) has ushered in a new era for PDLC. However, the limitations associated with their use cannot be overlooked. The aggregation of nanoparticles, leading to premature aging of the material, can significantly shorten the lifespan of the device. To address this issue, Zhong’s team proposed a composite structure of a bilayer nanofiber film-doped system [132]. They employed a polymer-induced phase separation method to incorporate antimony-doped tin oxide (ATO) and Cs_xWO₃ into PDLC using a co-polymerizable monomer, KH570. The electro-optical and physical properties of the resulting films were extensively studied. The results showed that the inclusion of both nanoparticles significantly reduced the driving voltage and enhanced infrared absorption. Specifically, the infrared absorption efficiency in the scattering and transparent states was 12% and 7%, respectively. Furthermore, the films exhibited excellent stability under high-intensity ultraviolet (UV) and high-temperature irradiation. Their mechanical properties, such as tensile strength and fracture growth rate, remained at a high level. However, it is noteworthy that increasing the nanoparticle concentration resulted in a reduction in the contrast ratio. Therefore, maintaining high overall optical and physical performance remains a key area for further exploration.

In addition, doping semiconductor NPs such as quantum dots (QDs) into LC opens a new avenue for the applications of LC materials in optoelectronic fields and beyond [133,134,135]. Compared to other non-mesogens (dyes, nanoparticles, and polymers), QDs offer superior optical efficiency and stable luminescence, which further broadens the application range of LCs. QDs are nanoscale semiconductor crystals that exhibit unique optical properties. Gagandeep Kaur and colleagues combined ferroelectric LCs (FLCs) with CdSe QDs and investigated their interactions [136] (Figure 7a). The researchers found that the spontaneous polarization (P_s value) of the samples decreased with the doping of QDs and increasing temperature. As a result, FLC doped with 0.05 wt% CdSe QDs exhibited improved performance, working at a lower voltage (reduced from 12 V to 6 V) compared to pure FLC, as shown in Figure 7b. Meanwhile, the addition of quantum dots facilitates the loosening of molecular packing, resulting in a more responsive electric field effect compared to pure FLC. This modification also reduces rotational viscosity, decreases the optical tilt angle, and significantly enhances anchoring energy. As shown in Equation (5) [137], the increase in anchoring energy plays a crucial role in reducing the response time, thereby optimizing the optical performance of the system:

$$\mathrm{\tau }=\frac{\mathrm{\eta }}{\mathrm{K}{\mathrm{\pi }}^2}\left({\mathrm{d}}^2+\frac{4\mathrm{d}\mathrm{K}}{\mathrm{W}}\right),$$

(5)

where W is the anchoring energy, d is the cell gap, K is the effective elastic constant, and η is the rotational viscosity.

Table 3. Recent studies on nanomaterial doping in LCs for enhanced optoelectronic performance.

Materials	Methods	Features (Compared to Those Without NPs)	Refs
0.6 wt% Zirconium oxide NPs, IRG651, CHMA, UV6301, SLC1717	20 μm thick PD&PSLC, UV light (365 nm)	Vth: from 13.73 V to 10.099 V Vsat: from 29.61 V to 20.83 V	[82]
0.3 wt% AABM NPs, EDDET, LC HNG74100-000	25 μm thick RPSLC, UV light (10 mW/cm2, 365 nm)	Vth: from 18.2 V to 11.5 V CR: from 115.6 to 185.9	[86]
8 wt% SiO2-SH NPs, acrylate monomer, SLC1717	20 μm thick PDLC, UV light (5 mW/cm2, 365 nm)	Vth: from 72 V to 16 V Vsat: from 98 V to 28 V CR: from about 20 to 165	[138]
0.05 wt% CNP, NOA-65, NLC HPC21300-000	23 μm thick PDLC, UV light (15 W/cm2, 354 nm)	Vth: from 34.87 V to 9.26 V Vsat: from 46.96 V to 19.02 V CR: from 132.29 to 189.09	[106]
0.4 wt%/0.4 wt% Fe3O4/Cs0.33WO3 NPs, UV6301, CHMA, IRG651, SLC1717	20 μm thick PDLC, UV light (10 mW/cm2, 365 nm)	Vsat: from 30.14 V to 22.47 V CR: from 60.31 to 76.45	[139]
0.5 wt% TiO2 NPs, NOA65, NLC BL036, chiral dopant CB15	5 μm thick PSCLC, UV light (3 mW/cm2)	Vth: from 3.2 V to 1.4 V Vsat: from 10.5 V to 7.5 V	[31]
1 wt% ANWs, NLC E7, NOA65	6 μm thick PDLC, UV light (3 mW/cm2)	Vth: from 4 V to 1.5 V Vsat: from 38 V to 19.5 V	[140]
0.05 wt% CNPs, NLC E7, NOA-65	23 μm thick PDLC, UV light (8 mW/cm2, 354 nm)	Vth: from 37 V to 17 V Vsat: from 66 V to 43 V CR: from 464 to 822	[141]

provides an overview of recent studies on nanomaterial doping in LCs for enhanced optoelectronic performance.

3.3. Optimization of Doping Material Structures

From a macroscopic perspective, the addition of various dopants to enhance the optoelectronic properties of LC has become a mainstream direction in current research. However, altering the structure of dopants at the microscopic level may also serve as another potential breakthrough for investigation. Yang’s team, while optimizing polymer monomer formulations as much as possible, explored the impact of the chain length of acrylate monomers on the performance of PDLC films [142] (Figure 8a). They found that as the alkyl chain length of the acrylate monomers increased, the driving voltage of the PDLC samples significantly diminished. This improvement is attributed to the fact that, for the same mass fraction of the dopant, the molar fraction of long-chain monomers is smaller, resulting in a greater steric hindrance per unit group and fewer reactive groups [143,144]. Consequently, the system’s reaction rate slows down, allowing the LC molecules more time to diffuse and aggregate. This leads to an enlargement of the polymer network pores, which in turn creates anchoring effects at the LC–polymer interface, allowing LC molecules to realign under a lower voltage. However, improving the monomer structure does not necessarily mean that longer chain lengths are always beneficial. When the chain length becomes excessively long, the increased steric hindrance can hinder the diffusion and aggregation of the LC molecules, resulting in a reduction in the polymer network pore size and an increase in the driving voltage.

As previously mentioned, due to the higher polymer content and the strong anchoring effect of the polymer on the LC molecules, PDLC devices typically require higher operating voltages that may exceed safe voltage limits, leading to increased energy consumption and safety risks. Previous studies have mainly focused on reducing the driving voltage of PDLC by increasing the polymer network mesh size; however, this approach often leads to excessively large average pore sizes, which in turn reduces the contrast ratio [145,146]. To address this limitation, the most widely adopted solution is the use of silicon and fluorinated materials [147]. Fluorinated materials have a lower surface energy, and these low-surface-energy monomers dispersed at the polymer–LC interface can effectively reduce the surface tension and thereby lower the anchoring energy, reduce the reaction rate, and ultimately lead to a decrease in the driving voltage. Additionally, the presence of fluorine atoms, which have high electronegativity and a small atomic radius, imparts excellent thermal stability, antioxidative properties, wear resistance, and chemical stability. Previous studies have indicated that the incorporation of fluorinated monomers in PDLC films can lower the saturation voltage [148]. However, when applied to smart windows, these performances were found to be insufficient. After investigating the impact of the acrylate chain length on the LC device performance, Zhang et al. further systematically examined the influence of fluorinated monomers on the performance of PDLC films [88] (Figure 8b). They found that the number of fluorine atoms in the fluorinated monomer significantly impacted the optoelectronic properties of the LC films. As the number of fluorine atoms in the monomer’s terminal group increased, the surface energy of the monomer gradually decreased, which slowed down the reaction and diffusion rates. This resulted in an increase in the mesh size of the polymer network, thereby reducing the driving voltage, as shown in Figure 8c,d.

Table 4. The compositions of samples B0-B7. Reprinted with permission from [88]. Copyright 2024, Elsevier.

Sample	Composition	(wt%)
B0	BDDA/BA/LMA	6.0/12.0/12.0
B1	BDDA/BA/LMA/TFEMA	6.0/10.0/12.0/2.0
B2	BDDA/BA/LMA/TFOMA	6.0/10.0/12.0/2.0
B3	BDDA/BA/LMA/PFPMA	6.0/10.0/12.0/2.0
B4	BDDA/BA/LMA/HFBMA	6.0/10.0/12.0/2.0
B5	BDDA/BA/LMA/HFOMA	6.0/10.0/12.0/2.0
B6	BDDA/BA/LMA/OFOMA	6.0/10.0/12.0/2.0
B7	BDDA/BA/LMA/PFMA	6.0/10.0/12.0/2.0

shows the composition of samples B0-B7; the films doped with 2 wt% PFPMA exhibited the best optoelectronic performance, with a more than 30% reduction in both the V_th and V_sat, while maintaining a commendable contrast ratio (110). However, when the number of fluorine atoms was excessively high, the performance of the PDLC films gradually deteriorated. Besides the previously mentioned effect of the overly long chain lengths, excessive fluorination could cause fluorinated monomers to permeate into the LC phase, hindering the diffusion of the LC molecules and causing non-uniform polymer network structures, which in turn increased the driving voltage. Therefore, the selection and design of the dopant structures have a profound impact on the overall performance of LCs. Looking ahead, in addition to the structural studies of the conventional small molecules, the structural optimization of emerging materials, such as nanomaterials, could become a key research direction.

3.4. Selecting Appropriate Polymerization Temperature and UV Intensity

For both PDLC and PSLC, the polymerization stage in the fabrication process is critical and indispensable. In the widely used PIPS method, maintaining appropriate UV polymerization intensity and polymerization temperature is essential for optimizing the optoelectronic performance of LC devices. UV intensity and polymerization temperature primarily influence the photopolymerization reaction rate, thereby affecting the microstructure, topology, and mechanical properties of the polymer network. Ma et al. incorporated acrylate-based LC monomers (LCMs) into PDLC and investigated their optoelectronic properties at varying UV intensities and polymerization temperatures [76]. The study revealed that when the polymerization temperature was maintained below 35 °C, both the V_th and V_sat remained at very low levels with minimal variation. However, when the polymerization temperature exceeded 35 °C, both the V_th and V_sat increased with rising temperatures. This effect is attributed to the enhanced anchoring effect of the PDLC network on the LC molecules at higher polymerization temperatures, which in turn raises the driving voltage. Additionally, contrast performance was observed to be superior when the polymerization temperature was below 35 °C, suggesting that an optimal polymerization temperature for PDLC films is approximately 25 °C. Compared to the polymerization temperature, variations in UV intensity had a smaller impact on the driving voltage. It is well-known that the contrast ratio is primarily determined by transmittance in the dark state. The experimental results showed that a UV intensity of 25 mW/cm² provided the highest dark-state transmittance and contrast for the samples. Therefore, selecting suitable polymerization temperatures and UV intensities can significantly enhance the development of high-performance LC devices.

3.5. Adjusting the Driving Frequency

Previous studies have confirmed that, due to the dielectric anisotropy of LC, polymer-LC (PLC) systems can exhibit dynamic electro-optical responses to applied frequencies [149]. This discovery presents a novel approach for reducing the threshold voltage of LC devices. As mentioned earlier, multi-walled carbon nanotubes (MWCNTs), through their unique interactions with LCs, have been shown to significantly improve the optoelectronic performance of PDLC. In addition, researchers have introduced a new method that utilizes the frequency response characteristics of MWCNTs to adjust the transmittance and driving voltage of PDLC by modulating frequency [131]. Possibly due to relaxation oscillations, they observed that within the low-frequency range of 10–150 Hz, the V_th decreased with increasing frequency regardless of MWCNT doping; however, the V_th of R-PNLC samples doped with MWCNTs exhibited a more pronounced sensitivity to frequency. In the frequency range of 200–950 Hz, the V_th of undoped samples sharply increased with rising frequency, whereas MWCNT-doped samples showed only a slight decrease in V_th. When the frequency reached 1000 Hz, the V_th of MWCNT-doped LC samples reached its lowest point. At frequencies above this threshold, the V_th of the doped samples began to increase gradually, but the undoped PLC saturated earlier (Figure 9a,b). This broader frequency modulation range contributes significantly to reducing V_th and enables transmittance modulation at lower voltages.

4. Conclusions and Perspective

In this paper, we systematically discuss the principles and current state of the development of LC smart windows from an electrochromic perspective. Building on the significant and stable progress already achieved in response times, we focus on recent efforts over the past two years aimed at enhancing two key optoelectronic properties of LC smart windows: contrast and driving voltage. We categorize and explain specific strategies and methods for improving these metrics. In this section, we provide a summary evaluation of these approaches and outline the challenges and potential future directions in this field.

The addition of various dopants has long been a research hotspot in the development of LC smart windows. Introducing organic and inorganic materials into LC–polymer mixtures is a well-established approach for modifying the LC device properties. Common methods include traditional organic/inorganic doping, cholesteric (chiral) molecule doping, dye-doping (guest–host systems), and nanostructured material doping. Traditional organic/inorganic doping primarily aims to reduce operating voltage by modulating the interaction between LCs and dopants and minimizing the anchoring forces at the LC–polymer–substrate interfaces. However, while voltage reduction is achieved, contrast enhancement often remains unsatisfactory. To address this, dichroic dye compounds with unique light absorption characteristics have gained popularity. Building on the strong scattering function of chiral cholesteric LC, these dyes can absorb light at specific wavelengths when the LC is in an opaque state, effectively reducing transmittance and thereby increasing contrast. However, the choice of dye must be made with caution, considering several key characteristics. The selected dye should have optimal absorption along the molecular chain, as well as a high dichroic ratio, high-order parameters, and excellent chemical stability; it also should possess good solubility in LC molecules because greater solubility implies a higher performance potential. Generally, dyes with positive dielectric anisotropy are preferred as their absorption transition dipoles align along the long molecular axis, weakly absorbing incident light polarized perpendicular to the dye axis, while absorption is maximized when polarization is parallel. In addition to dye incorporation in cholesteric LC, combining dyes with polymer–LC systems represents a new advancement: the fast response and excellent operating voltage of polymer–LC systems, combined with the high contrast of dye-doped systems, significantly enhance the optoelectronic performance of LC smart windows. Nevertheless, the limitations of polymer systems stem from the polymer itself: aiming for a low driving voltage by lowering the polymer content can lead to LC instability. Therefore, various nanostructured dopants have been extensively used in research, effectively mitigating the adverse effects of reduced polymer content. These nanostructures include nanos, nanorods, metallic nanoparticles, carbon nanotubes, quantum dots, etc. Advances and innovations in nanomaterials not only improve the optoelectronic performance and efficiency of LC smart windows but also provide superior mechanical properties and stability, making them more suitable for industrial and consumer applications. Of course, balancing the dopant concentration to achieve optimal combined performance remains an ongoing research priority; cost control and energy consumption are also essential considerations.

In addition to lateral studies on different dopant types, it is feasible to conduct longitudinal self-optimization of specific dopant structures and to refine LC formulations from an external perspective. At the microscopic level, the effect of the molecular chain length on device performance is substantial; factors such as temperature and UV intensity, which influence the polymerization process of LC mixtures, are also closely linked to optoelectronic properties. Furthermore, the frequency response characteristics of LC suggest that frequency modulation may offer a promising approach to reducing driving voltage and lowering conventional energy consumption.

In fact, for wearable smart glasses and smart windows, a contrast ratio of 100 and a drive voltage below the human safety voltage (36 V) or the human continuous contact safety voltage (24 V) are sufficient to meet people’s daily needs. However, there are application scenarios where a high contrast ratio is of utmost importance. Scientists are still striving to improve their optoelectronic performance as higher contrast ratios allow wider modulation ranges, while lower drive voltages result in reduced energy consumption and safer operation. It is worth noting that as the dimensions of liquid crystal smart windows increase, the mechanical property and the uniformity of the performance remain topics worthy of further research in the future.

In summary, as a unique material, LC offers exciting possibilities in many fields nowadays. In addition to energy conservation and privacy protection, LC smart windows can also be used in smart sunglasses for vision care [97,98], which can adaptively adjust the transmission of light according to the actual light condition in the environment, and they can also be applied for clinical treatment of amblyopia [97,98,150]. In general optical systems, they can serve as adaptive optical attenuators to perform real-time dynamic tuning of light with a very large dynamic range [151]. They may also perform selective blocking functions and be applied in multiple areas such as augment display [151]. LC smart windows not only demonstrate remarkable operational convenience and user experience enhancement but also exhibit significant potential for integration with renewable energy solutions, thereby advancing environmental sustainability. This paper focuses on the key optoelectronic metrics of contrast and driving voltage, summarizing recent research progress to promote the future development and applications of LC smart windows.