Review of Angular-Selective Windows with Guest–Host Liquid Crystals for Static Window Applications

Abstract

This review focuses on the development and advancements in angular-selective smart windows, with particular emphasis on static windows utilizing guest–host liquid crystal (GHLC) systems. Angular-selective windows are designed to adjust their transmittance based on the angle of incident light, offering enhanced energy efficiency and visual comfort in both architectural and automotive applications. By leveraging the anisotropic absorption properties of dichroic dyes, GHLC-based windows can selectively block oblique sunlight while preserving clear visibility from normal viewing angles. Various liquid crystal (LC) alignment configurations, including vertically aligned, homogeneously aligned, hybrid aligned, uniformly lying helix, and twisted aligned LC cells, have been investigated to optimize light control for different installation angles, such as for automotive windshields and building windows. These advancements have demonstrated significant improvements in energy conservation and occupant comfort by reducing cooling demands and regulating sunlight penetration. This review summarizes key findings from recent studies, addresses the limitations of current technologies, and outlines potential future directions for further advancements in smart window technology.

1. Introduction

Windows play a critical role in modern architecture, balancing the need for natural light, ventilation, and thermal regulation. Smart windows, which can dynamically or statically modulate the amount of sunlight and solar heat entering a space, have emerged as a key technology for improving energy efficiency while maintaining visual comfort [1,2,3,4,5,6,7,8]. In the context of escalating concerns about global warming and energy consumption, the development of energy-efficient solutions in buildings has become paramount. Traditional solutions, such as tinted windows, often fall short because they cannot adjust based on the angle of sunlight, leading to issues like glare and heat buildup. To address this, both dynamic and static smart windows have been developed, each offering unique advantages depending on the application.

Dynamic smart windows utilize systems that incorporate chromic materials, including electrochromic, photochromic, and thermochromic technologies. These windows modify their transmittance in reaction to external stimuli, such as variations in light, heat, or applied voltage [9,10,11,12,13,14]. While these dynamic windows offer flexibility in controlling light and heat, they generally absorb light isotropically, blocking sunlight from all directions equally. This uniform control can limit visibility and does not account for the angle of incoming light, which can be problematic in situations requiring selective sunlight control.

In contrast, angular-selective smart windows, often referred to as static windows, offer a more targeted approach [15,16,17,18]. These windows, particularly those utilizing guest–host liquid crystal (GHLC) systems [19,20,21], are designed to selectively block sunlight from specific angles, such as oblique incident light, while maintaining a clear view from direct, normal angles. This angular dependence is achieved through the anisotropic absorption properties of dichroic dyes, making static windows particularly effective in managing glare and heat from specific directions. For instance, static smart windows can reduce intense solar heat and glare during peak sunlight hours without darkening the entire view, thus minimizing the need for artificial lighting or excessive cooling. This capability is especially relevant for zero-energy buildings, where maximizing energy efficiency is essential to meet stringent sustainability goals and reduce reliance on external energy sources.

The distinction between static and dynamic smart windows is crucial as it highlights different approaches to optimizing energy efficiency and visual comfort. While dynamic windows excel in providing adjustable transparency in response to changing external conditions, static windows offer more refined control based on the angle of sunlight. This makes static windows ideal for applications such as automotive windshields, where reducing glare from oblique angles is critical for safety, or in buildings with large window surfaces, where selective light transmission can significantly reduce cooling requirements while maintaining indoor comfort. In a broader context, integrating angular-selective smart windows into architectural design can contribute significantly to global efforts aimed at combating climate change by reducing energy consumption and promoting sustainable living.

In this review, we explore the advancements in angular-selective smart windows, with a focus on GHLC systems. We examine various design strategies, performance metrics, and energy-saving potential of these windows, highlighting their distinct advantages over traditional dynamic windows. We also explore existing technologies and future research strategies that have the potential to enhance energy efficiency and improve occupant comfort, thereby contributing to the creation of more sustainable built environments.

2. Static (Angular-Selective) Windows

Most conventional studies on light-absorbing windows have primarily focused on normal incidence, where light enters the window directly at a perpendicular angle. However, this approach does not adequately reflect real-world scenarios. In real-world applications, sunlight usually reaches windows at oblique angles due to varying solar elevations throughout the day and across different seasons [19]. According to the data of the sun path in Korea during the summer and winter solstices [22], in summer, the solar elevation angle peaks around midday, causing sunlight to hit windows from a higher angle. Conversely, during winter, the solar elevation is lower, resulting in sunlight entering at shallower angles. These seasonal variations not only influence the intensity of sunlight but also the quality of light that enters the building, affecting both the esthetic and functional aspects of interior spaces.

To achieve optimal energy efficiency, controlling the transmission of sunlight based on these varying angles is crucial [19,21]. During the summer, absorbing more sunlight at higher angles can help reduce the heat entering the building, thus lowering cooling energy demands. This is particularly important in regions with hot climates, where excessive heat can lead to discomfort and increased reliance on air conditioning systems, significantly driving up energy consumption. On the other hand, in the winter, transmitting more light at lower angles can reduce heating and artificial lighting requirements (Figure 1a). By allowing more sunlight to enter, buildings can harness natural light, which is beneficial not only for energy savings but also for creating a more pleasant indoor environment. This ability to adapt to both seasonal and daily changes in solar elevation makes angular-selective windows an effective solution for energy saving in both commercial and residential settings.

Beyond their application in buildings, static windows are also highly beneficial in automotive contexts (Figure 1b) [20]. Many drivers use tinted windows, sunglasses, or sun visors to reduce glare and block sunlight. However, safety regulations in countries such as the United States and across Europe often limit windshield tinting to a transmittance of 50–70%, which can be insufficient for blocking sunlight and glare during driving. This regulatory limitation highlights the need for innovative solutions that prioritize both safety and comfort. Traditional solutions like sunglasses and sun visors also come with their own drawbacks, including frequent adjustments and restricted visibility. Moreover, these conventional methods may not provide adequate protection against UV radiation, which can have harmful effects over prolonged exposure.

Static windows utilizing GHLC cells offer a more efficient alternative. These windows can block oblique sunlight while maintaining clear visibility through normal angles, making them ideal for both buildings and vehicles. For example, when the solar elevation angle exceeds 75°—common during certain times of the day and year—GHLC windows effectively block direct sunlight without obstructing the driver’s view. This feature is especially crucial during dawn and dusk when the sun is low on the horizon, as these are often the times when glare is most problematic for drivers. This angular selectivity offers a significant advantage over traditional static solutions, such as tinted glass, by dynamically adjusting the transmittance based on sunlight’s incident angle. This results in improved comfort and safety for drivers.

Overall, static windows provide a necessary solution for improving both energy efficiency and occupant comfort across various environments. As we confront global challenges like climate change and the urgent need for sustainable living practices, the adoption of such innovative window technologies becomes increasingly critical. By selectively absorbing or transmitting light based on the angle of incidence, these windows offer a flexible and efficient method for managing solar radiation, contributing to reduced energy consumption and enhanced user experience in both architectural and automotive applications. Their integration into building designs and vehicle manufacturing processes can significantly contribute to the broader goals of energy conservation and environmental sustainability.

3. Principles of Designing Static Windows Using GHLC

GHLC devices, which consist of a host liquid crystal (LC) and guest dichroic dye, have gained significant attention due to their high transmittance and polarizer-free structure [23,24]. This section outlines the fundamental principles that underlie the design and operation of GHLC static windows, highlighting their unique properties and potential applications. The unique optical characteristics of GHLC devices stem from the dichroism of incorporated dye molecules. The transmittance of a homogenously aligned (HA) dye-doped LC cell can be described using the following mathematical expression [25,26,27]:

$$T_{\parallel }=T_0{e}^{-{\alpha }_{\parallel }cd}$$

(1a)

$$T_{\perp }=T_0{e}^{-{\alpha }_{\perp }cd}$$

(1b)

where T_∥ and T_⊥ refer to the transmittance of the GHLC cell when the polarization is aligned parallel and perpendicular to the dye molecules’ absorption axis, respectively. α_∥ and α_⊥ represent the absorption coefficients of dye-doped LCs for polarization parallel and perpendicular to the absorption axis of the dye molecules, respectively. These dye molecules display a significant absorption of light polarized along their absorption axis, while exhibiting minimal absorption for light polarized orthogonally to this axis (Figure 2a). T₀ represents the transmittance of an LC cell without dye, while c and d denote the concentration of the dye gap and cell gap, respectively.

When designing GHLC static windows, it is essential to consider the angle of incident light as a variable. Both the absorption coefficient and effective cell gap are influenced by this angle, increasing as the angle of incidence rises. The effective absorption coefficient can be calculated as follows:

$${\displaystyle \frac{1}{{\alpha }_{eff}^2\left(\theta \right)}}={\displaystyle \frac{{sin}^2\theta }{{\alpha }_{\parallel }^2}}+{\displaystyle \frac{{cos}^2\theta }{{\alpha }_{\perp }^2}}$$

(2)

where θ is the angle of the incident light. As the incident angle increases, the tilt angle of the dye molecules also increases, leading to enhanced absorption for light polarized parallel to the absorption axis (Figure 2b).

Furthermore, the effective cell gap is affected by the angle of incident of the light and can be computed using basic trigonometric functions:

d_eff (θ) = d₀/cos θ

(3)

Here, d₀ is the cell gap perpendicular to the substrates, and d_eff indicates the effective cell gap as a function of the incident light angle. This demonstrates that as the angle of incidence increases, the effective cell gap also increases, which correlates with a decrease in transmittance (Figure 2b).

Figure 2c illustrates the transmittance contour of the vertically aligned (VA) GHLC cell. The VA cell shows a steady decrease in transmittance in all directions as the angle of incidence increases. GHLC films, in contrast, have shown promise for various applications, including privacy displays and reducing reflections on automotive windshields. While VA cells have traditionally been favored for their simplicity, it is crucial to recognize that they may compromise viewing angles for end-users.

The integration of GHLC technology into static windows offers a distinct advantage: it enables the efficient management of light transmission while maintaining high visual clarity. By leveraging the angular-selective absorption properties of GHLC devices, designers can create windows that not only optimize energy efficiency but also enhance user comfort by minimizing glare and maintaining clear visibility. In summary, the design principles of GHLC static windows revolve around the exploitation of dichroic properties, the consideration of angular dependence, and the optimization of performance characteristics. This innovative approach paves the way for developing advanced window solutions that address the needs of various applications while promoting energy conservation.

4. Effects of Different LC Alignments on Static Windows

Figure 3a is a 3D schematic representation of the GHLC device in the VA state. The incident angle of sunlight increases relative to the normal view (z-direction). The GHLC devices exhibit different transmittance depending on the liquid crystal alignment state (Figure 3b). The transmittances of LC cells in different alignment states—VA, HA, uniformly lying helix (ULH), and twist-aligned (TA)—can be expressed using Equation (4) [28]. We used unpolarized incident light for the calculations in the equation. Since the effective absorption coefficients differ in the x- and y-directions, we separated the incident light into these two directions. We then calculated the transmittance for each direction individually and superimposed the results to obtain the final transmittance. Each alignment state affects the behavior of dye molecules and their interaction with polarized light in unique ways, influencing the transmittance characteristics. The transmittances of states can be expressed, respectively, as

$$T_{\mathrm{V}\mathrm{A}}=T_0\mathit{exp}\left(-{\alpha }_{\perp }cd\right)$$

(4a)

$$T_{\mathrm{H}\mathrm{A}}=T_0\left[\mathit{exp}\left(-{\alpha }_{\parallel }cd\right)+\mathit{exp}\left(-{\alpha }_{\perp }cd\right)\right]/2$$

(4b)

$$T_{\mathrm{U}\mathrm{L}\mathrm{H}}=T_0\left[\mathit{exp}\left(-\left({\alpha }_{\parallel }+{\alpha }_{\perp }\right)cd/2\right)+\mathit{exp}\left(-{\alpha }_{\perp }cd\right)\right]/2$$

(4c)

$$T_{\mathrm{T}\mathrm{A}}=T_0\mathit{exp}\left[-\left({\alpha }_{\parallel }+{\alpha }_{\perp }\right)cd/2\right]$$

(4d)

In the VA state, dye molecules are aligned perpendicular to the substrate, making the window transparent at normal incidence. In the HA state, they selectively absorb light polarized along the rubbing direction. In the ULH state, LCs and dyes are twisted, absorbing light thar is polarized perpendicular to the helical axis and parallel to the dichroic dye’s principal axis. In the TA state, dye molecules absorb light regardless of their polarization.

For static windows, the aim is to maintain transparency at normal viewing angles and opacity at oblique angles. Therefore, the difference in transmittance between normal and oblique views is crucial for performance. The absorption coefficients for different LC states were calculated (Figure 3c–f), showing how molecular structure and absorption vary with the incidence angle in the x-, y-, and z-directions.

In the VA state, as the incidence angle increases, the molecules gradually shift from vertical to horizontal (HA state), leading to increased absorption (Figure 3c). In the HA state, the absorption coefficient remains unchanged in the x-direction but decreases in the y-direction as the angle increases (Figure 3d). In the ULH state, as the angle increases in the x-direction, the molecular structure shifts towards the TA state, resulting in increased absorption. In the y-direction, the structure remains constant, with no significant change in absorption (Figure 3e). In the TA state, the molecular alignment shifts to a helical structure at higher angles, reducing absorption in both the x- and y-directions (Figure 3f).

These variations in absorption coefficients based on LC alignment and incident angles are key to optimizing static window performance, allowing for control over light absorption while maintaining transparency at specific viewing angles.

5. Static Windows for Energy-Saving Building

In order to assess the angular transmittance of GHLC cells featuring various configurations, we utilized the “TechWiz LCD 2D” software, which employs the Berreman matrix method for light transmission calculations. Static windows do not rely on polarizers and are designed to block unpolarized sunlight; the transmittance of the GHLC cells was calculated by an unpolarized white light source. The primary objective was to determine the optimal dye concentration and cell gap by analyzing the transmittance variations in both the x- and y-directions. Using the algorithm for optimization [28], we set the combination of LC and dye that showed the highest transmittance difference. The most effective configuration included ML-1309 LC material (nematic phase at room temperature), characterized by an optical birefringence (Δn) of 0.1096 and a dielectric anisotropy (Δε) of −3.9 from Merck mixed with the black dye X12 from BASF, as shown in Figure 2a.

We confirmed the absorption coefficients using this specific LC-dye mixture. The process involved a horizontally aligned cell that allowed for the evaluation of the absorption coefficient for horizontally polarized light, assessed both parallel and perpendicular to the absorption axis. The X12 dye exhibited absorption coefficients of α_⊥ = 0.024 μm⁻¹ and α_∥ = 0.227 μm⁻¹ within the wavelength range of 400 to 700 nm. For the VA and HA cells, we maintained a cell gap of 10 μm with a dye concentration of 1 wt%. In the case of the ULH cell, we incorporated a chiral dopant (S811, HTP ~11) and established a pitch of 8.75 μm, noting that variations in pitch did not significantly influence transmittance. We selected a cell gap of 5.5 μm and a dye concentration of 0.6 wt%. For the TA cell, the same chiral dopant was utilized, and a pitch of 1000 nm was established to reduce light reflection. This cell configuration also had a cell gap of 10 μm, with a dye concentration of 2 wt%.

This section provides a detailed analysis of the transmittance properties of various GHLC cells—VA, HA, ULH, and TA—across different incident angles. As presented in Figure 4a,b, software simulations revealed substantial differences in transmittance based on cell structure and incident angle. For instance, the VA cell exhibited the largest transmittance difference between normal and oblique angles, especially in the z- and y-directions at 75°, showing transmittance values of 60.3% and 18.4%, respectively. This resulted in a discrepancy of 41.9%, making it the most effective at controlling sunlight. By comparison, the TA cell showed minimal transmittance differences, with values of 3.73% and 2.48%, respectively, resulting in a small discrepancy of 1.26%.

In the x-direction, both VA and TA cells demonstrated similar transmittance behaviors, limiting the field of view for occupants. However, the HA and ULH cells showed significantly lower transmittance differences of 8.85% and 12.5% in the z- and x-directions, ensuring a broader and more stable viewing angle, particularly in the horizontal plane (x-direction) within a ± 60° range. This feature of the HA and ULH cells provides a nearly uniform transmittance, maintaining the visual comfort of building occupants while effectively reducing glare.

Figure 4c–f further highlights the transmittance contours of these cells, with the VA cell showing a consistent decline in transmittance across all directions as the incident angle increases. In contrast, the HA and ULH cells exhibit more controlled transmittance variation, particularly in the x-direction, making them ideal candidates for applications requiring wide viewing angles and controlled light absorption. The TA cell, with its uniform transmittance across all angles, can be useful in applications where uniform light blocking is desired.

This comparison underscores that the ULH cell may serve as a promising candidate for building applications due to its balance between light control and a wide viewing angle, while the TA cell’s uniform transmittance makes it suitable for cases where blocking all incident light is required.

To gain a deeper understanding of how varying incident angles affect light absorption in VA, HA, ULH, and TA cells, we carried out a detailed series of simulations. These simulations were performed at incident angles ranging from 0° to 75°, with increments of 15° for each trial (refer to Figure 5 for visual representation). As the angles of incidence increased, distinct behaviors emerged for each cell type. In the case of the VA cell, the structure appeared progressively darker as the incident angle increased, showing uniform behavior regardless of the viewing direction. This consistent darkening suggests that VA cells are highly sensitive to variations in incident angles. On the other hand, the HA and ULH cells, particularly in the x-direction, showed significantly lower light absorption, maintaining their brightness even at incident angles as steep as 60°. This performance highlights the robustness of these cells in managing light absorption across a wide range of viewing angles.

Although the static window configuration we tested does not possess dynamic transmittance adjustment, it provides several practical benefits. Most notably, it can help reduce energy consumption by limiting heat from sunlight, thus lowering air conditioning costs, while simultaneously enhancing the overall visual comfort for users. Additionally, our findings suggest that HA and ULH cells exhibit higher transmittance in the y-direction compared to the VA cell, revealing an area that warrants further investigation. One promising future direction is exploring methods to increase absorption in the y-direction without compromising the notable transmittance levels in the z- and x-directions, especially for applications requiring uniform light control. However, it is important to acknowledge the HA cell’s limitations, particularly its relatively smaller transmittance differences, which may reduce its effectiveness in certain scenarios.

6. Static Windows for Automotive Applications

In automotive applications, drivers often rely on sunglasses or sun visors to reduce glare and block direct sunlight, as conventional tinted windows struggle to balance visibility with sunlight control [20]. Unlike static windows in energy-efficient buildings, which are optimized for maximum transmittance at normal incidence, automotive windshields present a unique challenge due to their installation angle. Traditional window designs, while effective for flat surfaces, face limitations when applied to windshields because the angle of installation affects the sunlight incidence throughout the day. This can lead to suboptimal sunlight control and reduced visibility in vehicles. Therefore, a new design approach is required—one that considers the windshield’s installation angle to ensure clear visibility for drivers while efficiently blocking sunlight and reducing glare.

To evaluate the angular transmittance of GHLC cells for automotive applications, we used the software “TechWiz LCD 2D”, maintaining consistent LC and dye material properties. For the tilted LC cell, we set a cell gap of 5.5 μm and a dye concentration of 1 wt%. The hybrid aligned nematic (HAN) cell also had a cell gap of 10 μm and a dye concentration of 0.3 wt%.

Figure 6a illustrates the tilted LC cell’s schematic for the proposed installation angle, which typically ranges from 40° to 50°. In this study, we set the installation angle at 45°. Figure 6b shows the calculated transmittance of the GHLC cell with a pre-tilt angle of 55°. The reason the maximum transmittance angle of the LC does not exactly match the pre-tilt angle and installation angle is that the pre-tilt angle affects both the absorption coefficient and the path length. The highest normal-view transmittance for the tilted LC cell was 70.8% at a pre-tilt angle of 55°. In contrast, the transmittance in the oblique view (incident angle of 75°) was 50.8%, resulting in a 20% difference. Figure 6c presents the transmittance contours. The normal-view transmittance for automotive windshields was calculated at 70%, but this value can be adjusted for different installation angles by modifying the dye concentration and cell gap.

Figure 6d shows the HAN cell’s schematic for the proposed installation angle. The HAN cell can adjust the highest transmittance angle by controlling the dye concentration or cell gap. Figure 6e presents the transmittance for the GHLC cell at a 45° installation angle. The highest normal-view transmittance for the HAN cell was also 69.7%, while the transmittance in the oblique view (75°) was 62.6%, again with a 7.1% difference. The transmittance contours are shown in Figure 6f. Similar to the tilted LC cell, the normal-view transmittance for automotive windshields was calculated to be 70%, but the transmittance can be tuned by adjusting the installation angle, dye concentration, and cell gap.

7. Conclusions and Future Perspectives

In conclusion, this study demonstrated the potential of various LC modes for static window applications, applicable to both energy-saving buildings and automotive environments. For building windows, we investigated VA, HA, ULH, and TA cells. Among these, the VA cell exhibited the largest transmittance difference between normal and oblique angles, making it particularly effective for sunlight control. In contrast, while the ULH cell showed a slightly smaller transmittance difference in the y-direction, it maintained stable transmittance in the x-direction, providing a wider viewing angle, which is advantageous for practical implementation.

In automotive applications, we addressed the limitations of traditional designs that focus on maximizing transmittance at a normal incidence—a strategy less effective for windshields due to their installation angle. To overcome this, we utilized tilted LC and HAN cells. The HAN cell, while easier to fabricate, exhibited relatively smaller transmittance differences, whereas the tilted LC cell provided more significant variation but required a more complex fabrication process. This trade-off between ease of manufacturing and performance highlights the need for specific design choices based on the intended application.

In addition to building energy savings and automotive applications, the angular-selective light absorption properties explored in this study have potential applications in fields where control over incident light angles is critical. One promising area is greenhouse agriculture, where managing the sunlight intensity and spectrum can significantly affect plant growth. Static windows with angular-selective properties can optimize natural light exposure throughout the day, reducing the need for artificial lighting and improving energy efficiency. Another possible application is in personal protective equipment, such as visors or eyewear, where selectively controlling light based on the viewing angle can enhance visibility and user comfort in varying light conditions.

Beyond the scope of visible light, it is important to note that near-infrared (NIR) light accounts for nearly 50% of the solar spectrum. However, the introduced GHLC devices in this study are limited in controlling NIR light due to the absorption range of the dyes used. To address this limitation, future designs should explore the integration of electrochromic devices that are capable of NIR control. However, electrochromic devices typically lack angular-selective properties, making them less suitable for static window applications. To overcome this challenge, research into reflection-based cholesteric LC devices may provide a viable solution. Such devices can selectively reflect or transmit NIR light depending on the angle of incidence, offering the potential for developing static windows that effectively control both visible and NIR light based on viewing angle.