Energy-Efficient Liquid Crystal Smart Window with a Clear View

Abstract

In this study, we enhance the angular-selective light absorption capabilities of guest–host liquid crystal (GHLC) cells by introducing a novel design featuring a uniform lying helix (ULH) structure. Previously GHLC cells, predominantly vertically aligned cells absorbed obliquely incident light but compromised x-direction visibility. In stark contrast, our ULH-based design allows incident light to seamlessly traverse transmittance in both z- and x-directions while efficiently obstructing oblique incident light in the y-direction. Our innovative ULH-based GHLC cell achieves an impressive optical performance. Specifically, it attains a substantial transmittance rate of 56.7% in the z-direction. Furthermore, in oblique views encompassing both the x- and y-directions, it maintains competitive transmittance rates of 44.2% and 29.5%, respectively. This strategic design not only ensures clear and unobstructed views for building occupants in the z- and x-directions but also contributes significantly to energy conservation by preventing oblique incident light from penetrating, thus reducing cooling requirements. Our ULH-based GHLC cell represents a breakthrough in smart window technology, offering an elegant solution to the challenge of balancing energy efficiency and occupant comfort in architectural settings. This advancement holds promising implications for sustainable building designs by enhancing indoor environmental quality while mitigating energy consumption for cooling, ultimately redefining the potential of smart windows in contemporary architecture.

1. Introduction

Windows plays a pivotal role in the architecture and design of buildings, serving as more than just apertures through which occupants glimpse the external world. They are integral components that provide multifaceted functionality, enhancing the overall quality of indoor spaces. One of their primary roles is to usher in natural light, fostering a well-lit and inviting environment that can significantly impact the mood, productivity, and well-being of individuals within. Moreover, windows facilitate ventilation, allowing fresh air to circulate and maintain indoor air quality, which is particularly crucial in modern, energy-efficient building designs.

However, this versatility comes with a conundrum. While windows are instrumental in introducing daylight and enabling cross-ventilation, they are also notorious for their potential drawbacks. Unmitigated sunlight penetration can lead to a myriad of issues. Excessive solar heat gain can result in uncomfortable indoor temperatures, forcing occupants to rely on energy-consuming heating, ventilation, and air conditioning (HVAC) systems to maintain comfort. Additionally, the intense glare caused by direct sunlight can create discomfort and hinder tasks that require screens or visual concentration. Recognizing these challenges, the realm of architectural and materials science has witnessed the development of innovative solutions: dynamic and static smart windows. These advanced window technologies have emerged as promising strategies to strike a balance between harnessing the benefits of natural light and mitigating its adverse effects, thereby improving both occupant comfort and energy efficiency [1,2,3,4,5].

Dynamic smart windows, particularly those based on chromic materials, have garnered considerable attention in recent decades. These materials exhibit the remarkable ability to change their optical properties in response to external stimuli, such as temperature, light, or an applied voltage [6,7,8,9,10,11,12,13,14,15]. By dynamically adjusting their tint or transparency, they can modulate the amount of incoming sunlight and glare, effectively controlling the indoor environment and reducing the need for energy-intensive climate control systems. In contrast, static smart windows offer a fixed level of shading or opacity, primarily aimed at energy conservation and glare reduction [15,16,17,18,19,20]. However, the challenge with static smart windows lies in their perennial opaqueness, regardless of the incident angle of sunlight. This limitation has spurred research into alternative solutions, and one such innovation is the application of guest–host liquid crystal (GHLC) cells [21,22]. GHLC cells stand out for their unique angular-selective absorption properties, driven by the anisotropic absorption of dichroic dyes. This characteristic allows them to selectively absorb sunlight in the y-direction, thus curbing energy consumption, while maintaining a clear view in the x-direction, where glare reduction and occupant comfort are paramount.

In this study, we introduce a novel GHLC cell design featuring a uniform lying helix (ULH) structure, representing a significant advancement in smart window technology. The ULH-based GHLC cell is engineered to significantly widen the x-direction field of view for building occupants while preserving its core functionalities. By maintaining a high level of transmittance in the x-direction and reducing transmittance in the y-direction, this innovation offers an unparalleled balance between energy efficiency and user comfort. Specifically, our proposed GHLC cell achieves transmittances of 44.2% in the x-direction, 29.5% in the y-direction, and an impressive 56.7% in the z-direction, setting new benchmarks for smart window technology in contemporary architectural designs. In the subsequent sections of this manuscript, we delve into the design principles, fabrication processes, and performance characteristics of the ULH-based GHLC cell, showcasing its potential as a transformative solution for optimizing indoor environments in buildings. Through rigorous experimentation and analysis, we aim to provide valuable insights that advance our understanding of smart window technology and contribute to the development of sustainable, user-centric architectural designs.

2. Principles of the Device

In the realm of smart windows, substantial research endeavors have been directed toward optimizing transmittance in the z-direction, primarily to address energy conservation and glare reduction concerns. Nevertheless, the real-world application of smart windows in building design necessitates a nuanced consideration: the incident angles at which sunlight irradiates the windows. Unlike idealized scenarios in which sunlight strikes windows perpendicularly, in practical situations, sunlight often approaches windows obliquely due to the changing position of the sun throughout the day. This variation in incident angles underscores the need for smart windows capable of accommodating such conditions. To tackle this complex challenge, the development of smart windows that take incident angles into account has gained traction. These innovative windows have been designed to selectively block sunlight when viewed from oblique angles while preserving a clear and unobstructed view in the z-direction for building occupants. This dual functionality not only contributes to energy savings but also fosters a comfortable indoor environment [21,22]. However, a key limitation of many existing smart windows with radial symmetry lies in their inadvertent absorption of incident light in the x-direction, even when not intended. This unintended absorption results in narrowed views for occupants (as depicted in Figure 1). To truly overcome this limitation and provide an ideal solution, a smart window design with biradial symmetry is essential (Figure 1). Such a window configuration would seamlessly offer a clear view in both the z-and x-directions while effectively blocking sunlight when necessary.

The distinctive optical properties of dichroic dyes utilized in GHLC cells are central to achieving this biradial symmetry. These dyes exhibit dichroism, a characteristic wherein they absorb light intensely when irradiated in parallel along their absorption axis and weakly when irradiated perpendicularly along their absorption axis (Figure 2a). When integrated into GHLC cells with a VA structure, these dyes inherently possess radial symmetry properties. This symmetry allows them to be divided into three or more sections when rotated through a center of rotation by angles ranging from more than 0° to less than 360° (as depicted in Figure 2b). From the normal view (z-direction), the dye molecules exhibit weak absorption of incident light because both the x- and y-axis components of light travel perpendicular to their absorption axis. However, in the oblique view (y-direction), the dye molecules display strong absorption characteristics. This is because the x-axis component of light passes perpendicular to its absorption axis, while the y-axis component runs parallel to it. Similarly, in the x-direction, the dye molecules also exhibit strong absorption, resulting in limited visibility for occupants in these directions.

In stark contrast, GHLC cells adopting a ULH structure introduce biradial symmetry properties. This means that they possess two additional axes or planes of symmetry at right angles to the anteroposterior axis: the median-vertical and transverse or cross-axes (as illustrated in Figure 2b). When viewed from an oblique angle in the x-direction, the dye molecules within ULH cells exhibit weak absorption of incident light. This occurs because both the x- and y-axis components of light pass either perpendicular to or parallel to their absorption axis, making them ideal for enhancing visibility in this direction. Furthermore, as the incident angle increases, the effective absorption coefficients and light path length play pivotal roles in determining visibility. In the case of unpolarized light, as the angle increases in the x-direction when viewed from the front, the VA cell exhibits an increase in absorption coefficient. In contrast, the effective absorption in the x-direction for the ULH cell remains constant as the incoming angle changes. The path length of light also increases as the angle increases (as exemplified in Figure 2d). This stability is a crucial advantage when striving to provide occupants with a consistent and unobstructed view while managing sunlight penetration. In the subsequent sections of this manuscript, we delve deeper into the fabrication, optical characteristics, and performance evaluation of GHLC cells employing the ULH structure. By dissecting the principles and advantages of this novel configuration, we aim to contribute valuable insights to the field of smart window technology and inspire advancements in building designs that prioritize both sustainability and occupant comfort.

3. Results and Discussion

To assess the optical performance of the GHLC cells with different structures, we employed the commercial software tool “TechWiz LCD 2D (Patch 1, 0, 23, 0126)” (Sanayi System Company, Ltd., Incheon, Republic of Korea), which utilizes the Berreman matrix method for light transmission calculations. Our experimentation aimed to determine the optimal dye concentration and cell gap for the GHLC cell by calculating transmittance differences in the x- and y-directions while systematically varying these parameters. Leveraging an optimization algorithm [23], we identified the LC-dye combination that yielded a high transmittance difference. Ultimately, our optimal configuration incorporated E7 LC material (with optical birefringence Δn: 0.2255, dielectric anisotropy Δε: 14.1, and transition temperature T_NI: 63.3 °C sourced from Merck, Darmstadt, Germany) paired with black dye X12 (provided by BASF, Ludwigshafen, Germany) (as depicted in Figure 3a) [24]. We measured the absorption coefficient using this mixture of liquid crystals. The measurement method involved using a horizontally oriented cell to measure the absorption coefficient when the incident light was horizontally polarized, both when it was parallel and perpendicular to the absorption axis. X12 exhibited absorption coefficients α_⊥ and α_∥ of 0.024 μm⁻¹ and 0.205 μm⁻¹, respectively, within the average range of 400 to 700 nm, which comprises a red azo dye, yellow azo dye, and blue anthraquinone dye. All chemicals were purchased from commercial suppliers and used without further purification. For the ULH cell, we added a chiral dopant (S811, HTP~11). We established the pitch at 8.75 μm, noting that variations in pitch did not significantly affect transmittance. For equitable comparisons, we adjusted the dye concentration and cell gap of the vertically aligned (VA) GHLC cell to match the transmittance of the uniform lying helix (ULH) GHLC cell in the z-direction. This calibration led us to employ a cell gap of 12.5 μm and a dye concentration of 1 wt% for the VA cell. Conversely, for the ULH cell, we opted for a cell gap of 5.5 μm and a dye concentration of 0.6 wt%. Polarized optical microscopy (POM) of the fabricated VA and ULH cells between the cross-polar is presented in Figure 3b. Both cells employed the homeotropic alignment layer (AL64168, JSR Korea, Gongju, Republic of Korea) with a pretilt angle of 87.5° was spin-coated on the indium-tin-oxide-coated glass substrates. Spin-coating was performed for 10 s at 1250 rpm, and then 50 s at 4500 rpm. After spin-coating, the coated substrates were baked for 1 h at 250 °C, resulting in distinct visual characteristics. The VA cell displayed a dark image between crossed polarizers, a hallmark of its vertical alignment. In contrast, the ULH cell exhibited intriguing diffraction patterns attributable to its inherent chirality. Importantly, when viewed without polarization, both cells can provide a clear view without scattering in the z-direction. To demonstrate the absence of scattering, we attached two self-made cells to a KNU logo and then took a photograph (Figure 3c).

Figure 4 presents a calculated comprehensive overview of the transmittance characteristics of GHLC cells with both VA and ULH structures across a range of incident angles using a software tool. To ensure a fair and meaningful comparison between these configurations, we meticulously adjusted the dye concentration and cell gap of the VA cell to closely match the transmittance exhibited by the ULH cell in the z-directions. In the specific context of z- and y-directions (evaluated at a 75° incident angle), the VA cell demonstrated transmittance values of 56.1% and 11.3%, respectively, yielding a substantial discrepancy of 44.8%. However, in the x-direction, the VA cell’s transmittance closely paralleled that of the y-direction, resulting in a constrained field of view for occupants. In stark contrast, the ULH cell showcased transmittance rates of 56.7% and 29.5% in the z- and y-directions, respectively. This represented a significantly reduced disparity of 27.2% compared to the VA cell. However, the transmittance of ULH cells is 44.2%, which is 32.9% higher than that of VA cells in x-directions. What is particularly noteworthy is the ULH cell’s ability to maintain a minimal transmittance difference between the z- and x-directions, registering at just 12.5%. This exceptional attribute translated to nearly uniform transmittance across the horizontal plane within a range spanning ±60°, ensuring building occupants enjoyed an unhindered view of the external environment, all while effectively mitigating light absorption. It is paramount to underscore that the proposed GHLC cell, with its distinctive characteristics, serves as a smart window solution that not only delivers crystal-clear views in both z- and x-directions but also actively contributes to the pursuit of energy conservation by adeptly managing the intrusion of sunlight.

In our pursuit of comprehending the influence of incident angles on light absorption within the Vertically Aligned (VA) and Uniform Lying Helix (ULH) cells, we conducted a meticulous series of simulations, ranging from 0° to +75° at intervals of 15° (as illustrated in Figure 5). As these incident angles gradually increased, distinct characteristics emerged within both cell types. The VA cell progressively adopted a darker appearance with each incremental increase in the incident angle, regardless of the viewing direction under consideration. Meanwhile, in the x-direction, the ULH cell exhibited low light absorption, remarkably preserving its brightness even up to a 60° incident angle. It is worth noting that our static window configuration, while not endowed with automatic transmittance adjustment capabilities, delivers dual benefits: it curtails air conditioning costs and optimizes the visual experience for occupants. Nevertheless, an important consideration arises from these findings; namely, the ULH cell’s higher transmittance in the y-direction when compared to the VA cell. Addressing this disparity presents an exciting avenue for future research endeavors. The exploration of innovative methodologies to enhance absorption in the y-direction while simultaneously preserving the impressive transmittance rates observed in both the z- and x-directions holds promise and should be a focus of future investigations.

In Figure 6, we present the transmittance contours of both the Vertically Aligned (VA) and Uniform Lying Helix (ULH) cells. Notably, the VA cell displayed a gradual decline in transmittance as the incident angles increased across all directions. In stark contrast, the ULH cell exhibited a more controlled reduction in transmittance in the x-direction compared to the y-direction. This intrinsic biradial symmetry property inherent in the ULH cell configuration holds immense promise, particularly for applications in display technologies, as evidenced by prior studies [25,26]. GHLC (Guest–Host Liquid Crystal) films have garnered considerable attention for their utility in diverse applications, including privacy displays and the suppression of reflections on automotive windshields. Traditionally, these applications have leaned toward the utilization of VA cells. However, it is important to acknowledge that this preference sometimes comes at the cost of compromised viewing angles for end-users. In stark contrast, the proposed ULH cell emerges as a game-changer. It skillfully and selectively absorbs light in the desired direction while adeptly maintaining a wide viewing angle, a characteristic that sparks excitement and opens up new horizons for future display technologies. These experimental findings underscore the substantial advancements achieved through the implementation of ULH-based GHLC cells. Their potential transcends smart windows, extending into the realm of display technologies, where a broad viewing angle is indispensable. The trajectory of further research endeavors will continue to explore and refine these innovative optical attributes and their practical implications in diverse real-world scenarios.

4. Discussion and Conclusions

In this study, we have introduced a novel Guest–Host Liquid Crystal (GHLC) cell that represents a significant advancement in smart window technology. This innovative GHLC cell design is engineered to substantially enhance visibility in the x-direction, thereby addressing a critical challenge in the field of architectural design and sustainable building solutions. Central to the success of this GHLC cell is its ability to exploit the unique absorption anisotropy exhibited by guest dichroic dyes. This unique property allows the device to differentiate between the transmittance of incident light in the x- and y-directions, contingent upon the arrangement state of the dye molecules within the cell. As a result, our calculations have demonstrated the remarkable optical performance of the proposed GHLC cell. Under the z-direction, the GHLC cell achieved an impressive transmittance rate of 56.7%. However, when subjected to oblique views, specifically in the x- and y-directions, the transmittance exhibited different characteristics, reducing to 44.2% and 29.5%, respectively. These results underscore the GHLC cell’s efficacy in selectively managing incident light based on the angle of approach. This selective transmittance property is a key feature that positions the GHLC cell as a transformative element in architectural design. The practical implications of our findings are significant, particularly in the context of energy-efficient building design. When integrated into windows, the proposed GHLC cell offers building occupants a clear and unobstructed view in both the z- and x-directions. Simultaneously, it effectively blocks obliquely incident light in the y-direction, a critical factor in reducing the energy consumption required for cooling indoor spaces. This dual functionality not only contributes to improved occupant comfort but also aligns with the imperative of sustainable building practices. Furthermore, our study highlights the versatile applications of the GHLC cell beyond smart windows. The potential to enhance viewing angles and selectively manage light absorption opens new x-direction for privacy displays and display technologies. Privacy displays, in particular, stand to benefit from the GHLC cell’s ability to maintain wide viewing angles while ensuring the confidentiality of on-screen content. As with any innovative technology, there are areas for further improvement and exploration. Future research endeavors should concentrate on enhancing absorption in the vertical direction while preserving the high transmittance rates observed in the z- and x-directions. Achieving this balance will be instrumental in maximizing the utility of GHLC cells in various real-world scenarios. In conclusion, our study introduces a GHLC cell that not only improves visibility in the x-direction but also demonstrates its potential to revolutionize energy-efficient building designs and display technologies. With its unique capacity to selectively manage light transmission based on incident angles, this GHLC cell paves the way for user-centric, sustainable architectural solutions and innovative displays. The continued evolution and refinement of GHLC cell technology promise to redefine the possibilities of energy-efficient, user-friendly design in both the built environment and the realm of display applications.