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Abstract
Dichroic absorption dye-doped liquid crystal switching is preferred for transmittance control with maintaining visual clarity. In this paper, we present a parametric analysis of chiral guest-host liquid crystal (C-GHLC) switching for an enhanced transmittance-tunable smart window. Further analysis of the chiral twist power resulted in the proposal of a new modified transmittance governing formula for C-GHLC. The optimal C-GHLC cell design was determined through a comprehensive examination of the electro-optic transmittance change between transparent and opaque states by optimizing the chiral twist power in terms of ‘d/p’. Along with the theoretical parametric design of the C-GHLC cell, an optimal condition for the C-GHLC cell which can use commercial display driving environments was experimentally demonstrated for the first time. Consequently, an improved transmittance control (ΔT ≈ 40.5%) with a low voltage (Von ≈ 18 V) and with a sufficiently fast response time (τ ≈ 12 ms) suitable for 60 Hz (< 16.7 ms) was confirmed.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, as demand for smart living in a transparent environment grows, on-demand transmission control has become a desirable technical capability. As a result, smart window (or smart optical shutter) technologies which can control the optical transparency between transparent and opaque states are attracting intensive research attention for various applications, such as privacy protection and energy saving in transparent environments [1–6].
This optical transmittance control can be achieved using a variety of electro-optical technologies, including electrochromic devices (ECDs) [7–11], suspended particle devices (SPDs) [12,13], phase change materials (PCMs) [14–16], and liquid crystals (LCs) switching [17–28]. Also, some interesting approach using a power-free smart window with LC was reported [29–31]. However, controlling transparency in see-through applications requires dimming transmittance control while maintaining clear visibility.
There are two distinct technical types of transparent control for optical smart shutters. One is dynamic switching between transparent and opaque states with significant optical scattering. SPD and polymer dispersed liquid crystal (PDLC) technologies are particle-based Mie-scattering induced opaque control. Therefore, these particles including technologies can be used for scattering switching type optical shutter. It should be noted, however, that the scattered opaque state lacks clarity in a transparent environment. Moreover, these technologies require a high switching voltage of up to 100 V to control scattering. A non-scattering type dimming transmittance control with clarity is desirable for various transparent applications. In particular, this maintaining clarity to circumvent scattering is required for the applications such as automobiles due to safety issues. Also, from the viewpoint of visual and aesthetic characteristics in various transparent environments, a low scattered transmission switching is preferred. ECD has been actively approached for this type of transparency control as a representative technology. However, due to the intrinsic electrical oxidation or reduction process used in electrochromic control, ECD has a technical disadvantage of having a slow response time down to several minutes and not uniform transparent switching in large-sized applications [9,10]. Although a PCM has been reported for optical shutter technology recently, this method is also slow in decay time due to the slow thermal cooling process to recover the initial crystal phase [14–16]. In contrast, utilizing liquid crystal switching technologies such as PDLC, polymer network liquid crystal (PNLC), and dye-doped guest-host liquid crystal (GHLC) enables faster switching response time and switching uniformity in large-scale applications. Recently LC optical transmission control evolves so-called ‘nano-micro LC’ using a complex of nano-porous microparticles in a liquid crystal of low concentration is being studied recently [32–36]. In particular, these LC-using technologies can manipulate conventional liquid crystal display (LCD) processes and drive technical infrastructure.
However, it should be noted that applications, such as automotive head-up displays, smart windows, and eyeglasses, must maintain visual and optical clarity even in an opaque state. Therefore, smart window applications with such transparency require a technical approach which can circumvent scattering and minimize the haze effect. In view of these technical requirements, GHLC is preferred because of haze-free transmission switching in absence of particle-induced Mie scattering [37–39].
The GHLC cell is composed of host LCs and guest dichroic dyes that should template the alignment of host LC molecules. The alignment of LC can be obtained using well-developed surface alignment techniques in LCD. Traditional surface rubbing of polyimide and novel photo-alignment materials can be used for LC alignments [40–48].
In a homogenous alignment of LC with absorption dye, the transmitted light is strongly absorbed when the incident light is parallel to the absorption axis of the dye molecules. In contrast, when the incident light is perpendicular to the absorption axis, the dye molecules absorb weakly transmitting light [49–53]. Therefore, the directional dependency of dye absorption should be carefully considered. Note that when the transmitting light is unpolarized the opaque state of homogeneously-aligned GHLC (HA-GHLC) should mix maximum and minimum absorption without electric field (E-Off). By applying sufficient electric field (E-On) to have homeotropic alignment the transparent state can be switched with minimum absorption perpendicular to dichroic dye.
By doping chiral additives into achiral LCs, chiral liquid crystals (CLC) with self-assembled helical rotation of molecular twisting can be obtained. According to de Vries’ theoretical study [54], the optical properties of CLC molecules are altered by their rotating twist power. Therefore, CLC with helical power control attracts an intensive research interest for a variety of functional optical applications, such as photonic tunable bands [55–60], reflective displays [61,62], liquid crystal lasers [63–67], and optical sensors [68–71].
In particular, when CLC is used as the host medium for GHLC, improved transmitted light absorption can be obtained by optically twisted absorption of dye molecules in the opaque state, in contrast to HA-GHLC [72,73]. Notably, the transmittance of chiral GHLC (C-GHLC) cells is dependent on the cell gap and dye concentration to a recent report including one of the authors [73]. However, the transmittance relation was studied with a fixed pitch condition to avoid the wave-guiding effect. Therefore, the reported design method has limitations in application to all helical pitch conditions of C-GHLC.
Indeed, the transmittance of C-GHLC cells should be highly dependent on the pitch of the helical twisting [74–78]. Therefore, it is critical to optimize the transmittance of C-GHLC while taking chiral twisting power into account. Moreover, it should be noted that the increase in chiral elastic energy can lead to a higher operating voltage and a slower response time than HA-GHLC cells [78–81]. Given that the controllable transmittance difference, driving voltage, and response time of C-GHLC cells vary in complex ways as a function of helical pitch and also exhibit a trade-off relationship, it is important to investigate comprehensive parameters for optimal conditions of C-GHLC. However, unlike conventional HA-GHLC cells, no studies elucidating the clear formula relationship between the helix pitch and the driving properties of C-GHLC cells have not been reported yet.
In this study, we propose to optimize the cell conditions for C-GHLC based on the helical twisting power. A new chiral twisted cell design formula for C-GHLC was proposed. And the optical transmission difference ΔT of C-GHLC was investigated experimentally as a function of chiral twisting power ‘d/p’ (where d is the cell gap and p is the helical twisting pitch). Through a comprehensive study of driving voltage Von and response time τ parameters based on d/p, the optimal condition for C-GHLC was suggested using a new formula. Optimized C-GHLC cell parameters for better transmittance control under display operating conditions were investigated. As a result, we experimentally confirmed the improved tunable change of transmittance (ΔT ≈ 40.5%) under low voltage (Von ≈ 18 V) and fast response time (τ ≈ 12 ms), which are the driving environments of commercial displays.
2. Methods
2.1 Preparation of C-GHLCs
To begin, a positive LC (E7, Δn = 0.227, ne = 1.747, Δɛ = 12.7, γ1 = 56 mPa·s) was mixed with a chiral dopant (S811, Grandinchem) to create a chiral switching host for GHLC. Notably, the d/p conditions varied from 0.5 to 20 to locate the Bragg reflection and absorption in the infrared region while maintaining optical transmittance in the visible wavelength range. To obtain various helical power d/p from 0.5 to 20 was controlled by using different concentrations of chiral dopant with a fixed cell thickness of 10 µm. Detailed information of dopant concentration corresponding to each helical power was as follows. (d/p, wt%) = (0.5, 0.4448), (1, 0.8897), (2, 1.7794), (4, 3.5587), (8, 7.1174), (12, 10.6761), (16, 14.2349) and (20, 17.7936). To adjust the transmittance of C-GHLC control, the light absorption guest of a black dichroic dye (X11, BASF) in varying concertation from 1 wt% to 3 wt% was doped into the prepared CLC mixtures. The dichroic absorption coefficients of α|| and α⊥ were confirmed to be 0.1557 µm−1 and 0.0196 µm−1 at 550 nm, respectively, and the dichroic ratio was determined to be 7.944 at 550 nm (Supplemental Information Figure S1). The absorption coefficient of the dye was determined at room temperature using a HA-GHLC cell configuration with a 10 µm cell gap and a dye concentration of 2 wt%. To ensure the homogeneity of the prepared C-GHLC mixtures and stirred for 24 h at 100 °C. Finally, each C-GHLC cell was constructed by injecting each dye-doped CLC host mixture via capillary force into a homogeneous aligned empty cell with a thickness of 10 µm (LC-9.0, Instec) at room temperature. Note that for the alignment layer, an anti-parallel rubbed conventional homogeneous alignment layer with polyimide (KPI-300B) was used.
2.2 Electro-optical measurement of C-GHLCs
The electro-optic switching properties of the fabricated C-GHLC cells were investigated using an in-house microscope (BX53M, Olympus) equipped with an unpolarized white transmitting light source. A photodiode was used to detect the transmitted light (PDA36A-EC, Thorlabs). The transmission spectra of the C-GHLC cells in their transparent and opaque states were determined using a spectrometer (Flame-T, Ocean Optic) in the visible region between 420 nm and 700 nm. The driving characteristics of the C-GHLC cells were determined using a high voltage amplifier (9400A-DST, Tabor electronics) and an in-house developed LabVIEW system with a signal generator (PCIe-6251, National instruments). The haze value was measured using the haze meter (COH-400, Nippon denshoku).
3. Results and discussion
3.1 Theory of transmittance change in C-GHLC
When the guest (dichroic light absorption dye) molecules are incorporated into the HA-LC host, the transmittance of HA-GHLC cell should vary according to the incident angle parallel and perpendicular to the dichroic dye guest molecules, as described in Equations 1(a) and (b) [49–53]
where, T|| and T⊥ respectively are the transmittances of a GHLC cell for incident angle parallel and perpendicular to the absorption axis of the dye molecules. T0 is the transmittance of a host HA-LC cell without a dye, c is the dye concentration, and d is the cell gap. α|| and α⊥ respectively are the absorption coefficients of the dye for polarization parallel and perpendicular to the absorption axis of the dye molecules. The transmittance of the GHLC cells is controlled by c and d.In comparison, when the GHLC is in a chiral way within a CLC host, the helical rotating absorption of the dichroic dye should change the template chiral LC alignment in all directions and give an average absorption coefficient of (α|| + α⊥)/2. Therefore, according to the optical rotation effect of absorption, the twisted transmittance of C-GHLC (TTwist) in the opaque state can be expressed as Eq. (2) [72,82]
In addition, when the rod-shaped dichroic dye molecules align perpendicular to the substrates in the homeotropic state with the E-On state, the molecular absorption of dichroic dye should be minimized and the effective absorption should be symmetrical to the incident angle of transmitting light. As a result, this C-GHLC state should be transparent with a sufficient electric field. Therefore, the transmittance of this homeotropically aligned C-GHLC (THomeo) in the transparent state can be expressed as Eq. (3) [72,82]
In principle, the transmittance difference ΔT of C-GHLC can be controlled by the effective transmittance between TTwist and THomeo. To implement this, the configuration change of the dye molecules must change from the twisted configuration to the fully homeotropic state in the CLC host.
However, as illustrated in Fig. 1, the amount of helical twisting power should be considered as the actual transmittance of the C-GHLC cells in the homeotropic and twisted states. When the helical pitch is excessively long with weak chiral twisting of d/p, the wave-guiding effect should be enhanced. This would result in loss of effective light absorption with following higher transmittance in the opaque state than Eq. (2), which does not consider d/p in the previous report [72,82]. On the other hand, when the helical pitch is sufficiently short with strong d/p, enhanced chiral anchoring should affect the homeotropic state of C-GHLC to the transparent state. Due to the strong chiral anchoring, some residual twisting absorption should remain. This residual absorption leads to a further decrease of transmittance in the E-On state compared to THomeo in Eq. (3), which does not consider d/p. Moreover, d/p should be related to the operation voltage Von and response time τ. Therefore, to obtain the optimal conditions for C-GHLC, a more comprehensive analysis considering d/p is required.
Fig. 1. Schematic representation of the transmittance switching in C-GHLC between the twisted and homeotropic states. (a) C-GHLC switching with weak helical twisting power of low d/p condition and (b) C-GHLC switching with a strong helical twisting power of high d/p condition.
3.2 Experimental investigations of light transmission in C-GHLC
The actual electro-optical transmittance change behavior of C-GHLC was experimentally investigated in detail. As shown in Fig. 2, the transmittance of C-GHLC change according to the chiral power d/p was experimentally examined for the homeotropic state in the transparent state and the twisted state in the opaque state. The dye concentration and the chiral twisting power were adjusted differently for each condition of C-GHLC cells.
As c increased from 1 wt% to 3 wt%, the transmittances of C-GHLC decreased in both states for homeotropic and the twisted states as shown in Fig. 2. When a voltage is applied, the homeotropic state should transform into a transparent state with the LC molecules perpendicular to the substrate and the helix unwound. However, it should be noted that a dramatic decrease in the transmittance as a function of d/p for the homeotropic state was observed as shown in Fig. 2(a). Also, this decrease in the transmittance with d/p was observed in all dye concentration conditions. Since there is no inversion symmetry, a non-collinear proper vector can be associated with the chiral system, and the chirality forms the deviation of the equilibrium director in the rubbing direction [83]. Therefore, it can be assumed that this is due to the additional effect of chiral anchoring energy in C-GHLC. Moreover, when the chiral twisting power d/p is strong, the total anchoring energy should increase further. Therefore, it is assumed that some amount of CLC molecules and templating dichroic dye should remain in planar to the substrate even in a vertical electrical field. The increased surface anchoring energy density due to the increased chiral twisting power can be expressed using Equation (4a) as reported in recent studies [83, 84]. Now, rearrange Equation (4a) considering $p = \textrm{ }2\pi/{q_0},$ it could be found that the surface energy in CLC depends on d/p as described in Equation (4b).
Therefore, it was assumed that additional chiral anchoring energy contributed to the increase in elastic storing energy of CLC against dielectric switching for a perfect homeotropic state perpendicular to the surface.
As a result, some residual dye guest molecules that share the same direction as the CLC host molecules remain in the same direction at the surface, contributing further to the decrease in net transmittance relative to the ideal homeotropic state (THomeo). Our experimental results indicate that when d/p > 12, the transmittance of the homeotropic state decreases in radical. [Figure 2(a)].
When the helical twisting power of d/p was insufficient for the opaque state, the transmittance was increased further in the twisted state, as shown in Fig. 2(b). This was assumed as a result of the wave-guiding effect associated with weak twisting rotation. Considering that the wave-guiding effect occurs in the Mauguin regime, the twist rotation angle ϕ is less than the phase retardation Γ [85–88]. This Mauguin regime relationship can be expressed as d/p < Δnd/λ, where Δn is the birefringence and λ is the wavelength. For example, at λ = 550 nm, d = 10 µm, the wave-guiding effect is calculated to be found at d/p < 4.13. This trend was consistent with our experimental results, and transmittance increased when d/p < 4.13 was used in Fig. 2(b).
Fig. 2. Experimentally measured transmittance (filled circle) and fitted curves (solid line) of transmittance of the C-GHLC cells at (a) homeotropic state and (b) twisted state as a function of helical rotating power d/p for different dichroic dye concentration of c = 1, 1.5, 2 and 3 wt%.
Thus, the transmittance of the C-GHLC cell as a function of d/p should be modified from the expression represented in Eqs. (2) and (3), where d/p was not carefully considered as in the previous report [72,82]. Therefore, using the measured experimental data in Fig. 2, modified transmittance equations for homeotropic and twisted states in C-GHLC were proposed including consideration of the helical twisting power, ‘d/p’.
Both A1 and A2 were determined to be 0.297 and 0.936, respectively, from the experimental fitted values, while α was 0.189. Additionally, B1 and B2 were determined to be 18.005 and 0.086, respectively, while β had a value of 0.599. Notably, these transmittances of C-GHLC exhibited similar trends in both the homeotropic and twisted states for all dye concentrations.
The homeotropic state is used as a transparent state in a C-GHLC cell, whereas the twisted state is used as an opaque state. Therefore, the tunable transmittance difference (ΔT = TCLC-Homeo - TCLC-Twist) between homeotropic and twisted states is important and should be maximized. To find the optimal condition for obtaining the maximum transmittance difference, ΔT was mapped as a function of d/p at different dichroic dye concertation c, as illustrated in Fig. 3. As a result, it was found that when the chiral twisting power was insufficient (d/p < 4.13), the tunable ΔT was decreased significantly, Fig. 3(a). This was because the transmittance in the twisted state (TCLC-Twist) for the opaque state was increased excessively due to the assumed wave-guiding effect. In comparison, when the chiral power was too strong (d/p > 12), the tunable ΔT was decreased again. It was assumed as the transmittance in the homeotropic state (TCLC-Homeo) for the transparent state was drastically reduced due to the imperfect homeotropic state by the strong chiral anchoring effect.
Fig. 3. (a) Experimentally measured (filled circle) and fitted curves (solid line) of transmittance difference of C-GHLC as functions of the number of pitches d/p. (b) The transmittance difference contour maps of C-GHLC cells with comprehensive parameters considerations of the dye concentration c and chiral power d/p.
Using Eq. (5), a contour map of the controllable transmittance difference for C-GHLC was prepared with comprehensive consideration of c and d/p in Fig. 3(b). Because ΔT varies with c and d/p, controllable ΔT distribution conditions along with maximum conditions for C-GHLC cells can be intuitively found.
In addition to the transmittance change, dynamic electro-optical properties such as driving voltage (Von) and response time (τ) were investigated during the switching. Considering the use of display driving circuit conditions for such transparent control and the switching consistency with a transparent display, it is required to obtain an acceptable operating voltage (< 30 V) and a sufficiently fast switching response time (< 16.7 ms for 60 Hz driving) for practical applications. Particularly in C-GHLC, the chiral twisting power should also have an effect on the driving voltage and elastic conditions for response time. Therefore, finding optimal cell design conditions for C-GHLC is highly desired with comprehensive considerations for C-GHLC switching.
The driving voltage ‘Von’ of C-GHLC cells as a function of helical twisting power ‘d/p’ was shown in Fig. 4. The Von was found to increase as d/p increased, but no critical correlation of dye concentration was observed. When the electric field is beyond the critical electric field ‘EC’, the helix of the chiral would unwind align causing LC molecules to align homeotropically, as described by Eq. (6) [89–92].
Fig. 4. Experimentally measured driving voltage ${V_{on}}$ of C-GHLC, as functions of chiral power d/p.
Equation (7) demonstrates the elegant explanation of the linear increase of VC increases as a function of d/p. Critical voltage VC leads to an increase in driving voltage Von. Note that the driving voltage change of C-GHLC cells as a function of d/p was independent of c.
In addition, we investigated the response time properties of C-GHLC cells as functions of ‘d/p’, as shown in Fig. 5. The response time was determined by reading the elapsed time during a normalized transmittance change between 10% and 90% when the driving voltage Von was applied to achieve maximum transmittance. As a result, it was found that the decay time τoff was significantly decreased as a function of d/p, in contrast to the relatively stable and sufficiently fast rise time τon. Given that the elastic free energy density (felastic) in a chiral liquid crystal is expressed by Eq. (8) [87,88,92], we can assume that the elastic free energy density increases as the helical pitch of a C-GHLC cell decreases.
When the applied electric field is removed, the CLCs should revert to their twisted state due to the restoring effect of the elastic free energy. As the helical pitch decreases, the C-GHLC cell experiences an increase in the force required to return to the twisted state, resulting in a decrease in response time. Decay and rise time tended to decrease as a function of d/p, as expressed in Eqs. (9) and (10), respectively [93–96].
Note that the τon was complicated by the effects of variable electric conditions. Moreover, the chiral critical voltage VC and driving voltage Von should vary as a function of d/p. In general, sufficient chirality (d/p ≥ 8), was required to achieve a response time of less than 16.7 ms under 60 Hz signal driving conditions. The optical transmittance response tracking as a function of time at the minimum chiral condition of d/p = 8 revealed that the τoff = 3.26 ms and τon = 12.57 ms were observed, respectively (Supplemental information Figure S2).
Fig. 5. Experimentally measured decay time (τoff, filled circle) and rise time (τon, empty circle) of the C-GHLC cells, represented as functions of chiral twisting power of d/p.
The optimal targeting of C-GHLC designs was accomplished through additional stepwise filtering of complex transmittance changing conditions as a function of dye concentration ‘c’ and helix force ‘d/p’, as shown in Fig. 6. To begin, a contour map of the controllable transmittance difference ΔT was obtained by performing a thorough analysis of the transmittance change between the E-On and E-Off states. A comprehensive plotting of ΔT concerning c and d/p was prepared. In addition to this transmittance difference ΔT contour map, conditions for the display driving environment were applied to the ΔT contour map and filtered the C-GHLC parameter. Considering the existing display driving conditions, d/p was determined by filtering the environment to enable a low driving voltage of Von < 30 V and a sufficiently fast response time of τ < 16.7 ms capable of driving at 60 Hz as shown in Fig. 6(a). The dye concentration was further filtered in the second step by selecting a region with a controllable maximum transmittance difference ΔTmax. ΔTmax between the E-On and E-Off states of C-GHLC was observed in this study to be a maximum of 38% < ΔT < 41.2% as described in Fig. 6(b). Additional parameter filtering was performed in the following step, limiting the minimum required transparent and opaque state conditions. The parameterized boundary condition was chosen to satisfy both the transparent transmittance Tmax > 50% and the opaque transmittance Tmin < 15% as shown in Fig. 6(c). Finally, it was determined that the optimal parameter condition for C-GHLC was to set the dye concentration to 2 wt% under the chiral twisting power d/p = 8 or 9, as shown in Fig. 6(d). This approach resulted in the suggestion of a comprehensive optimal parameter design method for C-GHLC that considers the change of complex electro-optical properties.
Fig. 6. The proposed step design process for the optimum C-GHLC cell. (a) Transmittance-difference contour map of a C-GHLC cell as a function of d/p and screened filtering simultaneous display driving condition for τ < 16.7 ms and Von < 30 V in. (b) Filtered regions where driving voltage and response time were appropriately satisfied in accordance with the display driving environment. (c) The region that satisfied the high transmittance difference. (d) Regions that satisfy maximum transmittance in the transparent state and minimum transmittance in the opaque state.
Table 1 summarizes the comprehensive electro-optical properties of various conditioned C-GHLCs. Electrically tunable transmittance difference (ΔT), the maximum transmittance in the transparent state (Tmax), the minimum transmittance in the opaque state (Tmin), the driving voltage (Von), the rise time (τon), and the decay time (τoff) were summarized for each condition of C-GHLC switching. A weak chiral twisting condition of d/p = 0.5 results in a significant slow decay response time (τoff ≈ 526 ms). And small switching controllability in tunable transmittance difference (ΔT ≈ 28%) was observed although the driving voltage was low (Von = 11 V). In comparison, at a strong chiral condition of d/p = 20, the required driving voltage was significantly higher (Von = 44 V) than the desired voltage (< 30 V) for typical display circuit environments conditions. Moreover, the electrically controllable transmittance difference was also significantly smaller (ΔT ≈ 30%).
Table 1. Comprehensive electro-optical properties of C-GHLC with varied chiral twisting power of d/p, such as the tunable transmittance difference (ΔT), the transmittance of the transparent state (Tmax), transmittance in the opaque state (Tmin), driving voltage (Von), the decay time (τoff), and the rise time (τon).
The comprehensive electro-optical performance of C-GHLC could be optimized. By adjusting d/p = 8 or 9 from the step filtering design as described in Fig. 6, a greater transmittance difference (ΔT ≈ 40.5%), lower driving voltages (Von ≈ 18 V, 19 V), and a fast response time (τoff ≈ 10 ms, 12 ms) can be obtained for single C-GHLC in the same time. These optimized electro-optical properties can be operated using a commercial display driving circuit. A low voltage of less than 30 V and fast switching of less than 16.7 ms for 60 Hz display driving conditions can be directly approached. These characteristics with a commercial display driving environment, are expected to encourage the smart window's application expansion in a variety of applications.
The switching properties of C-GHLC were also confirmed by observing how it switched under each condition. Figure 7 compares the electro-optical switching control of transmittance for each C-GHLC cell with a low d/p, an optimized d/p, and a high d/p as specified in Table 1. In contrast to the insufficient transmittance change observed at low or high d/p values, the optimized C-GHLC cell (d/p = 8) demonstrated a vivid transmittance switching control between opaque and transparent states. Supplemental video also confirmed the optimized dynamic switching properties of the optimized C-GHLC (Kindly see the Visualization 1). Moreover, the optimal chiral condition was confirmed also by the full wavelength transmittance switching characteristics and unpolarized microscope images of C-GHLC cell between E-On and E-Off states as shown in Figures S3 and S4. The observed strong random crack lines of the strong chiral condition of d/p = 20 were assumed due to the strong chiral domains. The haze values of optimal C-GHLC cell were 6.5% in the E-Off case and 5.8% in the E-On case. This small value of haze was assumed due to the randomly distributed ball spacer for cell thickness from the microscope observation.
Fig. 7. Comparison of the optical transmittance switching control between the transparent and opaque states for each C-GHLC cell. (a) low d/p = 0.5, (b) optimized d/p = 8, (c) high d/p = 20 for each electrical operation voltage.
4. Conclusions
In this study, a comprehensive study was investigated to detail how to design an optimized chiral guest-host liquid crystal (C-GHLC) using polarization-independent chiral rotation for light transmittance control. Various electro-optical transmittance controls including careful consideration of the chiral twisting power. Along with dye concentration control, chiral twisting control was optimized in terms of ‘d/p’ for improved C-GHLC transmittance switching control. New modified governing equations for the transmittance changes in the transparent and opaque states of C-GHLC were proposed. In light of the chiral twisting power in terms of d/p, the C-GHLC design was optimized using these modified transmittances governing optical functions. By investigating electro-optical properties such as driving voltage, response time, and vivid transmittance difference as a function of d/p between transparent and opaque states. As a result of this analytical optimization, an improved transmittance switching (ΔT ≈ 40.5%) of C-GHLC was obtained with a low voltage (Von ≈ 18 V) and sufficiently fast response time (τoff ≈ 12 ms). Notably, these smart window properties of C-GHLC which can use the current display circuit at low voltage (< 30 V) and 60 Hz (< 16.7 ms) driving conditions were confirmed for the first time. It is expected that the detailed design of C-GHLC with improved switching properties and display operation conditions in this study would contribute to the following research of smart window technology for practical applications.
Funding
Ministry of Trade, Industry and Energy (Korea) (20016260); Samsung Science and Technology Foundation (SRFC-TC2103-01).
Acknowledgments
This research was funded by the Technology Innovation Program (20016260, Development of large size multi-function film with low operation voltage and high speed) funded by the Ministry of Trade, Industry and Energy (Korea) and Samsung Research Funding and Incubation Center of Samsung Electronics (SRFC-TC2103-01).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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