Hybrid Active–Passive Control of an Unconventional Smart Window Based on Dye‐Doped Dual‐Frequency Liquid Crystal

The allure of dual‐frequency liquid crystal (DFLC) stems from its unique feature of the switchable and revertible sign of dielectric anisotropy. In addition to the common frequency‐modulation method, this study focuses on manipulating the dielectric anisotropy in DFLC through change in temperature. Herein, the hybrid control of a dichroic‐dye‐doped DFLC smart window is proposed, which offers a passive‐control function by reacting to the temperature and automatically switching to one of the three distinct transmission levels: the transparent, variably gray (tinted or translucent), and opaque states. In contrast to conventional, passively controlled smart windows that typically exhibit a gray or opaque state at high temperatures and become transparent at low temperatures, the suggested smart window thermally operates in the opposite manner, presenting a unique characteristic that holds significant potential for a wide range of applications. This thermally reverse‐mode smart window allows one to actively control the degree of transparency by applied voltage at various frequencies as well. A parameter termed the “crossover temperature” is introduced as an indicator of the potency for the change in dielectric anisotropy of the DFLC by temperature. The operation scheme of the hybrid electrical‐and‐thermal control for the proposed vertically aligned dye‐doped DFLC smart window is summarized.

window by incorporating photoisomerizable (e.g., azobenzene), [36,47] photothermal (e.g., isobutyl-substituted diimmonium borate), [43] or photoconductive substances (e.g., zinc phthalocyanine) [48] that can respond to environmental stimuli.Although adding extra materials into a mesogenic host may offer merits for breakthrough applications in passively stimulated control of smart windows, there exist several potential problems for practical use, including the shortened switching lifetime, [43] degradation in optical performance, [48] and long response time. [49]bviously, it is highly desired to innovate a passive-control smart window enabled by a simple material system.
Dual-frequency LCs (DFLCs) are well known in LC science and technology due to their unique, electrically fast-switchable dielectric anisotropy properties.The dielectric anisotropy Δε in a DFLC changes the sign from positive to negative as the frequency of applied electric field increases from lower to higher than the crossover frequency f c . [50]Advantageously, this feature permits DFLC to be a great candidate for developing electrically controlled and fast-response smart windows. [51,52]DLCs containing host LCs and guest dichroic dyes are emerging candidates for smart-control windows by virtue of their uncomplicated attributes and dichroic absorption characteristics.The dye molecules strongly absorb incident light polarized parallel to their (long) absorption axis and weakly absorb light polarized perpendicular to it.These optical characteristics, which do not cause light scattering but allow viewers to see through them, have received considerable attention for various applications in windows, [35] tunable eyewear, [53] vehicles, [54] and buildings. [55]Their ability to achieve a "tinted" state in a polarizer-free scheme, similar to tinted glass for unobstructed view even when darkened, makes them ideal for such utilizations. [56,57]nfortunately, there are still certain challenges associated with DDLC-based smart windows.One major concern is their limited ability to achieve a fully opaque state for privacy protection because of their tendency to exhibit transparent or translucent states through varying levels of light absorption.Therefore, solving this problem while preserving a tinted state in DDLCs would hold great promise for their humanistic use in smart windows.
In this study, we explored a state-of-the-art switchable window cell based on a simple material system-dichroic-dye-doped DFLC-featuring both the electrically active and thermally passive operations.The proposed hybrid control gives rise to interchangeable functionality, allowing tuning or switching between the transparent, variably gray (tinted or translucent), and opaque states.Unlike the typical DDLC-based smart window, our device atypically exhibits high transparency at higher T and a tinted state at lower T. We believe that the reverse thermo-optical response of our device can find values for practical applications.For example, during the daytime with high T, the smart glass maintains its transparency, providing customer with clear view of the product showcases in shop windows.As T drops in the evening, the smart windows undergo a transition, transforming into a gray state.This change creates a more captivating and mysterious display effect.This innovation can be effectively exploited in various settings, such as commercial exhibition windows, retail stores, museums, and cultural centers.Alternatively, during daylight hours, the unconventional smart windows can retain their transparency, enabling diners to appreciate the external scenery while enjoying their meals.As evening falls, the windows respond to the lower T and, in turn, adjust to a tinted state.This feature renders diners with enhanced privacy and a comfortable ambiance, or sets a dimmer atmosphere, depending on the intended effect.Such an application is especially suitable for restaurants, cafes, bars, or nightclubs.

Device Function Design
The proposed thermally passive control for sensitively thermooptical adjustments is depicted in Figure 1.The window consists of two parallel pieces of glass sandwiching a vertical alignment (VA)-DDLC layer.Figure 1a shows the window's low transmission at night with a relatively low surrounding temperature.When a high-frequency electric field is applied, the DFLC and black dye molecules align themselves parallel to the glass substrates, leading to substantial absorption of light along their long axis.This causes the window to darken while still maintaining its translucency.By contrast, the self-switching smart window exhibits its optical transparency during the daytime, as illustrated in Figure 1b.When exposed to solar radiation, the smart window gradually heats up, prompting the DFLC molecules to self-adopt homeotropic alignment under the unchanged condition of applied voltage and resulting in least absorption.
Figure 2 illustrates the proposed electrically active control of the DDLC window that permits personal preference to arbitrarily switch among three different optical states at null or various nonzero applied voltages.Figure 2a displays the transparent state, which is a stable state achievable without an externally applied field.Figure 2b reveals a tinted state with a somewhat transparent black color sustained by voltage at a frequency beyond f c to change the sign of Δε in the DFLC, thereby imposing the orientation of LC and dye molecules to be perpendicular to the light propagation direction for effective absorption.Figure 2c shows the opaque state with turbulence in the DFLC layer, which is induced by the electrohydrodynamic (EHD) effect in the presence of an AC electric field.When an electric field at a specific frequency is applied, the movement of ions in the DDLC window disrupts the alignment of the LC and dichroic dye molecules, and the dynamically random disorder of the molecules brings about rapid fluctuations in refractive index and dichroic absorption over time.With both the dynamic scattering (DS) and moderate absorption in action, the window exhibits a milky opaque look.

Working Principle
To illustrate the feasibility of the hybrid control mode of the proposed DDLC smart window, we conducted a study to find the correlation between f c and T by measuring the complex dielectric spectra of the DDLC.The dielectric properties of the nematic host at four representative T are displayed in Figure 3a, showing the vertical (ε ⊥ ) and parallel (ε ∥ ) components of the real-part dielectric permittivity ε' acquired at zero bias (V DC = 0) and 40 V, respectively.Note that the cell gap of the homogeneously aligned cell used for the measurement is much thicker and the overlapped electrode area A much smaller to avoid the unwanted pseudodielectric behavior in the high-frequency range of interest, [58][59][60] enabling accurate grasp of the temperature dependence of f c .A common method for interpreting the dielectric response is through a single relaxation system characterized by a Debye-type relaxation mode model.ε 0 of the dielectric permittivity can be expressed as follows: [61] where ε L and ε H represent the dielectric constants in low-and high-frequency regions, respectively, ω (=2πf ) is the angular frequency, and τ R denotes the single relaxation time of the dielectric function, which is inversely proportional to the dielectric relaxation frequency f R , i.e., 2πf R • τ R = 1.The measured data (Figure 3a) were fitted to Equation ( 1   by the EHD effect.For example, when the DDLC cell is maintained at a constant T of 30 °C, the characteristic f c = 25.6 kHz, as shown in Figure 4.When an applied frequency f = 300.0kHz is utilized, the VA-DDLC window will appear tinted in the P state owing to the absorption of the black dye.As the applied f decreases, the window will become translucent as a result of growing EHD strength when f approaches f c until transitioning to opaque in the DS state at f = f c , as represented by the curve bearing a string of filled circles.Subsequently, as f continues to drop, say, to 5.0 kHz, the window will exhibit transparency in the H state.It is worth mentioning that f goes higher at high T in order to vary the sign of Δε.Second, let us focus on the thermal-control mode.To elucidate the working of switchability and tunability in the thermal control of the VA-DDLC window, the crossover temperature T c , which separates two T regions with opposite signs of Δε at a given f value, is introduced.Similarly, the passive mode permits tuning the sign of Δε from negative to positive by heat or increasing ambient temperature from T < T c to T > T c .For instance, at f = 25.6 kHz (where T c = 30 °C) and an arbitrary voltage amplitude, the DDLC window exhibits a black tint at T = 10 °C (<T c ) while it switches to a transparent H state at T = 50 °C (>T c ). Noticeably, the higher the driving frequency, the higher the T c value in that the relationship between f and T c in Kelvin satisfies the Arrhenius equation as does the behavior of the increase in f c with increasing T. The activation energy deduced through curve fitting is 0.74 eV for the DFLC material used in this work (data now shown).

Device Fabrication and Optimization
Figure 5a reveals the transmission spectra of three dissimilar types of DDLC cells in the wavelength range of λ = 400-780 nm at 25 °C under null applied electric field.It shows that the effective absorption by the dye takes place in the visible spectrum up to λ = 660 nm and that the VA cell exhibited the highest transmittance (of 67.6%) at λ = 500 nm.This result can be explained by the long absorption axis of the dichroic dye to be perpendicular to the VA cell substrates, conducing to the weakest absorption because of the electric field of emerging natural light vibrating perpendicularly to the long absorption axis during the propagation through the VA-DDLC cell. [54]Furthermore, the antiparallel (AP) and twisted-nematic (TN) cells possessed relatively low transparency, yielding T% values of 41.9% and 33.2% at 500 nm, respectively.It is obvious from Figure 5a that the AP cell outperformed the TN counterpart in terms of transmittance.This finding can be easily accounted for by the thorough absorption in all azimuthal directions in the 90°TN cell.Figure 5b depicts the DDLC cells used to produce Figure 5a.As shown in the photographs, the TN-DDLC cell exhibits the lowest level of visual transparency, while the VA cell displays the highest.
Figure 6a displays the spectra of five unperturbed VA-DDLC cells of different thicknesses at 25 °C.To characterize the overall transmission of a cell thickness for comparison, average transmittance at wavelengths spanning from 400 to 700 nm was calculated from each spectrum.The values thus obtained are 82.3%, 71.3%, 67.4%, 48.5%, and 32.1% for the VA cells of d = 4.2, 10, 15, 30, and 50 μm, respectively.As expected from the perspective of the optical path length, the experimental results indicate that thinner cells have higher transmittance, which falls with increasing cell gap.In line with the spectral data showing the cell-thickness dependence of transmission, the appearance of the VA-DDLC cell changes with its thickness, with thinner cells  appearing more transparent and thicker ones darker (Figure 6b).This phenomenon was observed from DDLC in the AP and TN cells as well.
Figure 7 explicitly discloses cell-thickness-varying transmittance averaged between 400 and 700 nm for the three aforementioned sample types (i.e., VA, AP, and TN cells).The transmission behaviors of the DDLC cells can be mathematically described as follows: [62] T VA ¼ T 0 e Àκ ⊥ d (2) where T 0 represents the transmission constant at zero thickness and κ ∥ and κ ⊥ are the parallel and perpendicular components of the effective extinction constant underlain by the dichroic absorption of the dye, respectively.These exponential equations allow one to perform curve fittings to the experimental data as presented in Figure 7. Here, the three thickness-transmittance plots unambiguously point out that transmittance decreases monotonically with increasing d for all three types of cells.In addition, it is evident that the VA-DDLC (TN-DDLC) cells consistently possessed the highest (lowest) transmittance under the same cellgap conditions.To ensure initially higher transmission in the field-off state as well as good gray-level tunability and opacity in field-on states, we chose the VA configuration for the cell mode and 15 μm for the cell thickness as our focus for followup experiments.This selection is particularly favored in that its fabrication does not entail rubbing for simplified process.

Device Electro-Optics Characterization
At selected frequencies ranging from 10 to 20 kHz, eight voltagetransmittance (at λ = 500 nm) curves derived from a 15 μm thick VA-DDLC cell are presented in Figure 8a.The transmission data, with a tolerance of AE1% caused by light fluctuation, were taken every 0.25 V rms in a square waveform at 25 °C where the characteristic f c is 15.2 kHz (Figure 4).One can see in Figure 8a that the transmittance T% (of %66%) remained unchanged at 10 and 14 kHz in spite of elevated voltage V. Intriguingly, when f increased to a value lying between 15 and 18 kHz, T% progressively declined as V increased.For fixed f = 15 kHz, T% dropped   to %40% at V = 18 V rms and further to %1% at V = 55 V rms from 66% at V ≤ 12 V rms , the threshold voltage V th .Upon closer inspection, we found that the decrease in T% at V > 12 V rms originated in part from the dynamic contribution of the parallel component of the long absorption axis of dye molecules induced by hydrodynamic instabilities or the EHD effect in the field-excited LC bulk, [63] giving rise to growing absorption as well as DS to deteriorate its transparency.As the intense DS effect at relatively high voltages can prompt very low transmittance, the contrast ratio (defined as the ratio of the highest transmittance to the lowest) obtained is higher than 1900 at f = 15 kHz, which was calculated between the transparent H and opaque DS states.Further, when f increased to 19 kHz and beyond, T% fell from %66% to 40% as V increased and persisted at T% = 40% as the voltage continued to rise.The contrast ratio obtained is less than 2 at f = 19 kHz, as calculated between the transparent H and tinted P states.To produce Figure 8b for the illustrative working zones, we defined the following three characteristic voltages: the threshold voltage V th , transition voltage V trans , and saturation voltage V sat at which T% = 65%, T% = 40%, and T% = 1% were first reached in the increasing-V process, respectively.Here, the transparency characteristics of the DDLC corresponding to f and V are unraveled by classifying transmittance into four different intervals and using colored sections to distinguish them.They are the transparent state (T% ≥ 65%) in region I, 40% ≤ T% < 65% as the tinted state in region II, the translucent state (1% ≤ T% < 39%) in region III, and T% < 1% as the whitish opaque state in region IV.Our results show that when 5 kHz < f < 15 kHz, the cell exhibited the transparent H state specified as region I regardless of V.In the frequency range of 15 kHz ≤ f ≤ 18 kHz, the transparency gradually decreases with increasing V up to 100 V rms in this study.It is worth noting that only in this frequency range the EHD effect can be induced with a higher operation voltage (see region IV).At 18 kHz < f ≤ 20 kHz (i.e., f > f c ), the EHD effect was not observed at voltages as high as 100 V rms .When f >> f c , switching of the devised smart cell can only occur between the transparent and tinted states on account of the dielectric effect in the guesthost system of Δε < 0, meaning that the field-induced director reorientation toward the P configuration or tinted state is retained even at increasing V.
To further evaluate the optical performance of the proposed DDLC smart window, haze H% in different states as a function of V was investigated and the experimental data are exhibited in Figure S1, Supporting Information.In terms of the thermally passive control, the relationship between temperature and transmittance at λ = 500 nm was investigated at fixed V of 25 V rms .The experimental results acquired in the heating process with a heating rate of 0.5 °C min À1 for five distinct frequencies are displayed in Figure 9a.At lower T, the DFLC possesses negative Δε so that the DDLC is in the tinted P state.For f = 60 kHz, T% remained constant (around 40%) until T approached T c at %40 °C (Figure 4).As T was continuously elevated from 37.5 to 40.0 °C, T % reduced with increasing T, exhibiting a translucent DS state governed by the EHD effect.When T increased further (T > 40 °C), the DDLC exhibited the transparent H state in consequence of the dielectric effect (Δε > 0). Figure 9a also implies that a higher T is required to force the smart double-glazed window to change its transmittance.This behavior is attributed to the fact that T c increases with increasing f, as depicted in Figure 4.These results undoubtedly manifest f-dependent tunability in transparency with T. Figure 9b illustrates the transmission spectra of the DDLC cell driven by V = 25 V rms and f = 60 kHz at 35, 38, 39, and 45 °C.Unlike Figure 9a showing transmission curves at a specific wavelength (λ = 500 nm), Figure 9b provides the overall spectral profiles in the visible range characterized by the dichroic dye in the nematic host.With photos taken from a 15 μm thick VA-DDLC cell situated 40 cm in front of a captured image of Taipei 101 Building, Figure 9c delineates the varying optical state dependent on temperature.All data presented in Figure 9 are reversible, viz., experimentally reproducible in the cooling process.

Operating Mechanism
Figure 10 summarizes the proposed smart window, which can operate in three states: the transparent H state, tinted P state, and opaque DS state.The arrows indicate the directions of operations for the electrically active and thermally passive control.For the active control, applying AC voltage at f = f c switches the VA-DDLC window to the opaque state enabled by the fieldinduced intense EHD effect.Further, the window can switch to a tinted P state, projecting a saturated T% value of %40AE1% at f > f c .Upon turning V off, the LC quickly transforms to its initial configuration, provoking the smart window to switch to the transparent H state with T% %66AE1%.Notably, switching between the three optical states can operate back and forth.Omitted in Figure 10, a tunable translucent state (taking on a slightly white tint) can be accomplished by setting f in the neighborhood of f c .Regarding the thermoresponsive control, the suggested DDLC smart window automatically regulates its transparency, displaying a tinted state, the opaque DS state, and the transparent H state at T < T c , T = T c , and T > T c , respectively.The thermal-control mode is reversible as well, allowing switching by heating or cooling the VA-DDLC window.

Conclusions
Based on dye-doped VA-DFLC, we have unambiguously demonstrated a hybrid (both electrically active and thermally passive) control approach to realization of an unconventional smart window highlighting the thermally reverse-mode function.As a deterministic f-dependent parameter, T c unquestionably designates the ability to adjust the temperature-dependent transmittance between tinted and transparent states.This feature allows users to tailor transparency according to their specific needs or the climatic conditions of their living environment.Through a comprehensive investigation of effective absorption in various LC configurations and cell thicknesses, we have determined that a VA-DDLC cell with a thickness of 15 μm is an optimal choice for the suggested smart window.The measured V-T% curves and the corresponding f-V plot yield valuable results, clarifying that the EHD effect can be induced by applying an electric-field frequency lower than f c , although higher applied voltage is required in such cases.Interestingly, we observed furious DS even when f ranged from 15 to 18 kHz, knowing that f c of the used DDLC material is 15.2 kHz at T = 25 °C.The highest haze  value observed in our cell device is 89%, clearly connoting its remarkable scattering performance in the opaque DS state.Moreover, we have offered a thorough illustration of the operational process for the devised VA-DDLC smart window with hybrid control.By adjusting the driving frequency (temperature) to different ranges, namely f < f c (T > T c ), f % f c (T % T c ), and f > f c (T < T c ), the introduced smart window can be effectively switched between the transparent, opaque, and tinted states, respectively.In the passive-control mode, the switching temperature is adjustable as desired, rendering a user-centered, humanbased feature in spite of the necessity of a sustaining electric field.The results depicted in this work paves a new avenue for the unconventional, thermally reverse-mode smart window to many new applications.

Experimental Section
The DDLC mixture used is the DFLC HEF951800-100 (HCCH Co.) impregnated with 1 wt% of the commercial dichroic dye S-428 (Mitsui Chemicals).The LC host is composed of plural chemical compounds, primarily with the cyano terminal groups.Most of the compounds have three or four rings in the core structures and have fluorine atoms as lateral substituents to introduce negative dielectric anisotropy at frequencies above %10 kHz.The DFLC exhibits dielectric anisotropy Δε ≡ ε ∥ -ε ⊥ = 9.1-7.0= 2.1 (at frequency f = 1 kHz and T = 25 °C) and birefringence Δn ≡ n e -n o = 1.718-1.496= 0.222 (as measured at the wavelength of λ = 589 nm and T = 25 °C).Its clearing point is observed at 104 °C.To obtain a homogeneous blend, the DDLC mixture was stirred at T % 120 °C (in the isotropic phase) for 2 h.Subsequently, precise volumes of LC in the microliter range were transferred using a micropipette, and the DDLCs were filled into various types of empty cells through capillary action.Each cell consists of a pair of glass substrates coated with transparent indium-tin-oxide electrodes.To prepare adequate substrates for assembling empty cells of various cell modes, we adopted two polyimides, AL-8395 (Daily Polymer) and SE-150 (Nissan Chemical), for spin coating onto conductive substrates to impose vertical and planar alignments of LC molecules, respectively.Note that the baked planar-alignment layers were successively subjected to mechanical rubbing to ensure the LC molecules aligned in a fixed azimuthal orientation or at a small specific pretilt angle.Consequently, three types of DDLC cells were fabricated, including VA cells without rubbing and, for comparison, 90°TN and 180°AP counterparts.To further investigate the relationship between light transmission and cell gap d, spacers of dissimilar diameters of 4.2, 10, and 15 μm and Mylar strips of thicknesses (of 30 and 50 μm) were introduced between two glass substrates.All cell gaps were determined by interference in the visible spectrum.An inductance-capacitance-resistance meter (Agilent E4980A) was used to measure the complex dielectric spectra of a DDLC cell whose temperature was regulated by a T-controlled system (Linkam T95-PE).The spectral characteristics in the visible range (λ = 400-780 nm) were acquired with a fiber-optic spectrometer (Ocean Optics HR2000þ) in conjunction with a halogen light source (Ocean Optics HL-2000).Application of various square-wave AC voltages across the cell thickness was enabled by a function generator (Tektronix AFG-3022B) along with a power amplifier (TREK Model 603).To analyze the light scattering nature of the proposed DDLC smart window in the visible spectrum, a haze meter (Nippon Denshoku COH-5500) was utilized to measure the total transmittance T t %, specular transmittance T s %, and diffuse transmittance T d % for the calculation of haze value H% ≡ (T d %/ T t %) Â 100%.Note that T t % = T d % þ T s %.T d % and T s % are the relative amounts of light transmitted at scattering angles greater and less than 2.5°i n the optical path, respectively.A polarized optical microscope (Olympus BX51) with crossed polarizers was exploited in the transmission mode for observing optical textures of DDLC cells under different electrical stimuli.
) to calculate f R at T = 10, 20, 30, and 40 °C, giving the corresponding values of 2.8, 8.4, 22.7, and 56.4 kHz, respectively.The fitting curves demonstrate excellent agreement with the experimental data as evidenced by the (four) high coefficients of determination, R 2 = 0.9999.The characteristic f c can be directly retrieved at the intersections between the four pairs of the ε ⊥ and ε ∥ curves, and they are 3.0, 8.6, 25.6, and 61.7 kHz at 10, 20, 30, and 40 °C, respectively.It is clear in Figure3athat the dielectric relaxation shifts toward higher frequencies with increasing T, causing a gradual increase in T-dependent f c .This phenomenon suggests a thermally feasible control over the orientation of LC molecules subjected to a fixed AC field as T-dependent Δε varies.Figure3bdisplays the frequency dependence of Δε and the red dashed line gives the frequencies where Δε vanishes at the four temperatures.As shown in the figure, Δε is negative at T = 20 °C but turns positive at T = 40 °C when an electric field oscillating at f = 20 kHz is applied.The frequency and temperature dependence of the dielectric properties of the nematic host makes the VA-DDLC smart window controllable in two control modes as delineated in Figure4.First, let us consider the electric-control mode.Δε can be tuned between two opposite signs by applying AC voltage of a specific frequency.The smart window works in three states: the planar (P) state at f > f c (Δε < 0), DS state at f = f c (Δε = 0), and homeotropic (H) state at f < f c (Δε > 0).Note that the DS state is induced

Figure 1 .
Figure 1.Illustration of the thermally passive-control function of the proposed VA-DDLC window at a fixed operation voltage.a) The window presents tinted appearance at a lower temperature.Conversely, b) it turns transparent at a higher ambient temperature.The device works reversibly.

Figure 2 .
Figure 2. Optical states in the electrically active-control VA-DDLC window: a) the transparent state at null applied voltage, b) a tinted state in an AC electric field at high frequency, and c) the opaque state at a particular frequency.

Figure 3 .
Figure 3. a) Parallel and perpendicular components of the real-part dielectric spectra of an AP-DDLC of 50 μm in thickness at various temperatures and b) the corresponding curves of f-dependent dielectric anisotropy.

Figure 4 .
Figure 4. Temperature versus frequency optical state diagram of the DFLC at temperatures ranging from 0 to 65 °C (data extended from Figure 3a).Setting the boundary between the top left zone of Δε > 0 and the bottom right zone of Δε < 0 at a giving T or f, the solid line corresponds to the condition of Δε = 0 (or f = f c or T = T c ).The temperature difference between two adjacent (i.e., nearest) symbols is 5 °C.

Figure 5 .
Figure 5. a) Transmission spectra of the DDLC in three different (VA, AP, and TN) cells and b) the photographs of the corresponding samples atop a piece of paper printed with eight black characters.

Figure 6 .
Figure 6.a) Transmission spectra of VA-DDLC in cells of various cell gaps and, correspondingly, b) their photos at 25 °C in the voltage-off condition.

Figure 7 .
Figure 7. Cell gap dependence of average transmittance of the DDLC measured from three different cell types.The cell thicknesses investigated are 4.2, 10, 15, 30, and 50 μm.The curves fitted to the experimental data are simulated in accordance with Equation (2)-(4).

Figure 8 .
Figure 8. a) Voltage-dependent transmittance of the VA-DDLC cell at various applied frequencies and b) voltage and frequency dependence of four optical states-the transparent, tinted, translucent, and opaque states-classified by four colored areas.

Figure 9 .
Figure 9. a) Temperature-dependent transmittance (at λ = 500 nm) of the 15 μm thick VA-DDLC cell driven by 25 V rms at various applied frequencies, b) transmission spectra in the condition of fixed V = 25 V rms and f = 60 kHz at four temperatures in a 10 °C range covering T c , and c) a series of photos taken at six T values under the condition of V = 25 V rms and f = 60 kHz.Uneven transparency shown in the photos is ascribed to the oblique (from the upper left to the bottom-right vertex) heat flow from a hair dryer to accommodate a front IR camera (FLIR ThermaCam E4) to read T at the center.

Figure 10 .
Figure10.Schematic of the working principle of a thermally reverse-mode VA-DDLC window whose transparency can be electrically and thermally modified.The field-off condition renders the transparent H state with high transmission.At a fixed applied frequency of AC voltage, the window appears transparent at higher temperature and yet switches to the tinted P state having moderate transmission at lower temperature.Omitted in this diagram is a tunable translucent state that appears in a narrow range of frequencies, voltages, or temperatures.