Introduction

When chiral molecules are introduced into nematic liquid crystals, they can induce a cholesteric liquid crystal (CLC) phase. The most distinctive optical property of the cholesteric phase is selective Bragg reflection. Because of the cholesteric helix structure, these liquid crystals can selectively reflect circularly polarized light, in accordance with Bragg’s law [1,2,3,4,5,6,7,8]. Additionally, CLCs can adopt three different states—planar, homeotropic, and focal conic—depending on their three-dimensional arrangement. Each state has unique optical properties, making cholesteric liquid crystals ideal for developing liquid crystal (LC) grating devices. However, sustaining or altering the cholesteric state requires an external force or an alignment layer [9,10,11,12,13,14,15,16,17,18].

To address this problem, various systems incorporating polymer networks into cholesteric liquid crystals, such as polymer-dispersed liquid crystals (PDLCs) and polymer-stabilized liquid crystals (PSLCs), have been studied to improve their stability and operational modes [19,20,21,22,23,24,25,26]. Bistable dual-frequency liquid crystals (BDFLCs), which operate in a novel manner, have also been developed. Additionally, some studies have indicated that incorporating a chiral dopant can reduce the operating voltage and response time [27,28,29,30,31,32,33,34,35].

A polymer-stabilized cholesteric texture (PSCT) cell can exhibit different states depending on the operating process. Initially, in the absence of an external electric field, the PSCT cell is in the planar state with all the helical axes perpendicular to the cell substrate. In this state, if the incident light wavelength is outside the selective light reflection range, the cell remains transparent. The application of a weak vertical electric field triggers a switch from the planar state to the focal conic state, in which the randomly arranged CLC helices scatter incident light, making the cell opaque [36,37,38,39,40,41].

Conventional color electronic books employ three layers of CLC films to reflect red (R), green (G), and blue (B) light and achieve full-color reflective displays, as shown in Fig. S1a [42,43,44,45]. To reduce power consumption, a bistable design is adopted for energy-efficient operation. In this study, a novel reflective bistable cholesteric liquid crystal display (CLCD) device was designed to simplify the fabrication process and enhance image sharpness, as demonstrated in Fig. S1b. Another design for reflective and back-lit displays is shown in Fig. S1c. With a color filter and a pellicle, the incident light can be reflected, and a backlight display can also be achieved. The Bragg reflection wavelength of the cholesteric cell must fall outside the visible range. In this way, the light transmitted by the LC film will appear transparent. The fabricated dual-frequency bistable liquid crystal devices exhibit stable opaque focal conic states and transparent planar states after a single switching cycle, requiring no energy to maintain their states.

We previously published a related paper in the Journal of Materials Chemistry C in 2021 [46], which focused on the synthesis of a novel chiral dopant and its applications in cholesteric liquid crystals and dual-frequency bistable liquid crystal (DFCLC) devices. In that work, we described only the application of DFCLC in the switching display of liquid crystal devices. The paper did not address how to shift the Bragg reflection out of the visible range, nor did it combine reflective color filters to achieve the display of color patterns. In addition, the use of a backlight source was not discussed. In the present study, DFCLC is combined with a transflective color filter to form a novel energy-saving color liquid crystal display device, which can be applied to the design of color electronic books and commercial advertising displays.

Experimental section

Materials

All reagents and chemicals were used as received without further purification. HEF951800-100 (HEF951) dual-frequency nematic liquid crystals, (S)-( + )-4′-(2-methylbutyl)-4-biphenyl-carbonitrile (CB15), and 4-methoxyphenyl 4-((6-(acryloyloxy) hexyl)oxy)benzoate (RM105) were obtained from Fusol Material Co., Ltd. (Tainan, Taiwan). 2,2-Dimethoxy-1,2-diphenylethan-1-one (IRG-651) 99% and polyvinyl alcohol (Mw = 89,000, (C₂H₄O)n) were obtained from Acros Organics (Geel, Belgium). S-( + )-2-octyl4-(4-hexyloxybenzoyl oxy)benzoate (S811) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Dichloromethane (DCM), anhydrous, 99.9%, and indium tin oxide (ITO) glass were obtained from Scharlau Chemicals (Sentmenat, Spain) and Uni-Onward Corp (New Taipei City, Taiwan), respectively. Spacers were provided by Shinkong Materials Technology Corp. (Taipei, Taiwan). Solvents, including ethanol (C₂H₅OH), dichloromethane (CH₂Cl₂), and acetone (CH₃COCH₃, 99.9 wt%), were obtained from Mallinckrodt (Hazelwood, Missouri, USA).

Instruments

The phase transitions of the liquid crystal mixtures were analyzed using a differential scanning calorimeter (DSC, PerkinElmer DSC 6000, Springfield, IL, USA) at a heating/cooling rate of 10 K·min⁻¹. Simultaneously, a Nikon ECLIPSE Ci POL (Tokyo, Japan) polarized optical microscope (POM) equipped with a hot stage (LINKAM T96-S, London, UK) was used, with the temperature scanning rate also set to 10 K·min⁻¹. To identify the isotropic phase of the liquid crystals, a quarter-wave retardation plate was inserted into the optical path of the POM. Thermal stability was evaluated under a nitrogen atmosphere using a thermogravimetric analyzer (TGA, PerkinElmer TGA 4000, Springfield, IL, USA) at a heating rate of 10 K·min⁻¹. The molecular structures of the synthesized compounds were confirmed by nuclear magnetic resonance spectroscopy (NMR, Bruker AV-500, Billerica, MA, USA) and Fourier transform infrared spectroscopy (FTIR, Jasco FT/IR-4600, Tokyo, Japan). Optical properties were measured using a UV–Vis–NIR spectrophotometer (PerkinElmer LAMBDA 950, Springfield, IL, USA). The surface alignment treatments were performed with a rubbing machine (LY-E, Unitech Computer Co., Ltd., Taichung, Taiwan). The electro-optical properties of the fabricated cells were estimated via a setup consisting of an arbitrary/function generator (Tektronix AFG3021B, Oregon, USA), a digital storage oscilloscope (Tektronix TDS 2014b, USA), a high-voltage amplifier (PINTEK HA-805, New Taipei, Taiwan), a photodetector (Electro-Optics Technology ET-2000 Silicon Pin Detector, Miami, USA) and a helium‒neon laser (Meredith Instruments, Arizona, USA).

Fabrication of cholesteric liquid crystal (CLC) cells

Cleaning of the glass substrate

ITO glass substrates (2.5 × 2.5 cm², 0.7 mm thick) were ultrasonically cleaned sequentially with neutral detergent, DI water, and acetone, followed by drying in an oven at 60 °C for 2 h to remove surface debris and oil.

Parallel alignment layer fabrication

A polyvinyl alcohol (PVA, Mw = 89,000–98,000) solution was prepared by dissolving 7.5 g of PVA in 500 mL of deionized water with stirring at 200 rpm for 24 h, followed by heating to 60 °C to ensure complete dissolution. After cooling, a cleaned glass substrate was immersed in the PVA solution for 1 min and dried in an oven at 60 °C for 30 min. The dried PVA-coated substrate was then rubbed three times at 2500 rpm to induce parallel alignment.

Liquid crystal mixture

The CLC mixture comprised the commercial unreactive dual-frequency nematic liquid crystal (DFNLC) HEF951800-100, commercial chiral dopants S811 and CB15, the monomer RM105, and the photoinitiator IRG-651. A dual-frequency CLC (DFCLC) mixture is prepared by mixing the DFNLC with chiral dopants. The physical properties of the commercial nematic liquid crystal HEF951 are detailed in Table S1, and the chemical structures of the utilized compounds are illustrated in Scheme 1.

Scheme 1
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Chemical structures of the chiral dopants CB15 and S811, monomer RM105, and photoinitiator IRG-651

Fabrication of cholesteric liquid crystal cells

Fabrication of parallel-aligned CLC cells

To create a parallel-aligned cell, two rubbed glass substrates were placed face to face in an antiparallel orientation. Spacers with a thickness of 16 μm were positioned at the two side edges of one glass substrate, and the other glass substrate was attached using epoxy glue. When the epoxy glue had set, a parallel-aligned cell was formed, with the gap determined by the 16 μm spacers.

Fabrication of nonaligned CLC cells

For the fabrication of a nonaligned glass cell, two glass substrates were joined using a 16 μm spacer and secured with epoxy glue, resulting in a gap of 16 μm.

The CLC phase of the predesigned CLC mixtures was verified using a DSC and a POM. An empty glass cell was placed on a hot plate and heated above the isotropic temperature of the CLC mixture. The preheated isotropic CLC mixture was then applied to one side of the glass cell, and the cell was allowed to fill by capillary action. Once filled, the glass cell was allowed to gradually cool to room temperature. The fabricated CLC glass cells were then reserved for further study.

Fabrication of polymer-stabilized dual-frequency CLC cells (PSDFCLCs)

After being filled with the polymerizable DFCLC mixtures, the CLC cells exhibit specific electro-optical properties. Polymerization of the DFCLC mixtures was performed using UV irradiation under an electric field. The intensity of the 365 nm UV light was 5 mW/cm², and the exposure time was 10 min, as shown in Fig. 1.

Fig. 1
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Schematic illustration of the fabrication process of a PSDFCLC cell without an alignment layer

Measurement of electro-optical properties

The DFCLC and PSDFCLC cells were powered by AC power to examine their electro-optical properties at various electric frequencies and voltages. A function generator, digital storage oscilloscope, and high-voltage amplifier were used to generate electric fields of different strengths and frequencies. The transmittance of the DFCLC and PSDFCLC cells was measured using a photodetector that converted light intensity into electrical signals, with a helium-neon laser (632.8 nm) serving as the light source.

The entire setup is illustrated in Fig. S2. This system was used to evaluate the electro-optical bistable properties of the liquid crystal devices.

Results & discussion

Preparation of dual-frequency CLC samples

To fabricate bistable liquid crystal devices, the commercially unreactive DFNLC HEF951 was chosen because of its wide liquid crystal phase temperature range and its varying dielectric anisotropy at different frequencies. As shown in Table S1, HEF951 exhibits positive dielectric anisotropy at low frequencies and negative dielectric anisotropy at high frequencies, aligning perpendicular to the frequency changes in the cell. To create CLC phases, the chiral dopants CB15 and S811 were added, resulting in the observed cholesteric phases. All sample cells were designed as DFCLCs.

The chiral dopants CB15 and S811 are expected to exhibit lower driving voltages and faster response times when applied to CLC cells. To find the formula with the best transmittance, the parameters of the two chiral dopants were adjusted. Table 1 shows the detailed composition of the sample mixtures. On the basis of the results, it can be concluded that the chiral dopants shifted the reflected light.

Thermal properties of DFCLC mixtures

The phase transition temperatures of predesigned DFCLC mixtures in S1 to S4 cells were confirmed using DSC, as illustrated in Fig. 2. The peaks observed in the DSC curves indicate the transition of the mixtures from the CLC phase to the isotropic liquid phase. The DSC curves for S1 to S4 show isotropic transition temperatures of 91 °C, 83 °C, 81 °C, and 77 °C, respectively. These results demonstrate that the prepared mixtures have an LC phase between –10 °C and 91 °C. These findings indicate that the S1 to S4 mixtures hold considerable promise for applications in LCDs. In addition, increasing the concentration of the chiral dopant S811 decreases the TNI temperature. Sample S4 has the lowest phase transition temperature to the isotropic phase. These results suggest that an increase in the amount of chiral dopant may decrease the order parameter of LCs.

Fig. 2
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DSC thermograms of S1, S2, S3, and S4 mixtures with a heating/cooling rate of 5 °C/min under a nitrogen atmosphere

Reflection spectra of DFCLC cells

To achieve full transmittance, UV–Vis was used to study the reflection region of the LC mixtures. As shown in Fig. 3, an increase in the chiral dopant S811 causes a redshift in the reflected light. The light reflected from sample S4 shifted to the infrared region. Sample S4 was selected as a standard mixture for further study.

Fig. 3
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Reflection spectra of S1, S2, S3 and S4

Depending on the applied voltage, CLCD has several distinct states in an LC cell. Theoretically, as shown in Fig. S3, DFCLC cells exhibit three distinct states, revealing some specific optical properties.

In the case of the S4 sample, the following possible states were predicted. In Fig. S3a, the focal conic state appears opaque because of the random dispersion of cholesteric helices, preventing most incident light from passing through. In Fig. S3b, the planar state is semitransparent, allowing most incident light to pass through the planar structures, with only a small proportion selectively reflected outside the visible light region, resulting in a transparent planar state. In Fig. S3c, the homeotropic state forms under high voltage at low frequency because of the positive dielectric anisotropy of the DFCLC. The strong electric field aligns the LC molecules parallel to the field, making the cell transparent and allowing incident light to pass through. To achieve this goal, states (a) and (b) were designed via the prepared S4 sample mixture. Notably, as shown in Fig. 3, the reflected light was shifted to the infrared region, and the LC cell was almost fully transparent.

Electro-optical properties of DFCLC cells

To investigate the bistable properties of the DFCLC S4 sample cell, the transmittance of the S4 cell was measured at various electric voltages at frequencies of 60 Hz and 70 kHz, as shown in Fig. 4. On the basis of the results, 60 V was selected as the driving voltage for the sample cells. For 60 Hz operation, the transmittance decreased from transparent to opaque during both the increasing and decreasing cycles. In contrast, at 70 kHz, the sample cell transitioned from opaque to transparent during the same cycles. These results indicate that the fabricated DFCLC cell is bistable.

Fig. 4
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Voltage-dependent transmittance of the DFCLC. A driving voltage of 60 V was selected for the LC cell

A schematic illustration of the reversible bistable switching of a DFCLC S4 cell at a driving voltage of 60 V is provided in Fig. S4a. Initially, the focal-conic state transitions to a planar ON state under a 70 kHz driving frequency. To turn off the LC cell, a 60 Hz electric field at 60 V was applied. Notably, both the focal-conic and the planar states were maintained even after the electric power was removed. These results demonstrate that the fabricated LC cell exhibits bistable properties. Figure S4b, c shows real images of the DFCLC S4 cell in the (a) focal conic state and (b) planar state. RGB color strips were set behind the sample cell. As shown in Fig. 4, the contrast of the DFCLC S4 cells is not high.

Response time of bistable DFCLC cells

The response time of DFCLC S4 cells was measured and is presented in Fig. S4. The response time was calculated as the absolute value of t1 − t2, where t1 is the time at which 10% of the transmittance variation occurs and t2 is the time at which 90% of the transmittance variation occurs [42]. Theoretically, the order parameter in the planar state is greater than that in the focal conic state. Transitioning from the focal conic to the planar requires more power, whereas the reverse transition from the planar to the focal conic has a similar response time. This suggests that it is easier to transition from a higher-order arrangement to a lower-order one. The response times of the fabricated LC cells were estimated using the curves shown in Fig. S5. The response times of the fabricated DFCLC S4 cell from focal-conic to planar and from planar to focal-conic were estimated to be 0.61 and 0.8 ms, respectively. On the basis of these results, the fabricated DFCLC S4 cell is suitable for use in the design of television sets.

Fabrication of polymer-stabilized DFCLC cells

Preparation of DFCLC mixtures

To increase stability and enhance electro-optical properties, a polymer network was introduced into the DFCLC cells by adding 1 wt% of the photoinitiator IRG-651 to the DFCLC S4 mixture. The detailed ingredients are listed in Table 1. After polymerization, the fabricated DFCLC cells form PSDFCLC cells.

Table 1 Composition of dual-frequency CLC mixturesa
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Fabrication of PSDFCLC cells

POM analysis

The fabricated PSDFCLC cells showed both focal conic and planar states when 60 Hz and 70 kHz driving frequencies were applied. The textures of the sample cell in the focal conic and planar states under POM are shown in Fig. 5. Owing to the birefringence of the LCs, a colorful pattern is shown in Fig. 5a. In the case of the planar state, the polarized incident light was blocked by the second cross polarizer, as shown in Fig. 5b.

Fig. 5
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The textures of the PSDFCLC S4 cell in the (a) focal conic and b planar states under a POM

Fabrication of PSDFCLC cells

Theoretically, PSDFCLC cells exhibit three states: focal conic, planar, and homeotropic. The pretest results revealed that after the polymerization of DFCLC cells in the homeotropic state, the LC cells demonstrated dual-frequency switching. As shown in Fig. S6, polymer network formation in the focal conic state results in an opaque PSDFCLC cell, whereas polymerization in the planar state results in a transparent cell. Notably, the density of the fabricated PSDFCLC cells could not be readily controlled by the use of the two frequencies to trigger a switch between the opaque and transparent states. This is attributed to the strong interaction between the polymer matrix and liquid crystals. The data in Fig. S6a and S6b illustrate that after polymerization, the focal-conic and planar-aligned LC molecules are strongly fixed by the polymer network, leading to a loss of mobility. Only the polymer network formed in the homeotropic state, as shown in Fig. S6c, allows normal switching.

POM analysis at 60 Hz

The textures of a PSDFCLC cell under various voltages at 60 Hz under a POM are shown in Fig. 6. The changes in texture with applied voltage demonstrate the variations in LC molecule alignment within the cell. The differences in both color and pattern indicate changes in the order parameter of the LC molecules. These results suggest that the LC molecules are readily controlled by the applied voltage, resulting in distinct molecular alignments.

Fig. 6
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The textures of the PSDFCLC cell were observed under various voltages at a low frequency of 60 Hz using a POM

POM analysis at 70 kHz

The variation in textures in a PSDFCLC cell under a high frequency of 70 kHz, as observed by a POM, is shown in Fig. 7. The sample cell was initially subjected to a starting voltage of 0 V. As the voltage increased, changes in the colors and patterns of the textures were observed under a POM. However, at 72 V, a dark pattern was observed, indicating that the polarized light passing through the LC cell was blocked by the second polarizer. When the voltage was decreased to 0 V, the dark pattern persisted. The dark pattern was also maintained when the voltage was reduced to 60 V and 36 V. These results indicate that the alignment of the LC molecules in the LC cell was preserved.

Fig. 7
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The textures of the PSDFCLC cell under POM at various voltages and a high frequency of 70 kHz

As shown in Fig. 5b, the PSDFCLC liquid crystal cell appears nearly completely dark under POM. These results indicate that the CLC molecules remain arranged in a nearly uniform planar structure. The focal conic and planar molecular alignments of the PSDFCLC liquid crystal cell at 60 Hz and 70 kHz are shown in Figs. 6, 7, respectively.

Electro-optical properties of PSDFCLC cells

To highlight these differences, the cells were initially set to a planar state at low frequency and a focal conic state at high frequency. As the voltage at 60 Hz increased, the cells gradually transitioned to a focal conic state, with the transmittance decreasing to a minimum of 10%. Beyond this voltage, the transmittance increased sharply as the cells shifted to a homeotropic state, reaching 91.5%. At high frequency, increasing the voltage resulted in a maximum transmittance of 99.5%, indicating a planar state. The driving voltage of the sample cell was approximately 72 V (4.5 V/μm). The results are summarized in Fig. 8, confirming that the PSDFCLC cell can switch between focal conic and planar states. This demonstrates the bistable nature and stability of the cell in response to electric fields, making it suitable for various applications. Additionally, real images of the ON and OFF states for the PSDFCLC S4 cell are shown in Fig. 9, which reveals greater contrast between the focal-conic state and the planar state.

Fig. 8
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Voltage-dependent transmittance of polymerized dual-frequency CLC S4 cells measured under an electric field

Fig. 9
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a Schematic illustration of reversible bistable switching of the PSDFCLC S4 cell at a driving voltage of 72 V. Real images of the PSDFCLC S4 cell in the (b) focal conic state and c planar state

In this study, no alignment layer was applied on the ITO substrate. In principle, the planar alignment direction of the liquid crystals cannot be fixed under such conditions. However, intermolecular interactions generally occur among liquid crystal molecules, allowing most of them to adopt a nearly uniform alignment state. As shown in Fig. 5b, the PSDFCLC exhibits an almost completely dark state under POM, indicating that the CLC molecules adopt a nearly uniform planar alignment. In addition, the liquid crystal device used in this study does not employ polarizers. Therefore, in the planar state, regardless of the orientation of the cholesteric liquid crystal alignment—even if the planar alignment is multidirectional—the incident light can still fully pass through the liquid crystal cell, resulting in a transparent state. In this paper, to clarify the electro-optical characteristics of the cholesteric liquid crystal cell, the molecular alignment of the cholesteric liquid crystals in the pixel cell is illustrated as a uniformly aligned planar structure, as shown in Figs. 9, S3, S4, S6, and S8.

Effect of frequency on PSDFCLC cells

To identify the optimal operating frequency under an applied electric field, the transmittance of the PSDFCLC S4 cell was measured at various frequencies and driving voltages, as depicted in Fig. S7. As the frequency increased, the transmittance of the S4 cell progressively increased. Above 10 kHz, the transmittance increased rapidly, peaking at 70 kHz with a transmittance of 84.7%. Therefore, 60 Hz (low frequency) and 70 kHz (high frequency) were selected for operation. A schematic illustration of the reversible bistable switching of the PSDFCLC cell at driving frequencies of 60 Hz and 72 kHz and 72 V is shown in Fig. S8.

The operation of the actual fabricated PSDFCLC device is summarized in Fig. 10. The reflective PSDFCLC device is shown in Fig. 10a, b. The back-lit PSDFCLC device is shown in Fig. 10c, d. In Fig. 10d, a color filter and a pellicle were used. This study revealed that the incident light could be reflected, and a backlight display could also be achieved with PSDFCLC cells. Both systems are bistable dual-frequency systems for color image display. Video S1 and Video S2 show the “ON” and “OFF” states for reflective and back-lit devices, respectively. Because the PSDFCLC designed in this study does not employ polarizers, the restriction on the viewing angle is relatively small, resulting in a wider viewing angle range.

Fig. 10
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Panels a and c show images of PSDFCLC S4 cells in the opaque and focal conic states, respectively, and b and d present schematic illustrations of reflective and back-lit devices, respectively

The composition of a color pattern is achieved by generating images using thousands of pixels based on the three primary colors, RGB. The pixel size has evolved from approximately 1 μm in the early stages to approximately 0.6 μm. As shown in Fig. 10, the RGB elements can be regarded as individual pixels. If the RGB pixels are further reduced to sizes less than 1 μm and combined in various RGB configurations, a full-color display can be realized.

Reliability and stability of PSDFCLC cells

After polymerization, the polymer matrices stabilize the PSDFCLC cell, demonstrating high switching reliability. As shown in Fig. 11, the PSDFCLC S4 cell maintains bistable states, switching between opaque focal conic and transparent planar states over 14 days without additional energy input once the state is set. The stability of PSDFCLC cells in (a) opaque focal conic and (b) transparent planar states is shown in Fig. S9. Additionally, it remains bistable after 100 switching cycles. These findings indicate that the fabricated PSDFCLC S4 cell is highly reliable and well-suited for practical applications.

Fig. 11
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Switching reliability of the PSDFCLC cell at a driving voltage of 72 V with a low frequency of 60 Hz and a high frequency of 70 kHz

The response time of the PSDFCLC S4 cells was measured, and the results are displayed in Fig. S10. The estimated response times for switching the S4 cells “ON” and “OFF” were 0.84 ms and 2.8 ms, respectively. The response time of the PSDFCLC S4 cells was slower than it was before polymerization. These findings suggest that although the polymer matrices significantly interact with liquid crystal molecules, increasing bistable switching stability, they may also increase resistance during the reorganization of liquid crystal molecules. Additionally, the polymer network can increase the viscosity of the liquid crystal material, causing the molecules to move more slowly under an electric field, thereby extending the response time. However, the interaction forces can be overcome by externally adjusting the electric power, resulting in a shorter response time.

Conclusion

A single-layer CLC cell equipped with a reflector and a pellicle film was used to fabricate a bistable dual-frequency CLC display that can be used for long-term displays, for example, in e-books and commercial color displays. Compared with conventional devices, this predesigned novel dual-frequency single-layer CLC display has energy-saving properties and a high-brightness display. Both DFCLC and PSDFCLC cells were prepared and characterized. The response times of the PSDFCLC S4 cell were estimated to be 0.84 ms and 2.8 ms for the “ON” and “OFF” operations, respectively. The stability and reliability of the fabricated PSDFCLC cells were confirmed.