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This paper reports an electro-opto-thermal addressing bistable and re-addressable display device based on gelator-doped liquid crystals (LCs) in a poly(N-vinylcarbazole) film-coated LC cell. The bistability and re-addressability of the devices were achieved through the formation of a rubbery LC/gel mixture at room temperature. The desired patterns were addressed, erased, and re-addressed by controlling the temperature, applied voltage, and UV light illumination. Moreover, grayscales were obtained by adjusting UV light intensity. The initiation, relaxation, rise, and fall times of photoconductive poly(N-vinylcarbazole) via UV light illumination of various intensities were also examined.
© 2016 Optical Society of America
1. Introduction
Bistable liquid crystal display (LCD) technologies have attracted considerable interest in the past decade because of their capacity to save energy. These displays can retain displayed images even when the applied field is switched off. However, bistable displays consume power only when the displayed image content requires alteration. Several types of potential bistable/tri-stable/multi-stable LCDs have been developed. These LCDs include bistable cholesteric LCs (reflective planar and scattering focal conic textures) [1,2], bistable salt-doped cholesteric LCs [3], polymer-stabilized cholesteric textures [4], bistable twisted nematic LCs [5–7], bistable smectic A LCs [8,9], surfactant-additive polymer-dispersed LCs [10], surface-stabilized ferroelectric LCs, zenithal bistable displays [11], bistable surface nematic LCDs [12], supramolecular LC gel [13], and dynamic fingerprint chiral textures [14]. The operating principles and bistable mechanisms of the above-mentioned LCDs include the scattering, reflective/transmissive, and phase retardation modes. Interest in the development of approaches for optically addressed LC devices, such as displays [15–17], gratings [18], lenses [19], and apertures, has grown for several years. These LC applications have also been expanded to erasable, re-addressable, and multi-stable modes. This paper reports an electro–opto–thermal (EOT) addressing method to obtain bistable and re-addressable display devices by using a photoconductive material and thermo-reactive gelator. The proposed LCD devices include two bistable modes. The first mode is transmissive mode with homeotropic (dark) and twisted nematic (bright) structures. The second mode is scattering mode with homeotropic (transparency) and multi-domain (scattering) LC structures. The former requires two polarizers, and the latter is polarizer-free.
Photoconductive materials usually possess the properties of electrical insulators and become electrical conductors when irradiated with light. The electric conductivity of photoconductive materials can be activated by illuminated light. The absorption of electromagnetic irradiation, including visible light, ultraviolet (UV) light, infrared light, and gamma radiation, can be transferred to generate carriers with high quantum efficiency [20–22]. Most polymers possess extremely low charge generation efficiency and carrier mobility in the absence of an electromagnetic field (light irradiation). Poly(n-vinylcarbazole) (PVK) is a photoconductive polymer that functions as an effective DC insulator in the dark and visible spectrum range. PVK can sustain a DC electric field within a certain limit. The reason can be understood that the effective electric field can be reduced by the built-in field resulting from ion separation induced by the applied DC voltage. The efficiency of an electric insulator decreases as the frequency of the applied field increases. Moreover, PVK is an effective conductive polymer for the external application of DC voltage when irradiated with UV light [22]. Regarding the alignment ability of coated PVK layers on substrates, the coated PVK layer can be mechanically rubbed to induce the planar alignment of LCs with their easy axes perpendicular to the rubbing direction [23–25]. The key to aligning LCs perpendicular to the rubbing direction is the anchoring induced by the phenyl rings in the side chain fragments of PVK. The planar alignment direction of a rubbed PVK film switches toward the rubbing direction via thermal treatment within a specific temperature range. The temperature range is dependent on the LC materials. The alignment direction will be rotated to that parallel to the rubbing direction when the temperature is close to the clearing temperature of the used LCs [23,24]. Therefore, PVK served as a photoconductive polymer and an alignment layer in the present study.
Another key material in this study is the gelator, with thermo-reversible linked hydrogen bonds that connect the gelator molecules. Gelator-doped LCs present a rubbery gel at room temperature because of the hydrogen bonds, and the LCs can be aligned by reorienting the gelators. The intermolecular hydrogen bonds of the gelator can be reduced through thermal treatment. The connected gelator molecules can be separated and homogeneously dispersed in the LCs at a temperature higher than that for breaking the hydrogen bonds (TBR-H) in the selected LCs. Notably, if the clearing temperature (TC) of the selected LCs is higher than TBR-H, the LC directors retain their original direction. Subsequently, the hydrogen bonds are gradually reconnected along the direction of the LCs as the temperature of the gelator-doped LCs decreases. The orientation of the LCs can be stabilized with the application of an electric field for aligning the LCs along a specific direction at a temperature between TC and TBR-H during cooling. When the temperature is below TBR-H, the hydrogen bonds will reconnect to reform gelators. Stabilization to LCs is along the direction consistent to that induced by the applied field because of the reconnected gelators once the applied external field is turned off [26–28].
In this paper, we report an EOT addressing bistable and re-addressable display device based on gelator-doped LCs in a PVK film-coated LC cell. The PVK material served as the photoconductive layer, and the thermo-reversible gelator functioned as the stabilizer for LCs. Thus, the employed LCs (Δε > 0) in the region, illuminated with UV light, were stabilized along the direction parallel to the applied field to achieve two stable LC alignment modes. If the initial alignment of the LC was twisted nematics (TN), then the LCD device under cross-polarizer displayed bright (TN) and dark (homeotropic alignment, VA) states. Additionally, if the initial alignment of the LC was disordered, the LCD device presented scattering (multi-domain LCs) and bright (homeotropic alignment, VA) states. Therefore, the desired patterns could be addressed by the EOT addressing approach. The addressed patterns for these two modes could also be thermally erased and have another pattern readdressed onto it through the EOT addressing method. Furthermore, grayscales were obtained by adjusting UV light intensity.
2. Experiment
2.1 Preparation of materials and fabrication of LC cells
The nematic LCs, HFW59200-200 (Fusol-material), with a positive dielectric anisotropic constant (Δε > 0), and gelator, 12-Hydroxystearic Acid (Alfa Aesar), were used in this work. The ordinary and extraordinary refractive indices of the LCs were no = 1.489 and ne = 1.597, respectively, at 20°C and λ = 589 nm. The clear temperature (TC) of the nematic LCs was 113°C. The mixing weight ratio (LC: gelator) of these two materials was 98:2. TBR-H in this system was measured at approximately 90°C, which was lower than TC of the adopted nematic LCs. Moreover, the employed photoconductive material was PVK. Chlorobenzene as solvent and PVK powder were mixed homogeneously in a weight ratio of 98.36:1.64 to coat a PVK film onto an ITO-coated glass substrate. The solution was spin-coated onto an ITO glass slide. After spin coating, the glasses were pre-baked in the oven at 80°C for 20 min and then post-baked at 120°C for 120 min. Two substrates coated with PVK films were rubbed in two perpendicular directions to form a 90° TN cell, separated by 7 μm ball spacers. Finally, one empty cell was homogeneously filled with the prepared LC/gelator mixture at isotropic state and then cooled to form a TN mode gelator-doped LC cell. As described above, the alignment direction provided by the rubbed PVK layer was perpendicular to the rubbing direction. Under thermal treatment within a specific temperature range, the alignment direction was switched toward the rubbing direction. Thus, both of the alignment directions of the rubbed PVK layers were switched toward the rubbing direction upon injecting the LC mixture at isotropic state. Finally, the initial 90° TN cell with dual rotation of the alignment directions from PVK layers through thermal treatment became another 90° TN cell.
2.2 Experimental setups
Figures 1(a) and 1(b) present the experimental setups for measuring transmission versus applied DC voltage (T-V) curves and for addressing the patterns onto gelator-doped 90° TN LCs in a PVK film-coated LC cell, respectively. The LC cell was placed in a temperature controller to maintain its temperature, and the wavelength of the employed intensity-controlled unpolarized UV light was 365 nm. The abbreviations P, A, and D represent the polarizer, analyzer, and photodetector, respectively. In Fig. 1(a), a red probed laser beam derived from a He–Ne laser (λ = 632.8 nm) was normally incident onto the LC cell, and the transmitted light was collected by a photodetector behind the LC cell. The T-V curves of the LC cell, located between cross-polarizers, were measured during illumination at various UV light intensities (0, 1.0, 3.0, and 5.0 mW/cm2). In Fig. 1(b), one of the two homemade photomasks with transparent letters “TY” and “PVK” was placed in contact with a temperature-controlled LC cell. The patterns could be addressed and re-addressed onto the LC cell by controlling the temperature, DC voltage applied on the LC cell, and UV illumination through the photomask.
Fig. 1 Experimental setups for (a) measuring transmission versus applied DC voltage curves and (b) addressing patterns onto the LC cell.
3. Results and discussion
3.1 Effect of photo-conductive material-PVK
Figure 2 shows the transmission versus applied DC voltage (T-V) curve of the fabricated gelator-doped 90° TN LCs in a PVK film-coated LC cell under cross-polarizers with/without UV illumination (λ = 365 nm) at TBR-H (~90°C). The transmissive axes of these two polarizers were set parallel and perpendicular to one of the rubbing directions. This condition indicated that the TN LC cell was in normally white mode. The transmission decreased as the amplitude of the applied voltage increased. However, the conductivity of the PVK film could be increased through UV light exposure because PVK is a photoconductive polymer. Curve (a) in Fig. 2 depicts the T-V curve of the LC cell without UV illumination at TBR-H. The operating voltage to switch the LC cell from bright to dark state was measured at approximately 50 V, which was extremely high. Curves (b)−(d) in Fig. 2 present the T-V curves of the LC cell, simultaneously illuminated with UV light having intensities of 1.0, 3.0, and 5.0 mW/cm2, respectively. The operating voltages for curves (b)−(d) were significantly reduced to approximately 25, 22, and 20 V, respectively. This phenomenon indicated that a high UV light intensity (within the limit) corresponded to a low operating voltage. Regarding the mechanism of PVK [20–22], the impurity ions of LCs aggregated close to the electrodes because of the applied external DC voltage. Hence, an induced electric field with direction opposite to the applied DC voltage from the impurity ions was constructed to reduce the effective electric field. If the applied DC voltage was sufficiently high to eliminate the induced electric field, the LCs became aligned along the applied DC voltage by the effective electric field. Moreover, the induced ion transportation or the so-called mobility from PVK reduced the impurity ions from LCs under UV light illumination. This finding indicated that the induced electric field was reduced. Finally, the effective electric field increased. In other words, the insulated PVK could be switched to the conductive one via UV illumination. Based on the experimental results, UV light of 5.0 mW/cm2 and the application of DC (25 V) were selected to demonstrate the EOT addressing method.
Fig. 2 Variations in transmission of the LC cell under cross-polarizers as a function of the applied DC voltage when the sample is exposed to UV light of various intensities at TBR-H (~90°C): (a) without UV illumination and (b)−(d) with UV illumination (λ = 365 nm) at intensities of 1.0, 3.0, and 5.0 mW/cm2, respectively.
Figure 3 plots the dynamic transmission (switching response) of gelator-doped 90° TN LCs in a PVK film-coated LC cell under DC voltage (25 V) and UV light illumination. The experimental setup is shown in Fig. 1(a). The selected intensities of UV light were 1.0, 3.0, and 5.0 mW/cm2. Figures 3(a)–3(c) and Figs. 3(d)–3(f) depict the dynamics as the UV light at various intensities was turned on and off, respectively. Notably, the DC voltage of 25 V was continuously applied onto the LC cell during measurements. The initiation time (τini) was defined as the duration from the time we turned on the UV light to the time the transmission reduced to 95% of the maximum transmission. It showed the required time for switching the function of photoconductive PVK films from insulated to conductive polymer. Experimentally, the initiation times by irradiating with 1.0, 3.0, and 5.0 mW/cm2 of UV light were approximately 30.8, 22.66, and 17.32 ms, respectively. These findings indicated that the initiation time of the UV illumination-induced enhancement of PVK conductivity decreased with increasing UV light intensity. Moreover, the generation rate of the induced ion transportation from PVK increased with rising UV light intensity. In other words, a high UV light intensity corresponded to a high ion mobility rate. Considering the relationship between the LC response and amplitude of the applied voltage, the effective DC voltage with 5.0 mW/cm2 UV illumination was higher than those with 1.0 and 3.0 mW/cm2 UV illumination. According to [22], UV illumination-induced ion flow or the so-called electric current increases with the UV light intensity. The initiation time can be shortened by increasing the UV light intensity. Meanwhile, the relaxation time (τrel) was defined as the duration from the time we turned off the UV light to the time transmission increased to 5% of the maximum transmission. Experimentally, the relaxation times for the cases by irradiating with 1.0, 3.0, and 5.0 mW/cm2 of UV light were approximately 1400, 283.2, and 244.1 ms, respectively. The relaxation time represented the duration in which the PVK maintained its conductivity after the UV light was switched off. The relaxation time decreased with the UV light intensity, because the ion mobility rate resulting from a high UV light intensity was higher than that from a low UV intensity. In other words, the high ion mobility rate, which was caused by high UV intensity illumination, resulted in the high transportation rate of ions in PVK films. The ions induced by high UV illumination could be transported quickly so that the duration of keeping the UV illumination-induced photoconductor for the case of 5 mW/cm2 UV light was shorter than that of 3 mW/cm2 UV light. Hence, the relaxation time decreased with the increase in UV light intensity. Notably, the widths of orange stripes in Figs. 3(a)–3(c) and Figs. 3(d)–3(f) represent the initiation times (τini) and relaxation times (τrel) of the coated PVK layers, respectively.
Fig. 3 Dynamic transmission (response) of the LC cell applied with a DC voltage of 25 V, and turning on (a) [(b) and (c)] and off (d) [(e) and (f)] the simultaneous illumination of UV light at an intensity of 1.0 [3.5 and 5.0] mW/cm2. The widths of orange and green stripes represent the initiation (τini) and relaxation (τrel) times, as well as the rise (τrise) and fall (τfall) times, respectively.
In addition, the response times, including the rise (τrise) and fall (τfall) times, for these three cases were also obtained and discussed. The rise and fall times were defined as the periods required to change the LC cell transmission from 90% to 10% (turning on UV light) and from 10% to 90% (turning off UV light) of its maximum transmission, respectively. Thus, the rise time represented the required time to optically tune the PVK films from insulators to conductors, and the fall time showed the required time to spontaneously (naturally) switch the function of the PVK films from conductors back to insulators. The measured rise (fall) times for the cases with UV light intensities of 1.0, 3.0, and 5.0 mW/cm2 and DC voltage of 25 V were 204 (8823), 56.5 (635), and 29.7 (538) ms, respectively. The rates of UV-induced ion formation and ion transportation were proportional to the UV light intensity because of various ion mobility rates. Notably, the applied DC voltage of 25 V was higher than the threshold voltage without UV illumination, indicating that the LCs had begun to be rotated (Fig. 2). When the UV light was turned off, the LCs naturally rotated back to the orientation consistent with that under DC voltage of 25 V with the spontaneous decay of PVK conductivity. The fall time eventually decreased when the UV intensity increased, because the ion mobility rate initiated by the high UV intensity caused the high transportation rate of ions in PVK films. The widths of green stripes in Figs. 3(a)–3(c) and Figs. 3(d)–3(f) represent τrise and τfall of the transmission, respectively. According to the experimental results, such an EOT addressing method could be completed within the order of sub-seconds. Regarding the lowest transmission for the cases with various UV light intensities, the transmission was dependent on the amplitude of the applied electric field. At an external voltage of 25 V, the transmissions for the cases with 1.0, 3.0, and 5.0 mW/cm2 UV light were 5.52%, 3.71%, and 1.61%, respectively (Fig. 3). Moreover, with the application of higher external voltage, the lowest transmission for these cases could be obtained and the values were almost the same due to the same reorientation of LCs.
3.2 Electro-opto-thermal addressing method
Figure 4 presents a schematic of the operation principle of the EOT addressing method for obtaining bistable and re-addressable display devices by controlling the temperature, applied DC voltage, and UV illumination. Figure 4(a) shows a schematic of an initially fresh TN cell at room temperature. Subsequently, the TN cell was heated to TBR-H (~90°C) to obtain the structure shown in Fig. 4(b). The strength of the intermolecular hydrogen bonds of the doped gelators decreased so that the disconnected gelator molecules could disperse in the LC host homogeneously. Meanwhile, the orientation of LCs remained in the initial alignment direction, TN, because TC of the LCs was higher than TBR-H. The temperature of the LC cell was maintained at TBR-H when the cell was illuminated with UV (λ = 365 nm) light through a photomask with the desired patterns.
Fig. 4 Schematic of the EOT mechanism addressing of (a) initial fresh TN cell at Troom; (b) heated TN cell at TBR-H (~90°C); (c) heated LC cell with the application of DC voltage and simultaneous illumination of UV light through the photo-mask at TBR-H; (d) cooling the temperature of the heated LC cell to Troom; and (e) LC cell with addressed patterns at Troom.
During UV illumination, an external DC voltage was applied onto the LC cell. The UV-illuminated region of the PVK film became a conductive film because of UV light illumination. The LCs (Δε > 0) in the UV-exposed region were reoriented toward the direction of the applied DC field, whereas those in the other regions remained as TN structures [Fig. 4(c)]. The LC cell was then cooled to room temperature. Both UV illumination and DC voltage were continuously applied during cooling. The disconnected hydrogen bonds reconnected spontaneously during cooling, thereby allowing the gelators to reassemble along the direction of the LC alignment [Fig. 4(d)]. After cooling, the UV light and external field were turned off, and the alignments of LCs remained unchanged because of the stabilization caused by the reformed gelator molecules [Fig. 4(e)]. Eventually, stable binary LC alignments, including TN and VA, were obtained at room temperature. The TN and VA under cross-polarizers presented bright and dark states, respectively. The addressed patterns could be erased thermally, and the other desired patterns could be readdressed using the EOT addressing method.
3.3 Bistable and rewritable LC display devices
Figure 5 demonstrates the bistable and re-addressable LCD device based on gelator-doped LCs in a PVK film-coated LC cell by using the EOT addressing method in accordance with the experimental setup plotted in Fig. 1(b). Figure 5(a) illustrates the observed LC cell treated with the EOT addressing method under cross-polarizers at room temperature (~25°C). In brief, the LC cell was heated to approximately 90°C to break up the connected hydrogen bonds. This process facilitated the homogenous dispersal of the gelators in the LCs [Fig. 4(b)]. UV light intensity of 5.0 mW/cm2 was employed to expose the LC cell through a photomask with a transparent pattern “TY.” An external electric DC field of 25 V was then applied onto the LC cell. In accordance with the mechanism described above, the LCs (Δε > 0) in the region “TY” illuminated with UV light were aligned to the direction parallel to that of the applied external field. By contrast, the other regions maintained TN structures. The gelator-doped LC cell with an applied DC voltage of 25 V at 90°C was then cooled to room temperature. After cooling, the external field and UV light were switched off. The LC cell with the addressed pattern “TY” is shown in Fig. 5(a). Thus, the LCs with both TN and VA structures were permanently stabilized by the reformed gelators [Fig. 4(e)]. The TN and VA structures presented bright and dark states, respectively, as observed under cross-polarizers. The “TY” pattern was displayed in the transparent background. The contrast ratio of the display device was evaluated to be higher than 50. Subsequently, the addressed pattern “TY” was erased via thermal treatment. The LC cell was heated to >90°C, and the hydrogen bonds were broken again to achieve the homogenous dissolution of the gelators into the LCs. After the LC cell cooled to room temperature, the addressed “TY” patterns with VA structures were erased by thermal treatment. Finally, the orientation of the LCs could be switched to the initial transparent state, in which the LCs exhibited TN structures, consistent with the state plotted in Fig. 4(a) [26–28]. Another pattern, “PVK”, was rewritten onto the thermally treated LC cell by using the EOT addressing method. Figure 5(b) presents the image, photographed by a digital camera, of the re-addressed pattern onto the LC cell at room temperature observed under cross-polarizers. After repeating the EOT addressing method for more than 20 times, the performance (e.g., the operating voltage, UV light intensity, and transmission) of the device exhibited no significant aging effect. Such an advantage resulted from the stable properties of the disconnecting and connecting hydrogen bonds of the doped gelator.
Fig. 5 Images of the demonstrated EOT patternable, erasable, and rewritable display device observed under cross-polarizers at Troom photographed using a digital camera. (a) Addressed pattern “TY” via the EOT addressing method; (b) re-addressed pattern “PVK;” (c) and (d) edge portions of the red circles in Figs. 5(a) and 5(b) observed under a crossed-polarizer polarized optical microscope. VA and TN represent vertical alignment and twisted nematics, respectively.
Moreover, the edge portions of the addressed patterns “TY” and “PVK” at stable states were observed under cross-polarizer polarized optical microscopy (POM) at room temperature to examine the LC alignment of the patterns [Figs. 5(c) and 5(d)]. The edges of the addressed and re-addressed letters were extremely sharp. In brief, the bright edges of the addressed patterns were caused by phase retardation of the LC alignment because of the continuous elastic distribution of LCs. The LC orientation around the edges continuously distributed from TN to VA so that the transmissions were different. The transmission of light through an LCD device at phase retardation (δ = 2πdΔn/λ, where d, Δn and λ are cell gap, birefringence, and wavelength of light) mode under cross-polarizer is proportional to sin2(δ/2) [22]. The different color was observed because phase retardation is a function of wavelength, λ. On the one hand, the LCs in the UV-unexposed regions retained their TN structures and exhibited a bright state under cross-polarizer POM. On the other hand, the LCs in the UV-exposed regions with VA structures presented a dark state under cross-polarizer POM. LC alignments TN and VA were clearly stabilized by the reformed gelators. The UV-exposed regions were verified again.
Stable grayscales could be demonstrated by using the EOT addressing method with UV light illumination and/or different DC fields. The dissolved gelator molecules were adopted to stabilize the LCs aligned by the applied DC field. Figure 6 shows the stably demonstrated grayscales by applying a DC voltage of 15 V and UV light illumination at intensities of 1.0, 3.0, and 5.0 mW/cm2 during EOT addressing. In the present study, the transmission of the LC cell with an applied DC field of 25 V and UV illumination intensities of 1.0, 3.0, and 5.0 mW/cm2 were 5.52%, 3.71%, and 1.61%, respectively (Fig. 3). Moreover, with the higher and higher applied voltage, almost the same transmission for these cases were obtained due to the same reorientation of LCs. In this study, the application of DC 15 V but not 25 V was selected to demonstrate the grayscales. The corresponding transmissions of these obtained grayscales under cross-polarizer were measured at 57.1%, 11.9%, and 6.7%, as shown in Figs. 6(a) and 6(c), respectively. The cause of the obvious defects in Figs. 6(b) and 6(c) was the non-uniform UV light source. Notably, the transmission of the LCD device was independent of the UV illumination time, if the illumination time was longer than the switching time of the photoconductive polymer layer PVK as it switched from insulated to conductive polymer. Experimentally, the LCD device illuminated with different UV light intensities could not reach the same transmission even if the illumination time was long enough. As shown in Fig. 2, the effective electric field modified by UV illumination was dependent on the UV light intensity.
Fig. 6 Photographs of the LC display device with stably demonstrated grayscales addressed through the application of a DC voltage of 15 V and the simultaneous illumination of UV light with various intensities of (a) 1.0 (transmission of approximately 57.1%), (b) 3.0 (transmission of approximately 11.9%), and (c) 5.0 (transmission of approximately 6.7%) mW/cm2.
In addition to the bistable transmissive mode [requires two polarizers with homeotropic (dark) and twisted nematic (bright) structures], the bistable scattering mode (polarizer-free) with homeotropic (transparency) and multi-domain (scattering) LC structures was also demonstrated in this study. For this experiment, two substrates coated with PVK films without rubbing treatment were assembled to form a randomly aligned LC cell separated by 20 μm ball spacers. The mixing weight ratio (LC: gelator) for the adopted mixture of LCs and HSA was 97:3. The mixture was homogeneously filled into the LC cell at the isotropic state and then cooled to room temperature. The filled LC cell presented a multi-domain scattering-mode gelator-doped LC cell. A relatively high HSA concentration was selected to enhance the scattering performance. Figure 7(a) shows the fresh cell with multi-domain LC scattering mode. The EOT addressing method was then adopted to address the pattern by applying a DC voltage of 30 V, illuminating with UV light of 5 mW/cm2 at 90°C, and cooling to room temperature. Figure 7(b) shows a stable image of the addressed pattern “HI” onto the LC cell at room temperature. The bistable LC structures were homeotropic in the regions with UV illumination (transparent foreground) and multi-domain in the regions without UV illumination (scattering background). Figure 7(c) presents the edge portion of the addressed letter “I” observed under cross-polarizer POM. Evidently, the homeotropic and multi-domain structures appeared dark and bright, respectively, under cross-polarizer POM. The addressed pattern could also be erased by thermal treatment, and another pattern could be re-addressed by the EOT addressing method (data not shown).
Fig. 7 Images of the (a) fresh LC cell and (b) addressed pattern “HI” onto the bistable scattering mode gelator-doped LC cell. (c) Edge of the red circle in Fig. 7(b) observed under cross-polarizer POM.
4. Conclusions
This paper reports EOT addressing bistable and re-addressable display devices based on gelator-doped LCs in a PVK film-coated LC cell. The desired patterns were addressed onto the gelator-doped LC cell by controlling the temperature, applied DC voltage, and UV irradiation. The addressed patterns were also thermally erased, and another pattern could be electro-opto-thermally re-addressed onto the LC cell. Moreover, grayscales were obtained by adjusting UV light intensity. The times of initiation, relaxation, rise, and fall of the photoconductive PVK through UV light illumination at various intensities were examined. Studies on the induced electric currents and their electro-optical properties through the PVK films caused by UV illumination at various intensities are currently underway. The proposed approach can be applied to display devices (e.g., e-book and e-paper) that do not require real-time information update and consume power when the displayed image content requires alteration. Moreover, although the operating mechanism is somewhat complicated, such an EOT addressing method is successfully developed to achieve optical addressable, bistable, and rewritable LC display based on electrically controllable birefringence mode. Also, the display devices based on EOT addressing method do not need to be pixelated because of properties of photoconductive PVK. The gelators provide stabilization ability to LCs so that such display devices can be expectedly extended to flexible electronics and roll-to-roll printing.
Funding
Ministry of Science and Technology (MOST) of Taiwan for financially supporting this research under Grant No. MOST 103-2112-M-008-018-MY3.
Acknowledgments
We are also grateful to Prof. Jy-Shan Hsu at Chung Yuan Christian University in Taiwan for allowing full equipment usage. Moreover, we sincerely thank the reviewers for their valuable comments and suggestions.
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