1. Introduction
Transparent display is being favored as a new generation of display devices with the features of visualizing images on a panel and allowing the observer to see both sides of the view at the same time [
1]. In addition, their unique features like having a thin profile, low power consumption, light weight, and so on are enticing [
2]. Transparent display in particular has the potential to open up numerous new business dynamics for the ongoing display industry, as well as giving users a wonderful experience of a better quality. Over the last few years, a great deal of research has been conducted on transparent display technology, and a variety of approaches have been developed and put into practice, including plasma display [
3], liquid crystal display (LCD) [
4], electrowetting display [
5], organic light-emitting diodes (OLEDs) [
6], cholesteric liquid crystals (ChLCs) [
7], and polymer-dispersed liquid crystal (PDLC) [
8]. Among these candidates, the latter three are more widely used, yet OLEDs and CHLCs still have some existing drawbacks. In the case of OLEDs, a background of significant ambient light results in reduced visibility due to their self-emitting properties [
9]. As well, large sizes are challenging to fabricate, and their reliability is still not good enough. For the ChLCs, the designed driver circuits generally require an elevated reset voltage and a complicated driver mode in that the three states under the action of an electric field are a planar state, a focal conic state, and a homeotropic state.
Electrically switchable PDLCs are extensively applied in preparation for large-area displays and smart windows equipped with scattering and transparent states in view of their polarization–independence, simplicity of fabrication, and low cost [
10,
11,
12,
13,
14,
15,
16,
17,
18]. PDLCs can be formed using micro- or nano-sized liquid crystal droplets dispersed in a uniform polymer matrix and have emerged as an essential new class of materials for a variety of utilizations in optical devices [
19,
20,
21,
22,
23]. In general, PDLC presents as a milky-white scattering state on account of the refractive index mismatch existing between the polymer matrix and the liquid crystal (LC) droplets. When applying an electric field to it up to a certain strength, the PDLC film changes from the scattered state to the transparent state, a phenomenon that can be ascribed to the rearrangement of the LC molecules along the direction of the external electric field towards matching the refractive index of each phase within the system [
24,
25,
26,
27,
28,
29]. The reversible switching of electro-optical capability provided PDLC films with various possibilities in fields such as multi-color displays [
30], micro-lenses [
31], anti-pee** films [
32], chemical sensors [
33], and organic light-emitting diodes [
34].
There are four methods to achieve the preparation of PDLC films, which are polymerization-induced phase separation (PIPS), solvent-induced phase separation (SIPS), thermally induced phase separation (TIPS), and the microencapsulation process (MP) [
35,
36,
37]. Of these, PIPS is the most commonly used, as it is relatively simple and fast to produce, requiring only a UV curing process and no other tedious steps. Furthermore, it can control the UV-light intensity to modulate the micro-morphology of the sample to realize differentiated electro-optical properties. Typically, PDLC films tend to require low threshold voltages, fast response times associated with on/off state transition, and high levels of contrast ratios for on/off transmittance to realize the demands of safety in the application process [
38,
39,
40]. As for contemporary PDLC devices, drawbacks such as a high driving voltage, low contrast ratio, and inferior mechanical applicability are still restricting their further deployment and must be carefully considered. Extensive research is being conducted to optimize the properties of these PDLC devices, and various approaches such as regulating polymerization conditions, changing the morphology of the microstructure, and do** functional materials have been utilized to improve their performance [
41,
42,
43,
44,
45,
46,
47]. By do** various types of dichroic dyes with different concentrations in the PDLC formulation, Zhao et al. not only realized the adjustment of the morphology, the driving voltage, and the contrast ratio of the PDLC films, but also provided a theoretical basis for obtaining PDLC films with a wider color gamut [
48]. Li et al. utilized the method of do** rare-earth nanoparticle GeO
2, whereby the threshold voltage (V
th) of PDLC decreased by 36.8% and the contrast ratio increased by 53.7%, which significantly enhanced the electro-optical performance of PDLC film [
49]. Ahmad et al. prepared polymer-dispersed liquid crystal (PDLC) films using photo-induced phase separation at a wide range of UV intensities (I = 0.33–1.8 mW/cm
2) and curing times (t = 120–600 s). The results showed that the increase in UV-light intensity accelerated the phase separation and significantly influenced the final morphology of the LC droplets inside the PDLC. Similarly, enhanced phase separation was observed by extending the curing time [
50]. However, with the increasing maturity of the overall whole display technology, there is an urgent demand for the application of more ingenious methods to achieve advanced step-driven displays.
Conventional PDLC devices have been successful in their application as electronic switching screens for privacy management. However, at present, the integral full drive-based transparent display has been difficult to satisfy the demand for advanced displays, and the existing step-driven technology usually involves complicated alignment processes. Herein, we reported on the preparation and analysis of a novel PDLC film with the capability of producing a step-driven display using the partitioned polymerization strategy. First, the fluorescent material 7-Amino-4-methylcoumarin was introduced into the LC system, which was dedicated to the optimization of the driving voltage and contrast ratio. Thereafter inspired by the fact that light intensity enabled differentiation in the driving voltage, the PDLC device was prepared by exposing the different regions of the LC cell to different UV-light intensities, leading to different voltage–transmittance (V–T) responses of the PDLC device for different regions. Thus, by applying an appropriate driving voltage, three different states, total scattering, semi-transparent, and total transparent, can be realized, respectively. Notably, the novel PDLC device also exhibited brilliant UV-shielding and anti-aging properties, which can empower it to be promising in the advanced display field.
3. Mechanism and Applications
The operating principle of the proposed PDLC device is presented in
Figure 14.
Figure 14a displays the approach to realize partitioned polymerisation, where two regions of the PDLC film are polymerised using two different light intensities. The right region was polymerized at low light intensities and the left region was the opposite. In the voltage-off state, due to the mismatch of the refractive indexes between the LC molecules and the polymer matrix, the incident light was scattered by both regions of the as-polymerized PDLC film, as shown in
Figure 14b. The proposed device, in other words, exhibited plenary scattering. When applying a low-voltage V1 between the two ITO electrodes, a homogeneous electric field was applied to the PDLC film. The PDLC film in the region polymerized at high light intensity became transparent, while the PDLC in the region polymerized at low light intensity had a higher V
sat due to its smaller pore size and still remained in the scattering state, resulting in a patterned transparent mode, as shown in
Figure 14c. As the applied voltage continued to increase from V1 to V2, which was greater than the V
sat of PDLC polymerized at low light intensity, all LC molecules in the PDLC films were aligned parallel to the applied electric field, resulting in all the incident light transitioning through without scattering, as well as making the total-transparent state obtained, as presented in
Figure 14d.
In order to improve product reliability, anti-aging tests were conducted on samples B2 and C2 in this section. The aging conditions in this test were 25 °C and 0.18 w/m
2 UV-light intensity. The electro-optical properties of the samples were measured at intervals of 12 h and the experimental results are shown in
Figure 15.
Figure 15a,b show the V–T curves recorded for samples B2 and C2 after ageing at different durations, respectively. Sample B2 differed from sample C2 due to the addition of AMCA to the latter. From the figure, it can be seen that the electro-optical properties of B2 appeared to be slightly deteriorated at 60 h with the extension of the aging time, while the electro-optical properties of sample C2 were basically unchanged. The variations of the V
sat and CR of B2 and C2 are demonstrated under different durations in
Figure 15c,d, respectively. Noticeably, the C2 sample exhibited more minute fluctuations with aging time compared to sample B2 without AMCA, which suggested that the presence of AMCA conferred samples with better environmental tolerance. The reason for this phenomenon was that AMCA has a certain UV-absorption capacity, which can absorb part of the UV light and thus slowed down the aging damage of PDLC samples.
Thermochromic materials play an instrumental role in implementing dynamic variations in color, which refers to a functional material capable of changing its visible-light-absorption spectrum when exposed to hot or cold conditions, with the property that color varies with temperature. The temperature at which a color change occurs was called the thermochromic temperature (T
0). In this section, two thermochromic materials, R and G, with a T
0 of 45 °C, were selected. When the temperature was below 45 °C, R and G are curry color and brown color, respectively. As temperature rose above 45 °C, the former changed to red and the latter to green.
Figure 16 records the coloration behavior of two thermochromic materials using POM at different temperatures.
Figure 16a,c visually presents that when the temperature was at room temperature, 20 °C or below T
0, R and G showed curry and brown, respectively. Then, the former turned to red and the latter to green upon a gradual increase in temperature to 60 °C above T
0.
Figure 17 exhibits the SEM images of the two thermochromic materials, from which it can be seen that the morphology presented a rounded spherical shape, with particle sizes ranging from 1 to 8 micrometers and with a relatively uniform distribution.
Using the above two thermochromic materials as patterned backing substrates, optical display behaviors under different fields were investigated, as shown in
Figure 18. At the zero field, as illustrated in
Figure 18a,d,g, the PDLC device showed a scattering state, and the backside pattern behind it was not shown. When an electric field higher than V
sat was applied, the PDLC device changed from a scattering state to a transparent state, and an image patterned using a thermochromic material behind it appeared. By simultaneously applying a thermal field higher than T
0 in the presence of the electric field described above, the backside pattern underwent electrochromic variation and rich pattern variation was realized.
Figure 19 demonstrates the fluorescence display of PDLC films containing different levels of AMCA under UV irradiation, and the fluorescence brightness became stronger and stronger with the increase in AMCA content, which was in correspondence with the experimental results mentioned above.
Figure 5 demonstrates that AMCA has a distinguishable UV-absorption peak, thus the PDLC films containing AMCA can be used to absorb part of the incident UV light by using the absorption of AMCA during UV-light irradiation, providing UV-shielding properties to PDLC films. To verify this conjecture, the UV-control shielding experiments were carried out using a UV lamp with a UV-intensity measurement as shown in
Figure 20. The incident UV-light intensity was kept constant in the experiment by kee** the distance from the UV-emitting-light source to the UV sensor unchanged. When no sample was placed on the UV sensor as a control in
Figure 20a, the UV sensor measured the UV intensity emitted from the light source to be 5.06 mW/cm
2. When sample B2 without AMCA was placed on the UV sensor as shown in
Figure 20b, the UV intensity emitted from the light source measured using the UV sensor was 3.15 mW/cm
2. And, when sample C2 containing AMCA was placed on the UV sensor as shown in
Figure 20c, the UV intensity was 1.07 mW/cm
2. The UV-shielding efficiencies of samples B2 and C2 were 37.75% and 78.85%. The only difference between samples B2 and C2 was that the latter contained AMCA, and the results showed that the addition of AMCA increased the UV-shielding efficiency of the PDLC films by 41.1%. This result is attributed to the fact that the AMCA in the PDLC film reduced the amount of light transmitted through the sample by absorbing some incident UV light, thus achieving a certain degree of shielding from external UV light. It also better explained that the AMCA-containing sample C2 showed better anti-aging performance under the above UV-aging-test condition.
Figure 21 demonstrates a partitioned electrically controlled display utilizing PDLC films. As the voltage was gradually increased, different regions of the PDLC film were driven successively, and the subsequent images were revealed. These applications exemplified the potential of this new PDLC film for advanced displays.
5. Conclusions
In conclusion, a novel step-driven PDLC transparent display with fluorescent capacity using partitioned polymerization was proposed in this paper. Acting as an excellent fluorescent material, AMCA provided PDLC with outstanding electro-optical behavior while reducing Vsat by 14.2 V and increasing CR by 38.2, realizing a 35.8% decrease in Vsat and 65.4% improvement in CR. Meanwhile, it was found that the change of polymer pore size caused by varying components and experimental conditions had a remarkable influence on the electro-optical performance of PDLC film. Thereafter, inspired by the fact that light intensity enabled differentiation in the driving voltage, by exposing the different regions of the LC cell to different UV-light intensities, significant differentiation of the microscopic pore sizes in the corresponding regions of the PDLC was achieved, thus enabling modulation in the three states of total scattering, semi-transparent, and total transparent at appropriate voltages. In addition, the underlay of the transparent display was mapped using thermochromic materials, enabling the PDLC to demonstrate variable patterns in the presence of multiple external field superimpositions. Notably, the presence of AMCA endowed PDLC film with outstanding UV shielding of up to 78.8% and brilliant aging stability in the 60 h test. Therefore, this study provided a methodological reference to break the current single-PDLC driving model.