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Abstract
Quantum dots (QDs) have been used to make backlight, which provides a superior color gamut, for liquid crystal flat panel displays. In the backlight system, quantum dots, embedded in a polymer film and illuminated by blue light, emit red and green light with narrow bandwidths. There is, however, a problem with the system in that the quantum dots emit light in all directions, and most of the emitted light is in directions with large incident angles and cannot exit the film due to the total internal reflection at the film-air interface and is wasted. We propose to use an aligned polymer dispersed liquid crystal (APDLC) film to reduce the total internal reflection in the QD backlight and thus improve the light efficiency. A regular PDLC film, where the embedded liquid crystal droplets are randomly oriented, exhibits isotropic scattering and is not a good candidate for the enhancement of light efficiency of QD backlight. Through a two-step polymerization, we successfully developed an aligned polymer dispersed liquid crystal (APDLC) film where the liquid crystal droplets are permanently unidirectionally aligned in the film’s normal direction. It exhibits selective scattering: it scatters light with large incident angles but not light with small incident angles. When the APDLC film is laminated on the QD backlight film, a significant enhancement of the light efficiency of the QD backlight is achieved. The APDLC film can also be used to increase the light efficiency of other flat panel displays, such as organic light emitting diode (OLED) display and micro-light emitting diode (MLED) display.
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1. Introduction
Liquid crystal displays (LCDs) are the leading flat panel display technology because of their merits of high resolution, lightweight, low manufacturing cost, and long service life [1–4]. The liquid crystal layer in the displays does not emit light. A light source, such as backlight or edge light, provides light. The liquid crystal, controlled by applied voltages, modulates the outgoing light intensity with the help of polarizers. Nowadays, white or color light emitting diodes (LEDs) are usually used as the backlight (or edge light). Colored images are displayed with the help of red, green, and blue color filters. There are two major problems with the color filters. First, color filters absorb light, resulting in a low light efficiency. Second, their transmission bandwidths are not narrow, resulting in a small color gamut.
Quantum dots (QDs) backlight is an emerging technology which can greatly improve the color performance of LCDs [5–8]. QDs are nano-sized semiconductor crystals that emit colored light (red, green, and blue) under UV (or blue) light irradiation [9–11]. The emitted colored lights have narrow bandwidths. When a QD backlight is used to illuminate LCDs, it can greatly improve the color gamut of the displays. In a QD backlight, QDs are dispersed in a polymer film. When the QDs are illuminated by UV (or blue light), they absorb the short wavelength light and emit longer wavelength light in all directions through photoluminescence. Some light is emitted in directions with small incident angles [with respect to the normal of the film as shown in Fig. 1(a)]; it is refracted at the polymer-air interface and comes out of the film. Some light is emitted in directions with large incident angles, and it is totally internally reflected back at the polymer-air interface and then waveguided through the film. It is either absorbed when waveguided through the film or comes out at the edge of the film, and therefore it is wasted.
Fig. 1. Schematic diagram of quantum dot backlight. (a) without aligned PDLC film. (b) with aligned PDLC film.
We report an aligned polymer dispersed liquid crystal (APDLC) film which exhibits selective scattering. It scatters emitted light with large incident angles but not emitted light with small incident angles. When the film is laminated on the top of the QD film, it reduces the light loss due to the total internal reflection and thus significantly improves the light efficiency of the backlight.
2. Design and working principle
The structure of a QD backlight is schematically shown in Fig. 1(a). A blue LED is installed on the edge of the system. The light produced by the LED is coupled to a light waveguide plate and is waveguided through the plate. The plate has scattering centers (dispersed particles or punched holes). When the incident light encounters a scattering center, it is scattered and enters the QD film. When the entered light is absorbed by a QD, an electron is excited from the valence band to the conduction band, producing a pair of electron and hole. When the electron and hole recombine, light is emitted. Due to the confinement of the electron in the QD, the emitted light has a very pure color (with a wavelength bandwidth of 10 nm) [12]. Thus, when the QD backlight is used to illuminate LCDs, a large color gamut is achieved.
The QDs emit light in all directions. The direction of an emitted light can be specified by the incident angle $\theta$, which is defined with respect to the normal of the QD film, as shown in Fig. 1(a). Not all the emitted light rays can exit the QD film, depending on their incident angles. If the incident angle is small, the emitted light refracts at the film-air interface and comes out of the side toward the functional liquid crystal layer (which displays images and is not shown in the figure). If the incident angle is larger than a critical angle ${\theta _c}$, it is totally internally reflected back. Then it is waveguided through the film and does not come out of the side toward the functional liquid crystal layer and thus is not utilized. The refractive index of the polymer in the QD film is usually about 1.5. The refractive index of air is about 1.0. Therefore, the critical angle equals ${\theta _c} = \arcsin (1.0/1.5) = {42^o}$.
We propose an aligned polymer dispersed liquid crystal (APDLC) film to enhance the light efficiency of the QD backlight. The structure of the APDLC film is schematically shown in Fig. 1(b). It consists of a polymer film with dispersed liquid crystal droplets. The liquid crystal, consisting of elongated rod-like molecules, in the droplets is uniformly aligned in the normal direction of the film. The liquid crystal has an anisotropic optical property that it exhibits an ordinary refractive index ${n_o}$ for light polarized perpendicular to the long axis of the molecule and an extraordinary refractive index ${n_e}$ for light polarized parallel to the long molecular axis. The polymer is optically isotropic and has a refractive index ${n_p}$, which is chosen to equal ${n_o}$. The APDLC film is laminated on top of the QD film with the help of an optical adhesive. The refractive indices of the polymers in the QD film and APDLC film as well as that of the adhesive are about the same. There is no refraction and reflection at the interface between them. For the emitted light with zero or small incident angles, when it propagates through the APDLC film, it encounters the same refractive index in both the polymer and liquid crystal, and thus is not scattered. Its incident angle with respect to the film’s normal does not change, and it comes out of the side toward the functional liquid crystal layer. For the emitted light with large incident angles, when it propagates through the APDLC film, it encounters different refractive indices in the polymer and liquid crystal, respectively, and thus is scattered. When it is scattered into directions with a small incident angle, it comes out of the side toward the functional liquid crystal layer. Therefore, the APDLC film can enhance the light efficiency of the QD backlight.
3. Aligned polymer dispersed liquid crystal film
Polymer dispersed liquid crystals (PDLCs) are composites consisting of isotropic polymer and liquid crystals, and have been used to make switchable windows for privacy control, flat panel displays, and projection displays [13–19]. In a regular PDLC film, a liquid crystal forms micron-sized droplets, which are dispersed in an isotropic polymer, as shown in Fig. 2(a). The refractive index ${n_p}$ of the polymer is matched to the ordinary refractive index ${n_o}$ of the liquid crystal but smaller than the extraordinary refractive index ${n_e}$ of the liquid crystal. Inside a droplet, the liquid crystal molecules are more or less aligned along a common direction known as the droplet direction. When no external electric field is applied, the droplet direction is, however, random throughout the film. For an incident light, independent of its propagation direction, it encounters different refractive indices in the polymer and liquid crystal droplet and thus is scattered. When a sufficiently high voltage is applied across the film, the droplets are aligned along with the film's normal direction, as shown in Fig. 2(b). Now for a normally (in the film’s normal direction) incident light, it encounters the same refractive index ${n_p}( = {n_o})$ in the polymer and liquid crystal droplet, and therefore it passes the film without scattering. For an obliquely incident light, it still encounters different refractive indices in the polymer and liquid crystal droplet and thus is still scattered. Because of the selective scattering, this PDLC film with the liquid crystal droplets aligned in the film's normal direction can improve the light efficiency of the QD backlight. When the voltage is turned off, the liquid crystal droplets relax back to their initial random state.
As mentioned above, one way to align the liquid crystal droplet in the film's normal direction is to apply a voltage across the film. This way is, however, troublesome because it needs a voltage source and consumes energy. In order to overcome the difficulty, we propose to use a polymer network to achieve the unidirectional alignment of the liquid crystal droplets in the absence of applied voltages [20]. In this method, a homogeneous mixture, consisting of isotropic monomer, liquid crystal, and mesogenic monomer (and a small amount of photo-initiator), is made. Then the mixture is sandwiched between two substrates with ITO (Indium-Tin-Oxide) electrode coating to make a film. Finally, the film is irradiated by UV light to photo-polymerize the monomers in the presence of an applied voltage (defined as curing voltage). The polymerization is carried out in two steps. In the first step, the film is irradiated by a relatively low intensity UV light. The isotropic monomers are polymerized to form an isotropic polymer matrix, and the liquid crystal and mesogenic monomers phase separate from the polymer to form droplets. Because of the curing voltage, the formed droplets are uniformly aligned in the film's normal direction. In the second step, the film is irradiated by a relatively high intensity UV light. The mesogenic monomers are polymerized to form an anisotropic polymer network which keeps the droplets in the film's normal direction even when the curing voltage is removed.
We carried out a systematic study aiming to develop APDLCs with highly selective scattering at 0 V. In our experiment, the following materials were used: isotropic prepolymer NOA65 (from Norland Optical Adhesive Inc.), nematic liquid crystal E7 (from Merck), mesogenic monomer RM257 (from Merck), and a photo-initiator BME (from Polyscience Inc.). NOA65 is a mixture of acrylate flexible monomers and oligomers. E7 has a positive dielectric anisotropy of +13.8, and the molecules align parallel to the applied electric field. RM257 is a bifunctional mesogenic monomer that forms an anisotropic polymer network when polymerized [21–27]. The concentrations of the constituent components are 54.4 wt.% NOA65, 42.6 wt.% E7, 2.5 wt. % RM257 and 0.5 wt.% BME, unless otherwise specified. The materials were mixed and vortexed until the mixture became homogeneously clear. Samples were fabricated by sandwiching the mixture between two ITO-coated glass substrates. The cell gap was controlled by 20 µm spacers. After that, the samples were subjected to UV irradiation to polymerize the isotropic and mesogenic monomers. The UV light was produced by a LED illuminator. The light produced by the LED illuminator was peaked at 365 nm and unpolarized. The polymerization was divided into two steps. In the first step, the UV irradiation time was 1 minute, and no voltage was applied. Throughout the first UV exposure, the isotropic prepolymer formed isotropic polymer; and the liquid crystal and the mesogenic monomer started to phase separate from the isotropic polymer to form droplets. Due to the random orientation of the LC droplets, the samples gradually turned opaque. In the second step, the UV irradiation time was 30 minutes, and a curing voltage of a square wave of 1 kHz was applied across the film to align the LC droplets along the film's normal direction. Throughout the exposure, the mesogenic monomers were polymerized to form an anisotropic polymer network inside the LC droplets, which retained the droplets in the film's normal direction afterward. During the UV curing, the samples were placed on a working bench in an open space with good air circulation at room temperature. The temperature of the samples was close to room temperature. RM257 has better solubility in the LC than NOA65. Note that in the first step, some RM257 monomers were polymerized, and some NOA65 monomers were not polymerized. Because of the difference in their chemical structures and solubilities in LC, the percentage of NOA65 monomers polymerized in the first step is higher than that of RM257 monomers polymerized.
The electro-optical properties of the APDLC samples were studied by measuring their transmittance as a function of applied voltage. The applied voltage was AC voltage of 1 kHz frequency. The incident light was a collimated He-Ne laser light with a wavelength of 543 nm. The photo-detector was a photo-diode with a collection angle of 4°. The transmittance was normalized with respect to the light intensity before the sample. The measurement geometry is shown schematically by the inset in Fig. 3.
Fig. 3. Transmittance vs. applied voltage of the APDLCs cured under various UV light intensities in the first step of polymerization.
We first studied the effects of UV light intensity on the optical properties of the APDLCs. The UV light intensity in the first step was varied, as shown in Table 1. The UV light intensity in the second step was fixed at 2.5 mW/cm2. The curing voltage in the second step was 100 V. The transmittance vs. applied voltage curve of the samples is shown in Fig. 3. The transmittances of the aligned PDLCs at 0 V are much higher than unaligned PDLCs because the LC droplets are partially or well aligned along the film’s normal direction due to the curing voltage and the polymer network formed in the second step. The transmittances of the samples at 0 V are listed in Table 1. The maximum transmittance is obtained at the intermediate UV light intensity of 1.25 mW/cm2. The effects of the UV light intensity in the first step on the transmittance at 0 V are realized through three aspects. The first aspect is the LC droplet size. The higher the UV light intensity, the droplets with smaller sizes are formed. In the absence of applied voltage, there are two factors affecting the direction of the droplet: anchoring of polymer at the droplet surface and aligning effect of the polymer network. The anchoring tends to align the droplet randomly, and its effect is stronger in smaller droplets. The polymer network tends to align the droplet in the film's normal direction. The second aspect is the amount of unpolymerized mesogenic monomers (RM257) after the first step. In the first step, some of the mesogenic monomers are polymerized. The higher the UV light intensity, the fewer mesogenic monomers are unpolymerized and the polymer network formed in the second step is weaker. The third aspect is the amount of unpolymerized isotropic monomers (NOA65) after the first step. The unpolymerized isotropic monomers are polymerized in the second step to form isotropic beads, which tend to randomize the direction of the LC droplets. With the higher UV light intensity in the first step, the fewer isotropic monomers are unpolymerized, and the isotropic beads formed in the second step have a weaker randomizing effect. Under the overall effects of the three aspects, better aligned LC droplets are formed under intermediate UV light intensity in the first step.
Table 1. Transmittance of APDLCs at 0 V vs. UV light intensity in the first step of polymerization
As shown in Fig. 3, the transmittance of the APDLCs increases with the applied voltage in the measurement. As the UV light intensity in the first step is increased, the driving voltage (the saturation voltage to obtain the maximum transmittance) increases. Under higher UV light intensity in the first step, smaller LC droplets form, where the surface anchoring effect is stronger and the aligning effect of the anisotropic polymer network is weaker, and therefore a higher voltage is needed to align the LC droplets. Another effect of the UV light intensity is on the maximum transmittance (the saturated transmittance at the saturated voltage). As the UV light intensity in the first step is increased, the maximum transmittance increases. This is probably because after the first step, the amount of unpolymerized isotropic monomers decreases, and therefore there are fewer isotropic polymer beads inside the LC droplets. The maximum transmittance of sample A4 is about 83%. The light loss is 17%. The reflection from the glass-air, glass-ITO, and ITO-polymer interfaces causes about 10% light loss. Non-perfect match between the reactive indices of the polymer and liquid crystal is responsible for the rest of the light loss.
We then studied the effects of UV light intensity in the second step on the optical properties of the APDLCs. The UV light intensity in the first step was fixed at 1.25 mW/cm2. The UV light intensity in the second step was varied, as shown in Table 2. The curing voltage in the second step was 100 V. The transmittance vs. applied voltage curve of the samples is shown in Fig. 4. The transmittance at 0 V increases slightly with the UV light intensity in the second step, as listed in Table 2. The transmittance increases with increasing applied voltage. The driving voltage and maximum transmittance are almost independent of the UV intensity. As mentioned above, there are three aspects that affect the electro-optical properties of the APDLCs. Once the UV light intensity in the first step is fixed, all the three aspects (the LC droplet size, the amount of unpolymerized mesogenic monomers, and the amount of unpolymerized isotropic monomers) are fixed. Therefore, the electro-optical properties are almost fixed. The only effect of the UV light intensity in the second step is on the anisotropic polymer network formed in the second step. When the UV light intensity is increased, a denser and smaller lateral sized network forms, which has a stronger aligning effect on the LC [28,29]. Therefore, the LC droplets are aligned better in the film's normal direction, and the transmittance at 0 V is increased.
Fig. 4. Transmittance vs. applied voltage of the APDLCs cured under various UV light intensities in the second step of polymerization.
Table 2. Transmittance of APDLCs at 0 V vs. UV light intensity in the second step of polymerization
In order to keep the LC droplets in the film's normal direction, the polymer network must be in the film's normal direction. In order to achieve this goal, the curing voltage in the second step is critical. We studied the effects of the curing voltage in the second step. The UV light intensity in the first and second steps were 1.25 mW/cm2 and 2.5 mW/cm2, respectively. The transmittances at 0 V of the samples cured under various curing voltages are listed in Table 3. When the curing voltage in the second step is 0V, the transmittance is very low, about 2%, because the direction of the LC droplets is random throughout the sample, as in regular PDLCs. As the applied voltage in the curing is increased, the transmittance at 0 V increases. When the curing voltage is increased to 100 V, the transmittance saturates at 75%. The transmittance of the samples as a function of the applied voltage in the measuring is shown in Fig. 5. For the sample cured under 0 V, the transmittance increases dramatically with the applied voltage. For the samples cured at voltages higher than 25 V, the transmittance increases slightly with the applied voltage. One interesting point is that the sample cured under 25 V has the transmittance of 67% at 0 V, while the sample cured at 0 V has the transmittance of 4% when the applied voltage is 20 V. These different transmittances are caused by the polymer network in the LC droplets. Without the polymer network, the voltage needed to align the LC droplets is about 30 V (which will be discussed in detail in the paragraph below). If a sufficiently high curing voltage is applied in the second step, the LC droplets are aligned along with the film’s normal, and then the polymer network is formed in the film’s normal direction, which will keep the droplets in the aligned state when the applied voltage is removed. If no curing voltage is applied in the second step, the directions of the LC droplets are random, and then the polymer network is formed in random directions. When the voltage is removed, the polymer network will try to keep the LC droplets in the random state. After the curing, in order to overcome the aligning effect of the polymer network, 100 V, much higher than 30 V, is needed to align the LC droplet.
Fig. 5. Transmittance vs. applied voltage of the APDLCs cured under various applied voltages in the second step of polymerization.
Table 3. Transmittance of APDLCs at 0 V vs. applied voltage in the second step of polymerization
The anisotropic polymer network is critical to keep the LC droplets aligned in the film's normal direction after curing. We studied the effects of the polymer network by varying the mesogenic monomer (RM257) concentration. The UV light intensity in the first step and second steps were 1.25 mW/cm2 and 2.5 mW/cm2, respectively. The curing voltage in the second step was 100 V. The transmittances at 0 V of the samples with various concentrations of RM257 are listed in Table 4. When the mesogenic monomer is 0%, the transmittance is very low, about 3%, because there is no anisotropic polymer network. Although the LC droplets are aligned in the film's normal direction during the curing, they relax back to the random state when the curing voltage is removed. As the mesogenic monomer concentration is increased, a more dense polymer network is formed, which has a stronger aligning effect on the LC. When the concentration is below 2.5%, the polymer network is not strong enough to keep the LC droplets in the state aligned by the curing voltage in the second step. After the curing, when the curing voltage is removed, the LC droplets relax some from the well aligned state, and the transmittance becomes lower than the maximum transmittance. When the concentration is above 2.5%, the polymer network is strong enough to keep the LC droplets in the state aligned by the curing voltage. After the curing, when the curing voltage is removed, the LC droplets remain in the well aligned state, and the transmittance remains at the maximum value. The transmittance as a function of applied voltage of the samples with the various mesogenic monomer concentrations is shown in Fig. 6. For the sample with 0% mesogenic monomer, at 0 V, its transmittance is very low, about 3%. As the applied voltage is increased, its transmittance increases. The driving voltage is about 30 V, at which the maximum transmittance of about 80% is reached. This driving voltage is lower than that (100 V) of the sample (C1) with 2.5% mesogenic monomer but cured at 0V, where there is the polymer network which has an aligning effect on the LC against the aligning effect of the applied voltage. Note that when the applied voltage is 20 V, the transmittance is 65%, which is the same as that of the sample (C2) with 2.5% mesogenic monomer and cured at 20 V. When the mesogenic monomer concentration is higher than 2.5%, the formed polymer network is strong enough to keep the LC droplets in the well aligned state. After curing, the transmittance is high and does not change much with the applied voltage.
Fig. 6. Transmittance vs. applied voltage of the APDLCs with various mesogenic monomer concentration.
Table 4. Transmittance of APDLCs at 0 V vs. the concentration of the mesogenic monomer.
We examined the morphology of the APDLCs under a polarizing optical microscope with crossed polarizers. If the UV light intensity in the first step is higher than 1 mW/cm2, the LC droplet size is about 1 micron. Under the optical microscope, it is very difficult to study the textures of droplets of that size. Therefore, we used UV light with very low intensity (0.05 mW/cm2) to prepare unaligned and aligned PDLCs with large droplet sizes. When the samples were studied under the microscope, there was no applied voltage. The microphotograph of the unaligned PDLC, cured in the absence of applied voltage, is shown in Fig. 7(a). The droplet size is around 3 $\mu m$. The droplets have the typical texture of bipolar droplets, [13,30,31] indicating that the LC orients tangentially on the polymer surface of the droplet. The textures of the LC droplets are different from one another, indicating that the bipolar axis of the droplets is random throughout the sample. The microphotograph of the aligned PDLC sample, cured under 100 V, is shown in Fig. 7(b). The textures of the LC droplets are more or less the same. The texture has two dark crossed brushes, indicating the bipolar axis is aligned in the film's normal direction [32–34]. The aligned PDLC droplet exhibits small birefringence; therefore, the texture is less colorful than that of the unaligned PDLC droplet. The incomplete black texture indicates that the LC is not perfectly aligned along the film normal direction. Therefore, it still exhibits some weak scattering and causes some light loss.
It is impossible to observe the polymer network inside the LC droplet under the optical microscope because of its limited resolution. Therefore, we used a scanning electron microscope (SEM) to study the polymer network. The samples were cured under UV light with very low intensity (0.05 mW/cm2). In the preparation of the samples for the SEM study, the samples were frozen, and then were broken into small pieces. Then samples were immersed in hexane for 1.5 minutes to dissolve the LC, but not the isotropic polymer and the anisotropic polymer network. After that, samples were placed in a thermal oven at 70°C for 2 hours to evaporate the solvent. Subsequently, they were coated with a thin layer of gold by spattering. The SEM microphotograph of the unaligned PDLC sample (without the mesogenic monomer) is shown in Fig. 8(a). The relatively bright region is the isotropic polymer. The relatively dark regions are the voids which were occupied by the LC droplets before removal. There is nothing inside the voids. The SEM microphotograph of the aligned PDLC sample is shown in Fig. 8(b). In some voids, there are polymer networks in the film's normal direction. In the other voids, no polymer network is observed, probably because the polymer network is very thin and dissolved by the solvent in the preparation of the sample for SEM study.
The different transmittances of the unaligned and aligned PDLCs can be seen visually. Figure 9 shows the photographs of the PDLC samples under room light illumination. A paper with the letters “KSU” is placed 5 cm beneath the samples. The unaligned PDLC sample at 0 V is very scattering and has an opaque white appearance, as shown in Fig. 9(a). The paper beneath it cannot be seen. When 100 V is applied, it becomes transparent, and the letters “KSU” can be seen, as shown in Fig. 9(b). The aligned PDLC sample at 0 V is transparent (with the transmittance of 75%), and the letters “KSU” can be seen, as shown in Fig. 9(c). When 100 V is applied, it becomes more transparent (with the transmittance of 83%), the letters “KSU” appear clearer, as shown in Fig. 9(d).
Fig. 9. Photographs of the PDLCs. (a) Unaligned PDLC sample C1 at 0 V, (b) Unaligned PDLC sample at 100 V. (c) Aligned PDLC sample A4 at 0 V. (d) Aligned PDLC sample at 100 V.
In order to demonstrate the selective scattering of the aligned PDLC, we measured the transmittance of the PDLC sample as a function of incident light angle. In order to eliminate the factor of the variation of the reflection from the substrate-air interfaces with the incident light angle $\alpha$, the sample is placed in a cylinder with a refractive index matching oil of the refractive index of 1.5, as shown by the inset in Fig. 10. The aligned PDLC sample does not scatter normally incident light. Its transmittance is high (75%) for incident light with 0° incident angle. As the incident angle is increased, it gradually becomes scattering, and its transmittance decreases, as shown by curve (a) in Fig. 10. When the incident angle is 60°, its transmittance decreases to 10%. For comparison, we also measured the transmittance of the unaligned PDLC sample at various incident angles. It is very scattering for light with any incident angle, and its transmittance is always low, as shown by curve (b).
Fig. 10. Transmittance of the PDLCs at 0 V as a function of incident angle. Curve (a): the aligned PDLC sample A4. Curve (b): the unaligned PDLC sample C1.
4. Light efficiency enhancement of QD backlight by aligned polymer dispersed liquid crystal film
We used the aligned PDLC films to enhance the light efficiency of the QD backlight. In our experiment, the QD backlight was provided by BOE Technology Co., Ltd. The substrate used to make the PDLC films is a polyethylene terephthalate (PET) film with ITO coating. The fabricated PDLC film was laminated on the top of the QD backlight with the help of a refractive index matching oil. We measured the emitted light intensity as a function of the emission angle $\beta$, which is defined in the inset in Fig. 11(a). The light intensity at 0° emission angle is normalized to 1. In order to make the light enhancing effect of the aligned PDLC films stand out, we also measured the light enhancing effect of unaligned PDLC films. Figure 11 shows the emitted light intensity as a function of emission angle of the QD backlight incorporated with the PDLC films at 0 V. All the PDLC films, independent of whether aligned or not, increase the light output. The incorporation of the PDLC films increases the light intensity but does not change the shape of light intensity vs. emission angle curve. There are two important features worth pointing out. First, the light enhancing effect of aligned PDLC films (cured at 100 V) is significantly higher than the unaligned PDLC films (cured at 0 V). Second, the light enhancing effect increases with the PDLC film thickness. When the film thickness is increased, its scattering capability increases, and more light emitted in large incident angles is scattered toward the film's normal direction.
Fig. 11. (a) Emitted light intensity vs. emission angle of the QD backlight incorporated with PDLC films at 0 V. (b) Emitted light intensity vs. emission angle of the QD backlight incorporated with PDLC films at 100 V
As mentioned before, the LC droplets at 0 V are not perfectly aligned because the transmittance of the aligned PDLC at 0 V is about 75% and can be increased further with applied voltages. We applied 100 V to the PDLC films and measured the emitted light intensity. The result is shown in Fig. 11(b). For the unaligned PDLC films, the light intensity is increased significantly by the applied voltage in the measurement, while for the aligned PDLC films, the light intensity is increased slightly by the applied voltage. From the measured function $I(\beta )$ of light intensity I vs. emission angle $\beta$, we calculate the total emitted light intensity by using the equation
The total emitted light intensities of the QD backlight enhanced by the PDLC films are listed in Table 5. The light enhancement efficiency of the unaligned PDLC at 0 V is only 5.6%. The light enhancement efficiency of the 20 $\mu m\; $ aligned PDLC at 0 V is only 17.1%. When the film thickness is increased to 30 $\mu m$, the light enhancing efficiency is increased to 21.6%.
Table 5. Total light intensity of the QD backlight enhancement by unaligned and aligned PDLC films.
5. Conclusion and discussion
Light emitting flat panel displays all have a common problem that light is emitted in all directions, and only a small portion can come out of the viewing side due to total internal reflection at the substrate-air interface. One solution to overcome the problem is to laminate a scattering film on the top of the display. A regular isotropic scattering film scatters light with any incident angle. It is a double-edged sword. On one hand, it may scatter light emitted with a large incident angle into a direction with a small incident angle, and thus the light can come out of viewing side of the display. On the other hand, it may scatter light with a small incident angle into a direction with a large incident angle, and then the light cannot come out. Therefore, the light enhancement capability of regular isotropic scattering films is limited. The aligned PDLC film is better than regular scattering films. It exhibits selective scattering: it scatters light with large incident angles, but not light with small incident angles. Furthermore, in order to save energy and simplify display structure, it is highly desirable that the LC droplets in the PDLC film are unidirectionally aligned in the film's normal direction in the absence of applied voltage. We demonstrated that the alignment of LC droplets can be achieved by forming anisotropic polymer networks inside the droplets. The light enhancement achieved at this moment is about 20%. There are many factors, such as the LC droplet size and shape, refractive indices of the LC, PDLC film thickness, and substrate thickness, which affect the light enhancement. Therefore, there is still room for improvement of the light enhancement.
In a light emitting flat panel display, such as OLED or MLED, light is emitted in all directions. A major portion of the emitted light does not come of the viewing side of the display due to the total internal reflection at the panel-air interface, and therefore is wasted. The APDLC film exhibits selective scattering and can reduce the light loss due to the total internal reflection. Therefore, the APDLC film can be used to improve the light efficiency of OLED and MLED displays.
In summary, we have developed a simple and efficient method of integrating an aligned PDLC film on QD backlight to enhance the light outcoupling efficiency. The alignment of the PDLC droplets is realized by anisotropic polymer networks formed under applied electric fields. The aligned PDLC film exhibits selective scattering. It scatters light with large incident angles but not light with small incident angles. The aligned PDLC film can also be produced with a low-cost manufacturing process.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. Chen and S.-T. Wu, “Advanced Liquid Crystal Displays with Supreme Image Qualities,” Liquid Crystals Today 28(1), 4–11 (2019). [CrossRef]
2. H.-W. Chen, J.-H. Lee, B.-Y. Lin, S. Chen, and S.-T. Wu, “Liquid Crystal Display and Organic Light-Emitting Diode Display: Present Status and Future Perspectives,” Light: Sci. Appl. 7(3), 17168 (2018). [CrossRef]
3. K.-H. Kim and J.-K. Song, “Technical Evolution of Liquid Crystal Displays,” NPG Asia Mater. 1(1), 29–36 (2009). [CrossRef]
4. D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices, 2nd Ed., (John Wiley & Sons, Ltd., 2014).
5. J. Chen, V. Hardev, J. Hartlove, J. Hofler, and E. Lee, “A High-Efficiency Wide-Color-Gamut Solid-State Backlight System for Lcds Using Quantum Dot Enhancement Film,” in Digest of Technical Papers - SID International Symposium, Vol. 43 (Blackwell Publishing Ltd, 2012), pp. 895–896.
6. J. F. van Derlofske, J. M. Hillis, A. Lathrop, J. Wheatley, J. Thielen, and G. Benoit, “Illuminating the Value of Larger Color Gamuts for Quantum Dot Displays,” SID Symposium Digest of Technical Papers 45(1), 237–240 (2014). [CrossRef]
7. Y. Shu, X. Lin, H. Qin, Z. Hu, Y. Jin, and X. Peng, “Quantum Dots for Display Applications,” Angew. Chem. Int. Ed. 59(50), 22312–22323 (2020). [CrossRef]
8. Z. Luo, D. Xu, and S.-T. Wu, “Emerging Quantum-Dots-Enhanced LCDs,” J. Disp. Technol. 10(7), 526–539 (2014). [CrossRef]
9. D. V. Talapin and J. Steckel, “Quantum dot light-emitting devices,” MRS Bull. 38(9), 685–691 (2013). [CrossRef]
10. Y.-S. Kim, B.-Y. Noh, and I.-K. Park, “Light Emission from CdSe Quantum Dot and ZnO Nanorod Hybrid Structures,” J. Korean Phys. Soc. 65(1), 74–79 (2014). [CrossRef]
11. J. A. Smyder and T. D. Krauss, “Coming Attractions for Semiconductor Quantum Dots,” Mater. Today 14(9), 382–387 (2011). [CrossRef]
12. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics (Wiley, 2013).
13. P. S. Drzaic, Liquid crystal dispersions (World Scientific, 1995).
14. J. W. Doane, N. A. Vaz, B.-G. Wu, and S. Zumer, “Field controlled light scattering from nematic microdroplets,” Appl. Phys. Lett. 48(4), 269–271 (1986). [CrossRef]
15. J. W. Doane, A. Golemme, J. L. West, J. B. Whitehead, and B.-G. Wu, “Polymer Dispersed Liquid Crystals for Display Application,” Molecular Crystals and Liquid Crystals Incorporating Nonlinear Optics 165(1), 511–532 (1988). [CrossRef]
16. J. M. Sanchez-Pena, C. Vazquez, I. Perez, I. Rodriguez, and J. M. Oton, “Electro-Optic System for Online Light Transmission Control of Polymer-Dispersed Liquid Crystal Windows,” Opt. Eng. 41(7), 1608 (2002). [CrossRef]
17. H. Hassan-Ali, O. Pinshow, and D. Gal-Fuss, “Evaluation of Polymer Dispersed Liquid Crystal (PDLC) for Passive Rear Projection Screen Application,” rpm 1, 1 (2019). [CrossRef]
18. J. Qi and G. P. Crawford, “Holographically Formed Polymer Dispersed Liquid Crystal Displays,” Displays 25(5), 177–186 (2004). [CrossRef]
19. T. J. Bunning, L. V. Natarajan, V. P. Tondiglia, and R. L. Sutherland, “Holographic polymer-dispersed liquid crystals (H-PDLCs),” Annu. Rev. Mater. Sci. 30(1), 83–115 (2000). [CrossRef]
20. J. Jiang, G. McGraw, R. Ma, J. Brown, and D.-K. Yang, “Selective scattering polymer dispersed liquid crystal film for light enhancement of organic light emitting diode,” Opt. Express 25(4), 3327 (2017). [CrossRef]
21. D. -K. Yang, “Polymer Stabilized Cholesteric Liquid Crystal for Switchable Windows,” in Liquid Crystals Beyond Displays: Chemistry, Physics, and ApplicationsQ. Li, eds. (Wiley online library2012) pp.505–523.
22. D. J. Broer, “Liquid Crystalline Networks Formed by Photoinitiated Chain Cross-Linking,” in Liquid Crystals in Complex Geometries: Formed by Polymer and Porous NetworksG. P. Gawford and S. Zumer, eds. (Taylor & Francis1996) pp. 239.
23. D.-K. Yang, “Polymer stabilized liquid crystal displays,” in Progress in liquid crystal science and technology eds. H. -S. Kwok, S. Naemura, and H. L. Ong, (World Scientific, 2013), Chapter 25, pp.597–628.
24. G. P. Crawford and S. Zumer, Liquid crystals in complex geometries (Taylor & Francis, 1996).
25. D. J. Broer, R. G. Gossink, and R. A. M. Hikmet, “Oriented polymer networks obtained by photopolymerization of liquid crystal-crystalline monomers,” Die Angewandte Makromolekulare Chemie 183(1), 45–66 (1990). [CrossRef]
26. Y.K. Fung, D.-K. Yang, Y. Sun, L. C. Chien, S. Zumar, and J. W. Doane, “Polymer networks formed in liquid crystals,” Liq. Cryst. 19(6), 797–801 (1995). [CrossRef]
27. G. A. Held, L. L. Kosbar, I. Dierking, A. C. Lowe, G. Grinstein, V. Lee, and R. D. Miller, “Confocal microscopy study of texture transitions in a polymer stabilized cholesteric liquid crystal,” Phys. Rev. Lett. 79(18), 3443–3446 (1997). [CrossRef]
28. R.-Q. Ma and D.-K. Yang, “Fré Edericksz Transition in Polymer-Stabilized Nematic Liquid Crystals,” Phys. Rev. E 61(2), 1567–1573 (2000). [CrossRef]
29. D.-K. Yang, Y. Cui, H. Nemati, X. Zhou, and A. Moheghi, “Modeling Aligning Effect of Polymer Network in Polymer Stabilized Nematic Liquid Crystals,” J. Appl. Phys. 114(24), 243515 (2013). [CrossRef]
30. Y. G. Marinov, G. B. Hadjichristov, and A. G. Petrov, “Single-Layered Microscale Linear-Gradient PDLC Material for Electro-Optics,” Cryst. Res. Technol. 44(8), 870–878 (2009). [CrossRef]
31. J. Jiang and D.-K. Yang, “Bipolar to Toroidal Configuration Transition in Liquid Crystal Droplets,” Liq. Cryst. 45(1), 102–111 (2018). [CrossRef]
32. X. Wang, E. Bukusoglu, D. S. Miller, M. A. B. Pantoja, J. Xiang, O. D. Lavrentovich, and N. L. Abbott, “Synthesis of optically complex, porous, and anisometric polymeric microparticles by templating from liquid crystalline droplets,” Adv. Funct. Mater. 26(40), 7343–7351 (2016). [CrossRef]
33. S. C. Jain and D. K. Rout, “Electro-optic Response of Polymer Dispersed Liquid-crystal Films,” J. Appl. Phys. 70(11), 6988–6992 (1991). [CrossRef]
34. R. Ondris-Crawford, E. P. Boyko, B. G. Wagner, J. H. Erdmann, S. Zumer, and J. W. Doane, “Microscope textures of nematic droplets in polymer dispersed liquid crystals,” Journal of Applied Physics 69(9), 6380–6386 (1991). [CrossRef]
