Selective scattering polymer dispersed liquid crystal film for light enhancement of organic light emitting diode

Jinghua Jiang; Greg McGraw; Ruiqing Ma; Julie Brown; Deng-Ke Yang

doi:10.1364/OE.25.003327

1. Introduction

Organic light emitting diodes (OLEDs) are an emerging technology that has many applications such as flat panel displays and flat panel lighting devices [1–3]. OLEDs are made from synthesized organic materials that produce light through electroluminescence. It has the advantages of a flat thin profile and light weight. They do not use polarizers as in liquid crystal displays (LCDs) and are compatible with flexible plastic and metallic substrates. They are therefore light-weight and mechanically rugged. OLEDs can also be fabricated into curved shapes. Their power efficiency is, however, limited by total internal reflection within the OLED structure [4]. In most OLED architectures, the organic electroluminescence material is sandwiched between a thin metal oxide anode deposited on a transparent substrate (such as glass or plastic) and a metal cathode, as shown in Fig. 1. The refractive index of the substrate is usually around 1.5, much higher than the surrounding air (with refractive index close to 1). Light produced within the organic layer is emitted in all directions. When the emitted light hits the substrate-air interface with a small incident angle, it is refracted into air. When the emitted light hits the interface with a large angle (larger than the critical angle θ_c for total internal reflection), as shown in Fig. 1(a), it is totally reflected back into the substrate which acts as a waveguide [5]. Some trapped light is absorbed by the OLED when propagating through it, and some is absorbed by the cathode upon reflection from it [6]. The remaining trapped light comes out of the edge of the substrate and is also wasted. Figure 1(a) shows only the refraction and reflection of emitted light in the upward direction. Light emitted in the downward direction is reflected upward by the bottom metallic cathode, and then follows the same paths as light that originated traveling in an upward direction. Only light emitted within the emission cone with the cone angle θ_c comes out of the device as shown in Fig. 1(a). The solid angle of the emission cone (for both up and down emission) is

Δ Ω = [2 \int_{0}^{2 π} [\int_{0}^{θ_{c}} \sin θ d θ] d φ] = 4 π (1 - \cos θ_{c}),

where ϕ is the azimuthal angle. The total solid angle is 4π. Therefore, the light efficiency of OLED is approximately given by

E = \frac{Δ Ω}{4 π} = (1 - \cos θ_{c})

For a substrate with a refractive index of 1.5, θ_c = 42°, and the light efficiency is 25% [4]. There have been many efforts to increase the light outcoupling efficiency of OLEDs. There are two broad approaches. The first is to design different structures internal to the OLED layers [7–11]. The second one is to use novel films on top of the transparent substrate. In this paper we focus our effort on the second approach using scattering films. In this method, a scattering film with embedded scattering particles is placed on top of the OLED [12]. Emitted light with large incident angles (with respect to the normal of the substrate) is scattered into directions with small incident angles and thus outcouples to air. The scattering film method has a drawback that emitted light with small incident angles can be scattered into directions with large incident angles and be reflected back.

Fig. 1 (a) schematic structure of OLED and total internal reflection. (b) OLED + PDLC with randomly oriented liquid crystal droplets. (c) diagram showing liquid crystal droplet director, propagation direction and electric field direction of incident light. (d) diagram showing the selective scattering of liquid crystal droplet. (e) OLED + PDLC with unidirectionally oriented liquid crystal droplets.

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In this paper we report a novel light enhancement film from polymer dispersed liquid crystal (PDLC). With controlled liquid crystal droplet orientation, the film exhibits selective scattering in that it only scatters emitted light with large incident angles. Therefore, it can significantly increase the light efficiency of OLEDs.

2. Principle

We use a polymer dispersed liquid crystal (PDLC) film to enhance the light outcoupling efficiency of OLEDs. We first fabricate a PDLC film and then laminate it on top of the OLED as shown in Fig. 1(b). A refractive index-matching material is placed between the OLED and PDLC in order to prevent light reflection at the interface between them. In the PDLC, the liquid crystal phase separates to form micron-sized droplets [13–18]. In each droplet, the liquid crystal is approximately aligned along a common direction called the droplet direction $\vec{N}$ as shown in Fig. 1(c). In a regular PDLC film, the droplet orientation is random throughout the film. The glass substrate and the polymer matrix of the PDLC are optically isotropic and had refractive indices close to 1.5. The liquid crystal is birefringent and has an ordinary refractive index n_o and extraordinary refractive index n_e. The refractive index n_p of the polymer is matched to n_o, but smaller than n_e. When the propagation direction of the incident light makes an angle α with respect to the liquid crystal droplet direction, as shown in Fig. 1(c), the effective refractive index of the liquid crystal is below [19,20].

n_{e f f} = \frac{n_{o} n_{e}}{\sqrt{n_{e}^{2} \cos^{2} α + n_{o}^{2} \sin^{2} α}}

For oblique (with respect to the liquid crystal droplet director) incident light,

α \neq 0^{o}

, the encountered refractive index in the liquid crystal droplet is different from that of the polymer. The incident light is scattered and its propagation direction is deflected as shown in Fig. 1(c) [13, 21, 22]. For normal incident light, α = 0°, the encountered refractive index in the liquid crystal droplet was the same as that of the polymer, and the incident light is not scattered. If the liquid crystal droplets in the PDLC film are randomly oriented as shown in Fig. 1(b), it behaves similarly to the scattering film previously described. It scatters rays of light both into and out of the emission cone. For light emitted within the emission cone, some is scattered out of the emission cone, and thus is totally reflected and wasted. This scattering is undesirable and is a drawback. For light emitted outside the emission cone, some is scattered into the emission cone, and thus come out into air. The solid angle of the emission cone, given by Eq. (1), is much smaller than the entire solid angel of 4π, and therefore the overall effect of the PDLC is to increase the light efficiency of the OLED.

To further increase the enhancing effect of PDLC, we designed a new PDLC structure, where all the liquid crystal droplets are aligned along the normal of the film as shown in Fig. 1(e). For the light emitted within the emission cone ( $0 \leq α \leq θ_{c}$ ), the mismatch between the refractive index of the liquid crystal n_eff and the refractive index n_p of the polymer is now small. Light originating within the emission cone is not scattered out of it. Therefore the drawback of the randomly oriented PDLC is eliminated. For the emitted light outside the emission cone, the mismatch between the refractive index of the liquid crystal and that of the polymer is large. The light scattering is strong and some of the emitted light is scattered into the emission cone so that it outcouples. Therefore the unidirectionally aligned PDLC can increase the light efficiency of the OLED more than the randomly oriented PDLC as well as the conventional scattering film.

3. Experiment and results

We made PDLCs from nematic liquid crystal E7 (from Merck) and UV glue NOA 65 (from Norland Products, Inc.). The liquid crystal (43%) and the glue (57%) were mixed and then sandwiched between two thin plastic substrates with ITO (indium tin oxide) electrode. The sample was irradiated under UV light (with the intensity 1.52 mW/cm²) for 30 minutes to cure the glue. The liquid crystal phase separated from the glue to form micron-sized droplets. During the curing, there was no applied voltage, except as otherwise specified, and the liquid crystal droplets formed had random droplet orientation throughout the sample. The thickness of the PDLC film was controlled by spacers. Samples with three thicknesses, 10 µm, 25 µm and 50 µm, were fabricated.

The transmittance of the PDLC films was measured as a function of applied voltage. In the measurement a green He-Ne laser with wavelength 543 nm was used. The light was incident normally on the PDLC films. The collection angle of the detector was 4°. The measured transmittance was inline transmittance. An AC voltage of 1 kHz was applied. The result is shown in Fig. 2(a), with transmittance was normalized to the incident light. At 0 V, the liquid crystal droplets were randomly oriented, the scattering was strong, and therefore the transmittance was low. As the applied voltage was increased, the liquid crystal droplets gradually aligned along the film normal direction. The scattering of the normally incident light became weak, and therefore the transmittance increased. For the 10 µm PDLC film, the saturated maximum transmittance was about 90%. As the film thickness was increased, the number of scattering centers (liquid crystal droplets) along the optical path increased, and the film became more scattering. This resulted in lower off-state (under 0 volt) transmittance and lower saturated transmittance (at high voltages). The driving voltage required to achieve a given transmittance increased with the film thickness.

Fig. 2 (a) Transmittance vs. applied voltage of the PDLC films with various film thicknesses. (b) Light intensity vs. view angle of OLED coupled with PDLC films with three different film. thicknesses.

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The OLED used in our experiment was made by depositing a white organic light emitting diode (WOLED) material on a 120 nm thick transparent conductive indium tin oxide (ITO) anode [23]. The WOLEDs contained a hole injection layer and a hole transport layer. The emissive layer (EML) consisted of a host, a yellow green phosphorescent emitter with 1931 commission internationale de l'eclairage (CIE) co-ordinates (0.45, 0.53) and a red phosphorescent emitter (0.65, 0.35) deposited on the hole transport layer. A light-blue EML was deposited on the first EML; and it consisted of a host and a light-blue emitter (0.17, 0.37). The electron transport layer consisted of 30 nm to 50 nm of a high conductivity undoped organic material. The cathode consisted of 100 nm of Al deposited on 1 nm of LiF, and the 1 cm² active emitting areas were defined by an organic grid. All layers were deposited under high-vacuum conditions (1x10⁻⁷Torr). The WOLED was transferred directly from vacuum into an inert environment glove-box, where it was encapsulated using a UV-curable epoxy, and a glass lid with a moisture getter.

The PDLC films were laminated on top of the white OLED. The emitted light intensity I(β) was measured as a function of the viewing angle β (the angle between the normal direction of the OLED film and the detection direction). The result is shown in Fig. 2(b) where no voltage was applied to the PDLC films. The emitted light intensity of the pure OLED (without PDLC film) at β = 0 was defined as 1. As the PDLC film thickness was increased, the emitted light intensity increased, because the film became more scattering.

The total light output of the emitted light in all viewing directions is given by

I_{t o t a l} = \frac{\int_{0}^{2 π} [\int_{0}^{π / 2} I (β) \sin β d β] d α}{Δ Ω} = \frac{2 π}{Δ Ω} \int_{0}^{π / 2} I (β) \sin β d β,

where

Δ Ω

is the solid angle covered by the detector. From Fig. 2(b), the total light output was calculated and appears in Table 1 as a function of the PDLC film thickness. The light enhancement increased with the film thickness. The relative light outcoupling enhancement (RLOE) is defined as

R L O E = \frac{I_{t o t a l / P D L C} - I_{t o t a l / p u r e O L E D}}{I_{t o t a l / p u r e O L E D}} \times 100 %

When the film thickness was 10 µm, the relative light enhancement was 9.2%. When the film thickness was increased to 50 µm, the relative light enhancement was increased to 16.5%.

Table 1. Total optical power of emitted light vs. PDLC film thickness.

View Table

We studied the performance of aligned PDLCs and found that they can increase the light outcoupling efficiency of OLED more than randomly oriented PDLC. As demonstrated in the previous experiment, applied voltages can align the liquid crystal droplets in the film normal direction as shown in Fig. 1(e). We laminated the 50 µm thick PDLC film on top of the OLED. We then applied voltage across the film and measured the emitted light intensity as a function of the viewing angle. The result is shown in Fig. 3(a). When 100 V was applied, the light intensity was higher at small viewing angles, because the aligned liquid crystal droplets did not scatter incident light with small incident angles. At large viewing angles, the light intensity remained almost the same, because the scattering for light with large incident angle was unchanged and remained strong. When the voltage increased from 0 V to 100 V, the total intensity of emitted light increased from 3.45 units to 3.63 units; the relative light enhancement was increased from 16.5% to 22.3%.

Fig. 3 (a) Light intensity vs. viewing angle of OLED coupled with 50 µm PDLC film with two different applied voltages. (b) Transmission vs. incident light angle of the 50 μm aligned (cured with 150 V) and unaligned (cured with 0 V) PDLC films.

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The constant application of voltage to increase the light outcoupling efficiency in OLED devices is undesirable. To eliminate this drawback, we developed PDLC films with liquid crystal droplets permanently aligned unidirectionally along the film normal direction using the polymer stabilization method [24–28]. We prepared a mixture consisting of 42.9% liquid crystal E7, 54.4% glue NOA 65, 2.5%monomer RM257 (from Merck) and 0.2% photo-initiator BME (from PolyScience Inc.). RM257 is a mesogenic bifunctional monomer. The mixture was sandwiched between two parallel plastic substrates as described above. When the film was irradiated by UV light to photo-polymerize the glue, a voltage was applied to align the formed liquid crystal droplets along the film normal direction. The monomer molecule had an elongated rigid central core, the same as liquid crystal molecules, and two flexible hydrocarbon tails. At the end of the tails, there was acrylate chemical group which can be polymerized. The monomer had good solubility in the liquid crystal. During the polymerization, the monomer molecules and the liquid crystal molecules first phase separated from the cured glue to form droplets which were aligned along the film normal direction by the applied electric field; then the monomer molecules were polymerized to form a network which was dispersed inside the droplets as shown in Fig. 1(e). The polymer network held the liquid crystal droplets in the film normal direction when the applied voltage wasremovedafter the polymerization. In this way, PDLC films with permanent unidirectionally aligned liquid crystal droplets were fabricated.

We made a variety of PDLC films with unidirectionally aligned liquid crystal droplets. The electro-optical properties of the films were first characterized under normal incident laser light. The results are shown in Fig. 4(a). Here the transmittance was normalized to the transmittance of two plastic substrates. Note that the PDLC films, whose electro-optical properties are shown in Fig. 4(a), were different from the PDLC films whose electro-optical properties are shown in Fig. 2(a). The samples in Fig. 4(a) were cured with applied voltages and had polymer networks inside the dispersed liquid crystal droplets; the ones in Fig. 2(a) were cured without applied voltages and had no polymer networks inside the dispersed liquid crystal droplets. Therefore, they had different electro-optical properties. As shown in Fig. 4(a), at 0 V, the transmittance of the PDLC films (cured with applied voltages) was much higher than the transmittance of the PDLC films cured without applied voltages, but lower than the transmittance of the PDLC films (cured without applied voltages) at high voltages, as shown in Fig. 2(a). This indicates that the liquid crystal droplets were only partially (not completely) aligned along the film normal direction by the polymer network formed in the curing. As an example, let us consider the 50 μm thick PDLC films. For the PDLC film cured without applied voltage, the transmittance was 0% at 0 V, and 66% at 150 V. For the PDLC film cured with 150 V, the transmittance was 48% at 0 V, and 57% at 150 V. At 0 V, the transmittance of the PDLC film cured with 150 V was higher than that of the PDLC film cured without applied voltage because the liquid crystal droplets were partially aligned along the film normal by the polymer network inside the droplets. At 150 V, the transmittance of the PDLC film (with polymer network) cured with 150 V was lower than that of the PDLC film (without polymer network) cured without applied voltage, probably because the polymer network inside the LC droplet was not perfectly aligned. The polymer network had a strong aligning effect on the LC, which prevented the LC droplet from aligning perfectly along the film normal direction by the applied voltage. Furthermore, the presence of the polymer network changed the LC droplet size and shape.

Fig. 4 (a) Transmittance vs. applied voltage of the PDLC films cured with applied voltages. (b) Light intensity vs. view angle of OLED coupled with PDLC films cured with applied voltages.

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In order to see the selective scattering of the aligned PDLC, we measured the transmittance of the 50 μm thick PDLC film as a function of incident light angle. In the measurement, the PDLC film was placed inside a cylinder with index match oil (with refractive index of 1.5, the same as the plastic substrate) in order to prevent refraction and reflection of substrate-air interface. The incident light was a green light from a He-Ne laser. The incident angle $θ$ was defined as the angle between the incident light direction and the film normal direction. The result is shown in Fig. 3(b). When the incident angle was 0°, the polarization of the incident light was perpendicular to the droplet axis and the effective refractive index inside the LC droplets was close to that of the polymer. The scattering was weak, and therefore the transmission was high, about 48%. As the incident angle increased, the effective refractive index inside the LC droplets increased; the difference between the effective refractive indices of the LC and polymer became larger. The scattering became strong, and therefore the transmission decreased. This result clearly demonstrated that the aligned PDLC film selectively scattered light with larger incident angles. We also included the result for the 50 μm unaligned PDLC film for the purpose of comparison. The scattering by the unaligned film was always strong and the transmission was low, independent of the incident angle.

We then laminated the three PDLC films on top of the OLED, respectively. The light intensity was measured as a function of viewing angle. The result is shown in Fig. 4(b). No voltage was applied during the measurement. From Fig. 4(b), the total light output was calculated and is listed in Table 1. For the 10 µm thick PDLC film, the light intensity was about the same as that of the bare OLED. This film had no light enhancement, worse than that of the 10 µm PDLC film cured without voltage. For the 25 µm thick PDLC film, the light intensity was higher than that of the bare OLED. The total light output was 3.37 units and the relative enhancement was 13.9%, slightly higher than that of the 25 µm PDLC film cured without voltage. For the 50 µm thick PDLC film, the light intensity was the highest. The total light output was 3.75 units and the relative enhancement was 26.6%, much better than that of the 50 µm PDLC film cured without voltage. Note that although the 10 μm PDLC film had higher transmittance for normal incident light (with small incident angles), it did not enhance the light efficiency of the OLED, because it had weak scattering for incident light with large incident angles. The best PDLC film for light enhancement should not scatter light with small incident angles but strongly scatters light with large incident angles.

4. Discussion and Conclusion

We have achieved light outcoupling efficiency enhancement of 26.6%. Further improvements may be possible by increasing the degree of alignment of liquid crystal droplets along the film normal direction of the PDLC film. The transmittance at normal incidence is less than unity, as shown in Fig. 5, indicating imperfect alignment. When the droplets are not perfectly aligned, the film will scatter light with small incident angle, which is not desirable. The liquid crystal droplets can be better aligned by creating an elongated droplet shape, applying higher voltages during the curing and using better polymer network inside the liquid crystal droplets.

When OLEDs are used in display applications, a circular polarizer is sometimes used to eliminate reflection of ambient light from interfaces and thus increase the contrast ratio of the displays. If the PDLC film is used to enhance light efficiency, it will be sandwiched between the OLED and the circular polarizer. The circular polarizer will eliminate the reflection from the front PDLC-air interface. The refractive index of the polymer binder of the PDLC is around 1.5 and close to that of the OLED substrate, and the reflection from the OLED-PDLC interface is negligible. If there is reflection of ambient light from the OLED bottom substrate, the PDLC might partially polarize the reflected light and then cause some light leakage. The polarizing effect of the PDLC is not clear at this moment and will be studied in the near future. When OLEDs are used for solid state lighting, there is no need of a circular polarizer and the PDLC film will not cause any problem in this regard.

The scattering of the PDLC film depends on the following factors: the refractive indices of the liquid crystal and the polymer binder, the liquid crystal droplet size and the film thickness. For an incident light, the encountered refractive index in the polymer binder is n_p, while inside the liquid crystal droplet, the encountered refractive index n_eff is determined by the ordinary and extraordinary refractive indices of the liquid crystal and the propagation direction of the light with respect to the liquid crystal director. The scattering effect is proportional to (n_p-n_eff)² [15, 16, 29–31]. Regarding the droplet size, it should be close to the wavelength of the incident light in order to have strong scattering. The scattering increases with the film thickness, but the quantitative relation between the light enhancement and the film thickness is not clear at this moment. Further research is needed.

In summary, we demonstrated that a polymer dispersed liquid crystal (PDLC) layer can be used to enhance the light outcoupling efficiency of OLEDs, because it scatters light emitted from the OLED into directions with small incident angles and thus reduces the light loss due to total internal reflection. We also demonstrated that by aligning the liquid crystal droplets in the PDLC film along the film normal direction, the film can be tailored to selectively scatter emitted light with large incident angles but not light with small incident angles. This can further enhance the light efficiency.

5. Acknowledgments

This work was partially supported by Universal Display Corporation.

References and links

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PDLC film thickness	curing voltage /measuring voltage	total optical power	enhancement
0 µm (Pure OLED)	0 V/0V	2.96
10 µm PDLC	0 V/0 V	3.23	9.2%
10 µm PDLC	30 V/0 V	3.04	2.8%
25 µm PDLC	0 V/0 V	3.35	13.1%
25 µm PDLC	63 V/0 V	3.37	13.9%
50 µm PDLC	0 V/0 V	3.45	16.5%
	0 V/ 100 V	3.63	22.3%
	100 V/0 V	3.60	21.6%

Selective scattering polymer dispersed liquid crystal film for light enhancement of organic light emitting diode

Abstract

1. Introduction

2. Principle

3. Experiment and results

4. Discussion and Conclusion

5. Acknowledgments

References and links

Cited By

Figures (4)

Tables (1)

Equations (5)

Optics Express