This study introduces an efficient polarized, white phosphor-converted, light-emitting diode (pc-LED) using a remote phosphor film sandwiched between a reflective polarizer film (RPF) and a short-wavelength pass dichroic filter (SPDF). The on-axis brightness of polarized white light emission of a RPF/SPDF-sandwiched phosphor film over a blue LED, showed greater recovery than that of a conventional unpolarized remote phosphor film over blue LED, due to the recycling effect of yellow light from an SPDF. The relative luminous efficacy of an RPF/SPDF-sandwiched phosphor film was made 1.40 times better by adding an SPDF on the backside of an RPF-capped phosphor film. A polarization ratio of 0.84 was demonstrated for a white LED with an RPF/SPDF-sandwiched phosphor film, in good agreement with the measured results from the RPF-only sample.
© 2013 OSA
Use of solid-state lighting based on light-emitting diodes (LEDs) is expanding rapidly. The advantages of white-light sources based on phosphor-converted LEDs (pc-LEDs) include their high brightness, long durability, good color rendition, low power consumption, mercury-free composition and ease of manufacture. These advantages have led to the introduction of white pc-LEDs into a wide range of traditional lighting applications, including LCD backlighting, outdoor lighting, indoor lighting, and automobile lighting. However, the greatest impact of white light based on pc-LEDs will likely be in backlighting applications for liquid crystal displays (LCDs) that demand a polarized white-light source.
The radiation from native semiconductor or down-conversion LED is generally unpolarized, but the development of polarized LED was clearly needed in applications such as high-contrast imaging [1, 2], optical communications , and LCD backlighting . For example, a liquid crystal display (LCD) has low energy efficiency because the LCD backlight has to go through various optical sheets, a liquid crystal panel, and color filters, which result in a large energy loss. To overcome the problems of the large energy loss in LCDs, the CCFL backlight is changed to LEDs by reducing the energy consumption, and the optical sheets are added and/or eliminated by enhancing the efficiency of transmitted light from the backlight. If the backlight unit has polarized light, the LCD panel can reduce the optical sheets of a polarizer, which absorb 29.3% of the light efficiency. So, the LCD backlighting system needs a polarized backlight to get high-contrast imaging, to reduce energy consumption, and to enhance energy conversion efficiency. The key factors for the performance of the polarized light emission devices are the mechanism, structure, and fabrication process of a polarized LED to realize polarized light emission. Therefore, some groups have recently reported that polarized light emission has been obtained from blue LED structures grown on nonpolar [5, 6] or semipolar GaN substrates . Other groups have investigated different approaches. Some have used special reflector designs including a polarization selective encapsulation , or a polarization-enhancing reflector , and photonic crystals  to get polarized light emission from widely used white or colored LED structures. Quite recently, a wire grid polarizer (WGP) has been introduced on the emitting surface of the LED die [11, 12]. This reflective polarizer, WGP, transmits one component of the wave, with polarization perpendicular to the grating lines, and reflects the parallel polarization component. The light reflected by the bottom side of the WGP has another chance to be recycled from the bottom reflector of a GaN chip, if it is depolarized inside by diffusion and scattering. This combined technique of reflective polarizer and bottom reflector has practical uses in the backlighting system of LCDs, if the total polarized output of the LED is significantly higher than the output of a standard LED combined with a standard polarizer. The introduction of the above-mentioned polarized blue LEDs could open the way to polarized white-light emitters if their polarization is associated with polarization-preserving, down-converting phosphors.
For the development of polarized white phosphor-converted LEDs (pc-LEDs), it is necessary to extract the polarized light directly from the phosphors or the optically designed phosphor layer. Commercial white pc-LEDs use the partial down-conversion technique with yellow or green/red powder phosphors. It is well known that light emission from powdery phosphors is hard to polarize, owing to the unpolarized isotropic emission resulting from photoluminescence of micro-sized powders. It is difficult to preserve the polarization of emitted light of phosphors excited by the polarized blue light of an InGaN LED chip. Therefore, the use of novel optical designs is another possible way to polarize the white light from phosphor layer of pc-LEDs. In the present study, we introduce a facile and simple design for a powdery phosphor layer sandwiched between a reflective polarizer film (RPF) and a short-wavelength pass dichroic filter (SPDF) [13, 14] to polarize the emission and enhance the polarized emission from the phosphor layer of a white pc-LED, in the same way as an RPF employed in the backlighting system in LCDs [15, 16]. Meanwhile, the remote phosphor technology has attracted great attention for applications using white pc-LED, rather than conventional phosphor-in-package type LEDs. This is due to its uniform illumination property, freedom from binning and chromaticity variations, and excellent spectral stability over time . So, the RPF/SPDF-sandwiched remote phosphor film capped white LED can be used in polarized light emission sources of LCD backlighting systems by simply capping the RPF on SPDF inserted remote phosphor films to reduce the polarizer and to enhance the energy efficiency. Compared to previous polarized LED methods, such as using nanostructure and WGP, this new polarized white LED method in this study can realize similar or enhanced optical properties, reduce the price of fabrication, and allow easy manufacturing by capping the RPF on SPDF-inserted phosphor film.
Figure 1(a) shows the schematic diagram of an actual lighting fixture featuring remote-phosphor technology. It typically consists of an LED board containing blue LED chips, a mixing chamber, and a remote-phosphor film. To attain polarized phosphor emission, the RPF and SPDF layers are simply introduced at top and bottom of the remote phosphor layer, respectively, as shown in Fig. 1(b). The mixed light of parallel-polarized reflective light from the RPF polarizer and the backward light from the phosphor layer are reflected and depolarized together by the SPDF. This SPDF typically exhibits continuous rotational symmetry and simply takes the light emitted in different polarization directions, and reflects them forward. When two orthogonally polarized lights of equal intensity are combined by near the surface of an SPDF, the result is depolarized light. Therefore, we briefly analyzed the optical properties and performance levels of polarized white light created by an RPF- and SPDF-sandwiched remote phosphor layer over a blue LED, and compared the results to those of white light emitted from a traditional white pc-LED having a remote-type phosphor layer. This straightforward and facile concept for making polarized white LEDs using RPF/phosphor/SPDF layers could lead to further research and developments related to the new application of polarized white LED lighting in LCD backlighting systems.
2. Experimental methods
Fabrication of short-wavelength pass dichroic filter (SPDF): For design of the SPDF multilayer films, the characteristic matrix method was used to simulate the reflectance (R), transmittance (T), and absorption (A) [13, 14]. Dielectric SPDF was fabricated on a glass substrate of 0.7 mm thickness. For fabrication of the stacks, terminal eighth-wave-thick SiO2 and quarter-wave-thick TiO2 nano-multilayered films ((0.5SiO2/TiO2/0.5SiO2)9, SPDF) were coated onto glass substrates by e-beam evaporation at 250°C. The thicknesses of the 0.5SiO2 and TiO2 layers are 56 nm and 73 nm, respectively. SPDF was fabricated as a yellow light-recycling filter for enhance the luminous flux by reflecting backward yellow emission to forward. The blue line in Fig. 2 shows the transmittance spectrum of SPDF in the normal direction.
Fabrication of YAG yellow phosphor film: The Y3Al5O12:Ce3+ (YAG) yellow phosphor was purchased from a phosphor company (Merck Co. Ltd.). For fabrication of the YAG yellow phosphor film, the optimum amount of phosphor was dispersed in a silicon binder (Shin-Etsu Chemical Co., Ltd. KER-2500A, KER-2500B) to form a yellow phosphor paste. The yellow phosphor paste was printed on a glass or a SPDF with a 200 μm spacer. After printing the YAG phosphor paste, it was dried and hardened at 150°C for 1 hour. The black line in Fig. 2 shows the normalized emission spectrum of the yellow phosphor film on a blue LED in the normal direction.
Characterization of white pc-LEDs: For realization of white light, a blue LED chip (λmax = 450 nm, 14 lm/W at 350 mA) was used as a blue light source and an excitation source for a yellow phosphor film with an applied current of 350 mA. The blue LED chips were purchased from Dongbu LED, Inc. In this experiment, the room temperature electroluminescence (EL) was measured in the normal direction for characterization of conventional, RPF-capped, SPDF-inserted and RPF/SPDF-sandwiched remote phosphor layers (see Fig. 3). The yellow phosphor film, coated on glass or SPDF, was put on the blue LED with a 0.7mm spacer. The RPF (3M, DEBF-D400-DS) was located directly on the yellow phosphor film and a polarizer was located on the RPF or yellow phosphor film with a 2 mm spacer. The red line in Fig. 2 shows the transmittance spectrum of RPF in the normal direction. The EL intensity with angular orientation of the polarizer was measured as a function of the polarizer-rotating angle from −90° to 90° at 10° intervals. All measurement systems are measured in the normal direction with a charge coupled device (CCD) (PSI Co. Ltd.). The luminous flux, 1931 CIE color coordinates, and correlated color temperature (CCT) were measured and calculated using the Darsapro-5000 program (PSI Co. Ltd.).
3. Results and discussion
Figure 3 shows the schematic diagrams and photographs of four pc-LED configurations with differently designed remote phosphor layers for comparison: conventional, RPF-capped, SPDF-inserted and RPF/SPDF-sandwiched remote phosphor layer. The room temperature electroluminescent (EL) spectra of four phosphor layer-assisted pc-LEDs were measured in the normal direction at a forward current of 350 mA. EL spectra of conventional and RPF-capped remote phosphor layers show that an RPF reduces a little differently the intensity of both blue and yellow emission peaks of the phosphor layer coated on a glass substrate in a remote-type white pc-LED. The peak wavelength and shape of the blue and yellow emission spectra of the RPF-capped remote-phosphor LEDs are similar to those of the original LED. This result is consistent with the fact that the RPF was introduced to give significantly decreasing transmission efficiency within the whole visible range, but some fluctuations of transmission efficiency occur with increasing wavelength. Although the transmitted white light through an RPF is p-polarized light, the total forwarding efficiency is highly reduced compared to the unpolarized remote phosphor LED source (i.e., more than one third of the extracted light is lost). So, it is necessary to increase the polarized white emission from a white LED with an RPF-capped remote phosphor film.
As shown in Fig. 4(a), inserting a blue-passing and yellow-reflecting SPDF between the YAG:Ce phosphor layer and a blue LED cup increases the yellow spectrum from the phosphor layer without altering the blue spectrum and reduce the phosphor concentration to realize the same color temperature. This means that the conversion efficiency of SPDF-inserted remote phosphor device is higher than conventional remote phosphor device. As previously reported, an SPDF recycles and projects forward the backward yellow light in the backside architecture of an LED cup. In addition, almost no change of peak wavelength and shape of yellow emission spectrum, indicate that the SPDF is well designed for giving uniform and high reflectivity within the yellow wavelength range. EL spectra also show the increased yellow emission of the phosphor layer sandwiched between RPF and SPDF, compared to that of the RPF-capped remote phosphor film on an LED. This additional SPDF can recycle the reflected s-polarized yellow emission from an RPF-capped remote phosphor-type pc-LED, as well as the depolarized, backward-yellow emission from the phosphor layer. The relative intensity of yellow light is recovered at up to over 100% of the unpolarized conventional remote phosphor layer without changing the blue light. This is due to the additional recycling of yellow light by the SPDF, and the additional re-excitation of reflected blue light by the RPF. On the other hand, the shape of the yellow spectrum is a little changed from the unpolarized emission, owing to the concerted effect of the fluctuating reflectivity of both RPF and SPDF (See Fig. 4(b)).
Similar to the spectrum change of phosphor films, the relative luminous flux and the relative conversion efficacy show a similar trend after adding RFP and/or SPDF. The relative luminous flux and the relative conversion efficiency of white light from remote phosphor films on blue LEDs, are reduced by capping an RPF, but increased by inserting an SPDF. They can then be recovered to higher levels than that of unpolarized remote films by adding both RPF and SPDF, as shown in Fig. 5. The enhancement ratios of the luminous flux of the RPF-capped, SPDF-inserted, and RPF/SPDF-sandwiched phosphor layer on blue LEDs were 0.73, 1.40, and 1.39, respectively. The conversion efficiency of RPF/SPDF-sandwiched phosphor film reached 0.59, which exceeds that of the unpolarized, conventional remote-phosphor film (0.50). Therefore, the reduced effect of white emission from remote phosphor film by a reflective-type polarizer can be addressed by the use of a light recycling system that combines reflective-type polarizer, remote phosphor and blue-pass-yellow-mirror dichroic filter. The CIE color coordinates of conventional, RPF-capped, SPDF-inserted, and RPF/SPDF-sandwiched phosphor layer on blue LED shifted from bluish white to yellowish white owing to the increased ratio of yellow to blue in the emission spectrum (see Fig. 5(c)). Likewise, the correlated color temperature (CCT) was lowered to the level available to consumers by balancing the blue and yellow light of the remote phosphor layer by sandwiching between RPF and SPDF. These CCTs (Conventional: 111,110K, RPF-capped: 16,190K, SPDF-inserted: 10,110K, and RPF/SPDF-sandwiched: 4,860K) show a good agreement with the photograph of each white LED system (See Fig. 3 and Fig. 5(d)).
Figure 6 also shows that the CCT distributions of the four samples are decreased with an increasing viewing angle due to the change in the blue and yellow emission ratio. As the viewing angle increased, the light intensity of the blue LED rapidly decreased and the light intensity of the yellow phosphors slowly decreased owing to the different angular dependence between the yellow light from the phosphor layer and the blue light of the LED chip that passed through the phosphor layer. As a result, the ratio of blue LED light to yellow light decreases with an increase in the viewing angle and the CCT of white light decreases with an increase in the viewing angle. The conventional remote phosphor white LED starts the CCT of 111,110K at the normal direction and dramatically decreases the CCT as a function of the viewing angle because the emission spectrum has more blue emission, which rapidly changes with the viewing angle, than yellow. Otherwise, the RPF/SPDF-sandwiched remote phosphor white LED starts with a CCT of 4,860K in the normal direction and a little change in the CCT as a function of the viewing angle because the emission spectrum has more yellow emission, which slowly changes with the viewing angle, than blue.
Optically polarized characteristics of four different types of remote phosphor films on blue LEDs were observed by measuring the EL intensity as a function of the orientation angle of the linear polarizer placed between remote, phosphor-capped LEDs and the spectrometer (see Fig. 7(a)). The intensities of four samples were determined by measuring the central wavelength peak intensity of each spectrum taken under a different polarizer-rotating angle at 10° intervals. As shown in Fig. 7(b), the measured intensity of both RPF-capped and RPF/SPDF-sandwiched LEDs varies significantly with the polarizer rotation angle, revealing polarized light emission from the pc-LED. Otherwise the small variations in PL intensities of conventional and SPDF-assisted LEDs indicate unpolarized light emissions from LEDs. We confirmed that both LEDs without an RPF show completely unpolarized emission.
The effects of linear polarizers on the emissions from RPF-capped and RPF/SPDF-sandwiched remote phosphors, on blue LEDs were also compared. Figure 8 shows the EL spectra of RPF-capped and RPF/SPDF-sandwiched remote phosphors on blue LEDs, with and without a polarizer. The RPF-capped emission was reduced by 35.7%, while the loss for the RPF/SPDF-sandwiched emission was only 30.1%. The on-axis brightness of the RPF/SPDF-sandwiched phosphors was 2.05 and 1.90 times higher than that of an RPF-capped phosphor layer with and without a polarizer, respectively. Light intensities of both RPF-capped and RPF/SPDF-sandwiched phosphors were similarly reduced by the absorption of the polarization film.
Generally, the polarization ratio is defined as ρ = (Imax – Imin)/(Imax + Imin), where Imax and Imin are the maximum and minimum luminous intensities. This ratio indicates the degree of optical, linear polarization of the two orthogonal polarizations verify when the polarizer (analyzer) is rotated. The obtained polarization ratio (ρ = 0.84) of RPF/SPDF-sandwiched remote phosphor on LED is comparable with that (ρ = 0.87) of the RPF-capped remote phosphor on a LED. These results indicate that the insertion of SPDF increased the polarized light output of the remote phosphor layer but had a little effect on the polarization ratio. Therefore, the use of polarized white light from RPF/SPDF-sandwiched, remote-phosphor film on blue LED, could enhance the output efficiency of polarized, RPF-capped remote phosphor systems, thus potentially reducing power consumption of a polarized, white-lighting system.
We have developed a polarized, white pc-LED lighting system by sandwiching a remote phosphor layer between a reflective-type polarizer film (RPF) and a blue-pass-yellow-mirror dichroic filter (short-wavelength pass dichroic filter, SPDF). We measured the optical and polarizing properties of four different types of remote phosphor layers: conventional, RPF-capped, SPDF-inserted and RPF/SPDF-sandwiched remote phosphor layers over a blue LED for comparison. When an RPF is simply covered on the phosphor layer, it transmits p-polarized light, but s-polarized light is reflected back to the phosphor layer and the LED cup. The reflected, s-polarized, yellow light at the RPF, and the backward yellow emissions from the powder phosphors are recycled by adding an SPDF on the backside of an RPF-capped remote phosphor layer. As a result, the on-axis luminous efficacy of the RPF/SPDF-sandwiched phosphor was 1.90 times higher than that of the RPF-capped phosphor layer. The yellow light was recovered to over 100% of that of the unpolarized, conventional remote-phosphor layer without changing the blue light. A polarization ratio of 0.84 was experimentally demonstrated for a white pc-LED with a RPF/SPDF-sandwiched, remote-phosphor film. Although this polarization ratio is slightly lower than the expected value (0.87), it is close to the theoretical result based on use of the same RPF on a phosphor layer. The approach toward the creation of polarized white light demonstrated in this work has increased potential for allowing polarization control of white pc-LEDs using powdery phosphors, and for providing “smart” and efficient polarized lighting capabilities beyond those using unpolarized, remote-phosphors of traditional pc-LED lighting systems.
This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Korea government (MEST) (No. 2011-0017449).
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