Materials, Designs, Fabrications, and Applications of Organic Electronic DevicesView this Special Issue
Enabling Lambertian-Like Warm White Organic Light-Emitting Diodes with a Yellow Phosphor Embedded Flexible Film
We demonstrate in this report a new constructive method of fabricating white organic light-emitting devices (OLEDs) with a flexible plastic film embedded with yellow phosphor. The flexible film is composed of polydimethylsiloxane (PDMS) and fabricated by using spin coating followed by peeling technology. From the results, the resultant electroluminescent spectrum shows the white OLED to have chromatic coordinates of 0.38 and 0.54 and correlated color temperature of 4200 K. The warm white OLED exhibits the yield of 10.3 cd/A and the luminous power efficiency of 5.4 lm/W at a luminance of 1000 cd/m2. A desirable Lambertian-like far-field pattern is detected from the white OLEDs with the yellow phosphor containing PDMS film. This method is simple, reproducible, and cost-effective, proving to be a highly feasible approach to realize white OLED.
In recently years, there have been great demands to replace the traditional lighting sources with more energy efficient ones. White organic light-emitting devices (OLEDs) are one of the most promising technologies for display and lighting applications owing to their advantages of low power consumption, large viewing angle, high contrast ratio, and short response time. Thus, the OLED has been considered as a possible solution for the next generation of light sources [1–3]. To meet the required standard, the electroluminescence (EL) of OLEDs should contain broad emissions that cover the entire visible range of the spectrum with Commission International de l’Eclairage (CIE) chromaticity coordinates of (0.33, 0.33).
Many methods have been developed to generate the white light emission of OLED by the partial energy transfer, such as host materials that are doped with fluorescence/phosphorescence dyes [4–13]. Several researchers have developed the mixing of the EL from the host molecules with the excimer/exciplex emissions to yield white light emission [14–16]. Other methods are to stack red-, green-, and blue-emissive components to fabricate OLEDs. By stacking RGB components and charge generation layer (CGL), a tandem structure could demonstrate good luminous efficiency [17–22]. However, the tandem structure has a major challenge of carrier transport in the junction interface and the multiple-stacking structure is complex.
On the other hand, the white light LEDs combined blue GaN chip and phosphors are quite common  and commercially available these days. This technology is mature and applied widely in many fields. In this study, a novel method to combine the feature of OLED and phosphor-based technology is proposed and demonstrated.
The fabrication of the phosphor embedded PDMS film of white OLED is easy to correlate the ordinary process of white OLED. A simplest white OLED consists of two kinds of emissive layer which can emit the complementary color light: blue and yellow. Each layer of organic materials was subject to temperature-gradient sublimation under high vacuum before use. The organic layer and metal layers were deposited by vacuum vapor deposition in a vacuum chamber. Fabricating white OLED is complicated due to the modification of multi-emissive layers. The PDMS film is also easy to fabricate and has the ability to resist moisture and heat resistance. The large-area and cost-effective phosphor-embedded PDMS film is utilized to attach with regular OLED cell to provide a new direction for light sources. In the following test, the fabrication steps and results will be discussed in detail.
2. Materials and Fabrication Process
The blue fluorescence of the bottom-emitting OLEDs without PDMS film was fabricated as a reference. Each layer of organic materials was subject to temperature-gradient sublimation under high vacuum before use. OLEDs were fabricated on the ITO-coated glass substrates between the transparent bottom indium-tin-oxide (ITO) anode and the top metal cathode. The organic and metal layers were deposited by vacuum vapor deposition in a vacuum chamber with a base pressure of <106 torr. The deposition system permitted the fabrication of the complete device structure in a single vacuum pump-down without breaking vacuum. The deposition rate of organic layers was kept at ~0.1 nm/s. The active area of the device is cm2, which is defined by the shadow mask for cathode deposition. After fabricating the OLED devices, the method of glass-to-glass OLED package with desiccant was used in a glove box. Before packing, the glass would be pretreated by solvent cleaning and UV-curing in order to remove the moisture and particle, increasing the adhesion between the glass lid and substrate, simultaneously. Then we used the packing machine to drop the UV glue on the substrate and made the glass lid and substrate closer by exerting forward force. Finally, the device was treated with UV curing of 110 seconds.
The OLED device structures were glass substrate/ITO (150 nm)/HATCN (10 nm)/TAPC (30 nm)/mcbp: FIrpic (10 wt.%, 20 nm)/BmByPB (50 nm)/LiF (1 nm)/Al (100 nm)/. Indium-tin-oxide (ITO) was used as the bottom anode, Dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HATCN) as the hole-injection layer, 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine](TAPC) as the hole-transport layer, 3,5-di(9H-carbazol-9-yl)biphenyl(mcbp): bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III)(FIrpic) as the emitting layer, 1,3-bis[3,5-di (pyridin-3-yl)phenyl]benzene(BmByPB) as the electron-transport layer, thin LiF as the electron injection layer, and Al as the top cathode. Illustrations of the reference and white light OLEDs structures are shown in Figures 1(a) and 1(b), respectively.
The flexible PDMS film was fabricated using the spin coating and peeling technology as the following steps . First, yellow phosphor (Y3Al5O12:Ce3+) with the particle size of 10 μm is blended with the silicone to form the phosphor-suspension slurry. Then the solution is dropped and spin coated for 500 rpm for 30 s on the glass substrate, and then baked it in an oven at about 80°C. Finally, the PDMS film is peeled off from the glass substrate then put the film on the OLED’s glass substrate. Figure 2 shows the magnified image of the PDMS film from an angle-view. The size distribution of yellow phosphor embedded in PDMS film can also been seen in Figure 2, in which yellow phosphor has a size of 10 μm. The thickness of the PDMS layer is measured approximately 220 μm.
3. Results and Discussion
To characterize the light emission properties of white organic light-emitting devices, a UV/VIS spectrometer was used and two OLEDs devices (samples A and B) were prepared. Sample B is the organic light-emitting diode with a yellow phosphor embedded PDMS film and Sample A has an identical OLEDs device without flexible PDMS film. Figure 3(a) shows the electroluminescent (EL) measured spectra from OLEDs devices of Sample A (black curve) and Sample B (red curve) in the normal direction. The wide spectral of the Sample B becomes broad and the emitting wavelength has a red shift that can be seen as energy transfer between the blue OLED and phosphor.
As expected, the emission peaks of Sample A are 470 nm and 500 nm. The emission peak of Sample B is 525 nm, and the spectrum has the visible range from 400 nm to 750 nm due to color mixing.
The luminance current efficiency and luminous power efficiency as a function of current density of the OLEDs device were investigated. Figures 3(b), 3(c), and 3(d) show the results of Sample A and Sample B measured in the normal direction. As presented, Sample A has high luminance and maximum luminous efficiency over 5,000 cd/m2 and 16.5 cd/A. Sample B has a yield of 10.3 cd/A at a luminance of 1000 cd/m2. The maximum luminous power efficiencies of Sample A and Sample B are 9.4 lm/W with the current density of 7.9 mA/cm2 and 5.4 lm/W with the current density of 9.9 mA/cm2, respectively. The phosphoresce emission OLED containing FIrpic has the lifetime of 200 hr . The lifetime of both Sample A and Sample B is the same.
The measured far-field emission profiles of Sample A and Sample B are shown in Figure 4. As shown in Figure 4, Sample A has the Lambertian-like radiation pattern at the full-width at half-maximum of 30° along the surface normal view. A desirable Lambertian-like far-field pattern is also observed from Sample B due to the randomness particles of phosphor embedded flexible PDMS film. The CIE coordinates of EL at normal direction from Sample A and Sample B are at (0.19, 0.39) and (0.38, 0.54) with correlated color temperature of 4200 K, shown in Figure 5(a). The CIE coordinates of EL from sample A was obviously shifted and, hence, the corresponding emission color is changed.
Figures 5(b) and 5(c) show the photography of OLEDs devices during measurement. Blue light and warm white light regions are corresponding to Sample A and Sample B. The combination of OLEDs with a yellow phosphor embedded flexible film possesses many possibilities for in solid state lighting.
In summary, a warm white organic light-emitting diode was demonstrated by attaching a yellow phosphor embedded flexible PDMS film. The flexible PDMS film was fabricated using the spin coating and peeling technology. From the measured results, the warm white organic light-emitting diode has an efficiency of 10.3 cd/A, luminous power efficiency of 5.4 lm/W, and the CIE coordinate of (0.38, 0.54) with correlated color temperature of 4200 K at a luminance of 1000 cd/m2. We believe that this approach of the fabricating method offers a viable solution to realize that white OLEDs and Lambertian-like profile shaping are suitable for applications in lighting and displays.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to gratefully acknowledge the Bureau of Energy, Ministry of Economic Affairs of Taiwan, for financially supporting this research.
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