A novel randomly textured phosphor structure for highly efficient white light-emitting diodes
© Chen et al; licensee Springer. 2012
Received: 17 November 2011
Accepted: 16 March 2012
Published: 16 March 2012
We have successfully demonstrated the enhanced luminous flux and lumen efficiency in white light-emitting diodes by the randomly textured phosphor structure. The textured phosphor structure was fabricated by a simple imprinting technique, which does not need an expensive dry-etching machine or a complex patterned definition. The textured phosphor structure increases luminous flux by 5.4% and 2.5% at a driving current of 120 mA, compared with the flat phosphor and half-spherical lens structures, respectively. The increment was due to the scattering of textured surface and also the phosphor particles, leading to the enhancement of utilization efficiency of blue light. Furthermore, the textured phosphor structure has a larger view angle at the full width at half maximum (87°) than the reference LEDs.
Keywordswhite light-emitting diodes imprinting phosphor texture package
In recent years, white light-emitting diodes (LEDs) have become important sources of illumination because of their high brightness, reliability, low power consumption, and long lifetime compared to conventional lighting sources . By far, white LED composed of GaN-based chip and yellow phosphor is the most promising and efficient method . The principle of phosphor-converted white LEDs was employing a short-wavelength LED to excite the wavelength conversion phosphor and down-convert high-energy photons to longer wavelength ones, and the combined photons create a perceived white spectrum . The advantages of this method are low cost, simple fabrication, and high conversion efficiency, comparing to individual red, green, and blue LEDs mixing and UV-LEDs exciting red, green, and blue phosphors .
However, for high-power application, there are still some imperfections left to be optimized and improved, such as the limited extracted power due to lead-frame package, silicone encapsulant, and the backscattered light. The backscattered light is not only unusable but also leads to reliability issues. Therefore, some methods such as scattered photon extraction package , remote phosphor package [6–8], and ring remote phosphor structure  are developed to better utilize the emitted photons and the design of optical lens. Due to a large difference in the refractive index between GaN and air, most of the light emission is trapped internally in LEDs. Therefore, the roughening or nano-texturing of surface was frequently adopted in GaN-based blue LEDs to increase extraction efficiency [10–12]. This method can also excellently increase the amount of blue light to excite the yellow phosphor in white LEDs, but the blue and yellow light still experience total internal reflection at the interface of air and silicone glue, which leads to the limitation of lumen efficiency in white LEDs. On the other hand, high light extraction efficiency could be attained by the design with a half-spherical lens, but the cost of the mold machine is high, and application is limited due to the large volume of the lens.
In this work, we further improved the luminous flux and lumen efficiency of white LEDs by a simple imprinting technique to form the textured phosphor structure, denoted as LED III. The LEDs with flat phosphor and half-spherical lens structures were denoted as LEDs I and II. With this textured surface, the total internal reflection at the interface of air and silicone could be reduced. Furthermore, this surface increases the probability of blue light scattering, which is supported by haze measurement results. Accordingly, more blue light can excite phosphor again due to the enhancement of utilization efficiency of blue light. Meanwhile, more yellow light can be extracted by the textured phosphor structure. Additionally, we further investigate the far-field emission pattern for LED I, LED II, and LED III, respectively.
Results and discussion
According to the above equation (Equation 2), the total absorption of the flat and textured phosphor structures can be calculated, which were 24% and 30% at the wavelength of 450 nm. The total enhancement of absorption can reach 25%, which leads to more blue light trapped in the textured phosphor structure. Hence, more yellow light could be produced due to this recycling of photons. Therefore, we further measured the angular-dependent relative intensity of LED I and LED III at a driving current of 120 mA, as shown in Figure 3b. It was clear that the intensity of blue light in the textured phosphor structure is lower than in the flat phosphor structure, which could be attributed to the increased blue light scattering and re-absorption. The result indicates the enhancement of utilization efficiency in blue light by the textured phosphor structure, and more yellow light can be extracted. Therefore, LED III has the higher relative intensity of yellow light than LED I.
In summary, we successfully proposed a simple method to use imprinting technique with a wet-etched c-Si substrate to fabricate the textured phosphor structure. The haze intensity of the textured phosphor structure shows 78% enhancement, compared with the flat phosphor structure. With the textured phosphor structure, the total internal reflection at the interface of air and silicone could be reduced. Furthermore, this surface increases the probability of blue light scattering, which is supported by the haze measurement results. Accordingly, utilization efficiency of blue light is greatly enhanced. Meanwhile, more yellow light can be extracted by the textured phosphor structure. As a result, the enhancement of the luminous flux was 5.4% and 2.5% higher than in the flat phosphor and lens molding packages at a driving current of 120 mA, respectively. The advantages of the textured phosphor structure are more compact package, higher lumen output, and larger view angle, which could provide flexible designs to various applications.
atomic force microscopy
crystalline silicon: KOH: potassium hydroxide
- LED I:
flat phosphor package
- LED II:
lens molding package
- LED III:
textured phosphor package
scanning electron microscope
The authors would like to thank Kismart Corporation and Helio Opto for their technical support. This work was funded by the National Science Council in Taiwan under grant numbers NSC100-3113-E-009-001-CC2 and NSC-99-2120-M-009-007. Finally, the authors would like to gratefully acknowledge Prof. Ching Cherng Sun at National Central University for his fruitful suggestions.
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