Making white-light-emitting diodes without phosphors
Solid-state lighting technology has attracted attention recently because it offers the potential to save energy and protect the environment by producing light more efficiently. In particular, high-efficiency indium gallium nitride/gallium nitride (InGaN/GaN) quantum-well (QW) white-light-emitting diodes (LEDs) look promising. The major techniques for fabricating semiconductor-based white-light devices use phosphors to convert UV or blue LED output into longer-wavelength colors to mix to form white light. So far, the most widely used phosphor is cerium-doped yttrium aluminum garnet (YAG:Ce), which can effectively convert blue light at wavelengths around 460nm into yellow light near 560nm. The conversion quantum efficiency can be larger than 60%.1 Based on this technique, the efficiency of a white-light LED as high as 190lm/W has been reported.2 Because of the broad spectrum of YAG:Ce emission and the two-color nature of such an LED, however, the color-rendering index is not high, only in the range from 66 to 80. Recently, another technique that offers a higher color-rendering index has been widely implemented: output from a UV LED has been converted through three phosphors into blue, green, and red light, which are then mix to create white light. However, the conversion efficiencies of the phosphors—particularly the one that generates red light—are not as high as we would like.3 For this reason, technologies that avoid phosphors deserve intensive study.
Two alternative approaches for implementing white-light LEDs have been demonstrated. The first uses a novel technique for growing high-indium InGaN/GaN QW structures that allow efficient emission in the green-yellow band, or even in the orange-red range.4 If a high-quality yellow- or red-emitting QW LED can be grown, we could achieve white-light generation by stacking two QW layers that emit in the blue and yellow, or three layers emitting in the blue, green, and red.
The second approach uses nanocrystals (NCs) with a cadmium selenide (CdSe) core surrounded by a zinc sulfide (ZnS) shell that can convert short wavelengths to longer ones. Such crystals can effectively absorb UV-green light and emit green-red light.5 The absorption and emission spectra of these NCs can be tuned by controlling the diameter of the CdSe core and the thickness of the ZnS shell. Moreover, the NCs have the advantages of high quantum efficiency and photostability. In particular, mixing the NCs with gold nanoparticles induces coupling between CdSe/ZnS NCs and surface plasmons (SPs) on the gold that can enhance the color conversion efficiency.
The key to implementing white-light LEDs by stacking QWs of different indium contents for emitting different colors is a growth technique for increasing the indium incorporation without sacrificing crystal quality. Recently, we introduced a prestrained metallorganic chemical vapor deposition (MOCVD) growth technique.6 In this technique, a low-indium InGaN/GaN QW is grown before the designated light-emitting high-indium InGaN/GaN QWs to create tensile strain in the GaN barrier layer right above the low-indium QW. This makes the incorporation of indium during the growth of the subsequent QWs more effective. The increased indium content that results leads to efficient emissions of yellow, orange, and red colors.7,8 By stacking the yellow- and blue-emitting QWs on one chip, a white-light LED was realized with chromaticity coordinates of (0.334, 0.338) at an injection current of 50mA, which is close to the ideal condition.8 Also, the color temperature is 5600K at the same injection current, which is the value of sunlight at noon. Figure 1(a) shows the lit LED. Its output spectra at various injection-current levels are shown in Figure 1(b).
In using CdSe/ZnS NCs for color conversion, a white-light device was reported.9 However, an improvement of the conversion efficiency can lead to a higher overall device efficiency and a better white-light quality. The coupling of light-emitting NCs with SPs on metallic nanoparticles can enhance the color conversion efficiency. Recently, we demonstrated enhancing both the absorption and emission efficiency by coupling a light absorber or emitter with SPs on a nearby dielectric/metal interface.10,11 Basically, SPs can absorb incident light and transfer the energy into a light absorber to enhance its effective absorption.
A white-light device has been made by spin-coating the mixture of CdSe/ZnS NCs and gold nanoparticles on the top of a blue/green two-color InGaN/GaN QW LED. This device effectively converted blue and green emissions into red light. Localized SPs are induced on the gold nanoparticles to couple with the CdSe/ZnS NCs. The localized SPs can absorb green emission and effectively transfer the energy into the CdSe/ZnS NCs through the coupling process for enhancing red emission. This process increased the conversion efficiency from the blue/green range into red light by around 30%. The conversion quantum efficiency can reach 52.8%, which is higher than the reported value of phosphor for converting short-wavelength photons into red light. Figure 2(a) shows a lit LED under the fabrication conditions described above. The LED's output spectra at various injection-current levels are shown in Figure 2(b).
We have demonstrated that both the strained growth technique and SP interactions can generate efficient LEDs that produce broad-spectrum visible light. Enhancing the emission efficiency of these methods will require improving the crystal growth qualities of green-red emitting InGaN/GaN QWs. A better result in the green range will in turn increase the red conversion efficiency by CdSe/ZnS nanocrystals.
Chih-Chung Yang received his PhD degree in electrical engineering in 1984 from the University of Illinois, Urbana-Champaign. He served as a faculty member in the Department of Electrical Engineering at the Pennsylvania State University, during 1984-1993. In 1993, he joined the Institute of Photonics and Optoelectronics, National Taiwan University, and served as its chairman and director from 2001 to 2007. His research areas include MOCVD growth of nitride compounds and fabrication of white-light LEDs. He has published more than 600 journal and conference papers. He is a fellow of the Optical Society of America.
Dong-Ming Yeh is currently working on his PhD degree. His research areas include the use of surface plasmon coupling with QWs and nanoparticles to enhance the light emission efficiency of LEDs.
Chi-Feng Huang is currently working on his PhD degree. His research areas include prestrained MOCVD growth for fabricating phosphor-free white-light LEDs.
Chih-Feng Lu is currently working on his PhD degree. His research areas include the use of prestrained MOCVD-grown epitaxial structures for fabricating efficient LEDs.