Like fluorescent lighting, solid state lighting uses phosphors to realize the desired color output. Commercial white solid state lighting typically uses gallium nitride (GaN) semiconductor-based blue LEDs operating at wavelengths of 450–470nm. That blue light excites a yellow-emitting phosphor powder, such as cerium-doped yttrium aluminum garnet (YAG:Ce), which is blended or encapsulated with epoxy on top of the LED die (see Figure 1).1–7 This process results in some of the blue LED light being converted to a wavelength of about 560nm. The yellow light stimulates the red and green receptors of the eyes, and the resulting mix of blue and yellow gives the appearance of white light. This white light, however, typically has a blue tint, which is referred to as cold light. Increasing the phosphor-emitted light's wavelength so that it is closer to red can reduce or eliminate the blue tint, producing a warm light. However, while the YAG:Ce is very efficient in blue-to-yellow light conversion, the newer red phosphor blends reduce the light output's brightness and efficiency.8, 9 In addition, and more important, the light output of and heat produced by the GaN LED die has been shown to degrade the epoxy/phosphor material, thus reducing the device's lifetime.10
Figure 1. A commercial white LED.
Replacing the epoxy with a promising new material, deoxyribonucleic acid (DNA), has shown the potential for enhancing the light output and efficiency, as well as redshifting the light emission to render brighter, more efficient, warmer solid state lighting with longer lifetimes.11
The idea came from our earlier work, where we blended the fluorescent dye, 4-[4-(dimethylamino)stylyl]-1-dococylpyridinium bromide (DMASDPB), in both a poly(methyl methacrylate) (PMMA) host and a DNA-cationic surfactant complex hexadecyltrimethylammonium chloride (CTMA) biopolymer host. The materials were optically pumped at a wavelength of 325nm.12, 13 The fluorescence of the DNA-based film measured 100 times higher than that of the PMMA-based film.12, 13 The maximum fluorescence for the PMMA- and DNA-based films was at wavelengths of 540nm and 580nm, respectively, or a 40nm redshift in the color of the DNA-based material.12, 13
The DNA we used for these studies was acquired from Ogata Research in Hokkaido, Japan.14–16 It was processed from salmon roe and milt sacs, a waste product of the Japanese fishing industry that is abundant and inexpensive. The DNA was initially soluble only in water, so it was first precipitated with the CTMA to make it water insoluble, but dissolvable in organic solvents.14–16 After dissolving the DNA-CTMA in butanol, we simply blended the YAG:Ce powder with the DNA-CTMA-butanol.
Figure 2 shows a 45μL drop-cast film of DNA-CTMA and EPO-TEK epoxy, both doped with 33% Merck Isiphor YAG:Ce phosphor and positioned over a blue Photon Micro-Light LED. We set a Sony model α100 camera at a speed of 1/160s and an aperture of f/5.6 to prevent saturating the camera's CCD. The light from the DNA-based film is both significantly brighter and whiter than that from the epoxy-based film. We are currently quantifying the differences in color and brightness, as well as the lifetimes, but these preliminary results are very encouraging. More detailed results will be presented at the 2012 SPIE Optics + Photonics, NanoScience + Engineering Symposium in August.17
Figure 2. Photographs of light emission from a blue LED passing through (a) poly(methyl methacrylate)-based film and (b) DNA-based film, each doped with 33% of a yellow-emitting phosphor.
I wish to acknowledge the Air Force Research Laboratory and the Edison Materials Technology Center for supporting this work. I would also like to thank Naoya Ogata for providing the DNA and Merck for providing the YAG:Ce phosphor.
US Air Force Research Laboratory (AFRL)
Wright-Patterson Air Force Base, OH
James Grote is a principal electronics research engineer with the AFRL's Materials and Manufacturing Directorate, where he conducts research in polymer- and biopolymer-based photonics. He is also an adjunct professor at the University of Dayton and the University of Cincinnati.
1. N. Narendran, L. Deng, Performance characteristics of LEDs, IESNA Annu. Conf. Tech. Papers, p. 157-164, Illuminating Eng. Soc. of North America, New York, 2002.
3. LEDs for General Illumination: An OIDA Technology Roadmap Update 2002 , Optoelectronics Industry Development Assoc., Washington, DC, 2002.
4. N. Narendran, Y. Gu, J. Freyssinier-Nova, Y. Zhu, Extracting phosphor-scattered photons to improve white LED efficiency, Physica Status Solidi (a) 202(6), p. R60-R62, 2005.
5. N. Taskar, R. Bhargava, J. Barone, V. Chhabra, V. Chabra, D. Dorman, A. Ekimov, S. Herko, B. Kulkarni, Quantum-confined, atom-based nanophosphors for solid state lighting, Proc. SPIE
5187, p. 133-141, 2004. doi:10.1117/12.515432
6. K. Yamada, Y. Imai, K. Ishii, Optical simulation of light source devices composed of blue LEDs and YAG phosphor, J. Light Visual Environ. 27(2), p. 70-74, 2003.
7. Lighting Research Center at Rensselaer Polytechnic Institute. Second year final report: high efficiency nitride-based solid state lighting. Presentation to US Dept. of Energy for sponsored project "High-efficiency, nitride-based solid state lighting." Cooperative Agreement No. DE-FC26-01NT41203, November 17, 2003.
8. A. Duggal, Phosphors for white light generation from UV emitting diodes, US Patent 6,294,800 B1, 2001.
9. U. Reeh, Light-radiating semiconductor component with a luminescence conversion element, US Patent 6,576,930 B2, 2003.
10. N. Narendran, Y. Gu, J. Freyssinier, H. Yu, L. Deng, Solid-state lighting: failure analysis of white LEDs, J. Crys. Growth 268(3-4), p. 449-456, 2004.
11. J. Grote, Light emitting diode with a deoxyribonucleic acid (DNA) biopolymer, US Patent 8,093,802 B1, 2012.
12. G. Zhang, H. Takahashi, L. Wang, J. Yoshida, S. Kobayahi, S. Horinouchi, N. Ogata, Nonlinear optical materials derived from biopolymer (DNA)-surfactant-azo dye complex, Proc. SPIE 4905, p. 375-380, 2002.
13. J. Grote, N. Ogata, J. Hagen, E. Heckman, P. Yaney, M. Stone, D. Diggs, DNA photonics, Molec. Crys. Liq. Crys. 426, p. 3-17, 2005.
14. L. Wang, J. Yoshida, N. Ogata, S. Sasaki, T. Kajiyama, Self-assembled supramolecular films derived from marine deoxyribonucleic acid-cationic surfactant complexes: large-scale preparation and optical and thermal properties, Chem. Mater. 13(4), p. 1273-1281, 2001.
15. G. Zhang, L. Wang, N. Ogata, Optical and optoelectronic materials derived from biopolymer deoxyribonucleic acid, Proc. SPIE
4580, p. 337-346, 2001. doi:10.1117/12.444982
16. E. Heckman, P. Yaney, J. Hagen, J. Grote, F. Hopkins, Processing techniques for DNA: a new biopolymer for photonics applications, Appl. Phys. Lett. 87, p. 211115, 2005.
17. J. G. Grote, Solid state lighting using deoxyribonucleic acid-phosphor blend, Proc. SPIE 8464, 2012. (Invited paper.)