7 August 2012
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
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