In 1997, the first white light-emitting diode (LED) was fabricated using a structure based on gallium nitride (GaN) and the phosphorus cerium:yttrium aluminum garnet (Ce:YAG).1 While efficient and bright, this LED fabrication process used inorganic phosphorus materials that can be inconsistent and expensive. Alternatively, researchers from the University of Palermo (Palermo, Italy) and semiconductor foundry Novagan (Lausanne, Switzerland) have developed a simpler, less expensive method to fabricate a white-light LED using a source that acts as a pump for the photoluminescence of an organic substance—essentially, optical frequency downconversion using a perylene-based dye.2
Organic color conversion
The high-efficiency, cool-white LED light was obtained via yellow downconversion from a GaN/indium GaN (GaN/InGaN) blue LED. Rather than using a conventional inorganic color-conversion process, the blue LED was used to pump a Lumogen perylene-based dye from BASF (Charlotte, NC). The commercially available dye (or pigment) has an absorption maximum at 450 nm and a broad fluorescence peak around 500 nm.
First, standard InGaN-based blue LEDs were fabricated on a sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The peak emission wavelength of the LEDs was centered at 450 nm with chromatic coordinates (0.1477, 0.0338) in the CIE chromaticity diagram.
Lumogen dye was dissolved in a polymethylmethacrylate (PMMA) solution using ethyl acetate as a solvent, and different PMMA molecular weights were tested. The bare side of the sapphire substrate was coated with the dye solution by either spinning or dipping so that different coating thicknesses could be obtained. Emission spectra and chromatic coordinates were measured by a spectrometer and a calibrated photodiode with the LED being driven by a constant current (5, 10, or 20 mA).
Three solutions were prepared at different weight percentages—two at 8% (PMMA1 and PMMA2) and one at 11% (PMMA3)—in order to optimize the LED output parameters (see figure and table). Solutions PMMA1 and PMMA3 were prepared with a polymer of molecular weight 350,000, while PMMA2 used a polymer of molecular weight 996,000. Equal amounts of dye were used for each solution and the spinner velocity was 1200 rpm.