Warm white LEDs based on Lumogen® Red and Yellow

One of the most widespread solutions for the production of white LEDs is the frequency downconversion of a part of the light, coming from a blue source, by exciting one or more materials (typically Ce:YAG) that emit at longer wavelength [1]. In this work we report ona simple and less expensive method to fabricate warmwhite-light LEDsusingthe photoluminescence of Lumogen®, a perylene-based polymer dyecommercialized by BASF,that has already beenprovedto be a good substitute for conventi onal inorganic colour conversion [2],[3]. Standard InGaN-based blue LEDs (~ 450 nm) were fabricated on a sapphire substrate by metal organic chemical vapour deposition. Both Lumogen® Yellow, and Red dyes weredissolved in a poly-methyl-methacrylate (PMMA) solution, which was in turn dissolved in ethyl acetate (16% w/w concentration). The bare side of the sapphire substrate was coated with the dye solution by dippingat different extraction speed,so that different coating thicknesses couldbe obtained. Dipping solutions were prepared by mixing the content of yellow and red dyes, with Lumogen® Yellow concentration fixed at0.5 mg/mL and Lumogen® Red in a concentration variable from 0.05 to 0.5 mg/mL. In the experiments here reported, five different solutions were used,each with the following couples of concentrations of yellow and red dyes in PMMA (mg/mL of Lumogen® Yellow; mg/mL of Lumogen® Red):solution YELL (0.5 ; 0), solution MIX1 (0.5 ; 0.05), solution MIX2 (0.5 ; 0.1), solution MIX3 (0.5 ; 0.2), solution MIX4 (0.5 ; 0.5). The wafer was three-times dipped, the first two times in the solution of Lumogen® Yellow (0.5 mg/mL), and finally in a sol ution of mixed dyes (Lumogen® Yellow and Lumogen® Red in different proportions). The second layer was rapidly coated above the first one, after thishad been completely dried, in order to minimize the partial dissolution of the first layer during the second dipping.The same procedure was carried out for the third layer.Four samples were prepared, the first and the second coating being prepared by dipping the wafer in thesolution YELL, the thirdone by dipping on different solutionsas following: Sample W1, dipped in MIX1; sample W 2, dipped in MIX2; sample W3, dipped in MIX3; sample W 4, dipped in MIX4. As shown both by the chromaticity diagram (Fig. 1), and the emission spectra (Fig. 2), with the increase of red dye concentration, the emitted white light (at 20 mA of driving current) became warmer and the chromatic coordinates moved awayfrom the blue region. The data reporting the correlated colour temperature and colour rendering index (CRI)are shown in Table 1. Withthe increaseof the red content, the correlated color temperaturedecreased while the CRI increased. In particular, sample W4 emitteda warm white light with a high CRI (87.2) and a low correlated color temperature (3912 K). The chromatic coordinates (x; y) were(0.381 ; 0.369), very close to the Planckian locus, as provedby the high value of CRI. This work showed that Lumogen® Yellow and Lumogen® Red can be advantageously used to set up a simple and cheap technique for fabricating warm white LEDs. In particular, warm white LED light with high values of CRI and low correlated color temperature was obtained and the results were reported.