Warm white LED light by frequency down-conversion of mixed perylene-based dyes

The growing demand of the solid-state lighting market for the development of sources for illumination has led to the fabrication of the first white LED in 1997, which employed a blue LED coated by a Ce:YAG phosphor to mix the down-converted yellow light with the blue one. The white light appears cold due to the weakness of red components in the emission spectrum. In order to obtain a warmer white, one possible solution is to add a red phosphor to the yellow one to move the chromatic coordinates properly, though the luminous efficiency drastically decreases due to the increased light absorption of the coating layer. It is generally believed that the low efficacy of warm white LEDs is the main issue today.Using photoluminescence of Lumogen® F Yellow 083 (BASF), a perylene-based polymer dye, we obtained a high efficiency cold white LED by generation of yellow down-conversion from a standard 450 nm GaN/InGaN blue LED with record values of 9.37 lm of luminous flux and 118.23 lm/W of luminous efficiency. The intense cold white light turned warmer, by adding a small quantity of another perylene-based dye, Lumogen® F Red 305 (BASF). Different weight proportions of dyes were dissolved in solutions with equal amounts of poly-methyl-methacrylate (PMMA) in ethyl acetate. Finally, the LEDs were dip-coated in each solution and optically characterised.Adding 2 mg of red dye to 10 mL of PMMA solution with 5 mg of yellow dye, the light turned into a purplish white with a dramatic decrease of the luminous efficiency (46.72 lm/W). Decreasing the amount of red dye to 0.5 mg, the white light appeared warm with a negligible decrease of the efficiency (116.11 lm/W) and a luminous flux of 8.03 lm