Frequency-Downconversion Stability of PMMA Coatings in Hybrid White Light-Emitting Diodes

We report on the properties of a poly(methyl methacrylate)-based coating usedas a host for an organic dye in hybrid white light-emitting diodes. The device iscomposed by a pump source, which is a standard inorganic GaN/InGaN bluelight-emitting diode (LED) emitting at around 450 nm, and a spin-coatedconversion layer making use of Lumogen F Yellow 083. Under prolongedirradiation, the coating exhibits significant bleaching, thus degrading thecolor rendering performance of the LED. We present experimental results thatconfirm that the local temperature rise of the operating diode does not affectthe conversion layer. It is also proven that, during the test, the photostabilityof the organic dye is compromised, resulting in a chromatic shift from Commission Internationale de l’Eclairage (CIE) (x;y) coordinates (0.30;0.39) towards the color of the pump (0.15;0.04). Besides photodegradation of the dye, we address a phenomenon attributed to modification of the polymer matrixactivated by the LED’s blue light energy as confirmed by ultraviolet–visibleand Fourier-transform infrared spectroscopic analyses. Three methods forimproving the overall stability of the organic coating are presented