Human-Centric Lighting: Rare-Earth-Free Photoluminescent Materials for Correlated Color Temperature Tunable White LEDs

Abstract

Artificial lighting is ubiquitous in modern society, with detrimental effects on sleep and health. The reason for this is that light is responsible not only for vision but also for non-visual functions, such as the regulation of the circadian system. To avoid circadian disruption, artificial lighting should be dynamic, changing throughout the day in a manner comparable to natural light in terms of both light intensity and associated color temperature. This is one of the main goals of human-centric lighting. Regarding the type of materials, the majority of white light-emitting diodes (WLEDs) make use of rare-earth photoluminescent materials; therefore, WLED development is at serious risk due to the explosive growth in demand for these materials and a monopoly on sources of supply. Photoluminescent organic compounds are a considerable and promising alternative. In this article, we present several WLEDs that were manufactured using a blue LED chip as the excitation source and two photoluminescent organic dyes (Coumarin 6 and Nile Red) embedded in flexible layers, which function as spectral converters in a multilayer remote phosphor arrangement. The correlated color temperature (CCT) values range from 2975 K to 6261 K, while light quality is preserved with chromatic reproduction index (CRI) values superior to 80. Our findings illustrate for the first time the enormous potential of organic materials for supporting human-centric lighting.

Keywords: circadian rhythms; human-centric lighting; photoluminescent organic materials; rare-earth-free materials; spectral conversion; tunable spectrum of WLEDs.

Figure 1

Spectral power distribution (peaking at 450 nm) of selected blue LED chip.

Figure 2

For WLED1, (a) spectral power distribution showing a high blue peak and (b) CRI values reaching a CRI (R_a) value of 55.6.

Figure 3

For WLED16, (a) spectral power distribution showing a high blue peak and (b) CRI values reaching a CRI (R_a) value of 80.3.

Figure 4

For WLED9, (a) spectral power distribution showing a medium blue peak and (b) CRI values reaching a CRI (R_a) value of 84.9.

Figure 5

For WLED15, (a) spectral power distribution showing a short blue peak and (b) CRI values reaching a CRI (R_a) value of 80.6.

Figure 6

(a) Photoluminescence excitation spectrum (peaking at 462 nm) and (b) photoluminescence emission spectrum (peaking at 511 nm) of Coumarin 6 green-emitting converter embedded in an EVA polymer.

Figure 7

Three-dimensional photoluminescence spectra of Coumarin 6 dye embedded in EVA polymer at excitation wavelengths ranging from 410 nm to 500 nm and emission wavelengths ranging from 450 nm to 750 nm.

Figure 8

(a) Photoluminescence excitation spectrum (peaking at 535 nm) and (b) photoluminescence emission spectrum (peaking at 604 nm) of Nile Red red-emitting converter embedded in an EVA polymer.

Figure 9

Three-dimensional photoluminescence spectra of Nile Red dye embedded in EVA polymer at excitation wavelengths ranging from 410 nm to 740 nm and emission wavelengths ranging from 500 nm to 800 nm.