White light-emitting diodes based on quaternary Ag–In-Ga-S quantum dots and their influences on melatonin suppression index

Highlights

• We introduce an environmentally friendly Ag–In-Ga-S (AIGS) quantum dots (QDs).

• WLEDs based on red emission and green emission AIGS/ZnS QDs are fabricated.

• Melatonin suppression index of as fabricated WLEDs are calculated and measured.

• AIGS/ZnS QDs provides WLEDs with highly visual performances and tunable MSI.

Abstract

Light sources have effect on the circadian rhythms so as to the health of human. Therefore, both visual and non-visual effects should be considered when designing and fabricating light sources. Quantum dot (QD) materials have been extensively used as color converters to fabricate white light-emitting diodes (LEDs). However, most of the work on the photoluminescent QD-based LEDs (QLEDs) are focused on the improvement of visual performances. Here, quaternary Ag–In-Ga-S (AIGS)/ZnS QDs are synthesized for the application of white QLEDs taking both visual and non-visual performances into consideration. Theoretical calculation is carried out and the results show that QLEDs based on the combination of red emission QDs and green emission ones can exhibit a high color rendering index (CRI) of 90.8 and a color correlated temperature (CCT) of 5669 K, a melatonin suppression index (MSI) of 0.788. Furthermore, the combination of red 670 nm emission and green 550 nm AIGS/ZnS QDs are integrated with a blue-emission chip to fabricate QLEDs. The as fabricated devices exhibit a maximum CRI of 90.33, a CCT of 5947 K and an MSI of 0.7866, which are all consistent with the simulation results. These experimental results are comparable to those of some commercial white light sources. We discuss non-visual effects of QLEDs for the first time, and our research results show that AIGS/ZnS QDs, which are free-of heavy metal elements and have wide full-width of half maximums (FWHMs), are suitable for the fabrication white light sources with highly visual performances and tunable MSI values, which have many potential applications in the field of healthy lighting and environmentally friendly light sources.

Introduction

During the past several decades, light-emitting diodes (LEDs) have attracted extensive attention owing to its huge energy-saving potential. LEDs have advantages including high luminance, high efficiency, long lifetime, small size, low consumption, and fast response etc. Moreover, its programmability enables them to be combined with smart sensors easily, and show great potential applications to satisfy the demands of healthy illumination in the fields of artificial lighting and display, being significant for relieving the energy crisis and the environmental protection. Nowadays, artificial light sources including LEDs have been applied widely to the home lighting, the commercial lighting, the roadway lighting, the electronic display and so on, and people stay significantly more time in bright environments [[1], [2], [3]]. Many biological and medical research teams have reported that artificial lighting has influences on the circadian rhythm, the physiology, alertness, and cognitive level of humans [4]. Moreover, Improper light stimulus can lead to circadian disruption and disease risks [2,5,6].

For human eyes, cone and rod cells on the retina are responsible for visual effects, and intrinsically photosensitive retinal ganglion cells (ipRGCs) are for non-visual ones. The corresponding three functions are the photopic sensitivity action curve (V(λ)), the scotopic sensitivity action curve (V′(λ)) and circadian spectral sensitivity function (C(λ)). All these curves have their own maximum values at corresponding different wavelengths [[7], [8], [9]]. Therefore, for a light source, both the visual and the non-visual effects are related tightly to its light composition or spectral power distribution (SPD). Presently, white LEDs (WLEDs) are usually realized by integrating rare-earth phosphors with an LED chip emitting blue or near ultraviolet rays. However, unchangeable emission spectra restraining the adjustability of SPD and lack of red emission rare-earth phosphors leading to low color rendering indexes (CRIs) hinder the further development of phosphor-based WLEDs. Quantum dots (QDs) are new photoelectric materials with many unique properties such as tailorable bandgap, adjustable emission wavelength and high photoluminescence (PL) quantum yields (QYs) [[10], [11], [12]]. QDs have been used as emitters to assemble electroluminescent LEDs and photoluminescent ones of high performances, and considered as the most attractive candidate for the next generation of artificial light sources [[13], [14], [15]]. For the fabrication of WLEDs with pure QDs as emitters, combination of QDs of different emission wavelengths are usually employed. However, self-absorption among them leads to energy loss and result in performance degradation [16]. Co-doped QDs with dual-emission, such as Ag, Mn: Zn–In–S/ZnS, Ag, Mn:Zn-Ga-S etc. have been synthesized successfully and used as single emitters to assemble WLEDs. In these reported electroluminescent devices, the fluorescence quenching among the QDs mixtures is reduced substantially [[17], [18], [19]]. Besides, I-III-VI QDs usually have wide FWHMs and large stokes shifts, which are helpful to relieve the self-absorption so as to improve the device efficiency [16,20,21].

Up to now, most of the researches about WLEDs are focused on the visual effects, such as CRI, color corelated temperature (CCT), luminous efficacy of radiation (LER) etc. Since the non-visual effects of light sources have effects on the circadian rhythm so as to the health status of human body, it is necessary to study the luminescence spectra of photoluminescent QD-based LEDs (QLEDs) and their influence on the secretion and suppression of melatonin. When specifying the light stimulus, all of the light characteristics should be taken into consideration. In other words, quantity, spectrum, timing, duration and the response curve of the research object that related to the dose should be specified [5,9,22]. Herein this work, quaternary Ag–In-Ga-S (AIGS) QDs with emission wavelength covering the whole visible region are synthesized by using of a solution method. By using of a double Gaussian model, spectra of QLEDs based on the as synthesized QDs are calculated, as well as the visual and the non-visual performances. As far as the non-vision effect is concerned, our work focuses mainly on the effect of SPD on the melatonin suppression index (MSI). Based on the simulation results, WLEDs also fabricated and their performances are measured. The experimental results show similar varying trend with those of the theoretical simulation ones. QLEDs based on AIGS QDs exhibit good visual performance and adjustable MSI, indicating its potential application in health illumination.