Efficient white light emission from a single silicate host with promising color quality for white light emitting diodes

Abstract

Strontium silicate phosphors were synthesized via a solid-state route with different silicon sources as SiO₂ and Si₃N₄. The phosphors synthesized with Si₃N₄ exhibited improved luminescence properties for white emission because of the coexistence of Sr₃MgSi₂O₈ and Sr₂SiO₄ phases. Under UV excitation, white light was observed due to the combination of blue emission from Sr₃MgSi₂O₈: Eu²⁺ and yellow emission from Sr₂SiO₄: Eu²⁺. The broad Eu²⁺ emission was explained on the basis of the dependency of crystal field strength on the site selection. The variation of Eu²⁺ ion concentration controlled the quality of the emitted light and at 5 mol% Eu²⁺, near white emission (0.30, 0.31) was obtained with external and internal quantum efficiencies of 50% and 68%, respectively. The WLEDs were fabricated by coating the present phosphors on an InGaN UV- LED chips and the EL spectrum displayed near white chromaticity (0.31, 0.33) with color rendering index of 80 and CCT of 6850 K. The results demonstrated the promising single host phosphors for near-UV white-light-emitting diodes.

Introduction

White-light emitting diodes (WLEDs) have received much attention in recent years since WLEDs with a long lifetime, energy-saving and environmentally benign properties can replace traditional incandescent and fluorescent lamps [1], [2], [3]. WLEDs are mostly fabricated from the combination of yellow-emitting YAG: Ce³⁺ phosphors and blue LED chips [4]. The commercial WLEDs with poor CRI (R_a=75) and high CCT (>8000 K) restrict the application in residential lighting [4]. Therefore, a new strategy has been developed by combining an InGaN LED, yellow-emitting phosphors with broad spectra, and red/ orange emitting phosphors to achieve warm white light with compatible CRI and CCT values [5].

The commercial approaches are complicated as different phosphors make the color balance difficult to control [6]. Hence, the research on the white light emission from a single phosphor has drawn more attention. The white-light emitting phosphors can be developed based on energy transfer between co-dopant pairs [7], [8], [9], [10] and Mn²⁺ ions are commonly used as co-activators. However, feeble Mn²⁺ d-d transitions may lead to the quenching paths and restrain the quantum yield (QY) and thermal stability [11]. Therefore, white-light emitting phosphors doped with a single activator ion are exceedingly favored for WLED applications to suppress the energy loss during energy transfer. Dai et al. [12] discussed a facile solid solution technique to develop a single activator doped phosphors with variable compositions to explore white-light. Various novel phosphors with promising luminescence, color tunability, and luminescence efficacy are developed with this strategy [13], [14], [15]. However, the design of the solid solution for the synthesis of the phosphors is complex. Hence, a new strategy is developed with alkali silicate materials for the synthesis of white light emitting phosphors in the present research work.

The alkali silicate materials draw significant attention owing to the promising thermal and chemical stability [16]. Several research works have been reported on alkali silicates doped with Eu²⁺ ions with tunable color emission including Li₂SrSiO₄ (580 nm), Ba₉Sc₂Si₆O₂₄ (500–520 nm), Ca₃Si₂O₇ (600 nm) and Ba₂MgSi₂O₇ (505 nm) [16], [17], [18], [19]. A thorough investigation of Eu²⁺ doped M₃MgSi₂O₈ (M= Ca, Sr, Ba) system was discussed by Blasse et al. and the reported results indicated the suitability of the phosphors as a UV- pumped blue LEDs [20]. However, the investigation on the silicates phosphors doped with a single activator to emit white light is rare.

In the present research work, Eu²⁺ ion activated strontium silicate phosphors were synthesized to achieve white light with various silicon sources for the first time. The broad white light emission from a single host was revealed to the co-existence of variable phases and corresponding crystal field strength. The WLEDs were fabricated by encapsulating the phosphors with InGaN UV- LED chips. The obtained results demonstrated a promising approach for developing novel white-light emitting phosphors with high CRI and suitable CCT values.