Realization of warm white light emitting in single phase Gd(P_xV_1−x)O₄:y at% Sm³⁺,1 at% Bi³⁺ phosphor

Abstract

A series of single phase, warm white light emitting phosphors, Gd(P_xV_1–x)O₄:y at% Sm³⁺, with 1 at% Bi³⁺ doping concentration were synthesized by high temperature solid state method in this work. The experimental results indicate broadband cyan emission of Bi³⁺ and characteristic orange-red emission of Sm³⁺ can be effectively tuned by changing the ratios of PO₄³⁻/VO₄³⁻ in Gd(P_xV_1–x)O₄:1 at% Sm³⁺,1 at% Bi³⁺, and the energy transfer process among VO₄³⁻, Sm³⁺, Bi³⁺ also can be adjusted. Based on this, warm white light emitting can be realized by further optimizing the doping concentration of Sm³⁺ in the phosphors. At 423 K, the PL intensity of Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ remains ～84.3% of the initial value at 293 K, while the measured quantum efficiency is 67.8%. EL spectrum analysis results of the fabricated white light emitting diode (wLED) based on a 310 nm UV-chip and Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ phosphors imply low correlated color temperature (3132 K) and appropriate color-rending index (R_a = 82.7). These results demonstrate that Gd(P_0.7V_0.3)O₄:2 at% Sm³⁺,1 at% Bi³⁺ is a good candidate for manufacturing UV-activated warm white light emitting diodes.

Introduction

In recent decades, the shortage of fossil fuels and worsening of environmental pollution have become prominent global problems, as thus, white light emitting diodes (wLEDs) with overwhelming advantages of energy saving and environmental friendliness have been booming and attracting lots of attention.1, 2, 3 Nowadays, with the rapid development of LED chip and phosphors manufacturing techniques, phosphor-converted wLEDs are the mainstream strategies to fulfill white light emitting.^4,5 The most common commercial implementations of wLEDs are combining blue light chip with yellow phosphors YAG:Ce³⁺.^6,7 However, this technical solution for implementing wLEDs still has some drawbacks. The lack of red light component usually results in low color-rendering index (R_a < 75) and high correlated color temperature (CCT > 4500 K). In addition, blue light hazard and relatively low performance also limit the applications of blue chip-based wLEDs in some areas.^8,9 In order to achieve better color-rendering index and color temperature, an improved technique to realize wLEDs is combining ultraviolet (UV) chip with green, red, and blue phosphors.¹⁰ Nevertheless, this technical route also encountered restrictions in the application due to different luminescence stabilities and efficiency of the tricolor phosphors in the same UV chip condition.11, 12, 13 Moreover, the mixing of multiple phosphors will result in fabrication complexity and manufacturing cost increase.¹⁴ Therefore, an interesting strategy that can combine UV chip with single phase white light emitting phosphor, which can overcome the above disadvantages, must be considered.¹⁵

As the most important component of luminescence materials activator, rare earth ions or transition group ions are common choice. Among rare earth ions, Sm³⁺ possesses the sharp, linear emitting from 4f-4f electron transitions in different matrix environments, and is commonly used as an excellent orange-red phosphor.16, 17, 18 The doping of Sm³⁺ can help reducing CCT in the process of wLEDs realization. As for non-rare earth ion Bi³⁺, the luminescence color is greatly affected by the coordination environment, which can be tuned from UV to green and even red by regulating the matrix environment.^19,20 Consequently, the combination of Bi³⁺ and Sm³⁺ in suitable matrix environment is an excellent strategy for realization of single phase wLED phosphor.

Rare earth vanadate (REVO₄) is an important kind of phosphor matrix material. Compared with 4f-4f electron transition in rare earth ions, VO₄³⁻ has better absorption capacity in the UV region and can effectively transfer the absorbed energy to activators, which can greatly improve the luminescence efficiency of activators.²¹ Moreover, the high chemical and thermal stability of REVO₄ further expands its application as phosphors matrix material.²² Phosphors based on vanadate have been studied widely, such as Eu³⁺ doped YVO₄ which has been developed as excellent red phosphor.²³ An interesting thing here is that the activators doping into phosphate-vanadate co-doped matrix (RE (P_xV_1−x)O₄) have better luminescence performance, as the doping of PO₄³⁻ can affect energy absorption and transfer process of VO₄³⁻.²⁴ Our team has studied that Tm³⁺, Dy³⁺ and Eu³⁺ doped phosphate-vanadate phosphors show some unique luminescence properties and special application value.25, 26, 27

In this work, we attempt to fabricate a new kind of single phase warm white light emitting phosphor by regulating doping ratio between Sm³⁺ and Bi³⁺, as well as the ratio between VO₄³⁻ and PO₄³⁻ groups in Gd(P_xV_1–x)O₄. The luminescence properties, color-tuning performance and energy transfer mechanism of Gd(P_xV_1–x)O₄:y at% Sm³⁺,1 at% Bi³⁺ were analyzed and discussed in detail.