Blue-emitting Ca5(PO4)3Cl:Eu2+ phosphor for near-UV pumped light emitting diodes: Electronic structures, luminescence properties and LED fabrications

doi:10.1016/j.jallcom.2015.12.054

Journal of Alloys and Compounds

Volume 663, 5 April 2016, Pages 332-339

https://doi.org/10.1016/j.jallcom.2015.12.054 Get rights and content

Highlights

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The electronic structures of Ca₅(PO₄)₃Cl host were calculated via DFT.
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Temperature-dependent PL property of Ca₅(PO₄)₃Cl:Eu²⁺ was studied.
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Quantum efficiency behaviors of the samples were investigated.
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R_a of 96.65 at a warm CCT of 3902 K was obtained for fabricated WLEDs.

Abstract

A blue-emitting Ca₅(PO₄)₃Cl:Eu²⁺ phosphor was prepared via a conventional high temperature solid-state reaction method. Crystal and electronic structure properties of the Ca₅(PO₄)₃Cl:Eu²⁺ phosphor were investigated using X-ray diffraction and density functional theory (DFT), respectively. The micro-morphology, reflectance spectra, thermal stability and quantum efficiency of the Ca₅(PO₄)₃Cl:Eu²⁺ phosphor were also studied. The optimum Eu²⁺ concentration in Ca₅(PO₄)₃Cl was determined to be 2.0 mol% and the concentration quenching mechanism can be explained by the dipole–dipole interaction. The emission intensity of the Ca₅(PO₄)₃Cl:Eu²⁺ phosphor was 58.2% of the initial value when the measured temperature increased from 30 °C to 150 °C. The activation energy was determined to be 0.254 eV, suggesting the good stability of this phosphor. A bright blue LED was fabricated using an InGaN-based near-UV LED chip (385 nm) and a Ca₅(PO₄)₃Cl:Eu²⁺ phosphor, and has an excellent blue-emitting property with CIE coordinates of (0.1480, 0.0350). Furthermore, a bright near-UV warm white LED was fabricated using an InGaN-based near-UV LED chip (395 nm) in combination with the present blue phosphor and the commercial green and red phosphors, which exhibits an excellent color-rendering index (R_a = 96.65) at a warm correlated color temperature of 3902 K with CIE coordinates of (0.3781, 0.3879). All the results suggest that the Ca₅(PO₄)₃Cl:Eu²⁺ phosphor is a potential blue-emitting candidate for the application in the near-UV pumped blue and warm white LEDs.

Introduction

Nowadays, solid-state lighting based on the combination of light-emitting diode (LED) chips and phosphors has attracted significant attention [1], [2], [3]. Due to the satisfactory characteristics of energy saving properties, long lifetime, high efficiency and high material stability, the solid-state lighting for LEDs is considered to be the next generation lighting source [4], [5], [6]. A typical commercial white LED (w-LED) is generated by pumping the blue InGaN based-LED chip on a yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. Nevertheless, this type of white LEDs suffers many disadvantages such as a high correlated color temperature (CCT > 4500 K) as well as a low color-rendering index (CRI < 80) due to the lack of a red component in the visible spectrum [7], [8]. Another alternative approach for generating white light is by pumping tricolor phosphors (blue, green and red) with near-UV light chips (350–420 nm). This approach produces white light with excellent CRI values and suitable CCT. The white light obtained from this approach is close to the nature light and thus some application areas can be broadened [7], [9], [10]. Therefore, phosphor materials play a very important role in these LEDs and it is vital to develop the high-performance tricolor phosphors which can be efficiently excited by the near-UV light.

As a highly efficient activator with the allowed 4f-5d transitions, Eu²⁺ ion is the most used activator in the phosphor and has been widely investigated at present. The excitation and emission spectra in a specific host generally consist of broad bands due to the 4f-5d transitions of Eu²⁺ ions [8]. Therefore, many Eu²⁺ ion-doped phosphors for near-UV pumped LEDs have been developed, such as Ba₂Ca(PO₄)₂:Eu²⁺ [11], Sr₂SiO_4-3xN_x:Eu²⁺ [12], SrB₂O₄:Eu²⁺ [13], BaMgSiO₄ [14], Ba₃Si₆O₁₂N₃:Eu²⁺ [15], BaSi₃Al₃O₄N₅:Eu²⁺ [16], and so on. Recently, much attention has been drawn to the Eu²⁺ ion-activated phosphates due to the advantages of high physical and chemistry stability, low synthesis temperature, and high quantum efficiency [12], [17], [18], [19]. Among the reported Eu²⁺ ion-activated phosphates, Ca₅(PO₄)₃Cl:Eu²⁺ has attracted particular attention. Some previous works reported the luminescent properties of Ca₅(PO₄)₃Cl:Eu²⁺ [20], [21], [22], but to the best of our knowledge, there is no report dedicated to the electronic structures and temperature-dependent luminescence properties of Ca₅(PO₄)₃Cl:Eu²⁺ phosphors as well as the fabrication of Ca₅(PO₄)₃Cl:Eu²⁺ converted near-UV blue and white LEDs.

In this work, the electronic structures, reflectance spectra, thermal stability and quantum efficiency (QE) of the blue-emitting Ca₅(PO₄)₃Cl:Eu²⁺ phosphors have been studied in detail. Moreover, we have also successfully fabricated the near-UV blue and white LEDs, respectively. The blue LED was fabricated using an InGaN-based n-UV LED chip (385 nm) and a Ca₅(PO₄)₃Cl:Eu²⁺ phosphor. Meanwhile, the near-UV white LED was fabricated using an InGaN-based n-UV LED chip (395 nm) in combination with the present blue phosphor and the commercial green and red phosphors. The results suggest that the Ca₅(PO₄)₃Cl:Eu²⁺ phosphor is a promising blue-emitting material for the application in the near-UV pumped blue and warm white light emitting diodes.

Section snippets

Synthesis

Samples with a general formula of Ca_5-x(PO₄)₃Cl:xEu²⁺ (x = 0, 0.005, 0.01, 0.02, 0.03, 0.05 and 0.07) were prepared by solid state reaction method. NH₄H₂PO₄ (A.R. grade), CaCO₃ (A.R. grade), NH₄Cl (A.R. grade) and Eu₂O₃ (99.99% purity) were used as starting materials. The stoichiometric materials were weighed and ground together in an agate mortar. Then the mixture was put into a muffle furnace and precalcined at 400 °C for 1 h, and subsequently further sintered at 1000 °C for 5 h in the

Crystal structure and morphology characteristics of the Ca₅(PO₄)₃Cl:Eu²⁺ phosphors

The XRD patterns of the representative Ca₅(PO₄)₃Cl and Ca_4.98(PO₄)₃Cl:0.02Eu²⁺ samples are shown in Fig. 1. The diffraction peaks fit well with the standard data card ICSD#24237 of Ca₅(PO₄)₃Cl, indicating that the introducing of Eu²⁺ ions has not brought obvious change of the structure of the Ca₅(PO₄)₃Cl host. Some tiny impurity peaks marked with asterisks located at 27.78° and 31.01° for the Ca₅(PO₄)₃Cl and Ca_4.98(PO₄)₃Cl:0.02Eu²⁺ samples in Fig. 1 might be attributed to the presence of Ca₃(PO₄

Conclusions

In summary, a series of Eu²⁺ doped Ca₅(PO₄)₃Cl blue-emitting phosphors under near-UV excitation has been investigated for the potential application in the blue and warm LEDs. The electronic structures of the Ca₅(PO₄)₃Cl host matrix were calculated using DFT and a wide band gap (5.30 eV) of the Ca₅(PO₄)₃Cl host matrix enables it to accommodate Eu²⁺ ions as an luminescent center. The concentration quenching of Eu²⁺ ions in the Ca₅(PO₄)₃Cl host is determined to be 2.0 mol% and the mechanism of

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 61076056 and No. 51308481), Natural Science Foundation of Fujian Province, China (Grant No. 2013J05082), Fujian Provincial Department of Science & Technology (Grant No. 2015H0036), Fundamental Research Funds for the Central Universities (Grant No. 2013121031 and 2013SH004), Program for New Century Excellent Talents in Fujian Province University (NCETFJ), Scientific Research Foundation for the Returned

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Blue-emitting Ca5(PO4)3Cl:Eu2+ phosphor for near-UV pumped light emitting diodes: Electronic structures, luminescence properties and LED fabrications

Highlights

Abstract

Introduction

Section snippets

Synthesis

Crystal structure and morphology characteristics of the Ca5(PO4)3Cl:Eu2+ phosphors

Conclusions

Acknowledgments

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

Mater. Res. Bull.

J. Lumin

J. Alloys Compd.

J. Alloys Compd.

Opt. Laser Technol.

Comp. Mater. Sci.

J. Non-Cryst. Solids

J. Alloys Compd.

Phys. Lett. A

J. Solid State Chem.

J. Lumin

J. Solid State Chem.

Opt. Commun.

Opt. Commun.

J. Lumin

Mater. Chem. Phys.

ACS Appl. Mater. Interfaces

ACS Appl. Mater. Inter

Light Sci. Appl.

Blue-emitting Ca₅(PO₄)₃Cl:Eu²⁺ phosphor for near-UV pumped light emitting diodes: Electronic structures, luminescence properties and LED fabrications

Crystal structure and morphology characteristics of the Ca₅(PO₄)₃Cl:Eu²⁺ phosphors