Dopant preferential site occupation and high efficiency white emission in K2BaCa(PO4)2:Eu2+,Mn2+ phosphors for high quality white LED applications

Abstract

The realization of white-light luminescent materials with high quantum efficiency remains a challenge for light emitting diode (LED) applications. In this research, a series of novel color-tunable Eu²⁺/Mn²⁺ co-doped K₂BaCa(PO₄)₂ phosphors were successfully prepared using a solid-state method. The crystal and electronic structures, Eu²⁺/Mn²⁺ site occupancy and luminescence properties were investigated for the first time. The Rietveld refinement indicates that both Eu²⁺ and Mn²⁺ tend to locate at M3 sites because of their compact coordinated octahedrons. Excitation and emission spectra together with decay lifetime variation consistently confirm the occurrence of efficient energy transfer from Eu²⁺ to Mn²⁺ ions that leads to an adjustable luminescence. Emitting colors can be tuned from blue to white to yellow, by varying the Eu²⁺/Mn²⁺ concentration in the K₂BaCa(PO₄)₂:Eu²⁺,Mn²⁺ phosphor. The external quantum efficiency of the optimized K₂BaCa(PO₄)₂:Eu²⁺,Mn²⁺ phosphor is ∼70%. LEDs fabricated by combining a 365 nm chip with the K₂BaCa(PO₄)₂:Eu²⁺,Mn²⁺ phosphor exhibit bright white light (correlated colour temperature (CCT) = 6787 K, colour rendering index (R_a) = 74.4) with excellent current stability of up to 300 mA. This demonstrates that the obtained phosphors could be applied to phosphor converted LED devices.