Photoluminescence and energy transfer rates and efficiencies in Eu3+ activated Tb2Mo3O12

Abstract

Luminous efficacy (LE) and colour rendering index (CRI) of various simulated phosphor-converted warm white-light emitting diodes are calculated. Actual measured phosphor emission spectra are employed for this task. The efficacy and CRI of Eu³⁺ activated red emitting phosphors are superior to Eu²⁺ emitting nitride-based phosphors, however, Eu³⁺ suffers from low absorption strength in the blue spectral range. Tb³⁺ exhibits comparatively strong absorption in this range and can be used as a sensitizer for Eu³⁺. A solid solution series of (Tb_1−xEu_x)₂Mo₃O₁₂ (TM:Eu³⁺) powders and ceramic discs is prepared by conventional solid state synthesis. Complete transfer from Tb³⁺ to Eu³⁺ is achieved in the (Tb_0.8Eu_0.2)₂Mo₃O₁₂ sample and a red colour point is realized upon 487 nm (Tb^{3+ 7}F₆ → ⁵D₄) excitation with a quantum efficiency of 94%. Full conversion of a 380 nm LED and improved conversion of a 465 nm LED was achieved employing TM:Eu³⁺ ceramics. The position and temperature related shift of the Eu³⁺ Stark sublevels is determined from temperature-dependent emission and excitation spectra. These spectra also reveal excited state absorption from thermally excited Eu^{3+ 7}F₁, ⁷F₂ and ⁷F₃ states. High-temperature measurements in the range of 350 to 800 K show a T_0.5 of 627 K. Decay measurements exhibit a clearly visible rise time. A new method to determine energy transfer rates from rise time curves is developed. From the energy transfer rates the transfer mechanism and efficiency can be determined with a higher degree of confidence compared to methods based on luminescence intensities.