Study On Energy Transfer Modified YAG:Ce³⁺/R(R=Pr³⁺,Cr³⁺) Phosphors For Use In High Color Rendering White LEDs

At present, the main strategy for producing white light is to combine blue LED with the yellow emitting Y3Al5O12: Ce³⁺ (YAG: Ce³⁺) phosphor, which can strongly absorb the blue light and subsequently emit yellow light, originating from the transition from the lowest 5d state to the 2F5/2 and 2F7/2 ground states of Ce³⁺. However, YAG: Ce³⁺ has relatively weak emission in the red spectral region, leading to low color rendering index for the current white LEDs. To enhance the red component, we single and doubly doped Pr³⁺ and Cr³⁺ ions to Y3Al5O12: Ce³⁺. The main purpose of this work is to research their photoluminescence, energy transfer and fluorescence decay properties, the main results obtained are listed as follow:（1） Nonradiative mutual energy transfers occur between Ce³⁺ and Pr³⁺ in Y₃Al₅O₁₂ （YAG）. The Ce³⁺â†'Pr³⁺ energy transfer starts from the lowest 5d state of Ce³⁺ to the 1D2 level of Pr³⁺ rather than the 3P0 state of Pr³⁺. As the lowest 4f5d band of Pr³⁺ is excited at 288 nm, Pr³⁺â†'Ce³⁺ energy transfer can occur through two pathways. One performs a direct transfer from the lowest 4f5d state of Pr³⁺ to the 5d state of Ce³⁺. Another describes that the lowest 4f5d state relaxes down to the 3P0 levels which subsequently transfers to the lowest 5d state of Ce³⁺. The former pathway is dominant. Ce³⁺ - Pr³⁺ and Pr³⁺ - Pr³⁺ ET are both governed by electric dipole-dipole interaction. For Ce³⁺ concentration of 0.01, the corresponding rate constant and critical distance are evaluated to be 4.5×10^-36 cm6s^-1, 0.81 nm for Ce³⁺ - Pr³⁺ ET and 2.4×10^-38 cm6s^-1, 1.30 nm for Pr³⁺ - Pr³⁺ ET. Spectroscopic study also demonstrates a pronounced ET from the lowest 4f5d of Pr³⁺ to the 5d of Ce³⁺. The relative PL intensities of Ce³⁺ and Pr³⁺ for various Ce³⁺ and Pr³⁺ concentrations are in good agreement with the results evaluated from fluorescence decay data.（2） Upon 5d excitation of Ce³⁺ in YAG:Ce³⁺, Pr³⁺, the Ce³⁺ - Pr³⁺ ET results in a growth of the intensity ratio of the 1D2â†'3H4 red line of Pr³⁺ to the yellow band of Ce³⁺ with increasing Pr³⁺ concentration up to a critical concentration. Further increasing Pr³⁺ concentration leads to a reduction of the Red/Yellow ratio and an rapid shortening of the 1D2 lifetimes due to concentration quenching by a Pr³⁺â†'Pr³⁺ cross relaxation described by （1D2, 3H4）– （1G4, 3F4）. An increase of the Ce³⁺ - Pr³⁺ ET rate followed by the enhanced the Red/Yellow ratio on only increasing Ce³⁺ concentration is observed. This behavior is attributed to the increase of the spectral overlap integrals between Ce³⁺ emission and Pr³⁺ excitation due to the fact that the yellow band shifts to the red side with increasing Ce³⁺ concentration while the red line dose not move.（3） A proportional dependence of the initial transfer rate on acceptor concentration is observed in each of these ET pathways. The proportional coefficient as the averaged ET parameters for initial decay are determined to be 73Ï„10^-1 for ET from Ce³⁺ lowest 5d to Pr³⁺ 1D2, 1252Ï„20^-1 for ET from Pr³⁺ 1D2 to another Pr³⁺ in the ground state, and 237Ï„40^-1 for ET from Pr³⁺ lowest 4f5d to Ce³⁺ 5d states, respectively, meaning the ET efficiency for the same concentration of acceptors follows the order of Pr³⁺-Pr³⁺ > Pr³⁺- Ce³⁺ > Ce³⁺ - Pr³⁺.（4） Nonradiative mutual energy transfers occur between Ce³⁺ and Cr³⁺ in Y₃Al₅O₁₂ （YAG）.Ce³⁺ - Cr³⁺ is governed by electric dipole-dipole interaction. For Ce³⁺ concentration of 0.01, the corresponding rate constant and critical distance are evaluated to be 1.03×10^-36 cm6s^-1, 0.63 nm.（5） Y3Al5O12: Ce³⁺, Pr³⁺, Cr³⁺ phosphors are prepared by solid state reaction. Three typical emission bands: yellow emission from Ce³⁺, light red emission from Pr³⁺ and deep red emission from Cr³⁺ are achieved upon blue light excitation on Ce³⁺. The study of photoluminescence and fluorescence decay indicates that there are Ce³⁺â†'Cr³⁺ and Ce³⁺â†'Pr³⁺â†'Cr³⁺ energy transfers. The rate constant and critical distance of Pr³⁺â†'Cr³⁺ energy transfer are evaluated to be 7.3×10^-37 cm6s^-1, 0.22 nm.（6） For the same Cr³⁺ concentration, the macroscopic Ce³⁺â†'Cr³⁺ energy transfer rate in the triply doped phosphor is larger than that in the doubly doped one, which is attributed to the competition between Ce³⁺â†'Pr³⁺ energy transfer and Ce³⁺â†'Cr³⁺ energy transfer. A white LED fabricated using a blue LED chip with the triply doped phosphor shows a color rendering index of 81.4 that is higher than that either using Ce³⁺, Cr³⁺ doubly doped or Ce³⁺ singly doped phosphors.