Tuning of the emission color via energy transfer from Bi³⁺ to Eu³⁺ ion in the Eu³⁺, Bi³⁺ doped calcium germanate phosphor for warm white LEDs

In the lighting field, it is essential to seek the phosphor materials with tunable emission for warm white light-emitting diodes (wLEDs). Herein, we reported a new type of Eu³⁺, Bi³⁺ doped calcium germanate phosphor, which can exhibit a broad emission tuning from blue, white and to red. According to the XRD results, the structure purity of all as-obtained germanate phosphors are revealed to feature a trigonal structure with the space group of R₃(No. 146). Structural analysis shows that the dopants of Eu³⁺ and Bi³⁺ should tend to substitute the Lu sites in the crystal host. The photoluminescence (PL) spectroscopy shows that the Eu³⁺ and Bi³⁺ ions can emit the blue emission and the red emission upon excitation at 297 nm and 393 nm, respectively. In view of the significant spectral overlapping between the excitation wavelength of Eu³⁺ and the emission wavelength of Bi³⁺, we study the energy transfer from Bi³⁺ and Eu³⁺ ion and reveal that the energy transfer is happened via an electric dipole–dipole (d–d) interaction. Remarkably, when the doping content of Bi³⁺ ions is fixed, changing the doping content of Eu³⁺ ions can induce the emission color tuning from blue, white and to red. To explain better the PL observation, a mechanistic pattern, which bases on the energy transfer from Bi³⁺ and Eu³⁺ ion, has been also constructed, which is expected to provide a general idea for achieving the energy-transfer-controlled tunable phosphor materials for phosphor-converted wLEDs.