White light LEDs (light-emitting diodes) are rapidly replacing incandescent lamps and (compact) fluorescent tubes in all lighting markets (consumer, automotive, and professional). The most widely used type of white LEDs (w-LEDs) is a phosphor-converted w-LED (PC-LED), which is composed of an InGaN-based blue LED and visible light emitting inorganic phosphors, such as oxides and nitrides doped with Ce³⁺, Eu²⁺ , Mn⁴⁺ as an active center. These phosphors are required to have excellent thermal quenching behavior because the LED chip reaches temperatures up to ∼200 °C in recent high-power w-LED. In these phosphors, two processes of thermal quenching are considered generally. One is the thermally activated cross over from the excited state to the next lower state. The other is the thermal ionization from the excited state to the bottom of the conduction band (CB). In this study, we investigated the thermal quenching mechanism for the 5d-4f and 3d-3d luminescence in w-LED phosphors doped with Ce³⁺ or Mn⁴⁺.

Ce³⁺-doped Y₃Al₅O₁₂ garnet (yttrium aluminum garnet,YAG) is the most prominent phosphor in w-LEDs. The excellent thermal quenching behavior of the Ce³⁺ luminescence in YAG:Ce³⁺ is well established, but the mechanism for thermal quenching remains unclear. To elucidate the mechanism of thermal quenching of YAG:Ce³⁺, thermoluminescence excitation (TLE) spectra were recorded at room temperature and 300 °C. At room temperature, the lowest 5d₁ band at 450 nm does not contribute to the charging process for TL while at 300 °C, a temperature corresponding to the onset of thermal quenching of the Ce³⁺ luminescence, the excitation in 5d₁ band gives rise to a TL signal. This result indicates that thermal ionization is responsible for thermal quenching of the Ce³⁺luminescence.

We also investigated the thermal quenching process for calcium aluminum magnetoplumbite, CaAl₁₂O₁₉, doped with Mn⁴⁺ (CAO:Mn⁴⁺), which is narrow red luminescence phosphor. We tried to understand the quenching mechanism of Mn⁴⁺ compared with the luminescence quenching properties of Mn²⁺. From the temperature dependence of Mn⁴⁺: ²E-⁴A₂ red luminescence and Mn²⁺:⁴T_1-⁶A₁ green luminescence intensities in CaAl₁₂O₁₉, the quenching temperatures of Mn⁴⁺ and Mn²⁺ are 330K and 800K, respectively. There is a big difference between the quenching temperatures of Mn⁴⁺ and Mn²⁺ in spite of the same host material and the similar luminescence transtion energy. The Mn⁴⁺:²E-⁴A₂ red luminescence is quenched by the thermally activated crossover through the ⁴T₂ excited state whose potential curve possesses a large configurational offset in a configurational coordinate diagram. Mn²⁺ green luminescence is also quenched by the thermally activated crossover, but the activation energy for non-radiative process is much larger than that for Mn⁴⁺ because there are no other excited states related with the quenching process.