Saturation Mechanisms in Common LED Phosphors

  • 商用荧光粉的要求：
  • YAG:Ce3+， CaAlSiN3:Eu2+目前还是最好的商用荧光粉，outcompete rapidly developing alternatives。
    • 优点：a close to 100% internal quantum yield and prolonged LED operating hours without severe degradation.
    • 缺点：thermal quenching or saturation quenching (automotive head lights and projector devices)
  • 对于high-power applications应用场景，目前只有YAG:Ce3+满足要求，没有可用的红色荧光粉。

Two-level systems $$ \frac{\mathrm{d} p}{\mathrm{~d} t}=k_{\mathrm{exc}}[1-p(t)]-k_{\mathrm{decay}} p(t) $$(1) \(p(t)\) is the probability to find a center in the excited state at some moment in time \(t\);
(2) \(k_{\mathrm{exc}}=\sigma I_{\mathrm{exc}} / \hbar \omega\) is the rate constant for excitation, depends on the absorption cross-section \(\sigma\) of a center, , the excitation intensity \(I_{\mathrm{exc}}\), , and on the excitation photon energy \( \hbar \omega\);
(3) \(k_{\text {decay }}\) is the rate constant for decay, \(=k_{\text {rad }}+k_{\text {nonrad }}\).

两种情况：

  • \(t>t_{2}\)
    \(p(t)=p\left(t_{2}\right) e^{-k\tiny{\text {decay }}\left(t-t\tiny{2}\right)}\)
  • \(t_{1}<t<t_{2}\)
    \(p(t)=p\left(t_{2}\right)\left[1-e^{-k_{\text {rise }}\left(t-t_{1}\right)}\right]\)其中\(k_{\text {rise }}=k_{\text {exc }}+k_{\text {decay }}\)

可以被测定的参数：As the emission intensity from the sample is proportional to excited-state population, both \(k_{\text {exc }}\) and \(k_{\text {decay }}\) can be determined from a measurement of the luminescence dynamics with square-wave-modulated excitation.

KSF:Mn4+ Mild experiment conditions
(1) the grains were placed on an indium foil directly connected to a heat sink; (2) the peak excitation intensity was limited to a maximum of 0.7 W mm−2;
(3) the duty cycle was set to no more than 5%;
(4) the heat sink waskept at room temperature.

  • Mn4+的激发态寿命为8.5 ms，和之前光谱仪测试的一致，而且这里没有随着激发功率的变化而变化，说明没有在这个实验条件下没有open nnradiative decay pathways。
  • 激发光为blue，发射光为red，所以必然存在热的生成，但是寿命没有变化，说明在此条件下not sufficient to lead to thermal quenching.
  • rise dynamics accelerate with increasing excitation intensity，这很容易理解，因为\(k_{\text {rise }}=k_{\text {exc }}+k_{\text {decay }}\)

 

 

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